162208/fulltext01.pdf · to be published in tribologia no.4 2002, finnish journal of tribology....

2 Lars Kraft

Dissertation for the Degree of Doctor in Philosophy in Materials Science, presented at Uppsala University in 2002. ABSTRACT Kraft, L. 2002. Calcium-Aluminate Based Cement as Dental Restorative Materials. Acta Universitatis Upsaliensis. Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 775. 67 pp. Uppsala. ISBN 91-554-5465-8.

This thesis presents the results from the development process of a ceramic dental filling material based on calcium aluminate cement. The main focus of the work concerns dimensional changes during setting, hardening and curing and the understanding of the factors controlling the dimensional stability of the system. A range of compositions for research purposes and the composition of Doxadent – a dental product launched on the Swedish market in October 2000 – were evaluated. Furthermore hardness characteristics, flexural strength, porosity and microstructure studies are presented. The studies of dimensional changes led to a thorough investigation of the measuring devices used and their relevance. A split pin expander technique, very simple in function, has been evaluated and improved. The technique is considered to be adequate for detecting dimensional stability in restrained samples, thus mimicking the case for real fillings in most tooth cavities. The dimensional changes in the calcium-aluminate based cement system are mainly controlled by the grain size, the exact composition and the compaction degree. The expansion of the calcium-aluminate cement system was in the early work decreased from several percent down to only tenths of a percent. Results show that Doxadent has less than 0.2% in linear expansion after 200 days of storage in water. However, long-term tests have been unable to verify whether expansion stops with time. Long-term in-vitro studies of dimensional changes also affect the test equipment used, which is why the long-term behaviour of the dimensional stability has to be clinically evaluated. The material integrates excellently with the tooth structure, has hardness and thermal properties similar to those of enamel and dentine, and is also biocompatible during hardening. A patented process for the preparation of wet compacted specimens was also developed. Lars Kraft, Department of Materials Science, The Ångström Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden © Lars Kraft 2002 ISSN 1104-232X ISBN 91-554-5392-9

Printed in Sweden by Fyris-Tryck AB, Uppsala 2002

Calcium Aluminate Based Cement as Dental Restorative Materials 3

To our oldest son Benjamin, Saved by amazing grace from the threat of death, he daily me embraces and takes away my breath!

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List of enclosed papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals: I. A Method for the Examination of Geometrical Changes in a CAC

Based Cement Paste L. Kraft, L. Hermansson, G. Gomez-Ortega, pp. 401-413 in proceedings of Shrinkage of Concrete - Shrinkage 2000, Pro 17, RILEM, 16-17 October 2000, edited by V. Baroghel-Bouny and P-C. Aitcïn.

II. Early-Age Dimensional Changes, Drying Shrinkage and Thermal Dilation in a New Type of Dental Restorative Material Based on Calcium Aluminate Cement L. Kraft, H. Engqvist, L. Hermansson, Submitted to Cement and Concrete Research in September 2002.

III. Deformation Characteristics in Various Calcium Aluminate Cement Admixtures Investigated With Three Different Methods L. Kraft, L. Hermansson, in manuscript.

IV. Hardness and Dimensional Stability of a Bioceramic Dental Filling Material Based on Calcium Aluminate Cement L. Kraft, L. Hermansson, 26th Annual Conference on Composites, Advanced Ceramics, Materials, and Structures, Cocoa Beach, Florida, The American Ceramic Society. Vol 23B, Issue 4, 2002. Edited by H-T Lin, M. Singh.

V. Characteristics of the Contact Zone between Tooth Structure and a Dental Filling Material Based on Calcium Aluminate Cement L. Kraft, J. Li, L. Hermansson, submitted to Journal of Dentistry in July 2002.

VI. Characterization of Cold Isostatically Pressed Calcium Aluminate Cement L. Hermansson, L. Kraft and J. Li, submitted to Journal of Materials Science in September 2002.

VII. Flexural Strength of a Bioceramic Dental Filling Material H. Engqvist, L. Kraft, K. Lindquist, N-O. Ahnfelt, L. Hermansson, submitted to Advanced Materials in July 2002.

VIII. Abrasive Wear of a Bioceramic Dental Filling Material H. Engqvist, S. Uppström, L. Kraft, N-O. Ahnfelt, L. Hermansson and S. Hogmark. To be published in Tribologia no.4 2002, Finnish Journal of Tribology.


IX. Aspects of Biocompatibility and Chemical Stability of Calcium-Aluminate-Hydrate-Based Dental Restorative Materials Y. Liu, J. Li, L. Sahlberg, L. Kraft, N-O Ahnfelt, L. Hermansson and J. Ekstrand. Submitted to Biomaterials in November 2002.

Papers are reproduced with permission from the publishers. The author’s contribution to the papers: I, III Major part of planning, experimental work and evaluation. II, IV All planning, experimental work and major part of evaluation V Major part of planning, part of experimental work and major

part of evaluation VI, VII Part of planning, experimental work and evaluation VIII ,IX Part of planning and evaluation.

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Contents

List of enclosed papers ...................................................................................4

Contents ..........................................................................................................6

The Quest........................................................................................................9

1. Introduction...............................................................................................10 1.1 Scope of this thesis ...........................................................................12

2 Materials in Dentistry ................................................................................13 2.1 Dental filling materials .....................................................................13

2.1.1 Amalgam .................................................................................14 2.1.2 Composites ..............................................................................15 2.1.3 Glass-ionomer cements (GICs) ...............................................18 2.1.4 Compomers and resin-modified glass-ionomers .....................19

2.2 The oral environment........................................................................20 2.3 Characteristics of the ideal material .................................................20 2.4 The tooth structure............................................................................21

3 Hydraulic Cements ....................................................................................23 3.1 Different types of hydraulic cements................................................24 3.2 Setting characteristics of hydraulic cements.....................................25

3.2.1 Hydration of cement ................................................................26 3.2.2 Dimensional changes in hydrated cement ...............................26

3.3 Calcium aluminate cement ...............................................................28 3.3.1 The hydration and conversion mechanisms of CACs..............28

3.4 CAC in dental filling materials.........................................................30

4 Characteristics............................................................................................31 4.1 Influence of test methods on results .................................................31 4.2 Results from earlier work .................................................................32 4.3 Compositions ....................................................................................33 4.4 Specimen preparation .......................................................................34

4.4.1 Pre-pressed specimens.............................................................35 4.4.2 Prepared (compacted) specimens ............................................35


4.4.3 The wet-press method..............................................................35 4.5 The compaction degree.....................................................................36 4.6 Dimensional changes (Paper III) ......................................................37

4.6.1 Influence of the physical and chemical characteristics............37 4.6.1.1 The compaction degree ..................................................38 4.6.1.2 Influence of preparation technique.................................39 4.6.1.3 Influence of grain size ....................................................40 4.6.1.4 Chemical composition....................................................40 4.6.1.5 Filler ratio.......................................................................40

4.6.2 Influence of the measuring technique......................................41 4.6.2.1 Temperature, humidity and chemical environment........42 4.6.2.2 Sample position and specimen size ................................42 4.6.2.3 The differential hydration...............................................43

4.6.3 Conclusions from the deformation tests ..................................46 4.7 Mechanical properties.......................................................................47

4.7.1 Hardness and porosity (Paper IV) ...........................................47 4.7.2 Flexural strength and fracture toughness (Paper VII)..............49 4.7.3 Wear characteristics (Paper VIII) ............................................50

4.8 Retention properties (Paper V) .........................................................51 4.9 Phase stability aspects.......................................................................52 4.10 The microstructure..........................................................................54 4.11 Biocompatibility (Paper IX) ...........................................................56

5 The Future..................................................................................................58

6 Acknowledgements....................................................................................60

7. References.................................................................................................61

Appendix I - Classification of cavities..........................................................65

Appendix II – The BOVA model .................................................................66

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Abbreviations

H H2O C CaO A Al2O3 CA CaAl2O4 CA2 CaAl4O7 C12A7 Ca12Al14O33 AH3 2 Al(OH)3 C3AH6 3CaO Al2O3 6H2O C2AH8 2CaO Al2O38H2O CAC Calcium aluminate cement OPC Ordinary Portland cement


The Quest

“I have asked God for a dental filling material to replace amalgam. Why don’t you do something really positive with your knowledge of materials?” This question posed by a mother-of-seven and dental technician about 15 years ago was the seed behind this work. Leif Hermansson, who at that time was R&D manager of ASEA Cerama and who is now professor of Structural Ceramics at the Dept. of Materials Science – and my supervisor - saw his life turn in a new direction because of this question. Evidently his wife Irmeli was troubled over the toxicity of mercury in amalgam. At the school for dental technicians she was taught to carefully collect all the amalgam after practice, or the mercury might cause environmental problems. The logical contradiction of restoring people’s teeth by placing the substance in their mouths was frustrating.

“Is it possible to develop a dental filling material that resembles enamel?” I asked the professor at a lecture on ceramic materials nine years later. This small glimmer of interest and curiosity, was probably the small seed that later led to my being offered a PhD student position.

This thesis deals with two scientific topics; the first is calcium aluminate

cement, (CAC) and its properties with the focus on its dimensional stability, and the second is the application of a calcium aluminate-based material as a dental filling material.

Much of the work presented here is conducted using self-developed techniques and measurement methods. This is because this type of material is dependent on a preparation technique different from that of established dental filling materials such as composite, amalgam or glass-ionomer cements. For this reason some measurement techniques for dental material are not always practical or even relevant for this type of material. Furthermore, small-size samples (such as dental filling material sizes) of cement specimens do not fit into normal cement and concrete research experiments dealing with large specimens only.

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1. Introduction

More than 50% of all dental restorations today are restorations of old dental fillings. There is therefore a need for a dental filling material that will perform more satisfactorily. The major reason for the failure of existing dental materials is secondary caries that develops due to bacterial ingress at the interface of the filling and the tooth, Table 1. In order to prevent postoperative problems a perfect seal of the dental cavity is essential so that no gaps form whatsoever. Therefore the filling material should be dimensionally stable throughout its insertion, hardening and long-term performance. To achieve a perfect seal at the tooth/filling interface a material should interact chemically to accomplish a bond with the tooth substance. Such a material needs to be tooth-like and must display excellent biocompatibility to perform with an appropriate host response in this specific application.

The task of developing a new biocompatible dental filling material that performs satisfactorily, is environmentally friendly and does not have any negative side effects on dentists or patients i.e. hand allergies [1] or mercury Table 1. Reasons for replacement of amalgam, composite, glass-ionomer cement (GIC) and resin modified glass-ionomer (RMGi) restorations in permanent teeth of adults >19 years expressed as percentages. [2]. Clinical diagnosis Amalgam Composite GIC RMGi n = 5,731 n = 2,952 n = 287 n = 93 Secondary caries 57 47 50 50 Fracture of restoration 25 24 25 17 - bulk 21 19 20 12 - margin 4 5 5 5 Discoloration 0 15 3 10 - bulk 0 12 3 8 - margin 0 3 0 2 Lost restoration 0 0 0 1 Fracture of tooth 7 4 4 5 Poor anatomic form 1 2 9 3 Pain/sensitivity 2 1 1 0 Change of material 4 1 1 2 Other reasons 4 7 8 12


toxicity [3], is very complex. Dental filling materials are inserted in the tooth cavity at the surrounding temperature and require a transition from a formable state to a solid state in a short time to avoid dimensional changes or the release of poisonous substances.

A brief summary of historical attempts aimed at meeting these requirements - or perhaps more accurately - to improve dental health is presented in table 2 [4, 5, 6]. This is only a brief list of the dental materials used during the period and is not intended to be an exact account of all the approaches used.

Table 2. History of dental filling materials 600 B.C. Etruscan gold bridge work 600 A.D. Chinese amalgam 1480 First record of gold fillings in human teeth by Johannes Arculanus,

University of Bologna 1500 Ivory dentures are carved from wax models 1744 Duchateau makes the first recorded porcelain denture 1819 Mineral cement, consisting of bismuth, tin and lead invented in France.

Gave name to the Swedish word “plomb” for filling from the Latin “plumbum” (lead)

1826 Taveau of Paris suggests the use of silver and mercury to make pastes for filling teeth

1879 The first cement to set in the mouth, zinc phosphate, is introduced

1880 Silicate cements are developed 1895 G.V. Black publishes the first detailed study on the properties of

amalgams 1907 W.H. Taggart invents a practical method for casting gold inlays 1950 Introduction of acrylic resin for fillings and dentures 1970 Composites begin to replace silicate cements 1976 Glass-ionomer cements is invented by A. Wilson 1978 Light- activated composites appear on the market 1985 Development of dentine bonding agents 1990s Development of compomers and resin-modified glass-ionomer cements 2000 Calcium aluminate-based filling material is introduced on Swedish

market

This thesis presents an insight into parts of the development of the material last mentioned in the list above. The focus is on the dimensional stability for a perfect seal but also on some important physical properties and biocompatibility aspects.

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1.1 Scope of this thesis Chapter 2 presents an account of the commercial dental filling materials, their benefits, drawbacks, their handling and hardening characteristics and their clinical success. Dental professionals may proceed directly to chapter 3 which gives an introduction to hydraulic cements, and particularly calcium aluminate cements. Chapter 4 summarizes the findings presented in more detail in the papers I-IX. There, the focus is on the dimensional stability of Ca-aluminate-based materials and some important physical properties. Chapter 5 discusses the future of Ca-aluminate based cement systems used as dental filling materials.


2 Materials in Dentistry

There are many different kinds of applications for dental materials in the oral environment. Besides dental filling materials, the focus of this thesis, dentists also deal with inlays, onlays and crowns for the repair of single teeth, and with sealants, gels, varnishes and liners in preventive dental health care. Furthermore dentists make bridges, partial dentures, dentures, impressions – by use of impression materials, dies and replicas, temporary restoratives, retainers and appliances for orthodontics and implants for anchoring fixed prosthesis to the maxilla or mandible (the upper and lower jawbones respectively). It is therefore no exaggeration to state that dentists and dental technicians have a wider variety of materials at their disposal than most people in other professions. A summary of the aspects of existing dental materials with regard to chemistry, handling and clinical performance, which is based mainly upon some inspiring textbooks [7, 4, 8, 9], will be presented below.

2.1 Dental filling materials In materials science, materials are generally divided into three groups: metals, polymers and ceramics. A composite is a combination of two or more of these different types of materials. In odontology the term ‘composite’ defines resins (polymers) where at least 50% of the weight consists of ceramic fillers.

Since this thesis originates from a Materials Science department and not from an odontology department, a short description of the most important dental filling materials seems appropriate. Their chemistry, handling and performance are briefly explained and their advantages and drawbacks are summarized. Despite all the benefits of the dental materials available today, improvements can still be made, both with regard to choice of materials and in the handling of them. Added to that, the development, handling and use of existing materials represent a fundamental platform of established knowledge in understanding how to develop new and better dental restoratives.

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2.1.1 Amalgam Amalgam is still the most widely used dental filling material of all those available. Dental amalgam is a mixture of approximately 50% mercury and an alloyi consisting of silver, tin, copper, and other elements. Conventional amalgam, often referred to as low-copper amalgam, is produced by mixing mercury with a powdered alloy containing ~70% silver, 27% tin and 5% copper. High-copper amalgam was developed for improved clinical performance and this has been standard since 1986. Such amalgams contain less silver and considerably more copper in the alloy (~13-30%) and hence require less mercury.

Amalgams are supplied either in pre-dosed disposable capsules or in bulk as powders or tablets (powder compressed to look like aspirin tablets) that must be mixed in a reusable capsule with a precise amount of mercury. The pre-dosed capsules are the most popular because they are the most convenient and user-friendly, minimising both mercury spill and contamination at mixing. (The alloy powder resides in one end of the capsule separated from the liquid mercury by a breakable diaphragm.)

Mixing is accomplished by shaking the closed amalgam capsules in so-called triturators at high speed, 2000-4500 cycles per minute for about 15 seconds. After trituration the amalgam is dispensed into a container, from where an amalgam carrier transfers the material into the cavity for condensing. The plasticity of the amalgam, determined by the mercury/alloy ratio, and the particle shape of the alloy control the mouldability of the amalgam.

The condensation, achieved by condensers of different tip-shapes and diameters, adapts the amalgam intimately to the walls of the cavity, minimises the porosity and expresses excess mercury not needed for the chemical reaction in the hardening process.

The hardening reaction between mercury and alloy which follows mixing is termed an amalgamation. For conventional amalgam the reaction is as follows:

7 Ag3Sn + 28 Hg → Ag3Sn + 9 Ag2Hg3 + Sn7Hg alloy mercury γ γ1 γ2

i Alloys (from fr. alloy = combine) are substances with metallic properties that consists of a metal fused with one or more metals or non-metals. Alloys may be a homogeneous solid solution, a heterogeneous mixture of tiny crystals, a true chemical compound or a combination of these. Metallic glasses and crystalline alloys have also been developed, and metal alloys are sometimes bonded with ceramics, graphite, and organic materials as composites.


The phases called γ1 and γ2 have low strength and the γ2 phase corrodes easily. In high-copper amalgam the hardening reaction continues according to: γ2 + Ag-Cu → Cu6Sn5 + γ1

By eliminating the γ2 phase the mechanical properties are improved.

High-copper amalgam increases the materials strength, gives it faster hardening, reduces its shrinkage and increases the resistance to corrosion but may fail to adequately close the gap to tooth substance due to reduced production of corrosive products.

The main benefits of amalgam are its strength, durability and success in numerous clinical trials. The material is also cheap and comparatively easy to handle. Its main disadvantages are its need for undercut cavities (involving the removal of a lot of healthy tooth substance) bad aesthetics, and lack of adhesion to the tooth substance. Other disadvantages are its high electrical and thermal conductivity, its high thermal expansion coefficient, lack of biocompatibility and its toxicity for the environment. Like all other dental materials amalgam is sensitive to correct or incorrect handling. For example, correct condensation is essential for good performance and if the material becomes contaminated with moisture at insertion, it will lead to increased corrosion and delayed expansion that later might crack the tooth.

The major cause of failure in amalgam fillings, as for most restorative materials, is secondary caries. Other reasons for replacing amalgams are marginal breakdown and bulk failure. Another issue that has received a great deal of attention in recent years is amalgam’s lack of biocompatibility, seeing it consists of almost 50% mercury. However, the toxicity of mercury also serves as an effective weapon against bacterial attacks on the cavity interface. (Hence the toxicity of mercury is an advantage in the restricted area of a tooth cavity.) Although there is a general consensus regarding the lack of proof linking amalgam restorations to diseases in dental patients [10], everyone agrees that the general toxicity of amalgam is negative biologically and environmentally.

2.1.2 Composites Composites comprise the second largest group of materials in the dental market today. These materials are constantly gaining new market shares at the expense of amalgam. They are the leading materials for anterior tooth restorations due to a high degree of aesthetics. They consist of a blend of resins and ceramic particles as fillers. They also contain other additives, such as silane (used to form a bond between the fillers and the resin matrix), initiation systems, catalysts, inhibitors and/or stabilisers, for controlling the

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handling and hardening properties [11]. The hardening reaction is initiated by free radicals (easily activated organic compounds) that start a polymerisation process i.e. a conversion of monomers to long rigid polymers. Composites combine the excellent mouldability qualities of resins with the hardness and wear-resistance of ceramics. Their main drawback is shrinkage during polymerisation that might create gaps at the interface of the restorative and tooth substance. To counteract any gap formation, an advanced bonding technique has been developed in order to achieve a perfect seal and strong bond with composite restorations, a technique that must be regarded as somewhat complicated.

Dental composites are classified in different groups according to the polymerisation method, particle size distribution and filler ratio [9]. The resin matrix is composed of monomers such as bis-GMA, TEGDMA and UEDMA, which are bifunctional and thus capable of cross-linking the resin into a three-dimensional molecular network [1]. The ceramic filler particles are of different sizes, fractions and size distributions. The fillers may be quartz, glasses containing barium, strontium or zirconium or combinations of these.

The introduction of the light-curing technology for composite fillings considerably simplified the clinical procedure and increased the colour stability of the fillings. Most composites used today are light-cured [12]. Light-activated materials are generally supplied in a syringe as a single paste containing monomers, co-monomers, filler and an initiator which is unstable in the presence of either ultra-violet (UV) or high-intensity visible light. The operator extrudes sufficient material for a restoration, whereas the remaining material in the syringe, which is not exposed to light, can be used for the next restoration. For visible light-cured resins it is necessary to place increments not exceeding 2mm in depth to ensure adequate cure. This restricts the minimum treatment time for composite restorations, and particularly for large ones [9].

Composite resins allow a conservative cavity preparation with the removal of the caries lesion only. But as stated above, preparation for a composite is somewhat complex, requiring an etching of the cavity and application of both a primer and an adhesive resin in the dentine zone to achieve a sufficient bond between the tooth substance and the composite. The etchant removes the smear layer and exposes the underlying dentine by demineralisation of hydroxyapatite. In this way also a significantly roughened surface is produced to increase the bonding. After all the etchant is removed, a primer containing monomers and hydrophilic molecules (such


as HEMAii) serving as wetting agents to improve penetration of the permeable dentine surface by the monomers is applied with a brush onto the dentine surface. This phase is designed to alter the chemical nature of the dentine surface and to overcome the normal repulsion between the hydrophilic dentine and the hydrophobic resin. Subsequent to priming, the adhesive is brushed onto the dentine surface in a thin uniform layer. The adhesive is essentially an unfilled resin with the same composition as the resin in composites. Therefore it not only coats but also partially penetrates the dentine surface. When the adhesive is light-cured it forms an air-inhibited layer on its surface. This layer polymerises when the overlying composite restoration is placed and the two become joined. The adhesive bonds with the dentine mainly by mechanically locking into its rough surface once the monomers become polymerised [8, 9].

The main advantages of composites are their excellent aesthetics and tooth conserving preparation technique, their formability and ease of placement, and their strong bonding with the cavity. The main drawback of composites is shrinkage during polymerisation and hardening which necessitates the time-consuming acid-etch bonding technique. Moreover, despite the light-curing technology the application technique is sensitive to aspects such as humidity, incomplete curing and residual mechanical stress due to shrinkage and the bonding technique. Composites wear more quickly than amalgam. Furthermore, like amalgam, composites may occasionally cause health problems. In particular hand allergies among dental staff are reported [1]. Composite fillings are more expensive than amalgam fillings.

Clinical success with posterior composites is extremely variable. Although numerous controlled clinical studies have shown that composites can serve as excellent restorations in conservative cavities, private practitioners have not always reported similar success. Limited studies from private practices have shown that the average lifespan of a composite restoration is 3-5 years, which is only one third to one half the lifespan of an amalgam under similar conditions. The main reasons for failures are the same as those reported for amalgam, namely secondary caries and loss of anatomic form or marginal fracture. However, newer formulas and the continual development of composites suggest an even brighter future for these materials.

ii HEMA – hydroxylmethacrylate, with a hydroxyl group of hydrophilic nature making it effective as a priming agent.

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2.1.3 Glass-ionomer cements (GICs) Glass-ionomer cements (polyalkenoates), representing a third category of restorative materials, are characterised by promising adhesive properties but lack high chemical resistance and mechanical strength. The material is a hybrid of the silicate cementsiii and polycarboxylate cementsiv [4]. The intention was to produce a cement with characteristics of both the silicate cements (transluscency and fluoride release) and of polycarboxylate cements (ability to bond chemically to tooth structure and harmless to the pulp) [13].

Glass-ionomers consists of a glass powder and a liquid. The glass powder consists of SiO2, Al2O3 and CaF2. The liquid contains different types of polyacids such as acrylic, itaconic, tartaric and maleic acid. The glass and the liquid are mixed by hand, but it is well known that achieving the correct powder/liquid ratio can be a problem. For this reason pre-portioned capsules (which are triturated like amalgam capsules [8]) are used for ensuring the right powder/liquid ratio. Nowadays many powder blends also contain freeze-dried polyacids. Cement from these types of powders is obtained by adding the correct amount of distilled water [4].

The setting of the glass-ionomer cements is via an acid-base reaction and can be described in three different phases, dissolution, gelation and hardening [4, 9]: - Dissolution: The polyacid (or H+ ions) slowly degrades the outer layer of

the glass particles with a release of Ca2+ and Al3+ ions. Calcium ions are released more rapidly and are primarily responsible for reacting with the polyacid. Aluminium ions become involved in setting at a later stage. The outer layer of the glass particles becomes depleted so that only a silica gel layer remains.

- Gelation: The calcium ions react with the negatively charged groups on the polymer (carboxyl groups of the acid) and cross-link the polyacids into an amorphous network holding the glass particles in place. (This phase is very sensitive to both drying and moisture contamination).

- Hardening: After 30 minutes the aluminium ions start to cross-link the polymer molecules, and due to their trivalent nature they ensure a higher degree of cross-linking. This way they provide the final strength of the material. There is a continual formation of salt bridges and water

iiiConsists of alumino-silicate glass powder mixed with phosphate acid. Hardens in low pH by acid-base reactions. Were replaced by composites and glass-ionomer cements in the 1960-70s. ivConsists of finely ground zinc oxide mixed with an aqueous solution of polyacrylic acid. The setting reaction (acid-base) is similar to that for glass-ionomer cements.


becomes bound to the silica gel surrounding the residual core of each glass particle.

The setting reaction can be described as (MO - metaloxide and A - acid): MO•SiO2 + H2A → MA + SiO2 + H2O Glass polyacid salt silica gel A modern GIC filling should only take about 7 minutes to prepare, from mixing to finish. However, the operation must be performed with care because of its sensitivity to both drying and moisture contamination.

The benefits of glass-ionomer cement restoratives are their adhesive properties (ability to bind to tooth substance), excellent thermal properties (having a thermal expansion coefficient close to that of tooth substance), biocompatibility and fluoride release (believed to increase caries-resistance). Like composites, GIC preparations are also conservative and preserve tooth substance [11]. The drawbacks of GICs are their handling sensitivity, poor mechanical and abrasive properties, susceptibility to acid corrosion and lack of radio-opacity. In terms of appearance, glass-ionomers offer a reasonable match with the natural tooth, particularly to dentine, although not as good a match as composites [9]. Therefore glass-ionomer cement has limited use as a dental filling material. For posterior fillings, both amalgam and composites are better alternatives. For anterior fillings the composites generally produce a better result. Glass-ionomers are used preliminary in Class V cavities, in small Class I cavities, and as temporary fillings. (Appendix I)

2.1.4 Compomers and resin-modified glass-ionomers Compomers, or more precisely, polyacid modified composite resins (PAMCRs) are composites in which the usual types of filler have been replaced by an ion-leachable alumino-silicate glass to encourage fluoride release. In some materials the resin contains acidic groups to generate acid-base reactions. But at the hardening process no acid-base reaction takes place in these materials. Instead, the setting is only a polymerisation light- activated process, just as for ordinary composites. It is thought that some acid-base reaction may occur as the resin absorbs water and the acid groups become ionised to activate fluoride release. The characteristics of PAMCR are generally the same as those of composites.

In general, resin modified glass-ionomers (RMGICs) likewise display the same characteristics as glass-ionomers. These products are, however, considered to be true hybrids, combining some of the advantages from both the GICs and the composites. They differ from conventional GICs by a different liquid composition which, apart from polyacids and water, also

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contains metacrylate resin and HEMA (enables co-existence of resin, polyacids and water). The addition of light-activated resins has simplified handling. The resin makes the mix less vulnerable to contamination by moisture and gives the material a longer working time and a rapid set, once irradiated with visible light. Furthermore, the resin improves the mechanical properties of the material except the abrasive resistance. The hybrid material has partly replaced the use of conventional glass-ionomer cements.

2.2 The oral environment The difficulties in developing a durable and reliable material are basically related to the placement of a formable material in a tooth cavity where it must transform and harden in a short time interval.

All this is closely related to the chemical reactivity of the material. Normally, materials are produced at a high temperature and used at a lower temperature where the energy state is lower. Such materials are safer from a chemical reaction viewpoint. To a large extent this explains the problems of dental filling materials that are produced and used at the same temperature in the harsh oral environment. Reactivity is needed for only a short time during the filling operation, and minutes later the material is supposed to be stable and endure the oral environment including moisture, the presence of acids, wear particles and impact forces, rapid variations in temperature and continual encounters with foreign objects [8].

2.3 Characteristics of the ideal material The durability of a dental filling depends on the general property profile of the material with regard to chemistry, physics and biology. The acceptance of the material by the professions also depends on its ease of handling clinically.

If a dental material should display the features in Table 3 [8] it would make a candidate for a perfect - or almost perfect - dental filling material.


Table 3. Characteristics of an ideal (utopian?) dental filling material. Biocompatible Non-toxic; non-irritating, non-

allergenic Environmentally friendly Non-toxic

Durable

Fracture resistant, strong, chemically stable, stable in water/moisture

Dimensionally stable Not affected by time, temperature changes or by uptake of saliva

Good abrasion resistance Wear characteristics equal to enamel Good adhesion to tooth To bond chemically or mechanically to

the tooth substance for complete sealing

Cost and time effective

Within dentist’s and patient’s budget

Easy to manipulate Mouldable. Prepared in-situ within reasonable time and effort.

Cleanable and repairable Can be maintained or fixed

Aesthetic

Toothlike appearance

Minimal thermal and electrical conductivity

Insulators

2.4 The tooth structure The structure of the tooth includes enamel, dentine, pulp and other tissues, blood vessels and nerves embedded in the jawbone (Fig. 1). The visible portion of the tooth is the crown, which is made of enamel – the hardest tissue in the body. Just beneath the enamel is the dentine (tooth bone), a substance harder than bone. The gingiva surrounds the base of the tooth. The root of the tooth with its blood vessels and nerves extends down into the jawbone and provides circulation and innervation. The jawbone is the bone that contains all the teeth and provides stability and mobility for the mouth and teeth for chewing.

Enamel, dentine and cementum are all hard composite materials composed of hydroxyapatite and collagen. Hydroxyapatite (Ca5(PO4)3OH) consists of three phosphate groups for one hydroxyl group and five calcium ions. Enamel contains the highest concentration of mineral of all body tissue being 96 wieght%. The rest is proteins (1,5%) and water (2,5%).

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Figure 1. The tooth structure.

Dentine has less mineral but still contains about 70% hydroxyapatite, approximately 20% collagen and 10% water with some cellular extensions into pores. Cementum is more like bone in structure and its purpose is to act as the anchorage of the periodontal ligament - a cluster of collagen fibres that hold the tooth in place and act as a shock absorber between the tooth and the jawbone.

A tooth is thus a composite structure but each of its components is also a composite material, containing hard mineral and soft tissue. The idea is that the mineral gives the composite stiffness and strength, while the soft component with a lower elastic modulus gives the material a tougher characteristic. Therefore teeth are hard and unyielding on the outside and soft on the inside. The same thinking is also applied in making hammers of high quality: the front piece of the hammerhead is very hard, but the mass of the hammerhead is softer and tougher [14].


3 Hydraulic Cements

The rationale behind this work was the idea of developing a biocompatible ceramic dental filling material with good mechanical properties equal to those of amalgam and with as good adhesive properties as those of GICs. This would make fillings more durable than composites and give them a lower failure rate with regard to secondary caries. Since dental filling materials are inserted in the tooth cavity at the surrounding temperature and the product requires a transition from a formable to solid state in a short time, hydraulic cement was selected as a potential material to fulfil these requirements. Cement, mortar and concrete have demonstrated a tremendous reliability and durability as a construction material. Some successful historic examples are the Egyptian pyramids and the many famous Roman structures like the Colosseum and Pantheon in Rome.

The Egyptians used gypsum mortars and mortars of lime in the building of the pyramids [15]. The Romans used pozzolana cement from Pozzouli, Italy near Mt. Vesuvius 1,281 m (Fig. 2), as mortar. Roman builders discovered that volcanic ash, when added to lime mortar, cured to a rocklike hardness even under water. Mixed with sand and gravel, this material was the equivalent of modern concrete. At first concrete was used like a particularly durable mortar, or for places where the ability to harden without drying out was important, for example in the watercourses of aquaducts and for bridge pilings. Later they also used the concrete in walls of houses, by pouring it between facings of masonry [16].

Due to the long proven durability of the hydraulic cements - performing well in different climates for long time periods – they should also possess a high potential for performing well as a dental restorative material.

24 Lars Kraft

Figure 2. The Roman town Pompeii buried by ash in the 79 A.D. eruption of Vesuvius. The houses were built of brickwork. Vesuvius is seen in the background. (Photograph by Robert Decker, 1971.)

3.1 Different types of hydraulic cements Hydraulic cements consist mainly of calcium silicates or calcium aluminates and have the ability to set and harden under water. Most types of cement are based on different calcium silicates. Such cements are known as Portland cements. The name originates from the colour and the quality of the hardened cement resembling a limestone quarried in Dorset (England) called Portland stone. Almost all existing buildings are built with ordinary Portland cement (OPC) concrete and about 90% of all the cement used in the UK and the US is OPC [17] (p. 69). Other types of Portland cements are rapid-hardening and very rapid-hardening, low-heat and sulphate-resisting, as well as white cement. All these types of cements have their specific application which is indicated by their names.

At the beginning of the 20th century Bied invented Calcium Aluminate Cements (CACs) as a solution to resisting attack by sulphate bearing waters on the concrete [18]. As in the case of Portland Cements there are also different blends of CACs. General characteristics of calcium aluminate cements are rapid-hardening properties, high initial strength and good corrosion resistance. CACs are about 4-5 times higher in price compared with OPC, which makes them unprofitable for use in construction. Another disadvantage with CAC concretes is the well-known phenomenon (among materials scientists) of conversion, in which metastable phases first


form before transformation or conversion into stable phases with time[19]. (See section 3.3.1)

3.2 Setting characteristics of hydraulic cements There are several excellent books written about the properties of cement and concrete. [17, 20, 21] This thesis presents some work covering the mechanical properties of this cement based dental filling material, and some work concerning its chemical and biological properties. However, the focus is on the physical nature of dimensional changes in the material, which is a result of the material’s physics (e.g. particle size, particle size distribution and compaction degree), its chemistry (e.g. composition, additives and w/c ratio) and the boundary conditions. This is closely related to the chemical interactions occuring at the early age of the cement-water mix, although the long-term performance also sometimes exerts a great influence. Therefore some basic definitions will be given concerning the early-age behaviour and dimensional changes of cement based materials.

SiO2

CaO

Calcium Aluminate Cements

Figure 3. Composition ra

Portland Cements

Al2O3

nge of Portland cements and calcium aluminate cements.

26 Lars Kraft

3.2.1 Hydration of cement The hydration of cement is often divided into three different phases called the dormant phase, the setting phase and the hardening phase. In the dormant phase (almost) no hydration takes place; in the setting phase an increasing rate of hydration/precipitation is shown, and in the hardening phase, the main hydration takes place but with decreasing speed. The water dissolves the cement grains into ions, which at saturation precipitate and form hydrates. These three phases can also, and perhaps more correctly, be expressed as dissolution, gelation (precipitation), and hardening in analogy with the hardening of glass-ionomer cements, described in paragraph 2.1.3. The setting phase is when the cement mix - from being a liquid - transforms into a visco-elastic solid.

3.2.2 Dimensional changes in hydrated cement Most of the dimensional changes in cement materials are related to the early-age hydration mechanisms. When water and cement react there is always a considerable decrease of the absolute volume. This shrinkage is referred to as a chemical shrinkage. However the macroscopic volume change is seldom large. Instead, the chemical shrinkage induces porosity within the material. Before setting the chemical shrinkage is equal to the external volume shrinkage. Vertical length change in cementitious materials before setting, which is caused by bleeding and/or chemical shrinkage, is called subsidence. (Bleeding is a consequence of sedimentation in the liquid state of concrete. In mixes with low water to cement (w/c) ratios, bleeding does not occur. [17] (p.206).)

Figure 4. Schematic of early-age dim

Autogenous shrinkage

Chemical shrinkage

The setting point

Shrin

kage

Time (hours)
ensional changes in normal concrete [22].


Macroscopic dimensional changes in materials after setting, and which are not caused by loss or ingress of substances, temperature variations, or the external effect of any force, are defined as autogenous volume change [23]. The difference of the autogenous shrinkage and chemical shrinkage is explained by a porosity built up between the hydrate network (Figures 4 & 5) Autogenous shrinkage after setting is caused by self-desiccation, when pore water in the matrix reacts with anhydrous cement grains and thus changes the internal humidity [24].

If the cement product is cured in water a net increase in mass and volume occurs due to continuous water uptake and hydration of anhydrous cement grains. Such swelling is about ten times greater in magnitude for OPC pastes compared to concrete [17] (p.425).

Other dimensional changes in cementitious materials are related to thermal dilation and creep [25, 26, 27].

Chemical shrinkageSubsidence

Figure 5. Relation between chemicalvertical direction for concretes with l

Autogenousshrinkage

shrinkage and autogenous shrinkage in the ow w/c ratio. (Non-bleeding mixes.)

28 Lars Kraft

3.3 Calcium aluminate cement CACs have a different and much wider range of compositions than

Portland cement (Fig. 3). Highly aluminous or white CACs – containing 70% alumina or more, are usually made by sintering calcined alumina with quicklime (CaO) or high-purity limestone [20]. Despite their high cost compared to OPC and the drawback of conversion (described below) CACs are the only cements other than Portland cement in continuous long-term production [28].

One present-day application of calcium-aluminate cement is in floor levelling compounds due to its shrinkage compensating ability (with gypsum) [29], good abrasion resistance and rapid hardening properties. The material has also shown excellent performance in sewage networks in South Africa [30]. But due to their high temperature durability, the most widespread use of CACs is in refractory applications for high temperature processes, such as in metal, glass and cement making [10, 28].

The conversion of CAC concrete can sometimes even halve their initial strength after a period of time. Due to some failures of precast prestressed elements made of CAC concrete in buildings during the 70s in England, the use of CAC in bearing constructions was stopped. In 1996 the Concrete Society (UK) presented a re-assessment concerning CAC in construction. The report concluded: “It is doubtful whether, even if a basis for the safe use of CAC concretes can be formalised and agreed, they would supplant Portland cement concrete for the mainstream of structural applications….” But the report also gave the following recommendation: “Specifiers, users and clients should be encouraged to consider applications where calcium aluminate cements would have technical and commercial benefits either in concrete form or as specialist proprietary products.” [31]

3.3.1 The hydration and conversion mechanisms of CACs The dental filling material in focus in this thesis is based on high alumina cement (SECAR 71) that contains about 70% alumina. It is almost entirely (+99%) composed of calcium aluminate phases, of which half is calcium aluminate (CA) and the other half calcium di-aluminate (Grossite-CA2). Also fragments of C12A7 and free alumina are found.

Mixed with water the Ca-aluminates react in an acid-base reaction where the ceramic powder works as the base and water as the weak acid, supplying ions to the water. Water dissolves the calcium aluminates and formation of Ca2+, Al(OH)4

– and OH– ions takes place [20], which at low w/c ratios, is


followed by an almost immediate precipitation of hydrates due to saturation of the solution [32]. These precipitates grow until they meet each other and a skeleton (a connected cluster of hydrates) of hydrates is built up continually. The hydrates, of which the main parts initially are amorphous, continue to form and the hardness develops with time in the material. The material in this study has no conventional “setting” since the material is highly compacted and has a w/c ratio below 0.2.

For CA the reactions are as follows, with cement nomenclature [33]:

at 21-35°C 6CA + 33H → 3C2AH8 + 3AH3 above 35°C ↓ -9H 6CA + 24H → 2C3AH6 + 4AH3 The corresponding reactions for CA2 are

at 21-35°C 6CA2 + 51H → 3C2AH8 + 9AH3

above 35°C ↓ -9H 6CA2 + 42H → 2C3AH6 + 10AH3 To the right hand side it is seen that conversion of the 8-phase results in release of water.

Since the hydration products of CAC depend on the ambient temperature at precipitation, although gibbsite (AH3) develops at all temperatures, the formation of the meta-stable phases CAH10 (forms below 20°C) and C2AH8 can be avoided if the hydration takes place at a higher temperature [34]. Above 30ºC mainly C3AH6 (Katoite) develops but some C2AH8 is also formed. The chemical reaction at 37ºC is summarised as:

3 CaO•Al2O3 + 12 H2O → Ca3[Al(OH)4]2(OH)4 + 4Al(OH)3 Ca-aluminate water Katoite Gibbsite The conversion process is time-dependent [31]. Higher temperatures, higher concentrations of lime and alkalinity speed up the conversion. If the exposure to higher temperatures is intermittent, the conversion effect increases even more [17] (p 92, p 94).

One of the reasons for the slower formation of the stable hydrate is the cubic nature of Katoite. The high symmetry requires simultaneous organisation of the atoms in all three dimensions at nucleation. In contrast, phases having a strong orientation to their crystals, forming as plates or

30 Lars Kraft

needles, can nucleate and grow more easily as atoms are added in one or two dimensions [28].

It has been claimed that the formation of C3AH6 is always preceded by the transitory formation of some C2AH8, even at temperatures up to 90ºC, but the direct formation of C3AH6 from CA can take place once C3AH6 has nucleated [34].

3.4 CAC in dental filling materials For CAC-materials to be used as dental fillings, rapid-hardening, white colour and corrosion-resistant properties are needed (Table 3). Besides the rapid-hardening properties, the high (initial) strength and the excellent acid corrosion properties, calcium aluminates also have better abrasion resistance than OPC, lower pH than OPC at hydration and good characteristics as a biomaterial (paper IX). Moreover, dental fillings are made at a rather high temperature (36°C). Therefore almost no metastable hydrates are developed and conversion is avoided.

In Table 4 the rationale behind the selection of CAC-based materials as a candidate for dental restorative materials is summarized.

Table 4. The rationale behind the use of Ca-aluminates in dental restorative materials Chemistry Composed of frequently occurring (common) elements,

oxides, hydrophilic nature, similarities with apatite Biology Biocompatible (also during curing) and environmentally

friendly Thermal Expansion and conductivity comparable to tooth tissue

Mechanics Hardness and stiffness comparable to hard tissues and bone, strong in compression

Processing In-situ room-temperature preparation, adjustable rheology and curing time


4 Characteristics

Experiments and investigations were initially conducted on various types of experimental compositions based on calcium aluminate cement. In particular deformation, porosity and hardness measurements were conducted (papers I-IV).

After determination (formulation) of the composition of the first product called Doxadent, most of the findings presented here are accounts of this materials characteristic. Besides the expansion measurements the characteristic of the contact zone between the dental filling material and tooth substance (paper V), the flexural strength (paper VII), some wear characteristics (VIII) and the biocompatibility and chemical stability (paper IX) were studied.

The most crucial of the material-related properties of dental filling materials are strength and fracture toughness, wear resistance, chemical stability and dimensional stability (shrinkage or expansion) and the adhesive properties.

4.1 Influence of test methods on results Materials are tested according to standard test methods – or arbitrary methods in cases where no consensus exists. For different types of dental materials different standard test methods exist for the evaluation of the same property. Different test methods may give considerably different results [35]. This reflects how the test method relates to the given property for evaluation. The standard method selected for a given type of material is often a method which favours the type of material the standard is applied to (paper VII).

For instance, the ISO Standard 4049, having small sample geometries to assure complete polymerisation, and free test bars with no influence on internal tensile stresses that normally develop in the clinical situation due to comprehensive shrinkage, favours flexural strength values of polymer composites. When ceramics are tested according to the EN 843-1 or ball on disc (ASTM-F394) [36] the influence of corner defects is avoided by chamfering of the test bars or how the stress is applied. However, all these

32 Lars Kraft

methods reflect the internal material properties but not necessarily the clinical performance.

The same thing is valid for the in vitro deformation and the in vitro wear measurements. The analysing system and the environment will influence the results. For expansion studies one must make distinctions between free expansion and restrained expansion in dry, wet, moist or sealed environments. (The acrylic split pin-expander technique, the method that has been most frequently used for expansion analysis in this study, measures a combination of free and restrained expansion, although the restraining forces developed are very small.) In-vitro wear is highly dependent on the test methods employed and other external conditions. For example in the case of the materials based on Ca-aluminates, accelerated tests give no time for the material in the surface region to rehydrate. This leads to a chemical contribution to the wear, which is much less pronounced in the real situation.

4.2 Results from earlier work Previous work [37], conducted at the Karolinska Institute, Huddinge Sweden, showed that cold isostatically pressed (CIP) cement powder enhanced the strength development in the cement. Green bodiesv, of cement were immersed in water or kept in humid air for curing at various temperatures for different periods of time. The flexural and compressive strengths for 30 days of hydration were 50-80 MPa and > 200 MPa, respectively. Conventional CAC has an initial compressive strength of about 80 MPa. Due to conversion and the temperature this value drops to approximately 50 MPa after various periods of time depending on the curing temperature. The hardness of the CIP materials was doubled in comparison to control specimens conventionally prepared. Unpublished results from these studies also revealed development of a continuously declining expansion with time. The reason was thought to be due to a slow continuous hydration of unreacted cement grains [38].

Another study was concerned with how the content of aggregates, the water to cement ratio (w/c ratio) and the compaction degree in the mix influenced the porosity and the fraction of anhydrous cement in a hydrated material. A model called BOVA (Swe.) or “COFA” - Calculation of Optimal Fraction of Aggregates, was derived [39] and applied (Appendix II). The model has similarities with a model presented by [40] and partly with Power´s model for hardening [41]. However, these models do not take the

v In the manufacture of ceramics the ceramic body is called a “green body” before it is heat-treated (sintered)


compaction degree into account. The theoretical model accounts for the calculation of the lowest possible porosity, which is fundamental for the achievement of good mechanical properties in ceramic materials, and the remaining fraction of anhydrous cement as a function of the aggregate fraction. The materials with the lowest expansion still showed a high expansion of up to ~1.5% linearly (~ 4.5 vol%), independent on the filler particle fractions chosen, although the fraction of aggregates giving the lowest porosity according to BOVA also showed the lowest expansion. It was found that lower fractions of aggregate yielded a higher fraction of Katoite.

Additional work [42], where four different types of aggregates were studied with respect to the overall material porosity, gave the following information: - The theoretic model BOVA corresponds well with the experimental

findings. Therefore the credibility of the model was strengthened. - Calcite as a filler material causes a high expansion, which was assumed to

be due to formation of C4ACčH11. - The use of wooden instruments, both sucking water and compacting

powder at the same time. increased the compaction degree significantly. - Cement powder should be of smaller grain size. - The particle size of aggregates and cement should be matched. Among the different materials tested as fillers, alumina addition yielded the lowest expansion (~2% linear) and calcite addition the highest hardness.

4.3 Compositions Besides studies on experimental powder compositions, similar to the Doxadent composition, early studies were conducted on a pure CAC paste of different grain sizes. Mixtures of CAC with additions such as silica fume, Portland cement, alumina or combinations thereof were also investigated. Most of the early mixes had an inert filler content of 28-vol% alumina (Table 5). This volume fraction was calculated from BOVA to increase the w/c ratio and thereby the possible degree of hydration in the materials (Paper III).

To accelerate the hardening process a lithium salt is added to the solution corresponding to a lithium content of 30-90 ppm.

34 Lars Kraft

Table 5. Composition of the admixtures studied, quantitatively. Mix nr.

Secar 71 (Lafarge AB)

Microsilica additivesa)

Portland cement additionb)

Filler fraction (Alumina)

1 Coarse S71 (milling 24 h)

None None 28vol%

2 Fine S71 (milling 70 h)

None None 28vol%


Large fraction None 28vol%


Large fraction Large fraction 28vol%


Smaller fraction (half of the large)


28vol%




None

a) From Sigma-Aldrich b) Cementa, Std Cement, Slite The Doxadent powder is based on Secar 71, a calcium aluminate cement produced by Lafarge Aluminates, with additions of micro-silica and Portland cement. It has 15 vol% zirconia added to achieve appropriate radio opacity. Studies were also conducted on the binder phase exclusively (Mix 6), i.e. the Doxadent powder composition without the addition of zirconia filler. The typical chemical composition expressed as oxides in the cured material is presented in Table 6. Table 6. Chemical composition of hydrated (cured) calcium aluminate based restorative material in weight %.

Oxide Fraction (%) Al2O3 CaO H2O ZrO2 Others: (Si-, Fe- Mg-, Ti- and alkali-oxides)

43 19 15 19 <4

Total 100

4.4 Specimen preparation Two different types of specimens, pre-pressed (machine-made) and prepared/compacted (hand-made) specimens were used in the different hydration studies. The machine pre-pressed specimens were dipped into a catalyst solvent before they were put into water for storage. Likewise the (hand-made) prepared specimens were placed in water for storage.


4.4.1 Pre-pressed specimens Two variants of pre-pressed specimens were used, pins and tablets. Pre-pressed tablets were both machine-pressed and hand-pressed uni-axially using a pressure of approximately 150 MPa. Materials for the pre-pressed pins were manufactured by cold isostatic pressing (CIP) at the Swedish Ceramic Institute (SCI), Gothenburg. Different types of tablets and pins were used with both various compaction degrees and diameters in deformation tests and in flexural strength tests.

Pre-pressed tablets were investigated regarding deformation using a split-pin expander technique and an automatic laser radial micrometer technique (paper III).

The pre-pressed pins were sawn from cold isostatically pressed materials and ground into proper dimensions for the testing of dimensional change and flexural strength. The dimensional stability of pins were investigated with a digimatic micrometer.

4.4.2 Prepared (compacted) specimens Two different types of the prepared specimens were used - specimens made by hand compaction of tablets or wet-pressed powder (See 4.4.3). The preparation of specimens was conducted with dental instruments. The specimens were packed into moulds (papers II & IV). In compacted specimens the control of the compaction degree is less than in pre-pressed samples. However, the compaction degree difference of crushed wet tablets compared to wet pre-pressed tablets is small. Very few new contact zones are generated compared to all the contact zones between the individual grains. In the preparation of specimens, like for those used in the split-pin expanders with a diameter of 4 mm and with a thickness of 6 mm, the pre-pressed tablets are very practical. Such preparations correspond well to the procedure of many dental cavities. For preparation of larger specimens usually wet-pressed material was employed.

4.4.3 The wet-press method To secure a complete and isotropic wetting of all cement grains in the material and at the same time achieve a low w/c ratio (high compaction degree) a new process was developed. In this process the cement powder is thoroughly blended in a small pot with an excess of water during a minute. Thereafter the excess water is pressed out from the mix using porous ceramic plates, wooden knobs and a hand press (paper III). This process is called the wet-press process and it is patented [43].

36 Lars Kraft

Figure 6. The wet-pressing method generates a low w/c ratio cement paste without the use of any water-reducers or pre-pressed tablets.

4.5 The compaction degree The compaction degree, a synonym for the bulk density of green bodies affects the formation of hydrates in the material matrix and thereby the material properties.

In the development of a cement based dental filling product, a high compaction degree while maintaining the good handling properties is a critical issue. The reason for having a high compaction degree in the material is to ensure a dense material with low expansion, low porosity and thereby higher mechanical strength, high hardness, and durability [44]. Modern powder technology was applied to decrease the porosity and the grain size of the material [37]. The compaction degree is defined as:

ρV

mdegreeComp =. (1)

where m is the mass, V the total volume and ρ is the specific density of the powder mix.


To achieve a high compaction degree (or low porosity) and full hydration the BOVA-model was applied to set the conditions. A high compaction degree restricts the possible w/c-ratio interval because a pre-pressed pure cement paste body with, for example, 40 vol% porosity and no filler can only take up 40vol% of water. This gives a w/c ratio of about 0.23 if the density of CAC is 2.94 g/cm3. Then, by adding fillers into the matrix, it is possible to achieve a higher w/c ratio.

In the work of achieving higher compaction, the patented wet-press was developed. Later a tablet manufacturing process was introduced to ensure the desired density.

4.6 Dimensional changes (Paper III) The deformation development during curing of CAC-based cements is dependent on a large number of factors. If only few experiments are conducted they will most likely give an unclear answer to the question of the deformation situation in the system. In paper III a broad comparative approach has been taken in the evaluation of a critical issue for dental filling materials – the question of dimensional stability.

The microstructure developed during hydration is the overall controlling factor of deformation, shrinkage or expansion. Factors such as the compaction degree, the composition (cement type[-s] and filler content), the grain size and the surrounding environment control the micro structural development and thus the dimensional behaviour of cement-based materials during hydration and long-term performance. Furthermore, the measuring technique selected, requiring different sample positions and sample geometries and preparations, will also influence the results significantly. The dimensional stability has been investigated using three different methods: a split-pin expander technique, a digital micrometer and an automatic radial laser beam micrometer.

(All data presented are the mean value from at least two specimens.)

4.6.1 Influence of the physical and chemical characteristics The most important physical characteristics of the cement powders are the compaction degree, the w/c ratio, the grain size and the homogeneity of the powders. These properties will affect the deformation characteristics of the curing material.

38 Lars Kraft

4.6.1.1 The compaction degree The influence of the compaction degree was studied with all three methods: the split-pin expander technique, the digital micrometer and the automatic laser radial micrometer. The results obtained are summarized in Table 7. Table 7. Linear expansion in % after ~100 days of hydration in specimens of different compaction degree, measured by different methods. 55% 58% ~ 61,5% 65% 67% 69%

Split-pin tabl.a) 0.3 0.27 - -0.0 ? - -

Split-pin prep.b) - - 0.12 - - -

Digimatic - - 0.3 - 0.17 0.18

Laser tabl. 1.4 0.8 0.6 - - -

Laser prep. 0.8 1.0 1.8 - - - a) Pre-pressed tablets carefully mounted in expanders. b) Specimens (tablets) hand packed with dentist tools – the regular method The general finding is that the compaction degree influences the expansion only to a limited extent at high compaction degrees, i.e. at levels above 50 % where unreacted Ca-aluminate cement starts to be found. However, there is a slight tendency that higher compaction degrees contribute to a reduced expansion. The laser beam data are disregarded in the assessment of the influence of compaction degree on the expansion, since the high expansion is most likely caused by on-surface precipitation, which masks possible compaction degree influence. For example, a precipitated layer of about 15 microns on a 3 mm specimen corresponds to a geometrical change of 1% (linearly).

The preparation technique with which samples are produced will influence the compaction degree. Prepared specimens from tablets having a higher compaction degree of approximately 59% compared with samples prepared from wet-pressed powders of approximately 53% yielded somewhat lower expansion, Figure7. (See also Paper III)

From literature data [23] it is found that the shrinkage increases with lower w/c ratio. This means that the expansion is expected to decrease with increasing compaction degree.


4.6.1.2 Influence of preparation technique The preparation technique with which samples are produced will influence the compaction degree. Prepared specimens from tablets have a higher compaction degree than samples prepared from wet-pressed powder yielding somewhat lower expansion. Compare Figures 7A and 7B. The sudden shift in the expansion at 400 days may be an artefact.

0,0%

0,2%

0,4%

0,6%

0,8%

1,0%

0 200 400 600 800 1000Time (Days)

Line

ar e

xpan

sion

0,0%

0,2%

0,4%

0,6%

0,8%

1,0%

0 200 400 600 800 1000Time (Days)

Line

ar e

xpan

sion

A) B) Figure 7. Long-term values of linear expansion in Doxadent measured with split-pin expanders. Specimens prepared from tablets in A) and specimens prepared from wet-pressed powder in B).

Figure 8 illustrates discrepancies between specimens made from the same powder. These discrepancies are the result of differences in the packing process.

0,0%

0,2%

0,4%

0,6%

0,8%

1,0%

0 50 100 150 200 250Time (Days)

Line

ar e

xpan

sion

DD I DD II DD IIIMix 6 I Mix 6 II

Figure 8. Pins of the size 2*2*25 mm made from wet-pressed experimental Doxadent and Mix 6 powder, measured with a digimatic micrometer.

40 Lars Kraft

4.6.1.3 Influence of grain size The separate factor that influences the deformation to the highest degree is the grain size (Figures 9A & 9B.). Smaller grains result in smaller linear expansion. The large individual grains contribute to an inhomogeneous general microstructure. Inhomogeneities in the structure cause several situations which may contribute to an increased expansion. These involve large grains, large areas of unreacted phases, possible crack formations in the layers surrounding the original CA-grains, the formation of large hydrates etc. A fine-grained microstructure will keep the different possible expansion factors locally at low influence levels.

0,0%

1,0%

2,0%

3,0%

4,0%

0 200 400 600 800Time (Days)

Line

ar e

xpan

sion

Mix 1 Mix 2

0,0%

1,0%

2,0%

3,0%

4,0%

5,0%

0 10 20 30 40 50 60 70 80 90Time (Days)

Line

ar e

xpan

sion

Mix 1Mix 2Mix 6

A) B) Figure 9. The grain size influence on expansion, prepared from wet-pressed powder. Measured with split-pin expanders (A) and radial laser beam micrometer (B).

4.6.1.4 Chemical composition The separate factor that influences the deformation to the second highest extent is the addition of silica and/or silica-containing phases. Thus there is a great difference between the deformation of the pure cement system and the cement system with additives. In experiments with different high performance concrete blends, it has been shown that addition of silica fume into concrete mixes increases the shrinkage [45]. From Figure 9B (Mix 6) it is obvious that silica - in particular in low concentrations [46] - decreases the expansion for CAC-based materials that are stored in water. Also compare the long-term data in Figure 9A with the long-term data in Figure 10 measured with split-pin expanders of blends with silica and silica-containing phases.

4.6.1.5 Filler ratio The filler content (inert fillers) will decrease the hydration occurring in the cement. The higher the fraction of the fillers is in concrete mixes, the smaller


the resulting shrinkage or expansion is (Fig. 10.). Filler particles introduced change the material properties with regard to mechanical properties. The filler particles will also affect the amount of residual unreacted cement and the positions of hydrated phases. The final microstructure will be more homogeneous and fine-grained. This will in general lead to lower expansion [17, 23, 47].

-0,20%

0,00%

0,20%

0,40%

0,60%

0,80%

1,00%

0 100 200 300 400 500

Time (Days)

Line

ar e

xpan

sion

Mix 6 I Mix 6 IIMix 5 DoxadentLogg. (Mix 6 I) Logg. (Mix 6 II)Logg. (Mix 5) Logg. (Doxadent)

Figure 10. Composition (filler) and process (preparation) influence on the linear expansion characteristics. Mix 5 and Doxadent have filler grades of 28 and 15 vol% respectively. Only the Doxadent specimens were made from tablets. All other specimens were made using an early makeshift model of the wet-press method.

4.6.2 Influence of the measuring technique It is obvious from the test series measuring the curing deformation of materials with the same composition, and the same general conditions, that the deformation results obtained differ between the different measuring techniques used. Due to the different geometries required for the different measuring techniques, it has not been possible to make measurements on exactly the same type of specimens with all three methods. However, prepared specimens from wet-pressed experimental Doxadent powder, although of different geometries, have been measured in all three different methods. The split-pins gives an expansion of ~0.2% after 250 days. The digimatic micrometer gives an expansion of ~0.6% after 100 days and the laser micrometer give a linear expansion of at least 0.6% after 90 days of hydration depending on the specimen size. From these data it is concluded

42 Lars Kraft

that the free expansion measurements give at least three times higher values than those obtained by split-pin expanders. It is believed that differential hydration mechanisms, which take place in these types of materials [17] (p.438), are the main reason for these discrepancies. Therefore the storage of specimens as well as their geometry plays an important role, since the surface reactivity always differs from the bulk activity.

4.6.2.1 Temperature, humidity and chemical environment The environment has a crucial influence on the chemical reaction process and, in the end, the mature product, as discussed in the introduction [19, 48]. Also small additions of additives such as retarders, superplasticizers, accelerators or polymers influence the rate of hydration and thereby the microstructure and dimensions of the mature solid material.

The first hour dimensional behaviour was examined with a digimatic micrometer, and the importance of hydration in a humid environment is shown (paper II). In a dry environment the material shrinks linearly as much as up to 0,4% during the first hour (Table 8). But if the material hardens in water it remains dimensionally stable. Table 8. Net deformation in dry hydrated samples after 1 hour. (Paper II) Hydration condition

Prepressed Tablets

Compacted specimens

Average deformation

Wet 37°C -0.010% 0.017% 0.004% Dry 37°C -0.407% -0.372% -0.388%

Dry 24°C -0.342% -0.343% -0.342%

4.6.2.2 Sample position and specimen size The sample position may restrict the material from direct contact with the surrounding solution or the material may be in direct contact with the external solution. The hydration mechanism, involving dissolution, precipitation and formation of new crystals (hydrates) takes place within the material (in the original water-filled pores) but also in the surface region and on the surface. The abundance of water at the surface gives a different chemistry and no geometrical restrictions of the precipitation of hydrates. Upon free surfaces, layers are generated by precipitation from the surrounding solution Therefore the surface hydration contributes to the deformation detected differently in different tests with regard to sample size, coatings or not, and with regard to the degree of restricting stress in specimens. Therefore the surface hydration might influence the dimensional


behaviour of free surfaces differently compared to specimens only having one free surface or few free surfaces.

Thus the different measuring techniques pick up possible bulk deformation, in-surface deformation and on-surface deformation to different extents. The split pin expander method is deemed to be a reliable method in the evaluation of materials with respect to dimensional changes in the CAC-system with relevance to the application as dental filling materials, since only limited on-surface precipitation is possible there, just as in the case of fillings in cavities. This is despite the fact that this method has the highest relative error.

4.6.2.3 The differential hydration As shown in Figure 11A, pre-pressed pins of different lengths were compared. After 120 days of hydration a pattern is seen. The shorter the samples are, the higher the relative expansion appear to be. This can be explained by the development of a surface layer. By dividing the total expansion into bulk expansion and surface expansion, the total absolute expansion, ETOT, can be written as:

ETOT = EBULK + ESURF (2) where EBULK and ESURF are the absolute bulk and surface expansion, respectively. The volume change due to hydration is directly proportional to the original length of the pins, Lo, and equation 2 can hence be rewritten as ETOT = L0×P + ESURF (3)

where P is the relative bulk expansion due to hydration. If the measured total expansion, ETOT, is plotted as a function of the original length of the pins, L0, the derived slope represents the relative bulk expansion and the y- intercept represents the total thickness of the surface layer. In this way the surface layer was derived to be about 7 microns in the specimens stored in artificial saliva (phosphate buffer solution), and to be about 12 microns for pins stored in distilled water. [49] (Fig. 12).

44 Lars Kraft

0,0%

0,1%

0,2%

0,3%

0,4%

0 30 60 90 120Time (Days)

Line

ar e

xpan

sion

10 mm15 mm20 mm25 mm

0,0%

0,1%

0,2%

0,3%

0,4%

0 30 60 90 120Time (Days)

Line

ar e

xpan

sion

DD dist water

DD art. saliva

DD art. sal. change

A) B) Figure 11. The linear free expansion for Doxadent pins of different lengths stored in artificial saliva (A) and the relative free expansion of Doxadent stored in different environments – distilled water, artificial saliva and, exchanged artificial saliva (B).

y = 0,0013x + 0,0066R2 = 0,9935

0

0,01

0,02

0,03

0,04

0,05

0 5 10 15 20 25 30Length of pins (mm)

Abs

olut

e ex

pans

ion

(mm

)

Figure 12. The absolute expansion as a function original length of four DoxaDent pins stored in artificial saliva for 111 days. The results presented in Figure 11B originate from identical specimens stored in different environments.

SEM studies confirm that surface layers precipitate upon the free surfaces of specimens stored in water (Fig 13A.). However, for prepared pins stored in water-saturated air no distinct layers could be seen (Fig. 13B).

A) B) Figure 13. On surface precipitation on specimens (A) – and not (B).


Al Ca P

O Si Zr

Al Ca P

O Si Zr A) B) Figure 14. Cross sections of broken pins of Doxadent (A) and mix 6 (B) imaged with secondary and backscatterd electrons respectively, with corresponding EDS mappings. (Imaging by Richard Westergård.) The layers built up on the different specimens stored in artificial saliva and distilled water were studied in SEM and with EDS mapping analysis. See figures 14-15.

The SEM images in Figure 14 clearly show that surface layers are formed. For the Doxadent pin a very smooth layer of fine micrustructure is seen. The energy dispersive spectrums reveal that the layers consist of calcium, oxygen and phosphorous (from the fact that Mix 6 does not contain any zirconium). The in-surface layer has no aluminium. Furthermore, the area below this layer is depleted of calcium. This area displays a darker colour in the backscattered SEM-image.

Relatively large crystals are formed upon the surface. This is an on-surface formation of what most likely is calcite (CaCO3). The EDS mappings confirm the presence of calcium and oxygen.

46 Lars Kraft

Al Ca C

O Si Zr

Figure 15. Crystals on top of doxadent pins stored in distilled water. (Image by Richard Westergård.)

In investigations regarding differential hydration of bulk versus surface on old experimental specimens, it was revealed that surface material contained calcite whereas bulk material did not. Thus the calcite formation is due to surface phenomena and not the result of interior bulk conversion. The fraction of calcite in in-vitro specimens stored in water increases with time, due to the carbonation process and might be a problem in the long-term perspective of durability. However, the calcite principally forms upon the surface of specimens stored in stagnant water, as shown in Figures 13A and 15. This process of on-surface precipitation is assumed not to take place in the real tooth where a different chemistry is present and where there are limited possibilities of precipitation upon any surface due to continual mild wear in the mouth environment.

4.6.3 Conclusions from the deformation tests

- The dimensional change during curing of Ca-aluminate based materials can be controlled by material modifications to be in the interval 0.1-0.3 %.

- The main factors controlling the deformation are grain size, composition (filler content), and additives that affect the basic cement system.

- Other factors that control the dimensional stability are compaction degree, preparation technique, and the general environment. These factors will become important when optimising the dimension changes close to zero-expansion.

- Caution must be exercised in interpreting experimental data since different test methods may differ from each other, mainly with respect to how they respond to the different deformation mechanisms. Precipitation


upon free surfaces and surface initiated growth will contribute to the dimensional changes detected.

- The split pin expander method is deemed to be a reliable method in evaluation of dimensional changes in the CAC materials system with relevance to the application as dental filling materials.

- Digimatic micrometry and radial laser beam scanning show high accuracy in the measurements, but these tests are more influenced by external factors such as on-surface precipitation.

For optimising the dimensional stability further it is concluded that more data is needed for a more complete understanding. How the compaction degree, the role of specimen size, the environment (type of solution, water or saturated air) and the differential hydration influence the deformation mechanisms are not fully understood from the results obtained in this thesis.

4.7 Mechanical properties The composition of calcium aluminates, inert filler content, the distribution of grains, the grain size of the different constituents, and additions like dispersants will influence the compaction ability of the raw powder as well as the hydration process of the material. This also determines the mechanical properties of the mature material. From powder technology (the ideal powder concept) it is known that small grain sizes and high homogeneity in the powder yield higher probability of reduced strength-controlling defect sizes [50].

The mechanical properties hardness, flexural strength, fracture toughness and wear resistance of the Ca-aluminate based system has been studied in this thesis.

4.7.1 Hardness and porosity (Paper IV) The porosity has a fundamental influence on the mechanical properties of materials, especially hardness, strength and wear.

Both the hardness and porosity development over time were examined. For the porosity evaluation both compacted specimens, prepared from four (experimental Doxadent) tablets, and single tablets were put in water for hydration. The tablets used in this study were the first prototype tablets having a fairly uniform degree of compaction of approximately 60% (paper IV).

48 Lars Kraft

All hardness data presented are from individual samples hydrated for different time periods (Fig. 16). The porosity measurements after hydration for 2, 5, 10, 30, 90, and 180 days are presented in Figure 17.

Prepared specimens showed a more homogeneous characteristic in porosity than single tablets. The density of the compacted specimens over a period of time was almost constant. Thus the compaction of the prepared specimens contributes to a more homogenous material by averaging the variations in density in each single tablet. The density difference of the tablets is due to a slight difference of compaction degree in the manufacturing process of the prototype tablets.

0

50

100

150

200

250

0 50 100 150 200 250

Time (days)

HV

(kg/

mm

2)

topbottomaverage

Figure 16. Hardness development with time in a prototype dental filling material based on CAC. All data are collected from individual specimens stored different hydration times.

4,0

5,0

6,0

7,0

8,0

9,0

10,0

2 5 10 30 90 180Hydration time (days)

Poro

sity

%

2,50

2,60

2,70

2,80

2,90

3,00

Den

sity

(g/c

m3)

Specimen PorosityTablet PorositySpecimen DensityTablet Density

Figure 17. Porosity and density in compacted specimens and in tablets. (Data are the average of two samples, all hydrated for different times.)


The non-continuity of the hardness increase is probably caused by conversion of C2AH8 hydrates after 10-30 days of hydration.

The hardness increased with time and after 100 days it exceeded 170 kg/mm2 vi. The porosity decreased with time for the prepared specimens. This was most evident for specimens hydrated for longer time periods.

The hardness increases with time due to a continuous hydration. During hydration the overall porosity is continuously reduced, but an intrinsic formation of micro-porosity develops due to a volumetric contraction related to the hydrates formed [37]. However, the capillary forces within this micro-porosity system cause new water to penetrate leading to additional hydration and as a result, further reduction of porosity. The main part of the hydration is completed after approximately one week, but the asymptotic characteristic of the hydration process causes a continuous hardness increase for a few months. This process is also confirmed by the porosity data obtained for the prepared specimens (Fig. 17), since the porosity decreases for specimens cured long-term (90 and 180 days). Thus, there is a correlation between the hardness data and the porosity data.

4.7.2 Flexural strength and fracture toughness (Paper VII) It is important to emphasize that flexural strength is not a material property. A ceramic material is linear elastic, which means that upon loading and de-loading the material is unchanged. Failure occurs when the stress level reaches a threshold for crack propagation [51]. This value is included in the equation of fracture toughness, K1C, which can be considered to be a material parameter. Final fracture occurs due to stress concentrations at defects in the microstructure. Thus, to increase the fracture strength a minimization of the defects must be made. The relation between fracture toughness, fracture strength and maximum defect size is described as:

critfC cYK σ=1 (4)

K1C = critical stress intensity factor, i.e. fracture toughness Y = geometrical constant (1.12-1.98, depending on crack position and

shape, often about 1.7) σf = fracture strength ccrit = critical defect size

vi These specimens were compacted in a meticulous manner. The average hardness of prepared specimens rarely exceeds 150 HV.

50 Lars Kraft

Examples of flexural strength data are presented below. The results show the difficulty in performing flexural strength measurements. Doxadent and the glass-ionomer cement showed sensitivity to the test method. The composite material showed the highest strength and the lowest sensitivity to choice of test method. Table 9. Flexural strength of some dental materials in MPa. Standard.deviation within Brackets. Paper VII Test method DoxaDent Tetric Ceram Fuji II

(glass-ionomer)Harward (zinc phosphate)

ISO 4049 21 (2) 120 (13) 26 (4) 20 (2) EN 843-1 59 (5) Not measured. Not measured. Not measured Ball on disc 69 (9) 142 (7) 41 (11). 18 (3)

Equation 4 implies that the fracture strength is dependent on the defect

size. Since defects are often statistically distributed, brittle ceramics yield statistically based values. Therefore an approach based on probability must be applied to describe the strength of ceramic materials [51]. Equation 4 also underlines the fact that flexural strength testing reflects the size of the largest defect present in the sample.

The most commonly used method to experimentally measure K1C is through the use of Vickers hardness indentation [52]. The fracture toughness of Doxadent was measured and calculated to be between 0.7 and 1 MPam1/2 depending on the fracture toughness formula used [53].

4.7.3 Wear characteristics (Paper VIII) Wear of a material is controlled by the hardness (mild wear) and the fracture toughness (severe wear). For wear testing of ceramic dental filling materials there are no approved standard test methods. This is due to the difficulty to relate wear to a material, since wear is a system-related property also depending on external conditions such as test method and environment (liquid, pH, temperature, wear particle concentration and properties, contact periodicity, contact pressure, tank effects etc).

Wear results from two different tests indicate the wear of Doxadent to be of similar magnitude to that of composites.

. Table 10. Wear of two dental filling materials compared to dentin and enamel. (Paper VIII) [54] Test method Doxadent Tetric Ceram Dentin Enamel ACTA, Wear depth

53 µm 40 µm - -

Dimpler test, Wear volume

7400 µm3/Nmm

9600 µm3/Nmm

5700 µm3/Nmm

2000 µm3/Nmm


4.8 Retention properties (Paper V) When comparing dental materials with respect to adhesion, it is of fundamental importance to emphasise the different scenarios for materials that either shrink or expand somewhat during curing. An expansion or shrinkage will either attach or detach the filling to or from the tooth substance. Because of the shrinkage in dental composite materials, the bond strength of the contact zone to tooth substance must overcome the tensile stresses induced by the shrinkage in the polymerisation process [55]. As mentioned earlier advanced bonding systems have been developed to counteract this shrinkage. Contrary to polymer composites Doxadent has a slight expansion (Paper II and III). Retention of Doxadent involves a limited compression created by the slight two-week linear expansion of < 0.1% of the material, leading to a shear strength of 10-35 MPa in the interface between the filling and the tooth substance (Paper VII). The actual adhesion forces needed to retain a filling in the tooth correspond to shear strengths of approximately 5-10 MPa – also often found for dental implants in general [56]. A very tight contact zone between Doxadent and the tooth structure is observed even at high magnification (Figures 18-19.). This is attributed to the slight expansion during curing and due to the hydration process involving dissolution of calcium aluminate and precipitation of hydrates. The precipitation will occur in all spaces with saturated liquid including the small micro-spaces between the packed material and the tooth wall. The sealing of the interface is further improved with time due to continued precipitation.

52 Lars Kraft

A) B) Figure 18. The contact zone between Doxadent and enamel in magnification of 1000x (A), and dentin in magnification 500x (B), respectively. (Observe the scratch marks, from the polishing procedure).

A) B) Figure 19. The same contact zone in ESEM A) dentin, magnification 2000x B) enamel, magnification 2400x..

4.9 Phase stability aspects Due to the accelerator added in water the dissolution and precipitation of hydrates start almost immediately. This takes place at room temperature until the specimen, or the dental filling material, is placed in its final environment at 37ºC. Therefore some C2AH8 hydrate may form initially, together with Gibbsite and Katoite. As the temperature in the material increases after its placement, the formation of C2AH8 hydrate declines until it ceases after a period of time. Due to conversion the fraction of C2AH8 decreases with time. From Figures 16, 17, 20 and 21 the main part of this conversion seems to take place after approximately 20-50 days of hydration. The water released in this conversion will react with anhydrous grains and will further fill out the porosity.


The XRD diffractograms of experimental Doxadent powder and Mix 6 (without filler particles) are plotted below.

☻ ● ☼■ ◙☼ ☺

170 days

60 days

25 days

15 days

7 days

3 days

2 days

1 day of hydration

● ●

● ●

● ◙ ∆■

Figure 20. X-ray diffractogram for experimental doxadent powder hydrated 1-170 days.

120 days 50 days 35 days 20 days 10 days 3 days 1 day of hydration

☻ ☼ ◙

☺ ◙

■ ■

◙ ☼ ■ ☼ ☻

☺

Figure 21. X-ray diffractogram for “Mix 6” powder hydrated 1-120 days. ☺ - 3C2AH8 ; ☻ - C3AH6 ; ☼ - 2AH3; ● - ZrO2 ; ■ - CA ; ◙ - CA2 ; ∆ - CaCO3 ; ○ - Si (for calibration in Mix 6 series).

○

●∆ ☻

■

54 Lars Kraft

This limited hydrate conversion may lead to a slight temporary decrease in strength indicated by the hardness and porosity data in Figures 16 and 17. In Figures 20-21 the general hydration process is revealed. The fraction of the anhydrous cement phases CA and CA2 decreases continually with time, and the fraction of the hydrates Katoite and Gibbsite increases. In the experimental Doxadent series (Fig. 20) some calcite was also detected in the oldest in-vitro samples. This is mainly due to the on-surface precipitation earlier discussed in section 4.6.2. The possibility of conversion of Katoite to Calcite due to carbonation was also taken into account. However, material from a failed restoration - in an attempt to build up a crown - which had been placed in-vivo for about 130 days showed no calcite formation in the XRD analysis.

4.10 The microstructure In Figures 22A-C the SEM images show the typical microstructure developed. The large white grains in image A are anhydrous cement grains of calcium aluminates. The white areas in images B & C are small zirconia particles or agglomerates of zirconia. The light grey areas are the anhydrous grains, the grey areas are hydrates, the dark grey areas are most likely Gibbsite. Some submicron porosity is also found.

The optical microscope displays the excellent contact to the tooth and the potential mould ability of Doxadent (Fig. 23). In Figure D the light comes from below. The lack of translucency in the cement material is shown. Figure 23F is a SEM image of the Doxadent enamel interface from the rectangle in Fig. 23E.


A) B)

C) Figure 22. The microstructure of Doxadent specimens, secondary (A) and backscattering (B,C) electrons images. The specimens were carefully polished in a diamond slurry.

56 Lars Kraft

A) B)

C) D)

E) F) Figure 23. An in-vitro prepared tooth, filled with both a Doxadent filling and a composite filling in different magnification (A-E Light optical microscope; F-SEM ).

4.11 Biocompatibility (Paper IX) The calcium aluminate dental restorative material was reviewed with respect to the chemical stability and the biocompatibility. Materials - both partially and fully cured - were examined by following the pH and ion concentration changes in water and artificial saliva exposed to the material for different


periods of time at 37ºC. Specimens that were at different hydration times were also subjected to cytotoxicity testing by using primary cultures of human oral fibroblasts. A tissue culture insert retaining tested materials was assembled into a 12-well plate above the fibroblast monolayers. The cytotoxicity was determined by an MTTvii [57] reduction assay at various time points. A pulp viability test was conducted on ten human teeth before and after treatment with Doxadent, and compared with results for amalgam and composite fillings. No significant difference was observed between the different dental filling materials.

Table 11. Vitality test mean values with standard deviation, lowest and highest reading. Filling Tooth

position* Mean value

Standard deviation

Highest value

Lowest value

Intact 4.47 33.4 8.2 54 20 Exp. material 5.25 33.2 8.1 52 21 Amalgams 5.92 37.0 11.5 57 18 Composites 5.54 37.8 12.8 56 15 *Mean tooth position for all observations

For materials immersed in water and in saliva for different times, pH measured 10-11 and 9-10, respectively. After final hydration the pH-range was 7-8. The release of metal ions during initial hydration was below 50 ppb/mm2 for aluminium and below 300 ppb/mm2 for calcium in water, whereas somewhat higher aluminium content was measured in artificial saliva. Neither a significant decrease of cell survival during the hydration process nor a change of the pulp viability of human teeth was shown. From the tests conducted it was concluded that this new type of dental filling material showed no sign of toxicity for partially or fully hydrated materials, although increased pH was detected during the hydration period. The release of metal ions during hydration was low.

vii MTT = (3-[4,5-dimethyl-thiazol-2-yl]-2,5-diphenyl tetrazolium bromide)

58 Lars Kraft

5 The Future

The use of hydraulic cements in restoration of teeth is not an entirely new idea. For example, Dentsply recently introduced a root-filling material based on calcium silicates. However, no other dental filling product, based on hydraulic cements, with the indication for use as permanent fillings in Class I, II and V has been launched before Doxadent. This calcium aluminate based dental filling material seems to fulfil several of the basic requirements of dental filling materials, such as: • The ability to be placed and properly moulded in a cavity. • The property of hardening in a short time, making it possible for the

patient to use the filling one hour after treatment. • Restoring teeth with no or limited post-operative problems • X-ray opacity • Thermal properties similar to those of tooth substance • Slight dimensional changes – no shrinkage • Excellent biocompatibility • Development of a tight contact zone against tooth structure without

etching or bonding systems In Table 3 (page 21) the characteristics of a “utopian” dental filling

material were presented. The CAC-materials thus meet several of these criteria. However, areas for further material studies concern, amongst other things, mechanical properties such as fracture toughness, strength and wear. Also handling characteristics need to be improved to assure a rapid user learning curve of this type of material. The aesthetics is also an area of further improvement. The materials studied were all white and opaque. Long-term evaluation of material properties and long-term statistically relevant clinical evaluations are areas of great need.

One important aspect dealt with in this study is the influence of in-surface and on-surface precipitation in the system and cavity walls. This gives the material remarkable potential as a biomaterial, not only within odontology but also in orthopaedics and plastic surgery [58]. By certain modifications of the CAC-based materials it has recently been shown that in-situ apatite


formation is possible in the interface between these materials and hard tissue (Swedish patent pending).

60 Lars Kraft

6 Acknowledgements

My supervisor Leif Hermansson is acknowledged for his friendship, never-ending enthusiasm and energy. Without his energy, much of this work would not have been concluded. The time he has given me I value highly.

Håkan Engqvist, my collegue the last years, is also acknowledged for friendship and his positivism of believing that things are always (almost) done easily and fast. He also convinced me that sweat, pain and endurance are poetry - at least in a jogger’s perspective.

Thank you Gunilla, Noa, Yashar, Jesper, Stina, Anna, and Jonas for your fellowship and your contribution to this thesis. Thanks Emil and Lena S for being like “old friends” to me at the office, and Irmeli for believing in me.

All personnel at the department of Materials Science is acknowledged, the secretary Carin Palm for taking care of everyone, Jan-Åke for solving problems and Rein Kalm for his amusing attitude (“no more mercury in graveyard chimneys now?”). Sture, thanks for your general easiness and for activities like “målarskola”. Urban, Micke, Micke K, Lena, Olle, Tobias, Jocke, Ricko, Daniel etc. – thanks for fellowship. In particular, thank you Urban for help with Endnote (saved me a lot of time), Ricko for imaging assistance and Lena for your proofreading.

Also, I must acknowledge the little sign hidden above the toilet door at the department: “With work you ruin the whole day”.

My friends among the jogging, inline-hockey and hockey-bockey competitors are warmly acknowledged, for example Anders for “taking care” of me on the ice. Other memorable things are bleeding feet, poop breaks and snoweating in the ditch, phenomena occurring roundabout the 17km long Ångström-track. Runners are different. It’s been fun!

Thanks Mum and Dad, and Hans and Clary for your support. Benjamin, Viktor and Emil – my marvellous sons – a great hug to you!!

Finally I praise you Gabriela, my sweet wife, for your love and care. With love/ Lars


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http://www.rumford.com/articlemortar.html

http://ancienthistory.about.com/library/bl/uc_geary_rrc3.htm

62 Lars Kraft

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Hardening Concrete, Edited by DTU Dept. of Structural Engineering and Materials, pp. Chapter 2, 2000.

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25. Ø.E. Bjøntegaard, J. Sellevold, Interaction between thermal dilation and autogenous deformation in high performance concrete, Shrinkage 2000, Paris, pp. 43-55, 2000.

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28. K.L. Scrivener, Historical and Present Day Applications of Calcium Aluminate Cements, Calcium Aluminate Cements 2001, Edinburgh, pp. 3-23, 2001.

29. C. Evju, Phase development, heat of hydration and expansion in a blended cement with b-hemihydrate and anhydrite as calcium sulphate, PhD thesis, Lund University, Materials Chemistry, Lund, 2002.

30. M.G. Alexander, Acid Resistance of Calcium Aluminate Cement in Concrete Sewer Pipe Mixtures, Calcium Aluminate Cements 2001, Edinburgh, pp. 633-645, 2001.

31. J.N. Clarke (1997). Calcium Aluminate Cements in Construction, a re-assessment, The Concrete Society.

32. D. Sorrentino, F. Sorrentino, M. George, Mechanisms of hydration of calcium aluminate cements, Materials Science of Concrete IV, Edited by Skalny and S. Mindess, pp. 1995.

33. S.D. Majumdar, R. Sarkar, P.P. Vajifdar, S. Narayanan, R.M. Cursetji, A.K. Chatterjee, User friendly high refractory calcium aluminate cement, Calcium Aluminate Cements 2001, Edinburgh, pp. 467-476, 2001.

34. K.L. Scrivener, A. Capmas, Calcium Aluminate Cements, Lea's Chemistry of Cement and Concrete, Edited by P.C. Hewlett, Arnold, pp. 1998.

35. K.J. Anusavice, S. Ban, Influence of test method on failure stress of brittle dental materials, Journal of Dental Research 69 (1990) 1791-1799.


36. L. Hermansson, H.T. Larker, Tensile testing of brittle ceramics, Interceram 36 (40) (1987)

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Bioceramic Dental Filling Material based on Calcium Aluminate Cement, 26th Annual Conference on Composites, Advanced Ceramics, Materials, and Structures, Cocoa Beach, Florida, pp. 825-832, 2002.

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I: Principles and theorethical background, Cement and Concrete research Vol. 31 (No. 4) (2001) 647-654.


Appendix I - Classification of cavities

Class I: Occlusal surfaces of posterior teeth, including the occlusal 2/3 of the buccal and lingual surfaces. Incisal 2/3 of the lingual surfaces of the maxillary anterior teeth Class II and Class III: Class II- Proximal surfaces of the posterior teeth Class III- Proximal surfaces of the anterior teeth Class IV - VI: Class IV- Proximal and incisal angle of anterior teeth Class V- Gingival 1/3 of the facial or lingual surfaces of all teeth (anterior and posterior) Class VI- Cusp tip of posterior teeth Short dentist vocabulary:

Occlusal: The chewing or grinding surface of posterior teeth (molars and pre-molars). Incisal: The biting edge of your centrals and laterals. (Central: The two upper and two lower teeth in the very center of your mouth. Lateral: The teeth just adjacent to the centrals.) Buccal: The tooth surface which is next to your cheeks. Lingual: The tooth surface next to your tongue. Proximal: Refers to the surfaces of teeth that touch the next tooth.

66 Lars Kraft

Appendix II – The BOVA model

BOVA (Swe: Beräkning av Optimal Volymandel Aggregat)is a theoretical model for calculation of the optimal volume share of filler in the cement body in order to have as low fraction of anhydrous grains and porosity as possible – at the same time. The model gives a tool for understanding the parameters involved in the hydration of cement. It has been presented earlier. [39, 42] The model has some similarities with the fundamental work presented by Powers in 1949 [59]. Assuming that a powder mix of cement and filler material is compacted to a certain level, how much porosity will be developed and what content of anhydrous grains will left in the matrix? Let

x = vol% filler material (FM) in the powder mix y = volume fraction water in the compacted body z = volume fraction of non-hydrated cement after hydration zo = volume share of non hydrated cement in the initial powder mix k = w/c ( volume water to cement ratio required for total hydration) The share unreacted cement in the compacted body initially zi is, under the assumption that no cement is hydrated during compaction as follows: zi = zo ( 1 – y) (A1)

Since the volume share of cement that the water y can hydrate is zh = y/k (A2) the fraction of anhydrous cement after hydration will be : z = zi – zh = zi – y/k (A3)

Taking into account that zo + x =1, (A1) and (A3) give: z = (1-x)(1-y) – y/k (A4) Now the porosity in the hydrated body can be calculated as Vol% pores = y – zh * ε , (A5)


where ε is a volume increase factor = cement density / hydrate density. The hydrated cement has a lower density than the unreacted cement phases, so it expands and partially fills up the pores left by the water when it reacts with the cement. The equations A2, A3 and A4 now yield: Vol% pores = y - ε * ((1-x)*(1-y) – z) (A6) The k-value is different for different cement types.

162208/fulltext01.pdf · to be published in tribologia no.4 2002, finnish journal of tribology....

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