i

WEAR OF ENAMEL ANTAGONIST AGAINST CERAMIC RESTORATIVE

MATERIALS

by

TANEET GHUMAN

JOHN O. BURGESS, COMMITTEE CHAIR

DANIEL A. GIVAN

MARK. S. LITAKER

AMJAD JAVED

A THESIS

Submitted to the graduate faculty of The University of Alabama at Birmingham,

in partial fulfillment of the requirements for the degree of

Master of Science

BIRMINGHAM, ALABAMA

2010

ii

Copyright by

Taneet Ghuman

2010

iii

WEAR OF ENAMEL ANTAGONIST AGAINST CERAMIC RESTORATIVE

MATERIALS

Taneet Ghuman

MASTER OF SCIENCE IN CLINICAL DENTISTRY

ABSTRACT

Tooth wear is an ever-increasing problem, and is likely to continue as patients' de-

mands and expectations rise and as more natural teeth are retained into old age. Demands

for esthetic alternatives have led to increased development of new generations of ceramic

materials. Dental ceramics are known for their excellent chemical and optical properties.

The wear of human enamel and the restorative material are often a functional and estheti-

cal concern when selecting a restorative material for clinical restorative treatment. Ce-

ramic restorations have been known to cause wear of opposing enamel. Some restorative

materials wear the opposing tooth while some have significant wear. Wear of restorative

materials and enamel is often determined by wear simulators. These machines have limi-

tations and most have not been calibrated to in vivo wear. Additional controversy centers

around the force exerted by these simulators on the esthetic restorative material. General-

ly the load used is similar to the forces generated during mastication. Validation of the

wear machines should be done by comparing their results with published in vivo studies.

The objectives of this study were (1) to evaluate volumetric wear and depth loss of

the antagonist surface (1) to evaluate the volumetric wear and depth loss of occlusal ena-

iv

mel investigated at baseline and 200,000 cycles.

Impressions, employing a polyvinyl siloxane, were made of the shaped enamel styli and

poured with improved dental stone and separated. Both the enamel styli casts and the ce-

ramic restorative materials were scanned with a non-contact Profilometer for before and

after 200,000 cycles in the wear simulator to determine loss of volume and depth of the

restorative material.

Superimposition of imaged casts was done to evaluate wear volume loss. The data was

statistically analyzed for volume loss of the 3 ceramic restorative materials compared to

enamel styli by ANOVA and Tukey-Kramer post hoc test. Lava Core showed higher in

vitro volumetric wear loss than all other groups (p<0.0001). There was no in vitro volu-

metric wear or depth loss for antagonist surfaces in any of the groups (p>0. 6209). There

were no differences for enamel wear depth for any of the groups (p>0.2757).

Keywords: Wear, enamel, ceramic restorative materials, wear volume loss, depth

loss, 3-D Non-contact Profilometer.

v

ACKNOWLEDGEMENTS

I am grateful to the many individuals who contributed to my education, culminat-

ing in achieving this Master‘s degree.

First, I wish to recognize Dr. John O.Burgess, Chair of my graduate committee.

Without his vision and guidance, this work could not have been completed. I convey my

earnest appreciation and gratitude for his invariable support, patience and motivation. I

am highly honored to have worked under his guidance and would love to work with him

in the future.

I would like to express my sincere thankfulness to my committee members, Dr.

Daniel A. Givan, Dr. Mark S. Litaker, Dr. Amjad Javed, for their invaluable advice over

the period of my research project. They were very tolerant and determined to see me

through. They all provided helpful assistance and advice for this work

I wish to extend my utmost gratitude to Mr. Preston Beck, without whose exper-

tise, I would not have been able to finish this study in time. I cannot thank him enough

for all that he has done for me.

I also wish to thank Dr. Deniz Cakir whose thoughtful & meticulous organization

of my research project allowed me to be consistent.

Many others have provided support and encouragement, especially Dr.Pranita Kadam,

Dr. Arjun Sarof, Dr. Ramtin Sadid Zadeh and Dr. Prabhavati Jampani my friend‘s from

the Master‘s program.

Finally, I thank all my friends and the biomaterials residents, without the moral

support of whom I would not have been able to achieve this.

vi

I am grateful to all these individuals and countless others with whom I have had

the privilege of interacting during the past years, and this dissertation is dedicated in part

to them.

Ivoclar Vivadent and 3M ESPE generously donated the materials for the study.

vii

DEDICATION

I dedicate my thesis to parents,

Mrs. Simran and Mr. Gurvinder Ghuman.

Thank you for your unconditional love and devotion with which you have raised me into

this world. Thank you for giving me a chance to prove and improve myself through all

walks of life. I am honored to have you as my parents.

I dedicate my thesis to my grandparents, sister and brother,

Ameet and Jaikaran Ghuman.

Thank you for believing in me; for allowing me to further my studies. I would not have

been at this stage without your unvarying encouragement and support.

viii

TABLE OF CONTENTS

Page

ABSTRACT...................................................................................................................... iii

ACKNOWLEDGMENTS.................................................................................................. v

DEDICATION...................................................................................................................vii

LIST OF TABLES............................................................................................................. x

LIST OF FIGURES............................................................................................................xi

INTRODUCTION...............................................................................................................1

Ceramics Background......................................................................................................... 1

Dental Ceramics.................................................................................................................. 2

Wear Background............................................................................................................. 11

Clinical Significance of wear............................................................................................ 13

Mechanism of Wear...........................................................................................................15

Wear Simulating Devices................................................................................................. 18

ACTA wear machine........................................................................................................ 20

Oregon Health Sciences University Oral Wear Simulator............................................... 21

University of Alabama Dental Wear Simulator................................................................ 21

Zurich computer-controlled masticator............................................................................. 22

BIOMAT wear Simulator................................................................................................ 23

Minnesota wear Simulator............................................................................................... 23

Willytec Wear Simulator................................................................................................. 25

Contributing factors for in-vitro wear simulation............................................................. 27

Alabama Chewing Simulators......................................................................................... 31

OBJECTIVES AND HYPOTHESES.............................................................................. 33

MATERIALS and METHODS........................................................................................ 35

Specimen preparation........................................................................................................ 36

Determination of Wear..................................................................................................... 39

Impression protocol.......................................................................................................... 40

Casts.................................................................................................................................. 41

ix

TABLE OF CONTENTS

Page

Determination of volumetric wear & depth loss............................................................... 43

RESULTS ........................................................................................................................ 47

DISCUSSION................................................................................................................... 50

CONCLUSIONS ............................................................................................................. 58

SUGGESTIONS FOR FUTURE RESEARCH................................................................ 59

REFERENCES................................................................................................................. 60

APPENDIX

A IRB APPROVAL FORM ........................................................................................... 67

B Tables -Repeated measure ANOVA & Tukey-Kramer test.......................................... 69

C In vitro enamel styli scan at baseline & 200,000 cycles.............................................. 76

D In vitro scan of Ceramic surfaces…………..………………………………………… 80

E Superimposed Images of Enamel Styli at Baseline & 200,000 cycles……………...... 84

F Superimposed Images of Ceramic surfaces at Baseline & 200,000 cycles…..………. 88

x

LIST OF TABLES

Page

1. Commonly used Dental Ceramics.................................................................................. 3

2. Materials used............................................................................................................... 36

3. Wear on Antagonist Ceramic Surfaces......................................................................... 47

4. Wear on Enamel Styli................................................................................................... 47

xi

LIST OF FIGURES

Figure Page

1 IPS e.max crystals ............................................................................................................5

2 Two-Body Wear..............................................................................................................16

3 Three-Body Wear............................................................................................................17

4 Fatigue Wear...................................................................................................................18

5 ACTA Wear machine.................................................................................................... 20

6 Oregon Oral Wear Simulator......................................................................................... 21

7 University of Alabama Dental Wear Simulator............................................................. 22

8 BIOMAT wear simulator............................................................................................... 23

9 Minnesota wear simulator.............................................................................................. 24

10 Willytec Wear Simulator............................................................................................ 25

11 Study Design............................................................................................................... 35

12 Stainless Steel Holders................................................................................................ 36

13 Carbide Burs................................................................................................................ 37

14 Mounted Maxillary Premolar....................................................................................... 37

15 Ceramic Blocks............................................................................................................ 38

16 Mounted ceramic specimen......................................................................................... 39

17 Modified UAB Chewing Simulator............................................................................ 40

18 PVS Impressions.......................................................................................................... 41

xii

LIST OF FIGURES

Figure Page

19 Stone replicas of styli removed from PVS Impressions.............................................. 42

20 Proscan 2000............................................................................................................... 44

21 Scanned specimens using Proscan................................................................................45

22 Superimposition of Images ..........................................................................................46

23 Original Alabama wear machine................................................................................ 54

24 Modified Alabama Wear Machine............................................................................. 55

1

INTRODUCTION

Ceramics Background

Land introduced the first feldspathic porcelain crown in 1886 (1). Since then, ce-

ramic crowns have been a focus of interest in patients and clinicians because of their nat-

ural appearance. Several types of ceramics have been introduced for producing all-

ceramic reconstruction. The first attempt was not until 1965; when McLaren and Hughes

introduced a technique strengthen conventional feldspathic porcelain with aluminous

porcelain, that a more common use of ceramic restorations was seen (2). However, ce-

ramic crowns were mostly used to restore anterior teeth because of their brittleness and

low tensile strength. In order to combine good optical results with enhanced stability,

glass-infiltered alumina (Inceram alumina) was developed. By adding glass the optical

quality was improved, however the high crystals content led to high opacity. Therefore,

alumina was adopted as a core material. Despite its enhanced mechanical stability, high

failure rates were reported with glass-infiltered alumina in posterior regions.

More recently, densely sintered high strength ceramics with mechanical

properties superior to conventional ceramics have been developed for restorative denti-

stry. Zirconia, the most stable of these high strength ceramics has flexural strength and

fracture toughness values of 900-100 MPa and 9 MPa/m1/2

. These values are twice as

high as glass-ceramics and glass-infiltered alumina. Using cores with alumina or zirconia

2

veneered with feldspathic porcelain, it is now possible to make a ceramic crown with im-

proved mechanical properties (3).

Increased patient demand for esthetic dentistry has encouraged interest in all-

ceramic dental restorations. Improved materials and innovative techniques have led many

dentists to use all-ceramic crowns and inlays for the restoration of posterior occlusal sur-

faces (4).

Introduction of alumina and zirconia as high strength core materials for all ce-

ramics crowns has significantly increase their use. Ceramic crowns can now be used in

posterior teeth successfully. When selecting a restorative material for dental practice, a

major consideration is its mechanical properties. Since restorative materials are used to

replace missing tooth structure, it must have adequate strength to withstand the forces

generated during mastication.

Dental Ceramics

Ceramics are probably the best materials available for matching the esthetics of a

complex human tooth. Dental ceramics are basically oxide based glass-ceramic systems.

Sintered ceramics and glass-ceramics are widely used as biomaterials for dental restora-

tion, especially as dental inlays, onlays, veneers, crowns or bridges. Ceramic materials

have been extensively employed in the clinical practice of dentistry mostly as an esthetic

restorative material. Different types of glass-ceramics and ceramics are available and ne-

cessary today to fulfill customers' needs (patients, dentists and dental technicians) regard-

ing the properties of the biomaterials and the processing of the products.

3

Glass-ceramics are particularly suitable for fabricating inlays, crowns and small

bridges, as these materials achieve very strong, esthetic results. High-strength ceramics

are preferred in situations where the material is exposed to high masticatory forces. A

well designed and fabricated ceramic crown is often indistinguishable from the adjacent

nature tooth. Although commonly used to replace decayed tooth structure, the esthetic

ceramic material is also used to cover pathological conditions of the enamel and dentin

such as unsightly stains, malformations of the teeth, or improper calcification. They are

used to close spaces (diastemae) existing between teeth and as enamel/dentin bonded par-

tial or total coverage without macro-retention.

Table 1: Commonly used Dental Ceramics

FLEXURAL

STRENGTH (MPa)

FRACTURE

TOUGHNESS (Mpa-m1 / 2

)

Feldspathic Porcelain

80

1.1

IPS Empress 120 1.2

IPS Empress 2 350 2.5

IPS e.max 350-400 2.5

In Ceram Alumina 400 4.5

In Ceram Zirconia 550 5.5

Procera (Alumina) 600 6.0

Zirconia (CERCON, LAVA) 900-1100 7-11

Tooth: Enamel

65-75 1

Tooth: Dentin

16-20 2.5

4

Conventional prosthetic treatment options such as porcelain-fused-to-metal

(PFM) are increasingly being replaced by all-ceramic restorative systems. Long-term

clinical studies have shown that the survival rates of Porcelain fused to Metal (PFM) sin-

gle crowns are 59% to 84% after an observation period of 15 years (5). In 2000, Ker-

schbaum reported a survival rate of 29% for cast metal crowns after an observation pe-

riod of more than 25 years (6). Scurria et al and Creugers et al reported a survival rate of

conventional fixed partial dentures (FPD) ranging from 69% to 74% after 15 years of ob-

servation period (7, 8). All- ceramic materials exhibit high level of translucency and lu-

minousness resembling those of natural teeth (9).

Ivoclar Vivadent introduced lithium disilicate reinforced ceramic in 1998. This

glass ceramic introduced by Ivoclar as IPS Empress® II, is similar to their newest ceram-

ic (IPS e.max®), where the alumino-silicate glass has lithium oxide added, exhibiting a

flexural strength and fracture toughness of 350MPa and 2.5MPa.m0.5

(Table 1). IPS e.max

was introduced as a modification to Empress 2 with 70% Lithium Disilicate crystals in a

glass matrix (Figure 1). The lithium disilicate crystals forming within this material are

needle-shaped and comprise about two thirds of the volume of the glass ceramic. The

shape and volume of the crystals contribute increased flexural strength and fracture

toughness of this material compared to Empress 2. The material comes in a pressable and

machinable form (e.max). This material can be very translucent even with the high crys-

talline content; due to the relatively low refractive index of the lithium disilicate crystals.

5

Figure 1. IPS lithium disilicate crystals in e.max

This material is translucent enough to be used for full-contour restorations or, for

the highest esthetics, can be veneered with special porcelain. Porcelain veneering mate-

rials for lithium disilicate glass ceramics are also alumino-silicate glasses with fluoroapa-

tite rather than leucite crystals. Fluoroapatite is a fluoride-containing calcium phosphate

with the chemical formula Ca5 (PO4)3F. The fluoroapatite crystals contribute to the ve-

neering porcelain‘s optical properties and coefficient (CTE) of thermal expansion to pro-

duce a veneering material that matches the CTE of lithium disilicate pressable or machin-

able material. Both the veneering material and lithium disilicate material are etchable due

to the glassy phase. Initial clinical data for single restorations are excellent with this ma-

terial, especially if bonded (10-15).

The clinical success of Empress 2 is detailed in several studies. Toksavul et al eva-

luated clinical performance of IPS Empress 2 all-ceramic crowns at 12 to 60 months. 79

IPS Empress 2 crowns in 21 patients were evaluated using Kaplan-Meier survival rate.

95.24% of the crowns were satisfactory at follow-up with 1 crown fracturing and1 endo-

dontically treated tooth fracturing (16). Taskonak et al reported a clinical evaluation of 20

anterior or posterior crowns & 20 anterior or posterior 3-unit fixed partial dentures in 15

patients Evaluation: Marginal adaptation, color match, secondary caries and fractures. 10

6

(50%) catastrophic failures of fixed partial dentures were reported, with 5 (25%) failures

within 1-year and (25%) by 2 years (17). Fasbinder et al reported a clinical evaluation of

IPS e.max crowns 2 yrs 62 IPS e.max crowns were cemented with 2 types of adhesive

resin cements, Multilink sprint & experimental self-adhesive cement by Ivoclar Vivadent

evaluated at baseline, 6 months, 1year and 2 yrs. No clinically identified cases of crown

fracture or surface chipping. No reported sensitivity at 1 or 2 years with either cement

with an Alfa score: 86.9 percent for crowns cemented with Multilink sprint & experimen-

tal self-adhesive cements (18). Early results indicate that IPS e.max crowns may be an

effective option for all-ceramic crowns (18). In a three year clinical study, Etman et al

cemented 90 posterior teeth crowns in 48 patients randomized into 3 groups. IPS e.max,

Procera AllCeram & Metal ceramic veneered with IPS Classic Porcelain. Data was ana-

lyzed using the modified USPHS criteria. Crowns that developed visible cracks were sec-

tioned and removed, and the surfaces were analyzed using a scanning electron micro-

scope (SEM). The data were analyzed using the Kruskal-Wallis nonparametric statistical

test, followed by the Mann-Whitney test with Bonferroni correction (α=.05). IPS e.max

& metal ceramic crowns showed fewer clinical changes than Procera All Ceram. Visible

roughness, wear & deformity noticed in occlusal contact areas of Procera AllCeram

crowns. SEM images showed well defined wear facets in both ceramic crown systems.

IPS e.max's clinical behavior was comparable to Procera AllCeram & metal ceramic

crowns but wear resistance superior to the Procera AllCeram crowns (19).

While early ceramics had a failure mechanism of bulk fracture however newer ce-

ramic materials using aluminum oxide and zirconia-based frameworks exhibit very good

clinical data and superior esthetics.

7

Zirconia holds a unique place amongst oxide ceramics due to its excellent me-

chanical properties the most recognizable names is Lava, Vita YZ, and Cercon®. These

materials were designed as a PFM alternative for single crowns and three-unit bridges

anywhere in the mouth. Larger bridges have been discussed but no large sample clinical

documentation exists for this application. Solid-sintered, monophase ceramics are mate-

rials that are formed by directly sintering crystals together without any intervening matrix

to from a dense, glass-free, polycrystalline structure.

Solid-sintered ceramics (polycrystalline glass-free) have the highest potential for

strength and toughness, but because of high firing temperatures and sintering, shrinkage

techniques were not available until only recently to use as high-strength frameworks for

crowns and fixed partial dentures. Zirconia has unique mechanical properties that provide

flexural strength for this ceramic range from over 900 MPa to 1,100 MPa (Table 1) (20).

It is important to note there is no direct correlation between flexural strength (modulus of

rupture) and clinical performance. With all things being equal, it is better to have an inhe-

rently stronger material than a weaker one. A more important physical property is frac-

ture toughness, a measure of a material‘s ability to resist crack growth ranges from 8 MPa

and 10 MPa for zirconia (21). This is significantly higher than any previously reported

ceramic ranging from 1-3 MPa, and roughly twice the amount reported for the alumina

materials (4-6 MPa). Zirconia has the apparent physical properties to be used for post-

erior three-unit fixed partial dentures (Table -1). Clinical studies for zirconia fixed partial

dentures have not demonstrated a problem with the zirconia framework (22-24). Unfortu-

nately, veneered zirconia had some problems associated with chipping and cracking of

porcelain. McLaren et al and his team did a pilot study testing the cracking resistance of

8

porcelain fired to zirconia. Using a slow-cooling protocol at the glaze bake to equalize

the heat dissipation from the zirconia and porcelain increased the fracture resistance of

the porcelain by 20% (25).

More than 1,200 Lava or Vita YZ restorations have been placed at the UCLA Cen-

ter for Esthetic Dentistry over the last 5 years, with less than < 1% per year failure rate

for core fracture. Chipping of the porcelain was noted in > 6% of the restorations that

could be recalled that required replacement, with many more showing chipping not re-

quiring replacement. The slow-cooling firing treatment on the glaze bake has minimized

or almost eliminated this problem. McLaren et al concluded from his clinical data that if

the proper porcelain firing protocol is used, single restorations anywhere in the mouth

and three-unit bridges (specifically Lava and Vita YZ) have performed well as a PFM

substitute (25). Inherent brittleness, low flexural strength & fracture toughness of con-

ventional glass and alumina ceramics have been the main obstacles for its use. While still

in its infancy, zirconia technology advances the fabrication of new biocompatible all-

ceramic restorations with improved physical properties for a wide range of promising

clinical applications. In clinical studies, zirconia core showed an overall success rate of

about 90-100% where as Zirconia Veneer material has problems with the veneer chipping

or cracking with minor loss of material after only 1–2 years. Schmitt et al conducted a 3

year clinical evaluation of 3 or 4 unit Lava fixed partial dentures for 30 patients and re-

ported veneering fractures of 4% and no framework fracture (26). Raigrodski et al re-

ported veneer fracture of 25% at 3 years, and Sailer et al reported 13% veneer chipping at

3 years & 15.2% at 5 years (24, 27, 28).

9

In the search for alternative and esthetic restorative materials, many all-ceramic

systems have been introduced for the general practitioner. However, dentists have re-

mained suspicious of the structural longevity, potential abrasivity, and fit of ceramic res-

torations, although facture and wear are the main concerns and reasons for failure of all-

ceramic crowns and fixed partial dentures (29). Newer reinforced ceramics showed better

durability then the earlier fired ceramic materials. Ceramics are more enamel abrasive

than other restorative materials such as gold, composites or amalgam.

Full contour zirconia crowns have been marketed which reduce the occlusal re-

duction of the tooth from 2-2.5mm to one mm a significant clinical advantage. However,

there has been considerable concern as to how these materials, formulated for improved

strength, wear the opposing dentition compared to other materials. An esthetic restoration

should not wear an opposing enamel surface. In a survey of members of the American

Academy of Esthetic Dentistry, in 1986 Christensen found "less wear on opposing teeth"

to be the single most desirable need for change in posterior tooth-colored crowns (30). In

1971 Mahalick et al. reported in vitro enamel-porcelain wear to be 2.4 times greater than

enamel-acrylic resin wear and 17 times greater than enamel-gold (31). Monasky and Tay-

lor tested a variety of surface finishes of porcelain against tooth substance and concluded

that the rate of enamel wear was a function of porcelain roughness (32). Ekfeldt and 0ilo

et al, using a bruxing subject, studied occlusal wear of porcelain, gold, and resin in vivo

(33). They also verified that enamel surfaces exhibited the greatest loss when opposed by

feldspathic porcelain. These and other studies have led some clinicians to caution against

the use of porcelain occlusal surfaces where rapid enamel attrition might be predicted,

10

such as for a bruxer or complete-denture wearer having porcelain opposed by natural

teeth.

A ceramic restorative material that combines good strength without the disadvan-

tage of increased enamel wear would be a significant addition to clinical dental practice.

Lambrechts et al. using clinical measurements reported that enamel vertical wear was be-

tween 20 m to 40 m per year when opposing enamel in the premolar and molar re-

gions, respectively (34). Dental wear, at first considered a pathological condition, is now

regarded as a physiological mechanism of teeth adaptation to continuous masticatory

stresses. (35). Enamel is the main tissue subjected to wear; however, advanced enamel

wear exposes increasingly large areas of dentin. Enamel hardness and anisotropy are the

major factors contrasting wear and micro fractures. Anisotropy is mainly related to the

different orientation of prism bundles and of hydroxyapatite crystals. Enamel wear devel-

opment is also related to differences in microhardness, density, mineral composition and

protein distribution. Masticatory loads distributed along the enamel-dentin junction un-

iformly disperse in the underlying dentin (35). As with enamel or dentin, restorative ma-

terials wear, and the wear mode depends on the type of restorative material. The various

dental materials may be grouped in four different categories: metal alloys, ceramics,

composites and unfilled polymers. Clinical studies have shown that metal alloys and ce-

ramic materials are generally very wear-resistant, whereas composites and unfilled poly-

mers have lower wear resistance (36, 37).

Ceramic materials may damage the opposing enamel. In vivo studies have shown

that the enamel wear depends on the ceramic material used (38, 39) internal porosities

and surface defects (40). Staining ceramic materials on the occlusal surface can influence

11

the wear of a ceramic material, as metal oxides used in the ceramic stains are abrasive to

enamel (40). Furthermore, patient-related factors such as dietary habits, dysfunctional

occlusion, biting force and bruxism contribute to accelerated enamel antagonist loss. Ce-

ramic wear testing remains difficult to assess in both in vitro and in vivo. Ceramic inves-

tigations are often studied using flat ceramic specimens opposing either human cusps in

their natural anatomic state or flattened (ground) enamel. It has been noted that the sum

of the vertical loss of enamel and of the restorative material can be a key to evaluating

wear characteristics relative to clinical performance. Evaluating the wear of restorative

materials requires that both the material of interest and the opposing material be consi-

dered. Clinically, it is the combined wear that is important; especially if the opposing ma-

terial is enamel. For example, in an in vitro study done using the University of Minnesota

artificial oral environment, significant differences were found in the ranking of material

wear depending on whether the material alone or the combined wear of the material and

enamel were considered. Enamel wear was measured when it opposed enamel, amalgam,

and porcelain. If only the material was considered, then enamel wore more than porce-

lain, which wore more than amalgam. Combining material wear with the opposing ena-

mel wear found that the enamel–porcelain combination wore more than the enamel–

enamel combination and that both wore significantly more than the enamel–amalgam

combination (29).

Wear Background

Mastication is defined as the act of chewing foods. Muscles of mastication pro-

duce active forces that are perpendicular and tangential to the occlusal surfaces of teeth

12

by guiding the movement of the mandible in relation to the functional movements with

the upper jaw. These forces are responsible, in part, for the wear of restorative materials

in the mouth. The ratio of the force of friction between the two surfaces and the force

pressing them together is the coefficient of friction (41). The coefficient of friction

changes with materials, and during mastication, the coefficient can be lower due to lubri-

cation from foods, drinks, and saliva (42). Wearing of tooth surfaces has significant clini-

cal consequences both esthetically and functionally. Increasing levels of tooth wear are

significantly associated with age. The predicted percentage of adults presenting with se-

vere tooth wear increased from 3% at 20 years to 17% in patients 70 years old and older

(43). The teeth most severely affected by wear are mandibular molars, but the most

commonly affected teeth are anteriors (43).

Several facts about biting forces are important while restoring the chewing capa-

bilities of patients. While evaluating the effect of tooth contacts on structures of mastica-

tory system, two factors must be considered: magnitude and duration of contacts. Both

chewing and swallowing activities must be evaluated. Estimates indicate that during

chewing, an average of 58.7lb of force is applied to teeth in 115ms. This yields

6.75lb/sec/chew. People swallow approximately 146 times a day while eating. An esti-

mated 66.5lb of force is applied during each swallow; this comes to 5068 lb/sec/day. The

total force activity for chewing and swallowing is about 17,200 lb/sec/day (42). Chewing

speed ranges about 49 to 120 cycles/minute. The usual chewing rate is 70 to 80 chews per

minute. It is important to consider that while the average chewing forces are about 22 Kg,

some individuals have biting forces as high as 360 Kg (42).

13

Materials wear is a complex and unpredictable phenomenon. The experiences

from the past decade prove that tooth wear occurs in an increasing number of cases in

dental practice. Tooth surface losses or ‗tooth wear‘ refers to the pathological loss of

tooth tissue by a disease process other than dental caries. The different wear mechanisms

are involved with the biomechanical factors of mastication. An ideal wear simulation

would incorporate both abrasion and attrition since mastication involves both processes.

Abrasion occurs in the presence of food as the jaw closes. It begins when both mandibu-

lar and maxillary teeth contact the food bolus and ends when the two teeth contact each

other. Because the teeth do not come in direct contact during abrasion, this stage is

termed the contact free area (CFA) region of wear. This stage of wear involves abrasive,

adhesive and corrosive wear (39). Attrition begins when the mandibular and maxillary

teeth directly contact and ends when they separate. This is termed the occlusal contact

area (OCA) region of wear (44). This stage of mastication involves abrasive; adhesive

and fatigue wear (44). The primary variables affecting the mechanism of wear include

the properties of the two contacting materials and the surrounding and interfacial media.

The rehabilitation of the lost tooth material is often very difficult, irrespectively of

whether it is needed because of functional or esthetic causes.

Clinical Significance of wear

Clinical wear bears a multifactorial etiology, understanding the mechanism of ac-

tion is an important step in an appropriate restoration material selection. Each material

selected should meet the individual wear behavior and needs. Individual factors may en-

hance the wear rates: aggressive tooth brushing, parafunctions, diet, acidic/aqueous envi-

14

ronment, surface geometry, and diminished tooth support. Failure of ceramic restoration

due to creation of micro cracks/flaws, poor masticatory function (29), impaired aesthetic

appearance (45), sensitivity, secondary caries and systemic effects through ingested wear

products (44). Teeth wear has significant clinical consequences both esthetically and

functionally. As teeth wear, they continue to erupt, which led to the concept of ―wearing

into occlusion‖. Physiological wear is surface degradation that results in progressive, but

very slow loss of convexity of the cusps, which manifests as a flattening of cusp tips on

the posterior teeth and incisal edges of mammelons on the anterior teeth. Excessive wear

results in unacceptable damage to the occluding surfaces and alteration of the functional

path of masticatory movement (40). It may also destroy anterior tooth structure that is

essential to acceptable anterior guidance function or esthetics, resulting in increased hori-

zontal stresses on the masticatory system and associated temporomandibular joint re-

modeling. If wear continues unabated, the enamel will eventually be breached. Once

breached, both the enamel and exposed dentin wear at accelerated rates. Excessive wear

on multiple teeth can have disastrous consequences. Biological consequences are related

to pain of the temporomandibular joint (TMJ), elongation of antagonists, loss of peri-

odontal ligament and tilting and movement of adjacent teeth. In spite of it being frequent-

ly mentioned as possible consequences, there is little evidence that occlusal wear as such

leads to the dysfunction of the TMJ, to muscle pain or periodontal disease (46-51). Sup-

portive treatment following restoration is important to monitor wear rates such as the res-

toration and nightguards.

A major concern in clinical practice is the wear resistance of resin materials used

in restorations especially involving posterior occlusion. Two types of wear have been de-

15

scribed by Leinfelder (52). One of these is wear initiated by generalized conditions (the

type of wear generated by a food bolus during mastication) and the other is wear generat-

ed under localized conditions (represented by direct tooth to materials contact). Some au-

thors have suggested that localized wear may be a more important contributor to the

breakdown of a material and contact wear may be more than two times as great as that in

non-contact areas. Clinical studies offer the most meaningful data on the performance of

a given material the time involved and costs associated with clinical studies have caused

manufacturers to have a strong interest in the use of wear simulator to test of prototype

materials during development and as a predictor of clinical performance. Leinfelder et al

developed a laboratory simulator capable of evaluating both generalized and localized

wear (52). This system transfers masticatory stresses to a composite specimen by means

of a flattened polyacetal stylus (generalized wear) or a stainless steel conical stylus (loca-

lized wear) in the presence of slurry of polymethylmethacrylate beads (PMMA). This de-

vice has facilitated the development of in vitro studies capable of predicting in vivo per-

formance. Previous work showed a correlation between in vitro wear and in vivo genera-

lized wear of dental restorative materials.

Mechanisms of Wear

Wear takes place at two surfaces: occlusal surface and proximal surface

Wear at contact-free occlusal area- CFOA

Wear at the occlusal contact area- OCA

While chewing, opposing dentition traps a layer of food and grinds it as the teeth

move past one another. The chewing forces produced during this phase are modeled

16

ranges between 10-20N (53, 54). At the end of chewing cycle, sliding motion stops as the

teeth reach centric occlusion. The chewing force ranges between 50-150N (55).

Wear is defined as a complex phenomenon and an ‗overall effect‘ of a number of

inter-related processes. It is further described these with five terms.

Two-body Wear

In two-body abrasion, surfaces are rubbed away by direct contact. During this

movement, the asperities must either fracture or deform. At a microscopic level, no sur-

faces are smooth and hence they contact by the reunion of their asperities. If both surfac-

es are brittle, there is fracture of the asperities. If one surface is soft, then the harder sur-

face will plough into it, rising up chips, which eventually fracture away. Gradually, all

the asperities fracture and the cumulative effect of loss manifests as wear (56). In the oral

cavity, these conditions predominantly occur during non- masticatory tooth movement.

Attrition is a form of two-body abrasion tooth wear that can be considered physiological

as it has been described as a prerequisite for balanced occlusion. It is the physiological

wearing away of dental hard tissues as a result of tooth-to-tooth contact without the inter-

vening foreign substances that causes localized wear of occlusal contacts (57). The wear

rate of enamel at occlusal contact areas in molars is about 41 m per year (58).

Figure 2. Two Body Wear

17

Three Body Wear

In three body wear surfaces are rubbed away by intervening slurry of abrasive

particles. The pressure between the surfaces is transferred to the particles, which then cut

away the asperities. In the oral cavity, this type of wear occurs during mastication and is

common in patients who eat an abrasive diet. During the early stage, when the occlusal

surfaces are separated by the food bolus, the abrasive particles act as a slurry and abrade

the whole surface as in generalized wear. They abrade the surface in the food shedding

pathways because of the shearing action of food on contact stress. This process is very

common in restorations with buccal or palatal extensions, as these absorb the main force

of the masticatory slurry in the escape root of the groove. This process tends to hollow

out the softer regions on a surface producing the chipping defects seen in occlusal molar

dentin (56). As the teeth begin to approximate during the later stages of mastication, the

remaining slurry particles get trapped between the asperities, in pits and in surface

grooves. If both surfaces have similar morphology then the abrasive particles may trans-

fer between scratches and cause more or less equal loss of both surfaces (56).

Figure 3. Three Body Wear

Fatigue wear

Some of the movement of the surface molecules is transferred to the subsurface

causing rupture of intermolecular bonds and a zone of subsurface damage. Micro cracks

18

form within the subsurface and coalesce to the surface, therefore causing loss of a frag-

ment of material inducing fatigue wear.

Figure 4. Fatigue Wear

Tribochemical wear (dental erosion)

It is caused when chemicals weaken the inter-molecular bonds of the surface and

potentiate the wear processes. There is interplay of erosion, attrition and three-body abra-

sion. In the oral cavity, acids normally cause extrinsic effects such as dietary acids or in-

trinsic effects resulting from gastric reflux. These acids weaken only the surface mole-

cules. These are then rubbed away by the movement of the surfaces and immediately the

underlying surface is attacked by the acid (56).

Adhesive wear

This occurs when there is a high attraction between surfaces such that cold welds

between the asperities. As the movement continues these micro-welds fracture, but not

along their original line of fusion. This type of wear is normally seen in metals (56).

Wear simulation devices

Many research centers have developed a variety of wear testing devices, each

with different degrees of complexity. As more and more materials are developed, and is

difficult to evaluate them by expensive time-consuming clinical trials to obtain a mea-

ningful data. Several mechanical testing systems (machines) have been created to simu-

19

late in vivo occlusal wear on the influence of test materials on natural enamel. The FDA

established guidelines for non-clinical laboratory studies including: equipment should be

calibrated and its maintenance defined and ensured so that the generation, measurement,

and assessment of data should be adequately tested, calibrated, and/or standardized (58).

In vitro models are easier than measuring in patient‘s mouth and do not depend on

patient compliance. In vitro models can provide reproducible chewing patterns for wear

evaluation where force profile, gliding path, medium and number of chewing cycles are

controlled. The machines and support systems offer controls to vary pH, temperature, an-

tagonist material, contact area, frequency of load cycles, load/force, third body medium,

lubrication and friction, duration of tooth contact, sliding speed, and wear measurement

technique.

All of which have been shown to affect wear rates. However, available machines

do not take into account the presence/absence of a periodontal ligament, which absorbs

some of the forces and influences motions during mastication. Due to the complexities of

recreating the mechanisms and properties of the oral cavity, no in vitro device can fully

simulate in vivo conditions. Hence, an important purpose of these machines is to serve as

a comparison of materials and to rank restorative materials according to their wear resis-

tance.

Many wear simulator research centers are trying to mimic the oral environment

and biological variables intending to rank restorative material according to their wear re-

sistance in comparison to reference materials.

20

ACTA wear machine (59-64)

The ACTA device has two metal wheels rotating in different directions with a dif-

ference of about 15% in the circumferential speed while being in close contact with each

other. The test specimens are placed on the circumference of one wheel whereas the other

wheel serves as antagonist. The force with which the two wheels contact is adjusted with

weights or springs. The stylus is a textured and hardened steel counter-wheel. The typical

medium used rice/millet seed shells suspension. A sliding movement with 15–20 N of

modifiable force (ranging between 0–50 N), 1.0Hz frequency and 100,000-200,000

cycles is used. Set-up: sample chamber with multi-chambered sample wheel, holding up

to 12 sample materials. The rotational speed of both motors ranges between 0–170 rpm

and is independently adjustable. Variables are contact stress, moving speed, and mutual

slip (15%). Although this machine does not closely simulate the biomechanical processes

of dental wear and forces applied are not high enough to produce fatigue wear, this ma-

chine is highly automated and provides a significant amount of screening data in a rela-

tively short time.

Figure 5. ACTA Wear Machine

21

Oregon Health Sciences University Oral Wear Simulator (OHSU) (65-67)

The OHSU device was a breakthrough device incorporating a sliding load which

duplicated the forces measured on a regular basis with a force sensor of a 10Hz measur-

ing frequency. This system is a multi-mode simulator. Stylus is enamel and conical, Me-

dium used is poppy seeds and PMMA beads. Movement is impact + sliding, and loading

is electro-magnetic. Frequency is 1.2 Hz with 50,000–100,000 no. of cycles, does not

have an integrated force sensor and the forces for abrasion testing (20 N) from one device

to another (65). The sliding distance varies from 5.36 to 6.24mm, producing variation in

the contact time. There is no systematic force/time measurements carried out with the

OHSU device.

Figure 6. Oregon Oral Wear Simulator

University of Alabama Dental Wear Simulator (52, 68-69)

The Alabama wear simulator in vitro wear device is the most cited as determined

by literature review. The Alabama wear simulator uses springs to generate an appropriate

22

force. The stylus is a flat of polyacetal cylinder or conical stainless steel styli. A variety

of materials were used for localized occlusal contact wear simulation and the medium

used is PMMA beads. The stylus movement is impact and rotation of 30°. Force exerted

is variable but generally is set at 75.6 N, vertical with a frequency of 1.2 Hz. It is a four-

station device with number of cycles ranging from 100,000–200,000–400,000. In this

wear set-up, multiple wear patterns can be seen. Generalized wear as simulation of the

wear during mastication.

Antagonistic enamel wear is simulated by wear of enamel created by direct con-

tact with the restorative materials. Vertical wear is measured as enamel height loss, ma-

terial stylus height loss, and total vertical height loss (the sum of the enamel height loss

and the restorative material height loss).

Figure 7. University of Alabama Dental Wear Simulator

Zurich computer-controlled masticator (70)

The stylus in this simulator is enamel. Medium is water (+alcohol + tooth brush-

ing). Movement: impact (+sliding) with a lateral movement of 0.2 mm, Force: 49 N with

23

a frequency of 1.7 Hz. Loading is electro-magnetic. Number of cycles range from

120,000, 240,000, 640,000 and 1,200,000 load cycles. Set-up: masticator. Variable:

toothbrush/toothpaste abrasion and chemical degradation. Zurich masticator has a rubber

socket that simulates the periodontal ligament and produces a sliding movement of the

sample, leading to a softening effect during wear simulation. The elastic modulus of the

rubber dam changes over time and these changes are accelerated by thermocycling.

BIOMAT wear simulator (71)

The stylus is SS304 counter-body. Medium is water and has an impact (+sliding)

movement. Force is 20 MPa. Loading is by weights. Set-up: reciprocal compression-

sliding wear instrumentation. Variable: shock absorbing layer, Oral temperature is 37 ◦ C.

Figure 8. BIOMAT wear Simulator

Minnesota: MTS wear simulator (72)

The MTS chewing simulator is the only device with an incorporated force sensor,

exhibiting high force capability, high power per unit weight and volume, good mechani-

cal stiffness, and high dynamic response. The hydraulic actuator produces a force, which

24

is controlled within narrow limits in the descent/lifting and lateral movements; In addi-

tion, the force profile (―haversine waveform‖) is highly reproducible. Simulator control-

lers regulating the force (load cell), force profile (dual trace oscilloscope) and vertical and

lateral movements are incorporated into the testing device. The MTS chewing simulator

is an adapted version of the MTS device, which is widely used by medical companies for

the biomechanical testing of artificial hip and knee joints and implants. Stylus used is a

tooth, Medium: water giving a Movement: sliding. Force is 13.35 N and loading is hy-

draulic Number of cycles vary from 120,000, 240,000, 640,000 and 1,200,000 load

cycles. Set-up: masticator. Variables are contact stress, moving speed, mutual slip, and

third-body composition.

Figure 9. MTS wear Simulator

25

Willytec Wear simulator (73)

The Willytec chewing simulator has been commercially available since 1997.

This simulator is utilized to load crowns and bridges for fracture tests and to evaluate the

deterioration of the marginal integrity of restorations placed in extracted teeth. Stylus

used is enamel, empress (diameter 2.36 mm). Medium is water. Movement: gnashing,

slippage, striking with lateral movement of 0.7 mm. Force: 50 N with Loading by

weights. Number of cycles is 120,000 cycles. Set-up: masticator with programmable

Thermocycling (5–55 ◦ C). Variables: speed of impact, intensity of the impact, impact

and sliding load path.

Figure 10. Willytec Wear simulator

Simulators have been developed to measure the in vitro wear of dental materials;

each of these machines has their advantages and limitations.

In 2006, Heintz conducted a round robin study correlating the results of the dif-

ferent wear simulating devices with ACTA, OHSU, Willytec and Zurich wear devices on

eight different composites. Specimens were prepared at the Ivoclar Vivadent laboratories,

26

and sent to different testing sites and collected and data was analyzed (45). Heinz con-

cluded that the relative ranks of the materials differed significantly between the wear de-

vices. This is the only published paper to date in relation to validation of all systems. The

major limitation of the study was that the testing protocol and the method for measuring

wear at the individual test sites were not standardized. Variation would also arise from

the differences in antagonist material, number of cycles, temperature, pH, slurry medium,

and wear measurement technique. Each of these variables has been proven to affect wear

rates of composite materials. An accurate comparison is done between wear rankings for

the testing devices based on the load profiles and wear patterns, which differentiate the

wear devices. The Minnesota wear device is more expensive and complex than the other

devices and is located in only one test site. The BIOMAT wear device simulates only

two body wear. The Alabama wear simulating device has been used prolifically in the

United States. Existing oral wear devices have varying methods of simulating the abra-

sion and attrition phases of wear. The BIOMAT, OHSU, Minnesota and Willytec devic-

es have a stylus that impacts the specimen and slides a certain distance. The Leinfelder

wear testing device stylus impacts the specimen and rotates 30 degrees. These devices all

incorporate both the abrasion and attrition phases of mastication. The ACTA device has

two wheels, a wheel containing composite specimens and one steel counter surface wheel

that rotate next to each other at slightly different speeds. This device measures only the

abrasion phase of mastication. A round-robin test with the ACTA, OHSU, and Willytec

revealed that these devices measure different wear mechanisms. Despite prolific wear

testing by industry and academia with these devices, the International Standards Organi-

zation (ISO) has not specified a standard wear testing system.

27

Broadly, oral wear simulating devices incorporate three methods of producing

wear: sliding, sliding with impact, and rotation with impact. The effects of sliding wear

(abrasive and adhesive mechanisms) were compared with impact wear (abrasive, adhe-

sive and fatigue mechanisms) using a BIOMAT simulator. The comparative rankings of

seven restorative materials (including two composites) differed significantly between the

two methods, and the study concluded that ―there is no correlation between impact-cum-

sliding wear and non-impact sliding wear‖. There has not been a study that has analyzed

the effect of wear produced with impact and rotation to wear produced with impact and

sliding.

Contributing factors for in-vitro wear simulation

1. Standardization of the antagonist: Counter sample materials

The choice of the counter sample is a critical factor in establishing the pattern of

wear and in achieving an efficient in vitro wear testing system. A variety of factors in-

cluding hardness, wear surface evolution and frictional coefficients have to be consi-

dered, relative to the tribology of the in vivo situation. Assessment of potential counter

sample materials should be based on the essential tribological simulation supported by

investigations of mechanical, chemical and structural properties (74). Antagonists stan-

dardized for shape and size and according to materials should show mean values similar

to those found in natural, non-standardized cusps. Krejci et al. measured the shapes and

sizes of palatal cusps of non-erupted human upper third molars (74). Natural enamel an-

tagonists are preferable for the simulation of wear in the occlusal contact area.

2. Composition of the antagonist

28

A variety of antagonists have been used which include enamel, gold, ceramics,

stainless steel, annealed chromium-steel counter bodies, Alumina ball: diameter 10 mm

and Dental porcelain. A study by Heintz concluded that enamel provided similar wear

results as two different ceramic antagonists and produced no more variation in the wear

data (56).

3. Shape of the antagonist

A variety of antagonist such as flat, ball or rounded, flattened enamel surfaces,

enamel harvested from extracted human third molars and machined into cusps with a 5

mm spherical radius or hemi- spherically and Standardized human enamel cusps with a

uniform contact area have been used (75).

4. Load/force

In the load/force diagram several variations are possible. Static and/or sinusoidal

cyclic and dynamic, Contact loads ranging from 1, 10, 20, 25, 50, 75, 100 N, Contact

loads ranging from 1.7, 3.2, 4, 6.7, 9.95, 16.2 kgf/cm 2. Chewing force of 53 or 75.6 N

maximum force, Abrasion load: 20 N and attrition load: 90 N and Resiliency of the peri-

odontal ligament.

5. Contact area size: force per unit surface area. Facet area

The importance of the effect of contact area dimensions on the wear of composite

specimens and their opposing enamel cusps was evaluated in vitro by Krejci et al (74)

6. Number of cycles

In order to compare results different studies, number of cycles should be taken in-

to consideration. Ranging from 5000, 10,000, 25,000, 50,000, 100,000 to 120,000.

7. Chewing frequency: frequency of load cycles

29

The chewing frequency used in vitro studies varies from 1.2 to 1.7 Hz

8. Duration of tooth contact

The duration of tooth contact during the in vitro loading should mimic the in Vivo

situation. Load and time significantly influence wear.

9. Sliding speed: relative speed of opposing surfaces

The sliding speed (2.5 mm/s) during the in vitro simulation should be comparable

with the in vivo situation (40).

10. Temperature

Temperature plays a plasticizing effect. Constant temperature (20, 37 ◦ C) or

thermocycling (5–55 ◦ C) should be maintained.

11. Food bolus during mastication

Variety of food bolus or slurry can be used during mastication movement simulat-

ing such as Slurry of water and unplasticized PMMA beads, PMMA powder, hydroxya-

patite slurry and millet-seed/PMMA-beads mixture.

12. Lubricant and friction

Oral lubricants consist of saliva, plaque and pellicle. Together they form a boun-

dary lubrication system, because the thickness of the lubricant layer is insufficient to pre-

vent asperity contact through the film. Effectiveness of boundary lubricants is influenced

by their chemical properties than their viscosity. The buffering capacity of saliva and pla-

que is important in minimizing the corrosive effects of acids (thickness of 100–500 nm)

may act as a protective layer. Several liquids are incorporated in the three-body wear ma-

chines, like: Water, alcohol, acids, olive oil, olive oil/CaF slurry, artificial saliva, yes or

no inclusion of bacteria.

30

13. pH

pH conditions seem to influence dramatically the wear conditions and therefore

they should be controlled carefully during In vitro wear testing. Following pH levels (1.2,

3.3, and 7.0) are frequently used during wear simulation. They should mimic plaque ac-

ids, gastric acids and dietary acids. If human enamel is used as counter body, acidity of

the medium has an impact on the wear behavior. Interplay of abrasion, attrition and ero-

sion of human enamel under several different pH conditions has been tested. Combina-

tion of erosion and abrasion resulted in significantly greater wear than erosion alone. Si-

multaneous erosion and abrasion resulted in about 50% more wear than alternating ero-

sion and abrasion. Chewing of acidic foods with some abrasive properties might cause

enhanced tooth wear. Dentin is more susceptible than enamel to erosion and abrasion

alone or combined. Load and time significantly influence enamel wear both in acid and

neutral conditions. Depth of dentine erosion significantly increases non-linearly with time

and significantly decreases with increasing pH. Dentin is susceptible to erosion even at

relatively high pH, the tubule system is readily exposed and dentine, unlike enamel,

shows little propensity to remineralize (75-76).

14. Enzymes

Enzymes seem to have the potential to degrade the samples during in vitro testing.

de Gee et al. (55) used esterase solution in the ACTA wear machine. Chemical cycling

can induced a generalized swelling of the composite samples and a modified wear curve.

These enzymes can be generated in saliva and by bacterial metabolism.

15. Enamel structure and physiology related to microwear

31

Enamel structure has an effect on microwear. The micro structural element is im-

portant in direction of shearing force relative to enamel prisms and crystallite orientation.

The different responses of prismatic and non prismatic enamels to abrasion reflect the

influence of structure at the level of organization of crystallites rather than prisms per se.

Variation in crystallite orientation in prismatic enamels may contribute to optimal dental

function through the property of differential wear in functionally distinct regions of teeth

(77).

16. Wear debris

Impact of wear debris at the zone of impact and friction should be examined more

carefully.

Alabama Chewing Simulators

The original Alabama wear simulator in vitro wear device is the most cited as de-

termined by literature review. The University of Alabama Wear Simulator is a modifica-

tion of the Leinfelder type three-body wear device. Over the years there have been many

modifications of the original machine. In the first publication in 1989, a polyethylene

tape was used as intermediate substance, driven by a tape advancing system. The tape

was subsequently replaced by PMMA slurry. PMMA beads have been shown to produce

the maximum loss of material in the shortest amount of time. Also, they do not degrade in

water and they transfer the masticatory energy to the surface of the composite resin rather

than absorb it. The initial force used was 55N (12.4 lbs), which was increased to 75N

(16.8 lbs) during each cycle. This was consistent with conventional biting forces of 16.0

lbs. A 30° clockwise rotation was initiated as soon as the stylus touched the specimen

which was staged with the loading cycle. Originally, the restorative materials were incor-

32

porated into extracted molars that were trimmed flat, which simulated generalized wear.

More recently, localized wear has been simulated by using ceramics (alumina and zirco-

nia) and stainless steel balls as the actuator against 4mm deep restorations that are set into

brass holders filled with acrylic resin (68-69). Most studies using the Alabama wear ma-

chine are run from 100,000 to 400,000 cycles.

A new modified chewing simulator has been developed to simulate occlusal im-

pact overcoming the gradual loading and unloading of the Alabama Dental Wear Simula-

tor. Occlusal contact between opposing cusps may be considered as an impact loading of

two inelastic bodies. The Alabama Dental Wear Simulator utilizes a spring-loaded stylus

to apply a load, which is dissimilar to clinical occlusal contacts.

The modified Alabama chewing simulator brings a stylus into contact with a spe-

cimen at a controlled rate of sliding displacement with a axial load. The modified Ala-

bama chewing simulator can vary loads, antagonist material, No. of cycles, pH, thermo-

cycling and medium used. In the configuration proposed for this study, allows the stylus

to contact and slide up to 2 mm of translation with a 40 N axial load. This simulation is

thought to model deflective occlusal contact, and is under investigation for study beyond

this study.

33

OBJECTIVES AND HYPOTHESIS

The objective of this study is to evaluate the wear of occlusal enamel and ceramic

restorative materials. Both the wear of the stylus and the wear of the specimen surface

will be investigated.

Hypothesis I:

There is no difference in the volumetric wear & wear depth of Lava core, IPS

e.max, Lava veneer material against enamel styli after 200,000 cycles.

Specific Aim I:

To measure by non-contact 3D profilometry and compare the volumetric wear &

wear depth of three ceramic materials against enamel styli after 200,000 cycles using a

modified-UAB simulator for an impact-sliding load without rotation. Differences be-

tween groups were determined by repeated-measures ANOVA and Tukey-Kramer‘s post-

hoc tests.

Hypothesis II:

There is no difference in the volumetric wear & wear depth of enamel styli after

200,000 cycles when against Lava core, IPS e.max, Lava veneer materials.

Specific Aim II:

To measure by non-contact 3D profilometry and compare the volumetric wear &

wear depth of enamel styli after 200,000 cycles when against three ceramic materials us-

ing a modified-UAB simulator for an impact-sliding load without rotation. Differences

34

between groups were determined by repeated-measures ANOVA and Tukey-Kramer‘s

post-hoc tests.

35

MATERIALS AND METHODS

STUDY DESIGN:

Figure 11. Study Design

36

Table 2

Materials used:

Product Type Manufacturer

Enamel Control Maxillary Premolars

IPS e.max Lithium Disilicate Ivoclar Vivadent

Lava Veneer Feldspathic Porcelain 3M ESPE

Lave Core Yttrium stabilized Zirconia 3M ESPE

Specimen preparation

Enamel Styli

10 samples per group were prepared for each enamel styli. Forty caries-free ex-

tracted human maxillary premolars were selected for each antagonist material and stored

them in a 2 percent solution of sodium azide. Stainless Steel Holders (Figure 12) were

designed and milled in the UAB Machine Shop to mount the coronal portion of caries-

free maxillary premolars.

Figure 12. Stainless Steel Holders

37

The coronal portions of maxillary premolars were mounted on screw in tips using

Panavia F 2.0 resin cement. In order to standardize the size and shape of each stylus, a

carbide bur was used to assure parallel shaping of the cusp to the long axis of the cusp to

assure similar geometry (Figure 13).

Figure 13. Carbide Burs

The styli were mounted on the Modified UAB Chewing Simulator and secured

parallel to the long axis of the wear simulator plunger (Figure 14).

Figure 14. Mounted Maxillary Premolar

38

Antagonist Molar, Control group

Ten caries-free extracted mandibular human molars for each material studied and

stored them in a 2 percent solution of sodium azide. Coronal portions of the mandibular

molar were mounted in a brass specimen holder using a self-cured acrylic resin. Ten flat-

specimens were prepared of molar enamel. Molars were standardized; assuring flat shap-

ing of the cusps to maintain similar geometry. The teeth were polished with a series of

400-, 600- & 1200-grit SiC paper under water-spray. Specimens were wet-finished with

0.05µm-alumina slurry/polishing cloth using a rotational polishing device (Buehler Ltd.,

IL, USA) and cleaned in an ultrasonic bath (5min) in distilled water and rinsed with wa-

ter. The specimens will then be stored for 24hrs at 37° C.

Antagonist restorative materials

Ten flat-specimens were prepared of each material, IPSe-max porcelain, Yttria-

stabilized Zirconia LAVA core, LAVA veneering porcelain (Table 2).

Figure 15. Ceramic Blocks

Specimens were mounted it in a brass specimen holder using a self-cured acrylic

resin and polished with a series of 400-, 600-, and 1200-grit SiC paper under water-spray.

39

Specimens were wet-finished with 0.05µm-alumina slurry/polishing cloth using a rota-

tional polishing device (Buehler Ltd., IL, USA) and cleaned in an ultrasonic bath (5min)

in distilled water and rinsed with water (Figure 16). The specimens will then be stored for

24hrs at 37° C.

Figure 16. Mounted ceramic specimen

This procedure ensured that the flat occlusal surface was parallel to a horizontal

plane, as well as to the flat surface of the energy-generating stylus.

Determination of Wear

The specimens were then subjected to wear on the modified UAB Chewing simu-

lator (Figure 17). A load of 40N was applied using enamel styli for 200,000 cycles. The

styli were set such that upon impact with the specimen a 2 mm slide across the specimen

surface under the constant 40 N load will occur. As soon as the maximum load was

achieved, the stylus rotated clockwise 30 degrees and then counter-rotated. Then the sty-

lus moved vertically up into its original position. At this point, the surfaces of the sam-

ples were covered with slurry of water and PMMA beads. Testing was performed in a

40

medium of physiologic saline and slurry of 50µ PMMA spheres (15g beads for 9g water)

at 32° C for both the stylus and the enclosing water bath.

Figure 17. Modified UAB Chewing Simulator

At baseline and 200,000 cycles, polyvinyl siloxane impressions were made of the

specimen surface and the enamel styli.

Impression protocol

Aquasil Ultra LV (Type 3: Light-bodied consistency, Classification ISO 4823)

cartridge is inserted in a hand-powered Heraeus Kulzer Dispensing Gun 2 (Model # DS

50, Heraeus Kulzer D-41538 Dormagen SW). A mixing tip was installed on the cartridge

and the dispenser and a small amount of base and catalyst were dispensed to ensure even

flow of both the catalyst and the base (mixing ratio 1:1). The trigger was squeezed to dis-

pense the impression material evenly on the specimen surface (Figure 18). A single oper-

ator was involved in the impression making procedure to reduce the number of variables.

41

Vinyl gloves were worn during manipulation of the material because latex gloves inhibit

the polymerization of polyvinyl siloxane (PVS) materials. The impression was allowed to

polymerize for 5 minutes after seating at an ambient temperature of 21 ± 2° C and humid-

ity of 33%. At this temperature the setting time of the impression material would be in-

creased. According to the manufacturer at 72° F (22° C) Aquasil Ultra Regular Set Im-

pression Materials have a minimum work time of 2 minutes 15 seconds and a minimum

removal time of 5 minutes from start of mix. In this study the impression was seated on

the enamel styli for 6 minutes and 10 seconds.

Figure 18. PVS Impressions

Casts

All the impressions were poured to scan in the order they were tested. The im-

pressions were washed thoroughly with tap water, to remove any remaining slurry and

dried with air water syringe to ensure no excess liquid remained. 32 ml distilled water,

accurately measured in a measuring cylinder at 23 ± 2° C was placed in a wet vacuum

42

bowl (Whip Mix Corporation, Model #6500, Louisville, US). 140 grams of Silky-Rock

type IV die stone powder (Whip Mix Corporation, Louisville, US) was added to the wa-

ter. The mixture was hand spatulated for 10 seconds to incorporate the powder in the wa-

ter and then vacuum mixed for another 30 seconds under 27 psi/Hg in a Whip Mix Com-

bination unit (Whip Mix Corporation, Louisville, US). The mix was poured at an ambient

temperature of 23 ± 2° C and humidity of 34 ± 1%. Using a stone vibrator set in a slow

mode the mixed stone was slowly poured in the impressions, and care taken to prevent

distortion. A new enamel stylus was used for each wear simulation. The samples were

retrieved after one hour (30 minutes recommended by the manufacturer). They were in-

spected for bubbles and other visible defects in the area of the preparation (Figure 19).

Specimens with such defects were discarded and new impressions were made. The spe-

cimens were then carefully sectioned to obtain the desired occlusal surface, the margin of

the maxillary premolar were evaluated for any defects during sectioning. The samples

were sectioned to make it easier to scan the sample with the non-contact 3D profilometer.

Samples were kept in a sealed plastic container at room temperature to avoid moisture

contamination.

Figure 19. Stone replicas of styli removed from PVS Impressions

43

Determination of volumetric wear & depth loss

A highly accurate non-contact surface profilometer, the Proscan 2000 (Scantron

Industrial Products Ltd., England), was used to scan the surfaces of the antagonist surfac-

es and enamel styli impressions made thereof. The Proscan 2000 is capable of 3D surface

profiling down to 10 nm depth of field and a submicron resolution at a rate of 1000 points

per second. The use of the (S-Type) chromatic sensor allows examination of dark and

rough surfaces with the object viewed in any orientation or auto-leveled using the pro-

prietary software Proscan and Proform. It scans any surface over an area up to 150mm x

100mm. It uses a focal multiplexing sensor with up to 0.005μm resolution. Safe white

light is transmitted through a lens, which has a built in spectral aberration. Takes the

white light and divides it into the full spectral field, focusing each different color fre-

quency at a slightly different point through a defined measuring range. When an object is

placed within this range, only one particular color frequency reflects back from the sur-

face. Information passes back into processor where a spectrometer analyzes the signal

and converts it to a measurement. Proscan combines these measurements with the precise

location of a moving X and Y linear table, giving three co-ordinates from which a three

dimensional profile is created. Results of the surface profile appear immediately on the

computer monitor and an image of the graphical 3-D representation can be saved on the

computer (Figure 20).

44

Figure 20. Proscan 2000

Impressions of each of the enamel styli, made with the Aquasil LV impression

material, were scanned using the same scanning parameters used for substrates. A partic-

ular substrate-impression-condition group was placed on a flat metal block affixed to the

flat scanning platform of the Proscan 2000 by modeling wax. Impressions were aligned in

a straight line such that each horizontal line on the impression met its corresponding hori-

zontal line on the adjacent impression in a straight line. This would enable the Proscan

software to read off designated random points on the lines, which were determined using

the matrix scan feature of the Proscan software (Figure 21). The step size was set at 20

μm. Then the sample rate was set at 300Hz. The S29/12 sensor model was chosen be-

cause that is the best sensor for measuring multiple teeth or a full arch. The S29/12 sensor

has a measuring range 0.12mm, resolution of .28μm, and uses chromatic, no triangula-

tion, technology.

45

Figure 21. Scanned specimens using Proscan

Therefore, the total samples were scanned at baseline and after 200,000 cycles

using the Proscan software (Figure 21). Wear volumetric loss differences between base-

line impression and 200,000 cycles were obtained using the Proform software by supe-

rimposition of the original (baseline) scans over the modified (200,000 cycles) scans

(Figure 22). For an impression to be dimensionally accurate and reproduce surface details

of the original model precisely, we determined that the closest value obtained in terms of

volume loss and the least difference obtained in terms of volume loss, to the original

sample/specimen would indicate the best adaptation of the impression material over its

substrate. This technique reduced the error along the XY axes. The measurements were

repeated two times on the same tooth and recorded to check the operators‘ reliability.

46

Figure 22. Superimposition of Images

47

RESULTS

The statistical analysis method used to compare means was repeated measures

ANOVA. Separate analyses were conducted for volumetric wear and volume loss. Tu-

key–Kramer post hoc test was used for pair wise comparison of group means.

Table 3

Wear on Antagonist Ceramic Surfaces (mean ± standard deviation)

IPS e.max LAVA Veneer LAVA Core

Volume (mm3) 0 0 0

Depth ( m) 0 0 0

There was no In-vitro volumetric wear or depth loss for antagonist ceramic sur-

faces in any of the groups (p>0. 6209).

Table 4

Wear on Enamel Styli (mean ± standard deviation)

Enamel –Antagonist Materials Volume (mm3) Depth ( m)

Enamel Control 14.7±3 589.9±96

IPS e.max 16.6±4 1173.8±287

Lava Veneer 18.1±2 890.6±144

Lava Core 30.4±3 1209.8±234

Repeated ANOVA between groups had p-value < 0.0001 which proves that the

means of the groups differ. Tukey-Kramer post hoc test for multiple comparisons showed

48

that Lava core showed higher In-vitro volumetric wear loss than rest of the groups (p<

.0001) There was no differences for enamel wear depth for any of the groups (p>0.2757).

The results of this study are divided into two parts:

A. To measure by non-contact 3D profilometry and compare the volumetric wear

& wear depth of three ceramic materials against enamel styli after 200,000 cycles using a

modified-UAB simulator for an impact-sliding load without rotation.

B. To measure by non-contact 3D profilometry and compare the volumetric wear

& wear depth of enamel styli after 200,000 cycles when against three ceramic materials

using a modified-UAB simulator for an impact-sliding load without rotation.

Differences between groups were determined by repeated-measures ANOVA and

Tukey-Kramer‘s post-hoc tests.

Study I: Mean differences were measure and compare using repeated ANOVA

and Tukey-Kramer post-hoc test. There was no difference between the means in the

groups (p>0. 6209). Hence, we accept the null hypothesis stating that there is no differ-

ence in the volumetric wear & wear depth of Lava core, IPS e.max, Lava veneer material

against enamel styli after 200,000 cycles.

Study II: Mean differences were measure and compare using repeated ANOVA

and Tukey-Kramer post-hoc test. There was a difference between the means in the groups

(p< .0001). Hence, we failed to reject the null hypothesis stating that there is no differ-

49

ence in the volumetric wear & wear depth of enamel styli after 200,000 cycles when

against Lava core, IPS e.max, Lava veneer materials.

50

DISCUSSION

The three materials tested have each been developed to restore the anatomic form

and clinical function of human teeth. Under the limitations of the present study, the find-

ings indicate that ceramic materials are wear-resistant and caused wear of opposing ena-

mel occurred against all ceramic materials. IPS e.max & Lava Veneer material showed

no difference in terms of enamel wear compared to the Lava Core material (Zirconia)

To date only minimal data have been published regarding wear characteristics of

the ceramics evaluated. In a study by Suzuki et al IPS Empress (IPS e.max) is described

as having mechanical properties similar to those of enamel (78). Ratledge et al. report no

significant difference between in vitro wear data of unglazed IPS Empress opposing

enamel from their control: enamel opposing enamel (79). Seghi et al conducted a study

using OHSU oral wear simulator using enamel cusps (3rd

Molars) to the ceramic sub-

strates with the wear parameters: 20-N load, 1.0Hz, 8mm sliding action, 50,000 cycles,

Food-like slurry and concluded that IPS Empress 2 wear rate similar to Enamel (80).

Heintz et al evaluated the wear of enamel against IPS e.max using the Willytec chewing

simulator using enamel cusps (Maxillary Premolars) to the ceramic substrates with the

wear parameters: 120,000 cycles, 5 kg weight, 0.7 mm sliding action, 1.6 Hz; Speed was

60 mm/sec, Thermocycling and showed similar results as IPS. emax wear as similar to

enamel (81). Ceramic wear testing remains difficult to assess in both in vitro and in vivo

controlled evaluations.

Wear is a complex process that involves the removal of material and antagonist

51

material as a consequence of direct contact of the material and the stylus. Contributing

variables affecting wear volume loss can be mostly determined by factors as number of

cycles, sliding, surface hardness, surface roughness as well as resistance to fatigue and

fracture (82).

Number of cycles: 200,000 cycles were used in this study. According to Flanagan

et al swallowing cycle occurs approx. 590 times during a 24-hour period with 146 cycles

during eating, 394 cycles between meals while awake and 50 cycles during sleep. Hence,

200,000 cycles may represent 9.2 months swallowing tooth contacts (83, 84).

Sliding motion: According to Heintz et al the sliding distance in vivo has been rec-

orded as 0.3 mm in the anterior direction and 0.18 mm in the bucco-lingual direction. In

vitro devices have used sliding distances ranging from 0.2 mm – 8.0 mm. The slide in our

machine is about 2 mm. There is a lot of variation with other wear simulators, and we

know that during mastication, the grinding contact is much shorter than 2mm and tends

last about 0.2 seconds (81). Therefore, we felt this device would give a better approxima-

tion to the oral condition.

Hardness: Enamel has a Vickers hardness number of 336 VHN, IPS e.max: 591

VHN, Lava Veneer: 530 VHN & Lava Core: 1250 VHN. When considering the ceramic

materials in this study, they all are similar in physical and mechanical properties in that

they are hard and brittle. However, they also differ significantly in their hardness. Note,

that enamel is less hard than all the other materials. In wear simulation, the enamel was

worn while the ceramics demonstrated no measurable wear. Further, the enamel wear

against the zirconia ceramic was twice the volume of the lithium disilicate or the

feldspathic porcelain. This may be explained by a fundamental relationship of Tribology,

52

which is the study of wear.

The Archard wear equation describes the sliding abrasive wear of two surfaces as

they are worn.

Archard Wear Equation:

V= Volume of Wear Loss

W= Normal Load

H= Hardness of the softest contacting surfaces

K= Dimensional Constant

L= Sliding Length

Therefore, Hardness and Wear loss are inversely proportional. Enamel, being of

less hardness is more readily worn. However, this simplified equation does not account

for wear differences noted in this study.

Surface Roughness: All ceramic specimens were treated with the same finishing

and polishing protocol. However, the Zirconia ceramic has much higher hardness than the

other two materials. As finishing and polishing is in itself a wear process to progressively

reduce surface roughness to obtain a smoother surface, then, treatment of the Lava Core

material in the same way as a Feldspathic or a Lithium Disilicate will result in a rougher,

higher hardness surface, to oppose the enamel stylus. Therefore, because this material has

been proposed to be used for full contour restorations, this research demonstrates the

need for uniquely different finishing and polishing procedures for Zirconia.

Anisotropic Behavior of Enamel: The wear of the enamel should have a uniquely

VkWL

H

53

different mechanism of wear. Enamel is formed with rods or prisms in an organized ar-

rangement on a bed of lower modulus dentin. Translational forces which follow impact,

which we hypothesize are more similar to intraoral conditions, appeared to be effective in

degradation of enamel in wear testing. The ceramic materials do not have this type of mi-

crostructure, and this as well as the higher hardness, contributes to their excellent wear

resistance.

However, there is not so much work being done on the evaluation of effect of

these factors on wear of ceramic materials. It was expected that enamel styli would show

patterns of wear. This wear pattern could have been attributed to strength and hardness of

these materials in combination to the original shape of the enamel styli. As wear occurs,

the wear facet increases in size, the force per unit area decreases, and the potential wear

decreases. As wear facets increase in size, the wear pattern may be changing.

Majority of the existing wear simulators do not have consensus about the amount

of force exerted during mastication (81). With regard to the agreement of obtained results

in relation to certain ceramic materials, a comprehensive analysis cannot be performed, as

the test parameters contribute to their differences. These new materials have only been in

the market past few years and studies are still determining their survival rates. Clinical

trials are expensive and time consuming, so companies rely on results from laboratory

testing. It is easier and less expensive yet less accurate to work outside of the mouth

where conditions are standardized. Researchers cannot take wear results straight from in

vitro studies and use them to predict the amount of in vivo wear. Results from in vitro

studies can only be used to rank and compare materials.

The modified UAB Chewing Simulator is a new machine developed to closely

54

simulate the oral cavity. At this point, we do not have enough studies done with this sys-

tem to qualify this system. This modified wear machine can produce variations such as

the load, number of cycles, temperature, and medium. A good number of studies use pre-

pared enamel from extracted molars as the antagonist and flat polished ceramic speci-

mens. However, there was a huge variation in wear simulators in relation to the applied

force, the used force actuator, the number of cycles, and the frequency of cycles per time

as well as the number of specimens.

These variations may account for the fact that there is no agreement between the

studies with regard to the ranking of specific ceramic materials for enamel and material

wear. Modified UAB chewing simulator has been introduced to overcome the limitations

of the existing UAB wear machine. When examined for the force the Alabama wear ma-

chine does not show the impact loading of the force profile. Alabama wear machine has

gradual increase and decrease of load with twist. The modified Alabama wear machine

shows an impact loading pattern, designed a simulator with rapid load application and a

slide. The force decreases during the sliding motion. There is a frictional contact, and

this is where a sliding wear occurs.

Figure 23. Original Alabama wear machine

55

Figure 24. Modified Alabama Wear Machine

Another factor that could have caused variations in results is the load. In this

study, 40N of load was used which is much higher than average biting forces, which vary

from individual to individual. It is generally found that males can bite with more force

(118-142 lbs) than can females (79-99 lbs) (84).

Another factor worth mentioning is the type-of-wear quantification system. Many

studies used some sort of profilometry or two-dimensional microscopy and software for

wear quantification without, however, showing data on the validation of the method re-

garding the accuracy of results. This may cause inaccuracy and the inability to compare

test results or material rankings.

In 2001 the ISO technical specification on the wear by two-and/or three body con-

tact describes eight different wear methods without, however, differentiating between

composite and ceramic materials (85). Furthermore, no detailed assessment of the eight

methods had been carried out and no information has been given whether the methods are

validated and the devices with which the methods are performed are qualified for that

purpose. A current review on wear simulation in vitro concluded that – when following

56

the FDA guidelines on good laboratory practice (GLP) (86) – only the expensive MTS

wear simulator (Minnesota method) is a qualified machine for testing wear in vitro. The

force exerted by the hydraulic actuator is controlled and regulated during all movements

of the stylus. All the other simulators lack control and regulation of force development

during dynamic loading of the flat specimens. This may be an explanation for the high

coefficient of variation of the results in some wear simulators and the poor reproducibili-

ty of wear. A round robin test evaluating the wear of 10 dental materials (eight composite

materials, one ceramic and one amalgam) with five wear simulation methods (ACTA,

Munich, OHSU, Zurich, Ivoclar) showed that the results were not comparable, as all the

methods follow different wear testing concepts (87). There are concerns and problems

with different wear simulating devices as they exhibit different testing protocols & me-

thod for measuring wear is not standardized. There are variations in terms of differences

in antagonist material, number of cycles, temperature, pH, slurry medium, wear mea-

surement technique procedures. There is no standardization and consistency between Va-

riables: Force profile, number of cycles, medium, with very little correlation – Most wear

devices lack scientific evidence to correlate in vitro wear with in vivo wear. A good com-

promise with regard to costs, practicability and robustness is the Willytec chewing simu-

lator (88), which uses weights as the force actuator and stepping motors for vertical and

lateral movements (88). There is no clear tendency, which could lead to a sensible expla-

nation for the inconsistencies. We strived for standardizing as much of the test parameters

as possible, such as geometry of crowns and flat specimens, chewing simulator parame-

ters, etc. Most wear machines use test pieces with a flat surface, where as teeth and resto-

rations have complicated shapes which cause different stresses at various sites on the res-

57

toration surface. Kunzelmann et al. who used unprepared enamel cusps in their investiga-

tion described in their paper that preliminary unpublished tests showed that grinding and

polishing of enamel increased the wear properties of the antagonist compared to unpre-

pared cusps (89). This statement is in line with the results of the present study as more

variation of the results was detected for the prepared cusps. Furthermore, test results that

involved prepared cusps were inconsistent and sometimes illogical. As many of the re-

viewed studies concerning enamel wear for ceramic materials used prepared and polished

enamel cusps this fact may explain the different ranking of the same materials from one

study to another. The lack of correlation between clinical and laboratory studies should

not discourage the use of wear machines; but they should be used to investigate the un-

derlying mechanisms rather than to rank the potential clinical performance. All wear me-

thods lack the evidence of their clinical relevance because prospective studies correlating

in vitro with long-term in vivo results with identical materials are not available. In vivo

studies measuring enamel wear opposing ceramic materials with adequate equipment are

scarce. All wear methods lack the evidence of their clinical relevance because prospec-

tive studies correlating in vitro with long-term in vivo results with identical materials are

not available. In the beginning of this study, the goal was to have a larger sample size and

scan samples at baseline, 100,000 cycles and 200,000 cycles. Due to technical and time

issues, in the end, a representative sample was included in this study.

58

CONCLUSIONS

Within the limitations of this study, we conclude:

1) Enamel wear depends on antagonist restorative material used.

2) The modified UAB Wear Simulator appears to be effective in wear evaluation of

enamel against ceramic materials.

59

SUGGESTIONS FOR FUTURE RESEARCH

Future investigations should more closely duplicate the conditions in the oral cav-

ity and produce comparable wear variables to the ones found in vivo. By controlling va-

riables such as number of cycles, medium, force, temperature, and pH will do this. Anta-

gonist combinations found in the oral cavity as ceramics against ceramics, ceramics

against other restorative materials can be evaluated. Quality of the ceramic surface would

be another interesting factor to study. Future investigators should also look at changing

the mechanisms of a twist and slide rotation that would closely simulate the chewing pat-

terns in the oral cavity, and also in vitro studies on different chewing simulators with sim-

ilar variables & identical materials. Future investigations can try to assess differences in

clinical and in vitro wear and that the generations of the Alabama Wear Testing Devices

used to correlate to what is occurring in the mouth, and studies correlating in vitro with

long-term in vivo results with identical materials.

60

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Appendix 1

IRB Approval Form

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69

Appendix 2

Tables-Repeated Measure ANOVA &Tukey-Kramer post hoc test

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Table 1

Table 2

71

Table 3

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Table 4

73

Table 5

74

Table 6

75

Table 7

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Appendix 3

In Vitro Enamel styli Scans at Baseline & 200,000 cycles

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Scan 1

Scan 2

78

Scan 3

Scan 4

79

Scan 5

80

Appendix 4

In Vitro scans of Ceramic surfaces

81

Scan 1

Scan 2

82

Scan 3

Scan 4

83

Scan 5

84

Appendix 5

Superimposed Images of Enamel Styli at Baseline & 200,000 cycles

85

Image 1

Image 2

86

Image 3

Image 4

87

Image 5

88

Appendix 6

Superimposed Images of Ceramic surfaces at Baseline & 200,000 cycles

89

Image 1

Image 2

90

Image 3

Scan 4

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Image 5