extra-coronal rest - crowns revisited

5 1 6 Dental Update November 2003

R E S T O R A T I V E D E N T I S T R Y

Abstract: This paper overviews recent changes to making crowns. For the most part itpresents clinically relevant information using examples where necessary. The paperwill act as a useful reminder for the techniques involved with crown preparations andthe choices of materials available.

Dent Update 2003; 30: 516522

Clinical Relevance: This paper presents some useful hints and information forpractitioners making crowns.


David Bartlett, BDS, PhD, MRD, FDSRCS(Rest. Dent.), Senior Lecturer/HonoraryConsultant in Restorative Dentistry (GKT), Floor26, Division of Conservative Dentistry, Guys,Kings and St Thomas Dental Institute, GuysTower, London Bridge, London SE1 9RT.

rowns are an important part of practice life and may contribute

significantly to the income of dentalpractitioners. Therefore, getting it right isfundamental. Should you use a metalceramic crown and cement it with zincphosphate, or use an adhesive cementwith an all-ceramic crown? Whichcombination might achieve the optimumappearance? Do the preparations need todiffer between products and whatchanges to clinical techniques haveoccurred to make the process simpler?

THE BASICS

PreparationProbably the most important stage inmaking crowns is not what material to use

but to create sufficient space in the rightplace for the crown. Often preparationssent to laboratories are inadequatelyprepared, not because they have aninsufficient shoulder width, but becausethe contour of the preparation is notmatched to that of the tooth. Teeth havecurves! The burs we use do not. Burs canbe tapered to provide the optimum toothreduction for retention, but cliniciansoften fail to remove sufficient toothtissue along the mid-buccal and incisalregions necessary for the bestappearance (Figure 1). The result createsproblems for the technician who faces thedifficult dilemma of whether to overbuildthe incisal and buccal porcelain tomaintain the translucent appearance ofthe porcelain on the crown, or overbuildthe porcelain to preserve the appearancebut make the contour bulky. The idealsolution is to remove sufficient tooth tomake the job of the technician morestraightforward.

Most crowns gain retention from thelength of the preparation and its taper.The ideal taper for a conventional crownis around 15o, although most practitioners

prepare the tooth with greater tapers thanthis, and the combination of a rigidmaterial and the luting cement is usuallysufficient to retain the crown.

For the most part, restoring teeth withcrowns is relatively straightforward. Butit is more difficult to be dogmatic aboutthe indications, especially with thedevelopment of newer composites whichcan match the appearance of someporcelain crowns. However, there is still aplace for crowns to restore:

l Broken down teeth;l Repeatedly fracturing teeth;l Appearance especiallyreplacement of extensively restoredanterior teeth;l Treatment of short clinical crownsand toothwear;l Function.

Restoring broken down teeth, possiblyfollowing endodontics, is a routinedecision-making process. Theappearance of some extensively restoredanterior teeth can be improved bycrowns, even though the translucency of

Crowns RevisitedDAVID BARTLETT

C

Figure 1. The preparation on the premolar hasthree buccal planes a cervical, mid-buccal andincisal. This creates sufficient space for the metaland ceramic need for the crown.


Dental Update November 2003 5 1 7

the natural teeth can rarely be matched.Restoring short clinical crowns is more

difficult and should only be consideredas part of an overall treatment plan.Increasingly, the benefit of usingadhesive cements to supplement theretention of crowns has helpedpractitioners but, when they are the majorretentive feature, careful planning shouldbe undertaken and the occlusion shouldbe carefully assessed.

After teeth are extracted there is apotential for the adjacent and opposingteeth to drift into the edentulous space.When this movement changes theintercuspal position, or creates aninterference, the term unstable occlusionis used. Crowns can be used to removethe interference, possibly with occlusaladjustment, and even out the occlusalplane to produce a more stable occlusion.

Pre-preparation MatrixThis is an often under-appreciated part ofthe process, because the provisionalcrown will provide a lot of usefuldiagnostic information. Probably mostimportant is the assessment of whethersufficient tooth has been cut away duringthe crown preparation. But otherinformation includes:

l Occlusal reduction;l Buccal and palatal/lingual reduction;l Shape of the crown height andwidth;l Gingival contour.

Normally, a matrix is taken of the tootheither with alginate, silicone or pink wax.The pre-operative state of the tooth thenacts as a guide or matrix for the final

crown. Some problems arise when thepre-operative shape of the tooth is notquite what is planned. For instance, onmesially tilted lower molars, there is littleneed to cut away tooth when there is noopposing contact in that area. Thesensible thing is to leave the toothunprepared and only reduce the clinicalheight in those areas that need it. Butwhen the provisional crown is madeusing the original tooth shape there isinsufficient occlusal material. One way isto add a small amount of composite,without bonding, to the occlusal surfaceto increase the height of the tooth. Thematrix then records this and the otherdetails and the provisional crown iseasier to make. Another way is to usemodellers wax (Figure 2). Take animpression of the pre-operative state ofthe tooth in wax, use sufficient wax toform a bulky sausage shape and allow thewax to cool in the mouth. Remove the waxand then, with a sharp instrument, carveaway the fit surface of the wax. This is toincrease the thickness of the provisionalcrown. The wax is rigid enough and,when used in thick section, strongenough to support making theprovisional restoration. It is particularlyuseful for gold crowns when the toothreduction is minimal. The onlydisadvantage of the wax or alginatetechniques is that they can only be usedonce. Silicone impressions are better ifthere is a concern that the provisionalcrown may need replacing, as the materialis dimensionally stable.

The other main use of a provisionalrestoration is a test of the design for theplanned restorations. If the plan is toaccept most aspects of the tooth shape,then the existing tooth can be used for

the shape of the matrix. If, on the otherhand, there are more comprehensivechanges planned, then a diagnostic wax-up is needed to inform the patient of theplanned change and also to provide aform for the provisional restorations. Thediagnostic wax-up can be duplicated anda 2 mm vacuum-formed splint madearound it, or a putty impression taken.The advantage of the clear splint is that itis possible to see the presence ofairblows in the splint whilst filling thetooth spaces and so avoid them, whereaswith the putty matrix this is not possible.

A final use of a matrix is to check to seeif sufficient tooth has been removedalong the mid-line of the prepared tooth(Figure 3). After the tooth has beenprepared, cut the matrix along the midlineof the silicone and replace the sectionedmatrix into the mouth. Then inspect thegap between the tooth and the matrix tosee if it is even. Particular care should betaken around the incisal and mid-buccalregions as this is an area, for reasonsmentioned earlier, that is often underprepared. You obviously do not need todo it every time, but it is worthwhileauditing your own preparation techniqueevery so often, in which case you wouldneed to take two matrix impressions onefor the provisional and one for theassessment.

Gingival Retraction MethodsThe other important choice is how toobtain adequate retraction of the gingivaltissues, the most commonly used methodbeing impression cord. There are anumber of makes on the market with a

Figure 2. A wax sausage is taken of the pre-operative state of the tooth and used to form thematrix.

Figure 3. The matrix is sectioned along the mid-line of the tooth and re-inserted into the mouth.The gap between the original outline of the toothand the preparation indicates where the toothhas been reduced and, if so, by how much.



range of sizes available to suit the clinicalneed (Ultradent, Utah, USA), these beingmore adaptable than the single-sizedvarieties (Racestyptine, Septodont,France). Retraction cord is oftenunnecessary, especially if the preparationis at or just below the gingival margin. It isneeded if the preparation is sub-gingival,when the cord displaces the gingivaltissues so that the impression materialflows around and beneath the margin.Some clinicians leave the cord in place,others remove it just before the impressionis syringed into place, and some place twolayers of cord and remove one just beforetaking the impression. Removing the cordcan create gingival bleeding, which willhinder the flow of the impression, evenwith the most hydrophilic materials. Theauthor prefers to leave the cord in place,take the impression and then remove itbefore placing the provisional restoration.

A recently introduced product, whichuses a paste that is squeezed along thegingival margin with a specially madeapplicator to control bleeding and retractthe tissues, is Expasyl (Kerr/Hawes,Peterborough, UK). The material isintroduced at right angles into the gingivalcrevice, allowing the material to flow andgradually move along the margin, finallyleaving it in place for about two minutes.After washing, the impression is takenusing a normal technique (Figure 4). Theadvantage of this new product is that it

chemically prevents bleeding and, at thesame time, provides gingival retraction.

The ImpressionThe choice of impression material isusually personal. Silicones tend to bequite hydrophobic, although somerecently introduced by the manufacturersclaim not to be (Take One, Kerr/Hawes,Peterborough, UK; Affinis, Coltne/Whaledent, Surrey, UK). In reality, allimpression materials need a clean, drysurface to record the details accurately.They are available in a number ofdispensing methods and formulations.The low viscosity materials are generallydelivered with a mixing gun, whilst putty ispresented as a catalyst and base mixingtechnique. The putty supports the washand effectively provides a special tray. Ifyou are using a putty/wash technique,ensure that the low viscosity materialcovers all the teeth, not just thepreparation, as this will ensure theaccuracy of the impression for theocclusion.

Silicones or polyethers (Impregum, 3MESPE, Seefeld, Germany) are available assingle stage materials. 3M ESPEdeveloped an automixer (Pentamix: 3MESPE, Seefeld, Germany) to dispenseImpregum because some clinicianscriticized the difficulty of mixing; this newsystem has significantly improved its

handling properties. The automixersimplifies the process and provides anevenly mixed material. It has recently beenadapted to be used with silicones from 3MESPE and other manufacturers materials.

Your impression must provide sufficientinformation for the technician to constructthe crown:

l The margin should be clearly visiblearound the periphery of thepreparation;l There should be no drags or tearsanywhere on the impression,especially around the impression ofthe prepared tooth/teeth;l Shiny, reflective surfaces around thetooth preparation often representinaccuracies, possibly representingmoisture contamination;l The occlusal morphology of adjacentteeth should be clearly seen;l Ensure that the impression is firmlyattached to the tray.

The choice of material may not be ascritical as the choice of impression tray.1,2

Rigid trays are essential for accurateimpressions; it should not distort whenthe impression is seated and removed fromthe mouth. A tray adhesive is used toprevent partial displacement of theimpression from the tray and eventualdistortion of the impression. Someoperators prefer metal trays and, whilstthese can be re-usable, they appear to beno more accurate than a rigid plastic one.1

One of the most demanding situationsis the most distal standing tooth. In lessrigid trays there is a tendency for flexure ofthe tray to occur on seating, especiallywith more viscous materials. Commonly,pressure is applied on the anterior part ofthe tray or over the premolar region andconsequently this may distort the mostposterior or distal aspect. This distortionreturns to its pre-elastic state after thepressure is released. The end result is thata preparation around the 2nd or 3rd molarregion is inaccurate and the fit of thecrown may not be achievable.

THE TYPE OF CROWNThe most common type of crown is acombination of metal and ceramic. Whilst

Figure 4. Expasyl (Kerr/Hawes,Peterborough, UK) is a new retraction systemutilizing a haemostatic paste which is injectedalong the gingival margin forcing the tissueaside and creating space for the impressionmaterial to flow.



this produces an acceptable appearancemost of the time, occasionally the opacityof the underlying metal becomes aproblem. The natural tooth is translucentand, whilst the metal provides strength, itstops light transmission. The technicianhides this by placing opaque porcelainonto the metal surface but, in doing so,makes the crown appear bright, increasingthe value. Adding more layers of dentineand enamel porcelain reduces this but thenatural appearance of teeth is rarelyachieved. Therefore, providing sufficienttooth reduction is imperative for thetechnician to hide this opaque layer.

There is extensive choice available forall-ceramic crowns and the more recenttypes appear to perform well in mostmouths. Theoretically, porcelain crownsare not as strong as metal ceramic ones,but the optimum strength for a crown isalmost impossible to predict. The strengthreported by the manufacturers willnormally be based on laboratory studiesand, whilst these will give an indication of

its performance, it will never replace theclinical test. The following is a list of someof the new porcelain materials:i. Ceramic cores

a. Inceramb. Procera

ii. Castable ceramicsa. Empress Ib. Empress II

iii. CAD CAMa. Cerec I, II, III

iv. Non-castable metal ceramic crownsa. Captek

Ceramic Cores

Inceram (Vita Zahnfabrik, BadSackingen, Germany)

The crown consists of two layers, an innercore made from Inceram to providestrength and an outer layer made fromconventional feldspathic porcelains toimprove the appearance (Figure 5). Thecrown is made of an absorbent refractorydie over which molten glass is poured toproduce a strong and dense crystallinecore with a relatively poor appearance.Conventional porcelains are needed tocreate an aesthetically pleasing crown.The high content of aluminia makesadhesive bonding difficult as the surfaceis resistant to most acids and so non-adhesive cements are used to lute thecrowns.

Procera (Nobel Biocare, Goteborg, Sweden)

The concept is not unlike Inceram but, inthis case, computer-controlled technologyis used to make the crown. The workingdie is scanned by sapphire probe andconverted into data which is sent by

e-mail to a laboratory where a computer-controlled milling machine makes the corefrom an aluminium oxide powder. Theouter surface, like Inceram, is made fromconventional porcelains (Figure 6). Thematerial can be adhesively bonded toteeth. In theory, because the Procera hasan opaque core, it can block outdiscoloured areas such as mildly stainedteeth. But the potential disadvantage ofthe technique, even with the thinner coresize of 0.4 mm, is that more tooth needs tobe cut away.

Castable Ceramics

Empress I and II (IvoclarVivadent, Schaan, Liechtenstein)

The crown again has two layers, an innermaterial made from Empress and an outerlayer made from conventional porcelains(Figure 7). Unlike Inceram the outer layeris much thinner and characterizes thesurface finish. The Empress core is madeusing the lost wax technique. Porcelain isforced under pressure into the shape leftafter the wax is burnt away, eventuallycooling to form the crown. The fit surfacecan be acid-etched with hydrofluoric acidto allow adhesive bonding. The advantageof the Empress system is that, since thecolour of the crown is consistentthroughout the restoration, in a casewhere horizontal space is at a premium, theEmpress might be a better option to usethan, for example, a Procera which stillneeds space for a core. However, withthinner crowns and the need to use onlythe intrinsic colour of the Empress, therewould be less space for characterization ofthe tooth.

Figure 5. Inceram crowns. This patient wantedextremely white teeth after she had bleaching.The rather monochromatic colour of the toothrepresents the colour choice of the patient ratherthan the material. Different surfacecharacteristics could have been added to theouter surface with conventional porcelains toachieve a better surface finish.

Figure 7. Empress crowns on the two centralincisors there is little distinction between thisand the Procera material and both provide avery good result.

Figure 6. Procera crowns on the incisors have been used to replace the discoloured andextensively restored teeth. Note the translucency of the crowns.



CAD-CAM

Cerec (Sirona, Bensheim, Germany)

This is a computer-controlled millingmachine but, unlike Procera, the wholecrown is milled from a block of standardporcelain. The only area where there issome difficulty is the characterization andthe occlusal form.

Non-castable Metal CeramicCrowns

Captek (Davis, Schottlander & Davis,Letchworth, UK)

The crowns basic structure is similar toconventional metal ceramic crowns in thatthe core is metal and the outer part isporcelain. Unlike a metal ceramic crown,there is no need for a casting machine. Aseries of different wax strips containinggold or platinum are sequentially appliedover an investment model and the wax isburnt away leaving the metal to form acore over which is layered porcelain. Theunderlying gold is said to give the crownsa warmer appearance than conventionalmetal ceramic crowns. Until recently,adhesively bonding gold to teeth wasunreliable but a new material hasovercome this problem.

Not only is the choice of porcelainimportant, but the tooth preparation mustmatch the material used. Virtually all theall-ceramic crowns need extensive toothreduction over the whole tooth, unlikemetal ceramic restorations where lingualand, occasionally, occlusal preparationscan be reduced if a metal surface ispreferred. The sapphire scanner used formaking Procera crowns cannot read

sharp edges on preparations andtherefore they must be avoided. Themanufacturer provides correctly shapedburs and advises significant toothpreparation to achieve the optimumresults they are not conservative oftooth tissue! The shoulder should berounded as should the core margins(Figure 8). The all-porcelain crownsappear more translucent and vital thanthe metal ceramic ones, especially aroundthe gingival margin.

For most patients, this theoreticalreduction in strength of all-ceramiccrowns is probably unimportant but,where a parafunctional habit is present,the load transferred to the crown may beexcessive leading to cracks or fractures.The porcelains are suitable for anteriorthree-unit bridges but their useposteriorly increases the risk and largerspans are contraindicated.3 Proceraprovide blanks onto which is addedconventional porcelain to produce anecessary tooth shape and the resultingtooth is glued to the adjacent crowns.Inceram and Empress use similartechniques for three-unit bridges. Thegold connector for Captec is solderedonto the adjacent crowns to provide thebridge.

WHICH CROWN MATERIALTO USE AND WHENProbably the most important factor in thischoice is the ability of the technician. Areally good technician will produce amagnificent metal ceramic crown whichwill match any all-ceramic. Conversely, thechoice of an all-ceramic crown does notnecessarily mean that the result is goingto be superior. Another important factor iswhat system does your normal techniciansupply? For instance, if you are contentwith the standards of your normaltechnician, then he/she may only supplyone particular type and therefore thedecision is personal. Finally, as mentionedabove, the different types of all-ceramiccrowns have individual advantages.

CEMENTATIONFor the most part, provided thepreparation fulfils optimum retention,

non-adhesive cements can produceclinically acceptable results. If thepreparation is unretentive, for a varietyof reasons, the cement lute may becomemore important in securing the crown tothe tooth than the shape of thepreparation.4 In such cases, adhesivecementation may be of value, using aresin-based luting system inconjunction with the treatment of theprepared tooth with a dentine-bondingagent. Such cement lutes are generallydual-cured systems and all containpriming agents, such as silanes, toincrease the wetting and bonding of thelute to the rough fitting surface of thecrown or retainer.

Until recently it was difficult to bondadhesive cements to Type III gold usedfor crowns and bridges. Tin plating andoxide layer formation were described toprovide a suitable surface for bonding,but the technique introduced additionaltechnical stages and was not convenient.Panavia F (Kuraray, Japan) is suppliedwith a metal primer allowing directcementation of high gold content alloysto the cement lute.

SUMMARYl Ensure that the crown preparationfollows the shape of the naturalcrown and follows the basicprinciples of retention and resistanceform.l Retraction cord can be helpful in sub-gingival preparations but is notnecessary for supra preparations orthose to the gingival margin.l Adhesive cements can supplementunretentive crown preparations, ascan slots and grooves.

REFERENCES1. Wassell RW. Plastic trays and accurate impressions. Br

Dent J 1998; 184: 266.2. Abuasi HA, Wassell RW. Comparison of a range of

addition silicone putty wash impression materialsused in the one-stage technique. Eur J ProsthodontRest Dent 1994; 2: 117.

3. Giordano R. A comparison of all-ceramic restorativesystems, Part 1. Gen Dent 1999; 47: 566.

4. Bartlett DW. Adapting crown preparations toadhesive materials. Dent Update 2000; 27: 460463.

Figure 8. The preparation for all-ceramiccrowns needs to be at least 1.3 mm plus forsufficient space, and preferably more. Theinternal line angles need to be rounded.

extra-coronal rest - crowns revisited

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