Prosthodontics

Distortion of metal-ceramic fixed partial dentures

resulting from metal-conditioning firing

Deniz Gemalmaz*/Semih Berksun**/Çigdem Kasapoglu***/Hasan Necdet Alkumm****

Abstract The aim of this study was to invesiigate the distortion of meial-ceramic prosthesesthat occurs during metal-conditioningßring and to examine whether the distortionmight be controlled by embedding the castings in an investment bulk and applyinga prior heat treatment. The distortion of palladium-copper and nickel-chromiumframeworks was evaluated by means of inner fit and horizontal linear measure-ments of the framework length. There was no statistically .'/ignißcant differencebetween palladium-copper and nickel-chromium frame-works in the changes ininner fit. ¡n addition, there was no statistically significant difference between theinvested test group and tiie control group in changes in fit. However, the meandeformation in framework length recorded for invested palladium-copper frame-works (-20.20 \Lm) was significantly smaller than that recorded for controlpailadiuin-copperframeworks(52.20\im).(,QumtesssnceÏQX 1996,27:193-201.)

Clinical relevance

Embedding castings in an investment bulk andapplying a prior heat treatment before metal-conditioning firing was found to be effective ineliminating distortion of tiiin marginal areas ofpalladium-copper frameworks.

Introduction

The initial fit of metal-ceramic restorations deterior-ates during the firing cycle of porceiain. The hypothe-sis has been that stresses that result from a positivethermal contraction mismatch between metal andporcelain cause deformation in the metal framework.''*

* Lecturer, Department of Prosthoduntjcs, Marmara Llujversily,Faculti' of Dentistry, Istanbul, Turkey.

** Associate Professor, Department of Prosthodontics, Ankara Uni-versity, Faculty of Dentistry, Ankara, Turkey,

'"•' Research Assistant, Department of Prosttiodontics, MarmaratJniversity, Faculty of Dentistry, Istanbul, Turkey.

' " " Profes5or, Department of Prosthodontics, Marmara University.Faculty of Dentistry, Istanbul, Turkey,

Reprint requests: Dr Deniz Gernaljnaz, Marmara University, Faculty ofDeniistiy, 80200 Nisantaii, Istanbul, Turkey.

The thermal cycling distortion has also been attributedto other factors, such as (!) plastic flow and creep ofthe alloy under high temperatures-'" ^ (2) contamina-tion of the casting that reduces melting temperaturesand causes the grain of the alloy to grow**; (3) formationof a layer of oxide on the internal surface of thecoping'; (4) reduction in the resilience of the metalbecause of the rigidity of porcelain'''*'; (5) inadequatesupport of the framework during firing"; (6) contam-ination of the internal surface of the coping with por-celain""-'"' '•'; (7) relaxation of solidification stresses'"'-' ;and (8) design of the tooth preparation and metal sub-structure-*'' •"' Although researchers^-^- ''"''=• '•'•"--'•agree that deformation occurs during thermal cycling,considerable controversy continues to exist withregard to the elfect of these factors.

Iwashita et ai'" concluded that the largest changes inmarginal gap were produced as a result of porcelainfiring cycles, and they indicated that distortion pri-marily occurred because of the stresses resulting fromthermal contraction mismatch. Bridger and Nicholls,'who measured the distortion of six-unit fixed partialdentures, reported that the contraction of porcelainhas a significant effect on distortion of the framework.Their finding is in agreement with the results ofIwashita et ah'*

3/1996 193

Gemalmaz et al

In contrast. De Hoíf and Anusavice,'" who usedfinite-element analysis to evaluate the effect of thermalcontraction differences on the distortion of metalcopings, recorded relatively low marginal distortions,indicating that stresses resulting from thermal contrac-tion mismatch are not the primary cause of distortion.

In a separate study, Anusavice and Carroll''' mea-sured the marginal gap of metal-ceramic crowns underconditions designed to exaggerate distortion effects;they used thin metal copings made of an alloy withpoor creep resistance and a porcelain with a largethermai contraction mismatch. The changes in mar-ginal gap resulting from the porcelain firing cycleswere remarkably small, which indicates that thethermal contraction differences are not the primaryetiologic factor in distortion of copings.

Many ¡nvestigators''''^''''''*'"'"'-''^''^'' have reportedthat distortion primarily occurs during the initialoxidation of the alloy before porcelain application.This finding suggests that porcelain-metal incompat-ibility stresses are not the primary cause of distortionand that distortion occurs because of some factorsrelated to the heat treatment for metal conditioning.

A currently popular theory indicates that the releaseof solidification stresses may be the primary etiologicfactor in the deformation of metal ceramics duringthermal cycling. Kulmer et al" demonstrated that thedimensional changes in metal copings that have beenfirst embedded in investment are significantly less thanchanges in those that are directly heated for oxidation.The authors concluded that the relaxation of solidi-fication stresses is the primary etiologic factor incoping distortion and that distortion might be con-trolled by investing the casting and performing a priorheat treatment.

Campbell and Pelletier^^ developed a simplifiedexperimental approach to examine changes in fit byuse of a one-walled metal-ceramic casting. The authorsobserved a significant reduction in distortion when theinitial thermal cycling was completed before thespecimens were cold worked, and they concluded thatthe release of casting and cold-working-inducedstresses have a synergistic effect.

The purposes ofthe present in vitro study were (1)to examine distortion in three-unit metal-ceramicfixed partial dentures during the metai-conditiotiingfiring cycle and (2) to evaluate the effectiveness ofthemethodology developed by Kulmer et al'^ to minimizedeformation resulting from relaxation of solidificationstresses-

Method and materials

Fabrication of frameworks

A master model representing a clinical mandibularposterior fixed partial denture was constructed fromnickel-chromium (Ni-Cr) alloy (Wirolloy, Bego). Themandibular right first molar, representing the missingtooth, and the second premolar and second molar,simulating the abutments, were machined to a height of7 mm with a taper of 4 degrees. A chamfer marginaldesign was constructed (Fig 1).

To create a wax pattern of 0.4-mm uniform thick-ness. Adapta deep drawing system (Bego) was used.To create uniform pontics, a master pontic wasdeveloped to fit the related edentulous space; uniformpontics were duplicated in a silicone mold. Thejunction area ofthe pontic to the abutment teeth wasadjusted to 3 mm in both buccolinguai and occluso-cervical dimensions.

A total of 20 wax patterns were invested in aphosphate-bonded investment (BellavestT, Bego) andwere cast according to the standardized techniquesin a Nautilius MP-microprocessor-controlled high-frequency vacuum pressure casting machine (Bego).Half of the copings were cast in a Nl-Cr alloy (Wiron99, Bego) and the other half in a palladium-copper(Pd-Cu) alloy (Begopal, Bego) (Fig 2).

The castings were divested in a Duostai corundutnblaster (Bego) with 250-^m aluminum oxide abrasive(Korox 250, Bego). Their fitting surfaces were exam-ined for irregularities. The irregularities were removedwith a small round bur. Begopal and Wiron 99 frame-works were sandblasted with 25-[im and 250-fimaluminum oxide abrasives, respectively, in Topstarcorundum blaster (Bego). Finally, each framework wascleaned ultrasonic ally in distilled water for 10 minutes.

Five randomly selected frameworks from each alloygroup served as the control specimens. These frame-works were subjected to metal-conditioning firingaccording to the manufacrurer's recommendations. Allofthe castings were fired under air pressure at 960°C.Holding times for Begopal and Wiron 99 alloys were 3and 10 minutes, respectively. Sandblasting is recom-mended for Wron 99 alloy to remove the excess oxidelayer. However, to avoid possible loss of metal duringsandblasting, this procedure was eliminated.

The remaining five frameworks from each alloygroup were used as the test group to assess theeffectiveness of embedding in investment to minimizedeformation. A silicone mold was fabricated so that3.5 mm space was left around the framework; when

194 Quintessence Internalional Volume 27, Number 3/1996

Gemalmaz et al

Fig 1 Master model of the leeth prepared for a posteriorfixed partiai denture.

Fig 2 Buccai view of a cast metal framewori< seated onthe master modei.

investment (Bellavest T) was poured into this mold,each framework was covered by a 3,5-mm-thick.uniform investment bulk. After setting of the invest-ment, the frameworks were heated at 85O°C for 30minutes under air pressure in a porcelain ftimace(Programat P90, Ivoclar). Following the firing, theframeworks were cooled to room temperature anddivested by carving the investment. Sandblasting wasavoided in the divesting procedure to eliminate pos-sible loss of metal from internal surfaces. The speci-mens were cleaned ultrasonically in distilled water for10 minutes.

The control and the test specimens were measuredbefore and after the first firing stage. The controlspecimens were subjected to only one firing becausethey simulated the conventional firing procednre. Thetest specimens were subjected to two firing cycles, furstin investment for the controlled relaxation of residualstresses and second in the metal-conditioning firingunder the same conditions as the firing of controlspecimens. Thus, measurement was repeated after thesecond firing of the test group.

Measurements

The distortion of the control and the test specimenswas measured by means of inner fit and horizontallinear measurements of the framework length.

To measure the distance between the casting and thewalls of the retainers, replicas were made of theintermediate space between the inner surface of theframework and the metal die surfaces. Eirst, the innersurface of the metal framework was coated with a thinlayer of light-bodied addition silicone material (Ex-

trude, Kerr). The denture was then placed onto themaster model and a standard load of 5 kg was applied.After setting of the impression material, the frameworkwas removed, and a thin film of light-bodied materialadhered to the outer surfaces of the abutments,representing the discrepancy between the frameworkand the retainers.

For the purpose of stabilization, a heavy-bodiedsilicone material (Extrude) was applied over theabutments by using an acrylic resin index. Thisprocedure made it possible to remove and handle theintermediate replica of the light-bodied material. Afterthese silicone materials were removed from the mastermodel, a medium-viscosity silicone material (Extrude)that adhered to the light-bodied film was injected intothe abutment spaces.

The replica specimens were bisectioned with ascalpel in a mesiodistal direction. To standardize thebisectioning procedure, fixed splits were made on themesial and distal sides of the outer acrylic resin indexand the silicone replicas were sectioned in accordancewith these splits. A reflection microscope (Nikkon)was used at xlOO magnification for measurements.Measurements of the film thicknesses were performedalong the mesial, occiusal, and distal walls at 1-mmintervals.

The distance between the mesial finish line of thepremolar retainer and the distal finish line of the moiarretainer, the total length of the framework (Fig 3), wasmeasured with the help of certain reference marks witha digital micrometer. The linear distance measure-ments were made for both control and test specimensin the same order as used for measurements of innerfit.

rirr'111''"'"' ' ' '""'' ' "•••''•""• *'T 'í«"^^'"3/1996 195

Gemalmaz et ai

Table I Mean changes in inner fit, as function of alloy type, investing, retainer, and axial wall

Change in inner fit (|im)

First firing Second firing

Alloy Investing Retainer Axial wall Mean SD SE Mean SD SE

111111111111222222222222

111111222222111111222

r-l

22

4.489.294.870.65

13.842.507.29

21.261.62

12.038.123.45

10.637.18

23.874.94

27.1117.9319.2014.592.882.853.141.64

16.6145.21

9.6910.2844.1027.7118.7838.5118.9818.7427.3817.9720.8814.999.83

26.5278.5042.84

9.9739.58

5.6710.5437.50

9.81

7.4220.21

4.334.59

19.7212.398.40

17.228.488.38

12.248.049,346.704.39

11.8635.1019.164.46

17.702.534.71

16.774.38

*

***

-1.27-2.64

1.17-8.48-1.046.77

******

1.939-95

-2.036-56

13.162-69

*

•II

*

26.9329.2911.0811.0923.3825.48

***

**

7.0230.0518.0328.0312.649.51

****

12.0413.104.954.96

10.4511.39

**

***

3.1413.448.06

12.535.654.25

SD = standard deviation; SE ' slandard error.For purposes of compiiler programming, codes were assigned lo the test cotid ¡tions. Alloy; ! -Ni-Cr, 2 -Pd-Cu; investing: t »control. 2 = test (invested);retainer: i - premolar. 2 - molar; axial vialh 1 = mesial, 2 = oeelusal, 3 - distal.* Only one firing was applied to ttie eontrol group.

Fig 3 Cervicai view of a cast metal framework The lineardistance between A and B indicates the totai length of theframeworiv.

The results were compiled on a data and analysis ofvariance was used for observed differences at aconfidence level of -95 ( cc = .05).

Results

The mean changes in inner fit as a function of alloytype, investing, retainer, and axial wall are foundin Table 1. The mean differences between the initialinner adaptation and the fit after firing ranges from-21.11 im to 23-87 iini- The values listed reflect thechanges between prefiring measurements and themetal-conditioning tiring cycle; thns, positive andnegative values are possible. Positive values imply anincrease in gap and a poorer fit, whereas negative

196 Quintessence Internationai Volume 27, Number 3/1996

Gemalmaz et al

Fig 4 Axial wall changes in nickel-chromium (Ni-Cr) and palladium-copper (Pd-Cu) alloys. Displace-ment pattern of mesial, occiusai,and distal walls of retainers afterfiring (anows). Only one fifing wasapplied to the control groups.

M-Cr ( CüntroJ |

Ni-Cr ( Test )

Pd-Cg ( Control )

Pd-Cu 1 Test 1

Second Firing

-/tan-

values indicate a decrease in gap and better adaptationof the axial wall.

For example, the mean change in fit values of-2,85 |im and -1.64 p.m recorded for the mesial anddistal walls of the molar retainer of the Pd-Cu testgroup implied inward displacement of these walls. Thechange in fit of 3.14 im observed for the occlusal wallof the retainer reflected an increase in gap at theocclusal surface. Therefore, the incomplete seating ofthe retainer occurred at occlusal surface due to aninward displacement of mesial and distal waUs. Thepremolar retainer of the same group had changes in fitof -19.20 |im, -14.59 |im, and 2.88 jim for themesial, occlusal, and distal walls, respectively. Thesevalues implied inward displacement of the mesial andocclusal walls toward the abutment tooth and outwarddisplacement of the distal wall, resulting in an in-creased inner space at the distal axial wall.

Changes in the axial waU that were recorded inpremolar and molar retainers of the experimentalgroups are shown in Fig 4. Although considerablevariation was observed between the displacementpatterns of the axial walls of each retainer, there weresome similarities among the patterns of displacementobserved for premolar and molar retainers of the fourexperimental groups. Inward displacement of mesialand occlusal walls and outward displacement of distalwall were observed for both premolar and molar

retainers of Ni-Cr test frameworks. In addition, theframeworks showed the same pattern of distortionfollowing the second firing. However, a single patternof distortion carmot be defined for all cases.

Table 2 summarizes the analysis of variance resultsfor changes in inner fit. None of the sources of varianceexamined generated differences that were statisticallysignificant.

The mean horizontal change in framework length inrelation to alloy type and investing is presented inTable 3. When all of the groups were compared, thesmallest change was recorded for the second firing ofPd-Cu test frameworks ( 13.80 [im); and the greatest forthe initial firing of Ni-Cr control group (57.ÜÜ (lm).The mean value of 13.80 [im for Pd-Cu test frame-works indicated that the mesial margin of the premolarabutment and the distal margin of the molar abutmentwere farther away from each other because of dete-rioration of these margins during firing.

Analysis of variance results for horizontal changesare shown in Table 4. There was no statisticallysignificant difference in the deformation of variouscomparisons. However, significant differences werenoted between the control and the test groups ofPd-Cu frameworks after first firing. Invested (test)specimens had significantly less deformation (-20.20|im) than did the control frameworks (52.20 |im).

Quintessencejnlgmatöoal VnliimeJ^Z, Number 3/1996 197

Gemaltnaz et al

Table 2 Analysis of variance of mean changes in internal fit

Source of variance First firing

0.7900.5340.5440.2920.5160.9980.9430.0890.5640.5000.5080.5010.2250.5220.654

Firing cycleSecond firing

0.248*

0,6100,663

*0,6090,518

0,924

*0,910

**

AlloyInvestingRetainerAxial wallAlloy X investingAlloy X retainerAlloy X axial wallInvesting x retainerInvesting x axial wallRetainer x axial wallAlloy X investing x retainerAlloy X investing x axial wallAlloy X retainer x axial wallInvesting x retainer x axial wallAlloy X investing x retainer x axial wall

• Only the invested test group ' subjected to a secünd firing, so Ihe investing variable does not enisl for the second firing.

Table 3 Mean changes in the length of the frameworks, as a function of ahoy type and investing

Alloy Investing

Change in framework length

First firing

57,0045,9320,5441,6033,0614,7852,2056,0925,08

-20,2038,4117,17

Second firing

*

**

26,6019.118,54***

13,8024,4710.94

Ni-Cr

Ni-Cr

Pd-Cu

Pd-Cu

Control

Test

Control

Test

MeanSDSEMeanSDSEMeanSDSEMeanSDSE

SD = standard deviation; SE - standard ertot,* Only one fmng was applied to tbe control group.

198 Quintessence International Volume 27, Number 3/i9g6

Gemalmaz et al

Table 4 Analysis of variance of mean changes inframework length

Source ofvariance

A x BA x CB x CD x ED x FE x F

df

I11111

Sum ofsquares

592.92310.4

562.513104.43686.42890.0

F value

0.371.870.775.671.972.79

P>¥

.51

.21

.52

.04*

.20

.13

A - Ni-Cr control group, first firing; B = Ni-Cr test group, first finng;C - Ni-Cr test group, second tiring; D - Pd-Cu eontrol group, first firing;E = Pd-Cu lest group, first firing; F = Pd-Cu tesi group, second firing.* Statistically significant difference.

Discussion

Compared to the initial fit ofthe casting, frameworksfor metal-ceramic fixed partial dentures exhibit apoorer intraoral fit after the metal-conditioning firing.This alteration in ñt may be due to a product ofchanges in the thin metal margins or due to generalizeddistortion in the framework body. The distortion ofthecasting is multidimensional, and thus it is hard toassess the distortion of a framework that has complexgeometry. An increase in marginal gap width or heightcouid result from an inward radial displacement oftheaxial wall, which would produce a tighter fit andincomplete seating. An outward and upward displace-ment ofthe marginal area alone couid cause a simiiarchange in the gap width or height. The initiai innerspace that occurs between the framework and theabutment tooth has also an important role in the finalseating of the framework, because inward radialdisplacement of axial walls may be tolerated to adegree by using these spaces. In any event, metal-ceramic frameworks show complex patterns of distor-tion because of their multiwalled configuration, and itis hard to define a single pattern of distortion for allcases.

The present study aimed to assess the changes ininternal fit of metal frameworks. Because the three-dimensional geometry of conventional fixed pros-theses complicates the study of the thermal cyclingdistortion, a replica method was used. An assessmentof restoration-tooth inner discrepancy by replicamethod will result in some inherent méthodologieerrors related to the replica material and to the method

of measurement. These drawbacks, however, havebeen discussed and accounted for previously"'-^

The values recorded for the alteration in fit, whetherpositive or negative, were relatively low, with amaximal displacement value of 27.11 pm. Thesechanges are not considered to be of clinical significance, because the optimal seating of crowns inclinical practice is limited by the minimal film thick-ness of commercial dental cements, which ranges from20 to 25 um. However, in a coping with a 5-degreetaper, a 6-iim radial displacement of the internalsurface ofthe coping can cause an opening of 70 [im atthe margin. Therefore, although the changes in innerfit are small, they may cause marginal openings withinthe limits of clinical significance. In addition, therewas no statistically significant difference in the chan-ges in inner fit related to alloy type, investingpretreatment, abutment, or axial wail, nor was there adifference with interactions of the four variables.

The distortion of thin metal margins was alsoassessed by measuring the changes in length of theframeworks. The average changes in mesiodistal lengthofthe frameworks were relatively higher than the meanvalues for changes in inner fit. The greatest change,57.00 ^m, recorded for the Ni-Cr control frameworks,indicated that the thin margins distorted to a greaterdegree than did the axial wails of the retainer.Additionally, the mean distortion recorded for the testgroup of Pd-Cu alloy (-20.20 (am) was significantlyless than that recorded for the Pd-Cu control group(52.20 i^m). Thus, initial heating of Pd-Cu alloy in aninvestment bulk was effective in preventing the distor-tion of thin margins. The results ofthe present studycorroborate the findings of Kulmer et al,'^ whodemonstrated that distortion oía casting is eliminatedby a prior heat treatment with help of an investmentsupport.

In a recent study, Campbell and Pelletier," aftercasting and die formation, reinvested the castings andheat treated them at 1,900°F for 20 minutes. Theyconcluded that the heat treatment of invested castingsresults in a significant reduction in the thermal cyclingdistortion ofthe alloy.

Because Terada^' assessed behavior at various stagesofthe firing cycle for porcelain-fused-to-metal crownsand concluded that polishing causes high compressionstress in metal strips that is released by degassing, itwould be more appropriate to make prior heat treat-ment in investment after cold working is completed. Inreverse to the procedure of Campbell and Pelletier,- ^who applied heat treatment just after casting and

umber 3/1996 199

Gemalmaz et ai

before cold working, reinvesting and heat treatmentwere made after cold working of the metal framework,in the present study. In this way, both solidificationand cold-working-induced stresses were released un-der control of an investment support.

This study was designed to evaluate a currentlypopular mechanism for reduction of marginal andgeneralized distortion of fratiieworks. It is highlyprobable that the solidification stresses and cold-working-induced stresses were released during initialheating of the framework, resulting in distortion of thinmetal margins. Although the reinvesting procedureseemed to be effective in reducing distortion in thinmarginal areas, it is a time-consuming procedure, andthe need for a secondary divesting process (sandblast-ing) may cause discrepancies because of metal loss.-*This phenomenon should be considered in futurestudies.

Summary

The distortion of three-unit metal-ceramic prosthesesthat occurs during me tal-condition ing firing wasevaluated in Pd-Cu and Ni-Cr frameworks. The studyalso examined whether distortion might be controlledby preheating the castings in an investment bulk. Thefollowing conclusions can be drawn from the presentstudy:

1. AU of the metal-ceramic frameworks distortedduring the thermal cycling process.

2. No statistically significant difference was foundbetween alloy types in the distortion of the metal-ceramic framework.

3. The alterations in fit recorded at the axial walls ofthe retainers were relatively low compared with thedistortion of thin margins.

4. Although the reinvesting process did not seem tobe effective in all cases, it was effective in reducingdistortion in thin mai^inal areas of Pd-Cu frame-works.

5. Metal-ceramic frameworks showed complex pat-terns of distortion because of their multiwalledconfiguration. Thus it was hard to defme a singlepattern of distortion for all cases.

Acknowledgments

The authors express particular apprecialion lo [he Bego Company forsupporting This research.

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200 Quintessence Intemalional Volume 27, Nijmber 3/1996

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^1 [intr-T-^oinr ititprr^"'""''' " ' '*•'"" f i',-Mi""hpr 3/1996 201

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