Thermal cycling II -Etiology
distortion
of metal
ceramics:
Part
Stephen D. Campbell, DDS, MMSqa and Lionel 53. Pelletier, DDS, MM%, DMDb Harvard School of Dental Medicine, Boston, Mass. The three-dimensional geometry of conventional fixed prostheses complicates the study of the thermal cycling distortion in metal ceramic alloys. Any explanation of the etiology of thermal cycling distortion in metal ceramic restorations must account for the observed magnitude, timing, and direction of the deformation. The simplified experimental geometry developed in Part I was applied to elucidate the etiologic factors involved in metal ceramic deformation. Techniques to minimize the thermal cycling distortion were also studied. It was found that all of the significant distortion occurred during the first thermal cycling of the alloy (oxidation) and that np, distortion resulted from the application of body porcelain. The specimens that were cold worked and then oxidized had significantly more distortion than any other group. A significant reduction in distortion was observed when the initial thermal cycling was completed before the specimens were cold worked. It was determined that the release of casting- and cold working-induced stresses had a synergistic effect. (J PROSTKET DENT 1992;68:284-9.)
t has been widely observed that the “as-cast” fit of metal ceramic restorations deteriorates during the high temperature firing cycles employed for porcelain veneer application.1-10 The etiology of this thermal cycling distortion has been the focus of extensive studies in the dental literature.l-I3 Many investigatorsg-12 have indicated that the firing shrinkage (12 % to 15 % ) of the porcelain is largely responsible for the distortion and the loss of fit associated with high temperature cycling of the metals. Others5,13 have suggested that the loss of fit is a result of the buildup of metal oxide films on the surface of the alloy, which interferes with the seating of the restoration. The poorer seating then results in a degradation of the marginal adaptation as measured extracoronally. A currently popular theory indicates that the loss of fit is the result of the differential in the thermal coefficient of expansion between the metal and porcelain.2, l4 This results in distortion of the metal framework during the cooling cycle, as the porcelain becomes rigid below its glass transition temperature. Any differential between the thermal coefficient of expansion of the alloy and that of the veneer porcelain will result in induced stresses and will increases the potential for distortion of the metal substructure. In a separate study, DeHoff and Anusavice6 concluded that this is not a primary source of the distortion but
Supported Harvard Presented aAssociate doctoral bInstructor 10/l/36187 284
in part by a grant from the William F. Milton Fund, Medical School, Boston, Mass. at the Academy of Prosthodontics, Wintergreen, Va. Professor of Prosthetic Dentistry, Co-Director of PostProsthodontics. of Prosthetic Dentistry.
. is only partially contributory. Others4, 6 have proposed that the distortion is primarily caused by the release of stresses resulting from casting solidification. It has even been suggested that the alloy type (noble versus base metal) may influence the resistance to thermal cycling distortion.5, 6 This is explained by the difference in the oxide film thicknesses and the modulus of elasticity of the metals. Interrelating all of these theories is the realization that the metal framework design (that is, collar width) and tooth preparation (that is, bulk of metal in the marginal areas) may be integral in resisting the deformation and loss of fit through their potential for maximizing the substructure rigidity. 3,10,16-17 What is clear from the literature is the lack of agreement on the actual cause of the thermal cycling distortion of metal ceramic restorations. A detailed explanation for this lack of consensus was described in Part I of this study. In summary, these included: (1) the complex three-dimensional geometry of crown forms; (2) the inclusion of the casting variables; (3) the failure to identify and isolate the variables involved in thermal cycling distortion (for instance, cold working); (4) the failure to measure the casting deformation directly (measuring the effect of deformation on the marginal seat of the restoration); (5) the on-and-off inaccuracies of the direct view extracoronal measuring techniques; and (6) the resultant lack of statistical sensitivity. The literature does indicate agreement in two areas: (1) distortion occurs during the thermal cycling processl-lo and (2) the timing of the deformation is such that most of it occurs during the initial oxidation of the alloy (before porcelain application).1-6 However, it has been observed that small changes continue during the subsequent heating and porcelain applications. AUGUST
1992
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2. Location of the 10 measurementsites(da&, resin die; light, casting). Fig.
Fig.
1. Casting on the resin measuringdie.
The purpose of this researchwasto apply the methodology developed in Part I to study the etiology of thermal cycling distortion in a metal ceramicalloy. The resultswere used to develop novel solutions to minimize deformation. MATERIALS
AND
METHODS
The samespecimengeometry and formation techniques developed in Part I were usedfor this study (Fig. 1). Fiftysix castingswere completedwith -uniform0.4 mm thick axial walls. The facial collar widths were maintained at 0.4 mm throughout and all castingswere formed with a gold palladium alloy (Olympia, Jelenko, Armonk, N.Y.) according to manufacturer-recommended techniques. This resulted in a standardized geometry, size, and thickness for each of the specimens. All castingswere air-abraded with a 50 pm alumina oxide abrasive (Belle de St. Claire, Van Nuys, Calif.). Measuring dies were then fabricated for eachof the specimens by pouring an autopolymerizing resin (Duralay, Reliance Dental, Chicago,Ill.) directly to the fit surfaces(shoulder and axial wall) of the castings.After 24 hours,the resin dies weretrimmed and.progressivelypolishedwith 400and 600 alumina oxide abrasion paper (Fig. 1). The specimensand dies were numbered and separated. The castingswere replaced on their respective resin dies and marginal openingsweremeasuredat 10predetermined locations (Fig. 2). A microscope with a filar eyepiece (American Optical, Buffalo, N.Y.) accurate to 1 pm was used for this purpose. A seriesof different finishing methodologieswas then applied to assesstheir effect on the magnitude, etiology, timing, and direction of the thermal cycling distortion. The castingswere divided into seven groups of eight and were treated as follows. THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Group 1. The external surfacewasfinished sequentially with green and white stones (Dura-Green Stone TC2, Dura-White Stone TCI, SHP, Shofu, Kyoto, Japan) to simulate a conventional method of shaping and preparing the alloy for porcelain application (Fig. 3). Sampleswerethen air-abraded with 50pm alumina oxide and initial thermal cycling (oxidation) was completed. This was followed by opaque and body porcelain applications. Group 2. Specimenswere oxidized immediately after divestment with no manipulation of the surface. Following this initial heat treatment, they werefinished with the samemethod asin group 1 above and were again oxidized. This alloweda determination of the role cold working (surfacefinishing) plays in thermal cycling distortion. Group 3. The specimenswere prepared as in group 1 above and, following the opaqueapplication, a secondresin measuringdie was fabricated directly to the castings.This wasdone to determine exactly how much distortion occurred as a result of the body porcelain addition. Group 4. A control group was treated as in group 1 above,except a secondresinmeasuringdie was formed directly to the casting after the initial oxidation. The metal wasthen oxidized a second and third time without porcelain application. The purposewas to determine more accurately how much distortion would occur after the initial oxidation when porcelain was not applied. Group 5. Specimens were fabricated as in group 1, except the fit surfacewas cold worked (as opposed to the external surface). Following the cold working, a secondresin die wasformed to 285
CAMPBELL
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Measurements were completed and were recorded following any manipulation or thermal cycling of the specimens. All oxidation cycles were carried out at 1900” F in air. The surface grain morphology of the as-cast, alumina oxide air-abraded, and cold-worked (finished with stones) surfaces were also studied.
RESULTS
Fig. 3. Cold working
of the casting with a white stone.
I. Mean marginal openings (in microns) for all measurement sites
Table
Group
Cold work and initial oxidation Oxidation only Cold work after initial oxidation (new die) Body porcelain (new die) Second oxidation (new die) Third oxidation (new die)
no.
12
3
4
5
6
14
14
13
5
13
1 0
3
0 0
7
0
0 2
0
the casting. The effect cold working had on the direction of the distortion could then be determined. Group 6. The geometry of the castings was altered by eliminating the seating groove. These specimens were manipulated as in group 1. This was done to determine if the groove geometry was influencing the magnitude or direction of the observed distortion. Group 7. Following casting and resin die formation, the specimens were reinvested (Complete, Jelenko) and heat treated at 1900’ F for 20 minutes. The samples were treated as in group 1. This determined if the thermal cycling distortion could be reduced by restraining the casting. 286
All castings fit their respective resin measuring dies perfectly before the thermal cycling of the alloys. The results appear in Table I. The mean marginal opening for all measurement sites following initial oxidation of the specimens that had their surfaces finished with the green and white stones (group 1) was 14 pm. The samples that did not have their surface finished prior to initial oxidation (group 2) had a mean marginal opening of 3 pm. Subsequent surface finishing and reoxidation of these unfinished specimens resulted in no additional distortion (mean = 3 pm). The group 3 specimens revealed that body porcelain application resulted in no additional distortion of the alloy. The group 4 samples showed that a mean opening of 2 grn resulted from a second oxidation cycle at 1900’ F and that no subsequent change occurred from a third oxidation cycle. Oxidation of the castings that had their fit side cold worked (group 5) resulted in a 5 pm mean opening. Altering the geometry of the castings by eliminating the groove (group 6) and then cold working and oxidizing the samples resulted in a mean opening of 13 pm. Examination of groups 5 and 6 revealed no apparent change in the direction of the observed distortion. Heat treating the reinvested castings at 1900” F before cold working (group 7) resulted in a mean opening of 1 pm. No subsequent change was noted when the specimens were cold worked and reoxidized. Statistical evaluation by analysis of variance (ANOVA) and multiple comparison testing (Tukey test) was completed to determine if there were significant differences within or between the experimental groups or measurement locations. This analysis revealed no differences except for the specimens that had their surface finished (cold worked) before the initial oxidation. Surface finishing before the initial thermal cycling resulted in significantly more distortion than in all other groups (p < 0.001) (Q = 8.88).
DISCUSSION Any explanation of the etiology of thermal cycling distortion in metal ceramic restorations must account for the observed magnitude, timing, and direction of the deformation. For example, the theory that the porcelain firing shrinkage is a significant causative factor in the distortion processis questionable.This is becausethe timing of the distortion hasbeendemonstratedto occur primarily during the initial oxidation processof the alloy (before porcelain application). Furthermore, the suppositionthat
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the densification shrinkage of the porcelain causes the metal to distort seems to predate the findings in the extensive literature currently available on dental porcelains. The temperature at which dental porcelain behaves as a viscous fluid has been determined to be between 600’ and 700’ C.r7 This is well below the high temperatures (900” C to 1000’ 6) used for the densification firing of dental porcelain. At these elevated temperatures it seems more likely that the porcelain would relieve the shrinkage stresses by flowing, rather than by stressing and distorting the alloy. The greatest magnitude of the observed distortion in the present study was clearly demonstrated to occur during the initial oxidation cycle of the alloys. This is in agreement with the findings in most of the dental literature.le6 The primary etiologic factors must therefore account for the majority of distortion occurring during the oxidation cycle. This effectively eliminates any process that depends on the presence of the porcelain veneer (for example, differential in the thermal coefficient of expansion of the alloy and porcelain, relaxation of dislocation defects (creep) in the alloy as a result of elevated temperatures, and applied stress of the porcelain). In addition, processes that would be expected to reoccur during each thermal cycling with only small changes in their magnitude could not readily account for the timing or magnitude of the distortion (for instance, build-up of oxide film, or alloy composition changes caused by diffusion of metal atoms at elevated temperatures). However, all of these processes may readily account for the small amount of distortion that has been observed subsequent to the oxidation thermal cycling for metal ceramic restorations. The fit of metal ceramic restorations may also be affected by factors resulting from the thermal cycling process but that do not cause deformation of the alloy. This is a result of variables inherent in the fit of a complex three-dimensional crown form onto a preparation. For example, build-up of the oxide film on metal ceramic alloys during sequential heating may result in the incomplete seating of the restoration.5, l3 Porcelain application also causes an increased rigidity of the metal ceramic prostheses. This may result in a further loss of the fit since the alloy can no longer flex as it is being seated.4: l1 These variables would not affect the results of the present study, since the geometry of the opposing walls was eliminated in the experimental design. This is especially true of the specimens that had the keying groove eliminated. The only known etiologic factors that can adequately account for the timing and magnitude of the observed distortion are: (1) release of stresses resulting from the solidification processes of the casting technique (for example, restriction of the alloy shrinkage in the casting investment) and (2) release of the stresses introduced by the cold working of the surface in preparation for porcelain application.18 While the other theories can contribute to the loss of fit, they cannot individually account for the magnitude and timing observed in metal frameworks.
THE
JOURNAL
OF PROSTHETIC
DENTBTRY
When a metal is plastically deformed at temperatures that are low relative to its melting point, it is said to be cold-worked (strain-hardened). Most of the energy expended in cold work appears in the form of heat, but a portion is stored in the metal as strain energy associated with various lattice defects created by the deformation. Stored energy increases with increasing deformation. Cold working is known to greatly increase the number of dislocations in a metal. Increasing the dislocation density increases the strain energy of the metal. The creation of point defects during plastic deformation is also recognized as a source of retained energy in cold-worked metals. The release of this stored energy occurs at elevated temperatures in the process known as annealing. These large energy releases appear simultaneously with the growth of an entirely new set of essentially strain-free crystals that grow at the expense of the originally badly deformed crystals (result of cold working). When a metal is cold worked, changes occur in almost all of its physical and mechanical properties. Working increases strength and hardness and decreases ductility. The annealing process of cold-worked metals occurs at elevated temperatures and results in the recovery, recrystallization, and grain growth of the deformed crystals, The recrystallization process is a time- and temperature-dependent process that is extremely sensitive (exponential) to small changes in temperature.lg The present study revealed that individually neither the release of the casting- nor the cold working-induced stresses were sufficient to cause significant distortion of the metal during their thermal cycling. Kowever, a synergistic relationship appears to exist between the casting and cold working-induced stresses, resulting in a statistically significant increase (p < 0.001) in the deformation of the cast alloy. When these components are separated and are not introduced during the same thermal cycling step, a statistically significant reduction (p < 0.001) in the distortion occurs. The direction of the observed distortion appeared to be the same throughout this study. The castings lifted away from the measuring die at all marginal areas. The only contact with the die was in some central region of the casting. Cold working of the fit surface did not change the direction of the deformation; however, it did result in a statistically significant decrease in the magnitude of the distortion (mean = 5 pm) (p < 0.001). This suggests that while a synergistic relationship exists between the casting and cold-working stresses, the underlying controlling factor is either the result of the geometry (second plane of the facial collar) or casting-induced variables (cooling pattern of the alloy, sprue location). This is attributed to: (1) the common direction of distortion regardless of the surface that was cold-worked; (2) distortion of all castings during their initial thermal cycling whether they had or had not been cold-worked (cold working affected the magnitude of the distortion, not its existence); (3) no thermal cycling distortion was observed from cold working alone (when cold 287
CA;MPBELL
working was completed after the initial oxidation); and (4) elimination of the keying groove had no effect on the magnitude, timing, and direction of the distortion. Therefore cold working affected only the magnitude of the distortion during the initial thermal cycling of the alloy and not the timing of the deformation. The reinvestment heat treatment technique was evaluated to determine if it could limit the distortion that results from the relaxation of the casting-induced stresses and therefore minimize one of the primary etiologic factors in the thermal cycling deformation of metal ceramics. By restraining the casting during a homogenizing anneal, the alloy can approach an equilibrium structure and potentially release stresses without distorting. Further thermal cycling of the homogenized structure would then be unlikely to cause subsequent distortion. The reinvested heat-treated castings (mean = 1 pm) did have significantly less distortion (p < 0.001) than the samples that were cold-worked and oxidized (mean = 14 pm). No further distortion was noted when the reinvested samples were subsequently cold-worked and thermal-cycled (1900’ F). Statistical evaluation (ANOVA and Tukey test) of the reinvested castings showed no significant difference from the other five groups. It is interesting to note the results of a statistical comparison between a hypothetical perfect casting group (no thermal cycling distortion), the reinvested samples (group 7), and the non-cold-worked, oxidized specimens (group 2). The reinvested heat-treated castings (mean = 1 pm) did not differ significantly from the samples in the hypothetical perfect casting group. The noncold-worked samples (mean = 3 pm) have a significantly greater amount of distortion than the hypothetical castings (p < 0.01) (Q = 4.7). CLINICAL
IMPLICATIONS
The results of the present studies suggestthat evaluating t,heintraoral fit of a metal ceramicrestoration can best be accomplishedby trying the casting on after the initial thermal cycling hasbeen completed. This would allow for a more appropriate assessmentof the final fit of the prostheses,sincethe majority of the thermal cycling distortion occurs during this initial heating. This study hasindicated the variables directly affecting the thermal cycling distortion of metal ceramicalloys. The use of one of two methodsmay minimize the deformation resulting from the initial oxidation of a metal ceramic alloy: (1) thermal cycling at the oxidation temperature immediately after divestment with no manipulation (no cold working) of the casting or (2) heat treatment of the casting (in the investment) for 20 minutes at the oxidation temperature before divestment. Either of thesetechniques will result in significantly less distortion of the casting during the thermal cycling process.The metal can then be prepared for porcelain application (cold-worked and oxidized) without any additional deformation. This study utilized an altered geometric form and con288
AND
PELLETIER
elusions relative to three-dimensional restorations should be limited. Clinical relevance is the ultimate test of any technique utilized in prosthodontics. Therefore, it seems prudent that the results be applied to appropriate clinical methodologies (crown forms) to verify that they significantly improve the fit of completed restorations. Such studies may also distinguish a difference between the proposed methods. SUMMARY The causes of thermal cycling distortion in metal ceramic alloys was examined. Methods for minimizing the distortion were then evaluated. It was determined that: 1. All of the significant distortion occurred during the first thermal cycling of the alloy (oxidation). 2. The specimens that were cold-worked and then oxidized had significantly more distortion than any other group. 3. A significant reduction in distortion was observed when the initial thermal cycling was completed before the specimens were cold-worked. 4. The primary cause of thermal cycling distortion was the release of casting-induced stresses, coupled with the synergistic effect of cold working. 5. The heat treatment of invested castings resulted in a significant reduction in the thermal cycling distortion of the alloy. The invested specimens did not differ statistically from a hypothetical group that had no distortion. 6. Cold working and thermal cycling after the initial oxidation resulted in no additional distortion. 7. Application of body porcelain resulted in no thermal cycling distortion. REFERENCES 1.
2.
3. 4. 5.
6. I.
8.
9. 10.
Ando N, Hakamura K, Namiki T, Sugata T, Suzuki T, Moriyama K. Deformation of porcelain bonded gold alloys. J Jpn Sot Appar Mater 1972;13:237-48. Iwashita AH, Kuriki H, Hasuo T, Ishikawa K, Hashimoto K, Harada H, Uochi T, Hata Y, Studies on dimensional accuracy of porcelain fused to precious metal crown. The influence of the porcelain to the metal coping on the porcelain fusing procedure. Shigaku 1977;65:110-25. Faucher RR, Nicholls JI. Distortion related to margin design in porcelain-fused-to-metal restorations, J PROSTHET DENT 1980;43:149-55. Bridger DV, Nicholls JI. Distortion of ceramometal fixed partial dentures during the firing cyc1e.J PROSTHET DENT 1981;45:507-14. Buchanan WT, Svare CW, Turner KA. The effect of repeated firings and strength on marginal distortion in two ceramometal systems. J PROSTMET DENT 1981;45:502-6. DeHoff PH, Anusavice KJ. Effect of metal design on marginal distortion of metal-ceramic crowns. J Dent Res 1984;63:132’7-31. Richter-Snapp K, Aquilino SA, Svare CW, Turner KA. Change in marginal fit as related to margin design, alloy type, and porcelain proximity in porcelain-fused-to-metal restorations. J PROSTHET DENT 1988;60:435-9. Belser UC, MacEntee MI, Richter WA. Fit of three porcelain-fused-tometal marginal designs in viva: a scanning electron microscope study. J PROSTHETDENT~~~~;~~:~~-~. Mumford G. The porcelain-fused-to-metal restoration. Dent Clin North Am 1965;4:241-49. Shillingburg HT, Hobo S, Fisher DW. Preparation design and marginal distortion in porcelain-fused-to-metal restorations. J PROSTHET DENT 1973:29:276-84.
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11. Silver M, Klein G, Howard MC. An evaluation and comparison of porcelain fused to cast metals, J PROSTHET DENT 1960;10:1055-64. 12. Shelby DS. Practical considerations and design of porcelain fused to metal. J PROSTHET DENT 1962;12:542-6. 13. Pascoe DF. Analysis of the geometry of finishing lines for full crown restorations. J PROSTHET DENT 1978;40:157-62. 14. Tuccillo JJ, Nielson JP. Shear stress measurements at a dental porce-
lain-gold bond interface. J Dent Res 1972;51:626-31. 15. Strating H, Pameijer CH, Gildenhuys RR. Evaluation of the marginal integrity of ceramometal restorations. Part I. J PROSTHET DENT 1981;46:59-65. 16. Mclean JW. The science and art of dental
Quintessence
THE
JOURNAL
Pub1 Co, 1979273-80.
OF PROSTHETIC
DENTISTRY
ceramics. vol 1. Chicago:
17. Twiggs SW, Searle JR, Ringle RD, Fairhurst CW. A rapid heating and cooling rate dilatometer for measuring thermal expansion in dental porcelain. J Dent Res 1989;68:1316-8. 18. Yamamoto M. Metal-ceramics. Principles and methods of Makoto Yamamoto. Chicago: Quintessence Pub1 Co, 1985:203-18. 19. Read-Hill RE. Physical metallurgy principles. 2nd ed. Boston:PWS Publishers, 1973:267-325. Reprint
requests
to:
DR. STEPHEN D. CAMPBELL HARVARD SCHOOL OF DENTAL 188 LONGWOOD AVE. BOSTON, MA 02115
MEDICINE
289
distortion
of metal
ceramics:
Part
Stephen D. Campbell, DDS, MMSqa and Lionel 53. Pelletier, DDS, MM%, DMDb Harvard School of Dental Medicine, Boston, Mass. The three-dimensional geometry of conventional fixed prostheses complicates the study of the thermal cycling distortion in metal ceramic alloys. Any explanation of the etiology of thermal cycling distortion in metal ceramic restorations must account for the observed magnitude, timing, and direction of the deformation. The simplified experimental geometry developed in Part I was applied to elucidate the etiologic factors involved in metal ceramic deformation. Techniques to minimize the thermal cycling distortion were also studied. It was found that all of the significant distortion occurred during the first thermal cycling of the alloy (oxidation) and that np, distortion resulted from the application of body porcelain. The specimens that were cold worked and then oxidized had significantly more distortion than any other group. A significant reduction in distortion was observed when the initial thermal cycling was completed before the specimens were cold worked. It was determined that the release of casting- and cold working-induced stresses had a synergistic effect. (J PROSTKET DENT 1992;68:284-9.)
t has been widely observed that the “as-cast” fit of metal ceramic restorations deteriorates during the high temperature firing cycles employed for porcelain veneer application.1-10 The etiology of this thermal cycling distortion has been the focus of extensive studies in the dental literature.l-I3 Many investigatorsg-12 have indicated that the firing shrinkage (12 % to 15 % ) of the porcelain is largely responsible for the distortion and the loss of fit associated with high temperature cycling of the metals. Others5,13 have suggested that the loss of fit is a result of the buildup of metal oxide films on the surface of the alloy, which interferes with the seating of the restoration. The poorer seating then results in a degradation of the marginal adaptation as measured extracoronally. A currently popular theory indicates that the loss of fit is the result of the differential in the thermal coefficient of expansion between the metal and porcelain.2, l4 This results in distortion of the metal framework during the cooling cycle, as the porcelain becomes rigid below its glass transition temperature. Any differential between the thermal coefficient of expansion of the alloy and that of the veneer porcelain will result in induced stresses and will increases the potential for distortion of the metal substructure. In a separate study, DeHoff and Anusavice6 concluded that this is not a primary source of the distortion but
Supported Harvard Presented aAssociate doctoral bInstructor 10/l/36187 284
in part by a grant from the William F. Milton Fund, Medical School, Boston, Mass. at the Academy of Prosthodontics, Wintergreen, Va. Professor of Prosthetic Dentistry, Co-Director of PostProsthodontics. of Prosthetic Dentistry.
. is only partially contributory. Others4, 6 have proposed that the distortion is primarily caused by the release of stresses resulting from casting solidification. It has even been suggested that the alloy type (noble versus base metal) may influence the resistance to thermal cycling distortion.5, 6 This is explained by the difference in the oxide film thicknesses and the modulus of elasticity of the metals. Interrelating all of these theories is the realization that the metal framework design (that is, collar width) and tooth preparation (that is, bulk of metal in the marginal areas) may be integral in resisting the deformation and loss of fit through their potential for maximizing the substructure rigidity. 3,10,16-17 What is clear from the literature is the lack of agreement on the actual cause of the thermal cycling distortion of metal ceramic restorations. A detailed explanation for this lack of consensus was described in Part I of this study. In summary, these included: (1) the complex three-dimensional geometry of crown forms; (2) the inclusion of the casting variables; (3) the failure to identify and isolate the variables involved in thermal cycling distortion (for instance, cold working); (4) the failure to measure the casting deformation directly (measuring the effect of deformation on the marginal seat of the restoration); (5) the on-and-off inaccuracies of the direct view extracoronal measuring techniques; and (6) the resultant lack of statistical sensitivity. The literature does indicate agreement in two areas: (1) distortion occurs during the thermal cycling processl-lo and (2) the timing of the deformation is such that most of it occurs during the initial oxidation of the alloy (before porcelain application).1-6 However, it has been observed that small changes continue during the subsequent heating and porcelain applications. AUGUST
1992
VOLURAE
68
NUMBER
2
DISTORTION
OF iMETAL
CERAMIC
ALLOYS:
PART
II
2. Location of the 10 measurementsites(da&, resin die; light, casting). Fig.
Fig.
1. Casting on the resin measuringdie.
The purpose of this researchwasto apply the methodology developed in Part I to study the etiology of thermal cycling distortion in a metal ceramicalloy. The resultswere used to develop novel solutions to minimize deformation. MATERIALS
AND
METHODS
The samespecimengeometry and formation techniques developed in Part I were usedfor this study (Fig. 1). Fiftysix castingswere completedwith -uniform0.4 mm thick axial walls. The facial collar widths were maintained at 0.4 mm throughout and all castingswere formed with a gold palladium alloy (Olympia, Jelenko, Armonk, N.Y.) according to manufacturer-recommended techniques. This resulted in a standardized geometry, size, and thickness for each of the specimens. All castingswere air-abraded with a 50 pm alumina oxide abrasive (Belle de St. Claire, Van Nuys, Calif.). Measuring dies were then fabricated for eachof the specimens by pouring an autopolymerizing resin (Duralay, Reliance Dental, Chicago,Ill.) directly to the fit surfaces(shoulder and axial wall) of the castings.After 24 hours,the resin dies weretrimmed and.progressivelypolishedwith 400and 600 alumina oxide abrasion paper (Fig. 1). The specimensand dies were numbered and separated. The castingswere replaced on their respective resin dies and marginal openingsweremeasuredat 10predetermined locations (Fig. 2). A microscope with a filar eyepiece (American Optical, Buffalo, N.Y.) accurate to 1 pm was used for this purpose. A seriesof different finishing methodologieswas then applied to assesstheir effect on the magnitude, etiology, timing, and direction of the thermal cycling distortion. The castingswere divided into seven groups of eight and were treated as follows. THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Group 1. The external surfacewasfinished sequentially with green and white stones (Dura-Green Stone TC2, Dura-White Stone TCI, SHP, Shofu, Kyoto, Japan) to simulate a conventional method of shaping and preparing the alloy for porcelain application (Fig. 3). Sampleswerethen air-abraded with 50pm alumina oxide and initial thermal cycling (oxidation) was completed. This was followed by opaque and body porcelain applications. Group 2. Specimenswere oxidized immediately after divestment with no manipulation of the surface. Following this initial heat treatment, they werefinished with the samemethod asin group 1 above and were again oxidized. This alloweda determination of the role cold working (surfacefinishing) plays in thermal cycling distortion. Group 3. The specimenswere prepared as in group 1 above and, following the opaqueapplication, a secondresin measuringdie was fabricated directly to the castings.This wasdone to determine exactly how much distortion occurred as a result of the body porcelain addition. Group 4. A control group was treated as in group 1 above,except a secondresinmeasuringdie was formed directly to the casting after the initial oxidation. The metal wasthen oxidized a second and third time without porcelain application. The purposewas to determine more accurately how much distortion would occur after the initial oxidation when porcelain was not applied. Group 5. Specimens were fabricated as in group 1, except the fit surfacewas cold worked (as opposed to the external surface). Following the cold working, a secondresin die wasformed to 285
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Measurements were completed and were recorded following any manipulation or thermal cycling of the specimens. All oxidation cycles were carried out at 1900” F in air. The surface grain morphology of the as-cast, alumina oxide air-abraded, and cold-worked (finished with stones) surfaces were also studied.
RESULTS
Fig. 3. Cold working
of the casting with a white stone.
I. Mean marginal openings (in microns) for all measurement sites
Table
Group
Cold work and initial oxidation Oxidation only Cold work after initial oxidation (new die) Body porcelain (new die) Second oxidation (new die) Third oxidation (new die)
no.
12
3
4
5
6
14
14
13
5
13
1 0
3
0 0
7
0
0 2
0
the casting. The effect cold working had on the direction of the distortion could then be determined. Group 6. The geometry of the castings was altered by eliminating the seating groove. These specimens were manipulated as in group 1. This was done to determine if the groove geometry was influencing the magnitude or direction of the observed distortion. Group 7. Following casting and resin die formation, the specimens were reinvested (Complete, Jelenko) and heat treated at 1900’ F for 20 minutes. The samples were treated as in group 1. This determined if the thermal cycling distortion could be reduced by restraining the casting. 286
All castings fit their respective resin measuring dies perfectly before the thermal cycling of the alloys. The results appear in Table I. The mean marginal opening for all measurement sites following initial oxidation of the specimens that had their surfaces finished with the green and white stones (group 1) was 14 pm. The samples that did not have their surface finished prior to initial oxidation (group 2) had a mean marginal opening of 3 pm. Subsequent surface finishing and reoxidation of these unfinished specimens resulted in no additional distortion (mean = 3 pm). The group 3 specimens revealed that body porcelain application resulted in no additional distortion of the alloy. The group 4 samples showed that a mean opening of 2 grn resulted from a second oxidation cycle at 1900’ F and that no subsequent change occurred from a third oxidation cycle. Oxidation of the castings that had their fit side cold worked (group 5) resulted in a 5 pm mean opening. Altering the geometry of the castings by eliminating the groove (group 6) and then cold working and oxidizing the samples resulted in a mean opening of 13 pm. Examination of groups 5 and 6 revealed no apparent change in the direction of the observed distortion. Heat treating the reinvested castings at 1900” F before cold working (group 7) resulted in a mean opening of 1 pm. No subsequent change was noted when the specimens were cold worked and reoxidized. Statistical evaluation by analysis of variance (ANOVA) and multiple comparison testing (Tukey test) was completed to determine if there were significant differences within or between the experimental groups or measurement locations. This analysis revealed no differences except for the specimens that had their surface finished (cold worked) before the initial oxidation. Surface finishing before the initial thermal cycling resulted in significantly more distortion than in all other groups (p < 0.001) (Q = 8.88).
DISCUSSION Any explanation of the etiology of thermal cycling distortion in metal ceramic restorations must account for the observed magnitude, timing, and direction of the deformation. For example, the theory that the porcelain firing shrinkage is a significant causative factor in the distortion processis questionable.This is becausethe timing of the distortion hasbeendemonstratedto occur primarily during the initial oxidation processof the alloy (before porcelain application). Furthermore, the suppositionthat
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the densification shrinkage of the porcelain causes the metal to distort seems to predate the findings in the extensive literature currently available on dental porcelains. The temperature at which dental porcelain behaves as a viscous fluid has been determined to be between 600’ and 700’ C.r7 This is well below the high temperatures (900” C to 1000’ 6) used for the densification firing of dental porcelain. At these elevated temperatures it seems more likely that the porcelain would relieve the shrinkage stresses by flowing, rather than by stressing and distorting the alloy. The greatest magnitude of the observed distortion in the present study was clearly demonstrated to occur during the initial oxidation cycle of the alloys. This is in agreement with the findings in most of the dental literature.le6 The primary etiologic factors must therefore account for the majority of distortion occurring during the oxidation cycle. This effectively eliminates any process that depends on the presence of the porcelain veneer (for example, differential in the thermal coefficient of expansion of the alloy and porcelain, relaxation of dislocation defects (creep) in the alloy as a result of elevated temperatures, and applied stress of the porcelain). In addition, processes that would be expected to reoccur during each thermal cycling with only small changes in their magnitude could not readily account for the timing or magnitude of the distortion (for instance, build-up of oxide film, or alloy composition changes caused by diffusion of metal atoms at elevated temperatures). However, all of these processes may readily account for the small amount of distortion that has been observed subsequent to the oxidation thermal cycling for metal ceramic restorations. The fit of metal ceramic restorations may also be affected by factors resulting from the thermal cycling process but that do not cause deformation of the alloy. This is a result of variables inherent in the fit of a complex three-dimensional crown form onto a preparation. For example, build-up of the oxide film on metal ceramic alloys during sequential heating may result in the incomplete seating of the restoration.5, l3 Porcelain application also causes an increased rigidity of the metal ceramic prostheses. This may result in a further loss of the fit since the alloy can no longer flex as it is being seated.4: l1 These variables would not affect the results of the present study, since the geometry of the opposing walls was eliminated in the experimental design. This is especially true of the specimens that had the keying groove eliminated. The only known etiologic factors that can adequately account for the timing and magnitude of the observed distortion are: (1) release of stresses resulting from the solidification processes of the casting technique (for example, restriction of the alloy shrinkage in the casting investment) and (2) release of the stresses introduced by the cold working of the surface in preparation for porcelain application.18 While the other theories can contribute to the loss of fit, they cannot individually account for the magnitude and timing observed in metal frameworks.
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When a metal is plastically deformed at temperatures that are low relative to its melting point, it is said to be cold-worked (strain-hardened). Most of the energy expended in cold work appears in the form of heat, but a portion is stored in the metal as strain energy associated with various lattice defects created by the deformation. Stored energy increases with increasing deformation. Cold working is known to greatly increase the number of dislocations in a metal. Increasing the dislocation density increases the strain energy of the metal. The creation of point defects during plastic deformation is also recognized as a source of retained energy in cold-worked metals. The release of this stored energy occurs at elevated temperatures in the process known as annealing. These large energy releases appear simultaneously with the growth of an entirely new set of essentially strain-free crystals that grow at the expense of the originally badly deformed crystals (result of cold working). When a metal is cold worked, changes occur in almost all of its physical and mechanical properties. Working increases strength and hardness and decreases ductility. The annealing process of cold-worked metals occurs at elevated temperatures and results in the recovery, recrystallization, and grain growth of the deformed crystals, The recrystallization process is a time- and temperature-dependent process that is extremely sensitive (exponential) to small changes in temperature.lg The present study revealed that individually neither the release of the casting- nor the cold working-induced stresses were sufficient to cause significant distortion of the metal during their thermal cycling. Kowever, a synergistic relationship appears to exist between the casting and cold working-induced stresses, resulting in a statistically significant increase (p < 0.001) in the deformation of the cast alloy. When these components are separated and are not introduced during the same thermal cycling step, a statistically significant reduction (p < 0.001) in the distortion occurs. The direction of the observed distortion appeared to be the same throughout this study. The castings lifted away from the measuring die at all marginal areas. The only contact with the die was in some central region of the casting. Cold working of the fit surface did not change the direction of the deformation; however, it did result in a statistically significant decrease in the magnitude of the distortion (mean = 5 pm) (p < 0.001). This suggests that while a synergistic relationship exists between the casting and cold-working stresses, the underlying controlling factor is either the result of the geometry (second plane of the facial collar) or casting-induced variables (cooling pattern of the alloy, sprue location). This is attributed to: (1) the common direction of distortion regardless of the surface that was cold-worked; (2) distortion of all castings during their initial thermal cycling whether they had or had not been cold-worked (cold working affected the magnitude of the distortion, not its existence); (3) no thermal cycling distortion was observed from cold working alone (when cold 287
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working was completed after the initial oxidation); and (4) elimination of the keying groove had no effect on the magnitude, timing, and direction of the distortion. Therefore cold working affected only the magnitude of the distortion during the initial thermal cycling of the alloy and not the timing of the deformation. The reinvestment heat treatment technique was evaluated to determine if it could limit the distortion that results from the relaxation of the casting-induced stresses and therefore minimize one of the primary etiologic factors in the thermal cycling deformation of metal ceramics. By restraining the casting during a homogenizing anneal, the alloy can approach an equilibrium structure and potentially release stresses without distorting. Further thermal cycling of the homogenized structure would then be unlikely to cause subsequent distortion. The reinvested heat-treated castings (mean = 1 pm) did have significantly less distortion (p < 0.001) than the samples that were cold-worked and oxidized (mean = 14 pm). No further distortion was noted when the reinvested samples were subsequently cold-worked and thermal-cycled (1900’ F). Statistical evaluation (ANOVA and Tukey test) of the reinvested castings showed no significant difference from the other five groups. It is interesting to note the results of a statistical comparison between a hypothetical perfect casting group (no thermal cycling distortion), the reinvested samples (group 7), and the non-cold-worked, oxidized specimens (group 2). The reinvested heat-treated castings (mean = 1 pm) did not differ significantly from the samples in the hypothetical perfect casting group. The noncold-worked samples (mean = 3 pm) have a significantly greater amount of distortion than the hypothetical castings (p < 0.01) (Q = 4.7). CLINICAL
IMPLICATIONS
The results of the present studies suggestthat evaluating t,heintraoral fit of a metal ceramicrestoration can best be accomplishedby trying the casting on after the initial thermal cycling hasbeen completed. This would allow for a more appropriate assessmentof the final fit of the prostheses,sincethe majority of the thermal cycling distortion occurs during this initial heating. This study hasindicated the variables directly affecting the thermal cycling distortion of metal ceramicalloys. The use of one of two methodsmay minimize the deformation resulting from the initial oxidation of a metal ceramic alloy: (1) thermal cycling at the oxidation temperature immediately after divestment with no manipulation (no cold working) of the casting or (2) heat treatment of the casting (in the investment) for 20 minutes at the oxidation temperature before divestment. Either of thesetechniques will result in significantly less distortion of the casting during the thermal cycling process.The metal can then be prepared for porcelain application (cold-worked and oxidized) without any additional deformation. This study utilized an altered geometric form and con288
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elusions relative to three-dimensional restorations should be limited. Clinical relevance is the ultimate test of any technique utilized in prosthodontics. Therefore, it seems prudent that the results be applied to appropriate clinical methodologies (crown forms) to verify that they significantly improve the fit of completed restorations. Such studies may also distinguish a difference between the proposed methods. SUMMARY The causes of thermal cycling distortion in metal ceramic alloys was examined. Methods for minimizing the distortion were then evaluated. It was determined that: 1. All of the significant distortion occurred during the first thermal cycling of the alloy (oxidation). 2. The specimens that were cold-worked and then oxidized had significantly more distortion than any other group. 3. A significant reduction in distortion was observed when the initial thermal cycling was completed before the specimens were cold-worked. 4. The primary cause of thermal cycling distortion was the release of casting-induced stresses, coupled with the synergistic effect of cold working. 5. The heat treatment of invested castings resulted in a significant reduction in the thermal cycling distortion of the alloy. The invested specimens did not differ statistically from a hypothetical group that had no distortion. 6. Cold working and thermal cycling after the initial oxidation resulted in no additional distortion. 7. Application of body porcelain resulted in no thermal cycling distortion. REFERENCES 1.
2.
3. 4. 5.
6. I.
8.
9. 10.
Ando N, Hakamura K, Namiki T, Sugata T, Suzuki T, Moriyama K. Deformation of porcelain bonded gold alloys. J Jpn Sot Appar Mater 1972;13:237-48. Iwashita AH, Kuriki H, Hasuo T, Ishikawa K, Hashimoto K, Harada H, Uochi T, Hata Y, Studies on dimensional accuracy of porcelain fused to precious metal crown. The influence of the porcelain to the metal coping on the porcelain fusing procedure. Shigaku 1977;65:110-25. Faucher RR, Nicholls JI. Distortion related to margin design in porcelain-fused-to-metal restorations, J PROSTHET DENT 1980;43:149-55. Bridger DV, Nicholls JI. Distortion of ceramometal fixed partial dentures during the firing cyc1e.J PROSTHET DENT 1981;45:507-14. Buchanan WT, Svare CW, Turner KA. The effect of repeated firings and strength on marginal distortion in two ceramometal systems. J PROSTMET DENT 1981;45:502-6. DeHoff PH, Anusavice KJ. Effect of metal design on marginal distortion of metal-ceramic crowns. J Dent Res 1984;63:132’7-31. Richter-Snapp K, Aquilino SA, Svare CW, Turner KA. Change in marginal fit as related to margin design, alloy type, and porcelain proximity in porcelain-fused-to-metal restorations. J PROSTHET DENT 1988;60:435-9. Belser UC, MacEntee MI, Richter WA. Fit of three porcelain-fused-tometal marginal designs in viva: a scanning electron microscope study. J PROSTHETDENT~~~~;~~:~~-~. Mumford G. The porcelain-fused-to-metal restoration. Dent Clin North Am 1965;4:241-49. Shillingburg HT, Hobo S, Fisher DW. Preparation design and marginal distortion in porcelain-fused-to-metal restorations. J PROSTHET DENT 1973:29:276-84.
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11. Silver M, Klein G, Howard MC. An evaluation and comparison of porcelain fused to cast metals, J PROSTHET DENT 1960;10:1055-64. 12. Shelby DS. Practical considerations and design of porcelain fused to metal. J PROSTHET DENT 1962;12:542-6. 13. Pascoe DF. Analysis of the geometry of finishing lines for full crown restorations. J PROSTHET DENT 1978;40:157-62. 14. Tuccillo JJ, Nielson JP. Shear stress measurements at a dental porce-
lain-gold bond interface. J Dent Res 1972;51:626-31. 15. Strating H, Pameijer CH, Gildenhuys RR. Evaluation of the marginal integrity of ceramometal restorations. Part I. J PROSTHET DENT 1981;46:59-65. 16. Mclean JW. The science and art of dental
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17. Twiggs SW, Searle JR, Ringle RD, Fairhurst CW. A rapid heating and cooling rate dilatometer for measuring thermal expansion in dental porcelain. J Dent Res 1989;68:1316-8. 18. Yamamoto M. Metal-ceramics. Principles and methods of Makoto Yamamoto. Chicago: Quintessence Pub1 Co, 1985:203-18. 19. Read-Hill RE. Physical metallurgy principles. 2nd ed. Boston:PWS Publishers, 1973:267-325. Reprint
requests
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DR. STEPHEN D. CAMPBELL HARVARD SCHOOL OF DENTAL 188 LONGWOOD AVE. BOSTON, MA 02115
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