rmd cycling r width
distortion
of metal
ceramics:
Part
Stephen D. Campbell, DDS, MMSc,” and Lionel B. Pelletier, DDS, MMSc, DMDb Harvard
Schooi
of Dental Medicine, Boston, Mass.
The complex three-dimensional geometry of conventional full crown restorations severely complicates the study of thermal cycling distortion in metal ceramic castings. A simplified experimental geometry was developed to (I) maximize the measuring sensitivity, (2) eliminate the casting variables, (3) allow the direct measurement of casting distortion, and (4) evaluate the thermal cycling distortion of a wide range of metal collar widths. It was found that all of the one-walled castings distorted during the initial thermal cycling (oxidation) of the alloy. There was no significant distortion associated with porcelain application or glazing. The castings with an 0.8 mm metal collar had significantly less distortion than those with 0.1 and 0.4 mm collars at 2 of the 10 sites measured (inferior or facial margins). (J PROSTHET DENT 1992;6’7:603-8.)
anagement of the facial marginal area of a metal ceramic restoration can be an esthetic problem for even the most skilled clinicians and technicians. This is primarily related to the metal collar of these restorations. In an attempt to maximize the esthetics, the facial metal collar is frequently thinned or eliminated. As a result, the marginal adaptation ofthe completed restoration could be adversely affected. This is largely because the “as-cast” fit of metal ceramic restorations deteriorates during the high temperature firing cycles that are employed for porcelain veneer application.1-g Contrary to popular belief, there appears to be few articles in the literature supporting the degradation of fit with the thinning of metal collars. It seemsthat this concept has been extrapolated from studies that examined the intluence of the marginal preparation design (for example, chamfer, shoulder) and did not isolate metal collar width as a variable.3j j, lo Several studies7-i2 have specifically evaluated the effect that porcelain proximity to the restoration margin (metal collar width) has on the thermal cycling distortion of the finished prosthesis. All of these studies concluded that metal collar width does not affect the marginal adaptation of -the completed restoration. The purpose of the current study was to (1) create an ex-
Supported in part by a grant from the William F. Milton Fund, Harvard Medical School, Boston, Mass. Presentedat the Greater New York Academyof Prosthodontics, New York, N.Y. =AssociateProfessorof Prosthetic Dentistry, Co-Director of Postdoctoral Prosthodontics. bInstructor of Prosthetic Dentistry. 10/l/36186
THE
JOURNAL
OF
PROSTHETIC
DENTISTRY
Fig.
1. Simplified geometry of master die design.
perimental design that would maximize the measuring sensitivity, eliminate the casting variables, and allow for the direct measurement of casting distortion and (2) apply this methodology to the evaluation of the thermal cycling distortion of a range of collar widths.
MATERIALS
AND METHO
A master die was machined to simplify the geometry sf a conventional preparation (Fig. 1). The die represented one wall (6 x 6 mm) of a full crown preparation with a 90degree shoulder (1 mm). A vertical orientation groove wa.s machined along the axial wall to aid in the accurate reseating of the castings. A vinyl polysiloxane impression material (Express Light Body, 3l’vI Dental Products, St. Paul, Minn.) was used to duplicate the machined preparation in an improved die stone (Die-Keen, Modern Materials, Columbus, Ohio). Wax patterns were then fabricated with 28 gauge casting sheet wax (Kerr Dental Mfg. Co., Detroit, Mich.). This resulted in uniform 0.4 mm thick axial walls. Three specimen groups were completed with 11 samples
CAMPBELL
AND
PELLETIER
Fig. 2. Experimental castings with 0.1, 0.4, and 0.8 mm facial collars on their respective resin measuring dies.
The castings were then replaced on their respective resin dies and marginal openings were measured at 10 predetermined locations (Fig. 3). A microscope with a filar eyepiece (American Optical, Buffalo, N.Y.) accurate to 1 Km was used for this purpose (100 power). Ten specimens each of the three collar widths were oxidized in air at 1900” F and were remeasured (Table I). Opaque was applied in two stages, followed by two applications of body porcelain and a final glaze firing. Measurements were completed and were recorded on the resin dies after each of the firing cycles. The remaining three samples were maintained as controls to observe any changes in fit that might have resulted from dimensional instability of the resin or from the process of taking the castings on and off the dies. RESULTS Fig. 3. Location of the 10 measurement sites. Locations 7 and 8 represent the facial marginal areas (dark, resin die; light, casting).
each of an 0.1, 0.4, and 0.8 mm metal collar (33 total). Castings were then formed with a gold-palladium alloy (Olympia, Jelenko, Armonk, N.Y.) according to manufacturer-recommended techniques. This resulted in a uniform geometry, size, and thickness for each of the specimens and collar width variables (Fig. 2). All castings were air-abraded with a 50 pm alumina oxide abrasive (Belle de St. Claire, Van Nuys, Calif.). Direct dies were then made for each of the specimens by pouring a measured volume of 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 were trimmed and progressively polished with 320, 400, and 600 alumina oxide abrasion paper (Fig. 2). All specimens and dies were numbered and separated.
604
Before the first thermal cycling of the alloy (oxidation), all 33 castings fit their respective resin dies with no observable marginal openings at the limits of the light microscope (Figs. 4 and 5). The three control specimens showed no change during the course of the investigation. All 30 of the one-walled experimental samples distorted upon the first thermal cycling process (oxidation). The mean marginal opening for all specimens and measurement sites was 14 pm following the initial oxidation of the alloy (Figs. 6 and 7). The standard deviations of individual measurement sites and sample groups were in the 5 to 9 ,umrange throughout, with most being around 6 to 7 pm. The initial oxidation cycle of the 0.1, 0.4, and 0.8 mm collar groups resulted in a mean opening of 14, 16, and 12 pm, respectively, for all measurement sites (Table I). Statistical evaluation by analysis of variance (ANOVA) and multiple comparison testing (Tukey test) was complete determine if there were any significant differences within or between the three collar width groups or 10 measurement sites. The analysis revealed no significant differences
AMAY
1992
VOLUME
67
NUMBER
5
DISTORTION
OF METAL
CERAMIC
ALLOYS:
PART
I
Fig. 4. Photomicrograph showing the perfect facial margin adaptation of a casting on its resin measuring die. (Original magnification x20.) A, Alloy; R, resin die.
Table I. Mean marginal openings (in microns) following initial thermal cycling (oxidation) Measurement Collar width (mm)
location
1
2
3
4
5
6
7
8
9
10
Mean
0.1
10
0.4 0.8
15 13
I1 14 11
19 19 13
16 19 14
14 16 14
15 16 15
13 16
14 13
8*
7*
14 17 12
14 18 11
14 16 12
*Statistically significant.
between the 0.1,0.4, and 0.8 mm collar width groups except for the facial collar width measurement sites (locations 7 and 8). The 0.8 mm collar mean opening of 8 pm was significantly better than the mean of 14 ym for the 0.1 mm Cp< 0.05) (Q = 3.4) and 0.4 mm (p < 0.025) (Q = 3.9) collar widths. There was no significant difference between the 0.1 and 0.4 mm collar widths. Combining the three collar width groups revealed no significant difference between measurement locations except the occlusal sites 3 and 4. The occlusal locations had a significantly (p < 0.05) greater opening (17 pm) than the mean of the remaining measurement sites (12 Km). The mean openings for all specimens at all sites for the first and second opaque applications, first and second body porcelain applications, and glaze firings were 15,15,14,15, and I5 pm, respectively, Statistical analysis revealed no significant change for any measurement site or collar width group following the initial oxidation distortion of the castings. The measurement sites and specimen group mean values for all thermal cycling (porcelain application) subsequent to the initial oxidation were, for the most part,
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identical to the oxidation values, and in no case did they vary by more than 1 or 2 pm.
The results of the present study differ from prior evaluations of the effect of collar width on marginal adaptation of metal ceramic restorations.7-12 This may in part be the result of the complex three-dimensional geometry of cast full veneer restorations. Many of the earlier studies ut,ilize direct external viewing of crown margins, since studying the effect of thermal cycling on metal ceramic alloys is most easily accomplished with a nondestructive methodology. This type of examination of the fit of a restoration is a measurement of the seat of the casting on the preparation. As described by Pilo et aLi3 and Pascoe,‘” the opposing wall geometry of a casting can result in large changes in the seat of a restoration, with only minor changes along the axial walls. FOF example, given a lo-degree axial wall taper, a change of 1 pm along an axial wall will result in the casting not seating by an additional 11 pm. Therefore measurements of the seat of a restoration at the external margins
605
CAMPBELL
Fig. 5. Photomicrograph showing the perfect axial wall adaptation of a casting on its resin measuring die. (Original magnification X5.)
are not a direct measurement of the alloy distortion. The direction of the distortion (toward or away from the axial walls), the initial adaptation of the casting to the axial walls, the magnitude of axial wall distortion, and the preparation geometry (degree of taper) can greatly affect the seating of the casting. This introduces variables that make an accurate assessment of the actual alloy distortion during thermal cycling difficult. Evaluating the distortion of a localized area of the casting geometry (marginal collar) becomes even more difficult. The direct view measurement of the seat of a cast restoration can also introduce errors caused by (1) the poor fit of the initial castings on the measuring dies, (2) the large standard deviation of the initial casting fit that results from the casting variables, (3) the technical difficulty and reproducibility of measuring the external curved surface of the margin of a crown, (4) the differential seating inaccuracies that result from taking the casting on and off the measuring die, and (5) the complex three-dimensional geometry, as previously described. These factors can introduce a large standard deviation in the measurements and may prevent detection of statistically significant differences in the data. The earlier studies7-l2 demonstrated as much as a 26 pm increase in the marginal seat opening when the facial collars were thinned. However, no statistical significance was found because of the large standard deviations. In addition, previous studies of collar widths may not 606
AND PELLETIER
have examined a sufficient range of sizes to isolate an effect on marginal adaptation during the thermal cycling process. One study9 compared a 0.5 mm collar with castings with a 0 mm collar. The 0.5 mm collar width may have been inadequate to demonstrate the decreased distortion observed in the thicker 0.8 mm collar of the current study. Another study7 compared a 0.75 mm facial collar with the thicker lingual collar on the same restoration. The 0.75 mm width may have been too great to identify a relationship between distortion and the proximity of porcelain to the margin. The current study observed no difference between the marginal adaptation of a casting witb a 0.1 or 0.4 mm collar. However, when the width was increased to 0.8 mm, a significant improvement in the fit was discerned. This suggests that there is a minimum width that must be achieved to effectively resist the marginal distortion that occurs during the thermal cycling process. The present study was undertaken to design a methodology to facilitate the direct measurement and observation of casting distortion that results from the thermal cycling process by (1) eliminating the casting variables through fabrication of a measuring die directly to the already formed casting and (2) simplifying the geometry of the preparation and casting to minimize the opposing axial wall effects and allow for a cross-seetional type view of the casting distortion (Figs. 4 and 5). Additionally, the sensitivity to detect statistically significant results would be enhanced by (1) improving the ease and accuracy of mea.surement with flat surfaces and (2) starting with castings that fit their respective dies with no observable marginal opening (Figs. 4 and 5). This allowed for the observation of alloy thermal cycling distortion in isolated areas of the casting geometry (facial margin). The altered geometry of the study specimens prevents conclusions as to how the overall fit of a three-dimensional casting will be affected. This could be better defined by applying the resin die technique to thermal cycling of full crown castings. However, any distortion that occurs is potentially deleterious and should be minimized. 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 (Figs. 6 and 7); the only contact with the die was in the central area of the casting. This suggests that some consistent etiologie force is present in all the castings. This may be due to (1) casting-induced stresses (for example, cooling patterns of the alloy), (2) sprue location on the side opposite the fit surface, (3) geometry of the specimens (for example, the bulk of the keying groove on the fit surface), and (4) the effect of surface treatment of the alloy (for example, cold working of the surface with stones in preparation for porcelain application). Increased understanding of the variables that cause the consistent direction of distortion may help elucidate the factors that contribute to the distortion. The significantly greater distortion of the occlusal locations for all specimens suggests that the facial collar provided resistance to the distortion. This may have been the MAY 1992
VOLUME
67
NUMBER
5
DISTORTION
OF METAL
CERAMIC
ALLOYS:
PART
I
Fig. 6. Photomicrograph showing distortion (facial margin opening) of the metal/ceramic alloy after the initial thermal cycling (oxidation) of a casting. (Original magnification x20,) A, Alloy; R, resin die.
result of the second plane geometry the collar introduced to an otherwise flat casting. Other geometric considerations (for example, gingival curvature) may also play a significant role in resisting or promoting distortion. The timing of the observed thermal cycling distortion was such that all of the significant loss in adaptation occurred during the initial thermal cycling (oxidation) of the alloy. Subsequent porcelain application had no significant effect on the distortion. CLINICAL
IMPLICATIONS
Based on the results of this study, thinning the facial collar beyond 0.8 mm could have the potential to cause increased marginal openings in metal ceramic restorations. However, other three-dimensional geometric determinants of actual clinical castings may also play a major role in distention resistance, thereby altering the actual collar width that is necessary to resist clinical marginal distortion. The decision to eliminate, minimize, or maximize facial collar widths is based on many factors including (1) esthetic concerns, (2) periodontal considerations, and (3) restorative considerations (for example, remaining coronal tooth structure). When metal frameworks are “tried in” before porcelain application, it is suggested that the initial thermal cycling (oxidation) of the alloy already be completed. This will provide a better indication of the fit of the finished restoration.
The effect of therms.1 cycling on distortion of a metal/ceramic alloy was examined for three facial collar widths on flat one-walled specimens (0.1, 0.4, and 0.8 mm). It was determined that: 1. All of the one-walled metal ceramic castings distorted during the thermal cycling process. THE
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DENTISTRY
Fig. 7. Photomicrograph showing distortion (axial wall opening) of the metal/ceramic alloy after the initial thermal cycling (oxidation) of a casting. (Original magnification X5.)
2. All of the distortion occurred during the first thermal cycling of the alloy (oxidation cycle). 3. No significant distortion resulted from the opaque and body porcelain application or glazing. 607
CAMPBELL
4. The 0.8 mm facial metal collar had significantly less distortion than the 0.1 or 0.4 mm collars at 2 of the 10 sites measured (facial margins) for these specimens. 5. Clinical restorations, because of their multiwalled configuration, may show different patterns of distortion. We thank Dr. Russel Giordano
7. Buchanan WT, Svare CW, Turner KA. The effect of repeated firings and strength on marginal distortion in two ceramometal systems. J PROSTHET DENT 1981;45:502-6.
8. DeHoff PH, Anusavice KJ. Effect of metai design on marginal distortion of metal-ceramic crowns. J Dent Res 1984;63:1327-31. 9. 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.
for his assistance.
REFERENCES 1. Mumford G. The porcelain-fuse-to-metal resoration. Dent Clin North Am 1965;14:241-9. 2. 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. 3. Shillingburg HT, Hobo S, Fisher DW. Preparation design and marginal distortion in porcelain-fused-to-metal restorations. J PROSTHETDENT 1973;29:276-84. 4. Iwashita AH, Kuriki H, Hasuo T, et al. Studies on dimensional accuracy of porcelain fuse to precious metal crowns. The influence of the porcelain to the metal coping on the porcelain fusing procedure. Shigaku 1977;65:110-25. 5. Faucher RR, Nicholls JI. Distortion related to margin design in porcelain-fused-to-metal restorations. J PROSTHETDENT 1980;43:149-55. 6. Bridger DV, Nicholls ,JI. Distortion of ceramometal fixed partial dentures during the firing cycle. J PROSTHETDENT 1981;45:507-14.
Bound
AND PELLETIER
volumes
available
10. McLean JW. The science and art of dental ceramics. Vol 1. Chicago: Quintessence Publishing Co, 1979:273-80. Il. Belser UC, MacEntee MI, Richter WA. Fit of three porcelain-fused-tometal marginal designs in viva: a scanning electron microscope study. J PROSTHET DENT 1985;53:24-9. 12. Strating H, Pameijer CH, Gildenhuys RR. Evaluation of the marginal integrity of ceramometal restorations. Part I. J PRO~THET DENT 1981;46:59-65.
13. Pilo R, Cardash HS, Baharav H, Helft M. Incomplete seating of cemented crowns: a literature review. J PROSTHETDENT 1988;59:429-33. 14. Pascoe DF. Analysis of the geometry of finishing lines for full crown restorations. J PROSTHET DENT 1978;40:157-62. Reprint requeststo: DR. STEPHEN D. CAMPBELL HARVARD SCHOOL OF DENTAL MEDICINE 188 LONGWOOD AVE. BOSTON, MA 02115
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MAY 1992
VOLUME
67
IiUMBER
5
distortion
of metal
ceramics:
Part
Stephen D. Campbell, DDS, MMSc,” and Lionel B. Pelletier, DDS, MMSc, DMDb Harvard
Schooi
of Dental Medicine, Boston, Mass.
The complex three-dimensional geometry of conventional full crown restorations severely complicates the study of thermal cycling distortion in metal ceramic castings. A simplified experimental geometry was developed to (I) maximize the measuring sensitivity, (2) eliminate the casting variables, (3) allow the direct measurement of casting distortion, and (4) evaluate the thermal cycling distortion of a wide range of metal collar widths. It was found that all of the one-walled castings distorted during the initial thermal cycling (oxidation) of the alloy. There was no significant distortion associated with porcelain application or glazing. The castings with an 0.8 mm metal collar had significantly less distortion than those with 0.1 and 0.4 mm collars at 2 of the 10 sites measured (inferior or facial margins). (J PROSTHET DENT 1992;6’7:603-8.)
anagement of the facial marginal area of a metal ceramic restoration can be an esthetic problem for even the most skilled clinicians and technicians. This is primarily related to the metal collar of these restorations. In an attempt to maximize the esthetics, the facial metal collar is frequently thinned or eliminated. As a result, the marginal adaptation ofthe completed restoration could be adversely affected. This is largely because the “as-cast” fit of metal ceramic restorations deteriorates during the high temperature firing cycles that are employed for porcelain veneer application.1-g Contrary to popular belief, there appears to be few articles in the literature supporting the degradation of fit with the thinning of metal collars. It seemsthat this concept has been extrapolated from studies that examined the intluence of the marginal preparation design (for example, chamfer, shoulder) and did not isolate metal collar width as a variable.3j j, lo Several studies7-i2 have specifically evaluated the effect that porcelain proximity to the restoration margin (metal collar width) has on the thermal cycling distortion of the finished prosthesis. All of these studies concluded that metal collar width does not affect the marginal adaptation of -the completed restoration. The purpose of the current study was to (1) create an ex-
Supported in part by a grant from the William F. Milton Fund, Harvard Medical School, Boston, Mass. Presentedat the Greater New York Academyof Prosthodontics, New York, N.Y. =AssociateProfessorof Prosthetic Dentistry, Co-Director of Postdoctoral Prosthodontics. bInstructor of Prosthetic Dentistry. 10/l/36186
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Fig.
1. Simplified geometry of master die design.
perimental design that would maximize the measuring sensitivity, eliminate the casting variables, and allow for the direct measurement of casting distortion and (2) apply this methodology to the evaluation of the thermal cycling distortion of a range of collar widths.
MATERIALS
AND METHO
A master die was machined to simplify the geometry sf a conventional preparation (Fig. 1). The die represented one wall (6 x 6 mm) of a full crown preparation with a 90degree shoulder (1 mm). A vertical orientation groove wa.s machined along the axial wall to aid in the accurate reseating of the castings. A vinyl polysiloxane impression material (Express Light Body, 3l’vI Dental Products, St. Paul, Minn.) was used to duplicate the machined preparation in an improved die stone (Die-Keen, Modern Materials, Columbus, Ohio). Wax patterns were then fabricated with 28 gauge casting sheet wax (Kerr Dental Mfg. Co., Detroit, Mich.). This resulted in uniform 0.4 mm thick axial walls. Three specimen groups were completed with 11 samples
CAMPBELL
AND
PELLETIER
Fig. 2. Experimental castings with 0.1, 0.4, and 0.8 mm facial collars on their respective resin measuring dies.
The castings were then replaced on their respective resin dies and marginal openings were measured at 10 predetermined locations (Fig. 3). A microscope with a filar eyepiece (American Optical, Buffalo, N.Y.) accurate to 1 Km was used for this purpose (100 power). Ten specimens each of the three collar widths were oxidized in air at 1900” F and were remeasured (Table I). Opaque was applied in two stages, followed by two applications of body porcelain and a final glaze firing. Measurements were completed and were recorded on the resin dies after each of the firing cycles. The remaining three samples were maintained as controls to observe any changes in fit that might have resulted from dimensional instability of the resin or from the process of taking the castings on and off the dies. RESULTS Fig. 3. Location of the 10 measurement sites. Locations 7 and 8 represent the facial marginal areas (dark, resin die; light, casting).
each of an 0.1, 0.4, and 0.8 mm metal collar (33 total). Castings were then formed with a gold-palladium alloy (Olympia, Jelenko, Armonk, N.Y.) according to manufacturer-recommended techniques. This resulted in a uniform geometry, size, and thickness for each of the specimens and collar width variables (Fig. 2). All castings were air-abraded with a 50 pm alumina oxide abrasive (Belle de St. Claire, Van Nuys, Calif.). Direct dies were then made for each of the specimens by pouring a measured volume of 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 were trimmed and progressively polished with 320, 400, and 600 alumina oxide abrasion paper (Fig. 2). All specimens and dies were numbered and separated.
604
Before the first thermal cycling of the alloy (oxidation), all 33 castings fit their respective resin dies with no observable marginal openings at the limits of the light microscope (Figs. 4 and 5). The three control specimens showed no change during the course of the investigation. All 30 of the one-walled experimental samples distorted upon the first thermal cycling process (oxidation). The mean marginal opening for all specimens and measurement sites was 14 pm following the initial oxidation of the alloy (Figs. 6 and 7). The standard deviations of individual measurement sites and sample groups were in the 5 to 9 ,umrange throughout, with most being around 6 to 7 pm. The initial oxidation cycle of the 0.1, 0.4, and 0.8 mm collar groups resulted in a mean opening of 14, 16, and 12 pm, respectively, for all measurement sites (Table I). Statistical evaluation by analysis of variance (ANOVA) and multiple comparison testing (Tukey test) was complete determine if there were any significant differences within or between the three collar width groups or 10 measurement sites. The analysis revealed no significant differences
AMAY
1992
VOLUME
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NUMBER
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DISTORTION
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CERAMIC
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PART
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Fig. 4. Photomicrograph showing the perfect facial margin adaptation of a casting on its resin measuring die. (Original magnification x20.) A, Alloy; R, resin die.
Table I. Mean marginal openings (in microns) following initial thermal cycling (oxidation) Measurement Collar width (mm)
location
1
2
3
4
5
6
7
8
9
10
Mean
0.1
10
0.4 0.8
15 13
I1 14 11
19 19 13
16 19 14
14 16 14
15 16 15
13 16
14 13
8*
7*
14 17 12
14 18 11
14 16 12
*Statistically significant.
between the 0.1,0.4, and 0.8 mm collar width groups except for the facial collar width measurement sites (locations 7 and 8). The 0.8 mm collar mean opening of 8 pm was significantly better than the mean of 14 ym for the 0.1 mm Cp< 0.05) (Q = 3.4) and 0.4 mm (p < 0.025) (Q = 3.9) collar widths. There was no significant difference between the 0.1 and 0.4 mm collar widths. Combining the three collar width groups revealed no significant difference between measurement locations except the occlusal sites 3 and 4. The occlusal locations had a significantly (p < 0.05) greater opening (17 pm) than the mean of the remaining measurement sites (12 Km). The mean openings for all specimens at all sites for the first and second opaque applications, first and second body porcelain applications, and glaze firings were 15,15,14,15, and I5 pm, respectively, Statistical analysis revealed no significant change for any measurement site or collar width group following the initial oxidation distortion of the castings. The measurement sites and specimen group mean values for all thermal cycling (porcelain application) subsequent to the initial oxidation were, for the most part,
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identical to the oxidation values, and in no case did they vary by more than 1 or 2 pm.
The results of the present study differ from prior evaluations of the effect of collar width on marginal adaptation of metal ceramic restorations.7-12 This may in part be the result of the complex three-dimensional geometry of cast full veneer restorations. Many of the earlier studies ut,ilize direct external viewing of crown margins, since studying the effect of thermal cycling on metal ceramic alloys is most easily accomplished with a nondestructive methodology. This type of examination of the fit of a restoration is a measurement of the seat of the casting on the preparation. As described by Pilo et aLi3 and Pascoe,‘” the opposing wall geometry of a casting can result in large changes in the seat of a restoration, with only minor changes along the axial walls. FOF example, given a lo-degree axial wall taper, a change of 1 pm along an axial wall will result in the casting not seating by an additional 11 pm. Therefore measurements of the seat of a restoration at the external margins
605
CAMPBELL
Fig. 5. Photomicrograph showing the perfect axial wall adaptation of a casting on its resin measuring die. (Original magnification X5.)
are not a direct measurement of the alloy distortion. The direction of the distortion (toward or away from the axial walls), the initial adaptation of the casting to the axial walls, the magnitude of axial wall distortion, and the preparation geometry (degree of taper) can greatly affect the seating of the casting. This introduces variables that make an accurate assessment of the actual alloy distortion during thermal cycling difficult. Evaluating the distortion of a localized area of the casting geometry (marginal collar) becomes even more difficult. The direct view measurement of the seat of a cast restoration can also introduce errors caused by (1) the poor fit of the initial castings on the measuring dies, (2) the large standard deviation of the initial casting fit that results from the casting variables, (3) the technical difficulty and reproducibility of measuring the external curved surface of the margin of a crown, (4) the differential seating inaccuracies that result from taking the casting on and off the measuring die, and (5) the complex three-dimensional geometry, as previously described. These factors can introduce a large standard deviation in the measurements and may prevent detection of statistically significant differences in the data. The earlier studies7-l2 demonstrated as much as a 26 pm increase in the marginal seat opening when the facial collars were thinned. However, no statistical significance was found because of the large standard deviations. In addition, previous studies of collar widths may not 606
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have examined a sufficient range of sizes to isolate an effect on marginal adaptation during the thermal cycling process. One study9 compared a 0.5 mm collar with castings with a 0 mm collar. The 0.5 mm collar width may have been inadequate to demonstrate the decreased distortion observed in the thicker 0.8 mm collar of the current study. Another study7 compared a 0.75 mm facial collar with the thicker lingual collar on the same restoration. The 0.75 mm width may have been too great to identify a relationship between distortion and the proximity of porcelain to the margin. The current study observed no difference between the marginal adaptation of a casting witb a 0.1 or 0.4 mm collar. However, when the width was increased to 0.8 mm, a significant improvement in the fit was discerned. This suggests that there is a minimum width that must be achieved to effectively resist the marginal distortion that occurs during the thermal cycling process. The present study was undertaken to design a methodology to facilitate the direct measurement and observation of casting distortion that results from the thermal cycling process by (1) eliminating the casting variables through fabrication of a measuring die directly to the already formed casting and (2) simplifying the geometry of the preparation and casting to minimize the opposing axial wall effects and allow for a cross-seetional type view of the casting distortion (Figs. 4 and 5). Additionally, the sensitivity to detect statistically significant results would be enhanced by (1) improving the ease and accuracy of mea.surement with flat surfaces and (2) starting with castings that fit their respective dies with no observable marginal opening (Figs. 4 and 5). This allowed for the observation of alloy thermal cycling distortion in isolated areas of the casting geometry (facial margin). The altered geometry of the study specimens prevents conclusions as to how the overall fit of a three-dimensional casting will be affected. This could be better defined by applying the resin die technique to thermal cycling of full crown castings. However, any distortion that occurs is potentially deleterious and should be minimized. 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 (Figs. 6 and 7); the only contact with the die was in the central area of the casting. This suggests that some consistent etiologie force is present in all the castings. This may be due to (1) casting-induced stresses (for example, cooling patterns of the alloy), (2) sprue location on the side opposite the fit surface, (3) geometry of the specimens (for example, the bulk of the keying groove on the fit surface), and (4) the effect of surface treatment of the alloy (for example, cold working of the surface with stones in preparation for porcelain application). Increased understanding of the variables that cause the consistent direction of distortion may help elucidate the factors that contribute to the distortion. The significantly greater distortion of the occlusal locations for all specimens suggests that the facial collar provided resistance to the distortion. This may have been the MAY 1992
VOLUME
67
NUMBER
5
DISTORTION
OF METAL
CERAMIC
ALLOYS:
PART
I
Fig. 6. Photomicrograph showing distortion (facial margin opening) of the metal/ceramic alloy after the initial thermal cycling (oxidation) of a casting. (Original magnification x20,) A, Alloy; R, resin die.
result of the second plane geometry the collar introduced to an otherwise flat casting. Other geometric considerations (for example, gingival curvature) may also play a significant role in resisting or promoting distortion. The timing of the observed thermal cycling distortion was such that all of the significant loss in adaptation occurred during the initial thermal cycling (oxidation) of the alloy. Subsequent porcelain application had no significant effect on the distortion. CLINICAL
IMPLICATIONS
Based on the results of this study, thinning the facial collar beyond 0.8 mm could have the potential to cause increased marginal openings in metal ceramic restorations. However, other three-dimensional geometric determinants of actual clinical castings may also play a major role in distention resistance, thereby altering the actual collar width that is necessary to resist clinical marginal distortion. The decision to eliminate, minimize, or maximize facial collar widths is based on many factors including (1) esthetic concerns, (2) periodontal considerations, and (3) restorative considerations (for example, remaining coronal tooth structure). When metal frameworks are “tried in” before porcelain application, it is suggested that the initial thermal cycling (oxidation) of the alloy already be completed. This will provide a better indication of the fit of the finished restoration.
The effect of therms.1 cycling on distortion of a metal/ceramic alloy was examined for three facial collar widths on flat one-walled specimens (0.1, 0.4, and 0.8 mm). It was determined that: 1. All of the one-walled metal ceramic castings distorted during the thermal cycling process. THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 7. Photomicrograph showing distortion (axial wall opening) of the metal/ceramic alloy after the initial thermal cycling (oxidation) of a casting. (Original magnification X5.)
2. All of the distortion occurred during the first thermal cycling of the alloy (oxidation cycle). 3. No significant distortion resulted from the opaque and body porcelain application or glazing. 607
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4. The 0.8 mm facial metal collar had significantly less distortion than the 0.1 or 0.4 mm collars at 2 of the 10 sites measured (facial margins) for these specimens. 5. Clinical restorations, because of their multiwalled configuration, may show different patterns of distortion. We thank Dr. Russel Giordano
7. Buchanan WT, Svare CW, Turner KA. The effect of repeated firings and strength on marginal distortion in two ceramometal systems. J PROSTHET DENT 1981;45:502-6.
8. DeHoff PH, Anusavice KJ. Effect of metai design on marginal distortion of metal-ceramic crowns. J Dent Res 1984;63:1327-31. 9. 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.
for his assistance.
REFERENCES 1. Mumford G. The porcelain-fuse-to-metal resoration. Dent Clin North Am 1965;14:241-9. 2. 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. 3. Shillingburg HT, Hobo S, Fisher DW. Preparation design and marginal distortion in porcelain-fused-to-metal restorations. J PROSTHETDENT 1973;29:276-84. 4. Iwashita AH, Kuriki H, Hasuo T, et al. Studies on dimensional accuracy of porcelain fuse to precious metal crowns. The influence of the porcelain to the metal coping on the porcelain fusing procedure. Shigaku 1977;65:110-25. 5. Faucher RR, Nicholls JI. Distortion related to margin design in porcelain-fused-to-metal restorations. J PROSTHETDENT 1980;43:149-55. 6. Bridger DV, Nicholls ,JI. Distortion of ceramometal fixed partial dentures during the firing cycle. J PROSTHETDENT 1981;45:507-14.
Bound
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10. McLean JW. The science and art of dental ceramics. Vol 1. Chicago: Quintessence Publishing Co, 1979:273-80. Il. Belser UC, MacEntee MI, Richter WA. Fit of three porcelain-fused-tometal marginal designs in viva: a scanning electron microscope study. J PROSTHET DENT 1985;53:24-9. 12. Strating H, Pameijer CH, Gildenhuys RR. Evaluation of the marginal integrity of ceramometal restorations. Part I. J PRO~THET DENT 1981;46:59-65.
13. Pilo R, Cardash HS, Baharav H, Helft M. Incomplete seating of cemented crowns: a literature review. J PROSTHETDENT 1988;59:429-33. 14. Pascoe DF. Analysis of the geometry of finishing lines for full crown restorations. J PROSTHET DENT 1978;40:157-62. Reprint requeststo: DR. STEPHEN D. CAMPBELL HARVARD SCHOOL OF DENTAL MEDICINE 188 LONGWOOD AVE. BOSTON, MA 02115
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MAY 1992
VOLUME
67
IiUMBER
5
