Evaluation of glass-cermet cores under cast crowns J.P. DeWald1 C.J. Arcoria1 J.L. Ferracane 2
~Department of Operative Dentistry Baylor College of Dentistry 3302 Gaston Avenue Dallas, Texas 75246 2Department of Dental Materials Science The Oregon Health Sciences University 611 S.W. Campus Dr. Portland, Oregon 97201 Received June 29, 1989 Accepted March 6, 1990 Dent Mater 6:129-132, April, 1990 Abstract-The goal of this study was to
compare two core materials-glasscermet and amalgam-in terms of crown retention when significant tooth structure is missing. The glass-cermet's ability to bond to teeth with and without the aid of retentive pins and/or grooves was examined. The effects of two storage solutions, formalin and sodium azide, on the bond of the glass-cermet were also evaluated. Sixty human molars were obtained and stored in either formalin or sodium azide. Thirty molars were prepared with one wall of tooth structure remaining, and the other 30 were prepared with two walls. Before being restored with glass-cermet, the preparations received either no retention, retentive grooves and pins, or retentive grooves only. Each preparation restored with amalgam received pins and grooves. Type III gold crowns were cemented in vitro with glass-ionomer luting agent, thermocycled 2500 times (5-50°C), and then removed under a tensile force. The glass-cermet material may be comparable to amalgam and has adequate adhesion to the tooth, without additional retention, when at least two walls of the tooth remain. However, when only one wall of the tooth remains, the amalgam core is superior under tensile forces. There was a significant difference between the sodium azide and formalin storage solutions with regard to bond strength in one group (two walls, no pins, no grooves). In general, the cores placed on teeth stored in sodium azide exhibited tensile bond strengths lower than those stored in formalin.
lass-cermet evolved from the glass-ionomer cements introduced in the early 1970's by Wilson and Kent (1972). The main advantages of glass ionomer include the ability of the material to bond chemically to tooth structure by an ionic interaction with the calcium ions in enamel and dentin and to leach fluoride. However, the cement does not possess adequate strength and stiffness for routine use as a loadbearing restorative. The addition of a metal alloy was the first step taken in an attempt to strengthen the glassionomer material so that it could be used as a core build-up (Simmons, 1983). Based on clinical observaLions, this admixture has been widely promoted (CRA Newsletter, 1983). It has been recommended that its clinical use, however, be limited to core build-ups where the material will receive support from tooth structure or pins (Arcoria et al., 1988). Recent attempts to improve the properties of the glass ionomers led to the development of the glass-cermet (McLean and Gasser, 1985). The silver powder in the glass-cermet cement is coated by glass through a process of high-temperature sintering. This has improved the abrasion resistance to a level which appears to be comparable to that of amalgam (Wilson and Prosser, 1984; McLean and Gasser, 1985). In addition, McLean and Gasser (1985) have clinically observed an improvement in fracture resistance. The compressive strength has been shown to be relatively high, but the material remains weak in response to tensile stress (Moore et al., 1985). Overall, the mechanical properties remain inferior to those of amalgam (Phillips, 1982; Swift, 1986), and it has been recommended that the use of cermets be limited to low-stress-bearing areas (McLean and Gasser, 1985; Marker eta/,, 1985; Swift, 1986; Walls et aL, 1987). Although McLean reported no unusual masticatory wear or marginal breakdown in the glass-
G
cermet restorations after two years in vivo, further research has been advocated before its use as a longt e r m restorative material can be recommended (McLean and Gasser, 1985; Moore et al., 1985; Jendresen et al., 1987). Despite this, glass-cermet has been promoted for restoration of Classes I, II, and III lesions in primary teeth (Croll and Phillips, 1986), Classes I and II lesions in permanent teeth (McLean and Gasser, 1985), and for core build-ups (McLean and Gasser, 1985; Swift, 1986; Donovan and Daftary, 1987; Taleghani and Leinfelder, 1988). Because of the low tensile s~'ength and fracture toughness of the material, mechanical retention is deemed to be important for core build-ups (Swift, 1986; CroU and Phillips, 1986; Donovan and Daftary, 1987). Thornton et al. (1986) recommends mechanical retention in clinical situations based on lower bond strengths obtained from a glass-cermet as compared with its glass-ionomer counterpart. Building up most of the coronal portion of a tooth with glass-cermet has also been contra-indicated because of the probability of failure under occlusal loads (Donovan and Daftary, 1987; Taleghani and Leinfelder, 1988). The objective of this study was to compare the tensile bond strengths of glass-cermet and amalgam when both materials were placed in core build-up preparations. The type of retention form, if any, required to prevent dislodgement of the glassc e r m e t material was also d e t e r mined. Finally, the effects of two storage solutions on the chemical bond between glass-cermet and tooth structure were evaluated. MATERIALS AND METHODS
Sixty, freshly extracted, non-carious molars were obtained. Forty teeth were stored in a dilute phosphate-buffered formaldehyde solution and the other 20 in a 2% sodium aside solution. This comparison was
Dental Materials/April 1990 129
PreparaUonA
Preparation B
Fig. Internal preparation designs.
made to determine the extent to which formaldehyde might alter the dentin surface and reduce the adhesive strength of the cermet to the tooth. Sodium azide has been used in another study for storing teeth prior to the evaluation of dentin bonding (Pashley et aL, 1988). The teeth were cleaned of debris and the roots embedded in epoxy resin to within 2 mm of the cemento-enamel junction. The samples were then placed in a jig and the teeth prepared to a chamfer margin with a No. 257.SB diamond (Premier Dental Co., Norristown, PA) in a highspeed handpiece that was placed in a Ney handpiece holder (J.M. Ney, H a r t f o r d , CT). Each tooth measured approximately 4 n~n in height, 8 mm in length, and 6 mm in width, with a taper ranging from 5 to 5.7°. Internal preparations, Preparation A (n = 30) arid Preparation B (n = 30), were then made with a No. 556 carbide bur (Fig.). The preparations received either no retention, retentive grooves only, or retentive grooves and pins. Each preparation was then restored with either Ketac-Silver glass-cermet (C) (ESPE-Premier Dental Products, Lynbrook, NY) or Tytin amalgam (A) (Kerr/Sybron, Emeryville, CA). Retention and restorative materials were placed as in Table 1. R e g u l a r s e l f - s h e a r i n g TMS (Whaledent, Int., New York, NY) pins were placed 1 mm inside each line angle. Retentive grooves were placed with a No. 1/4 round bur 2 mm in length where the axial wall met the existing adjacent lateral wall. The teeth were restored and tl"immed following manufacturer's directions. A chamfer margin was re-established. Each core was impressed with heavy-bodied vinyl polysiloxane ma-
terial ( C u t t e r P e r f o u r m , C u t t e r Dental, Miles Laboratories, Inc., Berkeley, CA) and stored in de-ionized water during crown fabrication. A die and wax pattern were made with a loop attached to the coronal portion, and a Type III gold crown (G-Cast, Degussa Dental, Long Island, NY) was cast for each core. Before crown cementation, each core was measured with a Boley gauge to the nearest 0.1 mm to determine surface area and taper. Crowns were fitted and then cemented with glassionomer Type I luting cement (GC Dental Industrial Corp., Tokyo, Japan) within one week after preparation and impression. The crowns were held under a constant load by a static weight of 2 Kg for three min. Two min later, the excess cement was removed, the samples placed in a humidor, and stored for 24 h at 37°C. They were then thermocycled in deionized water at between 5 and 50°C for 2500 cycles (30-second dwells in each bath). The samples were tested in tension in a Universal Testing Machine (Instron Corp., Canton, MA) at a cross-head speed of 2.54 mm/ rnin until failure (any decrease in load from the peak load). The results were analyzed by ANOVA (p = 0.05) and Scheff~'s test. A Student t test (p = 0.05) was used to evaluate the difference between storage treatments.
RESULTS The failure modes of the Tooth/Core/ Crown assembly are shown in Table 2. All 10 amalgam samples (A: Prep A and Prep B) exhibited an adhesive failure of the glass-ionomer cement. The crown pulled cleanly from the core in all samples. All 10 glass-cermet samples (CI: Prep A and Prep B), restored after pins and grooves were added for retention, exhibited good adhesion to the tooth, ~e., either the crown pulled cleanly from the core (8) or the core failed above the height of the pin. Of the 20 glass-cermet samples (C2: Prep A and Prep B) restored after two retentive grooves were placed, 19 cores, with or without tooth attached, separated from the preparation. In all cases, the core fractured at the retentive grooves, leaving some material within the grooves. One sample (Prep A) exhibited good adhesion of the core to
130 DeWALD et al./EVALUATION OF GLASS-CERMET CORES UNDER CAST CROWNS
the tooth, with over 80% of the core remaining a t t a c h e d to the tooth structure. Of the 20 glass-cermet samples restored with no retention (C3: Prep A and Prep B), 18 showed poor adhesion of the core to tooth s t r u c t u r e . One sample (Prep B) showed good adhesion,/.e., over 80% of the core remained attached to the tooth structure. One sample (Prep A) showed good adhesion of the core to the tooth structure,/~e., the crown pulled cleanly from the core. The mean values of bond strengths of the casting on the glass-cermet and amalgam cores are shown in Table 3. There were no significant differences between the groups for Prep A (two walls). Within the groups of P r e p B (one wall), the amalgam showed significantly more retention than the glass-cermet cores restored without pins (C2 and C3). There was no significant difference between the amalgam and the glass-cermet samples restored with pins (C1). Retentive s t r e n g t h s were significantly greater for glass-cermet when two walls (Prep A) of tooth remained as compared with one wall (Prep B), except when pins were placed. The sodium azide samples exhibited tensile bond strengths lower than those of the formalin samples for Preparation A (C3: no retention).
DISCUSSION This study supports earlier studies that have expressed reservations over building up most of the coronal portion of a tooth with glass-cermet (Donovan and Daftary, 1987; Taleghani and Leinfelder, 1988) and that suggest the use of mechanical retention (Croll and Phillips, 1986; Swift, 1986; Donovan and Daftary, 1987). When two walls remained, the glasscermet was adequately retained in the preparation under tensile forces as compared with amalgam. This was regardless of the type of retention placed. Even when no retention was placed, these samples adequately retained the glass-cermet restoration. However, when only one wall of tooth structure remained, the glass-cermet was not retained as well as the amalgam, except when pin-andgroove retention was used. From these results, it appears that retention in the form of pins and grooves is necessary when there is a limited
amount of tooth structure remaining to adhere to the glass-cermet. When the glass-cermet samples only were compared, the retentive strengths were significantly greater for the preparations having two walls as opposed to one wall. It appears that the material needs the support from surrounding tooth structure. This may also be due to the more available tooth structure for the glassionomer cement when the castings were cemented. This, however, would not explain why the amalgam samples with two walls of tooth remaining did not bond significantly better than did the amalgam samples with one wall. A possible explanation may be that the surface of the amalgam after crown preparation was as retentive to the luting agent as was the surface of the tooth. These results strongly suggest that the bonding of the glass-cermet is comparable with that of amalgam when two walls of tooth structure remain, r e g a r d l e s s of retention. These results, however, are limited to tensile tests. A further study needs to be conducted that utilizes tests in compression. The formalin and sodium azide storage solutions did not have an effect on bond strengths except in one group of samples where the sodium azide group was lower. The explanation for this is unclear. It was theorized that the formalin may affect the organic material of the dentin, thereby decreasing the bond of the glass-cermet or glass-ionomer to the tooth s t r u c t u r e . This study, however, did not support that hypothesis.
TABLE 1 PREPARATION, RESTORATIVE MATERIAL, RETENTION, AND STORAGE SOLUTION PRIOR TO PREPARATION PREP A 2 walls
PREP B 1 wall
C1 1 pin 2 grooves formalin n = 5
2 pins 2 grooves formalin n = 5
ACKNOWLEDGMENT
The authors would like to express
C3
2 grooves formalin n = 5 Na azide n = 5
formalin n = 5 Na azide n = 5
2 grooves formalin n = 5 Na azide n = 5
formalin n = 5 Na azide n = 5
A 1 pin 2 grooves formalin n = 5
2 pins 2 grooves formalin n = 5
TABLE 2 FAILURE MODES OF TOOTH/CORE/CROWN ASSEMBLY (FORMALIN AND SODIUM AZlDE) Prep A (2 walls; n = 30) Cement (3) 60% (adhesive failure)
01 Pin(s) 2 grooves
C2 No pin(s) 2 grooves
C3 No pin(s) No grooves
Core (above pins) (2)
40%
Tooth fracture/ core failure (3)
30%
Prep B (1 wall; n = 30) Cement (5) 100% (adhesive failure)
Tooth fracture/ core failure (9)
90%
Core/tooth (6) Core/core (1)* (cohesive failure)
60% 10%
Core/tooth
(1)
10%
Cement
10%
Tooth fracture/ core failure (8)
80%
(1)
Tooth fracture/ core failure (6) Core/tooth (3)
60% 30%
Core/tooth (1) Core/core (1)* (cohesive failure)
10% 10%
100% A Cement (5) 100% Cement (5) Pin(s) (adhesive failure) (adhesive failure) 2 grooves *Over 80% of core adhered to tooth structure. Samples had been stored in sodium azide prior to preparation. TABLE 3 MEAN VALUE* OF BOND STRENGTH (MPa) OF CASTING ON GLASS-CERMETAND AMALGAM CORES WITH GLASS-IONOMER CEMENT UTILIZING DIFFERENTMEANS OF RETENTION
CONCLUSIONS
As a core material, the glass-cermet is comparable to amalgam in tensile retentive strength when at least two walls of tooth structure remain. This was observed even when no retention was placed. However, when only one wall of tooth remains, the amalgam core is superior. It is therefore concluded that at least 50% of the tooth should be present to ensure adequate retention of glass-cermet as a core build-up material.
02
PREP A 2 walls
formalin
C1 2.69 - 0.50
sodium azide PREP B
formalin
2.39 __ 0.21
C2 2.73 _ 0.42
C3 2.90 ± 0.27
2.51 __ 0.19
2.55 ± 0.24
1.88 ± 0.31
2.14 _+ 0.27
A 2.96 _ 0.50
2.92 __ 0.28
1 wall 1.59 ± 0.22 sodium azide *Values are mean ± standard deviation for five (5) samples.
their gratitude to E S P E - P r e m i e r Dental Products, Lynbrook, NY, for the materials used in this study.
1.88 ± 0.29
REFERENCES A~CORZA, C.J.; DEWALD, J.P.; MOODY, C.R.; and FERRACANE, J.L. (1988): Effects of Thermocycling on Amalgam
Dental Materials~April 1990 131
and Alloy-Glass Ionomer Cores Luted to Cast Gold Crowns, Dent Mater 4: 155-157. CLINICAL RESEARCH ASSOCIATES NEWSLETTER (1983) 7: "Subject: Tooth
Buildup Materials and Techniques", p. 11. CROLL, T.P. and PHILLIPS,R.W. (1986): Glass Ionomer-Silver Cermet Restorations for Primary Teeth, Quint Int 17: 607-615. DONOVAN, T.E. and DAFTAR¥, F. (1987): Clinical Use of Glass-Ionomer Restorative Materials, Compend Con~in Educ Dent 8: 180-190. JENDRESEN, M.D.; KLOOSTER, J.; PHILLIPS, R.W.; SCHALLHORN,R.G.; and SULLrVAN, M.M. (1987): Report of the Committee on Scientific Investigation of the American Academy of Restorative Dentistry, J Prosthet Dent 57: 750-752. MARKER, V.A.; FERRACANE, J.L.; MILLER, D.; and WONG, N. (1985): Characterization of Metal-reinforced
Glass-ionomer Restorative Materials, J Dent Res 64: 297, Abstr. No. 1101.
MCLEAN, J.W. and GASSER, O. (1985): Glass-Cermet Cements, Quint Int 16: 333-343. MOORE, B.K.; SWARTZ, M.L.; and PHILLIPS, R.W. (1985): Abrasion Resistance of Metal-reinforced Glass-ionomer Materials, J Dent Res 64: 371, Abstr. No. 1766. PASHLEY, D.H.; DERKSON, G.D.; TAO, L.; DERKSON, M.; and KALATHOOR,S. (1988): The Effects of a Multi-Step Dentin Bonding System on Dentin Permeability, Dent Mater 4: 60-63. PHILLIPS, R.W. (1982): Science of Dental Materials, 8th ed., Philadelphia: W.B. Saunders, p. 321. SIMMONS, J.J. (1983): The Miracle Mixtnre: Glass Ionomer and Alloy Powder, TX Dent J 100: 6-12. SWIFT, E.J. (1986): Glass Ionomers: A Review for the Clinical Dentist, Gen Dent 34: 468--471. TALEGHANI, M. and LEINFELDER, K.F.
132 DeWALD et at/EVALUATION OF GLASS-CERMET CORES UNDER CAST CROWNS
(1988): Evaluation of a New Glass Ionomer with Silver as a Core Buildup Under Cast Restorations, Quint Int 19: 19-24. THORNTON, J.B.; RETIEF, D.H.; and BRADLEY, E.L. (1986): Fluoride Release from and Tensile Bond Strength of Ketac-Fil and Ketac-Silver to Enamel and Dentin, Dent Mater 2: 241245. WALLS, A.W.G.; ADAMSON, J.; MCCAPE, J.F.; and MURRAY, J.J. (1987): The Properties of a Glass Polyalkenoate (Ionomer) Cement Incorporating Sintered Metallic Particles, Dent Mater 3: 113-116. WILSON, A.D. and KENT, B.E. (1972): A New Translucent Cement for Dentistry. The Glass Ionomer Cement, Br Dent J 132: 133-135. WILSON, A.D. and PROSSER,H.J. (1984): A Survey of Inorganic and Polyelectrolyte Cements, Br Dent J 157: 449454.
~Department of Operative Dentistry Baylor College of Dentistry 3302 Gaston Avenue Dallas, Texas 75246 2Department of Dental Materials Science The Oregon Health Sciences University 611 S.W. Campus Dr. Portland, Oregon 97201 Received June 29, 1989 Accepted March 6, 1990 Dent Mater 6:129-132, April, 1990 Abstract-The goal of this study was to
compare two core materials-glasscermet and amalgam-in terms of crown retention when significant tooth structure is missing. The glass-cermet's ability to bond to teeth with and without the aid of retentive pins and/or grooves was examined. The effects of two storage solutions, formalin and sodium azide, on the bond of the glass-cermet were also evaluated. Sixty human molars were obtained and stored in either formalin or sodium azide. Thirty molars were prepared with one wall of tooth structure remaining, and the other 30 were prepared with two walls. Before being restored with glass-cermet, the preparations received either no retention, retentive grooves and pins, or retentive grooves only. Each preparation restored with amalgam received pins and grooves. Type III gold crowns were cemented in vitro with glass-ionomer luting agent, thermocycled 2500 times (5-50°C), and then removed under a tensile force. The glass-cermet material may be comparable to amalgam and has adequate adhesion to the tooth, without additional retention, when at least two walls of the tooth remain. However, when only one wall of the tooth remains, the amalgam core is superior under tensile forces. There was a significant difference between the sodium azide and formalin storage solutions with regard to bond strength in one group (two walls, no pins, no grooves). In general, the cores placed on teeth stored in sodium azide exhibited tensile bond strengths lower than those stored in formalin.
lass-cermet evolved from the glass-ionomer cements introduced in the early 1970's by Wilson and Kent (1972). The main advantages of glass ionomer include the ability of the material to bond chemically to tooth structure by an ionic interaction with the calcium ions in enamel and dentin and to leach fluoride. However, the cement does not possess adequate strength and stiffness for routine use as a loadbearing restorative. The addition of a metal alloy was the first step taken in an attempt to strengthen the glassionomer material so that it could be used as a core build-up (Simmons, 1983). Based on clinical observaLions, this admixture has been widely promoted (CRA Newsletter, 1983). It has been recommended that its clinical use, however, be limited to core build-ups where the material will receive support from tooth structure or pins (Arcoria et al., 1988). Recent attempts to improve the properties of the glass ionomers led to the development of the glass-cermet (McLean and Gasser, 1985). The silver powder in the glass-cermet cement is coated by glass through a process of high-temperature sintering. This has improved the abrasion resistance to a level which appears to be comparable to that of amalgam (Wilson and Prosser, 1984; McLean and Gasser, 1985). In addition, McLean and Gasser (1985) have clinically observed an improvement in fracture resistance. The compressive strength has been shown to be relatively high, but the material remains weak in response to tensile stress (Moore et al., 1985). Overall, the mechanical properties remain inferior to those of amalgam (Phillips, 1982; Swift, 1986), and it has been recommended that the use of cermets be limited to low-stress-bearing areas (McLean and Gasser, 1985; Marker eta/,, 1985; Swift, 1986; Walls et aL, 1987). Although McLean reported no unusual masticatory wear or marginal breakdown in the glass-
G
cermet restorations after two years in vivo, further research has been advocated before its use as a longt e r m restorative material can be recommended (McLean and Gasser, 1985; Moore et al., 1985; Jendresen et al., 1987). Despite this, glass-cermet has been promoted for restoration of Classes I, II, and III lesions in primary teeth (Croll and Phillips, 1986), Classes I and II lesions in permanent teeth (McLean and Gasser, 1985), and for core build-ups (McLean and Gasser, 1985; Swift, 1986; Donovan and Daftary, 1987; Taleghani and Leinfelder, 1988). Because of the low tensile s~'ength and fracture toughness of the material, mechanical retention is deemed to be important for core build-ups (Swift, 1986; CroU and Phillips, 1986; Donovan and Daftary, 1987). Thornton et al. (1986) recommends mechanical retention in clinical situations based on lower bond strengths obtained from a glass-cermet as compared with its glass-ionomer counterpart. Building up most of the coronal portion of a tooth with glass-cermet has also been contra-indicated because of the probability of failure under occlusal loads (Donovan and Daftary, 1987; Taleghani and Leinfelder, 1988). The objective of this study was to compare the tensile bond strengths of glass-cermet and amalgam when both materials were placed in core build-up preparations. The type of retention form, if any, required to prevent dislodgement of the glassc e r m e t material was also d e t e r mined. Finally, the effects of two storage solutions on the chemical bond between glass-cermet and tooth structure were evaluated. MATERIALS AND METHODS
Sixty, freshly extracted, non-carious molars were obtained. Forty teeth were stored in a dilute phosphate-buffered formaldehyde solution and the other 20 in a 2% sodium aside solution. This comparison was
Dental Materials/April 1990 129
PreparaUonA
Preparation B
Fig. Internal preparation designs.
made to determine the extent to which formaldehyde might alter the dentin surface and reduce the adhesive strength of the cermet to the tooth. Sodium azide has been used in another study for storing teeth prior to the evaluation of dentin bonding (Pashley et aL, 1988). The teeth were cleaned of debris and the roots embedded in epoxy resin to within 2 mm of the cemento-enamel junction. The samples were then placed in a jig and the teeth prepared to a chamfer margin with a No. 257.SB diamond (Premier Dental Co., Norristown, PA) in a highspeed handpiece that was placed in a Ney handpiece holder (J.M. Ney, H a r t f o r d , CT). Each tooth measured approximately 4 n~n in height, 8 mm in length, and 6 mm in width, with a taper ranging from 5 to 5.7°. Internal preparations, Preparation A (n = 30) arid Preparation B (n = 30), were then made with a No. 556 carbide bur (Fig.). The preparations received either no retention, retentive grooves only, or retentive grooves and pins. Each preparation was then restored with either Ketac-Silver glass-cermet (C) (ESPE-Premier Dental Products, Lynbrook, NY) or Tytin amalgam (A) (Kerr/Sybron, Emeryville, CA). Retention and restorative materials were placed as in Table 1. R e g u l a r s e l f - s h e a r i n g TMS (Whaledent, Int., New York, NY) pins were placed 1 mm inside each line angle. Retentive grooves were placed with a No. 1/4 round bur 2 mm in length where the axial wall met the existing adjacent lateral wall. The teeth were restored and tl"immed following manufacturer's directions. A chamfer margin was re-established. Each core was impressed with heavy-bodied vinyl polysiloxane ma-
terial ( C u t t e r P e r f o u r m , C u t t e r Dental, Miles Laboratories, Inc., Berkeley, CA) and stored in de-ionized water during crown fabrication. A die and wax pattern were made with a loop attached to the coronal portion, and a Type III gold crown (G-Cast, Degussa Dental, Long Island, NY) was cast for each core. Before crown cementation, each core was measured with a Boley gauge to the nearest 0.1 mm to determine surface area and taper. Crowns were fitted and then cemented with glassionomer Type I luting cement (GC Dental Industrial Corp., Tokyo, Japan) within one week after preparation and impression. The crowns were held under a constant load by a static weight of 2 Kg for three min. Two min later, the excess cement was removed, the samples placed in a humidor, and stored for 24 h at 37°C. They were then thermocycled in deionized water at between 5 and 50°C for 2500 cycles (30-second dwells in each bath). The samples were tested in tension in a Universal Testing Machine (Instron Corp., Canton, MA) at a cross-head speed of 2.54 mm/ rnin until failure (any decrease in load from the peak load). The results were analyzed by ANOVA (p = 0.05) and Scheff~'s test. A Student t test (p = 0.05) was used to evaluate the difference between storage treatments.
RESULTS The failure modes of the Tooth/Core/ Crown assembly are shown in Table 2. All 10 amalgam samples (A: Prep A and Prep B) exhibited an adhesive failure of the glass-ionomer cement. The crown pulled cleanly from the core in all samples. All 10 glass-cermet samples (CI: Prep A and Prep B), restored after pins and grooves were added for retention, exhibited good adhesion to the tooth, ~e., either the crown pulled cleanly from the core (8) or the core failed above the height of the pin. Of the 20 glass-cermet samples (C2: Prep A and Prep B) restored after two retentive grooves were placed, 19 cores, with or without tooth attached, separated from the preparation. In all cases, the core fractured at the retentive grooves, leaving some material within the grooves. One sample (Prep A) exhibited good adhesion of the core to
130 DeWALD et al./EVALUATION OF GLASS-CERMET CORES UNDER CAST CROWNS
the tooth, with over 80% of the core remaining a t t a c h e d to the tooth structure. Of the 20 glass-cermet samples restored with no retention (C3: Prep A and Prep B), 18 showed poor adhesion of the core to tooth s t r u c t u r e . One sample (Prep B) showed good adhesion,/.e., over 80% of the core remained attached to the tooth structure. One sample (Prep A) showed good adhesion of the core to the tooth structure,/~e., the crown pulled cleanly from the core. The mean values of bond strengths of the casting on the glass-cermet and amalgam cores are shown in Table 3. There were no significant differences between the groups for Prep A (two walls). Within the groups of P r e p B (one wall), the amalgam showed significantly more retention than the glass-cermet cores restored without pins (C2 and C3). There was no significant difference between the amalgam and the glass-cermet samples restored with pins (C1). Retentive s t r e n g t h s were significantly greater for glass-cermet when two walls (Prep A) of tooth remained as compared with one wall (Prep B), except when pins were placed. The sodium azide samples exhibited tensile bond strengths lower than those of the formalin samples for Preparation A (C3: no retention).
DISCUSSION This study supports earlier studies that have expressed reservations over building up most of the coronal portion of a tooth with glass-cermet (Donovan and Daftary, 1987; Taleghani and Leinfelder, 1988) and that suggest the use of mechanical retention (Croll and Phillips, 1986; Swift, 1986; Donovan and Daftary, 1987). When two walls remained, the glasscermet was adequately retained in the preparation under tensile forces as compared with amalgam. This was regardless of the type of retention placed. Even when no retention was placed, these samples adequately retained the glass-cermet restoration. However, when only one wall of tooth structure remained, the glass-cermet was not retained as well as the amalgam, except when pin-andgroove retention was used. From these results, it appears that retention in the form of pins and grooves is necessary when there is a limited
amount of tooth structure remaining to adhere to the glass-cermet. When the glass-cermet samples only were compared, the retentive strengths were significantly greater for the preparations having two walls as opposed to one wall. It appears that the material needs the support from surrounding tooth structure. This may also be due to the more available tooth structure for the glassionomer cement when the castings were cemented. This, however, would not explain why the amalgam samples with two walls of tooth remaining did not bond significantly better than did the amalgam samples with one wall. A possible explanation may be that the surface of the amalgam after crown preparation was as retentive to the luting agent as was the surface of the tooth. These results strongly suggest that the bonding of the glass-cermet is comparable with that of amalgam when two walls of tooth structure remain, r e g a r d l e s s of retention. These results, however, are limited to tensile tests. A further study needs to be conducted that utilizes tests in compression. The formalin and sodium azide storage solutions did not have an effect on bond strengths except in one group of samples where the sodium azide group was lower. The explanation for this is unclear. It was theorized that the formalin may affect the organic material of the dentin, thereby decreasing the bond of the glass-cermet or glass-ionomer to the tooth s t r u c t u r e . This study, however, did not support that hypothesis.
TABLE 1 PREPARATION, RESTORATIVE MATERIAL, RETENTION, AND STORAGE SOLUTION PRIOR TO PREPARATION PREP A 2 walls
PREP B 1 wall
C1 1 pin 2 grooves formalin n = 5
2 pins 2 grooves formalin n = 5
ACKNOWLEDGMENT
The authors would like to express
C3
2 grooves formalin n = 5 Na azide n = 5
formalin n = 5 Na azide n = 5
2 grooves formalin n = 5 Na azide n = 5
formalin n = 5 Na azide n = 5
A 1 pin 2 grooves formalin n = 5
2 pins 2 grooves formalin n = 5
TABLE 2 FAILURE MODES OF TOOTH/CORE/CROWN ASSEMBLY (FORMALIN AND SODIUM AZlDE) Prep A (2 walls; n = 30) Cement (3) 60% (adhesive failure)
01 Pin(s) 2 grooves
C2 No pin(s) 2 grooves
C3 No pin(s) No grooves
Core (above pins) (2)
40%
Tooth fracture/ core failure (3)
30%
Prep B (1 wall; n = 30) Cement (5) 100% (adhesive failure)
Tooth fracture/ core failure (9)
90%
Core/tooth (6) Core/core (1)* (cohesive failure)
60% 10%
Core/tooth
(1)
10%
Cement
10%
Tooth fracture/ core failure (8)
80%
(1)
Tooth fracture/ core failure (6) Core/tooth (3)
60% 30%
Core/tooth (1) Core/core (1)* (cohesive failure)
10% 10%
100% A Cement (5) 100% Cement (5) Pin(s) (adhesive failure) (adhesive failure) 2 grooves *Over 80% of core adhered to tooth structure. Samples had been stored in sodium azide prior to preparation. TABLE 3 MEAN VALUE* OF BOND STRENGTH (MPa) OF CASTING ON GLASS-CERMETAND AMALGAM CORES WITH GLASS-IONOMER CEMENT UTILIZING DIFFERENTMEANS OF RETENTION
CONCLUSIONS
As a core material, the glass-cermet is comparable to amalgam in tensile retentive strength when at least two walls of tooth structure remain. This was observed even when no retention was placed. However, when only one wall of tooth remains, the amalgam core is superior. It is therefore concluded that at least 50% of the tooth should be present to ensure adequate retention of glass-cermet as a core build-up material.
02
PREP A 2 walls
formalin
C1 2.69 - 0.50
sodium azide PREP B
formalin
2.39 __ 0.21
C2 2.73 _ 0.42
C3 2.90 ± 0.27
2.51 __ 0.19
2.55 ± 0.24
1.88 ± 0.31
2.14 _+ 0.27
A 2.96 _ 0.50
2.92 __ 0.28
1 wall 1.59 ± 0.22 sodium azide *Values are mean ± standard deviation for five (5) samples.
their gratitude to E S P E - P r e m i e r Dental Products, Lynbrook, NY, for the materials used in this study.
1.88 ± 0.29
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