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Resistance to cusp fracture in endodontically treated teeth
Resistance to cusp fracture in endod0ntically treated teeth
S. L. W e n d t Jr., 1 B. M. Harrisfl T. E. H u n t 3 1Department of Biomaterials, University of Alabama at Birmingham School of Dentistry, 2Divisionof Endodontics, 3University of Tennessee, Memphis, College of Dentistry, U.S.A.
Wendt Jr SL, Harris BM, Hunt TE. Resistance to cusp fracture in endodontically treated teeth. Dent Mater 1987: 3: 232-235. - This study investigated the cusp fracture resistance of several methods advocated to restore endodontically treated teeth. It is apparent statistically, that more than one restoration could be used to restore endodontically treated teeth to the fracture resistance of sound natural teeth. Two of the restorative methods, gold onlays and a composite resin/glass ionomer cement in a beveled preparation, had higher mean forces to resist cusp fracture than did sound natural teeth. Two other restorative methods, a composite resin/Scotchbond and a silver-filled glass ionomer restorative, had lower mean forces to resist Cusp fractures than did sound natural teeth, but these were not significantly lower at the p < .05 level. Abstract
Teeth that receive endodontic treatment become brittle with time and become increasingly subject to fracture (1,2). Most of the methods for restoring these teeth involve the use of intracanal posts, coronal pins, subsequent coronal build-ups, and cast metal restorations (3-5). This may require radical removal of tooth structure which will become brittle with time. Simple intracoraonal preparations, either Class 1 or Class II, in sound natural teeth have been shown to reduce the resistance to cusp fracture (6, 7). The assumption can be drawn that intracoronal preparations will also reduce the resistance to cusp fracture of endodontically treated teeth. Restoration of the prepared sound natural teeth with posterior composite resins or with glass ionomer restoratives have been investigated and it has been shown that an increase in the resistance of cusp fracture can be achieved when compared to prepared unfilled sound natural teeth (8, 9). The purpose of this study was to investigate the resistance to cusp fracture of endodontically'treated teeth. These teeth were restored with either glass ionomer restorative material, posterior composite resin, posterior composite resin/glass ionomer cement combination or metal alloy restorations. Material and m e t h o d s
Ninety-four non-carious, extracted, human maxillary first bicuspid teeth were used in this study. The teeth were di-
Key words: endodontic,tooth fracture, composite resin, glass ionomer cement. Stanley L. Wendt Jr., University of Alabama at Birmingham, School of Dentistry, Box 49, Department of Biomaterials, Birmingham, AL 35294, U.S.A. Accepted for publication 15 November 1986
vided into 7 experimental groups of 10 endodontically treated teeth each, one group of 8 endodontically treated teeth, and 2 groups of 8 sound natural teeth with no endodontic treatment (see Table 1). One group of 8 natural teeth (Group A) received no treatment. One group o f 10 endodontically treated teeth (Group B) had access only. These groups were regarded as control groups. Eight of the teeth were natural teeth which received M O D preparations and were not restored (Group C). One group of 8 teeth that were endodontically treated, were prepared with a M O D slot preparation and not restored (Group D). The rest of the 60 teeth, all endodontically treated, were divided into 6 groups of 10 teeth each, prepared with a M O D slot preparation and restored by various methods. All teeth were tested in an Instron Universal Testing Machine* for resistance to cusp fracture. Seventy-eight teeth were marked for endodontic treatment and complete endodontic therapy was accomplished. The teeth were instrumented using the step-back technique and filled with gutta percha points and a zinc oxide/ eugenol cement. Zinc oxide and eugenol temporaries were placed in the occlusal accesses. All teeth were mounted in acrylic blocks. MOD (mesio-occluso-distal) slot preparations 3 mm in depth and approximately 1.5 mm in width were pre-
pared on all teeth except teeth in Groups A and B. A 330 bur was used to prepare all M O D slot preparations, except for onlay preparations, which required the use of a 701 tapered fissure bur and a 1A diamond bur for the cuspal reduction and margins. The experimental teeth were restored in the following manner. Ten teeth (Group E) were prepared with a M O D slot preparation and an occlusal reduction of 0.5 mm extending onto the facial and lingual cusps and ending with a feather margin on the facial and lingual aspects of the respective cusps. This group received the onlay restorations. Onlays were fabricated out of Type III gold utilizing the direct wax method of pattern fabrication. The onlays were cemented with polycarboxylate cement. The second group of 10 teeth (Group F) were restored with Tytin t amalgam in the usual procedure. The third group (Group G) were restored with Ketac Fil* silver-filled glass ionomer restorative material with no bevelling of the cavosurface margins. The preparations were washed with polyacrylic acid for 10 s and rinsed in running water for 30 s. A clear mylar matrix band was applied to the prepared tooth. The glass ionomer restorative was mixed in a triturator for 10 s and bulk loaded in its syringe apparatus into the preparation. The restorative was varnished and allowed to set undisturbed for 15 min. The restoration was
*Instron Corp., Canton, MA.
*S.SS White, Holmdel, NJ.
Res&tance to cusp fracture Table 1. Experimental design. Tooth treatment
N
Preparation
Restoration
Group A, Group B, Group C, Group D, Group E, Group F, Group G, Group H, Group I, Group J,
8 10 8 8 10 10 10 10 10 I0
None Access only MOD MOD Onlay MOD MOD MOD MOD MOD
None None None None Gold onlay Amalgam GIR/no bevel CR/GI/bevel CR/GI/no bevel CR/bevel
natural endo natural endo endo endo endo endo endo endo
CR denotes composite resin. GI denotes glass ionomer cement. GIR denotes silver-filled glass ionomer retorative. Table 2. Fracture resistance (MPa) and standard deviations. Experimental treatment
Max.
Min.
Mean
SD
N
Group A Group B Group C Group D Group E Group F Group G Group H Group I Group J
2.52 1.00 1.38 0.79 5.58 1.50 1.97 2.65 1.74 1.97
0.98 0.58 0.41 0.30 1.59 0.39 0.83 1.17 0.60 1.00
1.50 0.79 1.05 0.55 3.84 0.91 1.43 1.71 1.05 1.47
0.48 0.14 0.34 0.19 1.32 0.41 0.38 0.47 0.38 0.39
8 10 8 8 10 10 !0 10 10 10
CR denotes composite resin. GI denotes glass ionomer cement. GIR denotes silver-filled glass ionomer retorative.
finished flush to the margin of the tooth and varnish was applied to the restoration before storage. A fourth group of 10 teeth (Group H) was restored using Ketac* glass ionomer cement as a base covering the dentin. The preparation was washed with polyacrylic acid for 10 s and rinsed in running water for 30 s. The glass ionomer cement was mixed according to manufacturer's directions and applied to the tooth covering all of the dentin. The glass ionomer cement was allowed to set undisturbed for 7 min. The cavosurface margins were bevelled with a short bevel (0.5 mm) and the glass iohomer cement and the enamel margins were acid etched with phosphoric acid for 60 s. A clear mylar matrix band was affixed to the prepared tooth. Lightcured Scotchbond~was applied to the etched surfaces and cured for 20 s. The teeth were then bulk loaded with P-30 composite resin ~ and light cured with a
*Premiere Dental Products Company, Norristown, PA, U.S.A. ~3M Company St. Paul, MN.
COE-Lite polymerization unit IIfor 20 s from each direction. The composite resin was finished flush with the bevelled margins. A fifth group of 10 teeth (Group I) were restored in the same manner with the P-30 composite resin/glass ionorner cement base, with the exception that the enamel margins of the preparation were not bevelled. A sixth group of 10 teeth (Group J) were prepared with bevelled enamel margins and restored with P-30 composite resin. A clear mylar matrix band was affixed to the tooth and Scotchbond was applied to the dentin. The enamel was etched with phosphoric acid for 60 s. The P-30 composite resin was bulk loaded into the preparation and light cured using the COE-Lite for 20 s from each side. The composite resin was finished flush with the bevelled margins. All teeth were stored in water at 37~176 for one week prior to testing. The teeth were then thermocycled for one hundred cycles, 60 s in each water bath at 4~ and at 58~ during each cycle. All teeth were placed in the Iustron
233
and fractured. The upper platen in the crossbead assembly was removed and replaced with a 3.25 mm in diameter rod with a convex surface. This rod engaged the incline ridges of the cusps without touching the restoration (except in the case of the gold onlays) and acted as a wedge to promote cusp fracture along the long axis of the tooth. The acrylic blocks, in which the teeth were mounted, had their bottom surface s ground flat so the cusp tips of the teeth were parallel with the lower platen of the Instron. The teeth were loaded in a compressive mode in the Instron utilizing a crosshead speed of 0.02 inches/minute. The pounds of force were recorded at fracture of the teeth.
Results The data was collected and subjected to Analysis of Variance (ANOVA) and a Student-Neuman-Keuls test. Mean fracture resistance (MPa) and standard deviations for the experimental groups are shown in Table 2 (see Table 2). The fractures of the cusps were about evenly divided between those fractures of the cusps above the cemento-enamel junction and fractures than extended through the floor of the preparation from mesial to distal. In all cases, fracture of the restored teeth were at the tooth-restoration interface and not through the restorative material. At the p < .05 level of significance for ranks in the Student-Neuman-Keuls test, teeth restored with onlay restorations (Group E) had a significantly higher resistance to force of cusp fracture than did all other teeth tested. The restorative procedures, in the endodontically treated teeth, of composite resin and glass ionomer base in a bevelled preparation (Group H), composite resin in a bevelled preparation (Group J), and silver-filled glass ionomer restorative (Group G) had the same statistical significance for resistance to cusp fracture as did the natural unprepared tooth (Group A). The natural teeth prepared with the MOD slot preparations (Group C) were significantly (p < .05) less resistant to cusp fracture than the natural unprepared teeth and the above mentioned restorations. The composite resin and glass ionomer based, unbevelled restorations (Group I) and the amalgam restorations (Group F) had the same statistical significance as did the prepared sound rrCOE Laboratories, Inc., Chicago, IL.
234
Wendt Jr et al.
Table 3. Summary of significant groupings of ranks for the Student-Newman-Keuls Test. Experimental groups i Group E Group H Group A I Group J I Group G i] Group C Group Group Group Group
I F B D
Type of restoration Endo/gold onlay Endo/CR/GI/bevel noneS~ natural tooth/ Endo/CR/bevel Endo/GIF/no bevel Naturaltooth-MOD/ none Endo/CR/GI/no bevel Endo/amalgam Endo-access only/none Endo-MOD/none
CR denotes composite resin. GI denotes glass ionomer cement. GIR denotes silver-filled glass ionomer retorative. Groupings within the brackets are significant to each other and are arranged from highest to lowest fracture resistance. natural tooth (Group C). The endodontically treated teeth with access only (Group B) and with the unrestored MOD preparations (Group C) could be statistically grouped together at the p < .05 level. The endodontically treated teeth with the preparations were statistically less resistant to cusp fracture than any other grouping (see Table 3).
Discussion This in vitro study showed that restoration of posterior endodontically treated teeth with gold restorations, which cover both cusps, are superior to resisting fracture of the teeth. The gold onlays (Group E ) s h o w a significantly (p < .05) higher resistance to fracture than even the natural dentition. This is not a new observation and would have been expected. It was anticipated from the fiterature (8, 9), that composite resin restorations using P-30 and glass ionomer restoratives would increase the cusp fracture resistance of the endodontically treated teeth. It was not anticipated that the restorations would have the same statistically significant resistance to cusp fracture as sound unprepared natural teeth (Group A). This was indeed the case, although the force needed to fracture the cusps was slightly less than the force needed to fracture the teeth in Group A. It was also anticipated that the resistance to cusp fracture would decrease if a sound natural tooth lost the integrity of the marginal ridges in an M O D preparation. It was shown statistically at the
p < .05 level that prepared, sound natural teeth (Group C) were less resistant to cusp fracture. Resistance to cusp fracture of endodontically treated teeth, with access only (Group B), was significantly less at the p < .05 level than with Group A teeth, but in the same statistical grouping (p < .05) as the Group C teeth. The decrease of resistance to cusp fracture in Group B teeth is undoubtedly due to a loss of internal tooth structure in the in vitro investigation. It may be due, in vivo, not only to a loss of internal tooth structrue, but also to the brittleness of the tooth structure due to loss of moisture content. It would follow that a M O D preparation of the endodontically treated teeth would further decrease the resistance to cusp fracture. This was indeed the case and the decrease was significant at the p < .05 level when compared to all other experimental groups. As expected, attempts to increase the resistance to cusp fracture by employing amalgam as a restoration (Group F) did not significantly increase the cusp fracture resistance in endodonticaUy treated teeth. In a previous study (9) using sound prepared teeth, glass ionomer cement restorations did not statistically increase cusp fracture resistance when compared to the sound, prepared, unrestored teeth. In this study, silverfilled glass ionomer restorative (Group G), as expected, significantly (p < .05) increased the cusp fracture resistance of the endodontically treated teeth. The cusp fracture resistance was higher than in the Group B and Group D teeth. It was also higher than the Group C teeth, a result not anticipated due to findings in the previous study. The silver-filled glass ionomer restorative increased the resistance to cusp fracture nearly to the level of both the composite resin restored endodontically treated teeth (Group J) and the natural, unprepared, sound teeth (Group A). Using glass ionomer cement as a base under the composite resin restorations brought surprising results. It was anticipated from the knowledge that glass ionomer cement bonds to dentin and that glass ionomer cements can be etched with phosphoric acid to provide a substrate for composite resin bonding, that resistance to cusp fracture would be as high or slightly higher than the composite resin without the glass ionomer cement base. W h a t was not
anticipated was the magnitude of the increase. The mean forces needed to fracture the cusps of Group H teeth were 30 lbs higher than the mean forces needed to fracture cusps of Group A teeth and 35 lbs higher than the Group J teeth. The force was second only to the force needed to fracture Group E teeth. Although the increase of a bevelled preparation (Group H) to resistance of cusp fracture was higher, it was not significantly higher at the p < .05 level to the bevelled composite resin restoration without the glass ionomer base (Group J) nor to the sound natural teeth (Group A). It was not expected that teeth which were not bevelled, before restoration with glass ionomer cement bases and composite resin (Group I), whould show significantly (p < .05) lower forces needed to fracture cusps than did the Group J or Group H teeth. Not only were the unbevelled Group I teeth more susceptible to fracture than the beveled teeth, they were also significantly ( p < .05) more susceptible to cusp fracture than silver-filled glass ionomer (Group G). They showed nearly the same magnitude of force needed to fracture as did the prepared, unrestored sound natural teeth (Group C). In this investigation, the fractures of the teeth restored with onlays involved only cusps and did not split the tooth mesio-distally. Several of the fractures did not displace the cusp from under the onlay, but simply fractured the cusp. It is not unusual to see this clinically. Cusp fractures of the teeth treated by the other restorations investigated in this study were about equally divided between cusp fractures and mesio-distal splitting of teeth. The fractures always involved the restoration and tooth interface and were not apparent in the restorative materials themselves. The results of this investigation would suggest methods of restoring the endodontically treated teeth other than cast metal coverage of the cusps, might be clinically appropriate. The question arises clinically, does the endodontically treated tooth need to be restored to resist forces beyond the forces needed to fracture cusps of sound natural teeth. If the answer is yes, composite resin and etched glass ionomer cement in a bevelled preparation could be used in place of the cast metal coverage. If the answer is no, 2 other restorations, composite resin in a bevelled preparation and glass ionomer restorative, would provide nearly the same resistance to fracture as the sound, natu-
Resistance to cusp fracture
ral tooth. T h e r e are of course, several other factors to consider clinically when advocating restorations of either glass i o n o m e r restoratives or composite resins in restoring posterior teeth. This investigation has only attempted to show a relationship of resistance to cusp fracture in endodonticaUy treated teeth that w e r e restored in a varied n u m b e r of ways to both the prepared and access only, endodontically treated teeth, and to the prepared and sound natural teeth.
2.
3.
4.
5.
References 1. Baraban DJ. The restoration of pulpess
6.
teeth. Dent Clin North A m 1967: 11: 633653. Helfner AR, Melnick S, Schilder H. Determination of moisture content of vital and pulpess teeth. J Oral Surg 1972: 34: 661~i70. Halpen BG. Restoration of endodontically treated teeth. Dent Clin North A m 1982: 29(2): 293-303. Kantor ME, Pines MS. A comparative study of restorative techniques for pulpless teeth. J Prosthet Dent 1977: 38(2): 405--412. Talim ST, Gohil KS. Management of coronal fractures of permanent posterior teeth. J Prosthet Dent 1974: 31(2): 172178. Mondelli J. Steagall L, Ishikiriama A,
235
Navarro MF, Soares FB. Fracture strengthh of human teeth with cavity preparations. J Prosthet Dent 1980:43 (4): 419--422. 7. Bakke JC, Duke ES, Norling BK, Windler S, Mayhew RB. Fracture strength of Class II preparations with a posterior composite. J Dent Res 1985: 64: 350, Abstract 1578. 8. Eakle WS. Fracture resistance of teeth restored with Class II bonded composite resin. J Dent Res 1986: 65(2): 149-153. 9. Eakle WS. Increasing the resistance of teeth to fracture: bonded composite resin versus glass ionomer cement. Dent Mater 1985: 1: 228-230.
S. L. W e n d t Jr., 1 B. M. Harrisfl T. E. H u n t 3 1Department of Biomaterials, University of Alabama at Birmingham School of Dentistry, 2Divisionof Endodontics, 3University of Tennessee, Memphis, College of Dentistry, U.S.A.
Wendt Jr SL, Harris BM, Hunt TE. Resistance to cusp fracture in endodontically treated teeth. Dent Mater 1987: 3: 232-235. - This study investigated the cusp fracture resistance of several methods advocated to restore endodontically treated teeth. It is apparent statistically, that more than one restoration could be used to restore endodontically treated teeth to the fracture resistance of sound natural teeth. Two of the restorative methods, gold onlays and a composite resin/glass ionomer cement in a beveled preparation, had higher mean forces to resist cusp fracture than did sound natural teeth. Two other restorative methods, a composite resin/Scotchbond and a silver-filled glass ionomer restorative, had lower mean forces to resist Cusp fractures than did sound natural teeth, but these were not significantly lower at the p < .05 level. Abstract
Teeth that receive endodontic treatment become brittle with time and become increasingly subject to fracture (1,2). Most of the methods for restoring these teeth involve the use of intracanal posts, coronal pins, subsequent coronal build-ups, and cast metal restorations (3-5). This may require radical removal of tooth structure which will become brittle with time. Simple intracoraonal preparations, either Class 1 or Class II, in sound natural teeth have been shown to reduce the resistance to cusp fracture (6, 7). The assumption can be drawn that intracoronal preparations will also reduce the resistance to cusp fracture of endodontically treated teeth. Restoration of the prepared sound natural teeth with posterior composite resins or with glass ionomer restoratives have been investigated and it has been shown that an increase in the resistance of cusp fracture can be achieved when compared to prepared unfilled sound natural teeth (8, 9). The purpose of this study was to investigate the resistance to cusp fracture of endodontically'treated teeth. These teeth were restored with either glass ionomer restorative material, posterior composite resin, posterior composite resin/glass ionomer cement combination or metal alloy restorations. Material and m e t h o d s
Ninety-four non-carious, extracted, human maxillary first bicuspid teeth were used in this study. The teeth were di-
Key words: endodontic,tooth fracture, composite resin, glass ionomer cement. Stanley L. Wendt Jr., University of Alabama at Birmingham, School of Dentistry, Box 49, Department of Biomaterials, Birmingham, AL 35294, U.S.A. Accepted for publication 15 November 1986
vided into 7 experimental groups of 10 endodontically treated teeth each, one group of 8 endodontically treated teeth, and 2 groups of 8 sound natural teeth with no endodontic treatment (see Table 1). One group of 8 natural teeth (Group A) received no treatment. One group o f 10 endodontically treated teeth (Group B) had access only. These groups were regarded as control groups. Eight of the teeth were natural teeth which received M O D preparations and were not restored (Group C). One group of 8 teeth that were endodontically treated, were prepared with a M O D slot preparation and not restored (Group D). The rest of the 60 teeth, all endodontically treated, were divided into 6 groups of 10 teeth each, prepared with a M O D slot preparation and restored by various methods. All teeth were tested in an Instron Universal Testing Machine* for resistance to cusp fracture. Seventy-eight teeth were marked for endodontic treatment and complete endodontic therapy was accomplished. The teeth were instrumented using the step-back technique and filled with gutta percha points and a zinc oxide/ eugenol cement. Zinc oxide and eugenol temporaries were placed in the occlusal accesses. All teeth were mounted in acrylic blocks. MOD (mesio-occluso-distal) slot preparations 3 mm in depth and approximately 1.5 mm in width were pre-
pared on all teeth except teeth in Groups A and B. A 330 bur was used to prepare all M O D slot preparations, except for onlay preparations, which required the use of a 701 tapered fissure bur and a 1A diamond bur for the cuspal reduction and margins. The experimental teeth were restored in the following manner. Ten teeth (Group E) were prepared with a M O D slot preparation and an occlusal reduction of 0.5 mm extending onto the facial and lingual cusps and ending with a feather margin on the facial and lingual aspects of the respective cusps. This group received the onlay restorations. Onlays were fabricated out of Type III gold utilizing the direct wax method of pattern fabrication. The onlays were cemented with polycarboxylate cement. The second group of 10 teeth (Group F) were restored with Tytin t amalgam in the usual procedure. The third group (Group G) were restored with Ketac Fil* silver-filled glass ionomer restorative material with no bevelling of the cavosurface margins. The preparations were washed with polyacrylic acid for 10 s and rinsed in running water for 30 s. A clear mylar matrix band was applied to the prepared tooth. The glass ionomer restorative was mixed in a triturator for 10 s and bulk loaded in its syringe apparatus into the preparation. The restorative was varnished and allowed to set undisturbed for 15 min. The restoration was
*Instron Corp., Canton, MA.
*S.SS White, Holmdel, NJ.
Res&tance to cusp fracture Table 1. Experimental design. Tooth treatment
N
Preparation
Restoration
Group A, Group B, Group C, Group D, Group E, Group F, Group G, Group H, Group I, Group J,
8 10 8 8 10 10 10 10 10 I0
None Access only MOD MOD Onlay MOD MOD MOD MOD MOD
None None None None Gold onlay Amalgam GIR/no bevel CR/GI/bevel CR/GI/no bevel CR/bevel
natural endo natural endo endo endo endo endo endo endo
CR denotes composite resin. GI denotes glass ionomer cement. GIR denotes silver-filled glass ionomer retorative. Table 2. Fracture resistance (MPa) and standard deviations. Experimental treatment
Max.
Min.
Mean
SD
N
Group A Group B Group C Group D Group E Group F Group G Group H Group I Group J
2.52 1.00 1.38 0.79 5.58 1.50 1.97 2.65 1.74 1.97
0.98 0.58 0.41 0.30 1.59 0.39 0.83 1.17 0.60 1.00
1.50 0.79 1.05 0.55 3.84 0.91 1.43 1.71 1.05 1.47
0.48 0.14 0.34 0.19 1.32 0.41 0.38 0.47 0.38 0.39
8 10 8 8 10 10 !0 10 10 10
CR denotes composite resin. GI denotes glass ionomer cement. GIR denotes silver-filled glass ionomer retorative.
finished flush to the margin of the tooth and varnish was applied to the restoration before storage. A fourth group of 10 teeth (Group H) was restored using Ketac* glass ionomer cement as a base covering the dentin. The preparation was washed with polyacrylic acid for 10 s and rinsed in running water for 30 s. The glass ionomer cement was mixed according to manufacturer's directions and applied to the tooth covering all of the dentin. The glass ionomer cement was allowed to set undisturbed for 7 min. The cavosurface margins were bevelled with a short bevel (0.5 mm) and the glass iohomer cement and the enamel margins were acid etched with phosphoric acid for 60 s. A clear mylar matrix band was affixed to the prepared tooth. Lightcured Scotchbond~was applied to the etched surfaces and cured for 20 s. The teeth were then bulk loaded with P-30 composite resin ~ and light cured with a
*Premiere Dental Products Company, Norristown, PA, U.S.A. ~3M Company St. Paul, MN.
COE-Lite polymerization unit IIfor 20 s from each direction. The composite resin was finished flush with the bevelled margins. A fifth group of 10 teeth (Group I) were restored in the same manner with the P-30 composite resin/glass ionorner cement base, with the exception that the enamel margins of the preparation were not bevelled. A sixth group of 10 teeth (Group J) were prepared with bevelled enamel margins and restored with P-30 composite resin. A clear mylar matrix band was affixed to the tooth and Scotchbond was applied to the dentin. The enamel was etched with phosphoric acid for 60 s. The P-30 composite resin was bulk loaded into the preparation and light cured using the COE-Lite for 20 s from each side. The composite resin was finished flush with the bevelled margins. All teeth were stored in water at 37~176 for one week prior to testing. The teeth were then thermocycled for one hundred cycles, 60 s in each water bath at 4~ and at 58~ during each cycle. All teeth were placed in the Iustron
233
and fractured. The upper platen in the crossbead assembly was removed and replaced with a 3.25 mm in diameter rod with a convex surface. This rod engaged the incline ridges of the cusps without touching the restoration (except in the case of the gold onlays) and acted as a wedge to promote cusp fracture along the long axis of the tooth. The acrylic blocks, in which the teeth were mounted, had their bottom surface s ground flat so the cusp tips of the teeth were parallel with the lower platen of the Instron. The teeth were loaded in a compressive mode in the Instron utilizing a crosshead speed of 0.02 inches/minute. The pounds of force were recorded at fracture of the teeth.
Results The data was collected and subjected to Analysis of Variance (ANOVA) and a Student-Neuman-Keuls test. Mean fracture resistance (MPa) and standard deviations for the experimental groups are shown in Table 2 (see Table 2). The fractures of the cusps were about evenly divided between those fractures of the cusps above the cemento-enamel junction and fractures than extended through the floor of the preparation from mesial to distal. In all cases, fracture of the restored teeth were at the tooth-restoration interface and not through the restorative material. At the p < .05 level of significance for ranks in the Student-Neuman-Keuls test, teeth restored with onlay restorations (Group E) had a significantly higher resistance to force of cusp fracture than did all other teeth tested. The restorative procedures, in the endodontically treated teeth, of composite resin and glass ionomer base in a bevelled preparation (Group H), composite resin in a bevelled preparation (Group J), and silver-filled glass ionomer restorative (Group G) had the same statistical significance for resistance to cusp fracture as did the natural unprepared tooth (Group A). The natural teeth prepared with the MOD slot preparations (Group C) were significantly (p < .05) less resistant to cusp fracture than the natural unprepared teeth and the above mentioned restorations. The composite resin and glass ionomer based, unbevelled restorations (Group I) and the amalgam restorations (Group F) had the same statistical significance as did the prepared sound rrCOE Laboratories, Inc., Chicago, IL.
234
Wendt Jr et al.
Table 3. Summary of significant groupings of ranks for the Student-Newman-Keuls Test. Experimental groups i Group E Group H Group A I Group J I Group G i] Group C Group Group Group Group
I F B D
Type of restoration Endo/gold onlay Endo/CR/GI/bevel noneS~ natural tooth/ Endo/CR/bevel Endo/GIF/no bevel Naturaltooth-MOD/ none Endo/CR/GI/no bevel Endo/amalgam Endo-access only/none Endo-MOD/none
CR denotes composite resin. GI denotes glass ionomer cement. GIR denotes silver-filled glass ionomer retorative. Groupings within the brackets are significant to each other and are arranged from highest to lowest fracture resistance. natural tooth (Group C). The endodontically treated teeth with access only (Group B) and with the unrestored MOD preparations (Group C) could be statistically grouped together at the p < .05 level. The endodontically treated teeth with the preparations were statistically less resistant to cusp fracture than any other grouping (see Table 3).
Discussion This in vitro study showed that restoration of posterior endodontically treated teeth with gold restorations, which cover both cusps, are superior to resisting fracture of the teeth. The gold onlays (Group E ) s h o w a significantly (p < .05) higher resistance to fracture than even the natural dentition. This is not a new observation and would have been expected. It was anticipated from the fiterature (8, 9), that composite resin restorations using P-30 and glass ionomer restoratives would increase the cusp fracture resistance of the endodontically treated teeth. It was not anticipated that the restorations would have the same statistically significant resistance to cusp fracture as sound unprepared natural teeth (Group A). This was indeed the case, although the force needed to fracture the cusps was slightly less than the force needed to fracture the teeth in Group A. It was also anticipated that the resistance to cusp fracture would decrease if a sound natural tooth lost the integrity of the marginal ridges in an M O D preparation. It was shown statistically at the
p < .05 level that prepared, sound natural teeth (Group C) were less resistant to cusp fracture. Resistance to cusp fracture of endodontically treated teeth, with access only (Group B), was significantly less at the p < .05 level than with Group A teeth, but in the same statistical grouping (p < .05) as the Group C teeth. The decrease of resistance to cusp fracture in Group B teeth is undoubtedly due to a loss of internal tooth structure in the in vitro investigation. It may be due, in vivo, not only to a loss of internal tooth structrue, but also to the brittleness of the tooth structure due to loss of moisture content. It would follow that a M O D preparation of the endodontically treated teeth would further decrease the resistance to cusp fracture. This was indeed the case and the decrease was significant at the p < .05 level when compared to all other experimental groups. As expected, attempts to increase the resistance to cusp fracture by employing amalgam as a restoration (Group F) did not significantly increase the cusp fracture resistance in endodonticaUy treated teeth. In a previous study (9) using sound prepared teeth, glass ionomer cement restorations did not statistically increase cusp fracture resistance when compared to the sound, prepared, unrestored teeth. In this study, silverfilled glass ionomer restorative (Group G), as expected, significantly (p < .05) increased the cusp fracture resistance of the endodontically treated teeth. The cusp fracture resistance was higher than in the Group B and Group D teeth. It was also higher than the Group C teeth, a result not anticipated due to findings in the previous study. The silver-filled glass ionomer restorative increased the resistance to cusp fracture nearly to the level of both the composite resin restored endodontically treated teeth (Group J) and the natural, unprepared, sound teeth (Group A). Using glass ionomer cement as a base under the composite resin restorations brought surprising results. It was anticipated from the knowledge that glass ionomer cement bonds to dentin and that glass ionomer cements can be etched with phosphoric acid to provide a substrate for composite resin bonding, that resistance to cusp fracture would be as high or slightly higher than the composite resin without the glass ionomer cement base. W h a t was not
anticipated was the magnitude of the increase. The mean forces needed to fracture the cusps of Group H teeth were 30 lbs higher than the mean forces needed to fracture cusps of Group A teeth and 35 lbs higher than the Group J teeth. The force was second only to the force needed to fracture Group E teeth. Although the increase of a bevelled preparation (Group H) to resistance of cusp fracture was higher, it was not significantly higher at the p < .05 level to the bevelled composite resin restoration without the glass ionomer base (Group J) nor to the sound natural teeth (Group A). It was not expected that teeth which were not bevelled, before restoration with glass ionomer cement bases and composite resin (Group I), whould show significantly (p < .05) lower forces needed to fracture cusps than did the Group J or Group H teeth. Not only were the unbevelled Group I teeth more susceptible to fracture than the beveled teeth, they were also significantly ( p < .05) more susceptible to cusp fracture than silver-filled glass ionomer (Group G). They showed nearly the same magnitude of force needed to fracture as did the prepared, unrestored sound natural teeth (Group C). In this investigation, the fractures of the teeth restored with onlays involved only cusps and did not split the tooth mesio-distally. Several of the fractures did not displace the cusp from under the onlay, but simply fractured the cusp. It is not unusual to see this clinically. Cusp fractures of the teeth treated by the other restorations investigated in this study were about equally divided between cusp fractures and mesio-distal splitting of teeth. The fractures always involved the restoration and tooth interface and were not apparent in the restorative materials themselves. The results of this investigation would suggest methods of restoring the endodontically treated teeth other than cast metal coverage of the cusps, might be clinically appropriate. The question arises clinically, does the endodontically treated tooth need to be restored to resist forces beyond the forces needed to fracture cusps of sound natural teeth. If the answer is yes, composite resin and etched glass ionomer cement in a bevelled preparation could be used in place of the cast metal coverage. If the answer is no, 2 other restorations, composite resin in a bevelled preparation and glass ionomer restorative, would provide nearly the same resistance to fracture as the sound, natu-
Resistance to cusp fracture
ral tooth. T h e r e are of course, several other factors to consider clinically when advocating restorations of either glass i o n o m e r restoratives or composite resins in restoring posterior teeth. This investigation has only attempted to show a relationship of resistance to cusp fracture in endodonticaUy treated teeth that w e r e restored in a varied n u m b e r of ways to both the prepared and access only, endodontically treated teeth, and to the prepared and sound natural teeth.
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References 1. Baraban DJ. The restoration of pulpess
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