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The in vitro effect of pulpal pressure and luting agent on tensile bond strength of complete cast crowns
The in vitro effect of pulpal pressure and luting agent on tensile bond strength of complete cast crowns A. Nilgun Ozturk, DDS, PhD,a Sema Belli, DDS, PhD,b and Gurcan Eskitascioglu, DDS, PhDc Faculty of Dentistry, Selcuk University, Konya, Turkey Statement of problem. The degree to which pulpal pressure may affect bond strength of complete cast crowns is unknown. Purpose. The aim of this in vitro study was to evaluate the effect of simulated pulpal pressure on the tensile bond strength of complete cast crowns luted with 2 different cements. Material and methods. Forty-eight human mandibular canine teeth were cleaned and stored in saline solution. The crowns were prepared by 1 investigator, and standardization of the preparation was accomplished by fixing a dental handpiece in a parallelometer. Uniform grooves, 0.5 mm in depth, were prepared with burs with depth guides. The complete crowns were prepared with a 0.5-mm shoulder margin. Teeth were randomly divided into 2 groups of 24 teeth each (Group I and Group II). In Group I, bonding and tensile test procedures of the teeth were carried out under simulated pulpal pressure (15 cm of saline solution). In Group II, simulated pulpal pressure was not used. The roots were removed 1 mm below the cementoenamel junction to create direct communication with the pulp chamber. The remaining pulpal tissues were carefully removed, and crowns were embedded in acrylic resin. The acrylic resin was then penetrated by a stainless steel tube that connected the pulp chamber and the barrel of a disposable plastic 5-ml syringe. The pulp chambers were filled with physiological saline solution under elevated pressure to locate the area of greatest permeability on the dentinal surface. Crowns were cast (Co-Cr alloy) with a 20-mm bar to allow testing of the bond strength. Each of the 2 groups were further divided into 2 luting cement groups of 12 each (Group I, A and B, and Group II, A and B). Group IA/ IIA and Group IB/IIB specimens were luted with a polycarboxylate luting cement (Poly-F Plus) and an adhesive luting cement (Superbond C&B), respectively. After storage in distilled water for 24 hours, all specimens were subjected to a tensile bond test in a universal testing machine at a crosshead speed of 0.5 mm/min until failure. The maximum load at fracture (Newton) was recorded. The results were then evaluated with 2-way analysis of variance and Tukey’s honestly significant difference tests (a = .05). Results. Simulated pulpal pressure increased the bond strength of cast complete crowns cemented with an adhesive luting agent (P = .01). No significant difference was found in the bond strength of complete cast crowns cemented with polycarboxylate cement with or without pulpal pressure. Superbond C&B adhesive luting agent showed significantly higher bond strength values for Group I (388.9 6 32.7) and Group II (300.9 6 66.8), when compared with polycarboxylate cement for Group I (221.3 6 17.3) and Group II (186.8 6 38.5) (P = .001). Conclusion. Simulated pulpal pressure had a positive effect on the retention of complete cast crowns when cemented with Superbond C&B adhesive luting agent. Superbond C&B significantly increased the retention of crowns in either the presence or absence of pulpal pressure. (J Prosthet Dent 2004;91:253-7.)
CLINICAL IMPLICATIONS This in vitro study demonstrated that tensile bond strength values for adhesive luting cement increased when pulpal pressure was simulated for the testing procedures.
L
oss of crown retention is the second leading cause of crown failure.1,2 Although establishment of optimal resistance and retention forms in the tooth preparation
This study was supported by Selcuk University Research Foundation (Project number: 2002/192). a Assistant Professor, Department of Prosthodontics. b Associate Professor, and Chief, Department of Endodontics. c Professor, and Chief, Department of Prosthodontics.
MARCH 2004
are of primary importance, a dental cement may also act as a barrier against microbial leakage, sealing the interface between the tooth and restoration.3 An ideal cement should provide a durable bond between dissimilar materials and possess favorable compressive and tensile strengths.2,4 Numerous laboratory studies have tested the performance of luting cements to predict clinical success.5,6 Although in vitro studies provide knowledge of the physical properties of a material, in vivo conditions may change the performance of the THE JOURNAL OF PROSTHETIC DENTISTRY 253
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 1. Bonding procedure. Pulpal pressure of 15 cm of physiological saline solution was used.
product. Variables in such tests include the source of dentin (human or bovine), tooth age, depth into dentin of the surface to be bonded, and test method (macroshear, macrotensile, microtensile).5 No study has shown an exact correlation between in vitro test values for bond strength and clinical performance.5,7 For example, bonding to dentin has been more complicated because of the characteristics of the dentin substrates, including high organic content, tubular structure variations, and the presence of outward fluid movement.8-10 Previously, in several in vitro bonding tests, fluid movement was not investigated.5,7,9 For in vitro experiments, perfusing the teeth with a solute similar to dentinal fluid is a means of more accurately simulating in vivo conditions of vital teeth.6,8,10,11 Recently, simulated pulpal pressure has been used during bonding tests.12-18 Bond strength tests showed that adhesive resin luting agents demonstrate better performance when compared with the traditional luting cements.19,20 Although factors such as thermal cycling and aging were included in these laboratory studies to simulate in vivo conditions, the effect of pulpal pressure during crown cementation was not investigated. Therefore, the purpose of this study was to evaluate the effect of simulated pulpal pressure on tensile bond strength of complete cast crowns luted with both polycarboxylate and adhesive luting cements.
MATERIAL AND METHODS Forty-eight caries-free human mandibular canine teeth extracted for periodontal reasons were used in this study. The teeth were cleaned and stored in a saline solution at room temperature during the study. 254
OZTURK, BELLI, AND ESKITASCIOGLU
The crowns were prepared by 1 investigator, and standardized preparations were accomplished by fixing a dental handpiece in a surveyor (BEGO, Bremen, Germany). Uniform grooves, 0.5 mm in depth, were placed on the labial and incisal surfaces. Three labial grooves were prepared with burs with depth guides (Laminate veneer system; Brasseler USA, Savannah, Ga) parallel to the gingival one third of the labial surface. A second set of 2 grooves was made parallel to the incisal two thirds of the uncut labial surface. The crowns were prepared with a 0.5-mm shoulder margin that followed the cementoenamel junction. Preparation for complete cast crowns was finalized with a diamond rotary cutting instrument (number 837 KR; Komet, Besigheim, Germany). The teeth were randomly divided into 2 groups of 24 teeth each. For Group I specimens, bonding and tensile test procedures of the teeth were carried out under a simulated pulpal pressure of 15 cm of saline solution.13,18 For Group II specimens, simulated pulpal pressure was not used. The roots were removed 1 mm below the cementoenamel junction to create direct communication with the pulp chamber using a slow-speed diamond saw sectioning machine (Isomet; Buehler Ltd, Lake Bluff, Ill). The remaining pulpal tissue was carefully removed with a hand instrument (Excavator; Komet), taking care not to touch the walls of the pulp chamber. The remaining crowns were embedded into acrylic resin (Meliodent; Bayer Ltd, Newbury, United Kingdom), keeping the pulp chamber empty. The acrylic resin was then penetrated by an 18-gauge stainless steel tube (Tyco Healthcare, Mansfield, Mass) that connected the pulp chamber and the barrel of a disposable plastic 5-ml syringe (Tyco Healthcare). The pulp chambers were filled with physiological saline solution under elevated pressure to locate the area of greatest permeability on the dentinal surface (Fig. 1). A pulpal pressure of 15 cm of saline solution was used.
Cast crown preparation Impressions of all tooth preparations were made with vinyl polysiloxane impression material (Speedex; Coltene/Whaledent Inc, Cuyahoga Falls, Ohio) and poured in a vacuum-mixed polyurethane die material (Alpha Die MF; Schu¨ltz-Dental GmbH, Rosbach, Germany). Wax copings were refined, sprued, and vacuum invested in a phosphate-bonded investment material (BellaStar; BEGO). Metal castings were made from Co-Cr material (cobalt-chrome solder; BEGO). Each crown was cast with a 20-mm bar on its occlusal surface to allow testing of the bond strength. The intaglio surface of the crown was airborne-particle VOLUME 91 NUMBER 3
OZTURK, BELLI, AND ESKITASCIOGLU
THE JOURNAL OF PROSTHETIC DENTISTRY
abraded with 50-mm alumina (BEGO) before cementation of the crown to the tooth.
Table I. Analysis of variance tensile bond strength results
Crown cementation
Simulated pulpal pressure (Group I, II) Luting cements (Group A, B) Simulated pulpal pressure 3 Luting cement
Each of the 2 groups was further divided into 2 subgroups according to the luting cement used. For Group IA and IIA specimens, a polycarboxylate luting cement (Poly-F Plus; Dentsply International, York, Pa) was used. In Groups IB and IIB, an adhesive luting agent (Superbond C&B; Sun Medical Co, Ltd, Moriyama City, Japan) was used. For the Poly-F Plus groups, the teeth were washed with water and dried. Two drops of liquid were mixed with 1 scoop of polycarboxylate powder with a spatula for 30 seconds, according to the manufacturer instructions. The crowns were cemented to the teeth under a static load of 2.0 kg, applied for 15 minutes. The teeth were supported in cork stoppers and placed in a stainless steel device, which was loaded with the 2.0-kg load during the cementation process.14 For the Superbond C&B groups, the green activator was applied to the dentin surfaces of the teeth for 5 seconds. One drop of catalyst was mixed with 4 drops of monomer (Superbond C&B; Sun Medical Co, Ltd) to wet the bonding surfaces. The mixture was combined with 2 scoops of powder and applied to the bonding surface. The crowns were cemented to the teeth as previously described. In Group I, A and B, a pulpal pressure of 15 cm saline solution was used during the bonding and testing procedures, as described by Ciucchi et al.18 Pulpal pressure was not used during the bonding and testing procedures in Group II, A and B. All the specimens were stored in distilled water at 378C for 24 hours, and tensile bond strength values were measured with a universal testing machine (TSTM 02500; Elista Ltd Sxti, Istanbul, Turkey) at a crosshead speed of 0.5 mm/min. The maximum load at fracture (N) was recorded. The statistical analysis was performed using statistical software (SPSS for Windows 2000, 8.0; SPSS Inc, Chicago, Ill). The means and standard deviations of the bond strengths for the 2 different luting cements were calculated for Group I and Group II. The bond strength values were analyzed with 2-way analysis of variance (ANOVA) and Tukey’s honestly significant difference (HSD) tests to determine whether a relationship existed between the simulated pulpal pressure and the luting cements tested (a=.05).
RESULTS As seen in Table I, the 2-way ANOVA indicated that bond strength was significantly affected by simulated pulpal pressure (P = .001) and luting cements (P = MARCH 2004
df
MS
F
P
1
45018.75
24.59
.001
1 1
238008.33 8586.75
130.04 4.692
.001 .036
.001). The Tukey HSD test indicated that simulated pulpal pressure significantly increased the bond strength of complete cast crowns cemented with Superbond C&B adhesive luting cement (P = .01). No significant difference was found in the bond strengths of complete cast crowns cemented with polycarboxylate cement with and without pulpal pressure. Superbond C&B showed significantly higher bond strength values for Group I (388.9 6 32.7) and Group II (300.9 6 66.8) when compared with polycarboxylate cement for Group I (221.3 6 17.3) and Group II (186.8 6 38.5) (P = .001).
DISCUSSION Bond strength testing is a commonly used method to evaluate performance of luting cements.6,16 However, concern has been expressed with regard to whether bond strength testing is a good indicator for the clinical performance of materials, because there may be a poor correlation between in vitro and in vivo performance of materials.7,16 Many factors affect in vivo bond strength, including pulpal fluid. Some investigators reported that movement of pulpal fluid through dentin under pulpal pressure has an effect on bond strength.12-17 Simulation of pulpal pressure is desirable because it more closely resembles the in vivo condition.5 Nikaido et al13 reported that testing of bonding systems with a simulated pulpal fluid such as diluted bovine serum may produce results closer to those obtained in vivo. The authors also reported that systems with a self-etching primer showed higher bond strengths when bovine serum was used. However, similar results also were found when physiological saline solution was used as simulated pulpal fluid.15 In this study, physiological saline solution was used to create simulated pulpal pressure because determination of the effect of pulpal pressure was more important than the ingredients of the fluid. In previous bond strength studies, flat dentin surfaces were generally used.13,15,16,18 However, clinically, flat dentin surfaces are not used for adhesive restorations. Other factors affect the adhesion procedure, including C factor, preparation design, and preparation depth. C factor is defined as the ratio of bonded to unbonded surface areas.8 The stress generated in different 255
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preparation designs is proportional to the configuration (C factor) of the preparation.20 In an in vitro study, Belli et al16 reported that pulpal pressure had different effects on occlusal or axial dentin surfaces. During crown cementation, many surfaces are involved with the bonding procedure. Therefore, in the present study, cementation to flat surfaces was not used to predict the effect of pulpal pressure. Unlike the results of previous bond testing studies on flat surfaces, the results of the present study indicated that pulpal pressure has a positive effect on bond strength, thus demonstrating that testing bond strength on flat dentin surfaces may not be reliable. In previous in vitro studies, wettability affected adhesion negatively because the flat dentin surfaces did not have a marginal seal.13,15,16,18 In the present study, the peripheral seal was provided by complete cast crowns, similar to in vivo conditions. Consequently, it is hypothesized that bond strength increased with pulpal pressure because of the negative pressure in the dentinal tubulus. Thus, on the basis of the results of the present study, in vivo crown retention may be improved compared with in vitro conditions. Musajo et al12 evaluated the effects of environmental pressure on the retention of complete cast crowns and found that the retention of complete cast crowns cemented with zinc phosphate cement was reduced after pressure cycling. Zinc phosphate cement provides retention by mechanical means only. The mechanical retention provided by zinc phosphate cement, as opposed to the micromechanical retention of the resin luting agents or the chemical retention of polycarboxylate cement, was cited as one of the limiting factors for the retention of crowns in the Musajo et al12 study. Simulated pulpal pressure did not affect bond strength of cast complete crowns cemented with a polycarboxylate cement in the present study. Most conventional luting cements, such as zinc phosphate, polycarboxylate, and glass ionomer, are brittle and susceptible to tensile failure.19 Resin luting agents have higher tensile strengths and are therefore less susceptible to this mode of failure.19 Resin luting agents are manufactured specifically for the cementation of crowns19 and are generally used in conjunction with, or contain, dentin bonding agents. Superior strength and bonding to teeth, reduced microleakage, and increased retention of complete coverage castings have been reported for resin luting agents.19 In this study, Superbond C&B adhesive luting agent was used. Some studies have indicated that adhesive luting cements demonstrate strong bonds to moist dentin.17 It has been suggested that luting agents containing hydrophilic primer provide a stronger bond to wet dentin surfaces. The bond strength of the Superbond C&B was found to be 256
OZTURK, BELLI, AND ESKITASCIOGLU
higher than the polycarboxylate cement. On the other hand, simulated pulpal pressure had a positive effect on bond strength of complete cast crowns cemented with Superbond C&B adhesive luting cement in the present study. However, this was an in vitro study and only 2 cements were tested—a polycarboxylate and Superbond C&B. Clinical trials are necessary to validate the results of this in vitro study. Although factors such as thermal cycling, aging, and loading were included in laboratory studies to simulate in vivo conditions, these factors were eliminated in this study to investigate the effect of pulpal pressure alone.
CONCLUSION Within the limitations of this study, the following conclusions were drawn: 1. Simulated pulpal pressure had a positive effect on the retention of complete cast crowns cemented with the adhesive luting cement tested. 2. Tensile bond strength values of the adhesive luting cement were significantly higher with pulpal pressure, 388.9 6 32.7; and without pulpal pressure, 300.9 6 66.8, when compared with polycarboxylate cement with pulpal pressure, 221.3 6 17.3; and without pulpal pressure, 186.8 6 38.5 (P = .001).
REFERENCES 1. Schwartz NL, Whitsett LD, Berry TG, Stewart JL. Unserviceable crowns and fixed partial dentures: life span and causes for loss of serviceability. J Am Dent Assoc 1970;81:1395-401. 2. Diaz-Arnold AM, Vargas MA, Haselton DR. Current status of luting agents for fixed prosthodontics. J Prosthet Dent 1999;81:135-41. 3. Pameijer CH, Nilner K. Long term clinical evaluation of three luting materials. Swed Dent J 1994;18:59-67. 4. Smith DC. Dental cements: current status and future prospects. Dent Clin North Am 1983;27:763-92. 5. Allen EP, Bayne SC, Brodine AH, Cronin RJ Jr, Donovan TE, Kois JC, Summitt JB. Committee on Scientific Investigation of the American Academy of Restorative Dentistry. Annual review of selected dental literature: report of the Committee on Scientific Investigation of the American Academy of Restorative Dentistry. J Prosthet Dent 2002;88: 60-88. 6. Jendresen MD, Allen EP, Bayne SC, Donovan TE, Hansson TL, Klooster J, Kois JC. Annual review of selected dental literature: report of the Committee on Scientific Investigation of the American Academy of Restorative Dentistry. J Prosthet Dent 1995;74:60-99. 7. Hasegawa T, Retief DH, Russell CM, Denys FR. Shear bond strength and quantitative microleakage of a multipurpose dental adhesive system resin bonded to dentin. J Prosthet Dent 1995;73:432-8. 8. Van Meerbeek B, Lambrechts P, Inokoshi S, Braem M, Vanherle G. Factors affecting adhesion to mineralized tissues. Oper Dent 1992;5: 111-24. 9. Burrow MF, Tagami J, Negishi T, Nikaido T, Hosoda H. Early tensile bond strengths of several enamel and dentin bonding systems. J Dent Res 1994;73:522-8. 10. Swift EJ Jr, Perdigao J, Heymann HO. Bonding to enamel and dentin: a brief history and state of the art, 1995. Quintessence Int 1995;26:95110. 11. Pashley DH. Clinical correlations of dentin structure and function. J Prosthet Dent 1991;66:777-81.
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12. Musajo F, Passi P, Girardello GB, Rusca F. The influence of environmental pressure on retentiveness of prosthetic crowns:an experimental study. Quintessence Int 1992;23:367-9. 13. Nikaido T, Burrow MF, Tagami J, Takatsu T. Effect of pulpal pressure on adhesion of resin composite to dentin: bovine serum versus saline. Quintessence Int 1995;26:221-6. 14. Lyons KM, Rodda JC, Hood JA. The effect of environmental pressure changes during diving on the retentive strength of different luting cements for full cast crowns. J Prosthet Dent 1997;78:522-7. 15. Prati C, Pashley DH, Montanari G. Hydrostatic intrapulpal pressure and bond strength of bonding systems. Dent Mater 1991;7:54-8. ¨ zer F. Bonding strength to two different surfaces of ¨ nlu¨ N, O 16. Belli S, U dentin under simulated pulpal pressure. J Adhes Dent 2001;3:145-52. 17. Gwinnett AJ. Moist versus dry dentin: its effect on shear bond strength. Am J Dent 1992;5:127-9. 18. Ciucchi B, Bouillaguet S, Holz J, Pashley D. Dentinal fluid dynamics in human teeth, in vivo. J Endod 1995;21:191-4. 19. White SN, Yu Z. Physical properties of fixed prosthodontic, resin composite luting agents. Int J Prosthodont 1993;6:384-9.
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20. Hahn P, Attin T, Grofke M, Hellwig E. Influence of resin cement viscosity on microleakage of ceramic inlays. Dent Mater 2001;17:191-6. Reprint requests to: DR A. NILGUN OZTURK SELCUK UNIVERSITY DEPARTMENT OF PROSTHODONTICS FACULTY OF DENTISTRY CAMPUS, KONYA TURKEY FAX: 90-332-2410062 E-MAIL: [email protected] 0022-3913/$30.00 Copyright ª 2004 by the Editorial Council of The Journal of Prosthetic Dentistry
doi:10.1016/j.prosdent.2004.01.003
257
CLINICAL IMPLICATIONS This in vitro study demonstrated that tensile bond strength values for adhesive luting cement increased when pulpal pressure was simulated for the testing procedures.
L
oss of crown retention is the second leading cause of crown failure.1,2 Although establishment of optimal resistance and retention forms in the tooth preparation
This study was supported by Selcuk University Research Foundation (Project number: 2002/192). a Assistant Professor, Department of Prosthodontics. b Associate Professor, and Chief, Department of Endodontics. c Professor, and Chief, Department of Prosthodontics.
MARCH 2004
are of primary importance, a dental cement may also act as a barrier against microbial leakage, sealing the interface between the tooth and restoration.3 An ideal cement should provide a durable bond between dissimilar materials and possess favorable compressive and tensile strengths.2,4 Numerous laboratory studies have tested the performance of luting cements to predict clinical success.5,6 Although in vitro studies provide knowledge of the physical properties of a material, in vivo conditions may change the performance of the THE JOURNAL OF PROSTHETIC DENTISTRY 253
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 1. Bonding procedure. Pulpal pressure of 15 cm of physiological saline solution was used.
product. Variables in such tests include the source of dentin (human or bovine), tooth age, depth into dentin of the surface to be bonded, and test method (macroshear, macrotensile, microtensile).5 No study has shown an exact correlation between in vitro test values for bond strength and clinical performance.5,7 For example, bonding to dentin has been more complicated because of the characteristics of the dentin substrates, including high organic content, tubular structure variations, and the presence of outward fluid movement.8-10 Previously, in several in vitro bonding tests, fluid movement was not investigated.5,7,9 For in vitro experiments, perfusing the teeth with a solute similar to dentinal fluid is a means of more accurately simulating in vivo conditions of vital teeth.6,8,10,11 Recently, simulated pulpal pressure has been used during bonding tests.12-18 Bond strength tests showed that adhesive resin luting agents demonstrate better performance when compared with the traditional luting cements.19,20 Although factors such as thermal cycling and aging were included in these laboratory studies to simulate in vivo conditions, the effect of pulpal pressure during crown cementation was not investigated. Therefore, the purpose of this study was to evaluate the effect of simulated pulpal pressure on tensile bond strength of complete cast crowns luted with both polycarboxylate and adhesive luting cements.
MATERIAL AND METHODS Forty-eight caries-free human mandibular canine teeth extracted for periodontal reasons were used in this study. The teeth were cleaned and stored in a saline solution at room temperature during the study. 254
OZTURK, BELLI, AND ESKITASCIOGLU
The crowns were prepared by 1 investigator, and standardized preparations were accomplished by fixing a dental handpiece in a surveyor (BEGO, Bremen, Germany). Uniform grooves, 0.5 mm in depth, were placed on the labial and incisal surfaces. Three labial grooves were prepared with burs with depth guides (Laminate veneer system; Brasseler USA, Savannah, Ga) parallel to the gingival one third of the labial surface. A second set of 2 grooves was made parallel to the incisal two thirds of the uncut labial surface. The crowns were prepared with a 0.5-mm shoulder margin that followed the cementoenamel junction. Preparation for complete cast crowns was finalized with a diamond rotary cutting instrument (number 837 KR; Komet, Besigheim, Germany). The teeth were randomly divided into 2 groups of 24 teeth each. For Group I specimens, bonding and tensile test procedures of the teeth were carried out under a simulated pulpal pressure of 15 cm of saline solution.13,18 For Group II specimens, simulated pulpal pressure was not used. The roots were removed 1 mm below the cementoenamel junction to create direct communication with the pulp chamber using a slow-speed diamond saw sectioning machine (Isomet; Buehler Ltd, Lake Bluff, Ill). The remaining pulpal tissue was carefully removed with a hand instrument (Excavator; Komet), taking care not to touch the walls of the pulp chamber. The remaining crowns were embedded into acrylic resin (Meliodent; Bayer Ltd, Newbury, United Kingdom), keeping the pulp chamber empty. The acrylic resin was then penetrated by an 18-gauge stainless steel tube (Tyco Healthcare, Mansfield, Mass) that connected the pulp chamber and the barrel of a disposable plastic 5-ml syringe (Tyco Healthcare). The pulp chambers were filled with physiological saline solution under elevated pressure to locate the area of greatest permeability on the dentinal surface (Fig. 1). A pulpal pressure of 15 cm of saline solution was used.
Cast crown preparation Impressions of all tooth preparations were made with vinyl polysiloxane impression material (Speedex; Coltene/Whaledent Inc, Cuyahoga Falls, Ohio) and poured in a vacuum-mixed polyurethane die material (Alpha Die MF; Schu¨ltz-Dental GmbH, Rosbach, Germany). Wax copings were refined, sprued, and vacuum invested in a phosphate-bonded investment material (BellaStar; BEGO). Metal castings were made from Co-Cr material (cobalt-chrome solder; BEGO). Each crown was cast with a 20-mm bar on its occlusal surface to allow testing of the bond strength. The intaglio surface of the crown was airborne-particle VOLUME 91 NUMBER 3
OZTURK, BELLI, AND ESKITASCIOGLU
THE JOURNAL OF PROSTHETIC DENTISTRY
abraded with 50-mm alumina (BEGO) before cementation of the crown to the tooth.
Table I. Analysis of variance tensile bond strength results
Crown cementation
Simulated pulpal pressure (Group I, II) Luting cements (Group A, B) Simulated pulpal pressure 3 Luting cement
Each of the 2 groups was further divided into 2 subgroups according to the luting cement used. For Group IA and IIA specimens, a polycarboxylate luting cement (Poly-F Plus; Dentsply International, York, Pa) was used. In Groups IB and IIB, an adhesive luting agent (Superbond C&B; Sun Medical Co, Ltd, Moriyama City, Japan) was used. For the Poly-F Plus groups, the teeth were washed with water and dried. Two drops of liquid were mixed with 1 scoop of polycarboxylate powder with a spatula for 30 seconds, according to the manufacturer instructions. The crowns were cemented to the teeth under a static load of 2.0 kg, applied for 15 minutes. The teeth were supported in cork stoppers and placed in a stainless steel device, which was loaded with the 2.0-kg load during the cementation process.14 For the Superbond C&B groups, the green activator was applied to the dentin surfaces of the teeth for 5 seconds. One drop of catalyst was mixed with 4 drops of monomer (Superbond C&B; Sun Medical Co, Ltd) to wet the bonding surfaces. The mixture was combined with 2 scoops of powder and applied to the bonding surface. The crowns were cemented to the teeth as previously described. In Group I, A and B, a pulpal pressure of 15 cm saline solution was used during the bonding and testing procedures, as described by Ciucchi et al.18 Pulpal pressure was not used during the bonding and testing procedures in Group II, A and B. All the specimens were stored in distilled water at 378C for 24 hours, and tensile bond strength values were measured with a universal testing machine (TSTM 02500; Elista Ltd Sxti, Istanbul, Turkey) at a crosshead speed of 0.5 mm/min. The maximum load at fracture (N) was recorded. The statistical analysis was performed using statistical software (SPSS for Windows 2000, 8.0; SPSS Inc, Chicago, Ill). The means and standard deviations of the bond strengths for the 2 different luting cements were calculated for Group I and Group II. The bond strength values were analyzed with 2-way analysis of variance (ANOVA) and Tukey’s honestly significant difference (HSD) tests to determine whether a relationship existed between the simulated pulpal pressure and the luting cements tested (a=.05).
RESULTS As seen in Table I, the 2-way ANOVA indicated that bond strength was significantly affected by simulated pulpal pressure (P = .001) and luting cements (P = MARCH 2004
df
MS
F
P
1
45018.75
24.59
.001
1 1
238008.33 8586.75
130.04 4.692
.001 .036
.001). The Tukey HSD test indicated that simulated pulpal pressure significantly increased the bond strength of complete cast crowns cemented with Superbond C&B adhesive luting cement (P = .01). No significant difference was found in the bond strengths of complete cast crowns cemented with polycarboxylate cement with and without pulpal pressure. Superbond C&B showed significantly higher bond strength values for Group I (388.9 6 32.7) and Group II (300.9 6 66.8) when compared with polycarboxylate cement for Group I (221.3 6 17.3) and Group II (186.8 6 38.5) (P = .001).
DISCUSSION Bond strength testing is a commonly used method to evaluate performance of luting cements.6,16 However, concern has been expressed with regard to whether bond strength testing is a good indicator for the clinical performance of materials, because there may be a poor correlation between in vitro and in vivo performance of materials.7,16 Many factors affect in vivo bond strength, including pulpal fluid. Some investigators reported that movement of pulpal fluid through dentin under pulpal pressure has an effect on bond strength.12-17 Simulation of pulpal pressure is desirable because it more closely resembles the in vivo condition.5 Nikaido et al13 reported that testing of bonding systems with a simulated pulpal fluid such as diluted bovine serum may produce results closer to those obtained in vivo. The authors also reported that systems with a self-etching primer showed higher bond strengths when bovine serum was used. However, similar results also were found when physiological saline solution was used as simulated pulpal fluid.15 In this study, physiological saline solution was used to create simulated pulpal pressure because determination of the effect of pulpal pressure was more important than the ingredients of the fluid. In previous bond strength studies, flat dentin surfaces were generally used.13,15,16,18 However, clinically, flat dentin surfaces are not used for adhesive restorations. Other factors affect the adhesion procedure, including C factor, preparation design, and preparation depth. C factor is defined as the ratio of bonded to unbonded surface areas.8 The stress generated in different 255
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preparation designs is proportional to the configuration (C factor) of the preparation.20 In an in vitro study, Belli et al16 reported that pulpal pressure had different effects on occlusal or axial dentin surfaces. During crown cementation, many surfaces are involved with the bonding procedure. Therefore, in the present study, cementation to flat surfaces was not used to predict the effect of pulpal pressure. Unlike the results of previous bond testing studies on flat surfaces, the results of the present study indicated that pulpal pressure has a positive effect on bond strength, thus demonstrating that testing bond strength on flat dentin surfaces may not be reliable. In previous in vitro studies, wettability affected adhesion negatively because the flat dentin surfaces did not have a marginal seal.13,15,16,18 In the present study, the peripheral seal was provided by complete cast crowns, similar to in vivo conditions. Consequently, it is hypothesized that bond strength increased with pulpal pressure because of the negative pressure in the dentinal tubulus. Thus, on the basis of the results of the present study, in vivo crown retention may be improved compared with in vitro conditions. Musajo et al12 evaluated the effects of environmental pressure on the retention of complete cast crowns and found that the retention of complete cast crowns cemented with zinc phosphate cement was reduced after pressure cycling. Zinc phosphate cement provides retention by mechanical means only. The mechanical retention provided by zinc phosphate cement, as opposed to the micromechanical retention of the resin luting agents or the chemical retention of polycarboxylate cement, was cited as one of the limiting factors for the retention of crowns in the Musajo et al12 study. Simulated pulpal pressure did not affect bond strength of cast complete crowns cemented with a polycarboxylate cement in the present study. Most conventional luting cements, such as zinc phosphate, polycarboxylate, and glass ionomer, are brittle and susceptible to tensile failure.19 Resin luting agents have higher tensile strengths and are therefore less susceptible to this mode of failure.19 Resin luting agents are manufactured specifically for the cementation of crowns19 and are generally used in conjunction with, or contain, dentin bonding agents. Superior strength and bonding to teeth, reduced microleakage, and increased retention of complete coverage castings have been reported for resin luting agents.19 In this study, Superbond C&B adhesive luting agent was used. Some studies have indicated that adhesive luting cements demonstrate strong bonds to moist dentin.17 It has been suggested that luting agents containing hydrophilic primer provide a stronger bond to wet dentin surfaces. The bond strength of the Superbond C&B was found to be 256
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higher than the polycarboxylate cement. On the other hand, simulated pulpal pressure had a positive effect on bond strength of complete cast crowns cemented with Superbond C&B adhesive luting cement in the present study. However, this was an in vitro study and only 2 cements were tested—a polycarboxylate and Superbond C&B. Clinical trials are necessary to validate the results of this in vitro study. Although factors such as thermal cycling, aging, and loading were included in laboratory studies to simulate in vivo conditions, these factors were eliminated in this study to investigate the effect of pulpal pressure alone.
CONCLUSION Within the limitations of this study, the following conclusions were drawn: 1. Simulated pulpal pressure had a positive effect on the retention of complete cast crowns cemented with the adhesive luting cement tested. 2. Tensile bond strength values of the adhesive luting cement were significantly higher with pulpal pressure, 388.9 6 32.7; and without pulpal pressure, 300.9 6 66.8, when compared with polycarboxylate cement with pulpal pressure, 221.3 6 17.3; and without pulpal pressure, 186.8 6 38.5 (P = .001).
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12. Musajo F, Passi P, Girardello GB, Rusca F. The influence of environmental pressure on retentiveness of prosthetic crowns:an experimental study. Quintessence Int 1992;23:367-9. 13. Nikaido T, Burrow MF, Tagami J, Takatsu T. Effect of pulpal pressure on adhesion of resin composite to dentin: bovine serum versus saline. Quintessence Int 1995;26:221-6. 14. Lyons KM, Rodda JC, Hood JA. The effect of environmental pressure changes during diving on the retentive strength of different luting cements for full cast crowns. J Prosthet Dent 1997;78:522-7. 15. Prati C, Pashley DH, Montanari G. Hydrostatic intrapulpal pressure and bond strength of bonding systems. Dent Mater 1991;7:54-8. ¨ zer F. Bonding strength to two different surfaces of ¨ nlu¨ N, O 16. Belli S, U dentin under simulated pulpal pressure. J Adhes Dent 2001;3:145-52. 17. Gwinnett AJ. Moist versus dry dentin: its effect on shear bond strength. Am J Dent 1992;5:127-9. 18. Ciucchi B, Bouillaguet S, Holz J, Pashley D. Dentinal fluid dynamics in human teeth, in vivo. J Endod 1995;21:191-4. 19. White SN, Yu Z. Physical properties of fixed prosthodontic, resin composite luting agents. Int J Prosthodont 1993;6:384-9.
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20. Hahn P, Attin T, Grofke M, Hellwig E. Influence of resin cement viscosity on microleakage of ceramic inlays. Dent Mater 2001;17:191-6. Reprint requests to: DR A. NILGUN OZTURK SELCUK UNIVERSITY DEPARTMENT OF PROSTHODONTICS FACULTY OF DENTISTRY CAMPUS, KONYA TURKEY FAX: 90-332-2410062 E-MAIL: [email protected] 0022-3913/$30.00 Copyright ª 2004 by the Editorial Council of The Journal of Prosthetic Dentistry
doi:10.1016/j.prosdent.2004.01.003
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