The Effect of Autopolymerizing Acrylic Resin Thickness on the Bond Strength of a Repaired Denture Tooth Wendy Auclair Clark, DDS, MS1 & Yung-Tsung Hsu, DDS, MS2 1 2
Private Practice, Goldstein, Garber, & Salama LLC, Atlanta, GA Department of Prosthodontics and Biomaterials, UAB School of Dentistry, Birmingham, AL The article is associated with the American College of Prosthodontists’ journal-based continuing education program. It is accompanied by an online continuing education activity worth 1 credit. Please visit www.wileyhealthlearning.com/jopr to complete the activity and earn credit.
Keywords Poly(methyl methacrylate); PMMA; bonding; removable. Correspondence Wendy A. Clark, Goldstein, Garber, & Salama LLC, Private Practice, 600 Galleria Pkwy SE Ste 800, Atlanta, GA 30339. E-mail: [email protected] The authors deny any conflicts of interest. Accepted November 1, 2013 doi: 10.1111/jopr.12156
Abstract Purpose: This study analyzed the conventional method of rebonding a denture tooth, evaluating the effect of varied thickness of autopolymerizing acrylic resin on the bond strength and the failure mode. Materials and Methods: A total of 52 heat-polymerizing acrylic resin specimens were fabricated with an anterior denture tooth. A cantilever-type bending force was applied with a universal testing machine to each specimen until failure. The failure mode was determined, and cohesive failures were excluded from part II. Thirty specimens were randomly selected and divided into three groups (n = 10). For each group, resin was relieved from the bonding area to create a 0, 1, or 3 mm space. The tooth was repositioned using its matrix and reattached to its base, filling the relieved space with autopolymerizing acrylic resin. The repaired specimens were tested using the same parameters. Data were analyzed with paired t-tests, one-way ANOVA, and post hoc test. Statistical significance was determined at p < 0.05. Results: The mean peak load to failure for the part I group was 88.91 N. While the peak load to failure decreased to 71.96 N (19.69% loss of original bond strength), statistical analysis revealed no difference between the bond strength of the specimens repaired with a 0 mm thickness of autopolymerizing acrylic resin and the original (part I) group (p > 0.05). The bond strength was lower for the group repaired with a 1 mm thickness compared to the original (part I) group (p < 0.05), with 65.8 N load to failure (29.63% loss). The bond strength was even lower for the group repaired with a 3 mm thickness (p < 0.05), with 58.64 N load to failure (33.07% loss). Post hoc analysis revealed a significant difference between the 0 and 3 mm groups (p = 0.04). The most common failure mode in the original group was adhesive (56%), then combination (34%), then cohesive (9.8%). The repaired group (n = 30) had similar results, with 56.7% adhesive, 36.7% combination, and 6.7% cohesive failures. Conclusions: The bond strength of a replaced denture tooth is affected by the thickness of the autopolymerizing acrylic resin. The failure mode of a rebonded denture tooth follows the same trend of the original failure. If possible, replace teeth with no relief. If combination failure occurs, leave residual base acrylic resin on the ridge lap.
Clinically, one of the most common denture repairs is replacing debonded anterior denture teeth.1-5 Clinicians are seeing an increase in the incidence of debonded denture teeth with the growing popularity of implant-supported acrylic prosthetics. Patients with implant-supported prosthetics have an increased masticatory force, leading toward the likelihood of acrylic fracture and tooth debonding.6-9 Several studies have investigated factors influencing the bonding of an acrylic denture tooth to its denture base. The more highly cross-linked the matrix of the denture tooth, the more dif528
ficult for the solvent to penetrate and form a chemical bond.10-16 Chemical applications to improve bonding have demonstrated varying results. The application of dichloromethane appears to greatly improve bond strength.16,17 This chemical is highly toxic, however, so the clinical applicability is questionable. The addition of methacrylate-based bonding agents has also consistently demonstrated an increase in bond strength.13,18-20 The application of monomer (MMA) produces variable results.17,21 Several studies found that its application to the ridge lap surface before processing had no significant effect on the bond
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 23 (2014) 528–533
Clark and Hsu
strength.2,3,22 Morrow et al23 found that the application of monomer decreased the bond strength, and others found that monomer application improved the bond.21,24 The variation in results is likely due to difference in technique. Adding mechanical retention to the denture base acrylic resin has also been investigated. Removing the glazed surface of the ridge lap portion of the denture tooth with air abrasion or grinding likely increases the bond strength of the acrylic resin denture tooth to the acrylic resin denture base.18,21,25-27 Other techniques include adding retentive grooves,28 a diatoric recess,18 and a mechanical “cingulum ledge lock” to improve bonding strength strength.29 The standard way to replace a debonded tooth is to reattach it to the denture base with autopolymerizing acrylic resin. Several studies found a stronger bond with heat-activated versus autopolymerizing acrylic resin.17,30-32 Due to ease of use, however, autopolymerizing acrylic resin remains the standard laboratory technique for most denture repairs. While many studies have investigated the bond strength of acrylic resin denture teeth to the denture base, few have investigated the bond strength of acrylic resin denture teeth that have been replaced after debonding. Shen et al32 debonded denture teeth from heat-polymerized acrylic resin bases and relieved 2 mm of acrylic resin from the denture base. Various treatments were performed on the ridge lap surface of the tooth. The tooth was placed in the same position with a stone index, and the 2 mm space was filled with either heat-polymerizing acrylic resin or autopolymerizing acrylic resin. The specimens were then retested, and it was found that heat-polymerizing acrylic resin formed a stronger bond to the tooth than autopolymerizing acrylic resin. The same study also found that treatment with chloroform increased the strength of the repair.32 Vallittu et al17 fabricated heat-polymerized specimens and fabricated an aluminum mold to maintain the tooth position. The teeth were replaced with the aluminum mold, and MMA was applied to the ridge lap for varying amounts of time before testing. Their results confirmed that the bond strength of the repaired tooth is weaker than the original bond and that bond strength increased as the time that MMA was in contact with the tooth increased.17 Despite advances in materials, replacing debonded denture teeth remains one of the most common denture repairs. Due to the lack of literature involving teeth that are debonded and replaced, additional research is needed in this area. It is the goal of this in vitro study to determine if a relationship exists between the thickness of repair acrylic resin at its interface with a replaced denture tooth and its bonding strength. Additionally, the failure mode of the repaired specimens will be evaluated to analyze the weakest point of the repair. The null hypothesis states that the thickness of autopolymerizing acrylic resin used to repair a denture tooth will not affect its bonding strength.
Materials and methods Fabrication of molds
This study consisted of two parts. Part I tested heat-polymerized acrylic resin specimens; part II tested specimens repaired with autopolymerizing acrylic resin. To retain the denture tooth, a wax pattern was fabricated in a shape established during a pilot study done previously by the authors. This pattern was du-
Thickness of Repair Acrylic and Bond Strength
Figure 1 A master mold was fabricated out of a laboratory silicone and used to fabricate 50 wax patterns. These wax patterns would be processed with heat-polymerizing acrylic resin and serve as the denture bases for the study.
plicated using a silicone material (REDU-It; American Dental Supply Inc, Easton, PA) to fabricate a mold. This silicone mold was used to standardize the wax patterns for part I (Fig 1). A denture tooth was positioned in the center of the wax pattern, and the specimen was processed using standard laboratory technique to preserve this position. A laboratory silicone (Sil-Tech; Ivoclar Vivadent, Schaan, Liechtenstein) was adapted to the facial surface of the tooth and around the acrylic resin base,18 which became the mold to position the teeth for the specimens in part I (Fig 2). Specimen fabrication for part I
The ridge lap surfaces of 50 maxillary right central incisor denture teeth (BlueLine Mould A14; Ivoclar Vivadent, Amherst, NY) were airborne particle abraded for 10 seconds with 50-µm aluminum oxide particles to remove the glaze layer, then steam cleaned with distilled water to remove any residue. Fifty specimens were fabricated as described. Baseplate wax was heated and flowed into the master mold to form a wax pattern. This wax pattern was positioned in the silicone tooth positioning mold along with a prepared denture tooth, and the tooth was attached to the wax pattern using a heated instrument. The specimen was invested in a maxillary denture flask to simulate dental laboratory conditions.11,33,34 Each specimen was processed with a heat-polymerizing acrylic resin (ProBase
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 23 (2014) 528–533
529
Thickness of Repair Acrylic and Bond Strength
Clark and Hsu
Figure 3 Example of an adhesive failure. Note the lack of residual acrylic resin over the entire bonding surface.
Figure 2 A second silicone mold was fabricated to standardize tooth position. Each wax pattern was placed in the mold, and the denture tooth was attached with baseplate wax.
Hot; Ivoclar Vivadent, Amherst, NY), following manufacturer’s instructions. After divesting and finishing, plaster was adapted to each specimen to serve as a tooth repositioning index for part II of the study. Testing part I specimens
A metal table was bolted to the base of a universal testing machine (model 5575; Instron machine, Norwood, MA). A chisel-shaped attachment was inserted, and the specimen was placed so the attachment would contact at the junction of the middle and incisal third of the lingual surface of the central incisor, 3 mm from the incisal edge. The central incisor was then subjected to a cantilever-type bending force at a 5 mm/min crosshead speed at an angle of 127° to the surface of the tooth until fracture.9,35,36 Upon fracture, the specimen was removed from the testing machine, and the maximum applied load was recorded. To determine the failure mode, each debonded denture tooth was examined under 10× magnification using a stereomicroscope (Renfert Mobiliskop; Renfert GnbH, Hilzingen, Germany) by a second year prosthodontic resident, an experienced dental laboratory technician, and the author (WAC). Each observer examined the bonding surface of the denture tooth, di530
Figure 4 Example of a combination failure, with a grid to demonstrate how the bonding surface was divided to determine failure mode. This example had residual acrylic resin in six of nine areas of the grid.
vided the tooth into ninths, and marked the areas with retained acrylic resin. The failure mode was classified as adhesive failure (debonding at interface between denture tooth and denture base acrylic resin), cohesive failure (fracture completely within denture tooth or denture base acrylic resin), or a combination adhesive/cohesive failure (Figs 3 to 5). For purposes of this study, a failure was considered adhesive if acrylic resin was present on 0.05). The bond strength was lower for the group repaired with a 1 mm thickness of autopolymerizing acrylic resin compared to the original (part I) group (p < 0.05), with 65.8 N load to failure (a 29.63% loss in bond strength). The bond strength was even lower for the group repaired with a 3 mm thickness of autopolymerizing acrylic resin (p < 0.05), with 58.64 N load to failure (a 33.07% loss in bond strength). The failure mode was recorded for each group. The most common failure mode in the original group (part I) was adhesive failure (56.00%), followed by combination failure (34.00%). In the repaired group, the trend stayed the same with 56.67% of the failures being adhesive (Fig 6). Considering the failure mode of the repaired (part II) specimens, the original bond strength was lower for those that failed adhesively (Table 2).
Discussion For years, studies have investigated different chemical and mechanical variables to determine what may increase or decrease the bond strength of a denture tooth to its base. This study evaluated the relationship between the thickness of repair acrylic at its interface with a replaced denture tooth and its bond strength. The null hypothesis was not accepted, as the results indicate a relationship is present. As the thickness of the autopolymerizing acrylic resin at the interface increases, the bond strength of the repair decreases. The results are consistent with other studies demonstrating a lower bond strength between denture teeth and autopolymerizing acrylic resin versus heat-polymerizing acrylic resin.23,30-32 The bond strength cannot directly be compared to those of other investigators due to differences in methods. This is a common statement among investigators of bond strength of denture teeth. Several countries have their own national specifications, and they each have very different methods for evaluating bond strength. Several investigators have acknowledged the limited clinical applicability of these tests.5,37 Because of this, investigators are continually trying to make a more clinically applicable testing method.2,3,9,33,38 Results of this study indicated a majority of adhesive failures for both the original and repair groups. These results could
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 23 (2014) 528–533
531
Thickness of Repair Acrylic and Bond Strength
Clark and Hsu
Figure 6 Percent (%) failure mode for part I (original) and part II (repaired) groups. The failure mode trends are consistent whether it was the original or repaired failure.
Table 2 Loss of bond strength (%) based on failure mode Failure mode
Original
Repaired
Loss of bond strength
Adhesive Combination
73.819 N 104.001 N
63.037 N 67.891 N
14.60% 34.72%
Note: There was a greater loss of bond strength with the combination failure group.
also not be directly compared to those of other investigators, as there is not a consensus on the definition of adhesive and cohesive failures. Following Chung et al,21 this study defined an adhesive (interfacial) failure as retained denture base acrylic resin on
Private Practice, Goldstein, Garber, & Salama LLC, Atlanta, GA Department of Prosthodontics and Biomaterials, UAB School of Dentistry, Birmingham, AL The article is associated with the American College of Prosthodontists’ journal-based continuing education program. It is accompanied by an online continuing education activity worth 1 credit. Please visit www.wileyhealthlearning.com/jopr to complete the activity and earn credit.
Keywords Poly(methyl methacrylate); PMMA; bonding; removable. Correspondence Wendy A. Clark, Goldstein, Garber, & Salama LLC, Private Practice, 600 Galleria Pkwy SE Ste 800, Atlanta, GA 30339. E-mail: [email protected] The authors deny any conflicts of interest. Accepted November 1, 2013 doi: 10.1111/jopr.12156
Abstract Purpose: This study analyzed the conventional method of rebonding a denture tooth, evaluating the effect of varied thickness of autopolymerizing acrylic resin on the bond strength and the failure mode. Materials and Methods: A total of 52 heat-polymerizing acrylic resin specimens were fabricated with an anterior denture tooth. A cantilever-type bending force was applied with a universal testing machine to each specimen until failure. The failure mode was determined, and cohesive failures were excluded from part II. Thirty specimens were randomly selected and divided into three groups (n = 10). For each group, resin was relieved from the bonding area to create a 0, 1, or 3 mm space. The tooth was repositioned using its matrix and reattached to its base, filling the relieved space with autopolymerizing acrylic resin. The repaired specimens were tested using the same parameters. Data were analyzed with paired t-tests, one-way ANOVA, and post hoc test. Statistical significance was determined at p < 0.05. Results: The mean peak load to failure for the part I group was 88.91 N. While the peak load to failure decreased to 71.96 N (19.69% loss of original bond strength), statistical analysis revealed no difference between the bond strength of the specimens repaired with a 0 mm thickness of autopolymerizing acrylic resin and the original (part I) group (p > 0.05). The bond strength was lower for the group repaired with a 1 mm thickness compared to the original (part I) group (p < 0.05), with 65.8 N load to failure (29.63% loss). The bond strength was even lower for the group repaired with a 3 mm thickness (p < 0.05), with 58.64 N load to failure (33.07% loss). Post hoc analysis revealed a significant difference between the 0 and 3 mm groups (p = 0.04). The most common failure mode in the original group was adhesive (56%), then combination (34%), then cohesive (9.8%). The repaired group (n = 30) had similar results, with 56.7% adhesive, 36.7% combination, and 6.7% cohesive failures. Conclusions: The bond strength of a replaced denture tooth is affected by the thickness of the autopolymerizing acrylic resin. The failure mode of a rebonded denture tooth follows the same trend of the original failure. If possible, replace teeth with no relief. If combination failure occurs, leave residual base acrylic resin on the ridge lap.
Clinically, one of the most common denture repairs is replacing debonded anterior denture teeth.1-5 Clinicians are seeing an increase in the incidence of debonded denture teeth with the growing popularity of implant-supported acrylic prosthetics. Patients with implant-supported prosthetics have an increased masticatory force, leading toward the likelihood of acrylic fracture and tooth debonding.6-9 Several studies have investigated factors influencing the bonding of an acrylic denture tooth to its denture base. The more highly cross-linked the matrix of the denture tooth, the more dif528
ficult for the solvent to penetrate and form a chemical bond.10-16 Chemical applications to improve bonding have demonstrated varying results. The application of dichloromethane appears to greatly improve bond strength.16,17 This chemical is highly toxic, however, so the clinical applicability is questionable. The addition of methacrylate-based bonding agents has also consistently demonstrated an increase in bond strength.13,18-20 The application of monomer (MMA) produces variable results.17,21 Several studies found that its application to the ridge lap surface before processing had no significant effect on the bond
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 23 (2014) 528–533
Clark and Hsu
strength.2,3,22 Morrow et al23 found that the application of monomer decreased the bond strength, and others found that monomer application improved the bond.21,24 The variation in results is likely due to difference in technique. Adding mechanical retention to the denture base acrylic resin has also been investigated. Removing the glazed surface of the ridge lap portion of the denture tooth with air abrasion or grinding likely increases the bond strength of the acrylic resin denture tooth to the acrylic resin denture base.18,21,25-27 Other techniques include adding retentive grooves,28 a diatoric recess,18 and a mechanical “cingulum ledge lock” to improve bonding strength strength.29 The standard way to replace a debonded tooth is to reattach it to the denture base with autopolymerizing acrylic resin. Several studies found a stronger bond with heat-activated versus autopolymerizing acrylic resin.17,30-32 Due to ease of use, however, autopolymerizing acrylic resin remains the standard laboratory technique for most denture repairs. While many studies have investigated the bond strength of acrylic resin denture teeth to the denture base, few have investigated the bond strength of acrylic resin denture teeth that have been replaced after debonding. Shen et al32 debonded denture teeth from heat-polymerized acrylic resin bases and relieved 2 mm of acrylic resin from the denture base. Various treatments were performed on the ridge lap surface of the tooth. The tooth was placed in the same position with a stone index, and the 2 mm space was filled with either heat-polymerizing acrylic resin or autopolymerizing acrylic resin. The specimens were then retested, and it was found that heat-polymerizing acrylic resin formed a stronger bond to the tooth than autopolymerizing acrylic resin. The same study also found that treatment with chloroform increased the strength of the repair.32 Vallittu et al17 fabricated heat-polymerized specimens and fabricated an aluminum mold to maintain the tooth position. The teeth were replaced with the aluminum mold, and MMA was applied to the ridge lap for varying amounts of time before testing. Their results confirmed that the bond strength of the repaired tooth is weaker than the original bond and that bond strength increased as the time that MMA was in contact with the tooth increased.17 Despite advances in materials, replacing debonded denture teeth remains one of the most common denture repairs. Due to the lack of literature involving teeth that are debonded and replaced, additional research is needed in this area. It is the goal of this in vitro study to determine if a relationship exists between the thickness of repair acrylic resin at its interface with a replaced denture tooth and its bonding strength. Additionally, the failure mode of the repaired specimens will be evaluated to analyze the weakest point of the repair. The null hypothesis states that the thickness of autopolymerizing acrylic resin used to repair a denture tooth will not affect its bonding strength.
Materials and methods Fabrication of molds
This study consisted of two parts. Part I tested heat-polymerized acrylic resin specimens; part II tested specimens repaired with autopolymerizing acrylic resin. To retain the denture tooth, a wax pattern was fabricated in a shape established during a pilot study done previously by the authors. This pattern was du-
Thickness of Repair Acrylic and Bond Strength
Figure 1 A master mold was fabricated out of a laboratory silicone and used to fabricate 50 wax patterns. These wax patterns would be processed with heat-polymerizing acrylic resin and serve as the denture bases for the study.
plicated using a silicone material (REDU-It; American Dental Supply Inc, Easton, PA) to fabricate a mold. This silicone mold was used to standardize the wax patterns for part I (Fig 1). A denture tooth was positioned in the center of the wax pattern, and the specimen was processed using standard laboratory technique to preserve this position. A laboratory silicone (Sil-Tech; Ivoclar Vivadent, Schaan, Liechtenstein) was adapted to the facial surface of the tooth and around the acrylic resin base,18 which became the mold to position the teeth for the specimens in part I (Fig 2). Specimen fabrication for part I
The ridge lap surfaces of 50 maxillary right central incisor denture teeth (BlueLine Mould A14; Ivoclar Vivadent, Amherst, NY) were airborne particle abraded for 10 seconds with 50-µm aluminum oxide particles to remove the glaze layer, then steam cleaned with distilled water to remove any residue. Fifty specimens were fabricated as described. Baseplate wax was heated and flowed into the master mold to form a wax pattern. This wax pattern was positioned in the silicone tooth positioning mold along with a prepared denture tooth, and the tooth was attached to the wax pattern using a heated instrument. The specimen was invested in a maxillary denture flask to simulate dental laboratory conditions.11,33,34 Each specimen was processed with a heat-polymerizing acrylic resin (ProBase
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 23 (2014) 528–533
529
Thickness of Repair Acrylic and Bond Strength
Clark and Hsu
Figure 3 Example of an adhesive failure. Note the lack of residual acrylic resin over the entire bonding surface.
Figure 2 A second silicone mold was fabricated to standardize tooth position. Each wax pattern was placed in the mold, and the denture tooth was attached with baseplate wax.
Hot; Ivoclar Vivadent, Amherst, NY), following manufacturer’s instructions. After divesting and finishing, plaster was adapted to each specimen to serve as a tooth repositioning index for part II of the study. Testing part I specimens
A metal table was bolted to the base of a universal testing machine (model 5575; Instron machine, Norwood, MA). A chisel-shaped attachment was inserted, and the specimen was placed so the attachment would contact at the junction of the middle and incisal third of the lingual surface of the central incisor, 3 mm from the incisal edge. The central incisor was then subjected to a cantilever-type bending force at a 5 mm/min crosshead speed at an angle of 127° to the surface of the tooth until fracture.9,35,36 Upon fracture, the specimen was removed from the testing machine, and the maximum applied load was recorded. To determine the failure mode, each debonded denture tooth was examined under 10× magnification using a stereomicroscope (Renfert Mobiliskop; Renfert GnbH, Hilzingen, Germany) by a second year prosthodontic resident, an experienced dental laboratory technician, and the author (WAC). Each observer examined the bonding surface of the denture tooth, di530
Figure 4 Example of a combination failure, with a grid to demonstrate how the bonding surface was divided to determine failure mode. This example had residual acrylic resin in six of nine areas of the grid.
vided the tooth into ninths, and marked the areas with retained acrylic resin. The failure mode was classified as adhesive failure (debonding at interface between denture tooth and denture base acrylic resin), cohesive failure (fracture completely within denture tooth or denture base acrylic resin), or a combination adhesive/cohesive failure (Figs 3 to 5). For purposes of this study, a failure was considered adhesive if acrylic resin was present on 0.05). The bond strength was lower for the group repaired with a 1 mm thickness of autopolymerizing acrylic resin compared to the original (part I) group (p < 0.05), with 65.8 N load to failure (a 29.63% loss in bond strength). The bond strength was even lower for the group repaired with a 3 mm thickness of autopolymerizing acrylic resin (p < 0.05), with 58.64 N load to failure (a 33.07% loss in bond strength). The failure mode was recorded for each group. The most common failure mode in the original group (part I) was adhesive failure (56.00%), followed by combination failure (34.00%). In the repaired group, the trend stayed the same with 56.67% of the failures being adhesive (Fig 6). Considering the failure mode of the repaired (part II) specimens, the original bond strength was lower for those that failed adhesively (Table 2).
Discussion For years, studies have investigated different chemical and mechanical variables to determine what may increase or decrease the bond strength of a denture tooth to its base. This study evaluated the relationship between the thickness of repair acrylic at its interface with a replaced denture tooth and its bond strength. The null hypothesis was not accepted, as the results indicate a relationship is present. As the thickness of the autopolymerizing acrylic resin at the interface increases, the bond strength of the repair decreases. The results are consistent with other studies demonstrating a lower bond strength between denture teeth and autopolymerizing acrylic resin versus heat-polymerizing acrylic resin.23,30-32 The bond strength cannot directly be compared to those of other investigators due to differences in methods. This is a common statement among investigators of bond strength of denture teeth. Several countries have their own national specifications, and they each have very different methods for evaluating bond strength. Several investigators have acknowledged the limited clinical applicability of these tests.5,37 Because of this, investigators are continually trying to make a more clinically applicable testing method.2,3,9,33,38 Results of this study indicated a majority of adhesive failures for both the original and repair groups. These results could
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 23 (2014) 528–533
531
Thickness of Repair Acrylic and Bond Strength
Clark and Hsu
Figure 6 Percent (%) failure mode for part I (original) and part II (repaired) groups. The failure mode trends are consistent whether it was the original or repaired failure.
Table 2 Loss of bond strength (%) based on failure mode Failure mode
Original
Repaired
Loss of bond strength
Adhesive Combination
73.819 N 104.001 N
63.037 N 67.891 N
14.60% 34.72%
Note: There was a greater loss of bond strength with the combination failure group.
also not be directly compared to those of other investigators, as there is not a consensus on the definition of adhesive and cohesive failures. Following Chung et al,21 this study defined an adhesive (interfacial) failure as retained denture base acrylic resin on
