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This study evaluates a modified 4-META/MMA-TBB resin (M4M) as a candidate material for filling screw-retained implant access hole. Its characteristics were compared with a conventional composite resin (CR) with or without a bonding agent (BA) or a ceramic primer (CP). Ceramic blocks were divided into five groups, including (A) CR, (B) CR with BA, (C) CR with CP and BA, (D) M4M, and (E) M4M with CP. Shear bond strengths were measured after 5000 times of thermocycling. Groups A, B, and D were excluded from further tests as they showed no adhesion. A cylindrical cavity (2.5 mm diameter, 3 mm depth) simulating access hole was prepared in a ceramic block and glazed to evaluate micro-leakage and wear test of groups C and E. The results were statistically analyzed with Mann–Whitney test (p < 0.05). Shear bond strength of groups C (7.6 ± 2.2 MPa) and E (8.6 ± 1.0 MPa) was not significantly different. In micro-leakage analysis, average wear depth and wear volume, group E (7.5 ± 3.3%, 59.3 ± 12.9 μm, 0.16 ± 0.04 mm3) showed significantly lower values than those of group C (45.6 ± 24.4%, 76.0 ± 16.4 μm, 0.28 ± 0.03 mm3). It is suggested that the combination of CP and M4M can be one of feasible systems to fill the ceramic access holes of the implant upper structure. © 2014 The Authors. Journal of Biomedical Materials Research Part B: Applied Biomaterials Published by Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 103B: 1030–1036, 2015.
INTRODUCTION
Screw-retained implant restorations were preferentially used from the 1980s to the early 1990s, for its advantages of being retrievable and cement free, although the esthetic outcome and the control of the occlusion were regarded as drawbacks.[1] Also, chipping of the ceramic at the occlusal surface was reported.[2] Cement–retained-implant technique has since been developed, and the results that are close to the natural teeth restorations could be achieved. This procedure is esthetically easier and reduces cost and time3; however, it can be a major cause of peri-implantitis if the excess cement is not completely removed.[4-6] Since the computer-aided design and computer-aided manufacturing (CAD/CAM) technologies have been developed, screw-retained implant-supported fixed partial dentures placed directly on the implant without an abutment, became a new approach. This CAD/CAM fabrication provides a greater passivity of fit compared with conventional casting fabrication methods.[7] Screw retention is thus considered as an optimal choice by many clinicians.[8] Although screw access holes have mainly been filled with photo-polymerizing resin composites, the clinical results were disappointing. Within a few months after filling, clinicians often experienced leakages at the access hole margins.[9] Because of poor bond strength to glazed ceramic surfaces and insufficient wear resistance of the composite resin (CR), the loss of access hole filling was also reported as one of the common prosthesis-related complications.[10] There are few studies that refer to filling procedures of the screw access hole. CR combined with opaque shades appeared to give a better esthetic outcome.[11] A novel technique, which bonds a pressed ceramic plug into the screw access channel of an implant restoration[12] seemed to be efficient, although the cost performance and extra chair time should be considered for patients. Improving the filling of access holes with a suitable material might expand clinical indications of screw-retained implant-supported fixed partial dentures. Recently, a modified 4-META (4-methacryloxyethyl trimellitate anhydride)/MMA-TBB (methyl methacrylate-tri-n-butyl borane)-based resin was introduced. It is an acrylic-based adhesive resin containing reactive prepolymerized filler particles.[13] The material polymerization is initiated by tri-n-butylborane (TBB) which has been used in 4-META–based resins for a long time.[14] This material shows an excellent bonding performance to teeth, ceramics, and metals when appropriate primers are used.[15, 16] It also presents higher wear resistance compared with the conventional acrylic resins.[16] The purpose of this study was to evaluate bonding performance and wear rate of modified 4-META/MMA-TBB resin combined with a phosphoric acid monomer primer as a material filling access holes of screw-retained implant restorations.
MATERIALS AND METHODS
The materials used in this study are presented in Table 1. They include a ceramic block, VITA VMK Master EN1 (VITA, Bad Säckingen, Germany), a phosphoric acid monomer ceramic primer (CP): UCP (Super-Bond Universal Ceramic Primer, Sun Medical, Moriyama, Japan), a photo-polymerizing nanohybrid composite (Fantasista, shade A2, Sun Medical) and its accompanying photopolymerizing bonding agent (BA; Hybrid Bond, Sun Medical) and a modified 4-META/MMA-TBB resin (Bondfill SB, Sun Medical). The components of modified 4-META/MMA-TBB resin and phosphoric acid monomer CP are presented in Table 2. The ceramic blocks (n = 50) with 10 mm × 10 mm × 5 mm dimensions were prepared and fired in a ceramic oven (Cerafusion MS-1200, Sekisui-denshi, Nagahama, Japan) at 930°C. They were divided into five groups according to different pretreatments and materials (Table 3). For the micro-leakage and wear tests, only groups presenting adhesion to the ceramic surface were evaluated with 10 specimens for each group and test (n = 40).
Table 1. Materials UsedMaterialProduct NameBatch NumberManufacturerCeramicVITA VMK32850VITACeramic primerSuper-Bond UCPLiquid A: FS1Liquid B: FW1Sun MedicalCompositeFantasista(A2) VM11Sun MedicalBonding agentHybrid BondEW2Sun MedicalAdhesive compositeBondfill SBLiquid: ET1Powder Light: ER12 Catalyst V: ER2Sun MedicalTable 2. Components of Modified 4-META/MMA-TBB Resin and Ceramic Primer TestedMaterialCompositionLiquidMethyl methacrylate (MMA), 4-META, polyfunctional methacrylatePowderPMMA, TMPT prepolymerized filler, PigmentPolymerization initiatorTBB, TBB-O, HydrocarbonCeramic primerLiquid A: Methyl methacrylate, Phosphate ester monomerLiquid B: Methyl methacrylate, 3-(trimethoxysilyl)-propyl methacrylateTable 3. Conditions of Pretreatment, Bonding, Filling, and PolymerizationGroupPretreatmentBonding AgentFilling MaterialPolymerization ConditionANoneNoneNano-hybrid composite resin (Fantasista)Light irradiation 20sBNonePhotopolymerizing bonding agent (Hybrid Bond)Nano-hybrid composite resin (Fantasista)Light irradiation 20sCPhosphoric acid monomer ceramic primer (UCP)Photopolymerizing bonding agent (Hybrid Bond)Nano-hybrid composite resin (Fantasista)Light irradiation 20sDNoneNoneModified 4-META/MMA-TBB resin (Bondfill SB)Auto polymerizationEPhosphoric acid monomer ceramic primer (UCP)NoneModified 4-META/MMA-TBB resin (Bondfill SB)Auto polymerizationShear bond test
The shear bond strengths between the ceramic blocks and the filling materials in each group were tested first to determine groups which have interest to evaluate micro-leakage and wear resistance test. The test procedure is illustrated in a flow chart (Figure 1). Each ceramic block was fixed into a brass ring using a self-curing resin, and then the ceramic surface was ground flat with a #600 SiC paper (Carbimet Paper Discs, Buehler, Lake Bluff, IL). Bonding areas were restricted using a 10-μm-thick masking sheet (Double-stick tape, Nichiban Co., Tokyo, Japan) with a 4.8-mm diameter circular opening onto the flattened ceramic surfaces. Specimens were divided into five groups as follows: group A: composite resin (CR); group B: bonding agent (BA) and CR; group C: ceramic primer (CP), BA and CR; group D: modified 4-META/MMA-TBB resin (M4M); and group E: CP and M4M (Table 3). For groups A and D, no surface pretreatment was performed. For group B, only the BA was applied at the surface before the filling of CR. For groups C and E, CP was applied as follows: a mixture of an equal amount of liquid “A” and “B” of the CP was applied to the surface and air blown. For groups B and C, the BA was applied for 20 s and air blown for 5 s, then photopolymerized using a polymerizing unit (Translux CL, Panaheraeus, Osaka, Japan) for 3–5 s. A PTFE (polytetrafluoroethylene) ring mold (8 mm outer diameter × 4.8 mm inner diameter × 2 mm height) was placed over the bonding area using a double-stick tape. Each filling material was applied to the surface through the mold. For the modified 4-META/MMA-TBB resin groups (D and E), the base liquid was activated by adding TBB initiator (3:1 ratio), and powder/liquid mixture was applied to fill the space inside the PTFE mold ring, using a brush-dip technique until the filling was completed. The resin was left for 10 min to complete autopolymerization. For the nanohybrid composite groups (A, B, and C), the space was filled by incremental method. Each increment (1 mm thickness) was photopolymerized for 20 s. The mold was removed after the complete polymerization. For each group, 10 specimens were used for this test. The specimens were left for 1 h after the polymerization and then thermocycled (Thermal shock tester D type, Thomos Kagaku, Tokyo, Japan) at 5°C for 1 min and 55°C for 1 min alternatively for 5000 cycles. Then, their bond shear strengths were measured using a universal testing machine (SHIMAZU AG-IS, Shimazu, Kyoto, Japan) with a 1.0-mm/min crosshead speed. The data were statistically analyzed using Mann-Whitney test (p < 0.05).
Figure 1.
Flowchart of shear bond testing. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Micro-leakage test
Specimen groups (A, B, and D) that showed no bonding to the flattened ceramic surface (#600 grit) were excluded from micro-leakage and wear tests; therefore, CP/nanohybrid composite group (group C) and CP/modified 4-META/MMA-TBB resin group (group E) were compared. The procedure is illustrated in a flowchart (Figure 2). For each group, 10 ceramic blocks were used. To simulate the access hole, a cylindrical cavity with 2.5 mm in diameter and 3 mm in depth was prepared at the center of the ceramic block with a diamond bur (JM STERI DIA straight cylinder flat end type, NTI-Kahla, Kahla, Germany) with cooling water. The ceramic block was then placed in the ceramic oven set at 930°C to obtain a glazed surface inside the cavity. For these specimens, CP was applied to the glazed ceramic cavity surface and air blown. For group C specimens, the BA was applied to the cavity surface and left for 20 s, then the excess bond was removed by air blown for 5 s before photopolymerization of BA for 3–5 s. The cavity was filled by incremental method with the conventional resin composite and photopolymerized for 20 s for each layer (1 mm thickness). For group E specimens, the modified 4-META/MMA-TBB resin was filled into the cavity with a brush-dip technique after applying the CP. Subsequently, the restored surfaces were finished by wet polishing with #2000-grit SiC paper to obtain a sufficiently flat surface. All specimens were left for 1 h at room temperature and then subjected to 5000 cycles of thermocycling (Thermal shock tester D type, Thomos Kagaku, Tokyo, Japan). The specimens were then stored in a 1% methylene blue solution for 24 h, and rinsed with running water to clean the surface. They were then sectioned vertically with the Isomet saw (Buehler, Lake Bluff, IL), and the depths of dye penetration between the filling materials and the glazed ceramic were observed with a digital microscope (Keyence VHX-900, Keyence Corp., Osaka, Japan) of 50-fold magnification. The micro-leakage ratio (%) was obtained by comparing dye penetration depth against depth of the cavity. The mean value of the measurements obtained at left and right sides was used as the result of each specimen. The results were statistically analyzed with Mann–Whitney test (p < 0.05).
Figure 2.
Flowchart of micro-leakage testing. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Wear test
Ten other specimens were prepared in the same manner as the micro-leakage analysis for both groups C and E, except surface polishing with #2000-grit SiC paper to obtain a flat surface. Subsequently, they were subjected to the three-body generalized wear test.[17] The specimens were oriented to the center of specimen holders and mounted to the wear device. A polyacetal stylus (6.4 mm diameter) hit the specimens perpendicularly with a 75-N load at a rate of 1.5 times per second using a slurry of 50 µm PMMA powders as an artificial food bolus for 400,000 cycles (Figure 3). The worn surfaces of specimens were measured by a noncontact 3D scanner (Keyence KS-1100, Keyence Corp., Osaka, Japan) every 100 μm interval with 0.01 μm accuracy. Both average wear depth and wear volumes of the specimens were then calculated with the software: KS-Analyzer (Keyence Corp). The results were statistically analyzed with Mann–Whitney test (p < 0.05).
Figure 3.
Flowchart of wear testing. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
RESULTS
The results of bond shear test demonstrated that A, B, and D groups, in which the tested restorative materials were used without the ceramic pretreatment, had no bond strength regardless of the use of the BA for the conventional CR. On the contrary, when the CP treatment and BA were applied before the filling of composite (group C), it exhibited 7.6 ± 2.2 MPa bond strength. The CP combined with 4-META/MMA-TBB resin (group E) showed 8.6 ± 1.0 MPa bond strength. All specimens in groups C and E exhibited a significant cohesive fracture within the ceramic, not an adhesive failure mode. The statistical analysis showed that there was no significant difference between groups C and E (Mann–Whitney test, p = 0.13). The results of micro-leakage and wear tests are presented in Table 4. A typical specimen after dye penetration for each group was shown in Figures 4 and 5. In group C, the dye penetration ranged from 14.6 to 94.3%, with the average value being 45.6 ± 24.4%. On the other hand, those of group E ranged from 2.3 to 13.6% and 7.5 ± 3.3%, respectively. There was a significant difference between groups C and E (Mann–Whitney test, p < 0.05). The results of wear test showed that the average wear depth of group E (59.3 ± 12.9 μm) was significantly smaller compared with that of group C (76.0 ± 16.4 μm). The volumetric wear value of group E, ranged from 0.09 to 0.21 mm3, whereas those of group C, ranged from 0.23 to 0.32 mm3. The average wear volume of group E was 0.16 ± 0.04 mm3, whereas that of the group C was 0.28 ± 0.03 mm3. The statistical analysis showed that there was a significant difference between groups C and E (Mann–Whitney test, p < 0.05).
Table 4. Results of Micro-leakage and Wear TestsGroupDye Penetration (Mean ± SD) Unit: %Wear Depth (Mean ± SD) Unit: μmVolumetric Wear (Mean ± SD) Unit: mm3C45.6 ± 24.476.0 ± 16.40.28 ± 0.03E7.5 ± 3.359.3 ± 12.90.16 ± 0.04Figure 4.
Typical specimen after dye penetration (group C): (a) top view and (b) section view.
Figure 5.
Typical specimen after dye penetration (group E): (a) top view and (b) section view.
Figure 6.
Surface morphology of worn specimen (group C).
Figure 7.
Surface morphology of worn specimen (group E).
DISCUSSION
The access hole is cylindrical and comparable to Class-I cavity. This configuration creates higher stresses during the polymerization process of resin; therefore, the incremental filling method is adopted to minimize the stress.[18] However, a screw access hole is recognized as a small cavity, and the filling mode is believed to give little influence on the wall adhesion.[19] Clinically, access hole bottom is covered with a soft material such as gutta-percha, PTFE, or temporary cement. Photopolymerizing resin composites have been widely used to fill the ceramic access holes of the implant upper structures; however, they have unfortunately less favorable prognoses.[20-22] It is reported that the use of a CP is mandatory to achieve optimum bonding of the composite to ceramic surfaces.[23] In this study, a modified 4-META/MMA-TBB resin was selected as it was indicated to restore tooth wear and wedge-shaped defects. According to the literature, a proprietary CP enhances the bond strength of the modified 4-META/MMA-TBB resin to the ceramic surface.[24-28] As simulated access holes consisting of glazed ceramic surface were tested, a pretreatment with a CP containing phosphoric acid monomer : UCP was suggested. The values of shear bond strength obtained in this study were lower than those reported in the literature,[23] inherent values should be higher as all of the specimens exhibited cohesive failures inside the ceramic. This unexpected breakage of ceramic specimen could be explained by various reasons. The feldspathic ceramic used in this study was made in the same manner as conventional ceramic implant-supported fixed partial dentures and are mechanically weaker compared with machinable ceramic blocks used in other studies.[29, 30] In adult orthodontic practice, bonding of orthodontic brackets to glazed ceramic surface has been a challenge for years.[31] In this field, if the adhesion has bond shear strength of 6–8 MPa, the bonding system is considered as clinically acceptable.[32] In an access hole filling procedure, shear bond strength is not mandatory as in orthodontic treatment, although bonding ability and wear resistance are considered to be significant factors. Modification of the ceramic surfaces by silicate blasting or acid etching procedure gives a reliable adhesion.[33] However, in intraoral treatments, including the access hole filling, decontamination of blasted or acid-etched surfaces cannot be easily achieved by means of a ultrasonic bath immersion technique.[34] Moreover, in access hole filling, such treatment is biologically not suitable. Especially, a hydrofluoric acid treatment needs considerable precautions.[35] According to the previous study[36] with photopolymerized CR to the tooth surface, the depth of dye penetration is approximately 1 mm from the top of the cavity. In this study, group E showed a 7.5 ± 3.3% dye penetration (0.18 ± 0.08 mm), which remained at superficial areas. Group C demonstrated a 45.6 ± 24.4% dye penetration (1.07 ± 0.61 mm). This result is probably related to the adherent substrate, that is, glazed ceramic, which is most difficult to bond. The filling material evaluated in this study is a hybrid material consisting of a 4-META/MMA-TBB adhesive resin and trimethylol propane trimethacrylate (TMPT) prepolymerized filler. The photopolymerized CR used in this study has a resin matrix consisting of triethylene glycol dimethacrylate (TEGDMA) and urethane dimethacrylate (UDMA) polymers and various fillers, including TMPT filler, micro-filler, and nano-filler. The TMPT filler possesses capacity to copolymerize with resin matrix having abundant polymerizable double bonds on its surface.[13] An improved wear resistance of CR was reported to be achieved by adding TMPT filler.[37] In the case of modified 4-META/MMA-TBB resin, TMPT fillers are in a flexible acrylic matrix, although the amount of TMPT filler is lower than that in nanohybrid CR used in this study. The low wear values of group E in this study might be due to the flexibility of the acrylic matrix that has an aptitude in lower brittleness. It has been reported that the polymerization of MMA is not inhibited by exposure to air when TBB is used as a polymerization initiator.[14] These characteristics give to modified 4-META/MMA-TBB resin an advantage of avoiding gap formation between the cavity and the filling material compared with photopolymerizing CR during the polymerization process. It seems that the polymerization starts concomitantly inside the material, the surface of the cavity wall and the top of the access hole. An adequate sealing result of filling with this acrylic resin might be attributed to bonding potential of the specific phosphoric acid monomer CP and a unique polymerization character of TBB. The generalized wear test has been used to evaluate the wear rates of various restorative materials when they are used to restore occlusal cavities.[38, 39] Wear rate is an important factor to predict the functional outcome of an access hole filling material. Several materials were compared using the three-body wear test, and the wear volume results were 1.5 mm3 for an acrylic resin, 0.3 mm3 for a flowable CR, and 0.9 mm3 for a modified 4-META/MMA-TBB resin.[15] A tooth-brush wear test demonstrated the same tendency.[16] In the current study, the wear volume of group C and group E were 0.28 and 0.16 mm3, respectively. It was shown that group E exhibited significantly less wear value compared with previous value, whereas group C demonstrated values close to the previous wear test conducted on a large material surface. Perhaps the bonding of the filling material to the surrounding ceramic influenced the wear stress. The bonding property of the 4-META/TBB acrylic resin might reduce the polymerization stress at the cavity surface. Polymer structure also influences the wear rates. As this acrylic resin contains MMA monomer, the set polymer possesses linear polymer segment; therefore, the material has a superior flexibility compared with conventional CRs. The elastic modulus of this modified 4-META/MMA-TBB resin exhibited 1.9 MPa, which is equivalent to that of conventional acrylic resins,[40] whereas a conventional nanohybrid CR exhibited 7.9 MPa. The flexural strength of the modified 4-META/MMA-TBB resin (66 MPa) is close to an acrylic resin (60 MPa), which is lower than a flowable CR (115 MPa).[12] This low flexural strength might absorb the wear stress and keep the integrity of the cavity wall adhesion. The influence of elastic modulus and flexural strength on the wear resistance is also reported.[41] Regarding the surface morphology of worn specimens, the wear patterns of groups C and E were different. This difference may relate to the amount of filler; however, it probably was more likely to be due to the stress concentration. When marginal debonding occurs, concentrated stresses provoked by polymerization shrinkage will be released and resulted in accelerated wear at marginal area.[42] As the diameter of the access hole cavity is small (2.5 mm diameter), the wear defect pattern is spreading to the entire surface in group C Figure 6. On the other hand, the wear defect existed only at the center in group E Figure 7. In a small cavity configuration, polymerization stress in the material might be a key factor rather than the quantity of filler or hardness of the filling material. Wear depth was also different in groups C and E (Table 4). The average wear depth of group E was 59.3 ± 12.9 μm, whereas for group C, the value was 76.0 ± 16.4 μm with a significant differences (p < 0.05, p = 0.035). This difference of results is explained by the tenacity of the modified 4-META/MMA-TBB resin. Each grain of filler is strongly attached to the flexible acrylic matrix and difficult to be detached. Based on the results with the limitations of the current study, it is concluded that E group showed a significantly less micro-leakage value and lower wear values compared with group C. The combination of the modified 4-META/MMA-TBB resin and phosphoric acid monomer CP can be one of indicating systems to fill the access holes of the implant upper structure. Further clinical studies may confirm these results.
ACKNOWLEDGMENTS
The authors thank Sun Medical for their material supply.
Regards,
Prashant Deshmukh
Sunmedical
9870349989
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