| Since the preliminary studies on osseointegration, dental implants have been extensively used for the rehabilitation of completely and partial edentulous patients over the last three decades. Despite the high success rates reported by a vast number of clinical studies, early or late implant failure is unavoidable. Late implant failure are observed after prosthesis delivery and mainly related to biomechanical complications.Experimental techniques are often used to analyze complex stress problems. Current techniques employed to evaluate the biomechanical loads on implants comprises the use of photoelastic stress analysis, strain-gauge analysis, three-dimensional finite element stress analysis and fatigue testing analysis. In this study, the stress distribution of bone around the supported implant's cervical and apical area of two different designation implant dentures on vertical loads was analyzed with linear strain gauges. And â‘¡ A Clinical prospective study of comparing with one and two implants replacing a single molar was conducted. â‘¢ A comparative study was conducted to investigate the stability and the failure patterns of identical crowns retained by cement and screw under dynamic cyclic fatigue conditions. â‘£ The effects of cement type, cement thickness, loading magnitude, and loading direction on stress distribution of the single cement-retention implant denture were investigated by three dimensional finite element analysis.Part 1. Analysis of Stress Distribution in bone around implant in two different designing implant denturesIn the first study, the single implant supported crown and cantilevered bar modelwas constructed. The single implant was covered by the 0.25mm thickness of acrylic resin, and the linear miniature strain gauges were attached to the surface of cervical and apical area of the implant by epoxy glues. The vertical loads of ION, 20N, 30N were respectively applied at four points on the crown and cantilevered bar of model. The results of the stress were recorded at each strain gauge. The loads, the points of load and their interactions indicated statistically significant difference in influencing the distribution of stress. The greater stress values were founded in the implant's apical area far to loading points and the greatest stress values were showed in the implant's cervical area near to loading points.In the second study, two implants supported fixed bridge and cantilevered bar model was constructed. The vertical loads of 20N, 40N, 60N were respectively applied at six points on the bar of the two- implant-supported fixed prosthesis models. The linear miniature strain gauges were attached to the surface of cervical and apical area of two implants by epoxy glues. The results of the stress were recorded at each strain gauge. The stress was evenly distributed as loads were applied at the center of the implant-supported prosthesis. The magnitude of stress was directly proportional to the load and the cantilevered length. And the greatest stress values were displayed in the distant implant's cervical area near to cantilever.Part 2. Analysis of the influence of biomechanical effect in different retaining pattern of the fixed implant prosthesesA comparative study was conducted to investigate the stability of identical crowns retained by cement and screw under dynamic cyclic fatigue conditions. Preliminary results indicate that the initial PTV of the cement and screw retained samples were not significantly different using the t-test with P>0.05. There is also no significant difference in the stability of the two groups. The cement retained group failed on average at 2.60M cycles (SD=2.27M cycles) for n=20, while screw retained samples failed at 2.17M cycles (SD=1.27M cycles) whenn=18 (P>0.05). Two cement samples were omitted as a result of machine error. The mode of failure for the two groups is striking. Of the eighteen-cemented samples, fifteen fractured at either the fixture or abutment screw level. Cemented samples were more rigid throughout the test and would fracture unexpectedly. In screw-retained samples, the retaining screw would gradually become lose and were detected by increasing PTV values. There is also a consistent decrease in the residual torque of the abutment screw. When the screw samples were left to run continuously, it increased the probability of either fixture or abutment screw fracture. These results indicate that the load concentration in the two groups is quite different under the same test conditions.In the fifth study, four three-dimensional computer models of single cement-retention implant denture were generated in the right lower first molar area, respectively. Zinc phosphate and glass ionomer were used in the thickness of 25 and lOOum. Models were loaded axially (ION and 150N) on the occlusion, and loaded buccollingually (ION and 15ON) on the buccal edge of the occlusion. Thearea and level of stress concentration on the abutment, implant and within lutingagents were tested. The results show that the stress on the implant denture chiefly concentrated in the neck and the apical of the implant under the axial load, and the stress in the neck of the implant was bigger than that in the apical of it. But underthe buccollingual load, the stress on the implant denture chiefly concentrated inthe cervical cortical bone of the implant. And the stress under the buccollingual load was much higher than that under the axial load. The stress on the implant denture increased in proportion to the magnitude of the load. The difference of the stress on the implant denture was not significant in the models of the two luting agents. But under the same load, the stress in the same thickness ZP was much higher than that in the GI. The difference of the stress on the implant denture was not significant in the models of the two kinds of thickness luting agents. But underthe same load, the stress in the 25 um luting agent was much higher than in the 100 um in the same luting agent.Part 3 Five-year retrospective study of one and two implants replacing single molarThe comparative study between one and two BLB implants replacing the first molars was conducted through retrospective evaluating of the 64 included cases. It was observed that there were more mechanical complications, including abutment loosing, fixture mobility, and aseptic peri-implantitis, when the single crowns were supported by one implant. Meanwhile, no mechanical complication was noted in double implants group. 5 years after delivery of the prosthesis, no significant difference of the marginal bone resorption was found. This study suggests that implant-supported molars, both by one and double implants, can be effective therapy, and the results indicate that two implants provide biomechanically more advantageous support than does one.Conclusion:1. In the single implant supported crown and cantilevered bar model, the greaterstress values were founded in the implant's apical area far to loading points and the greatest stress values were showed in the implant's cervical area near to loading points.2. In two implants supported fixed bridge and cantilevered bar model, the stress was evenly distributed as loads were applied at the center of the implant-supported prosthesis. The magnitude of stress was directly proportional to the load and the cantilevered length. And the greatest stress values were displayed in the distant implant's cervical area near to cantilever.3. These results indicate that the load concentration in the two groups is quite different under the same test conditions.4. The type and thickness of the luting agents had little effect on the stress level and stress distribution of the implant.5. The clinical study suggests that implant-supported molars, both by one and double implants, can be effective therapy, and the results indicate that two implants provide biomechanically more advantageous support than does one.
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