Misfit and stress analysis of multi-unit implant-retained restorations

The longevity of implant-retained restorations is abbreviated when marginal misfit and stress injure the bone-implant junction. Objectives: the purpose of this study was: 1) to evaluate the marginal misfits of multi-unit cast titanium frameworks made from 1-piece castings and laser welding technique; 2) to compare the stress induced around the implants after framework installation; 3) to correlate marginal misfit and stress around the implants; and, 4) to evaluate the influence of spark erosion processes in reducing misfits and stress values. Methods: from a mandibular master cast, containing 05 conical abutment analogs, 10 frameworks were made from each casting technique, using calcinable plastic cylinders as patterns. Mean marginal fit values (in µm) were obtained through the 1-screw test in which one screw was tightened at a terminal abutment and discrepancies observed at the central and opposed abutments. Stress (in gf) was measured after each framework installation with the tightening of the 5 prosthetic screws using strain gauges attached to abutments. The marginal misfit and stress measurements were reproduced after spark erosion of the screw seat bases and cast cylinder fitting borders. Two-Way ANOVA and HSD Tukey tests were used to compare results (P<0.05). Results: 1-piece castings showed significantly higher values of marginal misfit (332±63µm) and stress (1071±263gf) than laser-welded frameworks (199±36µm of marginal misfit) (564±176gf of stress). For 1-piece castings and after spark erosion steps, the marginal misfit values (205±66µm) and the stress values (503±146gf) were significantly reduced. Conclusions: it was concluded that: 1) laser-welded frameworks showed smaller marginal misfits with reduced stress than 1-piece castings; 2) there was a positive correlation between marginal misfit and stress; 3) the spark erosion procedures were efficient in decreasing marginal misfits and stress for 1-piece castings with no upgrading of laser-welded frameworks. (Study supported by FAPESP – grant number 04/13429-6)