The prosthetic screw loosening of two-implant supported screw-retained fixed dental prostheses in the posterior region: A retrospective evaluation and finite element analysis.
The study was aimed to investigate the prosthetic screw loosening of two splinted implants-supported, screw-retained (2-4-unit) fixed dental prostheses (TIS-FDPs) in posterior region and to explore the underlying mechanism. In the retrospective study, a study group of TIS-FDPs (n = 23) presenting prosthetic screw loosening and a control group of TIS-FDPs (n = 32) absent of prosthetic screw loosening during observation period were included. The prosthesis height (PH), inter-implant distance (ID) and cantilever distance (CD) of TIS-FDPs were measured and compared within two groups. In the finite element analysis (FEA) part, three serials of models presenting different clinical scenarios were constructed based on the abovementioned PH, ID and CD values respectively. In the clinical evaluation, the values of pH and CD in study group were statistically higher than those in control group, whereas the values of ID had no significant difference. In the FEA, the results indicated that there was no linear correlation between the increased ID values and the maximum von Mises stresses and the rotation angles. On the other hand, the increased PH and CD values would result in a strong linear growth of the maximum von Mises stresses and the rotation angles. Besides, it was found that the regression coefficients in PH model were all higher than those in ID and CD models. When TIS-FDPs were delivered in posterior region, the PH and the CD, rather than the ID, seemed to have a significant impact on the stress concentration of the prosthetic screws and the incident of prosthetic screws loosening.