Abstract: Objective: To compare the difference of mechanical complications between the internal and external adhesion in multi-unit screw-retained implant prosthodontics. Methods: There were 205 patients (414 implants) with multi-unit screw retention implant-supported prostheses in posterior area for 3 years. Among them, 99 patients (198 implants) had inner metal crown and outer zirconia crown bonded by technician and screw fixed in mouth, and 106 patients (216 implants) were fixed the inner metal crown with screw which had completely passive fit and then bonded the outer crown by doctor in mouth. The incidence of mechanical complications was compared between two groups after 3-year observation. Results: 48 of 198 screws in the cases bonded by technician had mechanical complications at different levels, and the adhesive failed in 3 cases. 24 of 216 screws in the cases bonded by doctor had screw loose, and the adhesive failed in 15 cases. There was significant difference in the outcome of mechanical complications and adhesive failure between two retention methods (P<0.05). Conclusion: In the 3-year observation period when bonded in-mouth by doctor, the screw loosening probability was reduced, however, the possibility of adhesive failure increased.

Key words: dental implants prosthesis, dental prosthesis retention, screw-retained implant, cement-retained implant, passive fit