Biotribological Behavior of Two Retrieved Implant Abutment Screws after Long-Term Use In Vivo

Purpose: To evaluate the effect of functional cyclic loading on two retrieved abutment screws used with single implant supporting cement-retained porcelain-fused-to-metal crowns by characterizing the structure, properties, and biotribologic behavior after long-term use in vivo. Materials and Methods: Two abutment screws were retrieved from the same patient and same implant at two different times. An external hex implant was used to replace a missing central incisor. A traumatic incident occurred 9 months after insertion and displaced the implant palatally by bending the screw. A second, similar incident occurred 31 months after insertion. In both cases, the abutment screws were retrieved and subjected to thorough nondestructive and destructive testing. Results: Light and scanning electron microscopic examinations revealed very minimal surface deterioration of the thread profile for the first screw (in service for 9 months) and demonstrated advanced metal adhesive wear in the form of galling for the second screw (in service for 31 months). The galling led to moderate thinning of the thread profile. Both screws were composed of Au-based alloy, where the microstructure of the matrix consisted of homogeneous equiaxed fine grains with two different second phase particles. Conclusion: It appears that the occurrence of adhesive wear on abutment screws in the form of galling is highly related to the length of in-service time in the mouth. This biotribologic behavior was inevitable and considered to be a normal consequence of long-term use in vivo. Metallurgic analysis indicated that both screws were identical in terms of composition and microstructure. Keywords: abutment screw, adhesive wear, biotribological behavior, galling, retrieval analysis, thread wear