Influence of the positioning of a cementless glenoid prosthesis on its interface micromotions

The positioning of the glenoid component in total shoulder arthroplasty is complicated by the limited view during operation. Malalignment and/or motion of the glenoid component with respect to the bone can be a cause of, or contribute to, failure of the implant. The aim of this paper is to determine the effect of the positioning of a cementless glenoid component on the micromotions between the implant and the bone during normal loading after surgery. For this study a three-dimensional finite element model of a complete scapula with a cementless glenoid component was used. In total, eight positions of the upper arm in both abduction and anteflexion were chosen to represent the patient's arm movement postoperatively. A previously published musculoskeletal model was used to determine the joint and muscle forces on the scapula with implant in each arm position. Five different alignments of the glenoid component (neutral, anterior, inferior, posterior, and superior inclinations), two different implantation depths ('optimal' and 'deeper' implantations), and two bone qualities (healthy and rheumatoid arthritis (RA) bone) were considered. Inclinations of 10 degrees with respect to a neutral alignment did not affect the overall interface micromotions in the optimal implantation depth. However, when the implantation depth was 3 mm deeper, anterior and inferior inclinations were more favourable than a neutral alignment and other inclinations. Micromotions in RA bone were always larger than in healthy bone.