Early Migration of the R3 Uncemented Acetabular Component: A Prospective 2 Year Radiostereometric Analysis

Background: Radiostereometric Analysis (RSA) is a highly accurate and true three dimensional method that analyses implant micromotion following total joint replacement surgery. This methodology was used to assess the migration of a fourth generation acetabular shell: the R3 acetabular component (Smith & Nephew, Memphis, Tennessee) which utilises an enhanced porous ingrowth surface (StikTite). The StikTite surface has previously been evaluated in an RSA study, but on the Reflection Cup. Methods: Twenty patients undergoing primary total hip arthroplasty were recruited and all had the R3 acetabular cup inserted. RSA examinations were performed immediately post operatively and at 6, 12 and 24 months. One patient was removed as they required early revision, three for technical reasons (inaccurate bead placement) and two were lost to follow up. Data was analysed for the remaining fourteen patients. Eight patients received a cementless femoral stem (Anthology; Smith and Nephew) with oxynium femoral heads (32 mm) and twelve a cemented femoral stem (Spectron; Smith and Nephew) with cobalt chrome femoral heads. Results: RSA revealed no significant migration in all planes of translation and rotation. Mean translations at 2 years were 0.36 mm (x-axis), 0.39 mm (y-axis) and 0.35 mm (z-axis). Mean rotations were 0.68 deg (x-axis), 0.99 deg (y-axis) and 0.77 deg (z-axis). Micromotion along the proximal-distal translation (y-axis) plane represents proximal migration of the acetabular component. Conclusions: Mean translations and rotations were higher than previously reported for the StikTite coating applied to the older Reflection cup. The levels of early proximal migration two years postsurgery were within published ‘acceptable' levels, albeit within the ‘at risk' range of 0.2-1.0 mm. These findings support further investigation and analysis of the R3 acetabular system, and continuing correlation with registry data as it becomes available.