While Finite Element (FE) studies have made valuable contributions to the understanding of the acetabular component mechanical environment, there is a lack of direct comparison between FE predictions and the actual performance of the implant in vivo. Radiostereometric Analysis (RSA) has been widely used to accurately monitor implant migration in vivo. The aims of this study were to 1) develop an improved FE modelling work flow, that includes musculoskeletal modelling to estimate the patient specific forces, to predict acetabular component stability; and 2) validate FE model predictions against patient-matched in vivo measurements of acetabular component stability using RSA.
Eight patients underwent primary THR and received an uncemented acetabular component. Postoperative CT scans and RSA exams were performed on day three; and gait analysis and RSA exams were performed at six weeks, three months, one and two years post-surgery. Patient-specific hemi-pelvis FE models[1] were developed to calculate the mean elastic modulus (MEM) of cancellous bone, composite peak micromotion (CPM) and implant contact area during a complete level gait walking cycle. In vivo RSA 3D translation and rotation of the acetabular component were calculated using UmRSA software.
There was good conformity between predicted CPM and MEM and the in vivo 3D RSA migration at six weeks. The two cases with the largest predicted MEM (>920MPa) and lowest CPM (<20µm) were confirmed to have the lowest in vivo RSA 3D migration (<0.14mm). The two cases with the largest predicted CPM (>80µm) and least MEM (>480MPa) were confirmed to have the largest in vivo RSA 3D migration (>0.78mm).
The results of this study are the first patient-matched in vivo validation of FE predictions of implant stability. While the sample size of this pilot study was low, using prospectively matched CT, gait and RSA examinations may allow further improvements of FE model predictions.