RADIOSTEREOMETRIC ANALYSIS (RSA) OF TWO MOM CUPS: TWO-YEAR RESULTS FROM A RANDOMISED CLINICAL TRAIL (RCT)

Summary Despite high revision rates, the mean two year migration of the ASR TM cup is within an acceptable threshold. Slightly higher migration rates found for the M2a- Magnum™ Porous Coated Acetabular Component but longer follow up is needed to establish if this implant is at risk. Introduction RSA can detect the migration of an implant, and continuous migration is a predictor for failure (1). The ASR TM resurfacing implant was withdrawn from the marked due to excessive failure rate but showed initial femoral component stability. The aim of this study was to investigate the initial implant stability for the ASR cup as a possible explanation for the high revision rate, and to compare it to another metal on metal (MoM) cup. Patients and Methods 36 patients with primary osteoarthritis from a RCT received either an ASR TM (n=19) or an M2a-Magnum Cup TM (n=17) without markers. Ten 0.8mm tantalum markers were injected in the periacetabular bone. Using a UmRSA® 43Calibration Cage™ (RSA Biomedical, Sweden), supine RSA images were obtained within 3 days of surgery, at 8 w, 6 m, 1 and 2 years. A model based RSA system using an EGS Hemi-sphere model (Medis specials, Leiden) calculated migration. For the 2 year analysis 16 and 15 sets of images were available. ANOVA with repeated measurements analysed movement over time for each type of implant as well as difference between implants. Results Mean translations in the medial –lateral (X), proximal-distal (Y) and anterior-posterior (Z) direction are depicted. Initial movement was noted, especially for the M2a-Magnum cup, where the 2 year mean (sd) X translation was −0.307 (0.53)mm (p = 0.01), Y translation was 0.282 (0.36)mm (p Discussion/Conclusion The early migration of both cups were low. The ASR implant is slightly higher than a competing implant with better survival rates on the Z axis, but low at particularly the Y axis, where the mean 0.075 mm is well below the limit of 0.2mm recently suggested as an acceptable threshold. The M2a-Magnum cup has migration rates just above that, but longer clinical follow up is needed to establish if this cup has greater risk of revision. We will continue to monitor our patients to establish if the individual patient with excess migration is at risk. Continuous migration is not likely to explain failure of neither the cup nor the femoral side of the ASR hip.