Prospective Evaluation of Fixation of the Prime Acetabular Cup

The gold standard for measurement of implant fixation over time is radiostereometric analysis (RSA). At the time of surgery, 0.8-1.0 mm Tantalum beads (typically n = 6-8 per region of interest) are inserted into the bone surrounding the implant of interest, and occasionally also into the polyethylene liner or insert of the implant. RSA exams are then acquired within the first 2 weeks post-operation, and again at 6 weeks, 3 months, 6 months, 1 year, and 2 years. Each exam involves a stereo x-ray acquisition with a calibration unit to reconstruct the 3D location of the implant relative to the marker beads. Across each exam, the migration of the implant relative to the marker beads is tracked compared to the baseline exam acquired within the first 2 weeks. RSA is extremely precise and accurate, and is able to track migrations on the order of 10's of microns. Large early migrations within the first 1-2 years post-operation are predictive of eventual implant loosening and failure. For acetabular cups, proximal migration of greater than 1.0 mm within the first 2 years post-operation is considered unacceptable, with cups migrating between 0.2 and 1.0 mm at risk of having a revision rate exceeding 5% at 10 years.

An RSA study should be completed for any new implant under the principles of phased innovation in orthopaedics. This ensures the new device will achieve long-term, stable fixation prior to its widespread adoption. Many device manufacturers incorporate the results of the RSA migration study into their marketing material. The advantage of RSA is its high degree of accuracy and its well-established thresholds for implant migration, meaning a relatively small number of patients are required to provide meaningful results.

Several joint replacement companies (Microport excluded) have previously released new products to the marketplace without RSA evaluation and these implants have later been identified as having ingrowth problems. The scientific aim of this study is to prove efficacy of ingrowth with a recently released implant.

This study design will be a prospective cohort with a retrospective control group. The study will be conducted at a single centre (London Health Sciences Centre - University Hospital, London, Ontario, Canada) with all cases performed by a single, fellowship-trained high-volume surgeon.

The study group will receive a Prime cementless acetabular cup (manufactured by Microport located in Arlington, Tennessee). All patients will also receive a cementless Profemur femoral stem with a 32 mm Cobalt Chromium femoral head, articulating on a highly crosslinked acetabular liner. The control group will be a previously published cohort that received the Reflection cementless acetabular cup with Roughcoat coating (Smith & Nephew, Memphis, TN). This is a well-established cup with long-term survivorship available in multiple registries, making it an ideal comparator.