19 – ONE-YEAR RESULTS OF AN RCT USING RSA TO COMPARE MINIMALLY INVASIVE SURGERY (MIS) TO STANDARD EXPOSURE IN PRIMARY UNCEMENTED MODULAR THA

Abstract

Purpose: To determine if MIS for primary hip replacement surgery increases the risk of long term aseptic loosening as predicted by implant micromotion measured with radiostereometric analysis (RSA).

Method: Ninety patients undergoing primary THA for osteoarthritis (exclusion criteria: post-traumatic arthritis, rheumatoid arthritis, hip dysplasia, previous hip infection) were randomized to undergo THR surgery utilizing the standard direct lateral approach (n=45; 24 male; age=58 yrs; BMI=27) or MIS via a one-incision direct lateral approach using specific instrumentation (n=45; 23 male; age=55; BMI=29). Uncemented acetabular and femoral (ProfemurZ) components were used with ceramic on ceramic bearings. The femur was marked with 9 tantalum beads placed in the greater trochanter, lesser trochanter, and femoral shaft distal to the tip of the prosthesis. Post-operative care was be standardized according to the care maps at our institution. Primary outcome measure was femoral stem MTPM (maximum total point motion) measured using Model-based RSA. Stereo supine X-rays were taken before weight bearing and 3, 6, and 12 months postoperatively. At the same time intervals Harris Hip Score, Oxford-12, WOMAC, and SF36 questionnaires were administered. Rates of infection, dislocation and revision were recorded.

Results: Eleven patients were lost to follow-up (4 due to missing post-op exams; 5 did not have enough beads placed during surgery; 2 were revised due to failure of the ceramic femoral head). There were five long neck fractures at 17–30 months postop that are reported in detail in a related abstract. There were no differences between groups for all outcome measures. Mean MTPM at 12 months was 2.5mm (SD=1.8mm) for the MIS group and 2.6mm (SD=1.2mm) for the standard group.

Conclusion: No difference between groups at one year indicates MIS for uncemented primary THR through a direct lateral approach does not appear to negatively affect stability of the femoral stem. Although promising, these results require confirmation with 2-year RSA data.