Effect of a collar on subsidence and local micromotion of cementless femoral stems: in vitro comparative study based on micro-computerised tomography.

Primary stability was achieved for both stem designs, with a mean micromotion below the osseointegration threshold. Under loading conditions similar to those observed in normal daily activity and with good press-fit, the collar had no influence on subsidence or micromotion. Further studies are required to test the potential advantage of collar with higher loads, undersized stems, or osteoporotic femurs.