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

Purpose The aim of this study is to quantitatively compare the difference in primary stability between collarless and collared versions of the same femoral stem. Specifically, we tested differences in subsidence and micromotion. Methods Collarless and collared versions of the same cementless femoral stem were implanted in two groups of six fresh-frozen cadaveric femurs. Each implanted femur was then subsequently tested for axial compressive and torsional loadings. A micro-CT based technique was applied to quantify implant subsidence and compute the map of local micromotion around the femoral stems. Micromotion of collarless and collared stems was compared in each Gruen zone. Results Subsidence was higher but not significantly (p = 0.352) with collarless (41.0 +/- 29.9 mu m) than with collared stems (37.0 +/- 44.6 mu m). In compression, micromotion was lower (p = 0.257) with collarless (19.5 +/- 5 mu m) than with collared stems (43.3 +/- 33.1 mu m). In torsion, micromotion was also lower (p = 0.476) with collarless (96.9 +/- 59.8 mu m) than collared stems (118.7 +/- 45.0 mu m). Micromotion was only significantly lower (p = 0.001) in Gruen zone 1 and for compression with collarless (7.0 +/- 0.6 mu m) than with collared stems (22.6 +/- 25.5 mu m). Conclusions 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.