CLOSENESS OF FIT OF UNCEMENTED STEMS IMPROVES THE STRAIN DISTRIBUTION IN THE FEMUR

Differences in the cross-sectional shapes of intramedullary stems are expected to affect the strains in the femur, a close proximal fit being proposed as particularly advantageous. We compared uncemented femoral stems of different designs, as well as a cemented stem, using the intact femur as a control. The collarless stems were custom-made, standard asymmetric, symmetric (i.e., neutral), and cemented symmetric. These different types of stem were inserted in sequence into eight cadaver femurs. A photoelastic coating technique was used to give a measure of the shear strains over the bone surface. The mean reductions of maximum shear strains in the upper 30 mm of the proximal-medial regions for the custom, asymmetric, symmetric, and cemented stems were 11, 24, 6, and 75%, respectively. The strains for the symmetric stem were concentrated in the proximal-medial region just below the neck resection line and then decreased rapidly below that level. For the asymmetric stems, the relative strains in the proximal-anterior and proximal-medial regions were dependent on the tightness of fit in the anterior-posterior and medial-lateral directions. The custom stems consistently produced a pattern of strain distribution that, overall, was closer to normal than that of the other stems; extremes at the anterior and medial regions were avoided. The strains down the entire medial side of the cemented stem were significantly less than for the intact femur or for the uncemented stems.