Closeness of fit of uncemented stems improves the strain distribution in the femur

Abstract

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.

The Adaptiva custom-made stem—our reasons for not using it anymore

“…Differences in the cross-sectional shapes of intramedullary stems are thought to affect the strains in the femur, and therefore a close proximal fit has been proposed as particularly advantageous. Hua and Walker showed in their cadaver series that custom-made stems consistently produced a pattern of strain distribution, which was closer to normal than that of standard stems [13]. Hua and Walker also showed that custom-made stems exhibited low values for relative motion under load [12].…”

Influence of cementless hip stems on femoral cortical strain pattern depending on their extent of porous coating

“…At the height of the antecurvation of the asymmetric stem, 48 % higher strains were registered on the anterior side as compared with the intact femora. This effect was interpreted as a sign of a more direct load transfer and was absent with the symmetric stem [30] . Though partially structured stems aim for a proximal load transfer, whereas extensively coated implants should be integrated at their full length, the results of our study did not expose a signifi cant difference in femoral cortical strain pattern within the three stems tested.…”

“…These changes could not be proven to be significant. Hua and Walker inserted a symmetric and an asymmetric hip stem (experimental design) in photoelastically coated composite femora [30] . At the height of the antecurvation of the asymmetric stem, 48 % higher strains were registered on the anterior side as compared with the intact femora.…”

“…No literature data are available concerning the infl uence of repeated use of composite femora after removal and reinsertion of hip stems. However, with both composite and human femora, the photoelastic coating technique is well established for cortical strain measurement since double refraction properties of certain plastics have been discovered in the 1930s [21,28,30,32,44,46,57,58,64] .…”

Custom Cementless THA in Patients with Skeletal Dysplasia Results in Lower Apparent Revision Rates than Other Types of Femoral Fixation

“…2). Concurrent mathematical modeling using finite element analysis combined with dual-energy xray absorptiometry (DEXA) studies suggest optimizing the ''fit and fill'' of the proximal femoral canal by incorporating a lateral flare permits more natural physiologic loading and preferential adaptive bone remodeling and osseointegration [21]. The strain pattern in the proximal femur using the CAD-CAM stem is closer to that of normal bone than with other types of prostheses, which preserves bone mass [21,22].…”