Smooth uncemented femoral stems do not provide torsional stability of femurs with cortical defects

Abstract

Objectives: The objective of this study was to experimentally determine the optimal length of a cementless femoral component in total hip arthroplasty (THA) in the presence of a diaphyseal defect. Materials and methods: This study was performed in paired, forty canine femora. The study was divided into two phases. First reamed femurs with 50% circular defect and contralateral control intact femurs were tested to determine the weakening imposed by the defect. Secondly, the effect of stem bypass distance of one, two and three diaphyseal diameters beyond the defect on bone strength were tested. Pairs were torsionally stressed to failure on the testing system. Both paired t-test and analysis of variance were used for data analysis.
Results: The femurs with 50% cortical defect alone always experienced spiral fractures through the defect with significant reductions in maximum torque (p<0.002), angular deformation (p<0.01), and energy absorption (p<0.01) to failure. Although there was slight improvement, no significant difference in maximum torque, angular deformation, and energy absorption to failure was observed with cementless rod implantation and no significant difference was observed between the groups with different bypass lengths. Conclusion In cementless femoral components, bone-prosthesis interface is not fully bonded in the early postoperative period. A suggestion may be made that patient with THA who have a defect in the femur, until his bone regains strength, should be protected from activities which produce large torsional loads or the design of a prosthesis that would optimize the initial interference fit should be used for these cases.