Prediction of cement damage for the Stanmore and Charnley femoral stems

Abstract

Charnley and Stanmore cemented femoral stems are of the most studied stems and in vivo long-term results up to 15 and 20 years, respectively, have been published. The in vitro study hereby described correlates biomechanical factors such as stem position in the femoral canal, cement thickness and fatigue cracks with in vivo published results. Four of each femoral stems were loaded and tested in fatigue simulating stair climbing during one million cycles at 2 Hz. After the fatigue experiments all implanted femurs were sectioned and analyzed. Correlation between in vivo long-term published results with debonding and cracking was performed. Finite element models of the Stanmore and Charnley stems were used to predict tensile and compression stresses in the cement mantle to detect the critical regions and correlate these with experimental results. Within the analysis of the sections we observed some defects relatively to the positioning of the stems. The Stanmore stem was mainly placed in a valgus position while the Charnley stem was in a varus position. The number of cracks in the stem–cement interface was more visible for the Charnley stems. The lateral aspect was the most critical since more cracks were visible. Cement damage was more pronounced proximally and especially where cancellous bone presents higher thickness. The stems presented different behaviors in the cement–bone interface. The finite element results showed agreement between the maximum principal stresses and the regions of more cracks. The in vitro results are comparable with long-term in vivo published results. The experimental study hereby described can be used as a pre-clinical test to predict the performance of new implants before being launched into the market.