BONE REMODELLING AFTER TOTAL HIP ARTHROPLASTY WITH SL PLUS STEM AND SL PLUS MIA STEM

INTRODUCTION Implantation of total hip arthroplasty (THA) components caused a significant alteration in stress environment. Several studies have reported that bone mineral density (BMD) decreases after THA, especially in the proximal femur. This phenomenon is explained as an adaptive remodeling response of bone tissue to a significant alteration in its stress environment. SL-PLUS MIA stem (Smith & Nephew Orthopaedics AG) is a modified implant of Zweymuller type SL-PLUS standard stem (Smith & Nephew Orthopaedics AG). The major change is an omission of the trochanteric wing, which enables a bone-sparing and may lead to changes of femoral stress distribution and rotational stability. The change of stress distribution in the femur could affect BMD after THA. In the present study, we constructed finite element (FE) models of femurs and stems before and at 1week after THA and analyzed equivalent stresses in the femur. In addition, we measured BMD in the femur by dual-energy X-ray absorptiometry (DEXA) after THA. The purpose of this study was to investigate the equivalent stress in the femur and to compare the results of the FE analyses with changes in BMD after THA. METHODS Twenty-one patients (18 women and 3 men) who underwent primary cementless THA with SL-PLUS MIA stem or SL-PLUS standard stem formed the basis of this study. Eleven patients received SL-PLUS MIA stem and ten patients received SL-PLUS standard stem. The mean age of the patients at THA was 67 years (range: 48∼82). BMD was measured with DEXA at 1 week and 3, and 6 months after THA. Zones were defined according to Gruen9s system (zones 1∼7). Computed-tomography (CT) images of the femur of all patients were taken before and at 1 week after THA. FE models of the femur and prosthesis were obtained from CT data by Mechanical Finder (Research Center of Computational Mechanics Inc., Tokyo, Japan), software that creates FE models showing individual bone shape and density distribution. Equivalent stresses were analyzed in zones 1 to 7 and compared to the DEXA data. RESULTS FE studies revealed that there was no significant difference in equivalent stress between SL-PLUS MIA stem group and SL-PLUS standard stem group after THA. BMD was maintained at 6months after THA in zones 3, 4, and 5, whereas BMD decreased in zones 2, 6, and 7. This means that BMD had decreased, especially in the proximal femur after THA. In zone 1, BMD decreased in SL-PLUS MIA stem group by 15%, while BMD was maintained in SL-PLUS standard stem. There was a significant difference in BMD percentage changes in zone 1 between SL-PLUS MIA stem group and SL-PLUS standard stem group at 6months after THA. DISCUSSION The present study demonstrated that there was no significant difference in equivalent stress between SL-PLUS MIA stem group and SL-PLUS standard stem group after THA. In zone 1, BMD decreased in SL-PLUS MIA stem group by 15%, whereas BMD was maintained in SL-PLUS standard stem group. This result cannot be explained by our results of equivalent stress.