Evaluation of the Effects of the Metha® Short Stem on Periprosthetic Bone Remodelling in Total Hip Arthroplasties: Results at 48 Months.

INTRODUCTION Total hip arthroplasty is one of the most performed procedures in orthopaedic surgery. Implantation of a prosthesis determines changes in the distribution of loads on the host bone, and this phenomenon, known as stress shielding, is related to the biomechanical characteristics of the implant. Usually stress shielding involves the proximal portion of the femur by reducing the mechanical strength and ability to withstand the transmitted loads. The aim of our study is to demonstrate how the use of a short hip stem reduces the stress shielding phenomenon to the proximal femur. MATERIALS AND METHODS The study analyzed 20 patients undergoing hip prosthesis surgery with a short stem (Metha<, B. Braun Medical, Inc., Bethlehem, Pennsylvania) at the Ist Orthopaedic Division of Pisa University (between December 2008 and January 2010). Each patient was subjected to analysis of periprosthetic bone mineral density by a bone densitometry (dual emission X-ray absorptiometry [DEXA] with the metal removal software) at 0, 6, 12, 18, 24, and 36 months, following a protocol based on the evaluation of the changes of bone density in the seven Gruen zones. RESULTS We recorded minimal changes in bone mineral density (BMD) at the level of the greater trochanter (-1.44%) and at the level of the calcar (-3.7%). BMD increased significantly after four years at the level of the lateral distal regions (R2 +9.6% - R3 + 12.4%) and at the level of the distal medial regions (R5 + 8.2% - R6 + 13.1%). We compared the results obtained with the literature data at 12 and 24 months with the same stem (Metha<). At 12 months follow up, we did not see a significant difference between our data and the data published in the literature. However, after 48 months of follow-up, we recorded significant differences in the curves of periprosthetic bone reabsorption at the level of the greater trochanter (Zone 1) and at the level of the calcar (Zone 7). DISCUSSION The data obtained from our study are in agreement with other studies in the literature, which demonstrates how the use of short stems preserves the metaphyseal bone stock at the level of the proximal femur, reducing the stress shielding phenomenon. From our data, obtained at 24 months and confirmed at 36, stress shielding seems to minimally occur at the level of the calcar. At the level of the great trochanter, we saw a good load distribution that maintained the baseline BMD; these data are in opposition to the literature data that showed a high increase of BMD at the level of the calcar (+12.9%) and a decrease at the level of the great trochanter. From the analysis of the radiographic images of our cases, and of the cases published with the same stem, these differences in load transfer encountered between the great trochanter and the calcar seems to be related to the level of the femoral neck osteotomy and the consequent stem position (varus/valgus). CONCLUSION We conclude that the amount of periprostetic bone reabsorption around the Metha< stem seems to be strictly related to the surgical technique and the final implant position.