A FINITE ELEMENT ANALYSIS FOR A NEW SHORT STEM CONCEPT DESIGN WITH SPHERICAL BONE INTERFACE FOR HIP RESURFACING

Hip resurfacing is an option for the treatment of degenerative diseases like hip osteoarthritis, rheumatoid arthritis or femoral head necrosis. This type of hip surgery, minimally invasive, is adopted for young male patients due to its bone saving procedure that leave more possibilities in case of an hypothetical revision, however with controversial results. The design of a new short stem could help to reduce postoperative failure caused by the new stress distribution in the bone such as loosening and fractures. The aim of the study was to compare, by means of a finite element method (FEM) analysis, the structural behavior of the biomechanical system bone-prosthesis constituted by a femur and respectively a new short stem design and a traditional one. A numerical model of an intact femur was developed and structurally validated with experimental results reported in literature. The new short stem design was compared with a long stem design; the two stems are combined with two different resurfaced femoral caps. Under two types of loads, respectively simulating normal walking and stairs climbing proximal femur stress-strain distribution and bone-stem relative micromotions were analyzed and compared. In conclusion it was observed that shortening of the stem results in a stress and strain increase in correspondence of the peripheral area of support of the cap on the bone; this situation could be a potential cause of bone fracture in the immediate postoperative period if the load is not gradually released. On the other hand the load transfer from the stem to the bone occurs in a more physiological way along the femoral neck. In long stem model there is a change in strain and stress distribution both in correspondence of the femoral head and along the femoral neck.