A numerical study of failure mechanisms in the cemented resurfaced femur: effects of interface characteristics and bone remodelling

Failure mechanisms of the resurfaced femoral head include femoral neck fracture in the short term and stress shielding and implant loosening in the long term. In this study, finite element simulations of the resurfaced femur considering a debonded implant—cement interface, variable stem—bone interface conditions, and bone remodelling were used to study load transfer within the resurfaced femur and to investigate its relationship with known failure mechanisms. Realistic three-dimensional finite element models of an intact and resurfaced femur were used. Various conditions at the interface between the stem of the prosthesis and the bone were considered. Loading conditions included normal walking and stair climbing. For all stem—bone contact conditions, the tensile stresses in the cement mantle varied between 1 MPa and 5.4 MPa, except near the distal rim of the resurfacing component where they reached 5.4—7 MPa. In the case of full stem—bone contact, high von Mises stresses (114—121 MPa) were generated in the implant at the stem—cup junction. These stresses were considerably reduced (maximum von Mises stress, 76 MPa) where a gap was present at the stem—bone interface. Resurfacing led to strain shielding of the bone of the femoral head (20—75 per cent strain reductions) and periprosthetic bone resorption (50—80 per cent bone density reductions) for all interface stem—bone contact conditions. In the lateral femoral head and the proximal femoral shaft around the trochantric region, bone density reductions varied between 10 per cent and 50 per cent. Bone apposition was observed in the inferior—medial part of the femoral head and proximal femoral neck region. For full stem—bone contact, more load was transferred through the stem to the surrounding bone, exacerbating strain shielding. Although femoral hip resurfacing conserves bone stock at the primary operation, strain shielding and periprosthetic bone resorption might lead to eventual loosening over time. Post-operatively, the resurfacing procedure generated elevated strains (0.50—0.75 per cent strain) in the proximal femoral neck—component junction irrespective of the variation in interface conditions, indicating an initial risk of femoral neck fracture. Subsequent to bone remodelling, this strain concentration was considerably reduced (0.35—0.50 per cent strain), lowering the risk of neck fracture. In order to reduce the potential risk of neck fracture, patients should avoid activities which might induce high loading of the hip during the early post-operative period to allow the bone around the proximal femoral neck to remodel and heal.