[Biomechanical aspects of the implant fixation and kinematics of hip resurfacing systems]

Hip resurfacing is undergoing a resurgence in orthopaedic surgery with an increasing number of implantations. The objective of this article is to present the biomechanical basics of implant anchorage as well as the kinematics of hip resurfacing implants.Today, fixation of the femoral component onto the prepared femoral head is mainly done using bone cement. Depending on the implant design, the bone structures beneath the femoral component can be exposed to stress shielding, followed by degradation of the bone density and subsequent initiation of implant loosening. However, the trabecular bone has the ability to adapt itself to the fixation peg, to additional cement pegs, and to the elastic properties of the femoral component as well. The acetabular component is mainly inserted into the bone stock without using cement. Provided that large prosthetic heads will be applied, thin-walled acetabular cups are crucial for bone-saving preparation of the acetabular bone stock. Nearly all hip resurfacing systems are currently based on metal-on-metal wear-bearing couples. The acetabular components are mainly designed as monoblock implants, which can make subsequent revision difficult. Kinematic analyses show a significantly lower range of motion of hip resurfacing implants compared with modern standard (stemmed) total hip replacement systems. This difference originates from the small ratio of the resurfaced femoral head diameter and the relatively thick neck of the femur. Impingement of the femur neck onto the rim of the acetabular component can result in subluxation, deformation of the bearing surfaces, femoral neck fracture, and impairment of the bony anchorage of the hip resurfacing implants.