COMPARISON OF CONTACT MECHANICS BETWEEN A TOTAL HIP REPLACEMENT AND A HIP RESURFACING WITH A METAL-ON-METAL ARTICULATION

Abstract

Introduction: Laboratory simulator and clinical retrieval studies of metal-on-metal (MOM) total hip replacements have shown that the metallic alloy, the femoral head radius, the clearance between the acetabular cup and femoral head and the cup thickness can influence the contact mechanics, the lubrication and the wear of the articulation. MOM hip resurfacing procedures have received significant attention recently. The purpose of the present study was to compare the contact mechanics between a MOM hip resurfacing implant and a MOM total hip replacement under identical conditions.

Materials and Methods: A 50mm diameter DUROM^TM MOM hip resurfacing prosthesis and a 28mm diameter Metasul^TM MOM bearing system (Centerpulse Orthopedics, a Zimmer Company, Winterthur, Switzerland) were investigated. All implants were manufactured from wrought-forged high carbon cobalt chromium alloy (Pro-tasul 21WF^TM). The diameters of the DUROM^TM femoral head and acetabular cup were 50mm and 50.145mm respectively, and the corresponding wall thickness of the acetabular component was around 4mm. The diameters of the Metasul^TM femoral head and acetabular cup were 28mm and 28.12mm. Three-dimensional finite element models were created to simulate the contact between the bearing surfaces of both the femoral head and the acetabular cup fixed to a three dimensional anatomically positioned pelvic and femoral bone consisting of both cortical (with 1mm thickness) and cancellous regions. The load applied to both models was 3200N.

Results: The maximum contact pressure at the bearing surfaces was found to be around 22MPa for the DUROM^TM and the contact area between the femoral and acetabular components was predicted to be 237mm². For the Metasul^TM bearing under identical conditions, the maximum contact pressure and the contact area predicted were approximately 47MPa and 74mm² respectively.

Discussion: A large reduction in the contact pressure, which should improve overall tribological performances, was noted for the DUROM^TM hip resurfacing prosthesis, as compared with the Metasul^TM bearing. The main reasons for this reduction were the large diameter of the articulation and the small acetabular cup thickness of the DUROM^TM system. In contrast, the Metasul^TM bearing has a smaller head diameter, and relies on a polyethylene backing underneath the metallic cup inlay to reduce the contact pressure at the articulating surfaces.