title: Edge loading in metal-on-metal hips: low clearance is a new risk factor.
abstract: The revision rate of large head metal-on-metal and resurfacing hips are significantly higher than conventional total hip replacements. The revision of these components has been linked to high wear caused by edge loading; which occurs when the head-cup contact patch extends over the cup rim. There are two current explanations for this; first, there is loss of entrainment of synovial fluid resulting in breakdown of the lubricating film and second, edge loading results in a large local increase in contact pressure and consequent film thickness reduction at the cup rim, which causes an increase in wear. This paper develops a method to calculate the distance between the joint reaction force vector and the cup rim--the contact patch centre to rim (CPCR) distance. However, the critical distance for the risk of edge loading is the distance from the contact patch edge to rim (CPER) distance. An analysis of explanted hip components, divided into edge worn and non-edge-worn components showed that there was no statistical difference in CPCR values, but the CPER value was significantly lower for edge worn hips. Low clearance hips, which have a more conformal contact, have a larger diameter contact patch and thus are more at risk of edge loading for similarly positioned hips.
authors.affiliation: Department of Mechanical Engineering, Imperial College London, London, UK. runderwood@exponent.com
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Underwood
authors.forename: Angelos
authors.initials: A
authors.lastname: Zografos
authors.forename: Ritchie S
authors.initials: RS
authors.lastname: Sayles
authors.forename: Alister
authors.initials: A
authors.lastname: Hart
authors.forename: Philippa
authors.initials: P
authors.lastname: Cann
mesh_terms: D000077:Acetabulum / Q000503:physiopathology* / Q000601:surgery; D003198:Computer Simulation; D017076:Computer-Aided Design; D005269:Femur / Q000503:physiopathology* / Q000601:surgery; D017276:Friction; D006621:Hip Joint / Q000503:physiopathology* / Q000601:surgery; D006622:Hip Prosthesis; D006801:Humans; D008670:Metals; D008954:Models, Biological; D011474:Prosthesis Design; D011475:Prosthesis Failure; D012307:Risk Factors; D013314:Stress, Mechanical; D016474:Weight-Bearing
chemical_list: D008670:Metals