Abstract
Introduction The historical degradation of polyethylene produced a direct relationship between contact stress and wear in knee prostheses(1). However, with the recent introduction of stabilised polyethylene and designs with reduced contact stress, the significance of this relationship has not been re-assessed. The purpose of this study was to analyse the contact mechanics of three currently available knee designs (two rotating platform and one fixed bearing) prior to and after long term simulator wear testing.
Materials and Methods Implants (six of each design) were loaded with 2600N at flexion angles of 0°, 30° and 60°. Contact areas were measured using Fuji Pre-scale pressure sensitive film, which was scanned and digitised using Image Pro Plus software.
Results The average contact stresses of the worn knee components are shown in Figure 1. Previously reported wear results for the three designs are shown in Figure 2 (1,2). Stresses reduced slightly following wear testing.
Conclusion The three designs tested produced stresses below the elastic limit of the polymer at all flexion angles. The two rotating platform designs had significantly reduced stress compared to the fixed bearing design. Both rotating platform designs tested de-couple the rotation and produce reduced cross-shear compared to the fixed bearing design. It is postulated that both low stress and reduced cross-shear are important in reducing the wear of knee prostheses.
Correspondence should be addressed to Roger Smith, Honorary Secretary, BASK c/o Royal College of Surgeons, 35 – 43 Lincoln’s Inn Fields, London WC2A 3PN
References
1. Fisher et al, Current Orthopaedics, 15, pp399–405, 2001. Google Scholar
2. Bell et al, Proc 49thORS, poster1403, 2003. Google Scholar
