THE IMPORTANCE OF CONTACT STRESS IN KNEE HEMIARTHROPLASTY DESIGN: A TRIBOLOGICAL SIMULATION

An alternative type of joint replacement for the young, active patient is hemiarthroplasty, which preserves bone stock for future revision surgery. However, this treatment has had varied success to date [Amstutz, 1994; Sisto, 2005]. The tribological regime in both healthy and those replaced with a hemiarthroplasty is very complex. The mechanisms of wear and degradation of AC are not completely understood. The aim of this study was to investigate the effects of contact stress on friction and wear of articular cartilage, using a tribological simulation of the medial compartment of a bovine knee. Materials and Methods A pendulum friction simulator was used to apply physiological loads and motion, in this tribological simulation of the medial compartment of the natural knee. Bovine medial femoral condyles (18 month old and with approximate radius of 38-42 mm) were articulated against tibial surfaces or stainless steel plates of different conformity to create different stress conditions. Several simulations were carried out as shown in table 1. Simulation Tibial Counterface Natural Tibia +meniscus intact (AC-AC +meniscus) Meniscectomy Tibia (meniscus removed) (AC-AC no meniscus) Hemi (flat) Flat SS plate – AC-SSFP Hemi (50mm radius) 50mm conforming plate AC-SSCP50 Hemi (100mm radius) 100mm conforming plate AC-SSCP100 Table 1:Five different tibial counterfaces were used simulating the natural, meniscectomised and hemiarthroplasty-replaced joint. Each simulation was tested at high (1000N) and low (250N) load. The lubricant used was 25% bovine serum in saline. The loading and motion profiles were derived from BS 14243-3:2004. A dynamic axial load was applied (peak loads are referred to in this paper). A motion amplitude between -10o 13o was used, with a frequency of 1Hz. The coefficient of friction ( ) was measured through a piezoelectric transducer. Contact stresses between bearing surfaces were measured using pressure sensitive Fuji Film and a spectrodensitometer. Results