Isoelastic Distal Ulnar Head Prosthesis: An In Vitro Joint Simulator Study

Abstract

A prosthetic ulnar head that matches the mechanical properties of the sigmoid notch of the radius may produce a superior clinical outcome. In this study, we investigated the feasibility of an ultrahigh-molecular-weight polyethylene (UHMWPE) ulnar head prosthesis as a bearing material and compared the tribology (wear properties) with that of a cobalt chrome (CoCr) distal ulnar replacement. We compared the friction properties, termed lubricity of the UHMWPE ulnar head prosthesis and polished CoCr ulnar head in an in vitro joint simulator with physiologic saline maintained at body temperature. Wear of the UHMWPE ulnar head and polished CoCr ulnar head against a synthetic bone counterface for 1 million cycles was simulated in the in vitro joint simulator. Finally, we studied the fatigue of the UHMWPE prosthesis for 3.1 million cycles of cyclic loading. Both the fatigued and nonfatigued surface were studied with thermal analysis to characterize the UHMWPE prosthesis. The dynamic coefficient of friction for the UHMWPE prosthesis was 0.087 +/- 0.016 and was not significantly different from the polished CoCr coefficient. The synthetic counterface bone wear was 45 times less for the UHMWPE ulnar head prosthesis compared with the polished CoCr head. Both the thermal properties and the viscoelastic moduli for the fatigued UHMWPE remained unchanged compared with unloaded specimens. Ultrahigh-molecular-weight polyethylene is a low-friction material with stable fatigue properties. In addition, in light of its low modulus of elasticity, the UHMWPE ulnar head prosthesis demonstrated low synthetic bone counterface wear compared with the polished cobalt chrome alloy prosthesis. In vitro joint simulation showed that there is a potential role for the UHMWPE ulnar head prosthesis in distal radioulnar joint prosthetic arthroplasty.