Does the marker wire position contribute to risk of polyethylene bearing fracture

INTRODUCTION: A small number of unicompartmental knee bearing fractures have been observed after several years in vivo (mean 17y). The design of the bearing has a fully congruent spherical surface on the femoral side and a flat fully congruent surface on the tibial side. The fractured bearings all showed signs of wear and impingement, indicating poor surgical technique [1]. In early designs, the bearing had two embedded metal marker wires to help assess the position of the bearing on post-operative radiographs; the posterior wire has now been replaced by two balls.
OBJECTIVES: To assess the influence of the radio-opaque marker wire position on the tensile stresses within the bearing and examine whether the replacement of the marker wire with balls reduced the stresses.
METHODS: A simplified parametric model of a Phase I bearing was created in which the marker wire position could be varied or replaced with marker balls. Finite element analyses (FEA) were performed explicitly (ABAQUS, Simulia). The loading used for the model represented a worst-case scenario, and used a maximum load of 2400N, based upon an 82kg patient during walking (3x body weight). The femoral component was modelled as a rigid body and the a non-linear J2- plasticity model was used the represent the material behaviour of the bearing.
RESULTS: The FEA models demonstrated high tensile stresses transversely across the bearing when posterior marker wire was included; these stresses increased the closer the wire was to the centre and reduced significantly when replaced with marker balls. The contact pressure on the spherical surface has both horizontal as well as vertical components; these, in combination with the position and presence of the marker wire, may lead to the tensile stresses and the typical fracture pattern observed in the fractured bearings.
CONCLUSION: Changing the marker wire to marker balls in the polyethylene bearing significantly reduces the tensile stresses within the bearing, thereby reducing the fracture risk.