Marker wire position is an important factor in the risk of polyethlyene bearing fracture

Introduction
The Oxford Unicompartmental Knee Replacement (UKR) has a mobile bearing, which is known to reduce the wear
rate and reproduce normal knee kinematics. The design of the bearing has a fully congruent spherical surface on
the femoral side and a at fully congruent surface on the tibial side. Unfortunately a small number of ultra-high
molecular weight polyethylene (UHMWPE) bearings have fractured after several years (average 17 years) in vivo.
The fractured bearings all showed signs of wear and impingement, indicating surgical technique may be to blame.
In almost all cases the fracture was transverse across the centre of the bearing; these fractures were examined in a
previous study [1]. In phase I and phase II Oxford UKR designs, the bearing had two embedded metal marker wires
to help assess the position of the bearing on post-operative radiographs. This has been replaced by marker balls in
the current, phase III, design. The purpose of the present study was to examine whether there may be a mechanical
cause for the characteristic pattern of fracture observed.
Patients & Methods
A simplied 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), a
symmetric quarter model was examined. The load applied was 2.4 kN and was applied in one loading-unloading step
sinusoidally over 1 second. A J2 plasticity model was used for the UHMWPE, the marker wires were modelled as linear
elastic titanium and the femoral and tibial surfaces as rigid bodies.
Results
In models where the marker wire was near to the centre of the bearing, a line of tensile stresses were observed in the
same pattern as the fractures. A reduction in the tensile stress was found when the marker wires were replaced by
marker balls.
Discussion/Conclusion
These results indicate that the position and presence of the marker wire may be the cause of the typical fracture
pattern observed in the fractured bearings. In the phase III component design, the posterior marker wires were
replaced by two marker balls, after this a reduced incidence of bearing fracture was observed. Changing the marker
wire to marker balls in the polyethylene bearing signicantly reduces the tensile stresses within the bearing, thereby
reducing the fracture risk.
References
[1] Pegg E, Pandit H, Gill HS, et al. 2011. Examination of ten fractured Oxford unicompartmental knee bearings. JBJS
[Br] 93-B: 1610-1616.
Tensile