Unicompartmental Arthroplasty with the Oxford Knee. Buy the book here.
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Introduction
The description of the Oxford Knee starts with an explanation of the function of mobile bearings in knee prostheses. An obvious advantage is that the areas of contact between the joint surfaces are maximised. In this chapter, we shall show that wear at the polyethylene surfaces is thereby minimised and that optimal kinematics can be achieved with minimal risk of loosening. We will discuss the biomechanics of the cementless components and problems that may occur with the tibia.
Designing against wear
Articular surface shapes and contact pressures
Most surface replacements of the knee, total as well as unicompartmental, have articular surfaces like those shown in Figure 2.1, approximating to the shapes of the ends of the human femur and tibia. The metal femoral surfaces are convex and the polyethylene tibial surfaces are flat or shallowly concave. These shapes do not fit one another, in any relative position, and so only parts of their articular surfaces are in contact and able to transmit load.
Most prosthetic femoral condyles attempt to mimic nature and are polyradial, with the shortest radius posterior. Thus the area of contact is smaller in flexion than in extension (Fig. 2.1). However, the compressive loads transmitted across the interface are potentially greatest in flexion, attaining up to six times body weight during stair ascent and descent (Taylor et al., 2004). For a given load, the average contact pressure (load per unit area) at the articular surfaces is inversely proportional to the area of contact; therefore the less congruous the surfaces, the higher is the average pressure at their interface. The wear rate of ultra-high-molecular-weight polyethylene (referred to hereafter as ‘polyethylene’) is said to increase exponentially with increasing contact pressure, rather than linearly as would be expected from classical wear theory (Rostoker & Galante, 1979); conversely, wear rate has been found to decrease with increasing contact area (Sathasivam et al., 2001).
