The mechanical advantages conferred by the natural meniscus can be enjoyed by an artificial knee if it is provided with two joint interfaces instead of one. The design of the articular surfaces of the Oxford Knee has not changed since its first implantation in 1976 (Fig. 2.7). The femoral component made of metal has a spherical surface, the metal tibial component is flat. The polyethylene meniscal bearing has a spherical upper surface and a flat lower surface. The meniscofemoral interface (ball-in-socket) allows the angular movements of flexion–extension, the meniscotibial interface (flat-on-flat) allows translational movements (Fig. 2.8), and axial rotation is allowed by a combination of translation and spinning movement at both interfaces. The unconstrained mobile bearing does not resist the movements demanded by the soft tissues, muscles, and ligaments. Restoration of natural mobility and function may be expected (Chapter 3). The surfaces of the prosthesis experience mainly compressive forces, features which should minimise component loosening (Goodfellow and O’Connor, 1978). A low loosening rate is reflected in a high long-term survival rate (Chapter 10).
Figure 2.7 Components of the Oxford Knee (Phase 1).
Figure 2.8 Combined sliding at the femoro-meniscal and menisco-tibial interfaces during flexion–extension allow anteroposterior translations of the femur on the tibia under the control of the ligaments while maintaining full conformity between the components in all positions (see Chapter 3).
The method of load transmission through the polyethylene bearing is, of course, quite different from that through the natural meniscus, but the functions of the two structures are analogous. The prosthetic bearing converts one incongruous interface into two congruous interfaces, maximising the area available for load transmission without limiting the freedom of joint movement, a feature which should minimise polyethylene wear while restoring physiological function. These are the justifications for calling the Oxford Knee a ‘meniscal bearing’ implant.
Why use a spherical not a polyradial femoral condyle?
A rigid polyethylene bearing can model only the mobility of the natural meniscus, and not its compliance. It cannot change shape and therefore cannot fit more than one of the several radii offered by a polyradial condyle. The only pairs of shapes that can maintain congruity in all relative positions of the components are a sphere in a spherical socket and a flat surface on a flat surface. Figure 2.9 shows the medial half of a specimen of a distal femur, sectioned through the sulcus of the trochlear groove. A circle fits the cartilage surface at the base of the trochlear groove quite well. Another circle fits the posterior facets of the femoral condyle, although it does not match its most distal facet. Therefore, a spherical femoral prosthesis attached to the posterior condyle can reproduce the shape of all but the most anterior part of the medial condyle.
Figure 2.9 Sagittal section of the distal femur demonstrating that the sulcus of the trochlea and most of the medial condyle are circular. (This material has been reproduced from the Journal of Engineering in Medicine: Proceedings of the Institution of Mechanical Engineers Part H. 1989 Vol 216 Issue H4 pp 223–233. The geometry of the knee in the sagittal plane. O’Connor J, Goodfellow J, Shercliff T, Biden E. Permission is granted by the Council of the Institution of Mechanical Engineers.)
Other mobile-bearing designs
Since 1978, several designers have used mobile bearings in total and unicompartmental knee prostheses but with polyradial femoral condyles (Buechel and Pappas, 1986; Schlueter-Brust et al., 2014). In such implants, the concavity on the upper surface of the bearing must have a radius of curvature large enough to accommodate the largest radius of the femoral condyle (offered in extension) and therefore too large to match the smaller radii (offered in all positions of flexion when the compression forces between the components are greater). The contact pressures in flexion are likely to be large. Thus the function of such a mobile bearing is not analogous to that of the natural meniscus and is unlikely to minimise wear. Non-conforming mobile-bearing prostheses offer little theoretical advantage over non-conforming fixed-bearing designs.