The first ‘modern’ designs, the St Georg (1969) and the Marmor (1972), had polycentric metal femoral condyles articulating on flat (or nearly flat) polyethylene tibial components, both cemented to the bones (Marmor, 1985; Neider, 1991) (Fig. 1.1). The stated principles of Marmor’s design were to reproduce as accurately as possible the polycentric form of the natural femoral condyles; and to avoid constraint of the articulation by employing a non-conforming tibial plateau (Marmor, 1998) . Most of the models introduced since were designed on the same principles.
Figure 1.1 St Georg unicompartmental prosthesis.
Initially, problems were caused by loosening following distortion of the thinnest polyethylene components (6 mm thick), which were abandoned in favour of thicker ones (Marmor, 1976). The persisting problem of deformation of the all-polyethylene component led to the use of metal-backed tibial implants, but this, in turn, resulted in diminished thickness of polyethylene and sometimes further problems with wear (Palmer et al., 1998). However, the fundamental problem remained. A round femoral component makes contact with a flat tibial component on a very small contact area, with high contact stresses, so that problems of wear and deformation were inevitable (Ashraf et al., 2004; Collier et al., 2007). Using a more conforming tibial component introduces constraints which may not be compatible with ligament function (see Chapter 3).
