Most of the early complications following both cemented and cementless OUKA relate to the tibia. These include early pain, tibial plateau fractures and tibial subsidence. It is perhaps not surprising that complications occur more at the tibial side than the femoral side because the bone resection significantly weakens the tibia. The resection involves both a vertical and a horizontal cut. The resected bone includes all the subchondral plate which usually provides a tension band supporting the buttress of the medial tibial condyle. Once the bone resection has been done, the stresses in the underlying bone increase substantially which in turn increases the risk of fracture as well as of bone pain.
Finite element analyses have been undertaken to study these problems. These include studies that estimated the strain in the cortical bone a few centimeters below the joint anteromedially in an area where pain commonly occurs (Simpson et al., 2009), and studies estimating the strain within the bone in the region where tibial plateau fractures may occur (Pegg et al., 2013b). These analyses have shown that, even with perfectly performed UKA, bone stresses anteromedially in the tibia increase by about 60% which may explain why there is anteromedial pain (Fig. 2.22). With bone remodelling, the bone may strengthen and the strains decrease which probably explains why the strains settle. In addition, following UKA, the strain within the bone at the site of a possible plateau fracture increases. Both studies have shown that errors in surgical technique are likely to increase the risk of pain or fracture (Fig. 2.23). In particular, it has been shown that vertical saw cuts that go too deep, horizontal resections that are too low, damage to the back of the keel slot or vertical saw cuts that are too medial, all increase the risk of fracture or pain. Furthermore, a cadaver study by Seeger et al. (2012) has shown that the load to fracture is lower following a cementless rather than a cemented component. Although the tibia is undoubtedly weakened during its preparation, we have found that the risk of fracture or pain is very low if the surgery is undertaken carefully without any of the errors described above. Surgeons need to be aware of these risks and take care with tibial preparation.
Figure 2.22 Tibial strain increases by 60% following UKA and is a possible cause of anteromedial tibial pain. With remodelling strain returns to normal levels which explains why pain settles.
Figure 2.23 With a perfect cut (left) the risk of tibial plateau fracture is slightly higher than in a normal knee. It is not appreciably increased by an extended horizontal cut (centre). It is however high (grey) with a deep vertical cut (right).
Conclusion
We have demonstrated that the use of an unconstrained fully congruous mobile bearing in a unicompartmental knee prosthesis can minimise polyethylene wear provided that impingement of bone surfaces or cement against the polyethylene is avoided. This has remained true over the 40-year life of the prosthesis, despite several changes in the material and its processing over that time. A rare complication has been fracture of the bearing, which has been observed only when excessive wear caused by impingement occurred. The interface stresses under a mobile bearing are ideal, particularly for cementless fixation, which should help to minimise loosening. When complications occur they are frequently related to the tibia. Careful surgery is needed to avoid these problems. The theoretical basis for this, and for the surgical technique needed to minimise the risk, are discussed in Chapter 11.