Degenerative lateral meniscus
The presence of degenerative tears of the lateral meniscus has been regarded as a contraindication to UKA by some authors (Ritter et al., 2004). Arthroscopy has not been used as a routine preoperative investigation in any published series of OUKA and, as the state of the meniscus cannot be assessed adequately through the small incision now employed, we do not have data on its predictive significance. Nevertheless, as mentioned above, the most common cause for revision of OUKA (Phase 2) was arthritis of the lateral compartment, and it is possible that a more stringent approach than we have adopted would diminish the failure rate from that cause. However, necropsy studies have shown that degenerative lesions are common in both menisci in middle-aged and older people, and so they must usually be compatible with adequate function of the joint (Noble & Hamblen, 1975). Ritter et al. (2004) found the lateral meniscus to be ‘degenerative or absent’ in 69% of varus arthritic knees treated by joint replacement. Therefore, paying attention to the state of the lateral meniscus would deny the advantages of UKA to very many patients in the hope of diminishing the 1.7% 10-year cumulative revision rate (CRR) from lateral compartment failure.
Symptomatic lateral meniscal tears do rarely occur after OUKA and can be treated satisfactorily by arthroscopic meniscectomy. We do not know if these are related to the state of the lateral meniscus at the time of surgery.
Articular surface damage
Damage to the articular surface of the lateral compartment is considered to be a contraindication to UKA. We recommend that the lateral side should be assessed with a valgus stress radiograph. If this shows full thickness cartilage then we will ignore any damage to the lateral side seen at operation. The only exception to this, which is very rare, is that if a full-thickness defect in the central weight-bearing cartilage is seen, we would treat this as a contraindication (although we have no evidence to support this practice).
In about 30% of cases, there is articular cartilage damage seen on the medial side of the lateral femoral condyle (Kendrick et al., 2010). This is often a circumscribed area of cartilage erosion, which can be up to 1 cm wide and 3 cm long, with eburnated bone exposed in its floor (Fig 5.4).
Figure 5.4 Intra-operative picture of right knee with patella retracted laterally showing the full thickness ulcer on the medial side of the lateral femoral condyle.
This erosion is a consequence of the varus alignment causing the medial side of lateral femoral condyle to impinge on the tibial eminence. (Fig. 5.5(a)). Following surgery, when the varus is corrected, impingement no longer occurs (Fig. 5.5(b)). We have found that there is no difference in outcome between patients with and without articular cartilage damage on the medial side of the lateral condyle; we do not , therefore, consider it to be a contraindication to OUKA.