As the number of lateral unicompartmental replacements we have done is relatively small, we do not have clearly defined indications for lateral OUKA supported by strong data. Instead, we tend to base the indications on the indications for medial OUKA. We believe the requirements for a successful lateral unicompartmental arthroplasty are as follows:
➤ Bone-on-bone osteoarthritis in the lateral compartment. This is best demonstrated by a Rosenberg view with the knee flexed to 45° or a valgus stress radiograph with the knee at 45° (Fig. 12.7). Standing AP radiographs often underestimate the disease severity.
Figure 12.7 (a) Valgus stress radiograph taken at 45° flexion showing lateral OA with bone-on-bone. (b) This is not shown by a standing weightbearing radiograph taken in extension.
➤ Full thickness cartilage in the medial compartment. This is best demonstrated by a varus stress radiograph (Fig. 12.9). Although some believe that a Rosenberg is an adequate way of assessing the medial compartment, there is no evidence that this is the case. During the surgery it is difficult, if not impossible, to visualise the medial compartment. If there is any concern about its condition, it may be sensible to do an arthroscopy at the beginning of the procedure.
➤ There should be a correctable intra-articular deformity. This is best demonstrated by a varus stress radiograph (Fig. 12.9) which should show that the lateral joint space is of normal thickness or thicker.
Figure 12.8 Lateral weight bearing.
Figure 12.9 Varus stress 40° flexed.
➤ Anterior cruciate ligament. The ACL should be intact. We do not know a good way of determining the status of the anterior cruciate in lateral OA from radiographs. We would therefore make the decision at operation having seen the status of the anterior cruciate.
➤ No contraindications. Like the medial side, we would ignore age, activity, obesity and chondrocalcinosis. We would also ignore the patellofemoral joint unless there was substantial damage with grooving and bone loss.
