Radiography is the most useful adjunct to physical signs in demonstrating the suitability of a knee for OUKA.
Anteroposterior radiographs
Anteroposterior radiographs, taken in the standard way with the patient weight-bearing on the extended leg, can demonstrate loss of articular cartilage medially by showing that the condyles articulate ‘bone-on-bone’ (Ahlback stage 2 or more) (Ahlback, 1968). However, in some cases in which there is full-thickness cartilage loss, this method fails to reveal it. A better projection for this purpose is a Rosenberg view with the patient standing with the knee 45° flexed, with the X-ray beam appropriately tilted, to be parallel to the tibial plateau. A varus-stressed film is more reliable than either of these methods (Fig. 4.7).
Figure 4.7 Varus stressed radiograph of anteromedial OA.
Valgus-stressed radiographs
Valgus-stressed radiographs are used to ensure that there is a normal thickness of articular cartilage in the lateral compartment and to demonstrate that the intra-articular varus deformity is correctable (i.e. the MCL is not shortened). We have found no other method of investigation to be so satisfactory in confirming these two key requirements for successful unicompartmental arthroplasty (Gibson & Goodfellow, 1986).
When the patient stands on a knee with a varus deformity, body weight tends to distract the lateral joint surfaces (Thomas et al., 1975; Dacre et al. 1991). Therefore, to measure the thickness of the lateral compartment cartilage, the lateral condyles must be firmly apposed to one another by applying a valgus force to the otherwise unloaded limb.
