The pattern of cartilage and bone erosion seen in anteromedial arthritis is not found on the lateral side (Harman et al., 1998). Medial unicompartmental OA is a disease of extension whereas lateral unicompartmental OA is a disease of flexion. On the lateral side, the cartilage and bone erosions tend to begin in the central part of the tibial plateau which is in contact with the femur at about 45° flexion. On the femoral side, there is a kissing lesion at about 45° flexion. The lesion then expands both anteriorly and posteriorly. As a result there is usually some retained cartilage on the femur in extension and 90° flexion (Gulati et al., 2009).
Gulati et al. (2009) reported the patterns of osteoarthritis (OA) in both medial and lateral unicompartmental OA of the knee. Forty patients with medial and 20 with lateral unicompartmental knee osteoarthritis were studied to determine the location of full thickness cartilage lesions. Intra-operatively, the distance between margins of the lesion and reference lines were measured. The femoral measurements were transposed onto lateral radiographs to determine the relationship between the lesion site and knee flexion angles. Both tibial and femoral lesions were significantly (p<0.01) more posterior in lateral OA than medial OA. In medial OA, the lesion centre was, on average, at 11° (SD 3°) of flexion, whereas in lateral OA it was at 40° (SD 3°). The smallest medial femoral lesions were near full extension and, as they enlarged, they extended posteriorly. The smallest lateral femoral lesions extended from 20° to 60° flexion. As these lesions enlarged, they extended both anteriorly and posteriorly. There was a well-defined relationship between the site of the lesions and their size, suggesting that they develop and progress in a predictable manner (Fig. 12.2). The relationship was different for medial and lateral OA, suggesting that different mechanical factors are important in initiating the different types of OA. The lesions in medial OA occur in extension, perhaps initiated by events occurring at heel strike (Gill & O’Connor, 2003). The lesions in lateral OA begin at flexion angles above those occurring during the single leg stance phase of the gait cycle, so activities other than gait are likely to induce lateral OA.
