The lateral radiograph demonstrates the site and posterior extent of any bone erosion on the tibial plateau. It is a reliable indicator of the functional integrity of the ACL and therefore of the suitability of the knee for OUKA.
Technique
The patient lies on his or her side on the X-ray couch, with the knee flexed about 20°. The outer side of the knee is in contact with the plate and the X-ray source is about one metre distant. The femoral condyles should appear superimposed. If they are not, the radiograph is difficult to interpret and should be repeated.
Interpretation
The tibial plateaux can be distinguished from one another by the different shapes of their posterior margins, as described by Jacobsen (1981) (Fig. 4.10). Sclerosis of the subchondral bone medially makes this distinction more obvious in the arthritic than in the normal knee.
Figure 4.10 The distinctive profiles of the posterior margins of the medial and lateral tibial plateaux. (After K. Jacobsen, Acta Orthop Scand Suppl 1981; 194:1–263.)
If there is no bone erosion visible, the ACL is almost certainly intact. When there is bone erosion (Ahlback, 1968), a concave defect is seen. The erosion does not extend to the posterior margin of the plateau, the ACL is intact (95% probability) (Fig. 4.11(a) and (b), and compare with Fig. 4.4). If the bone erosion extends to the back of the tibial plateau, or if there is posterior subluxation of the femur, the ACL is almost certainly absent or severely damaged (Keyes et al., 1992) and OUKA is not appropriate (Fig. 4.11 (c) and (d)).
It should be noted that while MRI is useful for diagnosing traumatic lesions of the ACL, it has been found to have little value in characterising the functional integrity of the ligament in degenerative disease (Sharpe et al., 2001). Nor do we regularly employ preoperative arthroscopy, having found the radiographic evidence of the posterior extent of the tibial erosion a more reliable (if indirect) measure of the ligament’s functional efficacy.
