The importance of the valgus stress test in the diagnosis of posterolateral instability of the knee

Background: The diagnosis of posterolateral instability of the knee is often based on a typical indirect mechanism of injury, a history of "giving way" and a positive dial test. Our search of the English literature revealed no mention of including the valgus stress test in the diagnostic protocol for posterolateral instability. Hypothesis: Based on our experience, we hypothesised that a medial collateral ligament (MCL) tear will also produce a positive dial test and that a valgus stress test would provide differential diagnostic information. Methods: The MCL's of 14 fresh cadaveric knees (7 cadavers) were cut to simulate a grade 3 tear, taking care not to damage the medial retinaculum or the posteromedial stabilisersrs of the knee. The amount of tibial external rotation (the dial test) was measured for each knee before and after transection of the MCL. Results: The results of the dial test after transection of the MCL were similar to those stemming from a solitary injury to the posterolateral corner. There was a significant increase in external rotation of the knee in 30° and 90° of flexion. More over, external rotation in 30° was significantly greater than external rotation in 90° of knee flexion. Conclusions: Whenever suspecting a posterolateral complex injury, one has to carefully perform a valgus stress test in 0° and 30°. Although the support of a clinical study is needed in order to make a definite conclusion, the dial test is probably not reliable in the presence of medial instability, and alternative diagnostic measures should be used.