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 stabilisers 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 degrees and 90 degrees of flexion. More over, external rotation in 30 degrees was significantly greater than external rotation in 90 degrees of knee flexion. CONCLUSIONS Whenever suspecting a posterolateral complex injury, one has to carefully perform a valgus stress test in 0 degrees and 30 degrees. 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.