The valgus stress test at 30° is the workhorse test for determining if there is a medial knee injury present. The main structure which provides valgus stability to the knee at 20° – 30° of knee flexion is the superficial medial collateral ligament. This is the largest and most important structure on the medial aspect of the knee and is best assessed at this flexion angle.
To perform the valgus stress test, the knee is flexed between 20° – 30° and one’s fingers are placed directly over the joint line. A valgus stress is then applied to the knee by applying the stress through the patient’s foot or ankle. It is important not to apply the test solely through the distal tibia because one is then not isolating the potential rotational instability which is present with a medial knee injury and can result in an underestimation of the amount of medial gapping.
There are many different ways to both qualitatively and quantitatively assess for medial knee injuries. One of the most common ways is through the AMA classification which was devised in 1966. In this classification, pain, but not significant gapping, during application of a valgus stress test is indicative of a partial intrasubstance grade I tear. Increased gapping, but a definite endpoint, is indicative of a grade II tear, while gapping with no definitive endpoint is indicative of a grade III or complete medial knee injury. Further testing has evolved more recently and includes the use of valgus stress xrays which have found to be more accurate for the assessment of the degree of the medial knee injury. In general, increased medial knee gapping at 20° of knee flexion of 3.2 mm is indicative of a complete superficial medial collateral ligament injury, while increased knee gapping of 9.8 mm is indicative of a complete medial knee injury.