The varus stress test at 20-30° of knee flexion is the actual workhorse test to perform when one is assessing for posterolateral instability of the knee. This test isolates out the function of the fibular collateral ligament. In effect, there should be no increase in varus gapping of the knee at 20-30° of flexion when the fibular collateral ligament is intact. When other structures are concurrently injured with a fibular collateral ligament tear, such as the popliteus tendon or popliteofibular ligament, there will be an increased amount of varus gapping present. Assessment of varus instability can be quite accurate in an experienced clinician’s hands. Subtle increases in varus gapping can usually be assessed to determine the amount of instability. It has been found by stress radiographs that increases of 2.7 mm of lateral compartment gapping are indicative of a complete fibular collateral ligament tear, while increases of 4 mm of lateral compartment gapping are indicative of a complete posterolateral corner injury.
To perform the varus stress test at 30° of knee flexion, the leg is placed over the examining table with the knee flexed at between 20°-30°. One’s fingers are then placed over the joint line while the distal femur is stabilized. A varus stress is then applied to the knee while one is holding the foot and ankle. One’s fingers can then assess for the amount of gapping. Just like the medial side of the knee, it is important not to grab the distal tibia when applying this test and to apply the stress through the foot and ankle so one can determine the amount of increased rotational instability and determine the true amount of lateral compartment gapping.