The varus stress test checks for damage to the lateral collateral ligament (LCL), a tough band of tissue on the outer side of your knee. This ligament is the primary structure preventing your knee from bowing outward, and the test applies force in that direction to see if the joint opens up more than it should. It’s the most helpful clinical test for assessing LCL injury.
What the LCL Does
The lateral collateral ligament runs along the outside of your knee, connecting your thighbone to the smaller bone in your lower leg (the fibula). Its main job is twofold: it prevents the knee from angling outward under stress, and it limits excessive rotation of the lower leg. Every time you plant your foot and change direction, or absorb an impact from the inner side of your knee, the LCL is working to keep the joint stable.
The LCL doesn’t work alone. Several backup structures assist it, including the iliotibial band (the thick tissue running down the outside of your thigh), the popliteus tendon, the cruciate ligaments inside the knee, and tendons from the hamstring and calf muscles. But the LCL is the primary stabilizer against outward angulation at every degree of knee bend, which is why it gets its own dedicated test.
How the Test Is Performed
You lie on your back with your leg relaxed. The examiner places one hand on the inner side of your knee and the other hand around your ankle or foot. They then push inward at the ankle while stabilizing the knee, creating an outward-angling force across the joint. This is done at two positions: with the knee straight (0 degrees of flexion) and with it slightly bent to about 20 to 30 degrees.
Testing at slight flexion is particularly important. When the knee is fully straight, other structures like the cruciate ligaments and joint capsule help brace the outer side. Bending the knee slightly relaxes those secondary stabilizers and isolates the LCL more directly. If the outer side of the joint gaps open at 20 to 30 degrees but stays stable when straight, that points to an isolated LCL injury. If it gaps open at both positions, the damage likely extends beyond the LCL into deeper structures.
What a Positive Result Means
During the test, your provider feels for how much the outer joint space opens compared to your uninjured knee. They’ll also ask about pain and listen or feel for any popping. A positive result means the joint opens more than normal, the ligament feels loose, or you experience significant pain with the maneuver. A negative result means the examiner didn’t detect abnormal movement or symptoms.
Positive results are graded by how many millimeters of extra opening the examiner detects compared to the other knee:
- Grade 1: 0 to 5 mm of extra opening. The ligament is stretched but intact. You’ll feel pain on the outer knee, but the joint still has a firm stopping point.
- Grade 2: 6 to 10 mm of extra opening. This indicates a partial tear. The joint feels looser, though some resistance remains.
- Grade 3: More than 10 mm of extra opening with no firm endpoint. The ligament is completely torn. The examiner’s hands feel the joint hinge open without resistance.
Beyond the LCL: Posterolateral Corner Injuries
A varus stress test can also reveal damage to a group of structures called the posterolateral corner (PLC), which sits at the back-outside of your knee. This area includes the LCL, the popliteus tendon, and the popliteofibular ligament. These three structures work as a team, and injuries to one often involve the others.
When the varus stress test shows significant laxity, especially at full extension, it raises concern that more than just the LCL is involved. Research modeling progressive PLC injuries has shown that as each structure is cut in sequence (LCL first, then popliteus tendon, then popliteofibular ligament), the joint becomes progressively more unstable. ACL tears frequently accompany PLC injuries, which is why a provider finding a very loose lateral side will typically evaluate the cruciate ligaments as well. MRI is often the next step to map exactly which structures are torn.
What the Test Feels Like
The test itself takes less than a minute. You might feel mild pain or discomfort as pressure is applied, especially if the ligament is already injured. Some people describe a sensation of the outer knee “opening up” during the maneuver. The examiner compares both knees, so expect the test to be performed on your healthy side too. That comparison is essential for detecting subtle differences in laxity that might not be obvious on the injured side alone.
If your knee is significantly swollen or you’re guarding against pain with muscle tension, the test becomes harder to interpret. Swelling and muscle spasm can mask true laxity. In those cases, the exam is often delayed two to three days to let swelling subside, or it’s performed under anesthesia before surgery to get an accurate read. Fractures also need to be ruled out with X-rays before stress testing, since applying force to a broken bone could cause further damage.
How LCL Injuries Happen
LCL tears typically result from a blow to the inner side of the knee that forces the joint outward, or from a twisting injury during sports. Contact sports, skiing, and activities involving sudden direction changes carry the highest risk. Unlike the medial collateral ligament on the inner knee (which is one of the most commonly injured knee ligaments), isolated LCL injuries are relatively uncommon. When the LCL does tear, it’s frequently part of a larger injury pattern involving the posterolateral corner or the cruciate ligaments.
Symptoms that typically prompt a varus stress test include pain focused on the outer knee, a feeling of instability when walking or pivoting, swelling along the lateral side, and difficulty bearing weight. If you’ve experienced a knee injury and notice your knee buckling outward or feeling “loose” on the outside, the varus stress test is one of the first assessments a provider will use to figure out what’s going on.