The varus stress test is a physical exam technique used to check whether the ligament on the outer side of your knee is intact. A clinician pushes your lower leg inward while stabilizing your thigh, looking for abnormal gapping at the outer knee joint. It’s considered the most helpful clinical test for assessing injuries to the lateral collateral ligament (LCL), the tough band of tissue that prevents your knee from bowing outward.
What the Test Is Checking
Your knee is held stable on its outer side primarily by the LCL, which runs from the bottom of your thighbone to the top of the smaller bone in your lower leg (the fibula). The LCL acts as the primary restraint against the knee opening up on the lateral side in all positions of bending. Several secondary structures back it up, including the iliotibial band (the thick tissue running down the outside of your thigh), the cruciate ligaments deep inside the knee, and a group of ligaments and tendons collectively called the posterolateral corner.
When the varus stress test produces abnormal movement, it tells the examiner that at least the LCL is damaged. Depending on how much the joint opens and at what angle the knee is positioned during the test, it can also reveal whether those deeper, secondary structures are involved.
How the Test Is Performed
You lie on your back on an exam table. The clinician places one hand on the inner side of your knee, right at the joint line near the end of your thighbone. The other hand grips your ankle or lower leg. From that position, the examiner pushes inward at the ankle while holding the thigh steady, creating a force that tries to open the outer side of the knee joint. The examiner feels for any gapping or looseness at the joint line while applying that pressure.
The test is performed at two different knee positions, and each position tells a different story.
At 30 Degrees of Flexion
With a small cushion or bolster placed under the knee to bend it about 30 degrees, the test isolates the LCL more directly. At this angle, the other stabilizing structures are slightly relaxed, so if the joint gaps open, it points toward damage to the LCL specifically, and possibly to the posterolateral corner structures that work alongside it.
At Full Extension (0 Degrees)
The knee is then straightened completely and the same force is applied. In full extension, everything is taut: the LCL, the posterolateral corner, and the cruciate ligaments all work together to prevent the joint from opening. If the knee is stable at full extension but loose at 30 degrees, the injury is likely limited to the LCL alone. If the knee gaps open even in full extension, the damage is more extensive, involving a combination of the LCL, posterolateral corner structures, and often one or both cruciate ligaments.
A positive test at full extension also raises concern for nerve injury. The common peroneal nerve wraps around the top of the fibula right near the LCL, and significant trauma to this area can damage the nerve alongside the ligament. When a clinician suspects nerve involvement, they’ll apply only enough force to confirm the instability is present without pushing further.
What the Results Mean
The examiner evaluates two things: how far the joint opens compared to your uninjured knee, and what the “end feel” is like. A healthy ligament produces a firm stop when stretched. A damaged one may feel soft or mushy at the end of its range, or have no clear stopping point at all.
Ligament injuries are graded on a three-level scale based on how much the joint gaps open beyond normal:
- Grade I (0 to 5 mm of gapping): The ligament fibers are stretched but not torn through. The joint opens slightly more than the other knee, but there’s still a firm endpoint. This represents a mild sprain.
- Grade II (6 to 10 mm of gapping): A partial tear. The joint opens noticeably, and the endpoint feels softer. There’s significant laxity, but some structural integrity remains.
- Grade III (more than 10 mm of gapping): A complete tear. The joint opens widely with no firm endpoint. At this level, the examiner is often feeling for whether the ligament provides any resistance at all.
These measurements are always compared to the opposite knee, since everyone has a slightly different baseline level of natural looseness. What matters is the difference between sides, not the absolute number.
What a Positive Test Suggests
A positive result at 30 degrees of flexion that resolves at full extension points toward an isolated LCL injury. This is the simpler scenario, and isolated LCL sprains (particularly Grade I and II) often heal without surgery.
A positive result at both 30 degrees and full extension is a more serious finding. It suggests the LCL is torn along with posterolateral corner structures and likely a cruciate ligament. These combined injuries are more complex and almost always require imaging (typically an MRI) to map the full extent of the damage. It’s also not uncommon with this pattern to find that a piece of bone has been pulled away from the fibula where the ligaments attach.
The varus stress test can also be performed with X-ray imaging simultaneously, called a stress radiograph, to get an objective measurement of joint opening. In a clinical setting, handheld force during the exam is somewhat subjective, so specialized devices can apply a standardized force (up to about 150 newtons) for more consistent, measurable results. This is more common in research settings or when surgical planning requires precise numbers.
Limitations of the Test
Pain and muscle guarding can make the test harder to interpret. If you’re in significant pain, your muscles tighten reflexively around the knee, which can mask the laxity that would otherwise be apparent. Swelling in the acute phase of an injury can have a similar effect. In these cases, the examiner may need to repeat the test after swelling subsides or rely on imaging for a clearer picture.
The test also depends on the examiner’s experience. Judging the difference between a “firm” and “soft” endpoint is a subjective skill that varies between clinicians. Robotic testing devices have been developed to provide objective, quantitative measurements of endpoint stiffness, though these are primarily used in research rather than routine clinical exams. For most people, a skilled manual exam combined with MRI when needed provides a reliable diagnosis.