The valgus stress test is a physical examination maneuver used to assess the stability of the knee joint. This technique evaluates the integrity of the Medial Collateral Ligament (MCL), the main stabilizer on the inner side of the knee. By applying a controlled, outward-directed force, the test determines if excessive joint opening on the medial side suggests ligament damage.
Anatomy and Preparation for Testing
The Medial Collateral Ligament, or MCL, is a broad, flat band of connective tissue situated along the inner aspect of the knee. It runs from the medial epicondyle of the femur (thigh bone) to the tibia (shin bone), acting as a significant restraint against forces that push the knee inward, a motion known as valgus stress. This ligament provides up to 78% of the restraining force needed to prevent the knee from collapsing inward.
Proper preparation for the valgus stress test begins with positioning the patient lying face up, or supine, on an examination table. The test is performed at two distinct angles of knee flexion to isolate different stabilizing structures. The first position is with the knee in full extension (0 degrees of flexion), which stresses the MCL along with secondary stabilizers, including the posterior oblique ligament and the posteromedial capsule. The second position involves flexing the knee to approximately 20 to 30 degrees, which relaxes the secondary stabilizers. This slight flexion is performed to specifically isolate the MCL.
Step-by-Step Procedure
The initial maneuver is performed with the knee flexed 20 to 30 degrees to target the Medial Collateral Ligament specifically. The examiner stands on the side of the injured leg, stabilizing the patient’s distal thigh, just above the knee, with one hand placed on the lateral aspect of the femur. The other hand grasps the patient’s ankle or distal tibia, using the lower leg as a lever. A controlled valgus force is then applied by the hand on the ankle while the thigh is stabilized. The examiner simultaneously uses the stabilizing hand to palpate the medial joint line, feeling for the degree of joint space opening or gapping.
The force must be applied gradually to assess the quality of the “end point,” which is the point at which the joint stops opening. The procedure is then repeated with the patient’s leg brought into full extension (0 degrees of flexion). Testing in full extension is necessary because the posterior capsule and cruciate ligaments contribute significantly to stability in this position. The full extension test will often feel firmer than the test performed in slight flexion due to the engagement of these additional structures.
Analyzing the Findings
Interpretation of the valgus stress test relies on observing the amount of medial joint gapping and feeling the quality of the end point compared to the uninjured knee. A “positive” test is indicated by an excessive amount of joint space opening or laxity, often accompanied by pain along the course of the medial ligament. A “negative” test is characterized by a firm end point with minimal to no gapping, suggesting the MCL is structurally sound.
Ligamentous injury is typically graded based on the degree of laxity observed during the test. A Grade I injury presents with minimal gapping, usually less than 5 millimeters, but with a definite, firm end point, indicating a mild sprain. A Grade II injury shows moderate gapping, generally between 5 and 10 millimeters, but the end point is “soft,” meaning the ligament is partially torn and fails to provide full resistance.
A Grade III injury, which represents a severe or complete tear of the MCL, shows significant gapping, often exceeding 10 millimeters, with no discernible end point. This indicates complete structural failure of the ligament. The angle at which the test is positive provides further diagnostic information; gapping only at 30 degrees suggests an isolated MCL tear. However, if gapping occurs at both 0 degrees and 30 degrees, it suggests a more severe injury involving the MCL and other posterior structures, such as the posterior oblique ligament or possibly the anterior or posterior cruciate ligaments. It is important to understand that while this test guides assessment, only a trained healthcare professional, such as a physician or physical therapist, can accurately diagnose and grade the extent of a knee injury.