The Medial Collateral Ligament (MCL) is a flat band of connective tissue along the inner side of the knee joint, connecting the thigh bone (femur) to the large lower leg bone (tibia). It functions as a static stabilizer, primarily resisting valgus stress—force that pushes the knee inward. MCL injuries commonly occur from a direct blow to the outside of the knee, forcing the joint open on the inside, or through a sudden, sharp twisting motion. Diagnosing the presence and extent of an MCL tear is necessary for determining the appropriate course of treatment.
Recognizing Symptoms That Warrant Testing
An MCL injury typically causes immediate and noticeable symptoms that prompt a visit to a healthcare professional. The most frequent complaint is acute pain or tenderness located directly along the inner side of the knee joint. Pain severity depends on the tear’s extent, ranging from a mild ache to a sharp sensation. Swelling often develops around the injury site, sometimes localizing to the medial side of the knee. Many people report instability or “looseness,” feeling the knee might give way when bearing weight or pivoting. A distinct “pop” may also be heard or felt at the moment of injury, signaling a sudden tear.
The Standard Physical Exam Procedures
Diagnosis begins with a hands-on assessment combining palpation and specialized movements to test ligament integrity. The physician carefully feels along the MCL’s full length to pinpoint the exact location of tenderness, which corresponds to the tear site. While tenderness over the medial joint line can be confused with a meniscal injury, careful touch often identifies the tear’s specific location.
The most informative procedure is the Valgus Stress Test, which directly assesses MCL stability. The patient lies down, and the clinician positions the injured knee between 20 and 30 degrees of flexion. This slight bend relaxes the knee’s secondary stabilizers, isolating the stress primarily to the MCL.
The examiner applies a valgus force, pushing the knee inward while stabilizing the ankle and thigh, attempting to open the medial joint space. A positive result is indicated by pain or excessive gapping on the inner side compared to the uninjured leg. Excessive joint opening at 30 degrees of flexion suggests an isolated MCL injury.
The gapping observed during the Valgus Stress Test allows the clinician to grade the tear’s severity.
MCL Tear Grading
- Grade I tear: Involves pain but no noticeable joint opening or laxity, meaning the knee remains stable.
- Grade II tear: Shows some joint opening but maintains a definite endpoint, indicating a partial ligament tear.
- Grade III tear: Exhibits significant joint opening with no firm endpoint, signifying a complete ligament rupture.
Using Imaging to Confirm Diagnosis and Severity
Although the physical exam suggests an MCL tear, imaging confirms the diagnosis, grades severity, and rules out associated injuries. A standard X-ray is often the first test, used not to visualize the ligament, but to check for fractures. X-rays ensure there is no associated bone damage, such as an avulsion fracture where the ligament pulls a piece of bone away.
Magnetic Resonance Imaging (MRI) is the gold standard for visualizing soft tissues and confirming an MCL tear. MRI provides highly detailed images, allowing the physician to see the exact location and extent of ligament damage, distinguishing between a sprain, partial tear, or complete rupture. MRI is also crucial for identifying simultaneous injuries, such as tears to the anterior cruciate ligament (ACL) or the meniscus, which commonly occur alongside severe MCL tears.
Ultrasound may be used as a supplementary tool, offering a dynamic view of ligament integrity. It uses sound waves to create images and can be performed while applying valgus stress to the knee. This allows the clinician to visualize ligament movement and gapping in real-time, providing an immediate, radiation-free assessment of stability.