LCL Sprain

The lateral collateral ligament (LCL) is the cord-like ligament running along the outside of your knee from the outer end of the femur (lateral femoral condyle) to the top of the smaller leg bone (fibular head). Its job is to resist forces that push the knee inward and cause the outer side to gap open, a motion called varus stress. Unlike the medial collateral ligament on the inside, which is commonly sprained in isolation, the LCL rarely tears alone. More often it is part of a wider injury to the posterolateral corner of the knee, a group of structures on the outer and back-outer side that together control both varus opening and rotational stability.

The mechanism of injury is usually a high-energy blow to the inside of the knee (pushing the outer side into tension) or a hyperextension-twist combination, the type seen in contact sports collisions or certain falls. Because the peroneal nerve wraps around the fibular head right at the LCL attachment, a significant LCL or posterolateral corner injury can also stretch or damage the nerve, causing weakness of foot dorsiflexion (lifting the foot up) or numbness along the top of the foot and lower leg. Checking nerve function is a routine part of every LCL exam.

Sprains are graded by severity: grade 1 is a stretch with intact fibers, grade 2 is a partial tear, and grade 3 is a complete rupture. Grade 3 LCL injuries and most posterolateral corner tears do not heal reliably on their own and typically need surgical reconstruction to restore stability.

Pain and tenderness on the outer side of your knee, especially right over the bony bump at the top of the fibula where the LCL attaches, are the hallmark findings. Swelling along the outside of the knee typically develops within hours of the injury. With a more severe sprain (grade 2 or 3), you may feel your knee buckle or give way outward when you try to stand or walk.

When the posterolateral corner is also involved, the instability is more pronounced: your knee may twist or rotate in ways it should not, particularly when changing direction or going downstairs. Stiffness and limited range of motion are common in the first few days. Bruising may appear on the outer knee and can track downward along the lower leg.

Your surgeon checks for pain and tenderness on the outside of the knee, especially at the bony bump where the LCL attaches, and uses a focused physical exam of the knee to check whether the LCL and the rest of the posterolateral corner are intact. MRI shows all three layers of the outer side of the knee. Your surgeon will also carefully check the function of the peroneal nerve, which runs right behind the area and can be stretched in posterolateral corner injuries.

For most lower-grade sprains, the LCL heals on its own with protection and rehab. The treatments below are listed in the order we usually introduce them: the brace protects the ligament while it heals, and therapy rebuilds the strength and balance that keep the outside of the knee stable afterward.

Complete (grade 3) LCL tears and high-grade posterolateral corner injuries are considered for surgical repair or reconstruction. These structures have poor healing potential on their own, and untreated, they cause significant rotational instability that grinds down the joint.

External patient-education references and related OSI pages for additional background:

• AAOS OrthoInfo, patient education library
• OSI glossary, orthopedic terminology in plain language