The ligament on the outside of your knee is the lateral collateral ligament, commonly called the LCL. It’s a cord-like band of tissue that runs along the outer edge of the knee joint, connecting your thighbone to the smaller bone in your lower leg. Its main job is keeping your knee from bowing outward, and it’s one of the most important stabilizers on the lateral side of the joint.
Where the LCL Sits and What It Connects
The LCL starts at the bottom of your thighbone (femur), on a bony bump called the lateral femoral condyle. From there it travels downward and attaches to the upper portion of the fibula, the thin bone that runs along the outside of your lower leg. Unlike some knee ligaments that blend into surrounding tissue, the LCL is a distinct, rope-like cord you can sometimes feel through the skin when your leg is crossed.
It doesn’t sit alone. The LCL is part of a group of structures on the back and outer corner of the knee called the posterolateral corner. Two other key stabilizers in this area are the popliteus tendon and the popliteofibular ligament. Together, these three structures work as a team to control sideways and rotational movement. The common peroneal nerve, which controls your ability to lift your foot, also runs close by, wrapping around the top of the fibula just below where the LCL attaches.
How the LCL Stabilizes Your Knee
The LCL is the primary structure preventing your knee from bowing outward, a motion called varus stress. It provides about 55% of that resistance when your knee is nearly straight and roughly 69% when your knee is bent to about 30 degrees. It also acts as a secondary restraint against the shinbone rotating outward relative to the thigh.
One important detail: the LCL is taut when your knee is straight and loosens as you bend deeper. This means it does most of its stabilizing work during standing, walking, and the early phases of bending. In deep flexion, other structures take over more of the load. When your knee is fully extended, the LCL works alongside both cruciate ligaments (the ACL and PCL) to resist outward forces.
How LCL Injuries Happen
LCL injuries typically result from a direct blow to the inside of the knee that forces the joint outward, or from a sudden twisting motion. Contact sports, car accidents, and awkward landings are common causes. Because the LCL rarely tears in isolation, damage to it often comes with injuries to other posterolateral corner structures or to the cruciate ligaments, particularly the PCL.
Symptoms of an LCL Tear
The hallmark symptoms are pain and tenderness along the outer side of the knee, swelling, and bruising. What distinguishes an LCL injury from other causes of outer knee pain is instability. Your knee may feel like it’s about to give out, buckle, or lock up, especially when you’re putting weight on it or changing direction. That sensation of being unstable on your feet can persist even after the initial pain and swelling have improved.
If the nearby peroneal nerve is also affected, which happens in roughly 15 to 29 percent of posterolateral corner injuries, you may notice numbness or tingling along your shin or the top of your foot. In more severe cases, nerve damage can make it difficult or impossible to lift your foot upward at the ankle, leading to a distinctive gait pattern where you raise your knee higher than normal to keep your toes from dragging.
How It’s Different From IT Band Pain
Outer knee pain doesn’t always point to the LCL. The iliotibial (IT) band, a long strip of connective tissue running from your hip to just below the knee, can cause pain in a very similar location. The key difference is the mechanism and the feeling. IT band syndrome is an overuse injury that builds gradually, usually in runners or cyclists, and produces a burning or aching sensation on the outside of the knee during repetitive bending. There’s no instability, no buckling, and usually no bruising. An LCL injury, by contrast, typically follows a specific traumatic event, comes with immediate swelling, and creates a sense that the knee can’t support you laterally.
Diagnosis
The first test a clinician will use is the varus stress test. You’ll lie down while they apply gentle outward pressure just above your knee, first with your leg straight and then with it slightly bent. If your knee gaps open on the outer side, that suggests LCL damage. The test isn’t perfect on its own, though. A negative result doesn’t rule out a tear, and imaging (usually an MRI) is typically needed to confirm the diagnosis and check for damage to surrounding structures.
Treatment and Recovery
Mild to moderate LCL sprains, where the ligament is stretched or partially torn, are usually managed without surgery. The standard approach includes rest, ice, elevation, bracing, and physical therapy to restore strength, flexibility, and stability. Many people with partial tears recover well with this conservative approach over a period of several weeks.
Surgery becomes the recommendation when nonsurgical treatment doesn’t relieve symptoms or when the ligament is completely torn. The surgical approach depends on where and how the ligament tore. If it pulled away from its attachment point on the femur or fibula, surgeons can reattach it using sutures or staples. If the tear is in the middle of the ligament, or if the injury is more than about three weeks old, reconstruction with a tissue graft is the more reliable option.
Because missed posterolateral corner injuries are a common reason ACL reconstructions fail, getting an accurate diagnosis matters. If you’ve injured your LCL alongside a cruciate ligament, both problems typically need to be addressed for the knee to regain full stability.