The LCL, or lateral collateral ligament, is a tough band of tissue on the outer side of your knee that prevents the joint from bowing outward. It runs from the bottom of your thighbone (femur) to the top of the smaller bone in your lower leg (fibula), acting as the primary restraint against forces that push the knee sideways. Of the four major knee ligaments, the LCL is injured least often, but when it does tear, it can significantly affect your knee’s stability.
Where the LCL Sits in Your Knee
The LCL is a cord-like structure on the outside of your knee, separate from the joint capsule itself. It starts just above the bony bump on the outer edge of your femur (the lateral epicondyle) and angles slightly backward as it travels down. It attaches to the head of the fibula, covering nearly 38% of that bone’s top surface. Unlike the ligament on the inner side of your knee (the MCL), the LCL doesn’t attach to the cartilage inside the joint. This separation gives it a distinct, rope-like feel that a doctor can sometimes palpate directly through the skin.
The LCL doesn’t work alone. It’s part of a group of structures called the posterolateral corner, which includes tendons and smaller ligaments that collectively stabilize the outer and back portion of your knee. When surgeons evaluate an LCL injury, they’re always checking these neighboring structures too, because damage to the posterolateral corner changes the treatment plan considerably.
What the LCL Does
The LCL’s main job is resisting varus force, which is any pressure that tries to open the outer side of your knee. Imagine someone pushing the inside of your knee outward, or landing awkwardly with your weight shifting over the outside of your leg. The LCL is the first structure to resist that movement. It’s tightest when your knee is fully straight and loosens slightly as you bend, which is why many LCL injuries happen with the leg in a more extended position.
Beyond side-to-side stability, the LCL also plays a supporting role in controlling rotation. When your lower leg twists relative to your thigh, the LCL helps limit how far that rotation goes. Losing LCL function doesn’t just make the knee feel loose side to side. It can also create a subtle rotational instability that makes pivoting, cutting, and changing direction feel unreliable.
How LCL Injuries Happen
LCL tears most commonly result from a direct blow to the inside of the knee that forces the joint outward. This is a classic football injury, where a tackle strikes the inner knee while the foot is planted. But direct contact isn’t the only cause. Sports that involve hard cutting, quick direction changes, jumping, and twisting all put the LCL at risk. Football, soccer, and skiing are among the highest-risk activities.
Non-contact injuries are possible too. A bad landing from a jump, a sudden pivot, or hyperextension of the knee can all stretch or tear the LCL. The ligament can fail at its upper attachment near the femur, through its middle substance, or at its lower attachment on the fibula. Each tear pattern looks different on imaging, and where the tear occurs can influence whether repair is feasible or reconstruction is needed.
Symptoms of an LCL Injury
Pain along the outer side of the knee is the hallmark symptom. It typically shows up immediately after the injury and worsens when you try to straighten or fully bend the leg. You may also notice swelling on the outside of the knee, though LCL injuries generally produce less dramatic swelling than ACL tears because the LCL sits outside the joint capsule.
The more telling symptom is instability. With a partial tear, you might feel a vague looseness when walking on uneven ground or going downstairs. With a complete tear, the knee can feel like it’s giving way during everyday activities. Some people describe a sensation of the outer side of the knee “gapping open” when they bear weight. In severe injuries involving multiple ligaments or a knee dislocation, the common peroneal nerve, which runs close to the fibula, can also be damaged. This nerve injury occurs in roughly 5 to 20 percent of knee dislocations and can cause numbness, tingling, or difficulty lifting the foot (foot drop).
Injury Grades
LCL injuries are classified into three grades based on severity:
- Grade 1 (mild sprain): The ligament fibers are stretched but not torn. You’ll have pain and tenderness on the outer knee, but the joint still feels stable when tested. Most people can walk, though it’s uncomfortable.
- Grade 2 (partial tear): Some fibers are torn, and the knee shows mild looseness during a stress test. Pain is more significant, and you’ll likely notice some instability during activity.
- Grade 3 (complete tear): The ligament is fully torn. The knee opens noticeably on the outer side during testing, and weight-bearing activities feel unstable. Grade 3 tears are often accompanied by damage to other structures in the posterolateral corner or the cruciate ligaments.
How LCL Injuries Are Diagnosed
The most common hands-on test is the varus stress test. You lie on your back while a clinician holds your ankle and pushes inward against the outer side of your knee. If the joint gaps open more than the other knee, it suggests an LCL tear. The test is done with the knee at both full extension and about 30 degrees of bend. Increased opening at 30 degrees points to an isolated LCL injury, while opening at full extension suggests broader posterolateral corner damage.
MRI is the standard imaging tool for confirming the diagnosis and checking for associated injuries. It shows the exact location of the tear (upper, mid-substance, or lower) and reveals whether the cartilage, meniscus, or other ligaments are involved. That said, MRI is more sensitive for detecting acute tears than chronic ones. For injuries that have been present for weeks or months, varus stress X-rays, which are taken while a sideways force is applied to the knee, can actually be more accurate. Research comparing the two methods found MRI had a sensitivity of about 66% for complete tears, while varus stress radiographs reached around 70%, with the gap widening for chronic injuries.
Treatment Based on Severity
Grade 1 and grade 2 injuries are typically managed without surgery. Treatment involves a period of protected movement, often with a hinged knee brace to limit side-to-side stress while allowing controlled bending. Physical therapy focuses on gradually restoring range of motion, strengthening the muscles around the knee (especially the hamstrings and hip stabilizers), and retraining balance. Most people with mild to moderate sprains return to normal activity within a few weeks to a couple of months, depending on how much tissue is damaged.
Grade 3 tears are different. A completely torn LCL rarely heals well on its own because the two ends of the ligament retract apart from each other. Early surgical repair, where the torn ends are stitched back together, tends to produce better outcomes than waiting. If the injury is older or the tissue quality is poor, reconstruction using a tendon graft may be necessary. When other ligaments are torn at the same time, the surgical plan addresses all damaged structures in a single procedure or a staged approach.
Recovery after LCL surgery generally takes four to six months before a return to sport, though full confidence in the knee can take closer to nine months. The rehabilitation process follows a predictable arc: protected weight-bearing for the first several weeks, progressive strengthening over months two and three, sport-specific agility work starting around month four, and a gradual return to competition once strength and stability benchmarks are met.
Why the LCL Is Often Injured With Other Structures
Isolated LCL tears are relatively uncommon. Because the forces needed to rupture the LCL are substantial, they frequently damage neighboring structures at the same time. The posterolateral corner, the ACL, and the PCL are all vulnerable in high-energy injuries. Knee dislocations, which involve tearing of at least two major ligaments, are the most extreme example and carry the added risk of nerve and blood vessel damage.
This is why a thorough evaluation matters. If you’re told you have an LCL injury, the clinical workup should assess the entire knee, not just the outer side. Missing a posterolateral corner injury alongside an ACL reconstruction, for example, is one of the recognized reasons for surgical failure and persistent instability down the line.