The ligament on the outside of your knee is the lateral collateral ligament, commonly called the LCL. It’s a band of tissue that runs along the outer edge of your knee, connecting the bottom of your thighbone (femur) to the top of the smaller lower leg bone (fibula). Its job is to prevent your knee from bending too far outward and to keep the joint stable during side-to-side movements.
What the LCL Does
The LCL acts as the primary stabilizer on the outer side of your knee. Every time you plant your foot and change direction, or absorb a hit to the inside of your leg, the LCL is the structure that keeps the outer half of the joint from gapping open. It works alongside a group of supporting structures called the posterolateral corner, which includes the popliteus muscle and tendon, the posterolateral joint capsule, and several smaller ligaments. Together, these structures control rotation and prevent the knee from wobbling laterally.
The LCL is the counterpart to the medial collateral ligament (MCL) on the inner side of your knee. While the MCL is injured far more often, LCL injuries tend to be more complex because they frequently involve damage to those neighboring posterolateral corner structures as well.
How LCL Injuries Happen
The classic mechanism is a direct blow to the inside of the knee that forces the joint outward, stretching or tearing the LCL on the opposite side. This is common in contact sports like football and soccer. You can also injure it by hyperextending your knee, landing awkwardly from a jump, or twisting with your foot planted.
Because the LCL sits in a neighborhood of other stabilizing structures, a strong enough force rarely damages it alone. When two or more structures on the outer and back side of the knee tear, clinicians refer to it as a posterolateral corner injury, which is significantly harder to recover from than an isolated LCL sprain.
Symptoms of an LCL Injury
The hallmark symptoms are pain, swelling, and tenderness specifically along the outer edge of your knee. You might hear or feel a pop at the moment of injury. Bruising often develops within a day or two. The symptom that distinguishes an LCL injury from general knee soreness is instability: your knee may feel like it’s about to give out, buckle, or lock up, especially when you try to change direction or stand on that leg.
Mild sprains might only produce tenderness when you press on the outer knee, with no real sense of looseness. More severe tears make the knee feel genuinely wobbly, as though the outer side of the joint has nothing holding it together.
Injury Grades and What They Mean
LCL sprains are classified into three grades based on how much the joint opens up during a stress test, where a clinician bends your knee slightly and pushes the lower leg outward:
- Grade 1: Pain along the ligament but no measurable gapping or instability. The fibers are stretched but intact.
- Grade 2: Partial tear with 5 to 10 millimeters of joint opening. The knee feels loose but there’s still a definite endpoint when stressed.
- Grade 3: Complete tear with more than 10 millimeters of gapping and no firm endpoint. The knee feels openly unstable.
Grade 3 injuries are the ones most likely to involve damage to the posterolateral corner structures, not just the LCL itself. An MRI showing complete tears in two or more of these structures strongly suggests a posterolateral corner injury that will need more aggressive treatment.
Treatment: Bracing vs. Surgery
Grade 1 and most grade 2 LCL injuries heal without surgery. The standard approach is rest, ice, elevation, bracing, and physical therapy to reduce swelling and rebuild strength and flexibility around the knee. A hinged knee brace helps protect the healing ligament by limiting side-to-side movement while still allowing you to bend and straighten the joint.
Surgery becomes the recommendation when conservative treatment fails to restore stability, or when the tear is severe enough that healing on its own is unlikely. The type of surgery depends on where and when the tear occurred. If the ligament pulled away from its attachment point at the top or bottom of the bone, surgeons can often repair it directly using sutures or staples. If the tear is in the middle of the ligament, or if the injury is more than three weeks old, reconstruction is typically recommended instead. Reconstruction involves replacing the damaged ligament with a graft.
Recovery Timeline
Grade 1 sprains generally feel significantly better within two to three weeks, with a return to full activity in four to six weeks. Grade 2 injuries typically require six to eight weeks of rehab before the knee feels stable enough for sports or demanding physical work. Grade 3 tears treated conservatively can take three months or longer, and surgical reconstruction extends that timeline further, often to six months or more before a full return to sport.
Regardless of grade, physical therapy is the core of recovery. Early goals focus on reducing swelling and restoring range of motion. As healing progresses, the focus shifts to strengthening the muscles around the knee, particularly the quadriceps and hamstrings, which act as dynamic stabilizers that take pressure off the ligament. The final phase involves sport-specific drills and agility work to rebuild confidence in the knee before returning to full activity.
Other Causes of Outer Knee Pain
Not all pain on the outside of the knee comes from the LCL. IT band syndrome, where the thick band of tissue running down the outer thigh becomes irritated where it crosses the knee, is one of the most common causes of lateral knee pain, especially in runners. A lateral meniscus tear (damage to the cartilage pad on the outer half of the joint) can also produce pain in a similar location but tends to come with catching, clicking, or locking sensations rather than the side-to-side instability typical of an LCL injury.
The key differentiator is usually instability. If your outer knee hurts but feels solid, the LCL is probably fine and the issue is more likely soft tissue irritation or a meniscus problem. If the knee feels loose or wobbly, particularly when a force pushes it sideways, the LCL is the more likely culprit.