The lateral collateral ligament (LCL) is a strong band of connective tissue situated on the outside of the knee joint. It connects the thigh bone (femur) to the smaller lower leg bone (fibula). The LCL’s primary purpose is to stabilize the knee against forces that attempt to push the joint outward, known as varus stress. This prevents the knee from bowing away from the body’s midline, ensuring the joint remains properly aligned during movement and weight-bearing activities.
Mechanisms of Injury
The LCL is most commonly damaged by a sudden, forceful push to the inside of the knee when the foot is planted, creating a significant varus force across the joint. This direct impact abruptly stretches the ligament beyond its capacity, leading to a sprain or tear. Such injuries frequently occur in contact sports involving a blow to the inner aspect of the knee.
LCL injuries are also caused by non-contact forces, though less frequently than direct impact. These forces include excessive hyperextension, which straightens the leg beyond its normal range of motion. The ligament can also be damaged by sudden, uncontrolled twisting or pivoting movements while the foot remains fixed on the ground. These forces generate an indirect varus stress that overstretches the LCL.
The degree of injury is often categorized into three grades. A Grade I injury involves overstretching the ligament fibers with microscopic damage, causing mild pain but no instability. A Grade II injury is a partial tear, which results in noticeable looseness or laxity in the knee joint when tested. The most severe injury, Grade III, represents a complete rupture of the ligament, leading to significant joint instability and often involving damage to other surrounding structures.
Recognizing the Signs of an LCL Tear
A person who has sustained an LCL injury typically experiences immediate, sharp pain localized to the outside (lateral side) of the knee. This is often followed by tenderness along the ligament, which worsens when pressure is applied. Swelling around the knee joint is also a common sign, though it can be less extensive than with other ligament injuries.
The most defining symptom for moderate to severe tears is a feeling of instability, or the sensation that the knee is “giving way” or buckling. This instability is particularly noticeable when attempting to bear weight, walk, or pivot on the injured leg. For a mild, Grade I sprain, symptoms are usually limited to mild pain and tenderness without any feeling of looseness.
As the severity increases to a Grade II partial tear, pain and swelling become significantly more pronounced, and the knee may exhibit mild instability. A complete Grade III rupture causes excruciating pain and marked joint instability, making simple weight-bearing very difficult. In some instances, stretching of the nearby peroneal nerve can cause numbness or weakness that extends down into the foot.
Initial Steps and Medical Evaluation
Following a suspected LCL injury, a person should immediately begin conservative first aid measures, often summarized by the RICE protocol: Rest, Ice, Compression, and Elevation. Resting the leg and applying ice helps minimize swelling and pain. Elevating the limb above the heart assists in reducing fluid accumulation around the joint.
A medical professional will conduct a physical examination, including a specific assessment called the varus stress test. During this test, the clinician applies an inward (varus) force to the slightly bent knee to assess the degree of lateral gapping or laxity in the joint. Excessive movement or gapping compared to the uninjured knee indicates an LCL tear.
To confirm the diagnosis and check for associated damage, imaging tests are often utilized. X-rays rule out fractures, such as an avulsion fracture where a piece of bone is pulled off by the ligament. A Magnetic Resonance Imaging (MRI) scan is the preferred method for assessing the severity of the LCL injury and identifying concurrent damage to other ligaments, cartilage, or soft tissues within the knee.