The Lateral Collateral Ligament (LCL) is a strong band of tissue on the outer side of the knee joint, connecting the thigh bone (femur) to the smaller lower leg bone (fibula). Its primary function is to provide stability against forces that push the knee inward (varus stress), preventing the joint from opening sideways. An LCL tear, or sprain, results from the overstretching or tearing of this ligament, causing acute pain and functional issues.
The Immediate Experience of Injury
The tear typically occurs when a force impacts the inside of the knee, pushing the leg outward and causing the LCL to stretch past its limit. This mechanism is common in contact sports when an athlete receives a blow to the inside of the knee while the foot is planted, or during a hyperextension injury.
Many individuals report hearing or feeling a distinct “pop” or tearing sensation on the outer side of the knee. This is followed by sharp, sudden pain localized directly over the ligament’s path. The pain is focused on the lateral (outer) aspect of the joint.
Understanding Localized Pain and Swelling
Following the injury, the pain becomes a persistent ache and tenderness concentrated along the outer knee. The area directly over the LCL is sore to the touch. The knee may also feel stiff, leading to a decreased ability to fully bend or straighten the leg in the hours after the injury.
Swelling usually develops around the outer knee within a few hours. Unlike tears to ligaments inside the joint, LCL injuries often result in localized swelling on the side of the knee, rather than a generalized effusion. Bruising may also become visible along the outer joint line within 24 to 48 hours.
Recognizing Functional Instability
The most defining sensation of an LCL tear is functional instability, which is the feeling that the knee joint is loose. This sensation of “giving way” or “buckling” occurs when the knee moves from side to side under weight-bearing stress. A tear compromises the LCL’s ability to prevent the knee from opening up on the outer side.
The feeling is often described as the knee shifting or wobbling laterally, particularly when attempting to pivot, turn quickly, or walk on uneven surfaces. This instability can make walking or standing difficult, as the joint lacks the structural support to maintain proper alignment. In severe cases, an individual may experience a persistent sense of looseness, potentially leading to the knee collapsing entirely.
Severity Levels and When to Seek Care
The severity of an LCL tear is classified into three grades. A Grade I injury is a mild overstretching of the ligament fibers, causing mild pain and tenderness but no joint instability. A Grade II injury involves a partial tear, resulting in moderate pain, swelling, and noticeable looseness or mild instability when the knee is stressed.
A Grade III injury is a complete rupture of the ligament, leading to significant pain, swelling, and marked joint instability. This severe tear often occurs alongside injury to other structures in the knee and can cause numbness or weakness in the foot if the nearby peroneal nerve is stretched. Professional medical evaluation is necessary if you experience an inability to bear weight, a persistent feeling of severe instability, or if symptoms do not improve within 48 hours of the injury.