The knee joint relies on four primary ligaments to maintain stability, and the Lateral Collateral Ligament (LCL) is one of these restraints. The LCL is a strong, fibrous band of tissue. Its function is to govern the side-to-side movement of the joint, preventing excessive motion. Understanding the LCL’s structure and duties helps explain how the knee remains stable during everyday movements and athletic activity.
Where the LCL is Located
The LCL, also known as the fibular collateral ligament, is situated along the outer side of the knee joint. This cord-like structure connects the femur (the large thigh bone) to the fibula (the smaller bone of the lower leg). Specifically, it originates from the lateral femoral epicondyle on the femur and inserts onto the head of the fibula.
The LCL is typically described as a long, narrow, and rounded structure, often measuring around 50 to 70 millimeters in length. Unlike its medial counterpart, the LCL does not attach to the joint capsule or the lateral meniscus. This separation allows the LCL to be more flexible and influences how it responds to stress and injury.
The LCL’s Role in Knee Stability
The LCL functions as the knee’s primary defense against varus stress, which is a force that pushes the knee outward, causing the joint to bow away from the body’s midline. The ligament prevents the lower leg from moving excessively inward relative to the thigh bone. This action is most pronounced when the knee is close to full extension.
The LCL provides restraint against this outward-bowing force, particularly when the knee is slightly bent, contributing to stability across various degrees of flexion. When the knee is fully extended, the LCL becomes taut, working with the other ligaments to lock the joint and maintain overall alignment. Beyond preventing side-to-side instability, the LCL also serves a secondary function by helping to limit excessive external rotation of the tibia. This rotational control is an important part of the knee’s posterolateral corner.
Common Causes of LCL Injury
An LCL injury, often referred to as a sprain or tear, typically occurs when a sudden, powerful force overwhelms the ligament’s capacity to resist varus stress. The most common mechanism involves a direct blow to the inner side of the knee while the foot is planted. This impact forcefully pushes the knee joint outward, stretching or tearing the LCL on the outer side.
Non-contact forces can also injure the LCL, frequently involving scenarios that combine hyperextension or extreme twisting of the knee. Sports that involve quick changes in direction, pivoting, or high-velocity collisions, such as football, soccer, and skiing, carry a higher risk for this type of trauma. When the LCL is torn, it often occurs alongside damage to other structures in the posterolateral corner of the knee.
The LCL vs. the MCL
The LCL and the Medial Collateral Ligament (MCL) are the two collateral ligaments of the knee. The LCL is positioned on the outer (lateral) side of the knee, while the MCL is located on the inner (medial) side. This difference in placement dictates their primary function in side-to-side stability.
Functionally, the LCL resists varus stress, preventing the knee from bowing outward. In contrast, the MCL resists valgus stress, which is a force that pushes the knee inward toward the other leg. Structurally, the LCL is a cord-like ligament, while the MCL is a broader, fan-shaped structure that is closely integrated and attached to the joint capsule, which makes it more susceptible to injury.