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What Is the Fibular Collateral Ligament?

By Shelby Miller
Updated Feb 15, 2024
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Also referred to as the lateral collateral ligament (LCL), the fibular collateral ligament (FCL) is one of the major ligaments of the knee joint. An extrinsic ligament, meaning that it lies to the outside of the joint capsule, the FCL is found on the lateral side of the knee joint, running vertically past the outside of the knee between the femur bone of the thigh and the fibula bone of the lower leg. Along with the tibial collateral ligament (TCL), which runs vertically past the medial, or inner, side of the knee joint between the femur and the tibia bone, the fibular collateral ligament helps to stabilize the joint against horizontal forces.

A ligament is a type of connective tissue that links two or more bones together at a joint. It is necessarily a strong tissue made up of lengths of collagen fibers bound together like a rope. The fibular collateral ligament is one of five major ligaments and several additional minor ligaments holding together the bones forming the knee joint: the femur above, the tibia and fibula below, and the patella, or kneecap.

Of particular importance to the structure of the knee joint are its major ligaments, found both within and without the joint capsule. Within the capsule, which contains the cartilaginous disks known as menisci and the synovial fluid that lubricates the joint, are the anterior and posterior cruciate ligaments. These obliquely span the space between the bottom of the femur and the tops of the tibia and fibula bones, forming an X. To the outside of the capsule are the collateral ligaments, which run parallel to one another down either side of the knee. Crossing the front of the knee vertically between the collateral ligaments is the patellar ligament, which is found both above and below the patella and which holds that small, disk-shaped bone in place between the femur and tibia.

The collateral ligaments are necessary to the integrity of the knee because they provide the lateral stability that prevents the joint from buckling sideways. Originating on the outside of the lower femur on a surface known as the lateral epicondyle, the fibular collateral ligament extends across the lateral aspect of the knee, spanning the space like a bridge. It attaches below the joint on the outside aspect of the head of the fibula bone. Likewise, the TCL joins the medial epicondyle of the femur to the head of the tibia bone on the other side of the knee joint.

When confronted by a lateral force moving sideways toward the inside of the knee joint, the fibular collateral ligament acts to resist and absorb that force. For example, if a collision in a soccer game causes a player’s knee to be struck on the inside of the joint, the FCL helps to keep the outside of the joint from separating. In many cases, however, these ligaments can be strained or torn by such forces, although the knee is more likely to buckle inward than outward, thus causing damage to the TCL.

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