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The popliteal ligament is one of two ligaments found in the leg behind the knee joint. Both extracapsular ligaments, meaning that they are located to the outside of the joint capsule, the oblique popliteal ligament and arcuate popliteal ligament link the bones of the knee on their posterior sides. As such, they serve a dual purpose of maintaining the structural integrity of the knee joint while preventing hyperextension of the knee by forces directed at its front side as well as external rotation of the tibia bone in the shin, or the outward twisting of the tibia relative to the knee.
Converging at the knee joint are the three major bones of the leg: the femur, tibia, and fibula. The knee joint itself is formed by the lower end of the femur or thigh bone and the upper end of the tibia bone, with the top of the smaller fibular bone of the shin aligning with the top of the tibia to the lateral side of the joint. In addition, the small, disk-shaped patella bone, also known as the kneecap, covers the anterior portion of the joint. Behind the patella and between the femur and tibia is the joint capsule itself, which is lined with a membrane and contains the cartilaginous menisci, the joint fluid, and several intracapsular ligaments that include the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL).
To the outside of the joint are many more ligaments that function to surround and protect the joint, as well as hold together the bones of the knee. These include the collateral ligaments, which link the bones longitudinally to either side of the joint, and the popliteal ligaments behind the knee. Unlike their counterparts, however, which consist of linear bands of fibrous tissue linking one bone to an adjacent bone, the oblique and arcuate popliteal ligaments are more complex in their attachments.
Originating on one end from the medial condyle of the tibia, which is the knob-shaped portion of the top of the tibia bone that is found beneath the joint on the inside aspect of the knee, the main body of the oblique popliteal ligament directs vertically and laterally. In other words, the majority of this ligament’s fibers run from below the inside of the knee obliquely upward and outward across the back of the knee. These fibers attach to the lateral condyle of the femur, the rounded bump on the bottom of the femur that is located above the joint on the outside aspect of the knee, as well as to the intercondylar fossa or space between the femur’s condyles.
On its diagonal course across the back of the knee, however, some fibers of the oblique popliteal ligament take a detour. A short distance above the medial condyle of the tibia where the ligament begins, those fibers nearest the inside border of the knee direct upward and slightly backward, branching off the ligament’s main section at an almost 90-degree angle. These fibers merge with the lowest fibers of the tendon of the semimembranosus muscle, one of three muscles of the hamstring group. That is, after running down the medial aspect of the posterior thigh, the semimembranosus forms a tendon that intersects with the oblique popliteal ligament behind the knee.
Mirroring this detour of the medial fibers of the oblique popliteal ligament into the semimembranosus tendon is the arcuate popliteal ligament on the lateral side of the knee. Just as the medial fibers direct upward from the body of the oblique ligament on the inside aspect of the knee, the lateral fibers of the ligament direct downward to form the arcuate ligament. Branching off the oblique section beneath the lateral condyle of the femur, the arcuate ligament is a narrow band that descends to attach to the head of the fibula just below. On its course it gives off its own division, one that inserts on the intercondylar space on the top center of the tibia, giving the arcuate popliteal ligament the appearance of an upside-down Y.
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