The ligaments inside the knee-joint are more important than the intra-articular ligaments; of any other joint.

The Crucial ligaments are two rounded tendinous bands attached to the head of the tibia and the non-articular surfaces of the condyles of the femur; they are called anterior and posterior, according to their tibial attachment. The Anterior Crucial ligament is attached to the head of the tibia just in front of the spine, and passes upwards and backwards, to be attached to the posterior part of the inner surface of the external condyle. This ligament is tense during extension. The Posterior Crucial ligament is attached to the head of the tibia behind the spine, and passes upwards and forwards, to be attached to the anterior part of the outer surface of the internal condyle. This ligament is tense during flexion.

The Semilunar Cartilages are placed between the articulating surfaces of the tibia and femur; they each have a thick convex outer border attached to the surface of the tibia, and a concave inner border ending in horns, anterior and posterior. The internal one forms nearly a semicircle. The anterior horn is attached just in front of the anterior crucial ligament, and the posterior horn just in front of the posterior crucial ligament between it and the spine. The external one is nearly circular, its two horns being attached one each side of the spine.

The transverse ligament is a rounded band stretched between the anterior margins of the two semilunar cartilages.

The synovial membrane is very large and complicated. It completely invests the capsule, all the interarticular ligaments, and a pad of fat which is found beneath the patella, where it forms a fold attached to the patella and the anterior part of the intercondyloid notch. This is called the plica synovialis patellaris. In addition it forms at the patellar end wing-like expansions-the plica alares-which are usually loaded with fat.