Corpus: Articular capsule

The articular capsule is the connective tissue covering of a joint. As an extension of the periosteum, it forms a tubular structure that surrounds the joint on all sides.

The articular capsule consists of two layers: an outer coarse fibrous layer (membrana fibrosa) and an inner synovial layer (membrana synovialis).

2.1. Membrana fibrosa

The membrana fibrosa, also known as the stratum fibrosum, is composed of coarse fibers, which consist primarily of collagen (type I). Its thickness varies between joints. The membrana fibrosa reinforces the joint with functional ligament structures, such as the collateral ligaments in the knee joint. At the joint edges, the membrana fibrosa continues into the periosteum. In some joints (e.g. the knee joint), intermediate discs (menisci) may be attached to the capsule.

2.2. Membrana synovialis

The membrana synovialis, or synovial membrane, is composed of loose connective tissue with fewer fibers than the membrana fibrosa. It features a single-layered, sometimes multi-layered epithelium formed by spread-out fibrocytes known as synovial covering cells. The synovial membrane forms vascularized folds (synovial plicae) and fatty, vascularized villi (synovial villi), increasing its surface area. The synovial membrane is highly sensitive to pain due to its abundance of nerve fibers and receptors. It produces and reabsorbs synovial fluid.

Prolonged immobilization of a joint can lead to contracture due to shrinkage of the membrana fibrosa. Excessive production of synovial fluid by the synovial membrane can result in joint effusion.