Joint Capsule: Definition, Anatomy, Types, Function and Associated Pathologies

They are sheaths of a fibrous nature that cover the joints.

The joint capsules are intended to allow the joint surfaces to maintain contact, and contain the synovial fluid to bathe the joint.

The joint capsules form complete sheaths so that the joints are freely mobile.

The joint capsule identifies the layer of tissue that surrounds the synovial joints and defines a fluid-filled cavity.

Anatomy

Each capsule is made up of two layers: an outer fibrous layer attached to the periosteum of the adjacent bone and an inner synovial layer that has secretory layers.

These layers are called: the outer as a fibrous layer, composed of fibrous tissue, and the inner, as a layer of the synovial membrane, of loose connective tissue.

The second layer is usually described separately as the synovium.

The joint capsule is a fibrous sleeve that adheres to the immediate periphery of the articular surfaces close to the cartilaginous lining and that ensures coaptation of the articular surfaces with the ligaments.

The joint capsule joins the two bones that intervene in the joint.

It extends from the periosteum of one of the bones to the periosteum of the other bone.

It is formed mainly by condensed connective tissue; hence the name fibrous capsule that is often given.

It acquires a greater thickness at certain points of its extension and is more or less rigid depending on the degree of mobility of the joint.

Joints and their types

The joints that connect two bones to each other are called joints and must withstand a high mechanical load.

There are three types of joints:

  1. Synarthrosis without the possibility of movement: bones of the skull.
  2. Amphiarthrosis are real joints with possible but very limited movements, such as: the joints between vertebral bodies or the pubic symphysis.
  3. Diarthrosis: allows free movement, limited by the surrounding ligaments.
  • Enarthrosis: such as the hip joint and the scapulohumeral joint that allow movements such as flexion, extension, approach, separation and rotation.
  • Ellipsoidal: such as the wrist joint, the radiocarpal (an elongated head that fits into a gleonid socket), which allow movements such as flexion, extension, adduction, and abduction.
  • Saddle: such as the sternocostoclavicular joint and the carpometacarpal joint of the thumb (a concave and a convex surface), which allow movements such as flexion, extension, adduction and abduction.
  • The hinge joint : like the knee or elbow joints (femorotibial and ulnar humerus), which allow movements such as flexion, extension.
  • Trichoids: such as the superior radioulnar joint (a bony cylinder that rotates on its axis), whose only movement is that of rotation.
  • Arthrodias: such as the costovertebral and chondrosternal joints (they join two almost flat surfaces), and only allow gliding movements.

Articular cartilage

It is a hyaline cartilage that is not covered with perichondrium. The fibers are specially arranged to withstand high mechanical loads.

The horizontal fibers on the surface of the joint groove function as shock absorbers. The following areas can be distinguished:

  • Tangential zone.
  • Transition zone.
  • Perpendicular zone.
  • Mineralized cartilage.

In articular cartilage, it has different areas with different alignment of the collagen fibers.

Cartilage is of different thickness according to its function. At the distal end of the femur (knee joint) it varies between 2-5 mm.

The synovial membrane

The synovial membrane (inner layer) secretes synovial fluid. Aligns the deep side of the joint capsule and the bony surfaces of the joint.

This layer is highly vascularized by the many articular arteries. In some joints, you may have synovial stripes in the form of intra-articular growth.

Its histological structure is not uniform: it includes collagen fibers and solid elastic formations, but also looser areas where elastic fibers are few; it is these areas that will be the site of strains, ruptures, or cysts.

Synovial liquid

Usually a lubricant, the synovial fluid is in the joint. It is very viscous and covers the joint surface like a thin film of lubricant.

In a normal, healthy joint, there is only a small amount of synovium, for example in the knee joint, about 0.5 to 2 ml.

It is secreted by the synovium (inner layer of the joint). Inflammation of a joint, like arthritis, begins with the synovium.

Sinusovial fluid consists of a loose layer of flat cells without a basement membrane.

There are two types of cells:

  • A cells are macrophages that synthesize hyaluronic acid and keep the joint cavity clean.
  • B cells have specialized fibroblasts whose function is to synthesize collagen and glycoprotein.

Pathology

Rheumatoid arthritis : it is the activation of CD4 T cells (lymphocytes), by an unknown antigen. Production of tumor necrosis factor, interleukins and metalloproteinases. The synovium thickens, osteoclasts destroy bone, and articular cartilage is destroyed by metalloproteinases.

Joint degeneration: it is the loss of water in the cartilage that produces small cracks in the cartilaginous cushion that disappears. Reduced movements and pain in the joint, most often in the knee or hip.

Fibrous capsule function

Fibrous capsule refers to an outer layer that surrounds the joint capsule, at a synovial junction.

The objective of the fibrous capsule is to provide support and protection to the organ or joint that it covers.

Synovial joints allow an individual to bend their limbs and flex their back.

The connections between the vertebrae and the junction of the pelvis and vertebrae are also included as synovial joints.

Synovial joints are made up of two bony ends, which are covered by a flexible and elastic articular cartilage.

This cartilage allows friction-free movement of the joint. The stability of each joint is maintained by a fibrous capsule.

This capsule attaches to the bones and collateral ligaments, which are attached to the sides of the joint and gives stability to the entire structure.

The fibrous capsule, together with the synovium, is part of the joint capsule.

This capsule is essential for the optimal function of the synovial joints.

The capsule limits unnecessary movement, while providing stability to the joint.

Fibrous capsules are made up of thick fibrous connective tissue, which forms a protective sleeve around the joint.

The capsule is attached to the bones forming the synovial joint in specific areas.

The fibrous capsule comes in different thicknesses, depending on the amount of stress it is exposed to.

These capsules can incorporate tendons in specific joints.