Joint anatomy

Cartilage

Cellular component. Chondrocytes. Highly active metabolically however they do not undergo mitosis normally. Crucial role in regulating cartilage matrix breakdown and repair. Some evidence that under certain circumstances of cartilage breakdown that cell synthesis can be reinitiated.

Cartilage is divided into layers. The superficial gliding or tangential layer has a large amount of water, collagen fibres are oriented tangential to the joint surface and proteoglycan content is low. Chondrocytes are present and are very discoid and flattened. The next layer is the transitional zone with thicker oblique oriented collagen fibres and chondrocytes are found singly or in pairs as round type cells. The next layer is the radial layer with vertically oriented large collagen ficres and a high PG content. The tide mark is the point where calcification of the cartilage starts and forms an undulating layer tangential to the joint surface. This is an area of mechanical weakness in the cartilage. Below the tide mark calcification of the cartilage increases until the subchondral bone is reached.

Cartilage contains no blood or lymphatic vessels and no nerve endings. This means that all nutrients, enzymes, cytokines, and eicosonoids must either diffuse into the cartilage from the subchondral bone, the synovial fluid (slow) or be synthesised in situ by the chondrocytes (much faster). Cartilage thickness varies from species to species, joint to joint and even within joints roughly in proportion to normal loads on the joint surface.

Subchondral bone

The bony plate immediately below the articular cartilage. It consists of cancellous bone, which blends into the articular cartilage through the calcified zone which ends at the "tide mark" in the cartilage.

Synovium and Fibrous joint capsule

The synovium lines all diarthrodial joints and any structure within that joint except for articular cartilage. It consists of a thin layer, 1-3 cells thick of fibroblast like cells called synovial lining cells which sit on a loose connective tissue layer containing blood vessels, nerves and lymphatics. There appear to be at least three different sub types of synovial lining cells which appear to have different functions including phagocytosis and secretion of synovial fluid. The synovium forms microvillous processes in many areas of the joint. The fibrous joint capsule is closely adherent to the synovial lining and much thicker. Made up of fibrous and elastic connective tissue it seperates the joint from surrounding soft tissue structures as well as providing integrity and stability to the joint. The fibrous joint capsule is often congruent with joint stabilising ligaments such as collateral ligaments.

Synovial Fluid

Is a dialysate of blood to which has been added glycosaminoglycan. Main GAGs are hyaluronic acid. Another protein, lubricin, is important for articular cartilage lubrication.

Tendons

A number of synovial joints have intra-articular tendons.

Menisci

Cartilagenous structures found in the stifle joint of most species. They are also found in various other synovial joints depending on the species. In the dog the only other joint with a meniscus is the temporomandibular joint. In man there are menisci in the shoulder, hip and clavicular joints.They consist of chondrocytes and fibroblasts in an extracellular matrix of fibrocartilage. The outer part of the disc has a blood supply however the inner part does not.

Fat Pad

Collection of fatty tissue found in stifle and other joints. Located between the synovial membrane and the fibrous joint capsule they usually provide a cushion between joint structures.E.g. Patellar fat pad. Recent research work indicates the patellar fat pad may have a role in trapping macroscopic particles in the knee joint.