Joints may be synovial (with a cavity) or fibrous.

Five classes of tissue make up synovial joints:

Tensile tissues: ligament and tendon, attached at entheses

Fibrous joints lack synovium and synovial fluid.

The bone adjacent to a joint consists of an open spongy framework of calcified collagen in a tough outer shell. Immediately beneath articular cartilage there is a more or less continuous subchondral bony plate but this is often extremely thin. The bone can withstand thrust forces as long as it is covered in cartilage, which distributes load evenly. Without cartilage, the spongy bone collapses easily. Bone is constantly remodelling in response to stresses. Abnormal remodelling occurs in metabolic disease, with abnormal stresses, in local joint disease, Paget's disease or infection.

Hyaline cartilage

Hyaline cartilage is the skeletal growth tissue. In many, but not all, joints a thin layer remains as the bearing surface in the adult. Its primary function can be seen as defining and maintaining skeletal shape — in a sense it carries the shape memory. Hylaine cartilage is avascular. It contains type II collagen and giant molecular complexes of a proteoglycan called aggrecan. Loadbearing normally occurs over small areas, varying with joint position. Cartilage does not normally wear despite decades of use, but will do if its composition or joint mechanics are abnormal. It can regenerate, and will do so at the margins of damaged joints as part of the osteochondral swellings known as osteophytes, but loadbearing areas will rarely rethicken once damaged, so the tissue is unable to restore its normal shape after injury.

Fibrocartilage

Fibrocartilage occurs as intervertebral disc, and as discs, menisci or ring pads in many peripheral joints. It lacks the combination of collagen II and aggrecan seen in hyaline cartilage and blends in with fibrous synovial tissue. It will regenerate to fill a space.

Synovium

Synovium is the name given to the soft tissue lining the cavities of joints, tendon sheaths and bursae. It is like other connective tissue packing, being a mixture of fatty, areolar and fibrous tissue. It differs from other connective tissue in having a slippery smooth non-adherent surface which allows movement between tissue structures rather than within the tissue substance. The surface of synovium is permeable to water, small molecules and proteins, but not to hyaluronan, which is the molecule that makes synovial fluid viscous. This allows synovium to trap synovial fluid within the cavity (see under synovial fluid below).

Synovial Fluid

Normal synovial fluid is clear, colourless and noticeably thick and stringy, like eggwhite. Hence the name syn ovium (‘with egg’). Its viscous and elastic properties are due to hyaluronan, a long chain glycosaminoglycan carbohydrate with a molecular mass of about 1 million.

Synovial fluid is effectively a liquid connective tissue. Because there are no fibrous components to it, the water and the hyaluronan ground substance move around together within the synovial space, whereas in other tissues water moves and the ground substance stays put. However synovial fluid is not like a glandular secretion, since its components enter and leave the cavity by the same route. Water diffuses in and out of the synovial cavity more easily than hyaluronan. The amount of water in a joint depends on passive equilibration of plasma dialysate with vascular and lymphatic compartments, as for all connective tissue fluid. It goes up and down with exercise and rest. Water can enter the joint rapidly during inflammation but once mixed with hyaluronan cannot leave so rapidly unless the joint ruptures. The half life of hyaluronan in the cavity is about 24 hours. It seeps out by a process of "reptate diffusion" - a sort of molecular snaking through the tissue surface.

Abnormal processes affecting joint synovial tissue usually affect all types of synovial cavity, which include synovial tendon sheaths and bursae. The synovial linin g in these structures is similar to that within joints, with a slippery non-adherent surface allowing movement between planes of tissue. Synovial tendon sheaths line tendons only where they pass through narrow passages or retinacula, as in the palm, at the wrist and around the ankle. Elsewhere the tendon lies in a bed of loose fibrous tissue.

Bursae occur at sites of shearing in subcutaneous tissue or between deeper tissues such as muscle groups and fascia. Many bursae develop during growth but new or adventitious bursae can occur at sites of occupational friction.

Ligaments hold bones together. They are variably elastic. Tendons transmit muscle power to bones and are inelastic (except in kangaroo legs). Joint "capsules" are composed of a basket work of independently moving ligaments and tendons associated with sheets of fascia. Some joints, such as the sacroiliac, are largely surrounded by ligament, others, such as the shoulder, with its rotator cuff, are surrounded by tendon. Tendons may also pass through joint cavities, e.g. long head of biceps.

Entheses

The point where ligament, tendon or aponeurosis joins bone is called an enthesis. Entheses are important because they are the main target in a group of inflammatory disorders associated with the HLA-B27 Class I allotype - the seronegative spondarthropathies.

Fibrous joints have no cavity. The bones are joined by fibrocartilage (pubic symphysis, intervertebral discs). Intervertebral discs also contain a semiliquid central nucleus pulposus which can herniate through a damaged outer fibrous annulus.