By Dr Dan Laptoiu, MD, Bucharest, Romania
Five classes of tissue make up synovial joints:
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 a live tissue constantly remodelling in response to stresses.
- 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. 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 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 eg. in the case of repairing cartilage defects of the knee; it has lower mechanical properties compared to hialine.
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. 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. Beneath the surface cell layer is a net of small blood vessels, important in the development of synovial inflammation. Joint, tendon sheath and bursal synovium all have the same structure.
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. 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 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. If joints are stretched suddenly, even the fluid does not fill all the space and the lining may jump into the vacuum formed, which is how people "click" their finger joints.
Tendon sheaths and bursae
The synovial lining 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 and tendons
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.