TYPES OF JOINTS
Our body is supported by a strong framework of bones. The bone is a hard scaffolding structure made up of firm tissue (collagen matrix) on which calcium and phosphorus salts are laid to give strength. A joint is where two or more bones meet, and is essential for movement of different body parts.
On the basis of structure and functions, joints have been divided into three categories :
|According to Structure||According to Function|
|Fibrous joints||Immovable e.g. Tooth in jaw|
|Cartilaginous joints||Slightly movable e.g.Spinal joints|
|Synovial joints||Freely movable e.g. Shoulder joint|
The purpose of the first two is to give strength and stability. The third type of joint is intended almost solely for movement.
I. Fibrous Joints
In these joints, bones lie side by side and are connected to each other by fibrous tissue. There is no movement possible in these joints. E.g. the joints between skull bones (called sutures), teeth and sockets (called gomphosis).
II. Cartilaginous Joints
Here, the opposing surfaces of the two bones are connected by cartilage (intervertebral disc) and only limited movement may be possible.
III. Synovial Joints
In synovial jointsm the participating bones are joined in such a way that free movement is possible. The range of movement made possible is extremely wide; from mere gliding movements between the small bones of the hand, to a phenomenal range of movements at the shoulder joint, so admirably put to use by bowlers in a cricket game.
Most of the joints in the body, including practically all joints of the limbs, are synovial joints. They have certain special characteristics:
- The opposing ends of the bones are covered by smooth, firm cartilage (called articular cartilage) and are not attached to one another. The articular cartilage plays a protective role; it keeps the bone ends from being worn-off by constant movement. Since the cartilage does not have any blood supply, it gets its nutrition from the surrounding synovial fluid.
- Joint CavityThe bone ends are surrounded by a bag of membrane that secretes a lubricating fluid. The membrane is called the Synovial membrane. Outside the membrane, tying the bones together and totally enclosing the synovial bag, is a layer of tough, strong, flexible fibrous layer called Joint capsule. The synovial fluid (secreted by synovial membranes) bathes the joint and provides lubrication. It is like oiling a constantly working machine (constant movement in a joint tends to produce friction). The bone would be worn away if there were no lubrication from the synovia.
- In addition to the joint capsule, the joints are also supported by a variable number of ligaments (rope like structures), which serve to provide stability to what would otherwise be a very loosely connected joint.
- Depending on the kind of movement called for, there are may types of synovial joints. At the hip, for example, maximum movement is rendered possible by a ball-and-socket joint, whereas the elbows and knees are examples of a hinge joint that allows movement in one axis only.
- Disease of any of the constituents could hamper the free movement of the joint. Rheumatic diseases, for example, could do so through pain, swelling, destruction of cartilage and bone.