There are nearly 200 joints in the human body. Their purpose is to make it possible to move.

What is a joint?

Joints are essentially the junctions between our bones - the place where the ends of two or more bones meet.  They can absorb sudden shocks, repair themselves when injured, and replenish their own supply of lubricating synovial fluid.

Cartilage
Each joint consists of two facing bones.  On the end of each there is a layer of white, smooth gristle-like material called cartilage.  This resilient substance covers and cushions the bone ends, reducing friction and allowing the bones to move smoothly and evenly over one another.  It also acts a shock absorber. 

Synovium
Around the outside of each joint is a membrane called the synovium, which regularly produces a small amount of a sticky substance called synovial fluid.  This helps to keep the cartilage in good condition and acts as a lubricant. 

Joint capsule
Covering the membrane is the joint capsule.  This is a lot thicker and tougher than the membrane - and is designed to seal and protect the joint. 

In large joints such as the knee, the capsule contains additional thick fibres called ligaments to help give the joint extra stability and to restrict and control the amount of movement.

Tendons
Muscle attached to bone produces movement by contracting and then relaxing.  They are connected to the bones by tendons.  These are attached to the end of the bones for maximum advantage. 

Tendons can be easily damaged, particularly at the point where they are attached to the bone, as this is where they are thinnest.

Types of joint?

Joints that need a wide range of movement are termed synovial joints - they have a synovial membrane, synovial fluid and joint capsule.  Synovial joints can be hinge joints such as the knee and the elbow, or ball and socket joints such as the shoulder and the hips.

Not all joints need to be able to move to the same extent.  Some need to be much stiffer, such as the joints between your teeth, and between your shin bones (the tibia and fibula).  In this instance, the bone is fixed together by fibrous tissue and the joint is called a fibrous joint.

Cartilagenous joints lie between synovial and fibrous joints.  They allow a small amount of movement, whilst being extremely strong. 

Your spine is an example of a collection of cartilagenous joints.  The amount of movement between any two invertebral bones of the spine is quite small.  However, each bone is separated from its neighbour by cartilagenous invertebral discs that act as shock absorbers.  When the bones and the discs of the spine are added together, they are able to give your spine considerable movement whilst retaining its strength.

11/03/2009