Joints

Where bones meet, they form a joint, which is held together by bands of tough connective tissue called ligaments. Some joints are fixed and have no movement at all, such as the skull, whereas others have a limited range of movement, such as the ribs and spinal vertebrae. A third type of joint – the most common – allows a wide range of movement and is known as a synovial joint due to the synovial fluid which provides vital lubrication and acts as a shock absorber. Examples include the knees and wrists. At rest, synovial fluid is jelly-like in consistency, but the heat generated by movement makes it less viscous and increases its flexibility; this is why warming up before exercise is important.

Where two bones meet at a joint that allows movement, smooth flexible connective tissue called cartilage covers the ends of the bones to reduce friction. Cartilage does not contain blood vessels so grows and heals very slowly. It can also wear out, which results in the bones moving against each other and causing pain; this is osteoarthritis.

There are several different types of synovial joints:

• Ball and socket joints involve the round head of one bone nestling into the cup-shaped space of another, allowing for a lot of movement – for example, the hips and shoulders
• Ellipsoidal joints are similar to ball and socket joints, but the end of one bone is convex and nestled into the concave end of the next, such as in the fingers and wrists
• Saddle joints facilitate similar movement to ellipsoidal joints, but are only found in the thumbs
• Gliding joints are comprised of bones with flat surfaces that move by sliding over each other, such as in the ankles
• Hinge joints allow movement in only one plane – for example, the elbows and knees
• Pivot joints enable bones to rotate on each other, such as in the neck.