Joints

Bones are linked together by joints. Most joints allow different parts of the skeleton to move. The human skeleton has joints called synovial joints.

The synovial joint

If two bones just moved against each other, they would eventually wear away. This can happen in people who have a condition called arthritis. To stop this happening, the ends of the bones in a joint are covered with a tough, smooth substance called cartilage. This is kept slippery by a liquid called synovial fluid. Tough ligaments join the two bones in the joint and stop the joint falling apart.

Synovial joint: featuring bone, cartilage, ligament, synovial fluid and synovial membrane.The main features of a synovial joint

Movement

Different types of synovial joint allow different types of movement. The table describes two types of joint:

Type of jointExamplesMovement allowed
Hinge jointKnee, elbowThe same as opening and closing a door, with no rotation (turning)
Ball and socketHip, shoulderBack and forth in all directions, and rotation

The bones cannot move on their own - they need muscles for this to happen.