Joints

Our skeleton is made up of hard and strong bones. A bone cannot bend, but an arm can be bent, since it consists of more than one bone. The places where two bones meet are called joints. In our bodies, we have both movable and immovable joints. Movable joints are formed by bones held together by elastic fibres called ligaments.

Ligaments ensure that the bones don't twist too much or move too far apart. Bones in movable joints are also separated by a layer of strong tissue called cartilage. Cartilage prevents the bones rubbing together and wearing down. Cartilage is lubricated by a thick, slippery fluid called synovial fluid. This reduces friction and allows for smooth movement.

But beyond these features, not all movable joints are the same. Let’s take a closer look. First, hold your arm out straight. What direction can you bend your elbow? Your forearm can move all the way up to your shoulder, but it can’t bend easily in other directions.

This joint allows back and forth movement in one direction, similar to the way a door moves on its hinges. It’s a hinge joint. There are hinge joints in your toes, knees, ankles, and fingers, too. The joint at the base of your thumb is a little different: it’s very flexible! It allows your thumb to move towards and away from your index finger, and to cross over the palm towards your little finger.

See how the ends of each bone resemble a horse’s saddle? This is a saddle joint. You don’t have many saddle joints in your body; they are found only in the thumb, shoulder, and inner ear. Let’s move up to the top of the spine. The joint between the first two vertebrae allows your head to bend up and down and turn from side to side – to pivot.

This is a pivot joint. In pivot joints, one bone rotates around the other, a bit like a wheel on an axel. Elsewhere in the spine, there are joints that glide instead of rotating. These are gliding joints, and they’re found in your ankles and wrists, too. To see a gliding joint in action, hold your lower arm steady and bend your hand upward at the wrist.

Now, wave from side to side. Gliding joints allow this to happen! Next, hold your arm out to the side and rotate it. The joint in your shoulder allows this circular motion. The end of one bone has a round shape like a ball which fits into a hollow space in another bone, known as a socket.

This is a ball-and-socket joint. Ball-and-socket joints are found only in your shoulder and hips. They allow more movement than any other joint in the body: forwards and back, up and down, and round in a circle. There’s a similar joint in your wrist. It looks a bit like a ball-and-socket joint, but the end of this bone is more egg-shaped than ball-shaped.

This is a condyloid joint. Because of its shape, it can move forward-backward and side to side, but it can’t rotate. These are the six kinds of movable joints. But there’s another type of joint too, one that doesn’t move: immovable joints. Here in your skull, several bony plates are held tightly together by immovable joints.

These give the skull the strength it needs to support and protect the brain. Your adult teeth, too, are held in your jaw by immovable joints. And now we have our full list: immovable joints and moveable joints, with movable joints broken down into six types: hinge, saddle, pivot, gliding, ball-and-socket, and condyloid.