Joints are structures responsible for joining different bones. They support the weight of the body and allow the movement of bones.
According to their movement, articulations can be classified as:
- Synarthrosis: they do not allow any movement. The joints of the cranial bones are the most typical examples.
- Amphiarthrosis: they allow slight movements. The joints of the vertebrae are the most typical examples.
- Diarthrosis or synovial joints: they allow complete movements. The bones are linked by ligaments. And the part of the bones in contact with other bones are in covered by cartilage. The space between the cartilaginous pieces that cover the bones in the diarthrosis are filled with a liquid that prevent them from friction and it is called synovial fluid.
According to the type of movement, the synovial articulation can be classified as:
- Gliding (or plane) joints: they are made up of two flat surfaces that move, so they allow only gliding or sliding movements. The most typical examples are the joints of metacarpal bones.
- Hinge joints: they allow the complete movement of two bones but only in one plane, like a door's hinge. The most typical examples are the joints in the knee and the elbow.
- Ball and socket (universal or spherical) joints: one of the bones form a sort of ball in the articular zone that fit in a concave socket. This allows tridimensional movements.
- Condyloid joints: a condyle is received into an elliptical cavity, allowing movements in two planes.
- Saddle (or sellar) joints: they are similar to ball and socket joints, but the contact surface are not spherical. Due to this, they allow movements in two planes, but not rotation.
- Pivot joints: these joints allow one bone pivoting in one plane around another, so one bone rotates about another. The most typical one is the joint between the first and second vertebrae, that allows the semicircular movement of the neck.