In medicine and biology, a joint is the place where two or more bones connect. Joints are made of connective tissue and cartilage and can be classified as:
- fibrous joints (such as those in the skull) connect bones with fibrous connective tissue; they allow little or no movement.
- cartilaginous joints (vertebrae, for example) connect bones with cartilage and fibrous connective tissue; they also allow little or no] movement.
- synovial joints contain synovial fluid, which lubricates the joint and absorbs frictional heat created by the joint's movement. There are several sub-types of synovial joints:
- ball and socket joint: the rounded head of one bone fits into a socket-like cavity of another, such as the hip and shoulder joints. These joints allow free rotation
- hinge joint: elbow and ankles. These joints allow for flexion and extension.
- saddle joint: the thumb. Bone surfaces are concave, allowing movement in all directions but only limited rotation.
- ellipsoid joint: structurally similar to a ball and socket joint but without rotation.
- pivot joint: the skull on the axisof the spine; movement is limited to rotation.
- gliding joint: the wrist. Bone surfaces slide across each other, allowing a wide range of movements.
From the science dictionary at http://biotech.icmb.utexas.edu/