Any of several types of connections between bones. Joints are classified as fixed, cartilaginous, or synovial according to their structure and movability.
Fixed joints are immovable joints in which the bones are connected with a thin, fibrous tissue. The plates that form the skull are joined by fibrous joints.
Cartilaginous joints, in which cartilage unites the surfaces of bones, are only slightly movable. One example of a cartilaginous joint is the symphysis pubis, the joint between the two pubic bones, which has a fibrocartilage disk between the two bone surfaces.
Most of the joints of the body are synovial joints, and these are the joints most often affected by arthritis. In synovial joints, the end surfaces of bones are covered with cartilage and enclosed in a joint capsule made of tough fibrous tissue. Space between the bones gives the joint room to move. The joint capsule is lined with a thin membrane called the synovium that produces synovial fluid to nourish and lubricate the joint. Ligaments outside the synovium connect the bones to one another and help keep them properly aligned. Tendons attach muscles to the bones, and tendons and muscles together help keep the joint stable and provide the strength to move it. Fluid-filled sacs called bursae help tendons and muscles move smoothly over bone or other muscles.
All synovial joints have basically the same structure, but they don’t all work by exactly the same mechanism. Here’s one way of classifying the different mechanisms, with examples from each class:
- The hip and shoulder are ball-and-socket joints, in which one surface is somewhat ball-shaped and the other is cup-shaped. This type of joint has the widest range of movement — sideways, forward or backward, and rotation.
- The finger joints are hinge joints, in which a convex surface of one bone fits into a concave surface of another in a clasp arrangement. Movement is limited to bending and straightening. The knee and elbow are modified hinge joints; they permit some rotation as well.
- The first two cervical vertebrae of the spine, located where the back of the neck meets the skull, meet in a pivot joint. In this type of joint, a rounded part of one bone fits into a ring-like structure of another bone, allowing you to rotate a part of the body, as when you turn your head.
- In ellipsoidal joints, such as the joints in the wrist, an oval-shaped piece of bone fits into a gently curved cavity. The wrist bends and rotates.
- Saddle joints, such as the joint at the base of the thumb, are, as the name suggests, saddle-shaped. They can rotate and move back and forth and from side to side.
- In gliding joints, the bone ends that meet in the joint are essentially flat and movement is limited. The sacroiliac joint, where the spine meets the pelvis, is a gliding joint.