Most of the body’s 300 joints are synovial joints. They are located where two bones meet and allow movement, such as bending, straightening, and rotation.
In addition to allowing movement, synovial joints have a few defining characteristics:
- On the surfaces where they meet, the bones are covered in cartilage. This strong, smooth, slippery material reduces friction and protects the bones from each other during movement.
- Each synovial joint includes a joint capsule. The capsule creates a protective envelope around the area where the two bones meet. The inner layer of the capsule is lined with a thin, delicate membrane, called a synovial membrane. The outer layer of the capsule is made of tough, fibrous ligaments. The ligaments attach the joint’s bones together.
- The synovial membrane produces a viscous fluid, called synovial fluid or joint fluid. This fluid is contained in the joint capsule and helps keep the two cartilage-covered bone surfaces separated and lubricated.
Synovial joints vary in structure and size. For example, the hip is a large ball-and-socket joint that allows for a 360° range of motion, while the knuckles in the fingers are small hinge joints that only allow for bending and straightening. Synovial joints are susceptible to various types of arthritis.
Nonsynovial joints are also located where two bones meet, but the surrounding tissue structures allow for little to no movement. Examples of nonsynovial joints include where the ribs meet the sternum and where the bones of the skull have grown together.