Diarthrodial joint

A diarthrodial joint is a classification of a particular type of joint within an animal. They are composed of bones, soft tissue and synovial fluid. The bones provide the ability to bear weight on the limb. The soft tissue structures include cartilage, ligaments, the soft tissues around the joint, the joint capsule, and the synovial membrane.

The ligaments and soft tissues around the joint provide mechanical stability (side-to-side and front-to-back) to prevent injury. The joint capsule is composed of collagen and also provides mechanical stability. It has a good blood supply and is highly sensitive to painful stimuli due to the rich source of nerves which run through it. The synovial membrane contains cells (synoviocytes), which help remove any unwanted material (bacteria, etc) and produce synovial fluid (helps maintain a normal joint environment). The synovial membrane has the capacity to regenerate and can stretch and contract during joint motion. Like the joint capsule, the synovial membrane has a rich blood supply. However, it is relatively insensitive to painful stimuli. Finally, the normal cartilage within a diarthrodial joint is hyaline cartilage. It is made up of collagen and ground substance. The ground substance is composed primarily of proteoglycan, chondroitin sulfate, and hyaluronic acid. The collagen fibers and ground substance provide the cartilage with the ability to compress and expand during weight bearing, thereby acting as a shock-absorber. Cartilage, unlike the bone underneath, contains no nerve fibers. Therefore, as long as the cartilage is not damaged, the joint should not be painful.

When arthritis sets in, the cartilage no longer is intact, exposing the underlying bone. The bone contains many nerve endings which causes the pain associated with arthritic conditions.