Arthritis is a name for a group of conditions that affect the joints of cats and dogs, including the young. The joint is the place where bones connect with each other.(for example, the hip and elbow joints). The two bones meet inside a kind of capsule that surrounds and encloses the joint. The capsule is made of ligaments (bands of tissue, also called synovial membrane). That capsule is filled with fluid that lubricates the joint (synovial fluid). The ends of the bones are covered in smooth cartilage to cushion the bones. Thus, in a normal joint, the bones have cartilage and fluid between them to keep them moving freely and smoothly. The bones are moved by muscles, which are attached to the bones by tendons.
When a joint is affected by arthritis, parts of the joint (such as the tendons and the capsule around the joint) become inflamed (red, hot and swollen). Parts of the joint (such as the cartilage) can also be damaged, and wear thin. The bones themselves can eventually be damaged.
There are many different kinds of arthritis. Most arthritis is caused by wear and tear. Arthritis can also be caused by a disease and trauma to the joint.