Osteoarthritis is a condition once thought to be due simply to wear and tear on the cartilage of a joint; however, it is now known to be a complex problem that involves an active disease process.

Normal joint surfaces are covered with a smooth layer of cartilage. Healthy cartilage allows bones to glide over one another. It also absorbs energy from the shock of physical movement. In osteoarthritis, the surface layer of cartilage breaks down and wears away. This allows bones under the cartilage to rub together, causing pain, swelling, and loss of motion of the joint. Over time, the joint may lose its normal shape.  Also, bone spurs may grow on the edges of the joint. Bits of bone or cartilage can break off and float inside the joint space. This causes more pain and damage. Primary canine osteoarthritis is believed to have an inherited predisposition. Osteoarthritis is more common and arises from some insult of the joint such as the trauma of an accident, abnormal stresses on the joint surfaces or infection in a joint. 

Senior dogs can develop Osteoarthritis, a degenerative joint disease. Symptoms include an altered gait, reluctance to jump, obvious pain when walking and stiffness upon rising from a laying position.