Dogs, like people, are susceptible to arthritis.  There are many many diseases that can lead to and cause arthritis like symptoms, but true arthritis occurs most commonly either as osteoarthritis or rheumatoid.

Osteoarthritis affects joint cartilage1 and causes it to wear roughly.  This wearing away and degeneration of the cartilage leads to a cushionless joint. 

Osteoarthritis is brought on by joint instability that can be heredity, developmental, or caused by an injury.  Although not caused by old age alone, older dogs that have been very active in life would have put more wear on there joints.  Larger animals also put more stress on there joints than a smaller animal would.

Rheumatoid arthritis is not the wear and tear disease that Osteoarthritis is. It is a autoimmune disease that causes the body's own immune system to attack healthy joint tissue causing inflammation and joint damage. Fortunately, this type of arthritis is not common in dogs.

There are no cures for either osteoarthritis or rheumatoid arthritis, but there are many treatments that can slow the progression of arthritis and provide pain relief to a suffering pet.

1. What is cartilage?  Cartilage is tough and elastic and acts as a shock absorber and bearing surface between bones. One place cartilage is found is in movable joints. Moveable joints can be hinged to provide a forward and backward motion, such as the elbow and knee, or a ball and socket to allow rotary movement, like those at the hip and shoulder. Between the bones at these joints is cartilage.