Another ball and socket joint. During the first 9 months of life the skeleton has to undergo rapid growth from a cartilaginous start in the foetal stage to bone by maturity. This change from cartilage to bone is known as 'endochondral ossification '. When the puppy is born, the middle regions of the long bones have already turned to bone but the ends are still cartilaginous to allow the bones to lengthen. These bands of cartilage are sometimes known as 'growth plates'. The bone ends which articulate to form joints (as the elbow, for instance) are covered by a protective layer of cartilage which, in turn, is ossifying to become bone. Occasionally, something goes wrong with this process, the matrix becomes calcified but the final change to bone does not occur, an area of thickened, retained cartilage results. This flap may lie free (unattached) within the joint, when the condition is known as osteochondrosis dissecans' (O.C.D.). When the flap retains some attachment, depending on the point of attachment, the condition is known as 'ununited aconeal process' (U.A.P.) or fragmented cclronoid process (F.C.P.) - sometimes 'ununited oronoidprocess ' (U.C.P.). In each of these conditions there is pain and mobility is impaired. The only method of diagnosis is by X-ray. Mildly affected dogs can be controlled by drugs and careful exercise. Osteochondrosis (covering all three conditions) is known in man, horses, pigs and fowl, as well as dogs. There is a significant heritability factor, though the mode of inheritance is not fully understood.

Selective breeding can minimise the incidence of the condition.

Osteochondrosis can also occur in the shoulder and hock joints.