This is a common condition in dogs where the hip joints are poorly formed. Each hip joint is shaped like a ball and socket; the ball is the top of the thigh bone (femur), the socket is where the femur fits into the pelvis. In a healthy dog, the ball forms a nice, tight fit within the socket and wear and tear is minimal. But in a dysplastic dog, the fit is loose and the ball moves around too much within the socket, creating abnormal wear and tear, pain and arthritis.
Any breed of dog can be affected, but it is mostly seen in larger breeds. Symptoms can occur at any age, but commonly begin in dogs less than a year old.