Literally, hip dysplasia (HD), means "badly formed hip". In order to understand this complex problem it is necessary to be aware of the structure of the canine hip. The ball and socket joint consists of two basic parts - the acetabulum and the femur. The femur, or thigh bone, consists of the head (the ball) and the neck (the part of the femur that joins the long shaft of the bone to the head). The acetabulum forms the socket part of the joint and it is in this socket that the head of the femur rests.

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In unaffected dogs there is a good fit between ball and socket. An easy way to think about it is to imagine a tennis ball and a cup. If you put the ball into the cup there is a good fit. If you put the ball into a saucer, it will roll around loosely. This poor fit between femoral head and acetabulum is characteristic of dysplastic dogs.

Affected dogs are not born with HD, only with the makings of it. As puppies grow, laxity of the muscles and ligaments surrounding the joint and the poor fit between the bones produces excess movement of the ball in the acetabulum. The separation between the bones is called subluxation, and at its most severe it can become a total dislocation (the head of the femur leaves the acetabulum). The surfaces of the bones start out completely smooth, but with HD changes take place. Bone rubbing against bone causes irritation which results in irregular bone growth and wear on the articular surfaces. These irregular surfaces result in Osteoarthritis, which can cause significant pain. As the bone of the acetabular rim is ground away, the socket becomes shallower and it is more difficult to keep the head of the femur properly seated.

Outward signs of hip dysplasia can range from none to severe pain. The dog's movement may or may not be affected. Some outward signs that MAY be seen in affected dogs are:

* Difficulty getting up from a lying or sitting position or in climbing stairs.
* A bunny hop gait (moving both rear legs together).
* A painful reaction to extension of the rear legs.
* Pushing on the rump may cause the pelvis to drop.
* When rolled over, some affected dogs will resist having their legs spread into a frog-leg position.
* A side to side sway of the rump.
* Dogs may avoid jumping
* Lameness may be seen, especially after strenuous exercise.