The bad stuff - dysplasia
So ... hip dysplasia is a problem that happens during growth and results in an abnormally formed joint. The femoral head doesn't fit into the acetabulum well. Most commonly we see that the acetabulum is flattened and shallow. Instead of being a nice deep C shape, it's a half-hearted parenthesis ( or even a flat line | that doesn't provide a good seat for the femoral head. The femoral head bangs around like a ball in a bucket.
Normally cartilage covers the femoral head and lines the acetabulum and makes a beautiful, smooth, slippery gliding surface. When the femoral head bangs around, this surface gets damaged.
There is so much "play" in the joint that the structures trying to keep the head in place stretch. The round ligament attaching it to the acetabulum gets lax. The muscles around the hip may not grow and strengthen properly. The overall effect is for the femoral head to drift away from its home in the acetabulum. As well, there is abnormal stress on the growing bones. The attachment of the head to the rest of the femur (called the neck, go figure) often becomes thickened, trying to strengthen itself for the extra load being placed on it from the structural abnormalities.
You can see this beautifully in these x-rays. On the left (A) we have a normal 6 month old Labrador Retriever. Her hips are a joy to behold. She has nice C-shaped acetabula (the plural of acetabulum) and the heads of her femurs are nice and round. When we look at the original x-ray we can see a shadow made by the back of the socket, and more than half of the head lies within this line. Everything sits together nicely. When we test for laxity in the joint, there is none.
Compare that to the x-ray on the right (B) of a 6 month old Golden Retriever. Look back and forth between the two photos; if you do it fast enough you can almost see the heads move outward. The acetabula are shallow, providing no place for the femoral heads to sit. The heads are drifting off in space somewhere. The necks are thickened already and look quite different from the normal dog's. When I did a laxity test on this dog (as we do on all dogs at spay or neuter time) I could completely dislocate both of this dog's hips without using too much force.
I am hearing voices now. "But Doc," they say, "my dog didn't get hip dysplasia until he was old. What's up with that?" It's simple, dear Watson. Not every dog with hip dysplasia has trouble with pain or discomfort as a young dog. Some have no outward signs of problems at all. In fact, the Golden Retriever (above) with the crummy hips on the x-ray doesn't limp, cry, or walk funny. He's two now, and he looks pretty normal. He is, however, a ticking time bomb. He will develop degenerative arthritis in those hips as he ages. Exactly when is up in the air (we'll talk about the various "risk factors" in a bit). With good management it may not be until he is 8 or 9, but it could be next month or next year. There's just no good way to predict this.
These x-rays are from adult dogs, a 9 year old German Shepherd cross and a 7 year old Golden Retriever. Neither had any clinical signs of dysplasia as a puppy, but both have developed significant arthritis in the hips as they have aged. Compare the neck area (the narrowing right behind the femoral head) of these two dogs to the puppies above. In the picture on the right the dog has one hip that is worse than the other. One neck is very thick while the other is almost normal. All of this thickening and remodelling is a result of arthritis.
Basement: Introduction to hip dysplasia
First floor: Hip dysplasia - normal anatomy and normal x-rays
Third floor: The whys and wherefores of prevention (?) and recognition