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Canine Hip Dysplasia
In nauseating detail


Some normal anatomy and pictures for orientation

petname The picture to the right is of a plastic model of the pelvis and hip joint in approximately a normal position when the dog is standing. We are looking at the pelvis from the side. The dog's head would be to the right in the picture, and the tail would be coming out the back just over the word "pelvis". This thing must be a Rottweiler model, because there is no tail. The spine in obviously longer than in the model, and it would run parallel to the tabletop toward the head.


petname I wanted to show you this picture because the way we take x-rays of dog hips is weird and can be confusing to look at. We don't take pictures while the dog is standing, because ... well, it's just very difficult. Instead we have the dog lie on his back and we pull the legs straight. It ends up looking just like you would look if you were lying on your back on a table. This picture shows the pelvis in this position. The dog's head would be toward the top, and his toes toward the bottom.

petname This (normal) x-ray was taken with the dog lying down in the required position. I've pointed out all the pertinent bits so you can compare them to the plastic model.


petname

The hips are ball-and-socket joints. Here is a picture of the hip joint on the model. You can see from the pictures and x-ray above that the pelvis is a big, bony box that looks a bit like a window frame. This picture shows better the nice round socket (obviously there is one on either side). The thigh bones (femurs) each have a large, round ball at the top end, which perfectly fit into the round sockets in the pelvis.

You can also see that the socket is not so deep that it completely holds the ball. If it was, we wouldn't have the great range of circular motion that we do in the hip. The socket takes in about half of the ball. This fortunate arrangement allows the male dog to lift his leg to pee on a tree, and allows for full extension and thrust backward when the animal is at a dead run.

If there were no other structures to hold this joint together, the balls would just fall out of the sockets, so we need other things. There is a ligament that runs from the end of the ball to the center of inside of the socket. In the picture above it is an elastic cord, but in real life it is a relatively un-stretchy ligament, and is one of the reasons that it's hard to dislocate the hip. There are also a lot of muscles that attach to the top of the femur from the pelvis and back. They help to move the leg, but also hold the hip in.

Obviously there are lots of areas for potential problems. A small change in one thing, like the rate of bone growth or whether a ligament is tight enough, affects another part, which affects two more, and the problem grows. Before you know it, the dog is in big trouble from what seems like a small problem.

More big words that will impress your vet

petname Anatomists like to call round things on the ends of bones heads', and since this one is on the femur it got called the femoral head. The socket in the pelvis is called the acetabulum (ass-e-TAB-you-lum). When we talk about the ball and socket, we are more correctly discussing the femoral head and how it fits into the acetabulum. There. Now you can sound all educated and fancy-like.