How hips form and Wolff's Law

By Carol Beuchat PhD

It's tempting to assume that bad hips are the result of "bad hip genes", and that you can improve hips through selective breeding by breeding away from the troublesome hip genes.

But nobody can find those genes for bad hips. We can identify genes associated with hip phenotype (normal vs dysplasia) in a Labrador, for example, but those genes are not predictive of hip phenotype in German Shepherds. If there are different genes associated with hip dysplasia in each breed, we have a real mess - a genetic disease that is caused by different genes in every breed. This seems very unlikely.

It turns out that the shapes of the ball and socket are not genetically programmed. After birth, the acetabulum and head of the femur form to fit each other; the shape of the socket forms the shape of the ball, and the shape of the ball forms the shape of the socket. This process is an example of Wolff's law.

Wolff's law describes how bone and cartilage are shaped in response to stress. This is a process that allows tissue to adapt to biomechanical stress. It is why it is so important for the head of the femur to be well-seated in the socket. If the hip is "loose", the ball can be pulled away from the socket. This changes the distribution of forces on the ball and socket, and they can change shape in response.

This is why Riser says that the hip will not become dysplastic if the joint remains "coherent", with the ball snugly tucked into the hip socket.

This is a little tricky to understand, but a video demonstration will help make it clear with a familiar example. ​

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