A normal Hip
The head of the thigh bone conforms to the Acetabulum.
HipAge1
The cartilage dulls as it is exposed to abnormal stress.
HipAge4
The wear of cartilage eventually exposes the sensitive underlying bone causing pain.
This can occur before one year of age in the severely Dysplastic Pup.
Minimising the Risk of HD
Key pointers to minimising the risk of having a Dysplastic dog:
1. Buy your pup from a breeder who uses the BVA Hip Scoring scheme for both parents, the scores of both parents should be as low as possible.
2. Limit your pup's exercise to 5 minutes per month of life per session. Do not engage in extended play sessions and limit goal oriented activity such as ball chasing.
3. Feed a diet that is below 23% Protein. This will reduce the likelihood of very rapid growth between 5 and 7 months of age.
Dogs are born with normal hips. Those dogs that develop Hip Dysplasia are born with the genetic potential for the development of hip instability. This is not mediated by a single gene, but multiple genes that will code for Skeletal Growth Rate, Soft Tissue Development, Fat Deposition and so forth. Dogs that develop Hip Dysplasia do so as a result of asynchrony between skeletal and soft tissue development. This will generally manifest as a rapid growth “spurt” between 4 and 7 months of age.
The effect is to turn the hindlimb into a very long lever. This long lever produces greater forces at the top of the thigh bone than can be resisted by the immature muscles of the hip. The result is instability.
This instability causes the head of the femur to rest in a higher position, resting on the delicate rim of the socket. Instead of having even loading of the head of the thigh bone in the socket, we now have a small area of contact between the two surfaces. This leads to wear and damage to the cartilage of the head and the rim of the socket.
The mechanical effect of being unable to lock the head of the femur into the fulcrum of the socket is that it is more difficult to get up from rest. A force equal to nine times body weight is needed to get up from rest. This is easy if you can lever off a good socket but is very difficult if you can’t. As a result many dogs with Hip Dysplasia will find this “simple” manoeuvre difficult and may pull themselves along the floor rather than get up.
With the head of the thigh bone in this displaced position, the socket becomes filled with fibrous tissue and the socket begins to flatten. Eventually the socket becomes completely infilled with bone. The abnormal wear on the head of the thigh bone results in the head changing shape. The hip is now no longer a “ball and socket joint”, but now more resembles a cup and saucer. The lining of the joint may become inflamed as a result of the wear debris produced by this higher friction environment. The combination of inflammation and abnormal wear results in further damage to the cartilage of the head of the thigh bone. Exposure of the sensitive underlying bone can lead to pain. This is similar to losing the enamel on your teeth. Enamel, like cartilage has no nerve endings and as long as we have enamel to enamel contact we do not have toothache. When the sensitive cavity is exposed, this is like having the sensitive bone exposed and contact based pain occurs. The image to the far right ("A Damaged Femoral Head") shows exposed bone on the head of the thigh bone from a dog that has had a Total Hip Replacement. It has become polished due to friction.