What is hip dysplasia?

The word "Dysplasia" literally means abnormal, so hip dysplasia literally translates as the abnormal formation of the hip socket. The hip is a "ball" and "socket" joint, in a normal hip the ball fits snugly into the socket, forming a pivot point.

Dogs which have a genetic predisposition for hip dysplasia are born with normal hips. However, as the dog grows, the structure of the hip joint becomes badly formed, and the "ball" no longer fits snugly into the "socket" and therefore does not rotate smoothly. In severe cases the ball joint isn't even within the socket.


Normal Hip	Mild Dysplasia	Severe Dysplasia

The normal hip (above) shows the rounded "ball" (Femoral Head) of the joint within the hip "socket" (Cranial Acetabulem Edge and Caudal Acetabulem Edge) and the Dorsal Acetabular Edge is also within the "socket.” It should be noted that the shape of the femoral head varies from dog to dog, not all femoral heads are perfectly round and that is normal as they generally have a hip "socket" to match (and it's not a case of a "square peg in a round hole"). What does matter is the smoothness of the femoral head and its position within the structure of the hip "socket.”

The mildly dysplastic hip (above) shows the rounded "ball" within the "socket" but the Dorsal Acetabular Edge is not fully encased in the socket.

The Severely dysplastic hip (above) shows that the "socket" is shallow, there is bone deposition in the Acetabular Fossa and the Femoral Head and the Dorsal Acetabular Edge are separate from the "socket".

For a more detailed look at the anatomy of the hip and for an explanation of the terms used to describe the hip please visit our KC/BVA Hip Scheme page.

What are the symptoms?

The severity of the symptoms obviously depends upon the degree of the dysplasia. More often than not, a dog becomes lame and may be unwilling to run and play as much as before. They often start to have trouble getting up or climbing stairs. If an older dog, they may perform a "bunny hop" when running, or walk with a "waddle" and become reluctant to exercise.

Why does hip dysplasia cause lameness and a reluctance to exercise ?

When a dog has a normal hip, the rounded femoral head (the end of the femur leg bone) fits snugly into the hip "socket" and as the dog moves the femoral head rotates (pivots) smoothly within the socket. When a dog has dysplasia, the femoral head will not fit smoothly and snugly into the hip socket and the effect is of bone grating upon bone. The dogs body will attempt to add new bone to the inside of the hip socket in an attempt to "close" the gap between the femoral head (the "ball") and the "socket" which is the bodies attempt to ease the situation. However, it still results in bone rubbing against bone and this bone growth causes the femoral head to be further displaced and doesn't really aid the situation! The constant rubbing of bone against bone causes inflammation and swelling and this in turn leads to arthritis (inflammation of the joint which causes pain, stiffness and limitation of motion).

Diagnosis

There are many diseases which can display similar symptoms to hip dysplasia, therefore the only true way to diagnose hip dysplasia is by X-ray of the hips together with a complete physical and neurological examination.

What causes it?

In almost all cases hip dysplasia is genetic (inherited) and is thought to be caused by several genes (polygenic), in addition to this the trait has "threshold characteristics". The "threshold characteristics" represent a continuous gradient of variation within the population (meaning the severity of the dysplasia is dependent on how many "threshold characteristics" are inherited from each parent).

Occasionally (although rarely), hip dysplasia can result from trauma i.e. car accidents, broken pelvis etc. where the head of the femur and/or the hip socket are damaged due to trauma and displacement occurs.

How do threshold characteristics influence the degree of dysplasia?

There are multiple genes responsible for HD and these genes are influenced by "threshold traits" (characteristics). The absence of dysplasia through to severe dysplasia is determined by the number of "threshold characteristics" the dog inherits (some from the sire and some from the dam, not necessarily in equal numbers).

By way of an explanation let us hypothetically assume that genes responsible for hip dysplasia are influenced by five "threshold traits" (in reality as far as I'm aware, the number of genes and threshold traits involved hasn't been determined). For a dog to be dysplastic, he needs to inherit 7 traits for mild dysplasia and 10 traits for severe dysplasia. For the example, a puppy inherits 4 traits (2 from Mum and 2 from Dad), his "quota" of traits is below the minimum of 7 so he's free of dysplasia. If neither parent has a significant number of "threshold traits", which when combined in the offspring, all the litter should be HD free.

From another mating, a pup inherits 6 (3 from Mum and 3 from Dad) likewise he's free of HD. Again, another mating, a puppy inherits 8 (7 from Mum and 1 from Dad), as the "magic" number of 6 has been exceeded he is affected by HD but is not severely dysplastic.

How do threshold characteristics correlate to hip scores?

We understand that regardless of the actual number of threshold traits associated with HD, a dog with a low hip score is likely to possess few or no threshold traits for HD. Conversely, a dog possessing a high hip-score is expected to possess a higher number of "threshold traits.” We know this to be the case because of the study into how "threshold traits" affect the expression of genes under their influence.

The "threshold traits" form a gradient (rising) and therefore (again hypothetically) a dog with a hip-score of less than 4 is likely to have 1 trait, a dog with a score less than 10 is likely to have 2 traits etc. etc. Clearly one would expect a dog with a score greater than 35 to have 7 or more traits and a dog with a score of greater than 51 to have 9 - 10 traits. These are purely hypothetical figures and are merely used to simplify the gradient effect of the "threshold traits.”

When we mate two dogs with below BMS scores, we are predicting that both parents posses very few threshold traits for HD (if they had a significant number themselves they would be HD affected). When we mate a lower scoring dog to a higher scoring bitch we are "gambling" on the chance that the males low level of threshold traits will counter balance the bitches higher number and the offspring will inherit insufficient traits to be affected with HD even though they may inherit more than the combined total of one of the parents they should hopefully be HD free.

If we were unwise and mated two parent's who have high hip-scores we know the chances of each parent having the middle to top end number of threshold traits will give rise to offspring with a high number of these traits and therefore the chances of them being dysplastic is high.

Naturally the various combinations of the threshold traits and their effect on the HD genes will produce different degrees of hip dysplasia from no dysplasia (hip score 0) through the wide variety of scores culminating in severe dysplasia (hip score 106).

Because there is no accurate way to predict the number of HD "threshold traits" carried by an individual dog and likewise we don't know the specific HD genes it is important to breed from low scoring parents and to ensure (where possible) that the close relatives of each parent are also low scoring (i.e. parents, siblings and any existing offspring).

The expression of the genes (phenotype) can be aided or hindered by several factors:

Genetic inheritance (the number of threshold characters inherited from each parent)
Overfeeding: high protein and high calorie diet whilst the dog is a puppy/juvenile
Excessive calcium intake (i.e. overdosing with mineral supplements or feeding bone meal)
Excessive intake of Vitamin D
Inappropriate exercise (too much or too little)
Injuries to the hind legs and pelvic area

HD is a genetic condition and unless a dog suffers extensive trauma (car accident etc.) to the hips/pelvis he will not develop HD unless he inherits the causative genes. The degree of dysplasia is affected by the level of threshold characteristics and the extent to which the genes can be expressed in the phenotype (how the dog looks/clinical signs and symptoms) can be helped or hindered by environmental management (diet/exercise).

Nutrition has quite an effect on how a dysplastic dog presents i.e. how extreme are the symptoms that can be observed. Puppies should be kept lean and not fat; obviously, a puppy which is carrying around too much weight will exacerbate any degeneration of the joints due to the strain of body weight on the joints. Research has also shown that giving a diet too high in protein and calcium also exacerbates the condition. Rapid growth in a young puppy also contributes, and, in most cases, the rapid growth rate is directly related to feeding a high calorie diet to puppies. Over supplementation of calcium has likewise been shown to be a major factor in the development of skeletal disease in puppies.

Exercise is the other main contributor. Many people over-exercise young puppies, or give them the wrong type of exercise. The wrong type of exercise can include forced running for any distance and too much exercise on tarmac or other hard surfaces. Up to at least six months of age, exercise on hard surfaces should be kept at a minimum. Correct exercise for puppies includes running and playing in the garden or in a park, although games that involve jumping and very rough play should be avoided, and the puppy should be allowed to rest as soon as he has had enough and must not "over-do" it. Swimming is an excellent form of exercise which builds up the muscles without putting stress on the joints.

How is it treated?

The treatment depends a lot upon the severity of the hip dysplasia and the age of the dog concerned, and veterinary treatment must be sought.

Any dog with hip dysplasia should be kept fit and trim as any excess weight will obviously aggravate the condition, but good muscle tone will help to support the dog's weight. Swimming is an excellent form of exercise which builds up the muscle without stress to the joints.

Non-steroid anti-inflammatory drugs, such as aspirin and carprofen, can often help manage pain. Research has shown that Vitamin C can also reduce the inflammation in the affected joints. Some people have also reported success with holistic medicines.

One such procedure is called a femoral head osteotomy. This involves removing the head and neck of the femur so that the bone does not contact bone, and a fibrous scar tissue then forms a "false" joint. As the dog's muscles must be strong enough to support the dog's weight on the false joint, regular exercise is very important. Another surgical procedure is hip replacement. This is the same as the human hip replacement, the diseased joint is taken out and an artificial joint is inserted.

Can it be Prevented?

Hip dysplasia cannot be totally prevented (because we don't know how many threshold traits there are and we don't know how many each parent carriers). However it is well documented that scoring breeding animals and only breeding from parent's with low scores (scores below the BMS) significantly reduces the occurrence of hip dysplasia, in addition to this, a dog with dysplasia who has a low scoring parent or parents is likely to be less dysplastic than if the parents had higher scores.

The reason why hip dysplasia cannot be totally eradicated by hip-scoring dogs is because the genes that control hip-dysplasia are not straightforward. That is to say the mode of inheritance is not straightforward. The genes responsible for hip dysplasia are polygenic (meaning there are more than one set of genes involved) and they are also believed to have "threshold" characteristics (meaning genes that represent a continuous gradient - an accumulation of variation within the population).

Because of the characteristics (multiple genes with threshold traits and environmental factors - diet/exercise) it is possible for parent's with scores of 0 : 0 (the best score possible) to produce affected offspring and parent's with scores of 53 : 53 (the worst score possible) to produce higher scoring offspring but not necessarily with scores as high as the parents.

Nutrition and exercise factors have been discussed above, and it is known that with the correct diet and exercise it is possible to ensure that the hip dysplasia is not expressed in cases of mild dysplasia and cases of moderate dysplasia can be helped by correct diet and exercise. Each breed has different requirements, and advice on this should be sought from a veterinary surgeon.