Hip dysplasia (HD) is the most common heritable orthopaedic condition seen in veterinary practice, affecting virtually every breed. There is no effective cure for this condition, and treatment consists mainly of palliation.
Traditional methods for diagnosing HD have been associated with disappointing progress in reducing the incidence of the disease. Some of the schemes also had a voluntary submission policy, in other words xrays of really bad hips were never sent for scoring, which resulted in breed scores being better than they should have been.
The only practical means of eliminating or reducing the incidence of HD is through selective breeding, based on well-defined objective phenotypic (physical make up, such as xray appearance) criteria. In theory, these phenotypic criteria will identify those dogs that lack the genetic predisposition for HD, and therefore will be suitable for breeding.
Most traditional screening systems looked at a single view of the hips, with the hips fully extended. Since this method underestimates true hip laxity, it cannot effectively predict the dog’s susceptibility to HD later in life. In one study, 55% of dogs scored normal by a hip extended radiograph, went on to develop osteoarthritic hips later in life. It is likely that the prevalence of HD in a breed is 2-3 times higher that is actually started from traditional hip-extended scoring schemes.
Therefore, using the extended hip method for selecting breeding dogs will retain a high level of HD in the offspring.
There is also considerable variation in scores when using the hip extended views, both between examiners and from the same examiner.
Hip laxity (looseness) is a measurable objective component that can be used to predict an animal’s susceptibility to develop HD and the osteoarthritis (OA) of HD later in life. Hip laxity, combined with rapid growth and weight gain, increases the risk of OA in the hips.
PennHIP testing measures hip laxity.
PennHIP involves two radiographic views: a compression view with the femoral heads compressed deeply into acetabula (sockets), and distraction view obtained by levering a custom-designed device between the legs to maximally displace the femoral heads from the acetabula.
This is then read by a panel of experts in Pennsylvania to give a distraction index (DI), defined as a measure of how much the femoral head is distracted from the acetabulum when under stress. DI of 1.0 = full distraction. 0.5 = 50% distraction.
Distraction xray revealed 2.5 times more laxity than the hip extended xray, and is therefore much more sensitive at detecting laxity in hips, the prime phenotypic factor in predicting a dog’s susceptibility for developing OA of HD.
Lax hips on PennHIP more highly correlated with OA than lax hips on extended view.
Tight hips on PennHIP are extremely unlikely to develop OA.
|| Compression view
|| Extended view
Hips with DI < 0.29 are very unlikely to develop OA.
DI > 0.3 is termed hyperlaxity. These hips are at risk of OA, with an increasing risk as DI increases further. In one study 98% of Labrador Retrievers with DI > 30% developed OA later in life.
DI of 0.3 can be thought of as a biological threshold, separating normal disease-free hips from OA prone hips.
DI can be measured from 4 months of age, and is a good predictor of susceptibility to OA, but the strength of the prediction improves at 6 months and again at 1 year. As DI increases over 0.3, the risk of OA increases, but rate of OA risk increase is breed related. Some breeds such as the German Shepherd Dog, are much more likely to develop OA as their laxity increases beyond 0.3. Other breeds are much less likely.
Majority of dogs certified suitable for breeding by hip extended radiographic scheme were found under PennHIP to have hip laxities corresponding to OA susceptibility, showing the prevalence of hip dysplasia to be far higher than previously thought.
Heritability denotes the reliability of the phenotype in predicting the genotype. In other words, it indicates how reliable measuring a physical characteristic is (such as hip laxity) in predicting the genetic make up of that individual.
High heritability means all the variation in phenotype is explained by the genes, and no environmental component exists. So, if a factor was found to have a high heritability, its measurement would give a clear indication of the genetic make up, and there would be no influence from other factors such as diet.
The higher the heritability of a selected trait and the stricter the selection criteria chosen, the more rapid the genetic change per generation of breeding. So, a highly heritable trait, which then is very strictly chosen for when selecting dogs for breeding, will lead to a very rapid change in the breed.
However, given the extremely high incidence of hip laxity, if we were to select dogs for breeding which had very good scores only (<0.3) we would be left with a very small population of dogs to breed from. As an example, Golden Retrievers with distraction index less than 0.3 constitute less than 5% of the population. To avoid problems associated with inbreeding caused by breeding from such a small group, a more moderate approach has been taken.
In is therefore prudent to choose breeding stock from those in the breed who have a distraction index less than the median score for the breed. This then represents the “tightest half” of the breed.
This will then apply continuous selection pressure while maintaining genetic diversity. In time, the breed median will steadily change and the breed scores will improve.
More rapid genetic change could be achieved by applying stricter selection pressure, which individual breeders are more than welcome to do.
PennHIP does not evaluate dogs in a pass/fail manner, it just gives a measure of hip laxity that a breeder can use to make an informed choice about whether to breed from that individual dog. We will not advise whether the individual is suitable for breeding or not, since there are obviously many other factors taken into consideration when selecting a dog for breeding than its hip score alone.
It is the owner’s responsibility to decide whether to breed from individual dog or not.
As a guiding principle, tighter hips are better hips, and a good starting point would be to accept for breeding any dog with a DI less than the breed median for the preceding year. Breed medians are updated continuously.
All xrays taken are submitted for evaluation, even ones that are obviously dysplastic.
When taking PennHIP xrays, there is a risk of cavitation risk. This is due to bubbles of nitrogen gas forming in joint fluid as the joint is stretched, as happens when we crack our knuckles. Cavitation causes no harm, but can make scoring the hip laxity difficult or impossible. 4% of hips will have cavitation when distracted, which may increase joint laxity artificially. DI can be measured if only one hip cavitates, but cannot be done if both cavitate. This happens in less than 1% of cases. In such a case, the procedure would need to be repeated.
and Paul Eason
have both now been accredited to perform PennHIP testing. This means we can offer this effective means of testing the susceptibility of a dog developing OA from HD later in life, at both our Papakura and Pukekohe clinics.
Testing involves a full anaesthetic, and a day stay in the clinic. Results are normally available in a few weeks, since they have to be sent to the USA for reading.
For prices please contact either Papakura