Jerold S. Bell, DVM, Adjunct Professor of Clinical Genetics
Department of Clinical Sciences, Tufts Cummings School of Veterinary Medicine, N. Grafton, MA, USA
Canine hip dysplasia is a complexly inherited disorder that is seen in wolves and across all purebred and mixed-breed dogs. It is the number one cause of arthritis in all dogs. We observe this disorder in clinical practice as; hip pain/stiffness, decreased range of motion, altered hind limb gait, and later arthritis.
The diagnosis of hip dysplasia is through the phenotypic evaluation of the hips. While palpation may reveal laxity or crepitus, the standard for diagnosis is through hip radiographs. The hip-extended ventrodorsal hip radiograph under general anesthesia is the accepted standard in most of the world. Radiographic evaluation includes; subluxation, shallow acetabulae, bony remodeling - especially at the dorsal rim of the acetabulum and femoral neck, and osteoarthritic changes. Different countries have different hip grading systems, including the OFA, BVA/KC, and FCI. PennHIP measures the difference between a hip compressed and hip distracted view and generates a distraction index (DI) representing passive laxity of the hip. The dorsolateral subluxation (DSL) test generates a score using a dorsoventral radiograph with the hips flexed and bearing weight at the stifles on the x-ray table. DSL is mostly used as a research tool and is not widely utilized in genetic screening.
The age of assessment for hip dysplasia is important, as it is not a congenital disorder and develops over time. PennHIP recommends evaluation for laxity beyond 4 months of age. The DSL test is recommended after 8 months of age. The BVA/KC and FCI require dogs to be 1 year of age. The OFA used to certify hips at 1 year of age, but found that 95% accuracy of diagnosis only occurred after 2 years of age. PennHIP describes an accurate early age diagnosis of hip dysplasia, but a study of early-age OFA preliminary ratings show a similar predictability. All of the different radiographic evaluations for hip dysplasia measure phenotypic aspects, and are all found to be correlated to each other.
Lifetime studies on the hip status of dogs show that maintaining a lean body condition produces overall better hip confirmation and reduced hip arthritis. Restricted calorie loads in “large-breed puppy foods” promote a more uniform growth rate where the boney and soft tissue components can develop and mature in unison. Excessive compaction on the hips (jumping and landing with full body weight on the hind limbs) should be avoided in immature dogs when the skeletal components of the hip are still cartilaginous and liable to deformation.
Heredity of Hip Dysplasia
The evolutionary development of breeds has produced some with higher and lower liability for developing hip dysplasia, based on which dysplasia liability genes they have lost or retained over time. Different studies find that hip dysplasia is 20 to 40% heritable. This means that 20 to 40% of the variability of dysplastic development is due to genetic factors, and the rest of the variability is due to environmental factors. This classifies hip dysplasia as a moderately heritable disorder, comparable to other complexly inherited traits such as egg production in poultry, and milk production in cattle. Proper selection against hip dysplasia should result in a reduction of the frequency of affected dogs.
The inheritance of hip dysplasia is polygenic, meaning that the action of several genes must combine together to produce the disorder. The specific combination of genes that produce liability to dysplastic development will vary between breeds, familial clusters, and between individual dogs. Environmental variables that can alter the expression of the disorder can include dietary load and degree of activity/mechanical stress.
Genetic studies into hip dysplasia have broken down the phenotypic liability into several components that appear to be inherited separately. Some of these variables include; joint laxity, age of ossification, depth of the acetabulae, and liability for osteoarthritis.
Breed differences in prevalence of hip dysplasia often have to do with breed defining characteristics. Breeds that were established on a racing phenotype had extensive selective pressure for good hip conformation. Those who did not excel were not used for breeding. Other breed differences have to do with conformational morphology. Lighter-boned tight-muscled breeds have a lower prevalence of HD compared to heavier-boned course-muscled breeds.
Chondrodystrophic breeds and dogs have in general poorer radiographic hip joint conformation. However, their lower hip scores do not necessarily correlate to increased clinical disease. In a study of Pembroke Welsh Corgis, all dogs showed radiographic signs of hip dysplasia, but this was not correlated to their susceptibility to develop later osteoarthritis.
Studies on the genetic control of polygenically inherited traits show that selection based on phenotypic measurements of individuals show less improvement when compared with selection based on familial data. OFA data show that hip conformation scores are directly correlated to the scores of the parents, grandparents, and their siblings. Combined parent hip scores are linearly correlated to the production of offspring with hip dysplasia - showing its inheritance as an additive (quantitative) trait.
Familial data can also be computed as estimated breeding values (EBVs), based on the phenotype of the parents, siblings, siblings of parents, offspring, and other relatives. By utilizing phenotypical depth and breadth of pedigree, EBVs utilize information that can more accurately reflect the cumulative genetic influences passed down to the individual dog.
An issue with the accuracy of calculating EBVs involves dogs with missing phenotypes - as most breeds have less than 10% of breeding dogs or their siblings evaluated. To provide the most power, EBVs require data on all normal and abnormal sibs within litters. Without this, the accuracy and precision of EBVs is low. In an applied setting, dogs with high EBVs may also become popular sires thus putting pressures on gene pools that can affect genetic diversity. EBVs for hip dysplasia have been developed for several breeds in the UK by Dr. Lewis at the Kennel Club, in the USA by Dr. Todhunter at Cornell University, and by the Australian Kennel Club. EBVs should include power estimates based on pedigree completeness.
Genomic breeding values (GBVs) are based on DNA markers that segregate with hip dysplasia in experimental populations. These markers may or may not be correlated to specifically identified dysplasia liability genes. Todhunter’s group at Cornell and Dr. Distl’s group in Hanover, Germany are working on GBVs. At this time breed-specific genetic marker panels are specific to the populations being studied, but do not accurately predict phenotypic liability in larger populations of the same breed or in different breed populations. Therefore, current commercial hip dysplasia liability DNA marker panels should be viewed with caution as they may not correlate to other populations within the same breed, or other breeds.
GBVs have greater promise in producing improvement with hip dysplasia as they avoid the issues of missing phenotypic information required for EBVs, as well as phenotypic variation caused by environmental influence. However, the development of accurate GBVs requires full extended pedigree phenotypic information. While EBVs and GBVs appear to hold the most promise for improved genetic selection against hip dysplasia, their specific clinical use and validation are a work in progress.
Genetic Selection for Normal Hips
Studies on the response to selection based on individual phenotypes show mild improvement with hip dysplasia. It is known that the greater the selective pressure (percentage of the population eliminated from breeding due to phenotypic score), the greater the improvement. However, the greater the percentage of the population eliminated from breeding, the greater the loss of genetic diversity of the population. Therefore, selection must be combined with consideration of the breed gene pools.
The phenotypic evaluation of individual breeding dogs should be uniformly applied, and based on a properly executed hip radiograph on an anesthetized or deeply sedated dog. This allows for the best evaluation of both boney conformation and joint laxity.
Selection of breeding stock should be based on familial data - depth and breadth of hip normalcy - not just the phenotype of the individual dog. The dog’s own hip rating represents its phenotype, but the relative’s hip ratings are more representative of the dog’s genotype. For example: The individual dog’s OFA web page, and associated vertical pedigree as demonstrated on the OFA website (ofa.org) provides a good representation of the expectation of hip quality that can be passed on from the individual. A dog with excellent-rated hips but with a preponderance of fair-rated relatives would be expected to produce more like its relatives than itself.
Breaking down the hip phenotype for individual dogs allows the breeder to focus on aspects that need improvement in the next generation. If a quality dog shows some subluxation or laxity, it should be bred to a dog with tight hips. If a quality dog shows a slightly shallow acetabulum, it should be bred to a dog with deep acetabulae.
Selection based on the best available phenotypic imaging and incorporating familial breadth and depth of pedigree data should improve the hip and elbow status of individual dogs and thus their breeds.
References are available upon request.