Cummings School of Veterinary Medicine, Tufts University, North Grafton, MA, USA
Canine hip dysplasia is a complexly inherited disorder that is seen across all purebred and mixed-breed dogs. In young dogs, hip dysplasia can present with acute unilateral or bilateral hindlimb lameness during the first year of life. Pathologically this lameness is caused by microfractures in the cartilaginous rim of the acetabulum prior to complete ossification, caused by instability of the hip joint. In severe cases of hip dysplasia, owners may report hearing clicking or clunking sounds emanating from the hips while the dog is walking.
Dogs with clinical signs of hip dysplasia may show stiffness, difficulty rising, swaying of the hips, nail scuffing, bunny hopping, decreased range of motion, altered hindlimb gait, and later arthritis. Some dogs may bow their legs with pronation of the stifles to attempt to more deeply sit and relieve the discomfort in their hips. This can lead to secondary medial patella luxation.
The diagnosis of hip dysplasia is through the phenotypic evaluation of the hips. Many dogs that will develop clinical signs of hip dysplasia will have palpable laxity of their hips at an early age. A gentile standing Ortolani procedure can be performed on puppies during their vaccination visits to detect significant hip laxity. Clicks to the hips at a young age are not correlated to later hip dysplasia, and usually resolve over time.
An Ortolani procedure, as well as a modified Barden's manipulation of the hips should be performed on all young dogs undergoing anesthesia - especially during spaying and neutering. Knowledge of mild, moderate or severe hip laxity with or without a positive Ortolani test can be predictive of later clinical signs of hip dysplasia. This knowledge will be helpful if the dog has severe laxity, has later hip lameness, and in counseling your client that maintaining a lean body weight could be protective of later hip arthritis.
Diagnosis of hip dysplasia is through hip radiographs. The hip-extended ventrodorsal radiograph under general anesthesia is the standard in most of the world. Radiographic evaluation includes subluxation, shallow acetabulae, bony remodeling, and osteoarthritic changes. Hip grading systems vary in the world, including the OFA, BVA/KC, and FCI.
There are other established radiographic techniques to evaluate hips. PennHIP measures the difference between a hip compressed and 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.
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; however, 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.
Approximately 75% of dogs with various degrees of hip dysplasia are able to function and live quality lives with conservative management. The primary medical management of pain from hip dysplasia is with nonsteroidal anti-inflammatory drugs (NSAIDs). Chondroprotective supplements also provide some relief from discomfort, but are not found to have preventative effects. Maintaining a lean body condition significantly decreases the weight-stress on the hips and diminishes hip pain.
There are two interventional surgical procedures that are available for puppies with severe hip dysplasia. Both are designed to establish a more normal acetabular coverage of the femoral head to provide better hip support. Both of these procedures must be performed before the puppy has undergone significant growth and before any boney remodeling has occurred.
Juvenile pubic symphysiodesis (JPS) is a procedure that cauterizes and stops the expansion of the pubic growth plate. The resultant growth of the pelvis tilts both ilia outward to provide more coverage of the femoral head. JPS must be performed before 18 weeks of age, so age of diagnosis and severity are important decision factors.
Triple pelvic osteotomy (TPO) can be performed on one or both hips. This procedure isolates and rotates the acetabulum outward to improve femoral head coverage. To be effective, TPO must be performed prior to 10 months of age and prior to any arthritic changes to the hip joint.
Older dogs with hip dysplasia that cannot be controlled with conservative measures can have salvage procedures performed. A total hip replacement is the gold standard for treatment. In small dogs, a femoral head and neck osteotomy (FHO) removes the bone on bone contact causing arthritic pain, and forms a false joint in the gluteal muscles. FHO does not work as well in larger dogs due to gluteal muscle fatigue.
Lifetime studies on the hip status of dogs show that maintaining a lean body condition produces better hip confirmation and reduced hip arthritis. Restricted calorie loads in "large-breed puppy foods" promote more uniform growth 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 hindlimbs) should be avoided in immature dogs when the skeletal components of the hip are cartilaginous and liable to deformation.
Heredity of Hip Dysplasia
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. The inheritance of hip dysplasia is polygenic, meaning that the action of several genes must combine to produce the disorder. Genetic studies into hip dysplasia have broken down the phenotypic liability into several components that are inherited separately. Some of these variables include joint laxity, age of ossification, and liability for osteoarthritis.
Studies on the genetic control of polygenically inherited disease show that selection based on phenotypic measurements of individuals shows poor 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 scores are linearly correlated to hip dysplasia showing its inheritance as a quantitative trait.
Estimated breeding values (EBVs) that incorporate phenotypic data are shown to be highly predictive of later dysplastic development. To provide the most power, EBVs require data on all normal and abnormal siblings within litters, which is usually not available in dogs. EBVs for hip dysplasia are being developed for several breeds in the U.K. by the Kennel Club, and in the U.S. by Dr. Todhunter at Cornell.
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 specific known dysplasia liability genes. Todhunter's group is working on GBVs in their breeding colony and Dr. Distl in Hanover, Germany, has studied GBVs in German Shepherds and Bernese Mountain Dogs. The breed-specific DNA markers that they have found differ between the breeds, showing that GBVs are specific to the population being studied. Liability DNA marker panels may not correlate to other breeds, or even other populations within the same breed. 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
With the current knowledge and understanding that we have of dysplastic development, we can offer concrete genetic counseling recommendations to our breeder clients. 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. Vertical pedigrees as demonstrated on the OFA website provide a good representation of the expectation of hip quality that can be passed on from the individual. The dog's own hip rating represents its phenotype, but the relative's hip ratings are more representative of the dog's genotype. 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 acetabula.