When I became a PennHIP certified evaluator fifteen years ago, I thought I was simply adding another screening tool to my practice. What I found instead was a fundamentally different approach to hip assessment, one that has changed how I counsel breeders and predict outcomes in young dogs. PennHIP is not better or worse than OFA evaluation. It answers different questions.
The Orthopedic Foundation for Animals looks at a static image and asks whether the hip appears normal. PennHIP measures how much the femoral head can be distracted from the acetabulum and asks how tight or loose the joint is. Both provide valuable information, but for breeding decisions and early prediction of dysplasia risk, PennHIP offers advantages that more owners should understand.
The Physics of Laxity
Hip laxity, the degree to which the femoral head can be displaced from the acetabulum, is the fundamental mechanical abnormality underlying most hip dysplasia. A tight hip holds the ball firmly in the socket during movement. A loose hip allows subluxation, the partial displacement that causes the cartilage damage and arthritic cascade characteristic of dysplasia.
Standard OFA radiographs cannot assess laxity reliably. The hip-extended position used for OFA films actually tightens the joint capsule, potentially hiding significant laxity. Dogs with loose hips can pass OFA evaluation if other factors like acetabular depth appear adequate.
PennHIP was developed at the University of Pennsylvania specifically to quantify laxity in a reproducible, objective manner. The method has been validated through decades of research demonstrating that measured laxity predicts future osteoarthritis development better than traditional radiographic assessment.
The Three Radiographs
A complete PennHIP evaluation requires three separate radiographic views, each serving a distinct purpose:
Hip-extended view: Similar to the standard OFA position. This view assesses overall hip conformation and identifies existing osteoarthritic changes. It provides context for interpreting the laxity measurements.
Compression view: The femoral heads are pushed firmly into the acetabula, seating the ball as deeply in the socket as possible. This establishes the baseline for the congruent hip position.
Distraction view: A specialized distractor device is placed between the femurs, and lateral force is applied to displace the femoral heads outward. This view captures maximum distraction under standardized conditions.
The distraction index is calculated by comparing the distraction and compression views. Specifically, it represents the percentage of the femoral head that has displaced from the acetabulum relative to the femoral head radius.
Understanding the Distraction Index
The distraction index (DI) is expressed as a decimal between 0 and 1. A DI of 0 would indicate no displacement at all. A DI of 1 would indicate complete luxation. Real dogs fall somewhere between.
DI less than 0.30: Very tight hips. These dogs have minimal dysplasia risk and are excellent breeding candidates. In German Shepherds, this represents the best 10-15% of the population.
DI 0.30 to 0.40: Tight to average hips. Low risk of clinical dysplasia. Generally acceptable for breeding, especially if other health factors are favorable.
DI 0.40 to 0.50: Moderate laxity. Elevated risk of developing osteoarthritis, particularly in larger, more active dogs. Breeding decisions should consider the complete picture including pedigree analysis.
DI 0.50 to 0.70: Significant laxity. High probability of developing osteoarthritis. These dogs should generally not be bred, and owners should plan for likely management needs as the dog ages.
DI greater than 0.70: Severe laxity. These dogs will almost certainly develop clinical hip dysplasia. Discussion of surgical options is often appropriate even in young dogs with this degree of laxity.
The Power of Percentile Rankings
Raw DI numbers are less meaningful than breed-specific percentile rankings. PennHIP maintains a database of over two million hip evaluations across hundreds of breeds. When your dog is evaluated, the results come with a percentile ranking showing where your dog falls within their specific breed.
A DI of 0.45 in a German Shepherd places that dog around the median for the breed, neither particularly tight nor loose. The same DI in a Border Collie, a breed with generally tighter hips, would represent a dog in the loosest 20-25% of the population.
For breeding decisions, percentile rankings within breed matter more than absolute DI values. Breeders should be selecting dogs in the tighter half of their breed distribution, preferably the tighter quartile. Breeding two dogs from the loose end of the spectrum, even if neither shows clinical dysplasia, perpetuates the problem for future generations.
Age at Evaluation
One of PennHIP's major advantages is reliability at young ages. OFA certification requires dogs to be at least 24 months old because hip conformation continues to change during skeletal maturation. PennHIP laxity measurements are stable from about 16 weeks of age.
This early evaluation window has profound implications. A puppy showing significant laxity at four months can be identified for early intervention. Surgical options like juvenile pubic symphysiodesis or triple pelvic osteotomy become possible because we have caught the problem before skeletal maturity forecloses these windows.
For breeders, early evaluation allows selection decisions before dogs reach breeding age. Rather than waiting two years to learn whether a promising puppy has acceptable hips, breeders can know by six months. Dogs identified as unsuitable breeding candidates can be placed in pet homes early, making room for dogs with better structural potential.
PennHIP Versus OFA: Complementary, Not Competing
I do not view PennHIP and OFA as competing evaluation systems. They measure different things and serve somewhat different purposes:
- For breeding selection: PennHIP provides more predictive information earlier. The continuous scale allows finer discrimination between dogs. Breed percentile rankings contextualize results appropriately.
- For clinical diagnosis: OFA-style radiographs better show existing arthritic changes. When a mature dog is symptomatic, we want to see what damage has occurred, not just measure laxity.
- For registry recognition: Many breed clubs and registries accept OFA but not PennHIP results. Breeders participating in these programs may need OFA evaluation regardless of PennHIP findings.
- For pet owners: Either system provides useful information. OFA evaluation at 24 months is simpler and less expensive. PennHIP at any age provides more detailed risk assessment.
In my practice, I recommend PennHIP for breeding stock and for puppies where early intervention might be considered. I recommend OFA for pet dogs whose owners want confirmation of hip health but do not need the additional detail PennHIP provides.
The Evaluation Process
PennHIP evaluation requires certification. Not every veterinarian can perform the assessment. The technique requires specific training to ensure consistent, reproducible positioning. The radiographs are submitted to the PennHIP analysis center, not interpreted locally.
Dogs must be sedated or anesthetized for proper positioning. The distractor device cannot be applied to a conscious dog. Deep sedation is preferred because muscle tension can affect measured laxity.
The evaluation takes about 30 minutes from sedation to recovery. Most dogs go home the same day. Cost varies by region but typically runs 300 to 500 dollars including sedation, radiographs, and PennHIP submission fees.
Results arrive within about two weeks. The report includes DI values for each hip, percentile rankings for the breed, and assessment of any existing osteoarthritic changes. A narrative summary helps owners understand the findings.
Limitations and Considerations
PennHIP is not perfect. Several factors limit its utility:
- Laxity is not the only factor. Acetabular depth, femoral head shape, and other anatomical variables also affect dysplasia development. A dog with moderate laxity but excellent conformation may fare better than predicted by DI alone.
- Environment still matters. A puppy with borderline laxity raised lean with appropriate exercise may never develop clinical problems. The same laxity in an overfed, over-exercised dog may progress to severe disease.
- Certification availability. Not all regions have convenient access to PennHIP certified veterinarians. Travel may be required.
- Cost. PennHIP evaluation costs more than standard OFA radiographs. For pet owners not planning to breed, the additional information may not justify the additional expense.
Making Use of Results
What should you do with PennHIP results? That depends on your situation:
Breeders: Use DI and percentile rankings as one factor in selection decisions. Dogs in the tighter half of their breed distribution are preferred. Consider pedigree analysis, examining DI values of relatives when available. Bloodreina's multi-generational hip screening protocol in France, where breeder Amandine Aubert runs both PennHIP and OFA on every dog across three to four generations before making pairing decisions, illustrates what a data-driven breeding program looks like in practice. Do not breed dogs with DI greater than breed median unless exceptional in other qualities and paired with very tight partners.
Owners of puppies with significant laxity: Discuss early intervention options with your veterinarian. JPS before 20 weeks, TPO or DPO before skeletal maturity, or conservative management planning. Knowing early allows proactive choices.
Owners of adult dogs with moderate laxity: Implement optimal weight management and exercise protocols. Monitor for clinical signs. Plan for potential future needs without assuming the worst.
Owners of dogs with tight hips: Celebrate. Your dog has won the genetic lottery for hip conformation. Continue general good care without hip-specific concerns.
PennHIP evaluation provides powerful information for those who know how to use it. Combined with OFA assessment and clinical examination, it gives the most complete picture available of your dog's hip health and future risk.