When veterinarians perform radiographs to evaluate the hips of potential breeding dogs, there are two methods they may use: PennHip radiographs and the Orthopedic Foundation for Animals (OFA) hip radiographs. While these two methods of hip evaluation have similar goals, there are a number of differences between the two techniques.
Perhaps the biggest difference between the two is that OFA radiographs focus on anatomic abnormalities and natural laxity, while PennHip radiographs document the hips’ distractibility under pressure. This makes PennHip a more complex, but also more sensitive, predictor of hip dysplasia. A 2010 study found that many dogs that were judged clinically normal on OFA radiographs demonstrated laxity on PennHip radiographs, including 52% of dogs with hips that were ranked “excellent” on OFA radiographs.1 Additionally, studies have also shown that PennHip results typically remain consistent over a dog’s lifetime, compared to OFA scoring which can vary over time.
In addition to improved sensitivity, there are a number of other advantages to PennHip radiographs. First, these radiographs can be performed as early as 16 weeks of age. This is a distinct advantage over OFA radiographs, which cannot be graded until a dog reaches 24 months of age.
Additionally, PennHip radiographs are more objective than OFA radiographs. When the radiographs are examined, a distraction index is calculated by dividing the measured distance between the femoral head center and the acetabular center by the radius of the femoral head. A distraction index of 0 is ideal and a distraction index of 1 represents a fully luxated hip. Dogs with a distraction index of >0.4 are at increased risk of developing hip dysplasia.
PennHip also has disadvantages. Dogs must be fully anesthetized. Additionally, the veterinarian and veterinary technicians performing the radiographs must complete a training program. (Fortunately, this training program is now offered at no charge.) Finally, PennHip radiographs are often more expensive than OFA radiographs, because of the need for anesthesia and the fact that three separate radiographic views are required.
First, the patient must be anesthetized. Then, three separate radiographs are taken:
- Distraction view: A PennHip distractor is placed between the thighs. The hindlimbs are then pressed medially, into the distractor, so that the femoral heads are lifted out of the acetabulum.
- Compression view: In this view, the femoral heads are pressed firmly into the acetabula.
- Hip-extended view: This is a more typical ventrodorsal view of the pelvis, identical to the OFA view described below.
The distraction view is used to calculate the distraction index. The compression and hip-extended views are used to provide additional information about the hip anatomy and the fit between the acetabulum and femoral head.
OFA Hip Radiographs
In many ways, OFA radiographs may be considered “easier” than PennHip radiographs. OFA radiographs do not require specialized training or specific tools, such as a distractor. Patients are not required to be sedated or anesthetized, although sedation may be needed to improve the likelihood of obtaining diagnostic radiographs. Only a single radiographic view is required, which decreases the cost to the owner.
However, OFA radiographs do have limitations. They can only be performed on dogs over 24 months old and they cannot be performed in pregnant or in-estrus females. Grading is also subjective, which means that false positives and false negatives may occur. A given dog’s hip grading may vary over time and the same radiograph may be graded differently by two different radiologists.
While PennHip radiographs provide an objective numerical distraction index, OFA radiographs are graded according to the following scale: excellent, good, fair, borderline, or dysplastic (mild, moderate, severe).
OFA hip radiographs use a standard ventrodorsal view of the pelvis. While sedation is not required, it may be necessary to obtain a diagnostic radiograph.
The dog is placed in dorsal recumbency with the rear limbs extended and parallel to each other. The stifles are rotated internally and the pelvis should be symmetric. Evaluating the obturator foramina for symmetry is often the easiest way to ensure adequate positioning.
When the radiographs are evaluated, special attention is given to the craniolateral acetabular rim, cranial acetabular margin, femoral head, fovea capitis, acetabular notch, caudal acetabular rim, dorsal acetabular margin, junction of femoral head and neck, and trochanteric fossa. The appearance of these parts of the coxofemoral joint is used to assign a subjective grade.
Whether a client chooses to pursue PennHip or OFA radiographs will depend upon their goals and priorities. PennHip radiographs are more specific and may be more informative for breeders who are strongly focused to minimize the risk of hip dysplasia, but OFA can also offer significant benefits and may be more accessible for some clients.
- Powers MY, Karbe GT, Gregor TP, et al. Evaluation of the relationship between Orthopedic Foundation for Animals' hip joint scores and PennHIP distraction index values in dogs. J Am Vet Med Assoc. 2010;237(5):532-541. doi:10.2460/javma.237.5.532
- Smith GK, Gregor TP, Rhodes WH and Biery D. Coxofemoral joint laxity from distraction radiography and its contemporaneous and prospective correlation with laxity, subjective score and evidence of degenerative joint disease from conventional hip-extended radiography, Am J Vet Res. 1993; 54: 1021-1042.