Fast Facts - Behind the PennHIP Research
PennHIP receives many questions each day regarding different areas of the PennHIP method. Often, the published literature contains the answers, but the manuscripts may be difficult to access and written in scientific language that can overwhelm readers or hide salient points of the research. Some of the important features of the PennHIP method are included below with a brief synopsis of the literature available. Links to the abstracts for these studies have been included if you wish to read into them further!
1. The Distraction Index (DI) as determined by the PennHIP method is the most reliable indicator of future hip osteoarthritis.
In a study comparing factors such as age, breed, weight, gender, distraction index and Norberg angle (another method of measuring hip laxity based on the conventional OFA-type x-ray), it was found that the distraction index had the highest correlation (predictive power) with future development of osteoarthritis irrespective of age at the time of PennHIP evaluation.
Smith GK, Popovitch CA, Gregor TP. Evaluation of risk factors for degenerative joint disease associated with dysplasia in dogs, J Am Vet Med Assoc, 1995; 206: 642-647.
2. The distraction index does not change significantly over time.
A study of large breed dogs showed that the distraction index stayed the same over time (within acceptable statistical limits) and was much more reliable over time than other methods such as the Norberg angle and the OFA scoring method.
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. Abstract
3. Keeping your dog at a lean weight throughout life delays the onset of hip osteoarthritis related to hip dysplasia.
Restricted feeding to maintain a lean body condition delayed or prevented development of radiographic (x-ray) signs of hip joint osteoarthritis in a group of 48 Labrador Retrievers followed throughout life. Lifetime maintenance of 25% diet restriction delayed onset and reduced severity of hip joint osteoarthritis, thus favorably affecting both length and quality of life. This study also showed that hip osteoarthritis can develop at anytime throughout a dog’s life.
Smith GK, Paster ER, Powers MY, Lawler DF, Biery DN, Shofer FS, McKelvie PJ, Kealy RD. Lifelong diet restriction and radiographic evidence of osteoarthritis in the hip joints of dogs. J Am Vet Med Assoc. 2006 Sep 1; 229 (5); 690-3. Abstract
4. The PennHIP method can be reliably performed on a dog as young as 16 weeks old.
PennHIP has studied the efficacy of this method from eight weeks up to three years of age. For the present, it is recommended that dogs should not be evaluated before 16 weeks of age and that follow-up radiography at 6 months or 1 year of age is encouraged. However, the decision to have the method performed again is always that of the owner.
Smith GK , Gregor TP, Rhodes WH and Biery DN. 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. Abstract
Smith GK , Hill C, Gregor TP, Olsson K. Reliability of the hip distraction index in two-month-old German Shepherd dogs. J Am Vet Med Assoc, 1998;212:1560-1563. Abstract
5. 80% of dogs evaluated as “normal” by the OFA were found to have hip laxity by PennHIP testing that predisposed them to developing hip osteoarthritis in the future.
Dogs judged as normal by the OFA harbored clinically important passive hip joint laxity as determined by the PennHIP distraction index. Results suggested that OFA scoring radiographs (x-rays) underestimated susceptibility to osteoarthritis in dogs. The presence of these “false-normal” dogs in the breeding pool likely slows the progress of decreasing hip dysplasia prevalence.
Powers MY, Karbe GT, Gregor TP, McKelvie PJ, Culp WT, Fordyce HH, Smith GK. 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: 532-541. Abstract
6. PennHIP Biomechanics: Biomechanical testing determined the optimal patient position for measuring maximal hip laxity.
Hip laxity was found to be maximal with the hind legs held in weight-bearing position as used in the PennHIP method. Hip laxity was actually masked in the conventional hip-extended position.
Smith GK , Biery DN and Gregor TP. New concepts of coxofemoral joint stability and development of a clinical stress-radiographic method for quantitating hip joint laxity in the dog, J Am Vet Med Assoc. 1990;196:59-70. Abstract
Heyman J, Smith GK and Cofone MA. A biomechanical study of the effect of coxofemoral positioning on passive hip joint laxity in the dog. Am J Vet Res, 1993;54:210-215. Abstract
7. PennHIP continues to research inherent differences among breeds.
The breeds with the tightest hips as measured by DI have the lowest susceptibility to showing hip osteoarthritis. Within each of the 8 breeds (American bulldog, Bernese Mountain Dog, German Shepherd dog, Golden Retriever, Labrador Retriever, Newfoundland, Rottweiler, and Standard Poodle) studied thus far, the looser the hips, i.e., the greater the distraction index (DI), the greater the likelihood of showing hip osteoarthritis.
Runge JJ, Kelly SP, Gregor TP, Kotwal S, Smith GK. Distraction index as a risk factor for osteoarthritis associated with hip dysplasia in four large dog breeds. Journal of Small Animal Practice 2010;51:264-269. Abstract
Smith GK , Mayhew PD, Kapatkin AS, Shofer FS, Gregor TP. Evaluation of risk factors for degenerative joint disease associated with canine hip dysplasia in German Shepherd dogs, golden retrievers, Labrador retrievers, and Rottweilers. J Am Vet Med Assoc, 2001;219:1719-1724. Abstract
8. Hormonal effects on hip dysplasia diagnosis
Contrary to popular belief, estrus (being in heat) is of no consequence to hip scoring; the study showed definitively that estrus does not adversely affect the distraction index or any other hip scoring method, including the subjectively-read, hip-extended method. The effects of pregnancy, however, on hip scoring, have not been studied. The hormone relaxin remains present up to 8 weeks post whelping (although it varies by breed). We know of no study describing whether relaxin has an effect on DI or any other hip scoring method but to be on the safe side, we advise waiting 8 weeks post-lactation or 16 weeks post-whelping before hip testing.
Hassinger KA, Smith GK, Conzemius HM, Hill CM and Gregor TP. Effect of estrus cycle on coxofemoral joint laxity, Vet Comp Ortho Traum, 1997;10:69-74.
9. Within and Between Examiner Repeatability
Studies have shown that the PennHIP method has a very high degree of reproducibility between examiners. In other words, your dog should have similar distraction scores no matter which PennHIP certified veterinarian performs the radiographs. This high degree of consistency is attributable to the inherent biomechanics of the canine hip joint and to the quality-assurance training that all PennHIP network veterinarians must successfully complete.
Smith GK , LaFond E, Heyman SJ, Cofone MA and Gregor TP. Biomechanical characterization of passive laxity of the canine coxofemoral joint, Am J Vet Res, 1997;58:1078-1082. Abstract
Smith GK , LaFond E and Gregor TP. Within-and between-examiner repeatability of distraction indices of the hip joints in dogs, Am J Vet Res, 1997;58:1076-1077. Abstract
10. Cats and Hip Dysplasia
Did you know cats can get hip dysplasia? As in dogs, the distraction index is correlated with osteoarthritis; the greater the distraction index, the more likely a cat will develop osteoarthritis. However, further studies need be conducted to determine how well cats tolerate laxity.
Langenbach A,Giger U, Green P, Rhodes H, Gregor T, LaFond E, and Smith G. Relationship of degenerative joint disease and laxity in the coxofemoral joint by use of distraction index and Norberg angle measuement in a group of cats. J Am Vet Med Assoc 1998;213:1439-1443. Abstract
Murphy TP, Biery DN, Fordyce HH, Gregor TP, Shofer FS, and Smith GK. Radiographic prevalence of hip dysplasia in 121 Maine Coon Cats, Proc 27th An Conf Vet Ortho Soc, Val dIsere, France, 2000:p. 53.
11. Canine Hip Dysplasia Can Manifest Long After 2 Years of Age and the Distraction Index Accurately Predicted OA Susceptibility
In a unique lifespan study of 48 Labrador retrievers much was learned about the natural history of canine hip dysplasia. Coxofemoral subluxation, as identified on the HE radiograph occurred by 2 years of age and not thereafter. Accuracy of OFA-criteria scoring was poor: 55% of dogs scored “normal” at 2 years of age became radiographically dysplastic by the end of life; 92% of the dogs scored as normal by OFA-criteria scoring at 2 years of age developed histopathologic OA of CHD at end of life. In contrast the PennHIP DI at 2 years of age predicted all 48 dogs to be susceptible to OA of CHD and by the natural end of life 98% of these dogs had radiographic or histopathologic evidence of OA.
Smith GK, Lawler DF, Biery DN, Powers MY, Shofer F, Gregor TP, Karbe G, Evans RH, and Kealy RD. Chronology of hip dysplasia development in a cohort of 48 Labrador retrievers followed for life, Vet Surg 2012; 41: 20-33. Abstract