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Fast Facts - Behind the PennHIP Research


PennHIP has published more than 40 manuscripts over the past 20 years. 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 was the most significantly correlated 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.



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.



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.


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.



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 via the PennHIP distraction index. Results suggested that OFA scoring radiographs (x-rays) underestimated susceptibility to osteoarthritis in dogs. The presence of these “normal” dogs in the breeding pool may slow 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.



6.   PennHIP Biomechanics


Biomechanical testing determined the optimal patient position for measuring hip laxity. Hip laxity was found to be maximal in the non weight-bearing position used in the PennHIP method and is 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.


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.



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, 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.


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.



8.   Hormonal effects on hip dysplasia


Contrary to popular belief, estrus (being in heat) is of no consequence to hip scoring; the study performed showed definitively that estrus does not adversely affect the distraction index or any other hip scoring method. The hormone relaxin, however, remains present up to 8 weeks post whelping (although it varies by breed). We know of no study describing whether relaxin has any 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. 


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.


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.



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. As of June 2011, 131 cats are listed in the PennHIP database.


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.


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.