The points I wish to discuss in this thread:
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.
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.
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.
number 2 was interesting simply because everyone rushes to do OFA at 2 years of age "hurry, get the hips xray'ed"
number 5 was a jaw dropper for me... so "normal" ofa hips can be actually bad... not good news.
considering 9 and how bad positioning can make a "good" hipped dog look "poor"... the fact that you can take 2 films of the same dog, the same day, one less well positioned, and get an "excellent" on one and a "fair" on another is a big issue... if the test were ideal you should get "excellent" on one, and "inconclusive, reshoot" on the other.