A Winn Foundation Report On ...
Hip Dysplasia and Patellar Luxation in Cats
Report on Hip Dysplasia and Patellar Luxation in Cats from the
First International Feline Genetic Disease Conference held June 25-28, 1998 at the University of Pennsylvania, Philadelphia, PA (Updated with Grant Progress Report)
by Susan Little, DVM, Diplomate ABVP (Feline)
Dr. Gail Smith and his colleagues at the University of Pennsylvania have been instrumental in the investigation and characterization of hip dysplasia (HD) and patellar luxation (PL) in cats. Until recently these problems, while well recognized in dogs, have received little attention in cats. The following is a summary of Dr. Smith's presentation:
Canine hip dysplasia is commonly found in the large and giant breeds and patellar luxation in the toy and miniature breeds. HD is less frequent in cats and they are not commonly screened for this condition; therefore, the true incidence of hip dysplasia in cats is unknown. Patellar luxation has been sporadically reported in the literature, but not well studied. Some surveys have found an increased incidence of PL in breeds such as the Abyssinian and Devon Rex. Joint laxity has been found to be associated with HD in dogs and plays an important role in the development of degenerative joint disease (DJD) in this species.
Dr. Smith and his colleagues devised a study to examine hip dysplasia and patellar luxation in cats and to examine the relationship between DJD and joint laxity using three different scoring systems similar to those developed for dogs.
A commonly used screening technique in dogs is the Orthopedic Foundation for Animals (OFA) scoring system based on standard hip radiographs. Two other techniques that measure the degree of joint laxity, Norberg angle (NA) and distraction index (DI) are increasing in use. The Norberg Angle measures joint laxity based on the relative positions of the femoral head and the acetabular rim on standard extended hip radiographs. One previous study looked at NA measurements in cats. The DI measures maximal joint laxity from compression and distraction radiographic views of the pelvis made with the hip joints in a neutral position.
This is the first study to use the distraction index (DI) to determine hip joint laxity in cats and the first attempt to correlate joint laxity with DJD. In contrast, the relationship between hip joint laxity and DJD in dogs is well established. A total of 78 cats were included in the study. The cats were from breeders, clients, and owners of catteries in addition to some cats from the University's own established colony. To be eligible for the study, cats were at least six months old and could be of either sex. No attempt was made to select cats on the basis of breed or clinical signs related to HD or PL. A total of 43 males and 35 females were examined; their mean age was 2.5 years. Eight different breeds were represented, making up 75% of the cats in the study.
The cats were examined, and their medical history was explored. Under sedation, all the cats had radiographs taken of the hips and stifles. The radiographs of the hips were evaluated by a board-certified radiologist using a scoring system established by OFA. Joint laxity was also measured using the NA method and DI. Evidence for DJD was noted and the relationship between DJD and joint laxity was determined.
Of the 78 cats examined, 11 had clinical signs of disease in the pelvic limb. Medial luxation of the patella (MPL) was found in 45 of the 78 cats (58%) and 71% of these cats had both stifles affected. Most affected cats had Grade 1 luxation. Within the study group, the Abyssinian and Devon Rex cats were affected more frequently and more severely.
Using the OFA scoring system, the investigators found that 25 of the 78 cats (32%) had hip dysplasia (HD). Most of the cats had mild HD (19/25); only two had severe HD. The majority of the cats with HD had bilateral disease (72%). Of the 25 cats with HD, 15 (60%) also had signs of degenerative joint disease (DJD).
Measurements of joint laxity found a mean Norberg angle (NA) in the cats of 92.4 degrees. In dogs, a NA of greater than or equal to 105 degrees has been suggested to be normal. None of the cats had a NA over 105 degrees, indicating a higher degree of joint laxity than is considered normal for most dog breeds. The cats with DJD had a significantly lower NA measurement (84 degrees).
Using the distraction index (DI), the mean measurement for all the cats was 0.51 and the mean DI for cats with DJD was significantly higher than for those without. The values reported by these investigators for cats places them in the same category as dogs with high joint laxity, such as the Golden Retriever. Unlike Golden Retrievers, however, the small body size of cats (like that of small dogs) seems to act as a protective factor lowering the radiographic and clinical expression of HD. Until larger populations of cats are studied Dr. Smith was unable to determine whether the hip laxity profiles found in this particular pool of cats is representative of the cat population at large. Despite this uncertainty, however, the inescapable conclusion from this study was that cats with tighter hips (lower DI) had a much lower likelihood of developing the clinical and radiographic signs of hip dysplasia compared to cats with looser hips.
Interestingly, 24% of the cats had both HD and MPL. Cats with MPL had three times the risk of HD of those without MPL. Overall, 39 of 78 cats (50%) had HD, PL or both conditions. The chance of having DJD increased with an increasing DI, a decreasing NA, and a worsening OFA score. It appears that in cats, as in dogs, hip joint laxity is related to DJD. In fact, in dogs hip laxity has been shown to be the primary risk factor for DJD. A weak association between PL and HD in cats was found. It is likely that PL and HD occur in cats more frequently than has been suspected, although the incidence of clinical signs in cats is low. Dr. Smith and colleagues are currently studying the utility of hip laxity as a selection criterion to reduce the incidence of hip dysplasia in cats.
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