by:  Greg Keller, DVM, MS, DACVR

Hip Dysplasia (cont.)

Anchor Jump Menu:
Hip Joint Conformation  Joint Laxity  Physical or Chemical Restraint Nutrition  Hormonal Effects
 Physical Inactivity Recommendations for Buyers Impact of OFA Hip Evaluations  References  

Hip joint conformation

The OFA consulting radiologists make subjective evaluations of the hip status based on criteria previously described (p.17). Although the radiologists apply the criteria subjectively, a study demonstrated good correlation between the consensus grade assigned and two objective measurements used to assess hip phenotype. These measurements are percent coverage (PC) of the femoral head within the acetabulum and Norberg angle (NA) which also estimates degree of fit. The higher the numeric value the better the degree of fit. A retrospective study of OFA hip phenotypes by Tomlinson (2000) reported a distinct difference in both percent coverage and Norberg angle values between OFA hip grades and between breeds. The following numerical values (*) for each OFA classification are averages derived from that study.

Excellent - This classification is assigned for superior hip conformation in comparison to other animals of the same age and breed. There is a deep seated ball (femoral head) which fits tightly into a well-formed socket (acetabulum) with minimal joint space width. *PC=63% NA=110

Good (Fig. 4) - The most common normal grade reported regardless of breed is slightly less than superior but a well-formed congruent hip joint is visualized. The ball fits well into the socket and good coverage is present. *PC=58% NA=108

Fair - Assigned where minor irregularities in the hip joint exist. The hip joint space is wider than a good hip phenotype. This is due to the ball slipping slightly out of the socket, causing a minor degree of joint incongruency (called subluxation). There may also be slight inward deviation of the weight-bearing surface of the socket (dorsal acetabular rim) causing the socket to appear slightly shallow. This can also be a normal finding in some breeds, such as the Chinese Shar Pei, Chow Chow and Poodle.*PC=49 NA=104

Figure 4: Good hips

Figure 5: Moderate HD

keller 04 keller 05


The following categories are not eligible for an OFA breed number:

Borderline - There is no clear cut consensus among the radiologists to place the hip into a given category of normal or dysplastic. There is usually more incongruency present than the minor amount found in a fair, but there are no arthritic changes present that definitively diagnose the hip joint as dysplastic. There also may be bony changes present on any of the areas of the hip anatomy that cannot be accurately evaluated as either an abnormal arthritic change or a normal anatomic variant for that individual dog. To increase the accuracy of the diagnosis, it is recommended the radiographs be repeated at a later date (usually 6 months). This allows the radiologist to compare the initial film with the most recent film and assess for progressive changes that would be expected if the dog is dysplastic. Most dogs (over 50%) with this grade that show no interval change in hip conformation receive a normal hip rating upon resubmission, usually a fair hip phenotype.

Mild Hip Dysplasia - There is significant subluxation present wherein the ball is partially out of the socket, causing an incongruent and increased joint space. The socket is usually shallow, only partially covering the ball. There are usually no arthritic changes present with this classification. If the dog has other superior traits and/or a great deal of time and investment has been placed into training, there is an option to resubmit a radiograph when the dog is older so it can be reevaluated. Most dogs will remain dysplastic, showing progression of the disease with early arthritic changes. There are a few dogs however, that show improved hip conformation with increasing age. Since HD is a chronic, progressive disease, the older the dog, the more accurate the diagnosis of HD (or lack of HD). At 2 years of age, the reliability for a radiographic diagnosis of HD is 95%, and the reliability steadily increases as the dog ages. Radiographs should definitely be resubmitted if they were initially taken during times of possible detrimental environmental effects such as periods of physical inactivity, or high hormone levels related to time of a heat cycle which could lead to a "false" diagnosis of mild hip dysplasia. *PC=40% NA=97

Moderate HD (Fig.5) - There is significant subluxation present wherein the ball is barely seated into a shallow socket, causing joint incongruency. There are secondary arthritic bone changes, usually along the femoral neck and head (termed remodeling), acetabular rim changes (termed osteophytes or bone spurs), and various degrees of trabecular bone pattern changes (called sclerosis). Once arthritis is reported, there is only continued progression of arthritis over time, and the dog may or may not be lame. The onset of lameness is unpredictable and some dogs may go most of their lives without showing any signs of lameness whatsoever. *PC=30% NA=92

Severe HD - assigned where radiographic evidence of marked dysplasia exists. There is significant subluxation present, where the ball is partially or completely out of a shallow socket. Like moderate HD, there are also large amounts of secondary arthritic bone changes along the femoral neck and head, acetabular rim changes, and large amounts of abnormal bone pattern changes. *PC=21% NA=83

In addition to assessing the dog/cat hip conformation, the veterinary radiologist reports other radiographic findings that could have familial, inherited causes, such as transitional vertebra or spondylosis. Transitional vertebra is a congenital malformation of the spine that occurs at the junctions of major divisions of the spine (usually at the thoracic and lumbar vertebral junction or the lumbar and sacral vertebral junction). Transitional vertebra take on anatomic characteristics of the two divisions of the spine between which it occurs. The most common transitional vertebra reported by OFA is in the lumbo-sacral area. Transitional vertebra are usually not associated with clinical signs and the dog/cat can be used in a breeding program, but the OFA recommends breeding to a dog/cat that does not have transitional vertebra.

Spondylosis is an incidental radiographic finding in which smooth new bone production is visualized on vertebral bodies at the intervertebral disc space margins. The new bone production can vary in extent from formation of small bone spurs to complete bridging of adjacent vertebral bodies. Spondylosis may occur secondary to spinal instability but often it is of unknown cause and clinically insignificant. A familial basis for its development has been reported. As with transitional vertebra, dogs/cats with spondylosis can be used in a breeding program.

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The effect of age and the use of preliminary radiographs for early detection of hip dysplasia

Frequently, breeders want early knowledge of the hip status on puppies/kittens in a given litter. This allows early selection of animals for use as show/performance/breeding animals or animals that would be best suited for pet homes. The OFA accepts preliminary consultation radiographs on puppies and kittens as young as 4 months of age for evaluation of hip conformation. If the dog or cat is found to be dysplastic at an early age, the economic loss from cost of training, handling, showing, etc. can be minimized and the emotional loss reduced. Preliminary radiographs are read by the OFA staff veterinary radiologist and are not sent to outside radiologists as are the 24-month-old examinations. The same hip conformation grading scheme is used.

The OFA has performed a retrospective analysis of the reliability of early radiographic evaluation for canine hip dysplasia, using information in their database obtained from the standard ventrodorsal radiographic projection. Corley (1997) reported on a population of over 2,000 dogs from the four breeds with the greatest number of OFA submissions (Labrador Retrievers, Rottweilers, German Shepherds, and Golden Retrievers). The reliability of the preliminary evaluation (3 to 18 months) was determined by comparing the initial evaluation to a follow-up evaluation (> 24 months) of the same dog. The reliability of a normal preliminary hip joint phenotype was 100% for excellent, 97.9% for good and 76.9% for fair (Table 2). The reliability of a preliminary evaluation of canine hip dysplasia was 84.4% for mild, 97.4% for moderate and 100% for severe (Table 3). Reliability of preliminary evaluations increased significantly as age at the time of preliminary evaluation increased, regardless of whether dogs received a preliminary evaluation of normal phenotype or canine hip dysplasia (Tables 4 & 5).

For normal hip conformations, the reliability was 89.6% at 3-6-months, 93.8% at 7-12 months and 95.2% at 13-18 months for the four main breeds. Pooled data comparing preliminary OFA evaluations at various ages and in various breeds with final OFA evaluations at 24 months or older resulted in a similar reliability factor for preliminary evaluations of approximately 90%. The false positive rate (defined as a preliminary evaluation of HD for a dog with a follow-up evaluation of a normal phenotype) of OFA preliminary evaluations < 6 months of age was 18%; and the false negative rate (defined as a preliminary evaluation of normal phenotype for a dog with a follow-up evaluation of hip dysplasia) of OFA preliminary evaluation < 6 months of age was 9%. This suggests that OFA preliminary evaluations of hip joint status in dogs are generally reliable. However, dogs that receive a preliminary evaluation of fair or mild hip joint conformation should be reevaluated at an older age (24 months).

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Table 2: Reliability of normal preliminary evaluations by hip grade







71 1,369 360 1,800

No Change

71 1,340 277 1,688

Norm to Dys

-- 24 75 99

Norm to Borderline

-- 5 8 13


100% 97.9% 76.9% 93.8%

Cl Upper

100% 98.5% 81.2% 94.8%

Cl Lower

94.9% 96.9% 72.2% 92.6%

Norm = Normal; Dys = Dysplastic; Cl = Confidence Level


Table 3: Reliability of dysplastic preliminary evaluations by hip grade







390 38 1 429

No Change

329 37 1 367

Dys to Norm

47 1 -- 48

Dys to Borderline

14 -- -- 14


84.4% 97.4% 100% 85.5%

Cl Upper

87.8% 99.9% -- 88.7%

Cl Lower

80.4% 86.2% -- 81.9%

Norm = Normal; Dys = Dysplastic; Cl = Confidence Level


Table 4: Reliability of normal preliminary evaluations by age


< 6 mo.

7-12 mo.

13-18 mo.



278 714 808 1,800

No Change

249 670 769 1,688

Norm to Dys

25 43 31 99

Norm to Borderline

4 1 8 13


89.6% 93.8% 95.2% 93.8%

Cl Upper

92.9% 95.5% 96.5% 94.8%

Cl Lower

85.4% 91.8% 93.5% 92.6%

Norm = Normal; Dys = Dysplastic; Cl = Confidence Level


Table 5: Reliability of dysplastic preliminary evaluations by age


< 6 mo.

7-12 mo.

13-18 mo.



102 150 177 429

No Change

82 126 159 367

Dys to Norm

18 15 15 48

Dys to Borderline

2 9 3 14


80.4% 84.0% 89.8% 85.5%

Cl Upper

87.6% 89.5% 93.9% 88.7%

Cl Lower

71.4% 77.1% 84.4% 81.9%

Norm = Normal; Dys = Dysplastic; Cl = Confidence Level

Joint laxity

Laxity is generally considered to be one of the earliest pathologic findings in HD. The fact that joint laxity plays a role, but is not the only factor, in development of hip dysplasia and its secondary changes of degenerative joint disease has been recognized for over 30 years.

Joint laxity (looseness of the joint) is a dynamic state that may not be determined by routine radiography. The joint may appear radiographically normal, but in actual use it may be loose.

Some dogs demonstrate abnormal laxity (subluxation) radiographically, but do not develop the more definitive degenerative changes of dysplasia.

Some dogs demonstrate radiographically tight hips, but later develop the degenerative changes of dysplasia.

Palpation of the hips to demonstrate looseness is not generally accepted as a single diagnostic feature of HD. Stress radiography using a fulcrum or wedge (placing an object between the thighs and bringing the stifles together to force the head of the femur out of the acetabulum) has been investigated as a technique to demonstrate the degree of radiographic subluxation that is possible. Some measurement criterion such as Norberg angle, millimeters of displacement, distraction index (DI), or dorsal lateral subluxation measurement (DLS) is usually employed to calculate the amount of displacement of the femoral head when compared to a fixed anatomic structure or to a standard radiograph taken without the fulcrum or wedge. The differences in the measurements indicate the range of possible motion or joint laxity. Different devices, measurements, and positions have been developed at the University of Pennsylvania (PennHIP®), Cornell University and Michigan State University. Use of the fulcrum technique has demonstrated that some laxity is expected in the normal joint, but that many dogs with laxity beyond a certain amount later show the more definitive characteristic radiographic changes of dysplasia. The specific degree of laxity that is acceptable at a given age, and in various breeds of dogs and cats has not been determined and represents a major unanswered question.

Table 6 is a comparison of different early screening procedures, and with the exception of palpation, all yield similar false-negative results (initially reporting a dog as normal that is later evaluated as dysplastic). There is, however, a major difference in the comparison of false-positive results (initially reporting a dog as dysplastic that is later evaluated as normal). A later publication by Lust (2001) suggested that the strength of the hip extended view (OFA view) is its specificity. Specificity refers to the ability to correctly identify dogs without hip dysplasia and this study also noted that this is dependent on the expertise of the evaluator.

Table 6: False-negative and false-positive results for dysplasia from 4 studies




Palpation (1)

25% 33%

DI at 4 months @ .3 (2)

12% 48%

DI at 4 months @ .3 (3)

0% 45%

DI at 4 months @ .4 (3)

13% 43%

OFA Prelims @ < 6 months (4)

9% 18%

1 = Reviewed by Willis; 2 = Smith et al.; 3 = Lust et al.; 4 = Corley et al.

The degree of joint laxity - as demonstrated by forcing the head of the femur away from the acetabula either by palpation or by using a fulcrum/stress device - that can be normal, and what degree is abnormal (eventually leading to degenerative joint changes) is unknown.

A primary reason this is unknown is that stress radiographic techniques measure artificially forced laxity in a non-weight bearing position. Improved accuracy using laxity as the diagnostic finding might be possible with a technique that measures dynamic laxity (laxity that occurs during normal movement).

There is currently no explanation to account for adult animals with substantial joint laxity that do not develop degenerative joint disease.

There is no pathologic evidence available to determine what processes are occurring in the hips that are lax but do not develop degenerative joint disease, or in hips that are tight yet develop degenerative joint disease. Without this information, there is a deficiency of necessary data to support breeding or treatment recommendations based on laxity alone. It is obvious that dogs with "tight" hips tend to be normal and those with markedly "loose" hips tend to be abnormal. What happens between the two extremes remains unknown. Further research using carefully controlled scientific methods is needed to understand the full implication of joint laxity.

However, breeders have a phenotypic screening method (standard hip extended radiograph) readily available that is safe, accurate, of modest cost, and effective. As an example of effectiveness, Leighton reported that while the mean DI did not change, the incidence of hip dysplasia at The Seeing Eye Inc. was dramatically reduced over five generations using the standard hip extended position and a subjective hip score similar to OFA's. That breeding program also illustrates the importance of obtaining and considering information on the hip status of siblings as well as on the dam and sire with regard to selection of potential breeding animals.

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Physical restraint or chemical restraint

Chemical restraint permits easier, and as a rule, more accurate positioning and reduces potential radiation exposure risk to the patient and veterinary personnel. The types of chemical restraint, depth of general anesthesia, or use of manual restraint only are environmental variables that can affect the radiographic evaluation.

Anesthesia has been shown to influence the evaluation, as a few dogs have been found to appear normal without anesthesia and yet demonstrate subluxation with anesthesia. This probably is due to muscular relaxation. The current belief is that a dog who appears dysplastic with anesthetic and normal without, should be considered dysplastic, or at best of questionable breeding quality. However, there are some veterinarians and a few HD control programs that do not recommend anesthesia as they feel that subluxation noted under anesthesia results in a false-positive finding.

Preliminary OFA data indicates that chemical restraint does affect the radiographic appearance of the hip joints in some dogs. Current information, observations made on large numbers of dogs, and experience with follow-up studies on large numbers of dogs, supports the recommendation that chemical restraint to the point of relaxation, or general anesthesia, be used. This appears to give a truer evaluation of the hip status, but more research is needed on this controversial subject, as there is an absence of controlled scientific data.

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Kasstrom, and later Kealy, reported that a higher than needed caloric intake during the rapid growth phase may result in earlier and more severe dysplastic changes when the genetic potential for dysplasia is present. Lower caloric intake may minimize or delay the evidence of dysplasia in the same dog, but will not change the genotype. Without genetic predisposition however, environmental influences alone will not create hip dysplasia.

There is no evidence in the scientific literature that megadoses of vitamin C (Bennett, 1987) or any other multi-vitamin/mineral supplement is beneficial in reducing the effects of, or preventing hip dysplasia.

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Hormonal effects

Estrus appears to affect the reliability of diagnosis in some females. Some animals in season demonstrate a degree of subluxation (laxity) that is not present when the bitch is out of season, possibly due to the relaxation effects of estrogens on the ligaments and joint capsule. Radiography of these bitches may result in a false diagnosis of HD.

It is recommended that bitches not be examined for HD when in season and radiographs should be obtained one month prior or one month following the heat cycle. In addition, following a pregnancy the OFA recommends that the bitch's radiographs be taken at least one month after weaning the offspring.

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Physical inactivity

Periods of prolonged inactivity may affect the reliability of diagnosis. A few animals exhibit subluxation after prolonged periods of inactivity due to illness, weather conditions, etc. On later examination, when the animal is in good muscular tone, the hips appear normal. Therefore, radiography is recommended when the animal is in good health and muscle tone.

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Recommendations for buyers

To verify health information when considering a purchase from a particular breeder, the buyer can obtain a pedigree of the animal in question. Health information then can be verified on the sire, dam, various siblings, and other close relatives at the OFA web site, www.offa.org. Information in the OFA's database can be used as a tool to increase the probability for obtaining a normal dog when choosing dogs for breeding, competition, or as healthy pets. Overall, if there are a substantial number of relatives that do not have OFA numbers in the pedigree, they should be assumed to be abnormal until proven otherwise. The more animals in a pedigree with OFA numbers, and the greater the percentage of their siblings with OFA numbers, the better the genetic probability for healthy animals from a given breeding. Breedings for which 2 to 3 generations of this depth and breath of information is available and normal will usually demonstrate significantly reduced incidence of HD.

It also may be helpful to consider whether the breeding in question is a repeat breeding, a line breeding, or an outcross. With repeat breedings, there may be health information available on puppies from the previous litter resulting from the same genetic combination. In the case of line breedings, experienced, knowledgeable breeders often have extensive information about the phenotypes present in their lines, and therefore can make more informed breeding choices. Longtime health conscious breeders often have greatly reduced the incidence of disease in their breeding programs, and this will be reflected in their track record (as verifiable on the OFA web site). Outcross breedings require more diligence of the breeder to fully investigate the new lines that are brought into the pedigree, and again, information available on the OFA web site may greatly aid in this effort.

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Impact of OFA hip evaluations through multiple generations of a population subset

Retrospective studies covering the period of 1972-2000 have demonstrated steady and encouraging progress as a result of the collaborative efforts of responsible breeders and the OFA. The OFA database population represents a specific subset of the general population of animals, primarily show dogs and cats, and working/hunting dogs. Accumulated data clearly illustrates the impact that the focused efforts of conscientious breeders can have on reducing the frequency of HD, and further indicates that the hip status of progeny follows that of parents (Table 1).

Success in reducing HD in a breed depends first on breeders recognizing that a problem exists. This must then be followed by a commitment to solve the problem and dedication to consistent use of a standard hip evaluation protocol.

HD has been reported in all breeds of dogs and some cat breeds that have been evaluated by the OFA. The OFA database is an important tool that can provide breeders with information regarding changes in hip status of specific breeds over time. The frequency of HD in most breeds has steadily declined. Concurrently, the percentage of animals with excellent hip conformation has steadily increased (Graph 1) in most breeds. Within the OFA population of animals with normal hip conformation, there has been a steady decrease in the percentage of fair and an increase in the percentage of excellent (Graph 2). Within the OFA population of dysplastic animals, there has been a steady increase in the percentage of mild with a corresponding decrease in the percentage of moderate dysplasia (Graph 3).

Graph 1: Percent dysplastic & excellent by birth year
keller graph 01
 Graph 2: Percent excellent vs. fair by birth year
keller graph 02
 Graph 3: Percent mild vs. moderate by birth year
keller graph 03


While this may be surprising to some, it is also important to realize that some of the smaller sized breeds and mixed breeds have as high a percentage of HD as the larger breeds and purebreds. Generalizations that claim that dysplasia is limited to, or more common in, large dogs and pure breed dogs, are misleading.

HD appears to be perpetuated by breeder imposed breeding practices. However, when breeders and their breed clubs recognize HD as a problem and establish HD reduction as a priority, improvement of breed hip status can be accomplished without jeopardizing other desirable traits.

Although it is clear from the graphs that breeders have made steady progress toward reducing the frequency of hip dysplasia, some are concerned that this decline may reach a plateau. As with any polygenic disease, it is anticipated that HD will decline in an exponential manner. Therefore, after several generations, it may appear that progress has leveled out. This is to be expected when phenotypic data is used to place selection pressure against polygenic disease traits with moderate to high heritability estimates. However, Leighton has shown that rapid progress can be expected in the first 3 or 4 generations, and is followed by slower but continued progress in subsequent generations. In the future, a DNA based genetic test might overcome this, but meanwhile breeders can continue to make significant progress by committing to careful selective breeding practices.

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G. G. Keller, D.V.M., MS, Diplomate of A.V.C.R., is the Executive Director of Orthopedic Foundation for Animals, Inc. Dr. Keller received his Doctorate in Veterinary Medicine in 1973 and was in a small animal private practice until 1987 at which time he accepted the Associate Director position for the Orthopedic Foundation for Animals. He received the Masters degree in Veterinary Medicine and Surgery in 1990 and Diplomate status in the American College of Veterinary Radiology in 1994. He assumed the role of Executive Director for the Orthopedic Foundation for Animals in January, 1997.

This article was originally printed on the OFA web site as part of its introductory article, "The use of health databases and selective breeding", and may be viewed in its entirety at http://www.offa.org/. Copyright © 2003 with all rights reserved. This publication cannot be reproduced in any form or by any means without prior written approval from the author(s).