by:  Greg Keller, DVM, MS, DACVR

Hip Dysplasia

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Development of the Hip Joint Clinical Finding of Dysplasia Radiographic Assessment Unilateral Hip Dysplasia Hip Dysplasia Database

Hip dysplasia (HD), literally defined as an abnormal development of the hip joint, was first reported in the dog in 1935 by Dr. G.B. Schnelle. Little to no further information was added to his report over the following decade, due primarily to limited availability of radiographic equipment and radiographic expertise within the veterinary profession.

Popularity of the working dog, particularly the German Shepherd Dog, increased greatly in the late 1940s and the importance of HD became evident to breeders, dog owners, and the veterinary profession. Unrelated, but concurrently, veterinary education underwent an explosion in numbers of veterinary colleges and in quality of specialized education. Rapid advances in the veterinary profession made it difficult for most general practicing veterinarians to remain current with expanding knowledge in animal diseases. To provide the best possible diagnosis and patient care, multiple specialty colleges were formed, including the discipline of radiology which became a recognized specialty in 1966 through the American College of Veterinary Radiology (ACVR).

Hip dysplasia has been reported in man and in most domestic species of animals. In some breeds of dogs and cats, it is the most common cause of osteoarthritis (degenerative joint disease). In recent years, interest in canine HD research has been at an all-time high, as evidenced by the number of conferences focusing on the subject and by the number of new publications in scientific journals and popular magazines.

We now know that HD is a more complex disease than what was first thought. The complexity of the problem is expected to, and has produced, research findings that appear to be contradictory. These research reports, and anecdotal writings that continually appear in the popular press, contribute to confusion and frustration in breeders and veterinarians not familiar with the scientific literature. Thus, few diseases in animals have resulted in such extreme emotional reactions, controversy, or monetary expense as HD.

While it is useful to summarize results from the scientific literature, in the final analysis more research is needed to find answers to the many unresolved questions about HD.

Hip dysplasia is currently accepted to be an inherited disease caused by the interaction of many genes (polygenic). In animals that are genetically predisposed, there are unknown complex interactions of genes with the environment that bring about the degree of phenotypic expression (mild, moderate, or severely hip dysplasia) of these genes within an individual.

At this time, selectively breeding for normal hips is the only means to reduce the genetic frequency of HD.

Radiography is currently the accepted means for evaluating the hip status and it is well documented that the frequency of HD can be significantly reduced using the standard hip extended view.

It is expected that future research studies will refine these currently accepted tenets. For example, advances in molecular genetics may bring about DNA tests to replace radiography as the primary diagnostic tool, or environmental factors such as medical or nutritional treatments may be identified that will overcome the genetic expression of HD in an individual animal.

There are many debates surrounding the myriad of possible factors that may influence or initiate one or more aspect of HD. While interesting to consider, the breeder and veterinarian can most successfully pursue their mutual goals by maintaining their focus on current knowledge without becoming mired in the debate. The responsible breeder attempts to produce the best possible representatives of the breed. The veterinarian assists the breeder in accomplishing this objective by encouraging breeder education, maintaining the general health of the dog and cat, and providing the best possible treatment when appropriate.

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Development of the hip joint

The embryonic hip joint and its supporting structures begin to develop from an undifferentiated mass of embryonic tissue. The differentiation of this tissue into the distinct parts of the hip joints is predetermined by a genetic code. Embryonic tissues form muscles, a specialized connective tissue that encases the joint (the joint capsule), and joint ligaments. A cartilage mold forms the unique parts of the ball and socket joint with the acetabulum functioning as the socket and the head of the femur functioning as the ball. These structures continue to grow and differentiate as the embryo matures. Ossification (bone formation) begins at approximately 49 days of pregnancy but the degree of skeletal maturity at birth appears to be breed dependent. That is, ossification in some breeds is more advanced than in others, which contributes to the continued difference in rates of skeletal growth after birth.

The surfaces of the femoral head and acetabulum are covered with smooth articular cartilage. A thin layer of fluid (synovial fluid) serves as a lubricant for the joint, carries nourishment for the articular cartilage, and separates the opposing surfaces. The head of the femur is attached to the depth of the acetabulum by a ligament (round ligament). The joint capsule encases the joint by attaching to the neck of the femur and to the rim of the acetabulum and is lined by a specialized tissue, the synovial membrane, which produces the synovial fluid. Muscles encase the entire hip structure and serve to stabilize and move the joint. The major pelvic muscles exert a forward and upward pressure on the femoral head during movement and the head of the femur is held in the acetabulum by the pelvic muscles, the joint capsule, surface tension, and the round ligament. Proper development of the joint depends upon the head of the femur being held firmly within the acetabulum.

The hip joint of the dog is reported to be normal at birth. After birth, a complex interaction of multiple genetic and environmental factors can initiate incorrect fit or function of one or more of the parts of the hip joint, although the exact pathogenesis of these interactions is not fully understood at this time. It is likely that these factors may differ between genetic lines, since HD is caused by the interaction of many genes. Currently, any attempt to define the process in an exact sequence of events is speculative.

Regardless of what the initiating interaction of factors may be, abnormal looseness (joint laxity) is generally accepted to be the most common abnormality that results in the pathologic changes of HD. However, some dogs with tight hips but shallow acetabula have also been reported to develop dysplastic changes.

Many of the early (2-14 weeks) pathologic changes are not readily detectable by clinical or radiographic examination. These include: swelling, fraying, and possible rupture of the round ligament; inflammation of the synovial membrane (synovitis) resulting in synovial fluid changes; stretching of the joint capsule; and damage to the cartilage mold of the acetabulum and femoral head. These structural alterations result in joint instability and subluxation, which are followed by erosion of the articular cartilage, changes in the bone beneath the articular cartilage, micro fractures of the dorsal acetabular rim, filling in of the acetabulum, remodeling (change in size, shape or architecture) of the femoral head, neck and acetabular rims, and production of osteophytes (bone spurs) around the joint.

Depending on the individual dog and the initiating factors of joint instability, the changes occur at varying rates and to differing degrees. Severe cases can be detected radiographically as early as 8 to 12 weeks of age, while others may not be evident until later in life (greater than two years of age).

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Clinical finding of dysplasia

While most animals with HD do not exhibit clinical signs, those that do are usually first affected between three and 15 months of age. In some, the signs may not be observed until later in life. The signs vary from decreased exercise tolerance to severe crippling. They include: a reluctance or inability to go up or down stairs, difficulty in rising from a sitting or prone position, bunny-hopping gait when running, stiffness early in the morning that improves as the animal warms up, changes in disposition due to pain, lameness after exercise, a wobbly gait, a clicking sound when walking, and many others. Many animals will shift their center of gravity forward in an effort to relieve weight and pressure on the hips, thereby developing disproportionately greater muscle mass in the front limbs as compared to the rear limbs.

The hip joint is a weakened structure in dysplastic animals and is more prone to injury from normal activities such as jumping off a couch or rough housing with a playmate. Frequently, this results in an acute lameness that appears as if it might have been caused by injury, whereas the underlying dysplasia actually made the joint more susceptible to injury. Obviously, the normal hip can be injured, but radiographic examination can usually distinguish between a hip problem due to dysplasia and one due to other causes.

HD cannot be diagnosed by observing how the animal moves, acts, lies down, etc. Clinical signs may have other causes, and therefore a complete orthopedic and radiographic examination is required before arriving at the conclusion that the signs are caused by HD.

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Radiographic assessment of the hip joint

Modern breeds vary widely in body size, shape and pelvic conformation. Because of these differences, OFA classifications are based on comparisons among individuals of the same breed and age. Knowledge of hip phenotype can be valuable for the breeder in selection against hip dysplasia and in estimating the potential for an active working life. It is assumed that radiographs submitted to OFA are generally screened by the veterinarian and the more obvious cases of HD are probably not submitted. Therefore, the actual frequency of HD in the general population is not known, but has been approximated by Corley (1997) and Rettenmaier (2002) to be higher than reported by OFA. However, the main objective of the OFA is to identify phenotypically normal animals as potential breeding candidates. Thus, the OFA reported breed frequency of HD can be used as a benchmark for breeders to gauge their breeding program's relative position.

Historically, the diagnosis of HD has been determined by radiographic examination of the hips according to the protocol established by the American Veterinary Medical Association. In this standard hip extended position (ventrodorsal view), the animal is placed on its back with the pelvis symmetrical, both femurs extended and parallel, and with the stifles (knees) rotated internally placing the patellas (knee caps) on the midline. The radiograph should include the last two lumbar vertebra and the stifle joints. It is essential, particularly in marginal cases, to obtain proper position and radiographic technique.

The radiographic criteria of subluxation, shallow acetabula, remodeling, and/or secondary degenerative joint disease are well documented. However, interpretation and application of these criteria differ between breeds, age of evaluation and veterinarians. Figure 1 provides the nomenclature of the hip structures that are evaluated by the veterinary radiologist. The veterinary radiologist is concerned with deviations in these structures from the breed normal, and with evidence of subluxation and degenerative joint disease (also called arthritis, osteoarthritis, or osteoarthrosis).

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Multiple anatomic areas of the hip are evaluated (Fig. 1) including:

keller 01

  1. Craniolateral acetabular margin - Area where abnormal bone spurs (osteophytes) develop as the dysplastic joint attempts to stabilize the biomechanically unstable femoral head.

  2. Cranial acetabular margin - Area visualized in conjunction with the hip ball to assess the degree of congruity and confluence of the hip joint.

  3. Femoral head (hip ball) - Assessed to determine its fit into the socket and degree of congruity with the cranial acetabular margin forming the joint space.

  4. Fovea capitus - Normal flattened area on ball for attachment of the round ligament; can be mistaken for degenerative changes if there is lack of familiarity or inexperience in interpretation of hip radiographs.

  5. Acetabular notch - Area visualized to help assess depth of socket or "degree of fit".

  6. Caudal acetabular rim - Area where bone spurs can form.

  7. Dorsal acetabular margin - Area visualized to assess the depth of the hip socket (acetabulum) and percent coverage of the femoral head.

  8. Junction of femoral head and neck - Area visualized to assess size, shape, and architecture of the femoral head/neck. The neck of the hip ball is usually the earliest and most commonly affected area where degenerative changes occur in a dysplastic joint. In the dysplastic joint, new bone builds up at the site of attachment of the joint capsule and muscular attachments. This is a result of abnormal stress created by incongruent articulation of the ball with the acetabulum during movement.

  9. Trochanteric fossa - Area to assess for any microtrabecular bone changes or new bone proliferation.

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Unilateral hip dysplasia

Hip dysplasia may occur in only one hip (unilateral). In man, the left hip is reported to be involved more frequently than the right at a ratio of 10:1. Unilateral dysplasia in dogs follows a similar pattern, but the predominantly affected side is breed dependent. It occurs more frequently in the left hip of the Labrador Retriever, Newfoundland, Akita, and Golden Retriever, but more frequently in the right hip of the Rottweiler. The German Shepherd Dog does not appear to have a side (left or right) predilection. Frequency of unilateral HD is also independent of the frequency of HD in a breed.

The reported frequency of unilateral HD varies from 3% to more than 30% of the dysplastic dogs depending on the population studied. It appears that frequency of unilateral HD is higher in some genetic lines within a breed, than in other lines within the same breed. Furthermore, the same hip (right or left) is repeatedly involved within the line. That is, when several or influential ancestors have unilateral HD in, for example, the left hip then the progeny that are unilaterally affected will almost invariably show the abnormality in the left hip.

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Hip dysplasia database

The OFA hip dysplasia control database functions as a voluntary screening service and as a database of hip status for dogs and cats of all breeds. Information intended to aid breeders in reducing the incidence of this polygenic problem is made available from this resource. The necessity for such a central repository was recognized by the Golden Retriever Club of America and the German Shepherd Dog Club of America, which provided the impetus for formation of the OFA.

The owner or agent should notify the veterinarian, before the x-ray examination, that the purpose is for OFA evaluation. This is best done at the time of making an appointment in order to ensure that application forms are available and that the required procedures are followed. The owner also should provide the animal's registration certificate (or copy of this information) and the animal's tattoo or microchip number at the time of radiography.

General procedures

Age - Only dogs and cats that are 24 months of age or older at the time of radiography can qualify for an OFA breed registry number. The hip joint status of younger animals will be evaluated, but only a preliminary consultation report will be issued.

Restraint - Obtaining a properly positioned film may require chemical restraint. The type of restraint used - physical, sedative, tranquilizer, or general anesthesia - is best determined by the veterinarian. The dog should not be fed on the day of radiography.

Positioning - Dorsal recumbency with the rear legs extended and parallel to each other and the stifles rotated internally is the prescribed position (Fig. 2). This standard ventrodorsal view is accepted worldwide as the basis for evaluation of hip joint status with respect to hip dysplasia. Care should be exercised to be sure the patient is positioned correctly.

Film size - For large and giant breeds of dogs, 14 X 17 inch film size is recommended. Smaller film sizes can be used for smaller breeds if the area between the sacrum and stifles can be included.

Film Identification - Permanent animal identification in the film emulsion is required for radiographs to be eligible for OFA registration. Lead letters, an I.D. camera, or radio opaque tape can be used to identify the film with: a) the hospital or veterinarian's name, date taken and registered name or number of the dog, or b) the veterinarian's or hospital's identification number or case number. In this latter case (b), the radiograph must be accompanied by a signed note from the veterinarian referring to such film by its identification number, and stating the date taken, and registered name or number of the dog as in (a) above.

If the above required information is illegible or missing, the OFA cannot accept the film for registration purposes.

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Figure 2

keller 02

A standard position radiograph of the pelvis that has been appropriately positioned will have symmetrical obturator foreamen (long arrow), symmetrical wings of the ilium (arrowhead) and kneecaps that are centered over the knees (short arrow) with the legs extended parallel to one another.

Figure 3

keller 03

A standard position radiograph of the pelvis that has been inappropriately positioned will have asymmetrical obturator foreamen and asymmetrical wings of the ilium (long arrow). The distortion caused by poor positioning can inaccurately make one hip look worse than it actually is by creating a more shallow appearing hip socket (short arrow) and the opposite hip appear better than it actually is by creating more depth to the hip socket over the hip ball (arrowhead). The OFA will routinely mail poorly positioned films back to the referring veterinarian and request repeating the study.

Exposure - Good contrast is essential. Technique settings (low kVp and high mAs), film-screen combinations and use of grids are all considered in producing the desired contrast. Film contrast should be such that the microtrabecular pattern of the femoral head and neck are readily seen. The dorsal-lateral margin of the acetabulum must also be visible.

Radiation safety - Proper collimation and protection of attendants are the responsibility of the veterinarian. Gonadal shielding is recommended for male dogs. Radiography of females in season or pregnant should be avoided.

Application information - The owner or agent should complete and sign the OFA application form, and the information is best obtained directly from the animal's certificate or registration papers. It is also important to record the animal's tattoo or microchip number, and registration numbers of the sire and dam. Application forms are available on request from the OFA or can be downloaded from the OFA web site (www.offa.org). The radiograph, signed application form (which should include the owner's choice of open or semi-open database), and the service fee should be mailed to: Orthopedic Foundation for Animals, Inc., 2300 E. Nifong Blvd., Columbia, MO 65201-3856. All radiographic images are retained by the OFA for research and reference purposes.

Operational procedures

When a radiograph arrives at the OFA, the information on the radiograph is verified against information on the application form. The age of the dog in months is calculated and the submitted fee is recorded. The veterinary radiologist on staff at the OFA then evaluates the radiograph for diagnostic quality. If it is not of suitable diagnostic quality (the hip is tilted, too light or too dark, etc.) it is returned to the referring veterinarian with a written request that it be repeated (Figure 3). If the radiograph is accepted for evaluation, it is assigned an application number and given a "quality control" hip rating.

There is a pool of 20 to 25 board certified veterinary radiologists throughout the USA in private practice and academia that consult for the OFA. The radiographic images are forwarded to 3 radiologists. Each evaluation is independent -that is, no radiologist knows what interpretation was given by another. The only information they have is the radiograph, application number, breed, sex, and age. The breed, age, and sex of dog are important for the radiologists to know so that normal conformational differences among and within breeds, and differences related to degree of skeletal maturity, can be taken into consideration. Each radiologist grades the hips into one of seven phenotypic hip conformation categories: excellent, good, or fair (which are normal and receive an OFA hip number); borderline; or mild, moderate, or severe (which are abnormal). When results of over 1.5 million radiographic evaluations by 35 radiologists were analyzed, it was found that all 3 radiologists agreed as to whether the dog/cat should be classified as having a normal phenotype, borderline phenotype, or HD 94.9% of the time. In addition, 73.5% of the time, all 3 radiologists agreed on the same hip phenotype (excellent, good, fair, borderline, mild, moderate, or severe).

When the final evaluation is completed, the consensus of the three evaluations is formulated. Two evaluations of the same phenotype result in a consensus of that phenotype; 3 different evaluations (i.e., excellent, good, and fair) result in a consensus of the middle phenotype. If the consensus is phenotypically normal (excellent, good, or fair) an OFA registry number is assigned. The owner of record, referring veterinarian, AKC, and appropriate breed club are notified of the evaluation results. Dysplastic results are not in the public domain unless the owner of record gives explicit direction for the release of such information by initialing the appropriate space on the application form.

The time it takes to obtain three independent evaluations, arrive at the consensus, and type the final OFA report is dependent on a number of factors. It takes approximately a week to 10 days for the film to arrive at OFA via the mail service. Depending on the case load it takes 12 to 14 days from the time that OFA receives the film to completion of the consensus report.

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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).