WebCanine.com » PennHip vs OFA

hip dysplasia, identification, treatment, research, and breeding

mom — Sun, 05 Dec 2010 03:54:01 +0000

Hip Check:In the battle against canine hip dysplasia, identification, treatment, research, and careful breeding selection are the weapons of choice.
First printed in the July 2002 issue of the AKC Gazetteby Jerold S Bell, DVM, Tufts University School of Veterinary Medicine

For affected dogs, hip dysplasia can be a debilitating and painful disease. It has been one of the fancy’s great challenges to combat and treat this hereditary developmental disorder, whose signs can include hip-joint pain, hind-limb weakness, lameness, exercise intolerance, degenerative joint disease, and arthritis. The disorder can include several abnormalities of the hip joints, such as joint laxity, anatomical abnormalities, and a predisposition to arthritis. While hip dysplasia is commonly perceived to be a disorder of larger dogs, it also occurs in small breeds, mixed-breed dogs, and even cats. The Pug, for example, has a significant frequency of affected dogs, while the Siberian Husky has a relatively low frequency of dysplasia.

Most experts agree that the majority of dogs that develop hip dysplasia have outwardly normal hips when they are very young, and develop the anatomical or laxity changes associated with the disorder during the first year or two of life. Initial symptoms may appear from 4 months to 1 year of age,

and include joint pain, a swaying and unsteady gait, “bunny hopping”, difficulty rising from a sitting position, difficulty with stair-climbing, and an aggravation of these signs with exercise.

Diagnosing the Disease
There are several methods used to diagnose hip dysplasia. The standard procedure is the extended-leg (x-ray). This is a radiograph of the pelvis and hip joints taken with the dog on its back, with hind legs extended. This hip-evaluation method is the one used by the Orthopedic Foundation for Animals (OFA), as well as by most European and Canadian dysplasia-control programs.

The hip is a ball (femoral head) and socket (acetabulum) joint. The OFA evaluates the hip radiograph for nine anatomical aspects. These include; a round femoral head, a deep acetabulum, a prominent notch in the femoral neck, a straight up-and-down cranial rim of the acetabulum, and minimal joint laxity. Good hip conformation is determined by imagining a line (dashed line in the drawing) connecting the outer edges of the acetabulum, and observing at least half of the femoral head enclosed within the acetabulum. Bony remodeling and arthritic changes will fill in the notch at the femoral neck, and cause “lipping” – proliferation of bone at the cranial acetabular rim.

OFA will certify a dog’s hips (at 2 years of age or older) as excellent, good, fair, or borderline, or as mildly, moderately or severely dysplastic.

The British Veterinary Association/Kennel Club (BVA/KC) evaluates the same extended-leg radiograph at 1 year of age or older. Unlike OFA’s rating system, it separately scores the nine anatomical aspects of the pelvic radiograph for each hip. The nine scores are added up for each hip, then totaled for the dog’s final rating. A perfect rating would be zero; the worst would be 106 (53 for each hip).

The Pennsylvania Hip Improvement Program (PennHIP) method of evaluating the hip joint is based on laxity alone. This method utilizes two separate radiographs on an anesthetized dog to record the hips in compressed and distracted views. The distracted-view radiograph (to the right) is taken while applying a uniform force on the hips to measure the maximum distractibility of the hip joints. (Distractability is the distance that the soft tissue allows the head of the femur to come out of the acetabulum.) The measured difference between the compressed and distracted views is used to compute a distraction index (DI). A DI of zero indicates no laxity of the hips, and a DI of 1.0 indicates luxation of the hips. Dogs with a DI of under 0.3 almost always have normal hips, and those over 0.7 are almost always dysplastic. PennHIP was designed to create a selection tool for tighter hips. By computing a breed average of distractibility, and selecting for tighter hips than the breed average, it is believed that the incidence of hip dysplasia should decrease over time.

Fighting Back With Breeder Selection
Hip dysplasia is considered a moderately inherited disorder, with researchers computing heritability values of 28 to 40 percent. This means that 28 to 40 percent of the variation between affected and unaffected relatives is due to genes. While it is more difficult to manage disorders with this level of heritability, many traits in this range improve with proper selection.

Since hip dysplasia is a polygenic disorder controlled by several gene pairs, the disease affects individual dogs due to different genetic combinations. The two dogs in the radiographs to the left have severe hip dysplasia. The one at the top has severe laxity, while the other (bottom) has tight hips, but with shallow acetabula and severe bony changes. Both these dogs have hip dysplasia, but due to different genetic causes.

One reason selection offers limited results is that many breeders are selecting dogs based solely on their OFA hip rating, and not on the specific aspects of the hip radiograph. If a dog receives a fair hip-rating due to some shallowness in it’s acetabula, it should be bred to a dog with deep acetabula (in addition to considering all other factors if it is going to be bred). If a dog’s hips have demonstrable laxity, then it should be bred to a dog with tight hips.

The BVA/KC’s dysplasia rating allows breeders to identify precisely how their dog’s hip-rating points are calculated. With this system it is easier to select prospective mates that will correct and compliment the different elements of hip joint conformation. With the OFA system, it is up to the individual breeder to work with their veterinarian to break down the hip radiograph into these separate components. By selecting for individual components of the hip radiograph, you may be more directly selecting for specific “normal-hip” genes.

The most important factor in selecting against a polygenic disorder like hip dysplasia is to seek breadth of pedigree. Most breeders select normal parents with normal grandparents, and expect to produce all normal offspring. This is selection based on depth of pedigree. With polygenic traits, the hip status of breeding dogs’ siblings better represents the range of genes that can be present. With breadth of normalcy in the littermates of breeding dogs, and even of parents of breeding dogs, you are more efficiently selecting for a preponderance of those “normal-hip” genes.

To help control the disorder, the OFA has a longstanding hip dysplasia registry. Until recently, results were available only for dogs with normal-hip certification. The OFA has now moved to a semi-open registry, which allows owners the option of having their dogs’ hip status posted on the OFA website (www.offa.org), regardless of a normal or affected certification. The Institute for Genetic Disease Control (GDC) hip registry was developed with the open-registry concept, and, as of summer 2002, will be merged into the OFA database. Through the OFA’s online searchable registry, you can find the hip status of littermates and relatives to determine normality in the pedigree breadth. Both the OFA and PennHIP radiographic methods can report low levels of false positive and false negative predictions for future dysplastic development. The OFA radiograph documents anatomical abnormalities (shallow sockets, early bony-changes), but only natural laxity in a hip-extended view. The PennHIP radiograph documents maximum distractibility, but there are dogs with DIs between 0.3 and 0.7 that can end up being clinically normal or affected. For both methods, radiographic findings at an early age are highly correlated to dysplasia at a later age. OFA offers preliminary evaluations at any age, but does not give permanent certification until two years of age, when over 95 percent of affected dogs will show radiographic signs.

Treatment and Prevention
One significant factor in the development of hip dysplasia is the nutritional load, especially in growing large-breed dogs. Feeding high-calorie puppy food promotes rapid bone growth and weight gain. The soft-tissue components of the hips don’t mature and grow at the same rate as the bones. Often, by the time these soft-tissue components catch up, there can be bony deformity, due to a period of unstable hip joints. By switching to the lower-calorie large-breed puppy growth food, or switching to adult dog food after fourteen weeks of age, the growth rate can be slowed, and all of the components of the hip joints can mature in unison. The adult size of the dog is genetically determined, and reduced-calorie feeding will only alter the age when this size is attained.

Excessive jumping and compaction activity on the hip joints during the critical growth periods prior to skeletal maturation can also affect the degree of later dysplastic development in genetically predisposed dogs. Changing feeding protocols and managing excessive environmental stress will probably not prevent hip dysplasia in genetically predisposed dogs, just as reasonable overnutrition and activity will probably not cause hip dysplasia in genetically normal dogs. However, modifying feeding practices can alter the degree or severity of clinical signs in affected dogs. Breeders should evaluate prospective breeding dogs that have been raised under fairly uniform conditions, so that any differences between them are due to heredity, not environmental influences.

There are no scientifically proven drugs, vitamins, or food supplements that will protect the hips of genetically predisposed dogs from developing hip dysplasia. Joint-formula compounds (including glucosamine and chondroitin) are shown to diminish hip-joint pain in dogs. Nonsteroidal anti-inflammatory drugs (such as aspirin, Rimadyl, and Etogesic) are also effective for treating joint pain. Owners should discuss with their veterinarian which medications are appropriate for their dog’s condition. Avoiding overfeeding, and maintaining a lean body weight will also diminish hip pain in affected dogs.

There are two types of early-intervention surgery that attempt to prevent the progression of hip dysplasia in young dogs. These are designed to improve the integrity of the hip joints when there are shallow hip sockets or significant joint-laxity. They both act to rotate the acetabulum upward and outward, so it has greater coverage and support for the head of the femur. With a triple pelvic osteotomy (TPO), three cuts are made in the pelvis with a bone saw to isolate the hip socket. It is then rotated and reattached with metal plates. This surgery must be performed before any arthritic changes have begun.

The other surgery is an experimental procedure called a juvenile pubic symphysiodesis (JPS). An electro-scalpel is used to close the growth plate on the floor of the pelvis. With normal growth occurring in the rest of the pelvis, the hip sockets then rotate outward. This procedure must be performed on dogs between 12 and 20 weeks of age, before significant pelvic growth has occurred. The affect of the procedure beyond 2 years of age has not been studied.

Both the TPO and JPS require the early identification of candidates for surgery, before the bony changes of hip dysplasia occur. This creates a problem, as there is no accepted diagnostic test designed to predict with high certainty which dogs will develop debilitating hip dysplasia that will require surgery. Several surgeons recommend early intervention surgery based on a PennHIP measurement of laxity in young dogs. Dr. Gail Smith, professor of orthopedic surgery at the University of Pennsylvania School of Veterinary Medicine, believes that this is a misuse of the technique. “PennHIP was designed as a selection tool to quantify a probability or risk factor for developing later hip dysplasia,” says Smith, who developed the PennHIP method. “The technique wasn’t designed as an indicator for surgery.” He feels that a dog being considered for any type of hip dysplasia surgery should be demonstrating some clinical symptom of the condition.

The TPO procedure has a longer track record to measure its outcome in older dogs. While research shows that in many cases the procedure does not stop the radiographic progression of hip dysplasia or arthritis, dogs who have undergone the surgery appear to experience less discomfort. However, since studies have shown that 76 percent of all dogs with radiographic signs of hip arthritis do well without surgery, controlled studies still need to be undertaken to determine the true value of these early intervention surgeries.

There are two accepted surgical procedures for removing the pain and lameness caused by hip dysplasia: femoral head and neck excision, and total hip replacement. Both of these surgeries are considered salvage procedures, as they remove the arthritic bone-on-bone contact of the hip joint, thus relieving the pain associated with it.

A femoral head and neck excision removes the ball from the ball-and-socket joint, and uses the muscles of the pelvis to support the hind leg. This procedure works well in most dogs up to 50 pounds, though heavier dogs could also be helped. It requires good muscle strength in the leg and buttocks since these, rather than bone, provide the support.

Total hip replacement, while the more aggressive surgery, works very well on affected dogs. The head of the femur is replaced with a metal implant, and the acetabulum is replaced with a synthetic plastic implant. With updated materials and techniques, dogs receiving total hip replacement have few complications, and return to normal function without pain. Any dog whose condition is severe enough to require surgery should be spayed or neutered at the same time.

Current research
In addition to the exploration into techniques to identify and treat dogs with hip dysplasia, research is being conducted to find the genetic causes of the condition. Dr. George Brewer, of the University of Michigan Medical School, is conducting research to find genes that cause canine hip dysplasia. Using 12 breeds, he is investigating candidate genes that code for hip-related functions such as physiology and connective tissue. “We are hoping to find one or two major trigger genes for hip dysplasia in each breed,” says Brewer, whose research is supported by a grant from the AKC Canine Health Foundation.

At the Cornell University College of Veterinary Medicine, Dr. Rory Todhunter is attempting – in a different way – to identify genes that cause hip dysplasia. He bred normal Greyhounds to dysplastic Labrador Retrievers, and then bred their offspring back to either normal Greyhounds or dysplastic Labrador Retrievers. Through manipulating the genes in this breeding scheme, he is trying to identify hip dysplasia-causing genes in the normal and dysplastic crossbred offspring. The goal of both of these research efforts is to develop genetic tests that can be used to select for genetically normal breeding-dogs.

Canine hip dysplasia continues to be a serious disorder across breed lines. As breeders and owners learn the proper techniques to decrease the frequency of producing affected dogs, we can anticipate significant progress in the reduction of this damaging and costly condition.

Hip Dysplasia by Breed (OFA Statistics):

AKC Breeds Most Affected:		AKC Breeds Least Affected:
Bulldog 73.4%		Australian Terrier 0.0%
Pug 60.8%		Borzoi 1.8%
Otterhound 51.2%		Saluki 1.9%
Clumber Spaniel 49.6%		Siberian Husky 2.1%
Neopolitan Mastiff 47.6%		Ibizan Hound 2.2%
St. Bernard 47.0%		Canaan Dog 2.4%
Sussex Spaniel 41.2%		Pharoah Hound 2.7%
Bassett Hound 28.6%		Belgian Sheepdog 2.9%
Newfoundland 26.8%		Schipperke 3.0%
Bloodhound 26.1%		Basenji 3.0%

Canine Hip Dysplasia May Be Underreported,

mom — Sun, 05 Sep 2010 18:36:53 +0000

According To Penn Vet Comparative Study
04 Sep 2010

A study comparing a University of Pennsylvania method for evaluating a dog’s susceptibility to hip dysplasia to the traditional American method has shown that 80 percent of dogs judged to be normal by the traditional method are actually at risk for developing osteoarthritis and hip dysplasia, according to the Penn method.

The results indicate that traditional scoring of radiographs that certify dogs for breeding underestimate their osteoarthritis susceptibility. The results are of clinical importance to several populations, most notably veterinarians, breeders and pet owners.

The two hip screening methods — the standard Orthopedic Foundation for Animals, or OFA model, and Penn Vet’s PennHIP model — were applied to a sample of 439 dogs older than 2 years. The four most common breeds included in the study were German shepherds, Labrador retrievers, golden retrievers and Rottweilers, all breeds commonly susceptible to hip dysplasia.

According to Penn researchers, even if breeders were to selectively breed only those dogs having OFA-rated “excellent” hips — the highest ranking but in some breeds, a very small gene pool, the study suggests that 52-100 percent of the progeny, depending on breed, would be susceptible to hip dysplasia based on the Penn Vet scoring method.

“We believe the lower rates of hip laxity detection using the OFA methods are not the fault of the expert radiologist reading the radiograph but rather a deficiency of the radiographic view,” said veterinary surgeon Gail Smith, professor of orthopaedic surgery, lead author and director of the PennHIP Program. “We believe many veterinarians are not using the best test to control a disease. In many ways this is an animal-welfare issue.”

The findings point to a weakness in current breeding practices. If breeders continue to select breeding candidates based upon traditional scores, then, according to the Penn study, breeders will continue to pair susceptible dogs and fail to improve hip quality in future generations. Despite well intentioned hip-screening programs to reduce the frequency of the disease, canine hip dysplasia continues to have a high prevalence worldwide with no studies showing a significant reduction in disease frequency using mass selection.

Canine hip dysplasia, or CHD, is defined by the radiographic presence of hip joint laxity or osteoarthritis with hip subluxation (laxity) early in life. A developmental disease of complex inheritance, it is one of the most common orthopaedic diseases in large and giant-breed dogs and causes pain and loss of mobility.

The traditional OFA screening method relies heavily on conventional hip-extended, or HE, radiographs, which the study contends do not provide critical information needed to accurately assess passive hip joint laxity and therefore osteoarthritis susceptibility.

“We suspect that all hip-screening systems worldwide based on the HE radiograph have similar diagnostic deficiencies,” Smith said. “Hopefully, our results will motivate veterinarians and breeders to consider this newer approach.”

To achieve genetic control of CHD, researchers said, an accurate test must minimize false-negative diagnoses which mistakenly permit the breeding of dogs that carry genes coding for CHD. Particularly for a late-onset disease such as CHD, dogs remaining in the gene pool must not only be free of obvious signs of CHD at the time of evaluation (2 years of age for OFA) but ideally should not be susceptible to the osteoarthritis of CHD that occurs later in life.

The PennHIP method quantifies hip laxity using the distraction index, or DI, metric which ranges from a low of .08 to greater than 1.5. Smaller numbers mean better hips. The PennHIP DI has been shown in several studies at multiple institutions to be closely associated with the risk of osteoarthritis and canine hip dysplasia. It can be measured as early as 16 weeks of age without harm to the puppy.

Specifically, the PennHIP method considers a DI of less than .3 to be the threshold below which there is a near zero risk to develop hip osteoarthritis later in life. In contrast, dogs having hip laxity with DI higher than .3 show increasing risk to develop hip osteoarthritis, earlier and more severely, as the DI increases.

Comparing the overall results of the study, 52 percent of OFA-rated “excellent,” 82 percent of OFA-rated “good” and 94 percent of OFA-rated “fair” hips all fell above the PennHIP threshold of .3, making them all susceptible to the osteoarthritis of CHD though scored as “normal” by the OFA. Of the dogs the OFA scored as “dysplastic,” all had hip laxity above the PennHIP threshold of .3, meaning there was agreement between the two methods on dogs showing CHD or the susceptibility to CHD.

The key feature of the PennHIP radiographic method is its ability to determine which dogs may be susceptible to osteoarthritis later in life. Because dogs are recognized as excellent models for hip osteoarthritis in humans, the authors are interested in the prospect of applying this technology to humans. Knowing a dog’s risk for osteoarthritis early would allow veterinarians to prescribe proven preventive strategies, like weight loss, to lower the risk of this genetic disorder. Also, dog breeders now have a more informative measure to determine breeding quality to lower the risk of hip osteoarthritis in future generations of dogs.

“In humans, with appropriate studies of course, it is conceivable that mothers of susceptible children – and there are many – may adjust a child’s lifestyle, including diet, to delay the onset or lessen the severity of this genetic condition,” Smith said.

PennHIP is currently in common use by service-dog organizations such as the U.S. Air Force, the U.S. Army and numerous dog-guide schools. There are approximately 2,000 trained and certified members currently performing PennHIP procedure worldwide.

The study was conducted by Smith, Michelle Y. Powers, Georga T. Karbe, Thomas P. Gregor, Pamela McKelvie, William T. N. Culp and Hilary H. Fordyce of the Department of Clinical Studies at Penn Vet. Culp is currently with the School of Veterinary Medicine at the University of California, Davis.

The study was funded by the University of Pennsylvania, the National Institutes of Health, The Seeing Eye Inc., the Morris Animal Foundation and Nestle Purina Co. The article was published in the Journal of the American Veterinary Medical Association.

Smith, who is the inventor, and the University of Pennsylvania, which holds the patent, have a financial interest in the PennHIP method.

A video in which Dr. Gail Smith describes this study is available here.

Source:
Jordan Reese
University of Pennsylvania

Article URL: http://www.medicalnewstoday.com/articles/199963.php
Medical News Today

Intro to PennHip 2

mom — Wed, 02 May 2007 03:06:18 +0000

The PennHIP method is a novel way to assess, measure and interpret hip joint laxity. It consists of three separate radiographs: the distraction view, the compression view and the hip extended view.

It is the amount of displacement of the femoral head from the acetabulum during distraction radiography that has been termed passive hip laxity and that has been shown to be directly related to the probability that a hip will develop degenerative joint disease characteristic of hip dysplasia. The distraction view and compression view, are used to obtain accurate and precise measurements of joint laxity and congruity. The hip-extended view is used to obtain supplementary information regarding the existence of degenerative joint disease (DJD) of the hip joint. (The hip-extended view is the conventional radiographic view used to evaluate the integrity of the canine hip joint.)

The PennHIP technique is more accurate than the current standard and it has been shown to be a better predictor for the onset of DJD. The radiographs below are of the same dog, yet the hip joint laxties in each view look very different. Notice that the hips in the distraction view appear to be much looser than they do in the hip-extended view.

The obvious contrast in joint laxity between the distraction and hip-extended views demonstrates the fundamental difference between the two radiographs. The looser the joint on the distraction view, the greater is the chance that the hip will develop DJD. The hip-extended view tends to mask true hip joint laxity because the joint capsule is wound up into a tightened orientation when the hips are extended. This explains why measurable joint laxity on the distraction view is always greater than the measurable laxity from the hip-extended view. In fact, distraction laxity is up to 11 times greater depending on the breed of dog under study. To see how joint laxity is actually measured and interpreted, go to the PennHIP Research section on the PennHIP site.

The compression view is used to determine the “goodness of fit” of the femoral heads into the acetabula. In a hip with DJD, the remodeling that occurs in the acetabulum and/or the femoral head, will often result in an ill-fitting “ball” and “socket” (see Primer of Canine Hip dysplasia for an example of DJD).To summarize, the PennHIP method:

- Obtains DJD readings from the standard hip-extended view
- Obtains hip joint congruity readings from the compression view
- Obtains quantitative measurements of hip joint laxity from the distraction view

Why do the hips appear looser on the distraction view?

For a brief review of the anatomy of the canine hip joint, see the Primer on Canine hip dylsplasia article.

A ball and socket joint has the ability to rotate about the three orthogonal (XYZ) axes. With respect to the hip, the femur can rotate back and forth (flexion/extension), medial and lateral (adduction/abduction), and can rotate “toe in”/”toe out” (internal/external rotation). The diagram at the right illustrates these points. Palpation of the hip also demonstrates that the femoral head can be translated (displaced) laterally. That is, it moves out or away from the acetabular cup. This lateral displacement, although an important property of the hip joint, had not been adequately studied prior to 1983. The amount of the displacement is simply the laxity of the joint.The maximum amount of displacement of the femoral head in the neutral position is not limited by the length of the round ligament as once thought. Instead, it is dependent on the relative volume of the synovial fluid in the joint, acting in combination with the joint capsule.

Biomechanical testing in the laboratory showed that femoral head displacement is greatest when the femur is in a position approximating a neutral or “standing” position, also called the stance-phase of weight bearing. The legs are placed in this position for the distraction view.In the familiar hip-extended view, the femur is pulled into extension (see the diagram at left). The fibrous elements of the joint capsule get “wound up” so to speak, so that the resulting tension serves to drive the femoral head into the acetabulum (socket). This explains why the measurable joint laxity on the distraction view is always greater than the measurable laxity from the hip-extended view.It follows that a loose-hipped dog, such as the one at the beginning of this page, could falsely be considered to have “tight” or “normal” hips according to the hip-extended radiograph. Such a dog would likely be approved for breeding and therefore could pass this loose-hipped trait onto its offspring.

In the schematic below, figure A depicts a normal joint in the neutral position. The joint capsule and the round ligament are lax (relaxed, but not stretched) when there is no distractive force being applied. When a force is applied (figure B) the joint capsule invaginates reflecting a pressure differential (vacuum effect) across the joint capsule. The relative degree of displacement is dependent upon the amount of synovial fluid present. More synovial fluid volume means greater hip laxity. Notice that the round ligament remains lax in this position. In a dog with excessive synovial fluid volume (figure C) the capsule does not invaginate and the magnitude of femoral head displacement is limited by the the joint capsule, the surrounding musculature and the round ligament. The distraction radiographic procedure shows the relative displacement of the femoral head from the acetabulum when a distractive force is applied, however the synovial fluid and joint capsule are transparent to x-rays and therefore are not visible on the radiograph. It is the amount of displacement of the femoral head from the acetabulum during distraction radiography that has been termed “passive hip laxity” and that has been shown to be directly related to the probability that a hip will develop degenerative joint disease characteristic of hip dysplasia.

Summary of Pennhip research

mom — Wed, 02 May 2007 01:12:12 +0000

A detailed summary of how the PennHIP analysis is conducted for canines is presented. The reliability of method, risk of developing degenerative joint disease, the heritability of the disorder and the bio-mechanics of joint laxity will be discussed. The following is a summary of PennHIP-related research published by Smith and colleagues.

References to the actual publications are given at the end of each section. A complete reference list can be found in the Scientific Reports section.

Measuring Hip Joint Laxity

A unique method for the measurement of hip joint laxity, using an index, was developed for the PennHIP compression and distraction views. The method is quantitative (i.e., it assigns a number to joint laxity) as opposed to being qualitative or subjective where an ordinal score is used (e.g. excellent, good, fair, etc.). the former is not as vulnerable to inter- and intra-observer errors commonly associated with subjective measurement systems.

The index method is calculated by superimposing precision-machined circle gauges on the cortical margins (rims) of the acetabulum and femoral heads (see example at left) to find the respective geometric centers. On the compression view (see below), if the joint is free of osteoarthritis, the centers of the acetabulum and femoral head should coincide indicating that the joint is indeed concentric. On the distraction view, the distractive force causes separation between the centers. The distance, d, between the centers is a measure of hip joint laxity. However, d also varies with dog size (larger dogs would likely have larger d’s than smaller dogs), with age of the dog, and with magnification due to variation in hip-to-film distance. To circumvent these potential sources of variation, d is normalized with respect to all sizes of femoral heads and acetabula by dividing it by the radius of the femoral head, r. The resulting index, I = d/r, is a unitless number ranging from 0 to 1 (or more). The laxity index computed for the compression view is called a compression index (CI), likewise, the laxity index for the distraction view is called the distraction index (DI). The distraction index (and the compression index) is a measurement of hip joint laxity. It does not allude to a passing or failing score. Hips with DIs on the distraction view that are close to 0 are considered to be tight, while DIs close to 1 are considered to be very loose. The DI is an indication of the “percent out of joint” that the femoral head is displaced from the acetabulum. For example, DI=0.58 means the femoral head comes out of the joint by 58%, DI=0.75, 75% out of joint (see schematic above), and so on. This also makes interpretation of the DI more intuitive: a hip with a DI=0.50 is twice as lax as a hip with a DI=0.25.

To obtain proper diagnostic radiographs, the musculature around the hip must be completely relaxed and so the dog must be under deep sedation or general anesthesia. Laxity as determined by the DI is therefore called passive hip laxity, as opposed to functional hip laxity which is the pathological form of hip laxity that occurs in dysplastic hips during weight bearing. (Clearly, functional hip laxity is of greater diagnostic interest, but there are presently no means to measure it.)

Reference

Smith GK, Biery DN, Gregor TP. New concepts of coxofemoral joint stability and the development of a clinical stress-radiographic method for quantitating hip joint laxity in the dog. J Am Vet Med Assoc 1990;196(1):59-70.

Reliability of the Method

In a longitudinal study, dogs were radiographed in the standard hip-extended position and the compression/distraction position at 4, 6, 12, 24 and 36 months of age. The hip integrity was evaluated by three different methods.

1. The hip-extended view was subjectively evaluated by a board-certified veterinary radiologist using the 7-point OFA scoring scheme (Excellent, Good, Fair, Borderline, Mild HD, Moderate HD, Severe HD).

2. The hip-extended view was also evaluated quantitatively for laxity with the Norberg angle (NA). (Please refer to the reference at the end of this section for information about the NA.)

3. The distraction view was evaluated for passive hip laxity as measured by the DI.

One of the objectives of this study was to assess each method’s diagnostic reliability over the different time periods. That is, the comparability, or closeness, of one measurement at one time period to the same type of measurement at a different period in time was investigated. The data were statistically analyzed using the intra-class correlation coefficient (ICC), a statistic that is used to determine comparability or “sameness” between groups of data. The ICC is a number ranging from -1 to 1. The closer the ICC is to 1, the greater the degree of comparability. (Note: a special type of ICC, called the kappa statistic, is used for categorical data such as the subjective scoring scheme. The interpretation of kappa is the same as for the ICC.)

The table below summarizes the main results. For brevity, only the results from the 4 month and 12 month readings compared to the reading at 24 months are given.

	4 vs 24months	12 vs 24months
Subjective (OFA)	0.08*	0.39
Norberg Angle	0.51	0.78
Distr Index (DI)	0.85	0.91
* indicates statistic not significant (at 0.05 level)

The high ICC values derived from the distraction indices indicate that the DI was clearly more comparable than the corresponding subjective score (OFA score) or NA measurement. In other words, the DI at 4 months was much the same as the DI at 24 months. For this reason, the DI is considered the most reliable assessment of hip laxity for a dog as young as 16 weeks old. A similar study from an independent laboratory has recently been published confirming these results (see reference below). A recently published study from Penn of 8-week-old German shepherd puppies revealed that the DI, at 8 weeks, did not strongly correlate to DIs’ at 12 or 24 months of age. It was concluded that hip evaluation in German shepherd dogs should not begin before 16 weeks of age. Other breeds of dogs require similar longitudinal evaluation to determine the earliest meaningful age for radiographic testing. Interestingly, the OFA score at 4 months was not capable of predicting later hip score meaning that in this study there was no scientific support for using preliminary OFA score at 4 months as an early screening tool. Even when comparing 12-month OFA scores with 24-month OFA scores, the correlation of 0.39, although statistically significant, was too low to be clinically useful.

In Summary

The ability to receive an early estimate of a dog’s hip laxity is important whether the dog’s intended purpose will be for breeding, for working or as a family pet. The data, which are regularly compiled and analyzed, will allow breeders to identify the members of their breeding stock with the tightest hips, thereby facilitating optimum selection.

References

Smith GK, Gregor TP, Rhodes WH, 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 in dogs. Am J Vet Res 1993;54(7):1021-1042.

Adams WM, Dueland RT, Meinen J, O’Brien RT, Giuliano E, Nordhein EV: Early detection of canine hip dysplasia: comparison of two palpation and five radiographic methods. J Am An Hosp Assoc 1998; 34(4): 339-347.

The Risk of Developing DJD

For more than 35 years, it has been empirically accepted that hip joint laxity is related to the development of DJD. However, prior to the research conducted at the University of Pennsylvania, there existed little or no scientific evidence to support this view. The problem was approached in two ways: First, the relationship of hip joint laxity with the coexistence of DJD in a cross-section of adult dogs was examined (cross-sectional study). Next, the relationship of laxity at an early age with the appearance of DJD at a later time was investigated (longitudinal study). The results are summarized below.

Cross-sectional Study

An analysis of 142 dogs (mean age of 20 months) showed a direct relationship of hip laxity (as determined by the DI) to the radiographic existence of DJD. Hips with low DI’s, i.e. “tight hips”, were very unlikely to exhibit DJD. In this study only one hip with a DI less than 0.30 exhibited any evidence of DJD (DI = 0.29). The converse was not true; that is, not all hips with a DI greater than 0.30 necessarily showed radiographic evidence of DJD. However, as the DI increased, there was an increase in the frequency of DJD. (see the graph below)

It is interesting to note that more than 50% of the hips in this study had distraction indices below 0.30, yet only one hip in this group < 0.30 showed any radiographic evidence of DJD. There appears to be a cut-off point – DI approximately 0.30 – below which the canine hips is not susceptible to getting DJD.

Longitudinal Study

In this study, dogs were radiographed at 4 months, 12 months, 24 months of age. A logistic regression model was invoked to determine the contribution of factors such as DI, Norberg Angle (NA), subjective score (OFA), weight and sex at 4, 12 or 24 months to the risk of developing DJD at or before 3 years. The analysis indicated that the DI at all age groups was the most significant prognostic factor and that the strength of the predictive power improved with age. The sex, weight, NA and subjective (OFA) score were not found to be significant factors in this study.

The graph at below is from a study involving German Shepherd dogs. It demonstrates how the DI at a given age is associated with the probability for developing DJD later on. The DI can be thought of as a risk factor for the development of DJD. Tight-hipped dogs (those with small DI’s) are at a low risk and loose-hipped dog’s (those with high DI’s) are at a high risk. In other words, the larger the DI (at 4 months), the greater the risk for developing DJD by three years of age.

The results represented in this graph are for German Shepherds. For example, a separate probability curve exists for Rottweilers. However, regardless of the breed, the “tight hip / loose hip” susceptibility to DJD still holds.

Note: The DI as a risk factor for the development of DJD is analogous to the association of serum cholesterol with the risk of developing heart disease. The higher a person’s cholesterol level, the greater is the risk that the person will develop heart disease. Not all individuals, however, with high cholesterol level will necessarily have heart disease. Yet to be on the safe side most people would choose to have low rather than high serum cholesterol levels because the odds are in their favor that low cholesterol means better cardiovascular health. Similarly tighter hips equate to greater DJD resistance.

References

Cross Sectional Study

Smith GK, Gregor TP, Rhodes WH, 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 in dogs. Am J Vet Res 1993;54(7):1021-1042.

Longitudinal Study

Smith GK, Popovitch CA, Gregor TP, Shofer FS. Evaluation of risk factors for degenerative joint disease associated with hip dysplasia in dogs. J Am Vet Med Assoc 1995;206(5):642-647. Smith GK, Gregor TP, Rhodes WH, 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 in dogs. Am J Vet Res 1993;54(7):1021-1042.

See also Lust G, Williams AJ, Burton-Wurster N, Pijanowski GJ, Beck KA, Rubin G, Smith GK. Joint laxity and its association with hip dysplasia in Labrador retrievers. Am J Vet Res 1993;54(12):1990-1999.

Heritability

Heritability is an important statistic relating the variation of a trait attributable to additive genetic effects with the total phenotypic variation of the trait. In other words, heritability relates the genetic basis of a disease or trait (the genotype) with what is actually expressed or observed (the phenotype). Heritability is expressed as a number between 0 and 1. The higher the heritability, the more the phenotype represents the genotype and the greater the rate of genetic change that can be derived from selection pressure. The accuracy of a diagnostic test to determine disease (in this case CHD) can have an impact on the the value of the heritability statistic. An inaccurate (highly variable) diagnostic test can effectively lower the estimate of heritability. Such high diagnostic variability is believed to explain the low calculated heritabilities of the subjective hip score (OFA).

The heritability of the hip phenotype scored in the hip-extended view by the OFA in the United States has recently been studied in four breeds of dog (English Setters, Portuguese water dogs, Chinese Shar-peis and Bernese Mountain dogs). Mean direct heritabilities, estimated by use of midparent offspring analysis, were 0.17(Â±0.05), 0.30 (Â±0.06), 0.31 (Â±0.05) and 0.30 (Â±0.04) respectively. The pooled values for all four breeds was 0.26 (Â±0.03) (Reed et al. 2000). With heritability estimates this low the application of selection pressure will at best result in slow genetic improvement without hope of ever completely eradicating the condition. If similar low estimates for heritability of the OFA phenotype are found for the more popular breeds of dog, it would account for the fact that the percentages of dysplastic progeny decreased only slightly over the study period (1971-93).

PennHIP is working with many breed clubs with an interest in the heritability estimates for their particular breed. Estimates for the heritability of passive hip laxity (DI) drawn from analysis of full pedigrees for the breeds examined thus far have yielded high values, eg. for German Shepherd Dogs, heritability = 0.50; for Labrador Retrievers, heritability = 0.60 (see Leighton et al, 1994). Heritability of the DI has more recently been calculated to be 0.64 (Smith et al 2000)

References

Jessen CR, and Spurrel FA. Heritability of canine hip dysplasia, in Proceedings. Canine Hip Dysplasia Symp 1973;53-61.

Leighton EA, Linn JM, Willham RL, et al . A genetic study of canine hip dysplasia. Am J Vet Res 1977;38:241-244.

Leighton EA, Smith GK, McNeil M, Gregor TP. Heritability of the distraction index in German shepherd dogs and Labrador retrievers, in Proceedings. Molecular Genetics and Canine Genetic Health Conference, American Kennel Club;1994 Oct 7-8.

Reed AL, Keller GG, Vogt DW, Ellersieck MR, Corley EA. Effect of dam and sire qualitative hip conformation scores on progeny hip conformation. J Am Vet Med Assoc. 2000;217:675-680

SmithGK, Lafond E, Gschwend J, Fordyce H, Biery DN, Leighton EA and Gregor TP. Heritability estimates of hip scores in the Golden Retriever breed. In Proc 27th An Conf Vet Orthop Soc. Val D’Isere, France 2000

Biomechanics of Canine Hip Laxity

http://www.vet.upenn.edu/research/centers/pennhip/resch_sum.html#Biomechanics#Biomechanics

A detailed survey of the biomechanical properties of the hip joint is beyond the scope of this document. A summary is given in PennHIP Method section. References Heyman SJ, Smith GK, Cofone MA. Biomechanical study of the effect of coxofemoral positioning on passive hip joint laxity in dogs. Am J Vet Res 1993;54(2):210-215 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.

For a more general overview, see the PennHIP Method Section: http://www.vet.upenn.edu/research/centers/pennhip/ph_method.html

Â· Measuring Hip Joint Laxity http://www.vet.upenn.edu/research/centers/pennhip/resch_sum.html#meas#meas

Â· Reliability of the Method http://www.vet.upenn.edu/research/centers/pennhip/resch_sum.html#reliability#reliability

Â· The Risk of Developing DJD http://www.vet.upenn.edu/research/centers/pennhip/resch_sum.html#Risk#Risk

Â· Heritability http://www.vet.upenn.edu/research/centers/pennhip/resch_sum.html#heritability#heritability

Â· Biomechanics of Canine Hip Laxity http://www.vet.upenn.edu/research/centers/pennhip/resch_sum.html#Biomechanics#Biomechanics

other good websites relating to CHD and the OFA PennHip controversy are;

http://www.thedca.org/hipdisp.html

http://www.workingdogs.com/doc0090.htm<