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THE PennHIP PROCEDURE

In 1937 veterinarian Gerry Schnelle wrote a paper entitled Bilateral Congenital Subluxation of the Coxofemoral Joints of a Dog.   It was a landmark original description of what we today call Canine Hip Dysplasia.  CHD for short, there are very few topics among dog owners, breeders, and veterinarians that can rival the spectrum of emotions evoked by this abnormality of the dog’s coxofemoral (hip) joint.  The PennHIP procedure is a useful tool in determining the degree of hip joint laxity in dogs.

WHAT IS HIP DYSPLASIA
In short canine hip dysplasia (CHD) is a developmental abnormality of the hip joint.  CHD is the most common orthopedic disease is dogs and is a polygenic (influenced by a number of genes) inherited trait. The radiographic changes observed with CHD include luxation or subluxation of the hip and osteoarthritis (degenerative joint disease).   It is generally accepted and now scientifically proven that hip joint laxity (hip looseness) plays a major role in the development of osteoarthritis. Because osteoarthritis commonly causes pain and discomfort in the affected joints, the dog with CHD often has impaired mobility and a diminished quality of life.  Therefore, any method that objectively measures hip joint laxity may be helpful in evaluating a dog’s risk for developing osteoarthritis and CHD.  This information would assist the dog owner or breeder in deciding which animals not to breed as well as in selecting the best individual animals for breeding.  Before we go further in this discussion of PennHIP, there are a few terms that are important to know.  Take a look at the Definitions below and refer to them if you get confused.

CHD in many respects has been ignored by some, underestimated by a few and fervently analyzed by many.  The evidence collected over the past 65 years since Dr. Schnelle first brought it to our attention has demonstrated that the presence of hip dysplasia in a dog has definite genetic determiners.  More recently, data has been presented that shows the development of joint deterioration in a dog with CHD correlates well with the degree of joint laxity present in the dog.  If hip laxity is present, other environmental factors, such as nutritional, can influence the degree of expression of abnormal signs associated with Canine Hip Dysplasia.  The determination of the degree of CHD in a dog, the ultimate debilitative effects of any hip joint abnormality, the heritability factors for the potential offspring and the ethics of breeding a dog with CHD all factor into making this topic somewhat complicated… as the scientists say “It’s a multi-factorial subject”.  Fortunately, however, the determination of degree of joint laxity using PennHIP Distraction Index is not subjective, but an accurate quantitative measurement. 

TRADITIONAL EVALUATION 
The ventrodorsal (front to back), hip-extended (legs pulled straight rearward) radiographic view as approved by the American Veterinary Medical Association in 1966 has been a standard for positioning of the dog on the x-ray table.  The radiograph needs to be taken with the dog under sedation or anesthesia for proper and consistent positioning.  If the positioning is not correct, the asymmetrical images of the hip joints will make accurate interpretation nearly impossible.  This traditional and still widely accepted method of evaluating hip structure is used by many hip screening programs throughout the world, including Orthopedic Foundation for Animal (OFA), the Institute for Genetic Disease Control in Animals (GDC) and PennHIP.  The OFA system and registry has been one strategy attempting to improve the knowledge base and selection process for breeders interested in eliminating CHD from a breed. Many breeders and veterinarians agree that the incidence of CHD has noticeably decreased over the years since it was first described, partly because awareness of CHD among pet owners and breeders has reached fairly high levels. For years the OFA protocol has been the most commonly utilized method of screening and certifying dogs as having normal or dysplastic hips. Unfortunately, subjective variables can enter into the OFA evaluation process.  Although preliminary radiographs can be revealing, a potential drawback with the OFA protocol is the fact that for certification and definitive evaluation a dog must be 24 months of age or older at the time the hip radiographs are taken.  Additionally, occasional criticism is heard that some veterinarians unknowingly affect the evaluation of the radiograph by their subjective positioning of the dog at the time the radiographs are taken; plus individual interpretation of what is visible on the radiograph is sometimes challenged.  There exists the disappointing fact that outright fraud has occurred by unscrupulous individuals who misrepresent a radiograph as being of an animal other than the one actually radiographed. 

PennHIP:  (University of Pennsylvania Hip Improvement Program)
And that is precisely where the PennHIP topic becomes important.  Commercially available since 1993, this procedure has been and was developed as an objective method of evaluating dogs’ hip structure.  It evolved as a direct result of the subjectivity factors and age constraint (maturity) limitations inherent to evaluation and certification of dogs by the OFA and other screening programs.  PennHIP research published in peer reviewed journals has shown that different breeds have different susceptibility to osteoarthritis.  Therefore, in PennHIP evaluations each breed is compared to its own.

PennHIP incorporates an improved method for evaluating the integrity of the canine hip. It has great potential to lower the frequency of CHD when used as a breeding selection criterion. The technique assesses the quality of the canine hip and quantitatively measures canine hip joint laxity.  Originally developed in 1983 by Gail Smith, VMD, PhD and his colleagues at the University of Pennsylvania, School of Veterinary Medicine, the procedure was conceived and developed as a new scientific method for an earlier and more accurate diagnosis of CHD.  Years of research conducted in Smith’s laboratory proved the diagnostic method to be capable of estimating the risk (susceptibility) for CHD in dogs as young as sixteen weeks of age.

The PennHIP method is a novel way to assess, measure and interpret hip joint laxity and hip conformation. Three separate radiographs are taken of a sedated or anesthetized dog… 
*  the distraction view
*  the compression view
*  the standard ventrodorsal, hip-extended view. 
The distraction view and compression view are used to obtain accurate and precise numerical measurements of joint laxity and conformation. The hip-extended view is used to diagnose osteoarthritis using criteria similar to the OFA and other hip screening organizations.  Precise positioning of the dog by trained certified PennHIP members is imperative and any radiographs that do not conform to prescribed positioning are rejected and must be repeated. 

Interestingly, a dog's hip laxity at 16 weeks of age will be much the same at one year, two years and older.  For most breeds, the PennHIP evaluation can be done as early as four months of age.  Recommendations based upon peer reviewed and published research, though, state that the reliability of the PennHIP method slightly improves with age, with one year being marginally superior to six months, which in turn is marginally better than four months. The Distraction Index (DI), obtained via the distraction view radiograph, is not subjective.  It is an accurate numerical measurement that has been shown to be reliable.

DISTRACTION INDEX
Also called the DI, the Distraction Index is obtained with the aid of a fulcrum so that a radiograph can be taken with the head of the femur as far out of the hip socket as possible.  This distance is then measured.  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 (socket). For example, DI=0.58 means the femoral head comes out of the joint by 58%, DI=0.75, 75% out of joint and so on.  The Distraction Index of PennHIP is not subjective, but an accurate quantitative measurement that has been shown to be repeatable and reliable.  A dog with a DI approaching 1 has a very high degree of predictability for developing Canine Hip Dysplasia and subsequent osteoarthritis... and should not be considered for breeding purposes.

WHO DOES PennHIP PROCEDURES
Only certified individuals who have undergone special training and have successfully demonstrated their expertise in the technique are permitted to perform PennHIP procedures.  Since both PennHIP and OFA use the ventrodorsal, hip-extended view, your PennHIP certified veterinarian can make a copy of the hip-extended radiograph for submission to the OFA at the time your dog has the PennHIP procedure. You do not have to abandon OFA opinion nor schedule a separate appointment for an additional radiograph. 

As of November 2001, there were 1,076 certified PennHIP members.  977 members are in the United States and Canada and 99 in 19 other countries.  Darryl N. Biery, DVM, Professor of Radiology and a Board Certified Veterinary Radiologist at the School of Veterinary Medicine, University of Pennsylvania states, “All PennHIP certified veterinarians have passed a certification process that includes course work and quality control supervision.”

With the devastating effects of CHD, the PennHIP procedure is a welcomed addition to the opportunities dog breeders have in selecting only the best individuals for carrying on the ideal traits of each breed.  That selection flexibility is enhanced by the opportunity to go beyond the qualitative description of calling a hip joint  “normal” by actually applying a numerical value to “normal” and abnormal joints.  That numerical value is called the Distraction Index (DI).  With PennHIP the breeder can make a more appropriate choice among breeding individuals with “normal hips” by referring to the Distraction Index and selecting the best individuals in the “normal” group… the ones with the tightest hips.

The PennHIP Laboratory publishes its findings in scientific journals. Published information is disseminated to all PennHIP members; it is also shared with interested breed clubs and routinely appears in publications within the dog fancy.  Will the PennHIP protocol replace the traditional OFA protocol?  Only time will tell. But this newer method of evaluating dogs’ hip joints will be one more tool to improve future generations of dogs.  So for all of us who love and respect canines, the PennHIP procedure should be a welcomed addition to our diagnostic and predictive treasure chest.

ADVANTAGES OF PennHIP
There are a number of advantages associated with doing the PennHIP procedure rather than the traditional method of radiography:
The PennHIP Procedure is...
* A scientifically validated method.
* Performed on dogs as young as 16 weeks of age.
* 2.5 –11 times more hip laxity is measured compared to the conventional ventrodorsal, hip-extended radiographic evaluation method.  This objective measurement of hip joint laxity is critical since the amount of laxity detected has a direct relationship to the likelihood that a dog will develop arthritis (the looser the hip the greater the risk). 
* PennHIP radiographic score has a higher heritability coefficient, and therefore has a greater potential to improve hips through selective breeding than the ventrodorsal, hip-extended view alone. 
* Requires mandatory submission of hip films to minimize bias in database.  (Even hips showing obvious dysplasia are sent in and added to the database.)

PennHIP OR OFA?
What procedure should a dog owner select when attempting to gain valid information about a dog’s hip status?   I posed this question:  “What is the major difference between having a dog evaluated by the OFA as opposed to having a PennHIP evaluation done?”   In response Dr. Smith states, “The PennHIP radiographic evaluation has been shown scientifically to give a truer estimate of hip quality; it is more accurate, more reliable and as mentioned it can be performed as early as 16 weeks of age. The major advantage of the PennHIP method is its unique ability to identify true-normal dogs, dogs not susceptible to hip dysplasia.”  

Small animal practitioner Mara DiGrazia, DVM, of New Hyde Park, N.Y., although not PennHIP certified, states her approach to the question of which procedure to use:  “If a client has a young puppy that they are considering to use for breeding I would recommend the PennHIP radiographs rather than waiting for an OFA.  This is because PennHIP can predict as early as 4 months of age what degree of hip dysplasia the dog may develop.  Early OFA radiographs may not reveal the same degree of laxity in the joint.  Early identification of hip deformity can make a world of difference to the dog’s future life.  The PennHIP procedure can bring about a timely spay or neuter if the dog has been found to be unworthy of breeding due to poor hip conformation.  The dog’s lifestyle can then conform to its needs as a pet rather than as a working or breeding dog.“

Definition of terms:

Articular Cartilage, nourished by synovial fluid, is the cartilaginous surface of the end of a bone, which contributes to the function of a joint.

Osteoarthritis, Degenerative Joint Disease (DJD) is marked by degeneration or breakdown of the articular cartilage, hypertrophy of bone at the margins of the joint, and changes in the synovial membrane and joint capsule, and is accompanied by pain and stiffness.

Joint Laxity refers to the amount of looseness in a joint.  Generally the more laxity the greater the potential for DJD from the trauma associated with loosely fitting joint members.

Luxation means dislocation.  A luxated hip is one that has extreme laxity such that it displays dislocation of the two bones of the joint.  The x-ray image at the top right of this page demonstrates a fully luxated hip on the right side of the image.

Subluxation means a partial dislocation of two joint members.  The x-ray image at the top right of this page demonstrates a subluxated hip on the left side of the image.

The ventrodorsal, hip-extended radiographic view:  This refers to the standard positioning of the dog on its back on the X-ray table.  The radiograph is made with the beam projection from the front to the back of the dog while the rear legs are pulled symmetrically rearward.  To be done properly, the view is generally done under sedation or anesthesia.

There are lots more x-rays to see if you go back to Radiology Headquarters