HIP TESTING EXPLANATION AND COMPARISON

There are two types of hip evaluation performed in the U.S.: OFA (Orthopedic Foundation for Animals; http://www.offa.org/) and PennHIP (Pennsylvania Hip Improvement Program; http://www.pennhip.org/).  Both evaluations are utilized to determine the possibility or onset of canine hip dysplasia (CHD) [or OA, osteoarthritis of the hip]. 

OFA evaluation is done as qualitative or subjective.  Scores are given by a group of 3 veterinarians that provide scores such as fair, good, etc., leaving it vulnerable to to inter- and intra-observer errors commonly associated with subjective measurement systems.  PennHIP is a computerized analysis and provides a numerical calculation for each hip.   It uses a unique method, an index, to measure hip joint laxity. The method is quantitative (i.e., it assigns a number to joint laxity. 

OFA: (to come)

PENNHIP:  consists of 3 integral components:

* An evidence-based radiographic technology to accurately screen canine hips for the susceptibility to OA.
* A worldwide network of trained veterinarians to perform the procedure with competence.
* A database maintained at the University of Pennsylvania designed to accumulate and report on breed-specific trends in OA and hip dysplasia control.

PennHIP's mission is to develop and apply evidence based technology to direct appropriate breeding strategies aimed at reducing in frequency and severity the osteoarthritis of canine hip dysplasia. The beneficiaries of this effort will be the many dogs who suffer with this controllable genetic disease and, of course, the dogs' owners.

PennHIP incorporates a new method for evaluating the integrity of the canine hip. It is accurate in puppies as young as 16 weeks of age. It has great potential to lower the frequency of CHD when used as a selection criterion.

Hip joint laxity is the most important risk factor for the development of osteoarthritis. In other words, the amount of laxity or looseness in a hip joint is related to the chance that a hip will develop OA: the looser the hip, the greater the risk. For this reason, it is important to understand the difference between passive and functional hip laxity.

• Passive hip laxity is subjectively scored or measured on a hip radiograph of a dog while it is under heavy sedation or anesthesia. the PennHIP method measures passive laxity.
• Functional hip laxity is the pathologic form of laxity occurring during normal weightbearing in dogs with dysplastic hips. Current hip screening methods cannot assess functional hip laxity.

 Under normal conditions, the sum of the forces on the joint are spread out over a large surface area. When laxity is present in the joint, the force applied by the surrounding muscles actually increases to compensate for the laxity. The sum of the forces exerted on the dysplastic hip is greater than that the sum of the forces exerted on the normal hip. In addition, the forces on the dysplastic hip are applied over a smaller surface area. The high joint contact stresses produce injury and ultimately result in the loss of delicate articular cartilage. Over time, functional hip laxity results in erosion of the femoral head and flattening of the acetabulum.

PennHIP uses a unique method, an index, to measure hip joint laxity. The method is quantitative (i.e., it assigns a number to joint laxity) as opposed to being qualitative or subjective (e.g. excellent, good, fair, etc., which is how OFA scores). The index 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,  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 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 image at right), 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. Therefore, the laxity as determined by the DI is 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.)

BREED AVERAGES AND DIFFERENCES BETWEEN OVERSEAS REPORTING AND U.S. TESTING

There is no "pass or fail" score with PennHip.  It is reflective of breed averages, and the scoring system can not be arbitrarily applied to all breeds.  It is breed specific.  For example, one breed's average score may be .35 and its high score .18, while another breed's average can be .70 and its high score .50.  While .70 may seen high in itself as compared to .35 in another breed, it is relative to the breed, not a generality.  In short, some breeds may have looser hips than others.

The TDR, when presenting the Tamaskan "breed average", has not included ALL tested Tamaskan dogs' scores, as the worst of scores have been omitted from the averages, thereby skewing the true breed hips average.   The Tamaskan Breeders Association will maintain complete and accurate records of all registered Aatu Tamaskan dogs, and a breed average will be provided as soon as possible and continue to be updated accordingly with the addition of new dogs and litters tested.

PIONEERING THE PENNHIP IN THE U.S.

Takeia was the first Tamaskan to undergo the PennHIP procedure (12/2008) . Unlike the OFA scans, the dog does have to undergo anesthesia for a PennHIP exam.  That's why it is recommended that the procedure not be done on dogs under one year old, since there is a chance with any procedure that utilizes anesthesia.  [However, Takeia did go through the procedure like a champ and came out of the anesthesia without incident.]  Since we would like to create a comparative database with OFA, we also sent copies of her scans to OFA for preliminary scoring.  [Since Takeia went into heat 2 months early, her procedure was done only 3 weeks before her heat cycle began, so it is possible that her scores could have been higher if she was not undergoing the hormonal change of the heat cycle.]

As part of the Tamaskan Breeders Association, we encourage breeders to do the same if they want to have their dog tested before the required 24 months for certified OFA results:  PennHIP procedure with films sent to OFA for a preliminary reading.  This will allow us to compare the two in order to start to determine a breed average in the US with both procedures.

WHEN NOT TO DO ANY HIP TESTING ON A BITCH

Up to six weeks before a bitch's heat cycle and two months after her cycle ends, her system starts to undergo hormonal changes that can affect that laxity of the hips.  If a bitch has a litter, it is recommended that you wait at least eight week after weaning before performing a hip procedure for those same reasons.  These bodily changes within the bitch can seriously affect the outcome and results of the scoring, whether with OFA or PennHIP.