Roger...I know you like research so I thought I would share the following
overview of the literature.
I have been the process of evaluating the "widely used" systems that are
available for the breeder. I have reviewed websites and their related
publications and came up that it is inclusive which is the superior
predictor, at this point in time. I have found that there are the clinical
examination (which only demonstrated overt CHD/DJD changes). the subjective
system (OFA) and two objective systems (Norberg angle and Penn Hip).
Interesting that the Norberg angle which is widely used in Europe/OZ does
not correlate to the findings or predictors of OFA. The Penn Hip system is
unique due to its incorporation of a positioning device (like the Norberg
Angle does) and a qualitative evaluation like OFA (CHD/DJD qualitative
reading is taken from the standard hip extended view). This encourages
"congruent validity" in its findings.
Roger. ..with this great variation of these systems...it is difficult to
obtain a true incidence of the disease. If cost is not an issue, a breeder
may be best served by having both systems of evaluation performed. So in
other words, it is too early to predict which system will truly have an
impact on the incidence of disease (especially since we do not know what
that true incidence is)...
The following is "food for thought" only...This is an overall of these
systems by various researchers and specialists. It is up to each breeder to
decide what system they are comfortable with...
Regards,
Kathy Yonkers
Stuarthome CKCS
USA
Definitions:
Canine Hip Dysplasia (CHD)- abnormal structures of the hip and surrounding
area.
Degenerative Joint Disease (DJD)- presentation of osteoarthritis
Clinical Presentation of CHD: 5 months to 12 months for the severe form of
hip dysplasia; later for the chronic form (must have DJD changes to
demonstrate this level of symptoms)
* Abnormal Gait
* "Bunny-hopping" When Running
* Thigh Muscle Atrophy (loss of muscle mass)
* Pain (difficult to measure)
* Low Exercise Tolerance
* Reluctance to Climb Stairs
* Audible "click" When Walking or during manual manipulation of the hips
* Increased Intertrochanteric Width ("points of hips" are wider than normal)
These clinical signs can mimic other conditions...and it would only
demonstrate frank CHD when DJD becomes problematic (severe form).
The breeder will need radiographic film and/or series to determine the level
of hip joint laxity and/or appearance of DJD in their dogs...while
remembering radiographs will only demonstrate historical issues (hard tissue
changes), not current or active disease. You will need an orthopedist
specialist to perform a clinical examination and correlate this to the
radiographic examination. Past history of injuries need to be
addressed...injuries do impact on developing DJD, even in absent of familial
traits. Also environmental factors (as reviewed in the literature) needs to
be figured in such as: diet, history of trauma, level of exercise, and rate
of growth.
So here is where the maize begins...you have one (1) subjective system and
two (2) objective systems that are widely used...each has advantages and
disadvantages and they cannot be used interchangeable (apples and oranges):
There are three (3) methods for diagnosis and prediction of predisposition
to CHD/DJD in canines:
1) hip extension radiograph method (OFA),
2) stress radiographic diagnostic method (Norberg angle). and
3) combination (Penn Hip).
These methods are categorized based on the positioning of the dog while the
radiograph is taken. The first method (OFA) is the most common method in
which the dog, while lying on its back, has its rear legs fully extended
with the knees rotated inward. The second method (Norberg angle) requires
that the dog be anesthetized and while lying on its back, the dog has its
legs positioned as they would be if the dog were standing. A custom-designed
device is placed between the legs which forces them apart, thereby
displacing the ball of the femur from the hip socket and allowing for
observation of joint laxity. The third method (Penn Hip) combines both
positioning methods in a modified fashion (see below).
Subjective Type:
I. The OFA method
http://www.offa.org
OFA have a schematic scoring system of phenotypes that each radiologists
draw their independent decision on while viewing one standard hip extension
radiograph using a 7-point scale. This method is a qualitative (looks at
quality...examiners may have a different subjective evaluation of the same
radiograph) method because no form of measurement is utilized. Instead this
method is based on subjective visual criteria such as degree of joint laxity
(subluxation) and the presence of degenerative joint disease (DJD).
Problems with this systems...intra-rator (score made by same examiner on
same radiographs submitted under different conditions) grading can vary; No
standardized positioning device is used to prevent artifacts. No computer
assisted software is used to grade the radiographs or check for incorrect
positioning variation. No specific training beyond entry level training is
required of the veterinarian to have them �certified� in this performing OFA
radiographic positioning. Vet schools vary in level of instruction of the
technique, thus level of competency can vary between clinicians. Inter-rator
reliability (the scores between different examiners) of the three rators is
reported at 73.5% level calibration, which is below average for clinical
research purposes, thus indicating subjectivity. If radiographs are
resubmitted for a future re-evaluation...a prior ID # is required for the
case. this identification could create bias in the rator(s) and thus
influence the sensitivity/specificity (false positives and false negatives)
levels to stay within an acceptable range.
This schematic grading might give the breeder the feeling that they do not
have to be concern if the grading is good or better when there is subjective
variation within the system. The breeder might use it to market their dogs,
and thus create a false sense of security to the consumer. Example...Is your
good truly a good...etc??? Not all films taken are send in to the data bank
by clinicians...so they are unable to measure true success of system over
given time without question of bias.
The true advantage is the low level of cost to breeder; can be completed
without sedation/anesthesia. long history of service, number awarded on AKC
pedigree of phenotype, and is widely available and accepted by breeders.
Objective Types: The second group can then be divided into two (2) methods:
The two (2) numerate methods - Norberg angle and Distraction index are
quantitative and capable of being used for sound statistical analysis. The
later method is comparatively new and is being promoted by the University of
Pennsylvania SVM. The major disadvantage is that both methods are more
costly and required the risk of sedation and/or general anesthesia.
II. The Norberg Angle method (BVA method)
http://www.bva.co.uk/index.html
This method is a quantitative method because it is based on measurement of
the angle formed by connecting a point at the center of the femoral head
(ball of the hip) to the upper acetabular rim (hip socket). Hips are then
scored as the number of degrees in the formed angle. For example, scores
range between 55 to 115 degrees with low end scores indicative of greater
hip laxity and higher risk for CHD/DJD. Dogs receiving scores above 105
degrees are accepted as having normal hip-joint conformation with lower
predisposition to CHD/CJD.
Interesting point:
There is very poor correlation reported in the literature, between the
Norberg Angle method used in Europe and OFA method used in the the USA . In
other words...a passing score by one evaluation method, does not guarantee a
passing score by the other. All of which points to a lack of reliable data
in the area of CHD/DJD worldwide.
III. The Distraction method (Penn HIP method)
http://www.vet.upenn.edu/ResearchCenters/pennhip/
This is a quantitative method like the Norberg angle with a qualitative
component. However, this method measures the separation distance of the
femoral head center (ball of hip) from the acetabular center (socket) while
the hip joint is forced to luxate, then divides this measurement by the
radius of the femoral head (ball of hip). In effect, this method evaluates
how far the femoral head can be displaced from the acetabulum. The resulting
number is termed the distraction index (DI). The DI score can range from 0
to 1 with scores closer to 0 indicating less hip laxity. Dogs receiving a
score of less than 0.3 are considered to have normal hip-joint conformation
with little, if no risk for CHD/DJD.
A summary of Penn Hip method is:
It uses both the:
1. Hip extension radiographic method and
2. Stress radiographic diagnostic method.
It obtains:
1. Hip joint conquity reading from a compression view
2. Quantitative measurement of hip joint laxity from the distraction view
3. DJD qualitative reading is taken from the standard hip extended view
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