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.
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The index method is calculated by superimposing precision-machined circle gauges on the cortical margins (rims) of the acetabulum and femoral heads (see far left) to find the respective geometric centers. On the compression view (see nearleft), 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 ds than smaller dogs), with age of 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).