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.
|Distraction View||Compression View||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.|
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 Canine Hip Dysplasia for an example of DJD).
To summarize, the PennHIP method
For a brief review of the anatomy of the canine hip joint, see the Canine Hip Dysplasia section.
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.