Distraction index

Distraction index is a dynamic measurement used to assess the anatomy of the hip joint in dogs. It evaluates passive hip joint laxity using conventional radiography.

The dog is placed in dorsal recumbency and a distension device is placed between pelvic limbs. Both femurs are adducted against the distension device and slightly extended (± 10° extension, compared with the neutral position) to expose the acetabulum. The tibiae are kept parallel and a medially directed pressure is applied. As such, the distension device acts as a lever that allows demonstration of the laxity present in the hip joints.

To determine the distraction index, best fit circles delimiting the femoral head and the acetabulum are drawn and the centres of these circles are positioned. The distance, d, between the centers is divided by the radius of the femoral head, r. The resulting distraction index is an unitless number ranging from 0 to 1 (or more).

Powers et al. found that the mean value for the distraction index was 0.435 ± 0.139 for various breeds of dogs. Thirty-one (52%) OFA-rated excellent dogs had distraction index ≥ 0.30 (range 0.14 to 0.61), 183 (82%) OFA-rated good dogs had a distraction index ≥ 0.30 (range 0.10 to 0.77), and 79 (94%) OFA-rated fair dogs had a DI ≥ 0.30 (range 0.14 to 0.77).

It seems that there is a cut-off point - a distraction index of around 0.30 - below which canine hips are very little prone to osteoarthritis.