Sonographic measurement of instability of the subtalar joint.

A sonographic method of measuring instability of the subtalar joint was developed. After defining reference points, we determined a sonographic fibulo-trochlear angle. This angle was measured in a neutral position and under an inversion stress position. A ratio q representing the relationship between the fibulo-trochlear angle measured in the neutral position and under stress, was determined. The method was tested on volunteers with clinically and radiologically stable subtalar joints and a control group with unstable subtalar joints. The stress radiograph using the Zwipp method was utilized for radiologic correlation. A sonographic ratio q>1.6 correlated with a radiological unstable subtalar joint. The borderline zone in which there was an overlap of stable and unstable subtalar joints was characterized by a q value of 1.2-1.6. A ratio q<1.2 correlated with a stable subtalar joint.