Location of the instantaneous center of joint rotation in the normal canine stifle.

Locations of 3 instantaneous centers of joint rotation (ICR) were found in the stifles of 6 healthy dogs. A novel algorithm, verified by computer simulation, was used to determine ICR from radiographs. Comparisons of ICR locations were made between flexion and extension, between left and right limbs, and between living and dead dogs, using analysis of variance. All stifles were examined for evidence of lesions, but all were normal. The normal location of the ICR in the stifle is near the joint surface, indicating that there is a predominantly rolling type of motion. As the limb flexes, the ICR moves caudally; as the limb extends, the ICR moves cranially. Significant differences in ICR position were not found between left and right limbs or between living and dead dogs. The average ICR position was significantly more caudal during extension than it was during flexion. Knowing the normal ICR position is a preliminary to diagnosing joint abnormalities, using the ICR.