Rotational instability of the patella on radiographic images.

In the period from 1980 to 1981, knee joint radiographs were made in 82 patients following the clinical diagnosis of patellar chondropathy. A total of 119 knee joints with patellar chondropathy and 45 with no specific complaints on the contralateral side, in addition to a control cohort consisting of 28 persons, i.e., 56 knee joints with no complaints, were evaluated. Tangential radiographic images of the patellae enabled us to measure directly the rotation of the patella around the axis perpendicular to its center. It could be established that all healthy patellae underwent medial rotation between 45 and 60 degrees of knee flexion which was maintained up to 90 degrees, whereas the affected knee joints did not rotate.