Interobserver variability of the Insall-Salvati ratio

Thirty-six patients presenting with knee pain were enrolled in a prospective study to assess the variability of the radiographic measurement of patellar height by the traditional and modified Insall-Salvati ratios. The traditional and modified Insall-Salvati ratios were measured from a lateral knee radiograph using a standard hand-held ruler by four physicians in varying order. The observers were in agreement in classifying the radiographs in two thirds of the cases when the traditional method was used but in less than one-half of the cases when the modified ratio was used. The source of the difference in the two ratios is a greater variability in measuring the distance from the inferior margin of the articular surface to the tibial tubercle.