The Insall-Salvati ratio is the ratio of the patellar tendon length to the length of the patella. This can be measured on a lateral knee xray or sagittal MRI. Ideally, the knee is 30 degrees flexed.
In the modified Insall-Salvati ratio, patellar bone length is replaced with patellar articular cartilage length. The Insall-Salvati index, however, is the most widely accepted method because of its lack of dependence on the degree of knee flexion and its established MRI criteria. 
- Syed A. Ali, Robert Helmer and Michael R. Terk; Patella Alta: Lack of Correlation Between Patellotrochlear Cartilage Congruence and Commonly Used Patellar Height Ratios; AJR November 2009 vol. 193 no. 5 1361-1366. (Level of evidence: B)
- JEROLD E et al; Patella Alta and Patella Infera, THEIR ETIOLOGICAL ROLE IN PATELLAR DISLOCATION, CHONDROMALACIA, AND APOPHYSITIS OF THE TIBIAL TUBERCLE; J Bone Joint Surg Am. 1975 Dec;57(8):1112-5. (Level of evidence: B)