Measurements made to calculate the Insall-Salvati index (ISI) and the Guenego-Verwaerde index (GVI). Mediolateral radiograph of a normal contralateral stifle illustrating the measurements made to calculate the Insall-Salvati index (ISI) and the Guenego-Verwaerde index (GVI). The ISI is the ratio of the patellar ligament length (PLL) to the patellar length (PL), and the GVI is the ratio of D to the patellar joint surface length (PJSL). D, Distance between the point at the intersection of the MA and the tibial plateau and the line perpendicular to the distal aspect of the PJSL