Insall and Salvati measured lateral radiographs of 114 normal adult knees to develop a simple, reproducible index of patellar height. They compared patellar tendon length to the greatest diagonal patellar length and defined normal variation. The study also tested Blumensaat's line as a reference standard.
Before this paper, no practical radiographic standard existed for defining patella alta. Blumensaat's line required exact 30° positioning and proved unreliable even when that angle was achieved.
The Insall-Salvati ratio changed everyday practice: on any lateral knee radiograph taken between 20-70° of flexion, you measure two lines and divide. If LT exceeds LP by more than 20%, the patella is pathologically high.
This matters most before patellofemoral realignment surgery. Distalizing a patellar tendon insertion without confirming true alta risks overloading the patellofemoral joint and accelerating cartilage wear.
The ratio also anchors subsequent modifications (Caton-Deschamps, Blackburne-Peel) developed for specific clinical scenarios, such as post-arthroplasty or skeletally immature patients, where the Insall-Salvati index has known limitations.
Insall and Salvati measured lateral radiographs of 114 normal adult knees to develop a simple, reproducible index of patellar height. They compared patellar tendon length to the greatest diagonal patellar length and defined normal variation. The study also tested Blumensaat's line as a reference standard.
Before this paper, no practical radiographic standard existed for defining patella alta. Blumensaat's line required exact 30° positioning and proved unreliable even when that angle was achieved.
The Insall-Salvati ratio changed everyday practice: on any lateral knee radiograph taken between 20-70° of flexion, you measure two lines and divide. If LT exceeds LP by more than 20%, the patella is pathologically high.
This matters most before patellofemoral realignment surgery. Distalizing a patellar tendon insertion without confirming true alta risks overloading the patellofemoral joint and accelerating cartilage wear.
The ratio also anchors subsequent modifications (Caton-Deschamps, Blackburne-Peel) developed for specific clinical scenarios, such as post-arthroplasty or skeletally immature patients, where the Insall-Salvati index has known limitations.