The Insall-Salvati ratio or index is the ratio of the patella tendon length to the length of the patella and is used to determine patellar height.
The Insall-Salvati ratio is probably the most commonly used measurement to assess patellar height. It is affected by the presence of tibial tuberosity abnormalities (e.g. Osgood-Schlatter disease, osteotomies) and a different technique may be required when these are present .
The Insall-Salvati ratio was initially determined on a 30° flexed lateral knee x-ray and was later applied to sagittal MRI.
Distance lines are used to calculate Insall-Salvati ratio:
- A: patellar tendon length (TL): length of the posterior surface of the tendon from the lower pole of the patella to its insertion on the tibia
- B: patellar length (PL): greatest pole-to-pole length
Insall-Salvati ratio = A/B (or TL/PL)
On plain radiographs:
On MRI, different thresholds are used due to intermodality differences in measurement technique (e.g. indirect vs direct visualization of the patellar tendon). The following is usually accepted as reasonable :
- patella baja: <0.74
- normal: 0.74-1.5
- patella alta: >1.5