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.
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Usage
The Insall-Salvati ratio is probably the most commonly used measurement to assess patellar height, partially based on its simplicity. It can be 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 4,5.
Measurement
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)
Interpretation
On plain radiographs:
patella baja: <0.8
normal: 0.8-1.2
patella alta: >1.2
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 1:
patella baja: <0.74
normal: 0.74-1.5
patella alta: >1.5
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