Insall-Salvati ratio

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. 

Usage

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 ^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