Insall-Salvati ratio

The Insall-Salvati ratio or index is the ratio of the patella tendon length (TL) to the length of the patella (PL). 

Insall-Salvati ratio

This can be measured on a lateral knee x-ray or sagittal MRI. Ideally the knee is 30 degrees flexed. 

Distance lines used to calculate Insall-Salvati ratio (see Figure 1):

  1. 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 
  2. B: patellar length (PL): greatest pole-to-pole length

Insall-Salvati ratio = A/B (or TL/PL)

The Insall-Salvati ratio (TL/PL) is considered normal between 0.8 and 1.2: 

More recent literature 2 suggests that the true normal range is more forgiving: from as low as to 0.74 to as high as 1.50.

Modified Insall-Salvati ratio

Also applied on a lateral 30 degree flexed knee radiograph, the modified Insall-Salvati ratio is measured slightly differently:

  1. A: distance from the inferior margin of the patellar articular surface (as opposed to the lower pole of the patella itself) to the patellar tendon insertion
  2. B: length of the patellar articular surface (see diagram).  

Modified Insall-Salvati ratio = A/B

The modified Insall-Salvati ratio is considered normal around 1.25, abnormal when >2 which is considered diagnostic of patella alta.

See also

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