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

Edit Article Share
URL of Article

Article Information:

rID: 1503
System: Musculoskeletal
Section: Gamuts
Tag: knee
Synonyms or Alternate Spellings:
  • Insall-Salvatti ratio
  • Modified Insall-Salvatti index
  • Modified Insall-Salvati index
  • Insall Salvati ratio
  • Insall Salvati
  • Drag
    Insall-Salvati in...
    Figure 1: I-S index: red line/yellow line
    Drag here to reorder.
  • Drag
    Modified Insall-S...
    Figure 2: modified I-S index: red line/yellow line
    Drag here to reorder.
  • Drag
    Figure 3: normal patella
    Drag here to reorder.
  • Updating… Please wait.


    Error Unable to process the form. Check for errors and try again.

    Alert_accept Thank you for updating your details.