Modified Insall-Salvati ratio

Last revised by Dr MT Niknejad on 12 Oct 2021

The modified Insall-Salvati ratio or index is another technique to assess patellar height.

It was proposed to avoid mismeasurement due to the variable shape of the inferior patella pole 2, however, the Insall-Salvati ratio has been demonstrated to be more reproducible 3

The modified Insall-Salvati ratio is applied on a lateral 30° flexed knee radiograph like the Insall-Salvati ratio but is measured differently:

  • 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
  • B: length of the patellar articular surface

Modified Insall-Salvati ratio = A/B

  • normal: 1.25 (range 1.2-2.1) 2
  • patella alta: >2
    • at this cut-off, 3% of normal knees will be classified as patella alta 3

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Cases and figures

  • Figure 1: modified Insall-Salvati index
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  • Figure 2
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  • Figure 3: MRI modified Insall-Salvati index
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  • Case 1: patella alta (2.2)
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