Modified Insall-Salvati ratio

Dr Dai Roberts and Dr Henry Knipe et al.

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
Knee pathology

The knee is a complex synovial joint that can be affected by a range of pathologies:

Article information

rID: 74625
Synonyms or Alternate Spellings:
  • Modified Insall-Salvatti index
  • Modified Insall-Salvati index

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Cases and figures

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