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