The Insall-Salvati ratio is the ratio of the patella tendon length (TL) to the length of the patella (PL). This can be measured on a lateral knee xray or sagittal MRI. Ideally the knee is 30 degrees flexed.
The traditional number used to TL:PL is < 1.2 (between 0.8 and 1.2), although more recent literature 2 suggests that the true range of normal is more forgiving - up to 0.74 to 1.50.
Patellar length (PL) is the greatest pole - pole lenghth
Patellar tendon length (TL) is defined as the length of the post surface of the tendon from the lower pole of patella to its insertion on the tibia.
Modified Insall-Salvati index
Also applied on a lateral 30 degree flexed knee radiograph. Measures distance from inferior margin of patella articular surface to the patella tendon insertion and compares this to the length of the patella articular surface (see diagram). Normal mean of 1.25. Ratio more than 2 is diagnostic of patella alta.
Synonyms & Alternative Spellings
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