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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. 

Patellar length (PL) is the greatest pole-pole length. 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 (see Figure 1).

To calculate: TL (red line) / PL (yellow line).

The traditional number used to TL:PL is < 1.2 (between 0.8 and 1.2): 

More recent literature 2 suggests that the true range of normal is more forgiving - up to 0.74 to 1.50.

Modified Insall-Salvati ratio

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.

See also

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