Patellar height measurement becomes important in the diagnosis of patella alta and patella baja.
Patella alta is a condition where the patella is high riding than its normal position.
Patella baja refers to low lying position of the patella.
Both conditions have been discussed separately.
Measurement of patellar height can confirm or rule out the presence of patellar height anomalies.
Following methods are used commonly
- Insall-Salvati ratio and modified Insall-Salvati ratio
- Caton-Deschamps index
- Blackburne Peel Method
- Blumensaat Method
Insall-Salvati ratio can be measured on lateral view radiograph or a sagittal section of MRI. The should be flexed by 30 degrees.
Following distances are measured
A -Patellar tendon length (TL)
It is the length of the posterior surface of the tendon from the lower pole of the patella to its insertion on the tibia
B- Patellar length (PL)
It is the greatest pole-to-pole length between two poles of patella
Insall-Salvati ratio = A/B or TL/PL
The normal values are between 0.8 and 1.2:
- <0.8 is patella baja
- >1.2 is patella alta
Modified Insall-Salvati ratio
It is also applied on a lateral 30 degree flexed knee radiograph but the measurements are slightly different.
Distance from the inferior margin of the patellar articular surface (Instead of lower pole of the patella in the original index) to the patellar tendon insertion
Length of the patellar articular surface (Instead of the pole to pole distance).
Modified Insall-Salvati ratio = A/B
The normal value is 1.25.
>2 which is considered diagnostic of patella alta.
Caton-Deschamps index (knee)
The Caton-Deschamps index is the ratio between the distance between the lower pole of the patella and the upper limit of the tibia, and the length of the patellar joint surface.
Caton-Deschamps index is also measured on a lateral radiograph or sagittal knee CT or MRI reconstructions with the knee flexed at an angle of 30º.
The distance between the lower pole of the patella and upper limit of the tibia
Patellar articular surface
A / B gives us the index
0.6-1.3 is the normal range for this index.
- <0.6 – patella baja
- >1.3 – patella alta
The Blackburne-Peel ratio is done in x-ray with 30 degrees of flexion.
Measure distance along the patellar articular surface.
A horizontal line at the level of the tibial plateau is drawn.
Measure the distance between the horizontal line and the inferior aspect of the patellar articular surface.
The value is achieved by B/A
0.8 is the normal value.
>1.0 is considered as patella alta.
This method is also used on the lateral view of x-ray in 30 degrees of flexion.
It uses Blumensaat line or roof the intercondylar notch as a reference line
The position of the lower pole of the patella (above or below the Blumensaat line) is noted.
Normally lower pole of the patella touches this line in 30 degrees of flexion.
Moreover, the distance between the lower patellar pole and the Blumensat line in millimeters are noted