The patellophyseal (PP) index is an alternative technique to measure the patellar height on both x-ray and MRI as the physeal line can be seen on both imaging modalities 1.
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Usage
The PP index is a variation on the patellotrochlear index that can be used on radiographs. It is an indirect measure of the chondral articulation between the patella and the trochlear groove measuring the anterior femoral physeal scar, which is thought to end in proximity to the trochlear cartilage 1.
It was proposed to address unusual patellar morphology that potentially had normal Insall-Salvati scores despite a patella alta. This was because it measured the patella position against the femur, rather than the tibial bony landmark which has been used in measurements such as Insall-Salvati or modified Insall-Salvati 1,2.
It has been found to be a reliable measure of patella height, although has a better intra- and inter-observer reliability on MRI than x-ray 3.
Measurement
The patellophyseal index is applied on a lateral knee radiograph or sagittal MRI slice with the greatest patellar length with two measurements obtained 1,2:
A: length from the anterior physeal line of the femur to the superior margin of the patellar articular surface
B: length of the patellar articular surface
Patellophyseal index = A/B
Interpretation
normal: 0.37 (range 0.09–0.65) 1
patella alta: >0.65
patella baja: <0.09
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