Extension patellar engagement index: a new measurement of patellar height.

Introduction: Patellar instability is a multifactorial pathology requiring precise evaluation of its contributing factors, particularly patella alta. Patellotibial height measurement indexes, such as the Caton-Deschamps index, have the disadvantage of being referenced to the tibia. Patellotrochlear indexes are more appropriate but fail to account for variable knee flexion during magnetic resonance imaging (MRI).

Objective: This study aims to validate a new MRI-based patellotrochlear measurement, the extension patellar engagement (EPE) index, which is reproducible and measures patellar height relative to the trochlea independently of knee flexion during MRI.

Materials And Methods: A retrospective study was conducted on 77 MRIs (37 unstable patients with a history of patellar dislocation, 40 controls with meniscal injury). The EPE index is the ratio between the patellar engagement length on the cartilaginous trochlea and the patellar cartilage length, measured on a knee virtually extended. Four measurements on multiple sagittal slices were used to calculate the EPE index.

Results: The inteobserver intraclass correlation coefficient (ICC) was excellent (0.79). The mean EPE index's value for all knees was 0.22. It was significantly higher (p = 0.034) in the instability group (0.26) compared to the control group (0.19). The average knee flexion during MRI was 11 ° (range: -8 ° to 25 °).

Discussion: The EPE index is reproducible. It is an anatomical index to classify patella alta when the patella does not reach the trochlea in knee extension (EPE index ≤0). It is also a functional index, with a mean value of 0.19 in the control group.

Conclusion: The EPE index measures the patellar engagement on the trochlea taking into account the knee flexion during MRI. It may guide surgical decision-making for patellar distalization and quantify the required correction.

Level Of Evidence: III; retrospective comparative study.