Axial MRI index of patellar engagement: a new method to assess patellar instability.

[trochlear dysplasia]

The aim of this study was to define a new index to measure lateral patellar displacement (LPD ) using nuclear magnetic resonance imaging (MRI ), an axial index of engagement of the patella (AEI ) obtained from two different axial MRI views then to validate its use in a prospective series of patients presenting an objective patellar instability (OPI ).One hundred and thirty-five patients with OPI and no history of surgery of the patella were included in a prospective study organized by the French Society of Arthroscopy performed between June 2010 and August 2012 . All patients underwent axial and sagittal MRI . The AEI was obtained by projecting predefined patellar and trochlear landmarks (cartilaginous landmarks ) on 2 different axial MRI views (one trochlear and one patellar ). The results were compared with a series of controls (n =45 ).The preoperative AEI of the patella was 0 .94 ± 0 .09 for the control group and 0 .84 ± 0 .16 for OPI group (P =0 .000016 ). The AEI could be obtained in 100 % of the cases if it was measured on 2 MRI views while it could not be measured in 38 .5 % of the cases if the measurement was only obtained from one MRI view or whenever the widest part of the patella was not across from the femoral trochlea . The AEI did not significantly depend on dysplasia or the presence of a supratrochlear spur . The lowest AIE values were associated with trochlear dysplasia with a supratrochlear spur (P =0 .0023 ) and a more prominent trochlea (P =0 .0016 ). The AEI was correlated with patellar tilt (P <0 .000001 ) and TT-TG on MRI (P <0 .000001 ).AEI is a new index to measure LPD . It can be obtained in all cases because it is obtained from two different MRI views . The normal value is close to 1 . It can be used to measure patellar instability on the axial plane in patients with OPI , especially in the most severe cases .