INTRODUCTION:Patella alta is one of the primary factors of patellofemoral instability and its importance lies in the reduced engagement between patella and trochlea during the early degrees of flexion. The evaluation of patellar height is based on conventional x-rays, CT scan and, more recently, MRI. The objective of this multicentric prospective study is to describe a novel index to assess in the sagittal plane the functional engagement between patella and trochlea. MATERIALS AND METHODS:One hundred and thirty-five patients with objective patellar dislocation were prospectively enrolled between April 2010 and September 2011 and were compared with a second group of 45 controls. All patients underwent a standard MRI and a complete radiographic study. Sagittal engagement was measured as the ratio between the articular cartilage of the patella and the trochlear cartilage length measured on two different MRI slices. RESULTS:The mean Sagittal Patellofemoral Engagement (SPE) index was 0.43 ± 0.18 and ranged from 0.02 to 0.913 in the Objective Patellar Dislocation group versus 0.42 ± 0.11 range 0.22 to 0.55 in controls. In the Patellar Dislocation group the mean Caton-Deschamps index was 1.18 ± 0.21 (range 0.71 to 1.91). There were 58 patients with patella alta, in whom the mean SPE was 0.39 ± 0.18 (range 0.02 to 0.87). Sagittal engagement was significantly higher when compared with patients in the Patellar Dislocation group who had no patella alta (mean 0.46 ± 0.16, range 0.1-0.913). DISCUSSION:The present study introduces a new method to measure the SPE with the use of MRI. The evaluation of the functional engagement of the patella with the femoral trochlea in the sagittal plane can serve as a supplementary tool to the existing methods of evaluating patellar height, and may help to better identify the cases where inadequate engagement is recorded despite the absence of patella alta, so that the need for tibial tuberosity osteotomy may be re-assessed.

La patella alta est un des facteurs principaux de l’instabilité fémoro-patellaire. Son importance est liée à un engagement réduit de la patella dans la trochlée pendant les premiers degrés de flexion. L’évaluation de la hauteur patellaire est basée sur des radios conventionnelles, sur le scanner et plus récemment sur l’IRM. L’objectif de ce travail multicentrique prospectif est de décrire un nouvel index pour évaluer l’engagement sagittal fonctionnel de la patella dans la trochlée. Cent trente-cinq patients avec une luxation patellaire objective ont été prospectivement inclus dans une étude entre avril 2010 et septembre 2011. Ils ont été comparés à un groupe témoin de 45 patients. Tous les patients ont eu une IRM standard et un bilan radiographique complet. L’engagement sagittal était mesuré en établissant un ratio entre les surfaces articulaires patellaire et trochléenne mesurée sur 2 coupes IRM différentes. L’engagement sagittal moyen de la patella était de 0,43 ± 18. Il allait de 0,02 à 0,91 dans les instabilités patellaires objectives contre 0,42 ± 11 (0,22 à 0,55) dans le groupe témoin. Dans le groupe de luxation patellaire objective, l’index de Caton-Deschamps moyen était de 1,18 ± 21 (0,71 à 1,91). Il y avait 58 patients avec une patella alta dont l’index sagittal d’engagement était de 0,39 ± 18 (0,002 à 0,87). L’engagement sagittal était significativement plus haut dans le groupe de luxation patellaire objective qui n’avait pas de patella alta (0,46 ± 16) (0,1 à 0,91). Cette étude introduit une méthode pour quantifier l’engagement sagittal de la patella en utilisant l’IRM. L’engagement fonctionnel de la patella avec la trochlée dans le plan sagittal peut être un instrument supplémentaire aux méthodes de mesure de hauteur patellaire. La mesure combinée de la hauteur et de l’engagement permettront d’affiner les indications d’abaissement de la tubérosité tibiale. II, étude prospective non randomisée.

INTRODUCTION: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).MATERIALS AND METHODS: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).RESULTS: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).DISCUSSION: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.