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Introduction: A new factor for patellofemoral instability-external torsion of the tibial tubercle-has recently been described. The primary aim of this biomechanics study was to analyze the consequences of internal torsion tibial tubercle osteotomy (TTO) on an experimentally unstable patella. We hypothesized that internal TTO can stabilize an experimental patellar instability., Materials and Methods: This in vitro study was conducted on six fresh anatomical specimens. The knees were flexed to 25°. The patella was destabilized by transecting the patellar retinaculae and the vastus medialis tendon and by applying continuous oblique traction on the quadriceps tendon. A 3D stereovision system was used to record patellar displacement and tilt and to determine whether patellar dislocation occurred. The measurements were done before the osteotomy then repeated on the same knee after a triangular internal torsion 30° TTO was completed, without medialization., Results: There was a significant difference in the patellar displacement and tilt before and after the osteotomy (p<0.05). Patellar dislocation, which was induced by traction on all the knees before osteotomy, did not occur after the osteotomy was performed., Discussion: Internal torsion of the tibial tubercle improves patellar stability, confirming our hypothesis. These findings confirm the stabilizing effect of placing the tibial tuberosity in internal torsion. Although a knee without instability factors is not the perfect model for patellar instability, our findings suggest that tibial tubercle torsion influences patellar stability. Internal TTO may be justified as a surgical treatment of patellofemoral instability., (Copyright © 2020 Elsevier Masson SAS. All rights reserved.)

Introduction: External torsion of the anterior tibial tubercle (TT), defined as external rotation around a craniocaudal axis with respect to the posterior femoral condylar plane, may induce patellar instability. To our knowledge no studies have focused on this parameter. The present study aimed to perform an MRI analysis of TT torsion. The study hypothesis was that TT torsion correlates with patellar instability and with 3 of its components: tibial tubercle-trochlear groove (TT-TG) distance, axial engagement index of the patella (AEI), and patellar tilt., Material and Methods: Four observers performed MRI measurements for 2 groups: 37 patellar instability patients (PI group) with history of at least 2 patellar dislocations, and 50 control patients with meniscal lesion but free from patellofemoral pathology. All measurements were taken from 2 axial slices with the posterior condylar plane as reference., Results: The intra-class correlation coefficient (ICC) was 0.88. TT torsion correlated with patellar instability, with a mean 5.8̊ in controls and 17.9̊ in the PI group (P<0.001). There were also excellent correlations between TT torsion and TT-TG distance, patellar tilt and patellar lateralization (measured by AEI), with correlation coefficients greater than 0.85., Discussion: TT torsion is a reproducible measurement, with excellent ICC. It is significantly correlated with patellar instability, with a discrimination threshold of 11.5̊, and correlations with all 3 components of instability. These statistical correlations enable TT torsion to be added to the list of patellar instability factors. Further studies should determine its biomechanical role and assess the contribution of associating TT derotation to medialization or distalization procedures., Level of Evidence: III; case-control study., (Copyright © 2017 Elsevier Masson SAS. All rights reserved.)

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., (Copyright © 2013 Elsevier Masson SAS. All rights reserved.)