[Evaluation of anterior knee laxity on MRI]. / Évaluation de la laxité antérieure du genou en IRM: la laxi-IRM.

PURPOSE: Evaluation of the ACL and anterior knee laxity on MR during anterior tibial translation. PATIENTS AND METHODS: Three groups were identified based on clinical and arthrometric (KT-1000) data: normal ACL (n=12), complete tear (n=10) and partial tear (n=20). MRI was performed without and with anterior tibial translation (pneumatic device) with morphological and laximetric analysis: drawer tests and dynamic evaluation of ligamentous tension. RESULTS: Intra- and inter-observer reproducibility was excellent, correlated to arthrometric data and clinical tests (Lachman, pivot shift). The difference between the drawer signs of normal subjects and patients with ACL tear was significant for a threshold value of 1,1mm for the anterior drawer (sensitivity: 93.33%, specificity: 91.7%) and 2.8 mm for the posterior drawer (sensitivity: 86.7%, specificity: 100%). Dynamic evaluation of ligamentous tension was also reproducible, statistically correlated to the MR drawer tests and reliable for the diagnosis of ACL lesions. In this preliminary study, the distinction between complete and partial ACL tears could not be detected. CONCLUSION: Anterior cruciate ligament function can be demonstrated on MR. The predictive value of this morphological and functional association should be determined in the management of patients with partial tears.

Helicity redistribution during relaxation of astrophysical plasmas

We present the first 3D numerical MHD simulations that show that Taylor's relaxation conjecture is not satisfied in some MHD evolution of magnetic configurations encountered in solar physics. We show that magnetic helicity can be slowly injected through the boundary into a magnetic configuration which then evolves into a MHD disruption, with the formation in finite time of a current sheet through which reconnection occurs, leading to a release of magnetic energy. While helicity is well conserved during the process, it is shown that the relaxed state is far from the constant- alpha linear force-free field that would be predicted by Taylor's conjecture.

A Twisted Flux Rope Model for Coronal Mass Ejections and Two-Ribbon Flares.

We present a new approach to the theory of large-scale solar eruptive phenomena such as coronal mass ejections and two-ribbon flares, in which twisted flux tubes play a crucial role. We show that it is possible to create a highly nonlinear three-dimensional force-free configuration consisting of a twisted magnetic flux rope representing the magnetic structure of a prominence (surrounded by an overlaying, almost potential, arcade) and exhibiting an S-shaped structure, as observed in soft X-ray sigmoid structures. We also show that this magnetic configuration cannot stay in equilibrium and that a considerable amount of magnetic energy is released during its disruption. Unlike most previous models, the amount of magnetic energy stored in the configuration prior to its disruption is so large that it may become comparable to the energy of the open field.