Patellar instability, usually manifested by recurrent patellar dislocations, is a complex multifactorial pathology. Its treatment associates surgical gestures chosen according to a precise evaluation of the factors of the instability. Five factors are decisive, varying from one subject to another: rupture of the medial patellofemoral ligament, dysplasia of the trochlea, excessive lateralization of the anterior tibial tuberosity (ATT), external torsion of the ATT and patella alta, object of this study. The patella, when it is alta, is an important factor of instability because it is abnormally located above the trochlea in extension of the knee and is no longer guided at the start of flexion: it risks not engaging in the trochlear rail and thus to dislocate at the start of flexion.~The measurement of the patellar height is essential to decide and quantify the surgical correction to be made. The classic index is that of Cato and Deschamps. It has the advantage of its simplicity, with the measurement on a simple profile radiograph of the ratio between the distance from the patella to the tibia and the length of the patella. However, it has the disadvantage of measuring this height in relation to the tibia when the dislocation is located at the level of the patello-trochlear joint. The MRI index of sagittal patellar engagement on the trochlea (IES), described in 2012, does not have this drawback but it does not appear to be reliable: its value depends, in fact, on the flexion of the knee. This flexion, required by the MRI, is variable and is not specified by the radiologist. It is due to the use of an antenna that surrounds and lifts the knee.~The hypothesis of the study is that it is possible to determine and validate a new IES, in knee extension and therefore reproducible, thanks to the measurement of knee flexion on MRI. The precise value of this flexion can allow a virtual correction of the original IES to obtain, by mathematical calculation, the new IES in virtual extension.
The measurement of the patellar height is essential to decide and quantify the surgical correction to be made. The classic index is that of Cato and Deschamps. It has the advantage of its simplicity, with the measurement on a simple profile radiograph of the ratio between the distance from the patella to the tibia and the length of the patella. However, it has the disadvantage of measuring this height in relation to the tibia when the dislocation is located at the level of the patello-trochlear joint. The MRI index of sagittal patellar engagement on the trochlea (IES), described in 2012, does not have this drawback but it does not appear to be reliable: its value depends, in fact, on the flexion of the knee. This flexion, required by the MRI, is variable and is not specified by the radiologist. It is due to the use of an antenna that surrounds and lifts the knee.~The hypothesis of the study is that it is possible to determine and validate a new IES, in knee extension and therefore reproducible, thanks to the measurement of knee flexion on MRI. The precise value of this flexion can allow a virtual correction of the original IES to obtain, by mathematical calculation, the new IES in virtual extension.