[Imaging of external instabilities of the patella. State of the art]

External femoropatellar instability is a dynamic abnormality from various origins: osseous, cartilaginous or musculotendinous; X-rays films cannot give a precise enough description of this phenomenon. Attention is drawn by anterior pain or a sensation of instability. Clinical analysis distinguishes between permanent, traumatic or transient dislocations which are now more frequently discovered as part of a femoro-patellar syndrome with or without cartilage involvement. Conventional imaging, CT-scan and MR imaging are based on faultless techniques. Lateral views precisely report femoropatellar architectural abnormalities and patellar instability. Skyline views are able to quantify the various parts of the dysplasia. Dynamic tests increase the sensitivity of the plain films. But the main shortcoming of these techniques is the lack of visualization of the initial patellar engagement in the trochlea. The femoropatellar component of the knee arthrography visualizes rather large cartilaginous lesions. CT-scan, better than skyline views, allows examining the patellar bone without interference with the trochlea (extended knee), during the engagement (15 degrees flexed knee) and after the engagement (30 degrees flexed knee). However, the examination technique varies from one author to another according to his own pathophysiologic understanding. With the bicondylar plane reference, the reliability of the CT-scan measurements are better than skyline views. Like the dynamic tests during the beginning of the patellar engagement at 15 degrees, flexion is more sensitive than those at 30 degrees. Finally, CT-scan arthrography demonstrates thinner cartilaginous lesions than conventional arthrography. Presently the main contribution of MR imaging consists of detecting transient patellar dislocation that a single clinical examination cannot differentiate from other internal knee disorders. MR imaging is more precise in analyzing the cartilaginous structure. Kinematic MR imaging, still in an experimental stage, offers a new approach to the dynamic study of the patellar tracking.