IMAGING OF EXTERNAL PATELLAR INSTABILITIE S - STATE-OF-THE-ART

External femoropatellar instability is a dynamic abnormality from vari ous 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 ana lysis distinguishes between permanent, traumatic or transient dislocat ions which are now more frequently discovered as part of a femoro-pate llar syndrome with or without cartilage involvement. Conventional imag ing, 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. T he femoropatellar component of the knee arthrography visualizes rather large cartilaginous lesions. CT-scan, better than skyline views, allo ws examining the patellar bone without interference with the trochlea (extended knee), during the engagement (15 degrees flexed knee) and af ter the engagement (30 degrees flexed knee). However, the examination technique varies from one author to another according to his own patho physiologic understanding. With the bicondylar plane reference, the re liability of the CT-scan measurements are better than skyline views. L ike the dynamic tests during the beginning of the patellar engagement at 15 degrees, flexion is more sensitive than those at 30 degrees. Fin ally, 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 si ngle clinical examination cannot differenciate from other internal kne e disorders. MR imaging is more precise in analyzing the cartilaginous structure. Kinematic MR imaging, still in an experimental stage, offe rs a new approach to the dynamic study of the patellar tracking.