[Biomechanics and semiology of transient lateral patellar dislocation: reliability of magnetic resonance diagnosis].

Transient patellar dislocation is a rare finding. It is usually due to direct trauma in mediolateral direction or to indirect trauma related to violent quadriceps muscle contraction. The clinical diagnosis of transient forms is usually rather difficult because the associated signs (hemarthros and pain at the joint and/or at the vastus medialis insertion) are not specific. Conventional radiography and CT are very accurate in bone studies, but exhibit major limitations in the detection of capsular injuries with intramedullary and/or cartilaginous bone involvement. We investigated MR diagnostic reliability in the study of the injuries caused by transient patellar dislocation. 526 MR examinations of the knee were reviewed retrospectively; fifteen of them were positive for transient patellar dislocation. We used a .5 T superconductive MR unit with a circumferential extremity coil. Axial, coronal and sagittal T1-weighted spin echo, T2*-weighted gradient echo and axial STIR images were acquired. The following criteria were considered specific for the diagnosis of former lateral patellar dislocation: retinacular changes; patella site; signal intensity changes in femoral and patellar intramedullary bone; joint effusion; cartilage and subchondral bone changes. All the patients with transient patellar dislocation had hemarthros with inner retinaculum involvement. The femoropatellar joint was incongruous in 115 cases; patellar subluxation was external in 8/11 patients and lateral in the remaining 3 patients. Medullary bruises were found in 8/15 cases and cartilage injuries in 13/15. Finally, other joint components, such as the posterior horn of the medial meniscus or the anterior cruciate ligament were involved in 6/13 cases. Arthroscopy was performed in 6 patients with associated injuries and confirmed all MR findings (100% agreement). To conclude, in our experience MRI was a very reliable tool to study the injuries due to transient patellar dislocation because it showed specific changes and possible associated traumas, which helps choose the most suitable treatment.