Using sunrise to surmise acute transient patellar dislocation.

purposeTo determine if there is a comparative measurement on bilateral sunrise view radiographs that can help predict acute transient patellar dislocation (ATPD).

methodsA retrospective chart review from a single institution was conducted of two patient groups, a case group with ATPD diagnosed by MRI, and a control group with knee injury not involving the patella or medial retinaculum. Three readers blinded to the MRI diagnosis reviewed the sunrise view radiographs and reported three values for both knees: (1) medial trochlea - medial patella distance (MT-MP); (2) lateral trochlea -medial patella distance (LT-MP); and (3) medial patellofemoral angle (MPFA), (Image 2). Diagnostic accuracy was assessed utilizing empirical ROC curves and their corresponding area under the curves (AUC). Inter-reader reliability was determined using intraclass correlation coefficients (ICC). Sensitivity and specificity were calculated for varying differences in MPFA and MT-MP between affected knee and unaffected knee.

resultsOf the three measurements, increased MPFA measured on the injured knee was the most accurate predictor of ATPD with substantial inter-reader reliability (AUC 0.765, ICC 0.681). MT-MP (AUC 0.707, ICC 0.586) and LT-MP (AUC 0.593, 0.189) distances were less accurate and less reliable predictors. Asymmetric difference in the MPFA or MT-MP between affected knee and unaffected knee was a very specific, albeit not sensitive indicator of ATPD.

conclusionComparative sunrise view knee radiographs can help predict the diagnosis of ATPD with high specificity, prompting early MRI evaluation for definitive diagnosis, helping expedite treatment planning.