Prevalence and patterns of anatomical risk factors in patients after patellar dislocation: A case control study using MRI

Abstract

Objective: To assess anatomical risk factors in patients after lateral patellar dislocation (LPD) and controls using MRI. Methods: MR images of 186 knees after LPD and of 186 age- and gender-matched controls were analysed. The presence of trochlear dysplasia was assessed by evaluation of trochlear inclination, facet asymmetry, and trochlear depth; patella alta was evaluated by the Insall-Salvati index and Caton-Deschamps index; the lateralised force vector was measured by the tibial tuberosity-trochlear groove (TT-TG) distance. Results: Compared with controls, dislocators had significantly lower values for all three parameters of trochlear dysplasia (-32 %, -32 %, -44 %) and significantly higher values for patella alta (+14 %,+13 %) and TT-TG (+49 %) (all P < 0.001). Trochlear dysplasia was observed in 112 dislocators (66 %), of whom 61 (36 %) additionally had patella alta and 15 (9 %) an abnormal TT-TG. As isolated risk factors, patella alta (15 %) and abnormal TT-TG (1 %) were rare. Only 25 dislocators (15 %) had no anatomical risk factors. Trochlear dysplasia in conjunction with abnormal TT-TG or patella alta is associated with a 37- and 41-fold higher risk. Conclusion: Most dislocators have anatomical risk factors, varying in severity and constellation. Key Points: • Magnetic resonance imaging provides unique information about anatomical variation within the knee. • Anatomical variants increase the risk for lateral patellar dislocation (LPD) • Trochlear dysplasia is the main risk factor for LPD. • Patellar alta and abnormal tibial tuberosity-trochlear groove distance may be additional factors. • Patient-specific evaluation of risk factors following LPD may help future management. © 2012 European Society of Radiology.