A Comparative CT Study on Anatomical Characteristics of Pediatric Habitual Patellar Dislocation in Extension and Flexion.

Objective: Habitual patellar dislocation (HDP) in children and adolescents is divided into dislocation in flexion and dislocation in extension, but their pathogenesis remains unclear. Our purpose is to focus on investigating the anatomical difference between the extended dislocation and the flexed dislocation of HDP.

Methods: We retrospectively observed all patients diagnosed as HDP who underwent surgery at our institution from May 2016 to August 2023. Patients were categorized into two subgroups according to "J sign": the extension group and the flexion group. We measured and compared the various anatomical parameters on CT images, including patellar dysplasia, trochlear dysplasia, alignment of the patellofemoral joint, and torsional deformity of the knee joint. Continuous data were analyzed using the t-test, while categorical data were analyzed using Fisher exact test and Mann-Whitney U test.

Results: We enrolled 20 patients (average age of 9 years old, ranging from 6 to 15; 14 patients in the extension group and 6 patients in the flexion group) with 30 knees. The extension group exhibited significantly greater Wiberg index (0.62 ± 0.09 vs. 0.56 ± 0.05, p = 0.030), lateral patellar angle (17.83 ± 4.5 vs. 12.24 ± 5.7, p = 0.009), PT/FT ratio (1.45 ± 0.1 vs. 1.31 ± 0.1, p = 0.005), and Caton-Deschamps index (1.46 ± 0.3 vs. 1.17 ± 0.1, p = 0.015) than those in the flexion group. Meanwhile, the flexion group demonstrated greater patellar lateralization distance (24.03 ± 11.5 vs. 16.26 ± 7.3, p = 0.036), patellar inclination angle (50.75 ± 13.1 vs. 29.98 ± 14.5, p = 0.001), lateral patellofemoral angle (38.49 ± 13.5 vs. 18.32 ± 8.2, p = 0.000) and congruence angle (57.57 ± 23.3 vs. 28.15 ± 16.7, p = 0.001) when knees were extending but a smaller lateral trochlear inclination angle (18.84 ± 6.9 vs. 12.49 ± 5.2, p = 0.026) than those in the extension group. Additionally, the knees in the flexion group predominated the femoral trochlea of Dejour type C and D (100%) while only half of the extension group had Dejour type C and D (51%). The duration of symptoms before surgery was significantly longer in the extension group compared to the flexion group (25.93 ± 25.8 months vs. 9.33 ± 4.18 months, p = 0.034). Parameters related to torsional abnormality of the lower extremity showed no significant difference between the two groups.

Conclusions: Patients with HDP in extension had patella alta, more severe patellar dysplasia, while those with HDP in flexion had a more inclined patella, laterally positioned patella when knees extended, and a more dysplastic femoral trochlea. These differences provide reasonable references for doctors to better understand the mechanisms of dislocation and the design of optimal surgical strategies.

Clinical relevance: Improved understanding of anatomical features of HDP is critical to informing clinical treatment decisions.