O2097: The Natural History of Imaging Parameters of the Knee In Patellofemoral Instability Patients Throughout Skeletal Maturation

Background: Numerous radiographic anatomic risk factors for patellofemoral instability (PFI). been described and limited prior work has examined the evolution of these measures among skeletally immature patients with and without PFI. However, consideration for skeletal age, which is known to widely differ among patients and across sexes, and inclusion of newer predictive measures is lacking in this work. The purpose of this study was to broadly assess patellofemoral morphologic changes on magnetic resonance imaging (MRI) among PFI patients through skeletal maturation.

Methods: A retrospective cohort study was performed of PFI patients with knee MRIs at a single institution from 2011-2021. Patients undergoing prior surgery or with history of tumor, congenital or syndromic etiologies of patellar instability were excluded. Studies with a relative even age and gender distribution across patients of chronologic ages 9-18 years-old were identified. MRI measurements of axial alignment (Tibial Tubercle-Trochlear Groove [TT-TG], Patellar Tendon-Lateral Trochlear Ridge [PT-LTR], and Tibial Tubercle-Lateral Trochlear Ridge [TT-LTR] distances), patellar height (Caton-Deschamps Index [CDI] and Insall-Salvati Index [ISI]), and trochlear morphology (Trochlear Depth Index [TDI], Lateral Trochlear Inclination Angle [LTIA], and Sulcus Angle [SA]) were performed. Knee MRI-based skeletal ages were assigned and standardized across sexes with a 2-year adjustment . The relationship between radiographic indices and skeletal age were examined with linear correlation.

Results: Study criteria identified 178 patients (56% female) for inclusion with a mean chronologic age of 13.3 (Range: 9.4-18.9). Mean values for all imaging measures across the entire cohort were: TT-TG [16.0±5.0], TT-LTR [-5.1±5.1], PT-LTR [7.9±5.9], SA [162.0±12.2], LTIA [11.9±7.0], TDI [1.9±2.3], CDI [1.2±0.2] and ISI [1.4±0.2]. TDI demonstrated a very weak positive correlation with increasing skeletal age (r=0.170, p=0.023) while SA (r=-0.184, p=.014) and ISI (r=-0.211, p=.005) had very weak and weak negative correlations, respectively (Figure 1). TT-TG, TT-LTR, PT-LTR, LTIA, and CDI showed no significant correlation with skeletal age (p>0.05).

Conclusion: In contrast to non-PFI patients in prior literature, imaging anatomic risk factors for PFI are abnormal among PFI patients throughout maturation and show little evolution towards reducing risk during skeletal development. These findings reinforce the notion that patients with PFI may have both congenital and developmental factors contributing to their enhanced risk. Furthermore, this study bolsters the limited literature base with respect to the evolution of radiographic anatomic risk factors for PFI in patients during skeletal growth using skeletal age to standardize patients across sex and chronological age. The measures examined do not appear to follow the natural history reported in healthy knees