Magnetic Resonance Imaging Evaluation of Patellofemoral Instability with Emphasis on Tibial Tuberosity–Trochlear Groove Ratio: A Case Series

Abstract Patellar instability is a multifactorial condition influenced by trochlear morphology, patellar height, extensor mechanism alignment, and soft tissue integrity. The tibial tuberosity–trochlear groove (TT–TG) distance is commonly used to assess lateralization; however, borderline values remain difficult to interpret. The TT–TG ratio has been proposed as a normalized parameter to improve assessment. Our aim was to evaluate magnetic resonance imaging (MRI) based parameters of patellofemoral instability with emphasis on the role of the TT–TG ratio in interpreting borderline TT–TG values. This retrospective case series included three patients with clinically suspected patellar dislocation who underwent MRI evaluation. Parameters assessed included trochlear morphology (trochlear depth, lateral trochlear inclination, and sulcus angle), patellar height (Insall–Salvati ratio [ISR] and Caton–Deschamps index), patellar alignment (patellar tilt angle and bisect offset), and extensor mechanism alignment (TT–TG distance and TT–TG ratio). Soft tissue structures, including the medial patellofemoral ligament, were also evaluated. Two patients had recurrent dislocation following trivial trauma, while one had acute dislocation after significant trauma. Borderline trochlear morphology was observed in two cases. One patient demonstrated patella alta based on ISR. All cases showed lateral patellar maltracking. TT–TG distance ranged from 13 to 19.5 mm (normal to borderline), whereas TT–TG ratio ranged from 0.20 to 0.26, with one case showing abnormal lateralization. Higher TT–TG ratios corresponded with increased bisect offset and greater lateral patellar displacement. These findings suggest that patellar instability results from a combination of anatomical and biomechanical factors. The TT–TG ratio demonstrated a better correlation with lateral patellar maltracking compared to absolute TT–TG distance, particularly in borderline cases.