Increased tibiofemoral rotation on MRI with increasing clinical severity of patellar instability.

PURPOSE: Tibiofemoral rotation through the knee joint, specifically relative external tibial rotation, has been identified as a potential contributing factor to patellar instability. The purpose of this study is to investigate the relationship between severity of instability with degree of tibiofemoral rotation in three clinical cohorts: fixed or obligatory dislocators (in which the patella either is constantly laterally dislocated or laterally dislocates with every instance of knee flexion, respectively), standard traumatic instability patients, and normal controls. METHODS: A retrospective study was performed with three cohorts from April 2009 to February 2019: fixed or obligatory dislocators, standard traumatic instability patients, and controls with normal magnetic resonance imaging (MRI) of the knee. All fixed or obligatory dislocation patients from the study time frame were analyzed; controls and standard traumatic instability patients were randomly selected. Inclusion criteria were age under 18 years and qualifying diagnosis; exclusion criteria were outside institution MRI and previous MPFL reconstruction or tibial tubercle osteotomy. Tibiofemoral rotation was measured blindly on initial axial MRI using the posterior femoral and tibial condylar lines. Tibial tubercle to trochlear groove distance (TT-TG) was measured. Intraclass correlation coefficient (ICC) was calculated among four measurers. RESULTS: A total of 100 patients were included, 20 fixed or obligatory dislocators, 40 standard traumatic instability patients, and 40 controls. Median age was 13.2 years (range 10-17 years), with 55 females. Age was significantly higher in the standard traumatic instability group than both the control (p < 0.001) and fixed or obligatory dislocator groups (p = 0.003). ICC for TT-TG and tibiofemoral rotation were 0.92 and 0.96, respectively. Fixed or obligatory dislocator patients averaged 8.5° external tibiofemoral rotation, standard traumatic instability patients 1.6° external tibiofemoral rotation, and controls 3.8° internal tibiofemoral rotation. Both tibiofemoral rotation and TT-TG were highest in the fixed or obligatory dislocator cohort, followed by the standard traumatic instability cohort, and lowest in the controls (p < 0.0001 for tibiofemoral rotation and TT-TG). Multivariate analysis showed no correlation between age and tibiofemoral rotation. CONCLUSIONS: Measurement of tibiofemoral rotation was reproducible with excellent interrater reliability. The degree of tibiofemoral rotation is correlated with severity of patellar instability, with the greatest external tibiofemoral rotation in fixed or obligatory dislocator patients, followed by standard traumatic instability patients, and slight internal tibiofemoral rotation in controls. High external tibiofemoral rotation may be an important pathoanatomic factor in fixed or obligatory dislocators, and with further understanding may become a prognostic factor or surgical target. LEVEL OF EVIDENCE: III.