Is tibial tuberosity–trochlear groove distance an appropriate measure for the identification of knees with patellar instability?

Abstract

Purpose: Tibial tuberosity–trochlear groove distance (TT–TG) has been regarded as a useful tool for establishing therapeutic choices for patellar instability. Recently, it has been shown that TT–TG negatively correlated with the quadriceps angle, suggesting that if used individually, neither provide a valid measure of instability. This study aimed to compare TT–TG distance between both knees in patients with unilateral instability to assess whether this measurement is a decisive element in the management decisions for patellar instability. Methods: Sixty-two patients (18 male and 44 female), reporting to a specialist patella clinic for recurrent unilateral patellar instability, were included in the study. Patients underwent bilateral long leg computed tomography scan to determine TT–TG distance in both knees. Tibial TT–TG in symptomatic and asymptomatic knees in the same individual was compared statistically. Results: Mean TT–TG distance in the symptomatic knee was 16.9 (±4.9) mm, compared to 15.6 (±5.6) mm in the asymptomatic knee. Tibial TT–TG was not significantly different between stable and unstable knees (n.s.). Conclusions: The lack of difference in TT–TG distance between stable and unstable knees suggests that TT–TG distance alone may not be a decisive element in establishing therapeutic choices for patellar instability. It should, therefore, be interpreted with caution during clinical evaluations. Level of evidence: II.