Anteroposterior distance between the tibial tuberosity and trochlear groove in patients with patellar instability

Background: Our objective was to describe a measurement to assess sagittal tibial tuberosity (TT)–trochlear groove (TG) distance and to compare this between asymptomatic (control) patients and patients with symptomatic patellar instability. Methods: We compared static CT images of 22 fully extended knees of patients with symptomatic patellar instability with images of 22 asymptomatic knees. TT–TG distance was measured to quantify lateralization of the TT, and anteroposterior TT–TG distance was used to quantify the sagittal distance between these two points. Lateral trochlear inclination, sulcus angle, and trochlear depth were measured. Groups were compared using paired t tests (alpha = 0.05). Correlations of anteroposterior TT–TG distance with lateral trochlear inclination, sulcus angle, and trochlear depth were assessed using linear and multivariate regression. Results: Mean TT–TG distances were 19.9 ± 4.4 mm (symptomatic) and 16.8 ± 5.5 mm (control) (mean ± std deviation) (P = 0.002). Mean anteroposterior TT–TG distances were 8.3 ± 7.8 mm (symptomatic) and − 0.5 ± 4.6 mm (control) (P < 0.0001). The symptomatic group had greater measurements of trochlear dysplasia, with lower lateral trochlear inclination, greater sulcus angle, and lower trochlear depth (all P < 0.0001). Anteroposterior TT–TG distance and trochlear depth were strongly negatively correlated (r = 0.62, R² = 0.39, P < 0.0001). Conclusions: In asymptomatic patients, the anteroposterior TT–TG distance was − 0.5 mm, indicating that the TG and TT were nearly in the same coronal plane. In patients with symptomatic patellar instability, the TG was almost nine millimeters anterior, and this distance correlated with measurements of trochlear dysplasia. Level of evidence: III, case control study.