The Tibial Tubercle–to–Trochlear Groove Distance Is Reliable in the Setting of Trochlear Dysplasia, and Superior to the Tibial Tubercle–to–Posterior Cruciate Ligament Distance When Evaluating Coronal Malalignment in Patellofemoral Instability

Purpose To determine best practices for consistent and accurate evaluation of coronal alignment in patients with patellofemoral (PF) instability. Methods Six reviewers examined 239 knee magnetic resonance images (MRIs) in patients with PF instability and anterior cruciate ligament (ACL) rupture. Measurements included tibial tubercle–to–trochlear groove (TT-TG) distance measured at the most proximal and distal portions of the trochlea, tibial tubercle–to–PCL (TT-PCL) distance, and Dejour classification of trochlear dysplasia. Results Interrater reliability was low for Dejour classification (k = 0.289), but improved to moderate (k = 0.448) when patients were separated into normal/Dejour A and Dejour B/C/D. Interrater reliability was high for proximal and distal TT-TG measurements (interclass correlation coefficients [ICCs] = 0.807 and 0.936, respectively). TT-PCL was moderately reliable (ICC = 0.625), and correlated with TT-TG (r = 0.457, P <.001 proximal and r = 0.451, P <.001 distal). No significant difference was found between the proximal and distal measurements of TT-TG in each patient, though the PF group exhibited higher values than the ACL group (P <.001 for both). TT-PCL was significantly higher for the PF group than the ACL group (P =.015), but this difference lost significance when the group was divided by the TT-PCL cutoff of 24 mm (P =.371). Conclusions The proximal and distal techniques for measuring the TT-TG distance are similar to each other, and reliable despite level of reviewer training or presence of dysplasia. The TT-TG distance was predictive of patellofemoral instability. The TT-PCL distance was found to be less reliable than either method of measuring the TT-TG distance. Thus, this study demonstrated TT-TG to be superior to TT-PCL as a measurement of coronal malalignment. Given the variability in Dejour classification in this and other studies, a more reliable classification system for trochlear dysplasia as defined on cross-sectional imaging is warranted. Level of Evidence Level III, retrospective clinical trial.