Tibial Tubercle-to-Posterior Cruciate Ligament Measurement in Patellar Instability Is Influenced by Axial Slice Selection and Tibial Width Normalization.

PURPOSE: To quantify the variability in tibial tubercle-to-posterior cruciate ligament (TT-PCL) measurements using previously established methodology on axial magnetic resonance imaging scans in patients with and without a history of patellar instability based on placement of the dorsal condylar line. METHODS: A retrospective review was performed of patients undergoing isolated medial patellofemoral ligament reconstruction at a single institution (2018-2021). TT-PCL was measured using 3 distal-proximal placements of the dorsal condylar line (proximal, middle, and distal), and values were compared between the instability and control groups and across measurement locations. Analyses were repeated after normalization to tibial width. RESULTS: This study included 64 patients with patellar instability and 30 healthy controls, with no clinically meaningful differences in age or sex distribution. Mean proximal, middle, and distal TT-PCL distances were higher in the patellar instability cohort than controls (23.10 ± 3.86 vs 21.31 ± 3.98 mm; 23.58 ± 3.80 vs 21.56 ± 3.67 mm; 24.40 ± 4.43 vs 22.13 ± 4.13 mm, respectively), with significant between-group differences at all axial locations (all P < .05). Normalized TT-PCL/tibial width ratios also remained higher in the instability cohort (0.32 ± 0.06 vs 0.29 ± 0.05 proximal; 0.33 ± 0.06 vs 0.29 ± 0.06 middle; 0.34 ± 0.06 vs 0.30 ± 0.05 distal), with reduced variability. Receiver operating characteristic analysis showed moderate discrimination (area under the curve 0.67 proximal, 0.67 middle, and 0.69 distal). CONCLUSIONS: TT-PCL measurements increase as the dorsal tibial condylar line is selected more distally, underscoring the need for consistent reference point selection. Normalizing to tibial width reduces variability and enhances comparability across patients. Among the evaluated axial locations, distal TT-PCL measurements showed the strongest discriminatory performance for patellar instability, although the magnitude of this effect was modest. CLINICAL RELEVANCE: Future research using a TT-PCL measurement must report on and be consistent in selecting the axial height from which the magnetic resonance imaging cut is taken.