Underestimation of tibial tuberosity–trochlear groove distance in conventional knee magnetic resonance imaging compared to full-extension imaging

Purpose: To assess the effect of the knee flexion on the tibial tuberosity–trochlear groove (TT–TG) distance measurement by comparing conventional and whole-leg rotational magnetic resonance imaging (MRI). Methods: This retrospective cohort study included 45 patients with patellar dislocations who had undergone both conventional knee MRI with the knee in slight knee flexion (20°–30°) and full-length rotational MRI in extension (0°–5°). TT–TG distances were determined by two independent observers using standardised landmarks. Measurement reproducibility was evaluated using intraclass correlation coefficients (ICCs) for both intra and interrater reliability (3-month interval). Results: TT–TG distance values derived from conventional MRI were significantly smaller (13.4 ± 4.0 mm, 95% confidence interval [CI]: 12.2–14.6) compared to those from whole-leg rotational MRI (18.3 ± 4.4 mm [95% CI: 16.9–19.6]), yielding a mean difference of 4.8 ± 3.0 mm (95% CI: 3.9–5.8; p < 0.001). Both interrater (conventional MRI: ICC = 0.946 [95% CI: 0.901 – 0.971]; rotational MRI: ICC = 0.949 [95% CI: 0.899–0.973]) and intrarater reliability (conventional MRI: ICC = 0.995 [95% CI: 0.992–0.997]; rotational MRI: ICC = 0.991 [95% CI: 0.983–0.995]) were excellent across modalities. Conclusion: Conventional knee MRIs, performed with knee in slight flexion, are consistently smaller compared to those acquired in full extension whole-leg rotational MRI. Knee flexion, therefore, leads to a systematic underestimation of the TT–TG distance. Standardising knee position during imaging is essential to ensure accurate and comparable TT–TG assessments. Level of Evidence: Level II, cohort study (diagnosis).