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

Abstract

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). 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). 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. 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 II, cohort study (diagnosis).