Measurement of tibial tuberosity-trochlear groove distance by MRI. Assessment and correction of knee positioning errors

The tibial tuberosity-trochlear groove (TTTG) distance varies with the position of the knee in the MR or CT scanner. We present and assess a simple method for adjustment of adduction or abduction of the knee. MRI of the knee encompassing a three-dimensional (3D) sagittal sequence including ≥ 8 cm of the proximal tibia was analyzed (29 females, 17 males; median age 45 years). Using 3D visualization software, the central longitudinal axis of the proximal tibia (TA) was constructed, and the TTTG distance was measured before and after alignment of the TA. Observer reliability was assessed with inter- and intra-class correlation coefficient (ICC) and Bland-Altman plots. Adduction of the knee occurred in 26 examinations, mean 2.7° (range 0.0° to 9.4°), and abduction in 20 examinations, mean 2.6° (range 0.0° to 7.2°). Following adjustment, the mean TTTG distance increased 2.4 mm (range 0.0 to 6.7 mm) in the knees positioned in adduction and decreased 2.3 mm when in abduction (range 0.0 to 5.5 mm). The correlation coefficient (r${}^2$) between the deviation in adduction and abduction and the difference between TTTG unadjusted and adjusted was r${}^2$ = 0.96. ICCs were excellent, but limits of agreement were close to ± 3 mm. Measurement of the TTTG distance by MRI is influenced by a systematic technique-dependent error caused by knee positioning in adduction or abduction. We suggest a simple method for adjusting the positioning.