Systematic Technique-Dependent Differences in CT Versus MRI Measurement of the Tibial Tubercle–Trochlear Groove Distance

BACKGROUND: The tibial tubercle-trochlear groove (TTTG) distance is used to quantify the degree of lateralization of the patellar tendon insertion on the tibial tubercle relative to the deepest part of the trochlear groove. Disagreement exists as to whether the TTTG distance measured on computed tomography (CT) and magnetic resonance imaging (MRI) can be considered equivalent. PURPOSE: To compare TTTG distance as measured on axial CT and MRI and to investigate the potential effect of patient positioning between modalities. STUDY DESIGN: Cohort study (diagnosis); Level of evidence, 3. METHODS: Patients who received both CT and MRI of the same knee for any indication from August 2010 to April 2014 were included in this study. The TTTG distances were measured twice by 2 raters in a randomized order, with at least 30 days between ratings to minimize recall bias. Inter- and intrarater reliability of CT and MRI measurements and intermethod reliability were assessed with intraclass correlation coefficients (ICCs). Bland-Altman plots were also created to assess agreement. Differences in patient positioning were investigated to determine its effect on the TTTG distance. RESULTS: Fifty-nine patients (age, 32.8 ± 12.9 years) were included. Interrater ICCs were excellent for both CT and MRI measurements. Intrarater ICCs were excellent for both raters. Absolute agreement ICCs for intermethod reliability were fair to good, but consistency type agreement was excellent. A systematic bias of lower MRI distances (bias = -2.8 mm) compared with CT was observed. The investigation of CT versus MRI imaging techniques demonstrated that the standard MRI examination places the knee in approximately 4.6° of relative varus alignment compared with CT. CONCLUSION: A systematic bias toward lower TTTG distances on MRI compared with CT was found. This finding is likely dependent on imaging technique, including patient positioning. Patient knees were positioned in varus on the MRI compared with the CT examination, with resulting lower TTTG distances on MRI compared with CT. The TTTG distances on CT and MRI vary with imaging technique, which may be attributable to patient positioning and result in differences among imaging centers.