CT and MRI measurements of tibial tubercle-trochlear groove distances are not equivalent in patients with patellar instability

Background: Tibial tubercle-trochlear groove distance (TT-TG) is a commonly used measurement for surgical decision making in patients with patellofemoral malalignment and instability. This measurement has historically been performed utilizing axial computed tomography (CT). More recently, magnetic resonance imaging (MRI) has been proposed as an equivalent method, but this has not yet been fully validated. Purpose: To determine the reliability of TT-TG distance measurements on both MRI and CT and to determine whether the measurements are interchangeable with one another. Study Design: Cohort study (diagnosis); Level of evidence, 2. Methods: All patients with patellar instability who underwent both CT and MRI of the knee from 2003 to 2011 were included (n = 59 knees in 54 patients). Two fellowship-trained musculoskeletal radiologists measured the TT-TG distances for each patient by CT and MRI in a randomized, blinded fashion. Interobserver reliability was calculated between radiologists for both imaging modalities, and intermethod reliability was calculated between the 2 imaging modalities. The results are reported using intraclass correlation coefficients (ICCs) and Bland-Altman analysis. Results: The 59 knees had a mean TT-TG distance of 16.9 mm (range, 8.3-25.8 mm) by CT and 14.7 mm (range, 1.5-25.1 mm) by MRI. Interobserver reliability between the radiologists was considered excellent for both CT and MRI (ICC = 0.777 and 0.843, respectively). When comparing CT to MRI, the ICC was considered only fair for each of the raters (0.532 and 0.539). Eleven patients (19%) had a TT-TG distance of °20 mm on CT preoperatively and underwent distal realignment by tibial tubercle osteotomy. In this surgical subgroup, the mean TT-TG distance was 22.5 mm (range, 19.8-25.8 mm) by CT and only 18.7 mm (range, 14.4-22.8 mm) by MRI for a mean difference of 3.80 mm (P<.001). Conclusion: The TT-TG distance can be measured with excellent interrater reliability on both MRI and CT; however, the values derived from these 2 tests may not be interchangeable. This observation should be taken into consideration when MRI is used for surgical planning because MRI may underestimate the TT-TG distance when compared with CT.