End-stage extension of the knee and its influence on tibial tuberosity-trochlear groove distance (TTTG) in asymptomatic volunteers

Increased tibial tuberosity-trochlear groove distance (TTTG) is one potential correcting parameter in patients suffering from lateral patellar instability. It was hypothesized that end-stage extension of the knee might influence the TTTG distance on MR images. Transverse T1-weighted MR images of the knee were acquired at full extension, 15A degrees and 30A degrees flexion of the knee in 30 asymptomatic volunteers. MRI parameters: slice thickness: 3 mm, matrix: 256 x 384, FOV: 150 x 150 mm. Two observers independently measured the TTTG at all positions. Mean TTTG for observer 1 was 15.1 +/- A 3.2 mm at full extension, 10.0 +/- A 3.5 mm at 15A degrees flexion and 8.1 +/- A 3.4 mm at 30A degrees flexion. Mean TTTG for observer 2: 14.8 +/- A 3.3 mm at full extension, 9.4 +/- A 3.0 mm at 15A degrees flexion, 8.6 +/- A 3.4 mm at 30A degrees flexion. Mean values were significantly different (p < 0.001) between full extension and 15A degrees as well as 30A degrees flexion for both observers. Mean values were significantly different (p < 0.001) between 15A degrees and 30A degrees for observer 1, but not for observer 2 (n.s.). Interobserver agreement was very good (intraclass correlation coefficient: 0.87-0.88; p < 0.001). The TTTG increases significantly at the end-stage extension of the knee. Therefore, the comparability of published TTTG values measured on radiographs, CT and MRI at various flexion/extension angles of the knee are limited. Development of diagnostic criteria in a consecutive series of patients and a universally applied 'gold' standard, Level II.