The effect of native knee rotation on the tibial-tubercle-trochlear-groove distance in patients with patellar instability: an analysis of MRI and CT measurements

Background This study aimed to quantify the effect of lower limb rotational parameters on the difference in the tibial-tubercle-trochlear-groove (TTTG) distance when assessed with magnetic resonance imaging (MRI) and computed tomography (CT) in patients with patellar instability. It was hypothesized that an increased native knee rotation angle significantly contributes to an underestimation of TTTG by MRI. Methods Forty patients with patellar instability who had undergone standard radiographs, MRI and CT scans were included in this retrospective study. A musculoskeletal radiologist assessed all imaging for TTTG, femoral and tibial rotation, knee rotation and flexion angle, and trochlear dysplasia. Delta TTTG was defined as the TTTG measured on MRI subtracted from the TTTG measured on CT. Statistical analysis determined the effect of these parameters on the calculated difference between TTTG when measured on CT and MRI. Results Equal knee flexion in MRI and CT resulted in a Delta TTTG of 0.1 +/- 0.3 mm compared to 4.0 +/- 3.3 mm in patients with different knee flexion angles in both imaging acquisitions (p = 0.036). The knee rotation angle measured on CT (native knee rotation angle) was negatively correlated with Delta TTTG (r = - 0.365; p = 0.002), while neither tibial nor femoral rotation showed any associations with TTTG (n.s.). Trochlear dysplasia did not show any significant correlation with Delta TTTG, regardless of classification by Dejour or Lippacher (n.s.). Both the native knee rotation angle and the MRI knee flexion angle were independent predictors of Delta TTTG, yet with an opposing effect (knee rotation: 95% Confidence Interval [CI] for beta - 0.468 to - 0.154, p < 0.001; knee flexion 95% CI for beta 0.292 to 0.587, p < 0.001). Patients with a native knee rotation angle > 20 degrees showed a Delta TTTG of - 5.8 +/- 4.0 mm (MRI rather overestimates TTTG) compared to 0.9 +/- 4.1 mm Delta TTTG (MRI rather underestimates TTTG) in patients with < 20 degrees native knee rotation angle. Conclusion The native knee rotation angle is an independent, inversely correlated predictor of Delta TTTG, thus opposing the effect of knee flexion during MRI acquisition. Consequently, these results suggest that not only knee flexion but also knee rotation should be appreciated when assessing TTTG during patellar instability diagnostic evaluation as it can potentially lead to a false estimation of the TTTG distance on MRI.