Summary

The knee rotation angle is an independent, inversely correlated predictor of the difference between TTTG measured on MRI and CT in patients with patellar instability.

Abstract

Purpose

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 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. deltaTTTG 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 deltaTTTG of 0.1 +/- 0.3° compared to 4.0 +/- 3.3° in patients with different knee flexion angles in both imaging acquisitions (p=0.036). The knee rotation angle measured on CT was negatively correlated with deltaTTTG (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 deltaTTTG, regardless of classification by Dejour or Lippacher (n.s.). Both the CT knee rotation angle and the MRI knee flexion angle were independent predictors of deltaTTTG, 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 CT knee rotation angle >20° showed a deltaTTTG of -5.8 +/- 4.0° (MRI overestimates TTTG) compared to 0.9 +/- 4.1° deltaTTTG (MRI underestimates TTTG) in patients with <20° CT knee rotation angle.

Conclusion

The knee rotation angle is an independent, inversely correlated predictor of deltaTTTG, 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 an overestimation of the TTTG distance on MRI.