Tibial tuberosity-trochlea groove distance is dependent on flexion angle and intra-articular version in different magnetic resonance imaging recording techniques.

PURPOSE

The purpose of this study was to compare the tibial tuberosity-trochlea groove distance (TT-TG) and the tibial tuberosity-posterior cruciate distance (TT-PCL) between conventional and rotatory magnetic resonance imaging (MRI). It was hypothesised that the TT-TG varies between the investigated techniques, due to differences in knee flexion angle and intra-articular version. Variations in TT-TG could lead to misdiagnoses and consequently result in inappropriate surgical indications.

METHODS

Twenty-five patients with both a conventional knee MRI and a rotatory MRI (which allows full knee extension) due to recurrent patellar dislocation were included. TT-TG and TT-PCL, knee flexion angle and intra-articular version (external rotation) were determined in each scan. Inter-rater reliability was assessed using the intraclass correlation coefficient (ICC). Between-group differences were assessed using the Wilcoxon matched-pairs signed-rank test. Spearman's correlation coefficient was used to detect correlations between the TT-TG and TT-PCL with flexion angle and intra-articular version.

RESULTS

The TT-TG was significantly ( < 0.05) higher in rotatory MRI (median: 18.4 mm, interquartile range [IQR]: 7.3 mm), in comparison to conventional MRI (median: 12 mm, IQR: 5.7 mm), while no significant differences were observed for TT-PCL. Knee flexion angle was significantly higher in the conventional MRI (median: 21.4°, IQR: 8.6°) compared to rotatory MRI (median: 3.1°, IQR: 3.4°,  < 0.0001). The intra-articular version was significantly lower in the conventional MRI (median: 2°, IQR: 6.3°) compared to the rotatory MRI (median: 9°, IQR: 7.3°,  < 0.0001). Measurements showed excellent interrater agreement (ICC: 0.87-0.94).

CONCLUSION

TT-TG measurements are dependent on flexion angle and intra-articular version, which vary with differing MRI techniques. Extension of the knee with a simultaneous higher intra-articular version in the rotatory MRI technique, suggests increased TT-TG close to extension, due to the screw-home mechanism. This should be considered to avoid misdiagnosis due to the TT-TG.

LEVEL OF EVIDENCE

Level III.