The additional value of an oblique image plane for MRI of the anterior and posterior distal tibiofibular syndesmosis.

OBJECTIVE

The optimal MRI scan planes of collateral ligaments of the ankle have been described extensively, with the exception of the syndesmotic ligaments. We assessed the optimal scan plane for depicting the distal tibiofibular syndesmosis.

MATERIALS AND METHODS

In order to determine the optimal oblique caudal-cranial and lateral-medial MRI scan plane, two fresh frozen cadaveric ankles were used. The angle of the scan plane that demonstrated the anterior and posterior distal tibiofibular ligament uninterrupted in their full length was determined. In a prospective study this oblique scan plane was then used in addition to the axial and coronal planes, for MRI scans of both ankles in 21 healthy volunteers. Two observers independently evaluated the anterior tibiofibular ligament (ATIFL) and posterior tibiofibular ligament (PTIFL) regarding the continuity of the individual fascicles, thickness and wavy contour of the ligaments in both the axial and the oblique plane. Kappa was calculated to determine the interobserver agreement. McNemar's test was used to statistically quantify the significance of the two scan planes.

RESULTS

In the axial plane the ATIFL was in 31% (13/42) partly and in 69% (29/42) completely discontinuous; in the oblique plane the ATIFL was continuous in 88% (37/42) and partly discontinuous in 12% (5/42). Compared with the axial plane, the oblique plane demonstrated significantly less discontinuity (p<0.001), but not significantly less thickening (p=1.00) or less wavy contour (p=0.06) of the ATIFL. In the axial scan plane the PTIFL was continuous in 76% (32/42), partially discontinuous in 19% (8/42) and completely discontinuous in 5% (2/42); in the oblique plane the PTIFL was continuous in 100% (42/42). Compared with the axial plane, the oblique plane demonstrated significantly less discontinuity (p=0.002), but not significantly less thickening (p=1.00) or less wavy contour (p=0.50) of the PTIFL. The interobserver agreement score and kappa (κ) regarding the continuity for the ATIFL in the axial and oblique planes was 91% (κ=0.79) and 91% (κ=0.55) respectively; for the PTIFL it was 86% (κ=0.65) and 100% (κ = not defined).

CONCLUSION

The ATIFL and PTIFL are routinuely scanned in the orthogonal planes. The advantage of MRI scanning in an oblique image plane of about 45 degrees permits a better evaluation of the ligaments compared with the axial plane, particularly a better interpretation of ligament continuity, thickening and wavy contour. This may lead to a reduction in false-positive results, especially regarding partial or complete ligament ruptures. This can be of considerable aid in therapeutic management.