Lateral Ankle Sprain

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Diagnosis Definition

85% of ankle sprains are lateral, most commonly due to ankle inversion
The lateral collateral ligament consists of the anterior talofibular ligament (ATFL), the calcaneofibular ligament (CFL), and the posterior talofibular ligament (PTFL)
Lateral ankle sprains can be graded by the number of ligaments involved (Grade 1 – 1 ligament, usually ATFL; Grade 2 – 2 ligaments, usually ATFL and CFL; and Grade 3 – all 3 ligaments)

Imaging Findings

Routine ankle MR imaging is performed in the axial, coronal, and sagittal planes; plantar flexion allows better visualization of the calcaneofibular ligament; sequences include T1 and T2; marrow abnormalities are best evaluated with STIR
Normal ligaments are thin, linear, low-signal-intensity structures joining adjacent bones, delineated by contiguous high-signal-intensity fat
MRI of ligament injury shows discontinuity, detachment, thickening, thinning, or irregularity of the ligament
Heterogeneity and increased intraligamentous signal intensity on FS or T2-weighted images is indicative of intrasubstance edema or hemorrhage
Obliteration of the fat planes around the ligament, extravasation of joint fluid into the adjacent soft tissues, and talar contusions may also be seen

Pearls

Fluid within the peroneal tendon sheath can be a secondary sign of calcaneofibular ligament injury
Chronic ligamentous tears manifest as thickening, thinning, elongation, and wavy or irregular contour of the ligament with no residual marrow or soft-tissue edema or hemorrhage
Decreased signal intensity in the fat abutting the ligaments is indicative of scarring or synovial proliferation

References

Perrich KD, Goodwin DW, Hecht PJ, Cheung Y. Ankle Ligaments on MRI: Appearance of Normal and Injured Ligaments. AJR 2009; 193:687–695
Rosenberg ZS, Beltran J, Bencardino JT. Imaging of the Ankle and Foot. RadioGraphics 2000; 20:S153–S179

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