• Su Joa Ahn, Yu Mi Jeong, Beom Goo Lee, Jae Ang Sim, Hye-Young Choi, Jeong Ho Kim, and Sheen-Woo Lee.
• Department of Radiology, Gachon University, Incheon, Republic of Korea.
• Acta Radiol. 2016 Oct 1; 57 (10): 1251-60.

BackgroundReliable magnetic resonance imaging (MRI) diagnosis is important in cases of posterolateral corner (PLC) injury due to the limitations of physical examination in patients with multi-ligament injury.PurposeTo document the appearance of PLC of the knee on three-dimensional (3D) isotropic MR images, and to determine the significance of MRI findings in patients with confirmed posterolateral rotatory instability.Material And MethodsTwenty-five patients that underwent surgery for posterolateral instability, and 25 individuals with normal MRI constituted the study cohort. The PLC appearances (popliteofibular, fabellofibular, arcuate ligaments, popliteomeniscal fascicle) were analyzed using 3D isotropic proton density sequence and routine two-dimensional (2D) MRI. In addition, the "fibular cap" sign was evaluated. Statistical analysis was performed using the Chi-square and McNemar's tests.ResultsThickening of popliteofibular, fabellofibular, arcuate ligaments, and popliteomeniscal fascicle was significantly more frequent in the PLC injury group than in the control group (P < 0.05). The sensitivity and specificity of 3D MRI for popliteofibular, fabellofibular, arcuate ligaments, and popliteomeniscal fascicle injury were 63/92%, 54/100%, 46/100%, and 58/92%, respectively. On comparing 3D and 2D images with respect to injury detectability (grade 3 or 4), both modalities visualized injuries, but 3D detected grade 3 or grade 1 rather than grade 4 or 0, respectively. The fibular cap sign was observed significantly more frequently in PLC group, with 58% sensitivity and 100% specificity, and was better observed by 3D than 2D (P < 0.05).Conclusion3D MRI is a valid modality for detecting PLC abnormalities as it visualizes pathologies in each component and exhibits the positive fibular cap sign.© The Foundation Acta Radiologica 2016.

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