PMC4896228_01
Arcuate sign in Case 1, a 20-year-old man with posterior right knee dislocation following trauma incurred while riding a motorcycle. Post-reduction AP radiograph of the right knee shows a minimally displaced avulsion fracture of the fibular head.
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Right knee radiographs obtained at the time of transfer showed a minimally displaced avulsion fracture of the fibular head (Figure 1).
MR imaging of the right knee in Case 1, obtained 2 days following the traumatic event. A, Coronal fast spin-echo T2-weighted image and B, sagittal fast spin-echo T2-weighted image, demonstrate the minimally displaced fibular head avulsion fracture (arrows) with associated edema at the insertion site of the arcuate ligament complex. C, Coronal fast spin-echo T2-weighted image shows edema at the site of the popliteus tendon tear (asterisk), as well as an intact fibular collateral ligament (arrowheads). D, Sagittal fast spin-echo T2-weighted image shows a tear of the anterior cruciate ligament at its tibial attachment (open arrowhead). A torn posterior cruciate ligament is also partially seen (open arrow).
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MR imaging was performed two days later, re-demonstrating the fibular head avulsion fracture with edema in the fibular head at the insertion site of the arcuate ligament complex (Figure 2).
PMC4896228_02
Arcuate sign in Case 2, a 28-year-old man who suffered an anterior left knee dislocation after twisting his knee while chasing his dog down a flight of stairs. Post-reduction AP radiograph of the left knee shows a displaced avulsion fracture of the fibular head which is retracted proximally.
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Radiographs demonstrated an avulsion fracture of the fibular head, with proximal retraction of the fracture fragment (Figure 3).
MR imaging of the left knee in Case 2, obtained 1 day post-trauma. A, Coronal fast spin-echo T2-weighted image and B, sagittal fast spin-echo T2-weighted image show the displaced fibular head avulsion fracture (arrows) attached to the proximally retracted biceps femoris tendon and fibular collateral ligament (arrowheads). Edema in the popliteus muscle is compatible with muscle strain (asterisk in A). C and D, Sagittal fast spin-echo T2-weighted images demonstrate an anterior cruciate ligament tear, with no intact fibers visualized in the intercondylar notch (open arrowheads in C). The posterior cruciate ligament is also attenuated and torn (open arrow in D).
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MR imaging perfomed the next day again showed the fibular head avulsion fracture with retraction of the biceps femoris tendon and fibular collateral ligament (Figure 4).