What can be learned from this 17-year-old’s knee injury?

This 17-year-old male reports with knee pain following a football injury. Be sure to size up the case by looking at the first two images without arrows before looking at the same images with arrows to indicate key findings.

What are the anatomical structures indicated by the arrows? Hint: While this patient’s anterior cruciate ligament (ACL) is intact, these structures are very important in the setting of acute ACL tear.

Sagittal T2 FSE

Sagittal T2 FSE

Sagittal T2 FSE

Sagittal T2 FSE

The green arrow on image 3 indicates the popliteus muscle (there is a mytotendinous strain in this case), and the red arrows on image 4 point to the arcuate ligament (there is an arcuate ligament tear with fluid leaking from the joint).

Popliteus muscle injuries seldom occur in isolation and are an important ancillary finding of internal derangement of the knee joint. Associated injuries include cruciate and collateral ligament injuries as well as meniscal tears and bone bruises. It is important not to overlook posterolateral corner injuries in the presence of an anterior cruciate ligament tear because the reconstructed cruciate ligament will likely fail without repair of the posterolateral corner injuries.

Bonus Question: What is an arcuate sign?

Answer: The arcuate sign is an avulsion fracture of the proximal fibula at the site of insertion of the arcuate ligament complex and is usually associated with cruciate ligament injury, though not in this case. The avulsion fracture is usually small, less than one centimeter, and involves the styloid process of the fibula.

For more knee case review, head to MRI Online. For further reading on the arcuate sign, click here.

Dr. Stephen Pomeranz

Dr. Robert Wissman

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