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Background
Ruptures of the anterior cruciate ligament (ACL) are common injuries involving young individuals. These injuries occur most often during sports and exercise, with approximately 250,000 individuals suffering from an ACL rupture per year. Given the long-term complications of untreated ACL rupture, prompt surgical reconstruction of the ruptured ACL is typically performed. Understanding classic imaging features of injured ACLs, as well as postoperative appearances, allows for prompt treatment and planning by the orthopedic surgery team.

Educational Goals / Teaching Points
The goal of this educational exhibit is to discuss the normal architecture and biomechanics of an intact ACL and normal variants, as well as to demonstrate the various ways in which a ruptured ACL may appear on imaging. This exhibit will also review the normal appearance of a postoperative ACL reconstruction, as well as potential immediate and delayed graft complications.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Normal appearance of the ACL, including the attachment sites and two components (anteromedial and posterolateral bundles). Variation in appearance of the unruptured ACL, including mucoid degeneration and ACL ganglion cysts. Mechanism of injuries leading to ACL tearing. Imaging appearances of partial and complete ACL tears, including primary and secondary signs. Associated injuries with ACL tearing (unhappy triad, Segond fracture, menisocapsular separation). Basic information regarding the ACL reconstruction procedure and postoperative complications, such as re-tearing, arthrofibrosis, roof impingement, tunnel cysts, and hardware failure.

Conclusion
Given the frequency of ACL injury in young athletes, it is important to recognize both normal and abnormal appearances of the ACL. With surgical repair being the main treatment of the ruptured ACL, being aware of all potential postoperative complications and imaging manifestations is important to guide clinical management.