Flag sign: a case report of an unusual magnetic resonance imaging finding of a free lateral meniscal fragment

Background Meniscal injury is a common sports medicine condition. Magnetic resonance imaging (MRI) is widely used to effectively diagnose meniscal injury. A flag sign on MRI is usually typical of a meniscal root tear. We report the case of a “flag sign” caused by a free lateral meniscal fragment that mimicked the anterior cruciate ligament (ACL) signal on MRI. Case presentation This was a 21-year-old male patient who suffered a knee injury (swelling and pain) playing football. A physical examination revealed positive Lachman and lateral McMurray tests. MRI images showed an ACL injury and the flag sign, and a diagnosis of ACL tear and lateral meniscal injury was made. Arthroscopic lateral meniscal repair and ACL reconstruction were performed. Six months postoperatively, MRI showed that the injuries were well healed. In this particular case, the flag sign introduced interference to the MRI assessment of ACL injury. Conclusions This case illustrates that clinicians should carefully identify the morphological changes in the meniscus and the relationship of the meniscus with the femoral condyle before reaching a final diagnosis