Flag Sign: A Case Report of an Unusual Magnetic Resonance Imaging Finding of a Medial Meniscal Root Fragment in an Adolescent Water Polo Player

Introduction: A meniscus tear ranks among the most common sports-related injuries, especially among athletes. Sudden, twisting movements, such as pivoting to catch a ball, usually in contact sports, and can tear the meniscus. Magnetic resonance imaging (MRI) technology is extensively utilized for identifying meniscal tears. A flag sign on an MRI typically indicates a partial or complete meniscal root tear. We present a case where a “flag sign” was attributed to a medial meniscal root fragment in an adolescent water polo player. Case Report: A 15-year-old male patient suffered a knee injury (mild swelling and pain) while playing as a goalkeeper in water polo. A physical examination revealed negative Lachman and positive medial McMurray tests. MRI images showed a typical flag sign, without any other concomitant injuries. An arthroscopic medial meniscectomy was performed. 8 months post-operatively the injuries were well healed. In this instance, the flag sign caused interference with the MRI evaluation of an anterior cruciate ligament (ACL) injury Conclusion: This case highlights the importance for clinicians to meticulously assess both the morphological changes in the meniscus and its relationship with the femoral condyle before arriving at a definitive diagnosis even in the athletes of non-contact sports. The meniscal “flag sign” mimics the ACL signal on MRI. Keywords: Meniscus, tear, knee, arthroscopy, flag sign, injury.