Abstract： Background The medial plica may be caused by direct trauma or joint degeneration,which also could be iatrogenic.There have been few reports in the literature discussing incidence of the medial plica caused by an operation on the knee joint,specifically after the reconstruction of anterior cruciate ligament (ACL).In this study,we aimed to evaluate and analyze the relationship between the incidence of the medial plica and reconstruction of the ACL.Methods A retrospective case series study was conducted to review the findings of 1085 patients between 2003 and 2007,who underwent second-look arthroscopy after reconstruction of the ACL (between 2002 and 2006).The correlation of the incidence of medial plica with the stability of the knee joint,the time from onset of injury to reconstruction surgery,the associated injuries,and the rate of progress during postoperative rehabilitation were analyzed.Results We found that 722 patients had the structure of a medial plica.The incidence after reconstruction of the anterior cruciate ligament (66.5%) was significantly higher than usually reported.All these medial plica had avascular fibrotic and thickened edges.An excision of pathologic medial plica and fat pad synovial fringes were done.The incidences were significantly different between the two groups with their reconstruction operation time,from onset of injury to surgery (less than one month or over 2 years),and the progress rate of postoperative rehabilitation (knee flexion could not be over 90° in four weeks).The incidence was not different between the groups with knee stable conditions.Conclusions Medial plica is more common in patients after reconstruction of ACL.More associated injuries and more rehabilitation difficulties can increase the medial plica incidence.