The plica synovialis infrapatellaris is occasionally identified arthrographically and can be confused with an intact anterior cruciate ligament. In our series, the plica synovialis infrapatellaris was never found in association with an intact anterior cruciate ligament; proper recognition of this structure is diagnostic of a torn anterior cruciate ligament. The plica synovialis infrapatellaris can be distinguished from an intact anterior cruciate ligament primarily by its anteroinferior insertion point which is on the infrapatellar fat pad or anterior edge of the tibial plateau. In comparison, the anterior cruciate ligament is just anterior to the tibial spines. A secondary method for differentiating a plica synovialis infrapatellaris from an intact anterior cruciate ligament, the "apical angle," is also described.