The arthroscopic appearance of a normal anterior cruciate ligament in a posterior cruciate ligament-deficient knee: the posterolateral bundle (PLB) sign.

Partial anterior cruciate ligament (ACL) tears that result in functional instability are usually the result of a deficiency of the anteromedial bundle (AMB), and are evident arthroscopically by a prominent posterolateral bundle (PLB). As double-bundle ACL reconstructions are being suggested to recreate a more normal anatomy, the roles of each bundle are being more critically questioned. We present a case that describes the appearance of a normal ACL in a posterior cruciate ligament (PCL)-deficient knee in which the PLB of the ACL is prominent, giving the appearance of a complete tear of the AMB of the ACL, and scarring of this bundle to the PCL stump. On further inspection, and when the tibia was reduced, the normal appearance of the ACL returned, with visualization of the AMB obscuring the PLB. We have named this arthroscopic finding the "PLB sign." This finding confirms the individual roles of each bundle of the ACL. It is important to not misinterpret this abnormal appearance of the ACL as a partial ACL tear in a PCL-deficient knee.