**1. Introduction**

When it comes to the traumatic soft tissue injury of the knee, the patient is always afraid of having a meniscus or cruciate ligament lesion. Despite the widespread disrepute of meniscal, cruciate ligaments or isolated collateral ligaments tears, the management and treatment options are well-established with scientifically proved good results. More challenging remain acute and chronic rotatory instabilities of the knee which require a high grade of suspicion to be recognized, a broad knowledge of anatomy and biomechanics to determine injured structures and properly addressed them and, finally, have a debilitating influence on the whole knee joint when left unrecognized [1–3].

One of the most common rotatory instability pattern is a posterolateral rotatory instability (PLRI), which is a consequence of injuries to the structures of so-called

posterolateral corner (PLC) of the knee. This anatomical and functional region of the knee consists of many static and dynamic stabilizers from which the most important are three: fibular collateral ligament (FCL), popliteus tendon (PLT) and popliteofibular ligament (PFL). The others involve iliotibial band (ITB), biceps femoris tendon (BT), posterolateral knee capsule, fabello-fibular ligament [4, 5]. From three main stabilizers mentioned above, the FCL works as a primary restraint to varus stresses, especially close to knee extension, whereas PLT and PFL plays a crucial role in limitation of tibial external rotation. Furthermore, the PLT provides antero-posterior stability in 30° of knee flexion and, working as a dynamic stabilizer, actively rotates the knee internally [4, 6, 7].

Typical mechanisms of injury to the PLC involve knee hyperextension with varus deformation like for example direct hit to the anteromedial region of tibia, forced external rotation with the foot fixed on the ground, mostly during sport activities, but also motorbike or vehicle accidents as a part of complex knee injuries [4, 5, 8]. The PLC injuries account for 16% of all knee ligamentous injuries, but only 28% of them are isolated [5, 6]. Usually they are associated with anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) tears [5]. Non-recognition of PLRI concomitant with ACL or PCL tears may lead not only to unsatisfactory clinical results of surgical treatment, but also to reconstruction failures and further revision surgeries [9]. Thus, an adequate diagnosis and management of PLC injuries are essential for the knee joint well-being.
