PCL  Tear

Tears of the posterior cruciate ligament (PCL) are relatively uncommon. Rupture is usually caused be excessive rotation or hyperextension or by direct trauma whilst the knee is flexed. Commonest causes include road traffic accidents and contact sports e.g. football.
Normally the PCL is seen as a thick dark (low signal) structure throughout and forms a smooth arc.

In the first example the ligament is of increased signal (bright) in its centre and has an jagged  rather than smooth contour. On the coronal images, it can be seen to deviate medially to come to lie above the inner aspect of the medial meniscus, rather than attaching to the tibial spine.

Sagittal T1 and T2*, and coronal PD and STIR images are shown:

In the second example the ligament is markedly swollen and of greatly increased signal.
Sagittal T1 and T2* weighted images are shown: