The posteromedial corner is the region towards the back of the knee on the inner side.
The neglected corner
Much recent attention has been paid by radiologists and surgeons to the posterolateral corner, but the posteromedial corner is often neglected.
Reefing of the Posteromedial Capsule in Anteromedial Rotatory Instability. Offerhaus C, Balke M, Arner JW, Musahl V and Höher J. Arthrosc Tech. 2018 May; 7(5): e547–e551.