Abstract: A prosthetic knee (10) for a transfemoral amputee has a monocentric rotational joint (20) that rotatably attaches an upper housing (12) attachable to the amputee's residual limb to a lower housing (16) attachable to a below-the knee prosthesis. A locking mechanism (32) disposed in the lower housing (16) has a piston (34) biased in a locked position releasably engaging the upper housing (12) preventing rotation of the rotational joint (20). An unlocking mechanism (32) having a trigger (48) attached to a posterior side of the lower housing (16) by a pair of spaced-apart trigger links (52) is configured to unlock the locking mechanism (32). A piston rod (62) having a first piston-rod end (64) pivotably attached to one of the trigger links (52) has a second piston-rod end (68) pivotably attached to the piston (34).