A spinal anchor assembly for securing a spinal fixation element has an anchor element adapted for attachment to bone. The anchor element has an opening for receiving a spinal fixation element and side walls on opposed sides of the opening with a plurality of inferior surfaces defined on the side walls. A twist-lock closure closes the opening and captures the spinal fixation element. A locking element can be provided on the closure element and the anchor element. The locking element can be configured so that when the closure element is rotated from the open position to the closed position, the locking element engages to deter the closure element from twisting back to the open position.