US Patent 7125426 Locking cap assembly for spinal fixation instrumentation

A spinal anchor assembly for securing a fixation element includes an anchor element and a twist-in cap. The anchor element is configured for attachment to a bone, typically by comprising a mono- or polyaxial screw or hook, and includes a proximal portion having an open slot for receiving the fixation element and having radially inwardly protruding flange segments. The twist-in cap is received in and closes the open slot. The proximal portion of the anchor and the cap are adapted to twist-lock together by a partial rotation of the cap from an open position to a closed position to cover the slot so as to capture the fixation element and to lock the cap in the closed position.