Various implementations described herein include a screw locking receiver. The screw locking receiver includes a receiver frame which has a first surface, a second surface opposite and spaced apart from the first surface, and an inner surface which extends between the first surface and the second surface. The receiver frame defines a cylindrical inner channel having a central axis. The receiver includes a counterbore which extends along a first portion of the inner surface. The receiver includes a plurality of threads extending helically about a second portion of the inner surface. The counterbore has a diameter that is sufficient to accept a screw head having a non-parallel central axis with respect to a central axis of the inner channel when in a fastened position.

A surgical fixation system including a base plate, a plurality of anchors and a locking element. The base plate has at least a pair of fixation apertures configured to receive at least a portion of the anchors therethrough. The fixation apertures are located within the base plate such that upon proper placement of the base plate within a surgical target site, one of the fixation apertures is positioned over a first bone segment (e.g., a first vertebral body), and the other fixation aperture is positioned over a second bone segment (e.g., a second vertebral body).

A driver-specific back-out prevention mechanism is disclosed that includes a screw and a body. A first driver can be used for coupling and decoupling the screw with the body, and a second driver can only be used for adjusting the position of the screw in the receiving body. The body includes a cavity for receiving a head of the screw, with the cavity having an inward flange. The screw includes upper and lower sockets, with a channel extending though the head from the lower socket and a lock body disposed in the channel. Engagement with the first driver allows the locking body to move such that the fastener can be inserted and removed from the body. Engagement with the second driver displaces the locking body such that the locking body interferes with the flange such that screw fastener cannot be removed from the body by the second driver.