An apparatus includes a spacer, a proximal retention member, a distal retention member, and an actuator. The spacer is configured to engage adjacent spinous processes. The proximal retention member is coupled to the spacer such that a portion of the proximal retention member is in contact with a proximal surface of the spacer. An axis within a plane defined by a first surface of the distal retention member is non-parallel to and non-normal to a longitudinal axis of the spacer. The distal retention member is movably coupled to the spacer such that a second surface of the distal retention member contacts a distal surface of the spacer. The actuator is movably coupled to the spacer and is configured to move relative to the spacer along the longitudinal axis. An actuation surface of the actuator is slidably coupled to and substantially parallel to the first surface of the distal retention member.
