A modular pedicle screw assembly has a multi-part cam lock structure for fixing a threaded shank to a head subassembly. A modular pedicle screw assembly provides capture and release of a spherical end of a threaded shank by a head through rotation of closures radially disposed in an internal chamber of the head. Geometry of the closures cooperate with geometry of the internal chamber, providing a selective cam action between the closures and the internal chamber to increase and decrease size of an opening formed by the closures. Closure rotation produces an open state where the spherical end of the threaded shank can pass through the closures, releasing the spherical end of the threaded shank from the head, and a closed state where the spherical end of the threaded shank cannot pass through the closures thereby capturing the spherical end of the threaded shank in the head.
