A cortical bone screw assembly comprising a shaft with a threaded section (2) and a cutting and self-tapping end section (3). The assembly includes a nut (14) which includes a resilient body (16). The resilient body (16) can expand to allow translational movement of the nut (14) along the shaft, but when such radial expansion is prevented so is such translational movement. However, the nut (14) can still be moved along the shaft by rotation on the thread (2). This enables the nut (14) to be advanced rapidly along an exposed length of shaft, but when it enters a restricted volume which prevents expansion of the resilient body (16), further advance can only be achieved by relative rotation.