A screw formed of cortical bone for use in the human body with an implant having a screw hole for receiving at least a portion of a screw therethrough, includes a shaft with a thread along at least a portion of its length. The thread has an outer diameter dimensioned to pass through the screw hole in the implant. The trailing end of the screw is configured to cooperatively engage at least a portion of the screw hole of the implant so as to prevent the screw from linear motion along the mid-longitudinal axis of the shaft in a direction opposite to the direction of insertion when the screw is threaded through the screw hole to attach the implant to a bone portion of the human body. The screw is formed substantially of cortical bone of a single cortical thickness.