A fixation device (100) for bones comprises a member (1) which is to be fixed to one or more bones (40, 42) and has at least one bore (2a-c, 2', 2'') for receiving a bone screw (30), wherein the at least one bore comprises a first internal thread portion (22a-c； 22'). The bone screw for insertion through the bore into the bone has a first shaft section provided with a first external thread portion (32) arranged to cooperate with the internal thread portion of the at least one bore, and a head section (36) having a diameter larger than that of the shaft section to provide a catch arranged to engage with a stop formed in the bore. The bone screw further has a second shaft section which comprises a clearance groove (34a) extending between the catch of the head section and the external thread (32) of the first shaft section. The clearance groove allows disengagement of the two thread portions, such that the bone screw is prevented from being unscrewed off the bore when it is loosened within the adjacent bone, e.g., a vertebral body. The member may comprise a side wall (4) of a cage used in an intervertebral implant device, or may represent a plate of a bone plate assembly.
