The application concerns a bone anchor (116), comprising:an interconnection portion (172) to slidingly engage a spacer (112);a fixation portion (184) spaced apart from the interconnection portion (172) to fix the anchor (116) to a bone; anda leg (196, 198) connecting the fixation portion (184) to the interconnection portion (172),whereby the leg (196) includes a cutting edge (197) on the leading end (168) capable of cutting through bone.