Abstract: A cap (1) has a housing (9) adapted in order to receive one end of the pin (2), and a tapped hole (11) opening out transversely in this housing, adapted in order to receive a fixing screw (12) allowing the pin (2) and the cap (1) to be solidly attached together; the screw (12) is initially made in a single piece with the terminal part ( 4 ) of a handle (7) of the fixing instrument (3), an embrittled zone (13) being made at the transition between the screw and the terminal part (4). The surgeon first covers the pin (2) with the cap (1), then screws the screw (12) until it locks the pin (2), and the end of screwing causes the embrittled zone (13) to rupture, so that only the screw (12) remains locked in place in the cap (1) fixed to the pin (2). This cap prevents any migration of the pin and protects the neighbouring tissues.