The present invention relates to a surgical instrument for a technique for attaching bone fragments using a cannulated screw, the implantation of which is guided by a pin. The instrument (1) comprises: (i) a central channel (5) defining a longitudinal axis (6), enabling the slidable and rotatable insertion of said instrument (1) onto said guide pin (ii) a nesting structure (15) suitable for being inserted into a recess made in the head of said cannulated screw and for blocking the rotation thereof (iii) cutting edges (20, 21) suitable for boring at least one of said bone fragments by rotating said surgical instrument (1) about said longitudinal axis (6) and (iv) a means (11) for gauging the insertion depth of said guide pin in said bone fragments. Said instrument (1), which is conveniently inserted onto said guide pin and rotatably moved, enables either the boring of at least one of the bone fragments by means of said cutting edges (20, 21), or the screwing of said cannulated screw, inserted onto said guide pin, by engaging with said nesting structure (15).
