Comprises a pedicular screw (1), a washer (3) which secures the pedicular screw (1) to a cup (6), a bar (8) which comprises the means of attachment to other devices and a locking plug (9) associated with a locking screw (11) which is internally threaded on the cup (6) and determines the position of the bar (8). The locking plug (9) incorporates protuberances (12) which enclose within the cup (6) guiding the assembly formed by the locking plug (9) and the locking screw (11) during positioning and tightening of the locking screw (11) on the cup (6), pressing the bar (8) against the washer (3). The outer surface of the cup (6) and the locking plug (9) have an essentially circular configuration interrupted by individual flat rebates (13, 14) facing each other which help to reduce the width ofthe device in the axial direction of the bar (8).