A spreader system (10, 310) comprising: a first post (40, 340) to be coupled to a the first bone fixation element when the first bone fixation element is attached to a first bone part, the first post (40, 340) including a first post body (200, 400) that is elongate along a first direction and defines a proximal end and a distal end spaced from the proximal end along the first direction, the first post (40, 340) further including a first socket that extends into the distal end of the first post body (200, 400), and the first post (40, 340) includes a first expandable member disposed within the first socket such that as the first socket receives a head of the first bone fixation element the first expandable member expands to permit the first socket to receive the head, and after the first socket has received the head the first expandable member contracts to thereby couple the first bone fixation element to the first post (40, 340) characterised in that the first post (40, 340) is poly-axially pivotable relative to the head of the first bone fixation element when the first post (40, 340) is coupled to the first bone fixation element, and the first post body (200, 400) includes an internal surface that defines the first socket and an opening disposed at the distal end of the first post body and in which the internal surface defines a circumferential recess that extends radially into the internal surface proximate to the opening, the first expandable member being at least partially disposed within the circumferential recess; wherein the spreader system (10, 310) comprises a second post (44, 344) to be coupled to a second bone fixation element that is attached to a second bone part, wherein at least one of the first post (40, 340) and second post (44, 344) can move relative to the other after the first and second posts (40, 340, 44, 344) have been coupled to the first and second bone fixation elements (18a, 18b), respectively. a first post (40, 340) to be coupled to a the first bone fixation element when the first bone fixation element is attached to a first bone part, the first post (40, 340) including a first post body (200, 400) that is elongate along a first direction and defines a proximal end and a distal end spaced from the proximal end along the first direction, the first post (40, 340) further including a first socket that extends into the distal end of the first post body (200, 400), and the first post (40, 340) includes a first expandable member disposed within the first socket such that as the first socket receives a head of the first bone fixation element the first expandable member expands to permit the first socket to receive the head, and after the first socket has received the head the first expandable member contracts to thereby couple the first bone fixation element to the first post (40, 340) characterised in that the first post (40, 340) is poly-axially pivotable relative to the head of the first bone fixation element when the first post (40, 340) is coupled to the first bone fixation element, and the first post body (200, 400) includes an internal surface that defines the first socket and an opening disposed at the distal end of the first post body and in which the internal surface defines a circumferential recess that extends radially into the internal surface proximate to the opening, the first expandable member being at least partially disposed within the circumferential recess; wherein the spreader system (10, 310) comprises a second post (44, 344) to be coupled to a second bone fixation element that is attached to a second bone part, wherein at least one of the first post (40, 340) and second post (44, 344) can move relative to the other after the first and second posts (40, 340, 44, 344) have been coupled to the first and second bone fixation elements (18a, 18b), respectively.