The present invention provides a tensioning ancillary instrument (5) for tensioning an elongate element to fasten an implant on a portion of bone, by surrounding said portion of bone at least in part, said instrument (5) comprising a body (6) having proximal and distal ends (6a, 6b), the distal end (6a) being provided with a device for bearing against the implant, said body (6) extending along a longitudinal axis (L). The instrument (5) further comprising a carriage (7) that is movable along the longitudinal axis (L), and a fastener device (8) for fastening to the elongate element (2). The instrument (5) also includes a rotary shaft (9) rotatable about the longitudinal axis (L) of the body (6), the shaft (9) being configured to co-operate with the carriage (7) in such a manner that rotation of the shaft (9) drives movement of the carriage (7) in translation along the longitudinal axis (L) and correspondingly moves the carriage (7) away from the distal end (6b) of the body (6) and tensions the elongate element.
