1. A computer-implemented method for planning implant placement, the implant comprising a first implant in a first limb and a second implant in a second limb, the first and the second implant forming an artificial structure joining the first and second limb, the method comprising the steps: a) providing a first limb data set representing a first reference system in which the first limb is fixed; b) providing a second limb data set representing a second reference system in which the second limb is fixed; c) providing a representation of the implant for the head of the second limb in a known position in the second reference system; d) providing relative pose data representing a plurality of different positions and/or orientations, referred to as relative poses, of the first reference system relative to the second reference system; e) determining, using a processor, a data set representing a plurality of envelope points, said envelope points being generated by calculating for the plurality of relative poses locations of a point of the representation of the implant provided in step c in the first reference system; and f) placing, using a processor, a representation of the first implant in the first reference system by adjusting and/or matching pose of the representation of the first implant to the plurality of envelope points.