1. Method for planning implant placement, the implant comprising a first implant ( 50 ) in a first limb ( 20 ) and a second implant ( 40 ) in a second limb ( 30 ), 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 ( 40 ) 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 relative positions and/or orientations, referred to as relative poses, of the first and second reference system; e) determining a data set representing a plurality of envelope points, said envelope points being generated by calculating the locations of a point ( 62 , 63 ) of the second implant representation in the first reference system for the plurality of relative poses; and f) placing a representation of the first implant ( 50 ) in the first reference system by adjusting and/or matching pose of the representation of the first implant to the plurality of envelope points.