A data processing method performed by a computer for determining implant orientation data representing a range of functional orientations of a first implant part relative to a first bone, the first implant part being part of an implant pair which further comprises a second implant part for a second bone, the implant pair being envisaged to be implanted in a patient, wherein a functional orientation results in a desired functionality of an artificial joint formed by the implant pair, comprising the steps of: - acquiring second implant orientation data representing the orientation of the second implant part relative to the second bone, - acquiring implant shape data representing the shapes of the first and second implant parts, and - acquiring activity data representing at least one desired activity of the patient to be possible after implanting the implant pair, wherein each desired activity has an associated range of motion between the first bone and the second bone, characterized by - calculating the implant orientation data from the second implant orientation data, the implant shape data and the activity data by determining, for a trial orientation, whether or not the ranges of motion of all desired activities can be achieved with an implant joint formed by the implant pair for the trial orientation and adding the trial orientation to the implant orientation data if this is the case, wherein calculating the implant orientation data involves calculating a zero joint orientation of the implant joint, in which the first and second bone are in a neutral position relative to each other, from the trial orientation and the second implant orientation data and determining whether or not a range of motion of a desired activity can be achieved by the implant joint involves determining whether or not the implant joint can reach the range of motion from the zero joint orientation. - acquiring second implant orientation data representing the orientation of the second implant part relative to the second bone, - acquiring implant shape data representing the shapes of the first and second implant parts, and - acquiring activity data representing at least one desired activity of the patient to be possible after implanting the implant pair, wherein each desired activity has an associated range of motion between the first bone and the second bone, characterized by - calculating the implant orientation data from the second implant orientation data, the implant shape data and the activity data by determining, for a trial orientation, whether or not the ranges of motion of all desired activities can be achieved with an implant joint formed by the implant pair for the trial orientation and adding the trial orientation to the implant orientation data if this is the case, wherein calculating the implant orientation data involves calculating a zero joint orientation of the implant joint, in which the first and second bone are in a neutral position relative to each other, from the trial orientation and the second implant orientation data and determining whether or not a range of motion of a desired activity can be achieved by the implant joint involves determining whether or not the implant joint can reach the range of motion from the zero joint orientation.