A computer implemented method (100, 200) for determining the intra-operative deviation from a planned implant position, comprising: generating a three dimensional model of at least the surface of a bone of a patient into which an implant is to be implanted, the three dimensional model including a plurality of anatomical features of the bone; creating a planned position of the implant in the patient's bone from the three dimensional model, wherein the planned position of the implant in the patient's bone is defined by a plurality of planned implant position parameters; determining the distance of each of the plurality of anatomical features relative to the implant for the implant at a plurality of different positions in the three dimensional bone model characterized in that the method comprises: receiving as input intra-operatively measured distances of the implant to each of the plurality of anatomical features of the patient's bone; and comparing the determined distances at the plurality of different positions in the three dimensional bone model and the intra-operatively measured distances of the implant to each of the plurality of anatomical features to determine an approximate intra-operative position, being the one of the plurality of positions closest to the intra-operative position of the implant in the bone; and determining any differences in the planned position of the implant and the intra-operative position of the implant in the bone from the differences in the planned implant position parameters and the implant position parameters corresponding to the approximate intra-operative position. generating a three dimensional model of at least the surface of a bone of a patient into which an implant is to be implanted, the three dimensional model including a plurality of anatomical features of the bone; creating a planned position of the implant in the patient's bone from the three dimensional model, wherein the planned position of the implant in the patient's bone is defined by a plurality of planned implant position parameters; determining the distance of each of the plurality of anatomical features relative to the implant for the implant at a plurality of different positions in the three dimensional bone model characterized in that the method comprises: receiving as input intra-operatively measured distances of the implant to each of the plurality of anatomical features of the patient's bone; and comparing the determined distances at the plurality of different positions in the three dimensional bone model and the intra-operatively measured distances of the implant to each of the plurality of anatomical features to determine an approximate intra-operative position, being the one of the plurality of positions closest to the intra-operative position of the implant in the bone; and receiving as input intra-operatively measured distances of the implant to each of the plurality of anatomical features of the patient's bone; and comparing the determined distances at the plurality of different positions in the three dimensional bone model and the intra-operatively measured distances of the implant to each of the plurality of anatomical features to determine an approximate intra-operative position, being the one of the plurality of positions closest to the intra-operative position of the implant in the bone; and determining any differences in the planned position of the implant and the intra-operative position of the implant in the bone from the differences in the planned implant position parameters and the implant position parameters corresponding to the approximate intra-operative position.