A smooth mating surface model defining a mating surface of a customized arthroplasty jig is generated. For example, sagittal slices of a volumetric image of a patient bone are segmented with segmentation splines. An anatomically accurate model of the patient bone is generated from the segmentation splines. The anatomically accurate model includes anatomically accurate segmentation splines. The anatomically accurate segmentation splines are transformed into mating surface contours. Any inadequate segments of the mating surface contours are modified to obtain modified mating surface contours. A mating surface model of the patient bone is generated based on the mating surface contours and the modified mating surface contours. Three-dimensional cross-sections of the mating surface model are smoothed to generate the smooth mating surface model.