A custom arthroplasty guide and a method of manufacturing such a guide are disclosed herein. The method of manufacturing the custom arthroplasty guide includes: a) generating medical imaging slices of the portion of the patient bone; b) identifying landmarks on bone boundaries in the medical imaging slices; c) providing model data including image data associated with a bone other than the patient bone; d) adjusting the model data to match the landmarks; e) using the adjusted model data to generate a three dimensional computer model of the portion of the patient bone; f) using the three dimensional computer model to generate design data associated with the custom arthroplasty guide; and g) using the design data in manufacturing the custom arthroplasty guide.