1. A method for designing a customised surgical instrument or prosthesis for a body part of a specific patient, the body part having a body part surface, the method comprising the step of: processing at least one x-ray image of the body part to generate a processed patient image, wherein processing the at least one x-ray image includes filtering the at least one x-ray image, creating a differential image corresponding to differences in intensity of the filtered at least one x-ray image and normalizing peaks in the differential image by applying a normalizer function to the differential image; instantiating a statistical model generated using a minimum description length approach and having a set of anatomical correspondence points across the statistical shape model using image data derived from the processed patient image to generate a patient specific model of the body part having a modeled surface, the instantiating step comprising: (i) generating a pseudo x-ray image of the body part from a CT image reconstructed from the statistical model using parameters of a model of the body; (ii) processing the pseudo x-ray image of the body part to generate a processed pseudo x-ray image, wherein processing the pseudo x-ray image includes filtering the pseudo x-ray image, creating a differential image corresponding to differences in intensity of the filtered pseudo x-ray image, and normalizing peaks in the differential image by applying a sigmoid function to the differential image; (iii) comparing the processed patient image and processed pseudo x-ray images to determine the differences between the processed patient image and the processed pseudo x-ray image; and (iv) repeating steps (i) to (iii) using different parameters of the model of the body part to minimize a cost function indicating the differences between the processed patient image and the processed pseudo x-ray image and generate the patient specific model; and generating a customised surgical instrument or prosthesis having a contact derived from the modeled surface of the patient specific model.