A Method to Determine the Nominal Diameter for Hip Joint with No “Pre-wear” Data

This paper presents a methodology for determining the initial diameter of the explanted hip prosthesis based on scanning/measuring using coordinate measuring machines (CMM) and CAD processing of the obtained data. ISO 14242- 2:2000 shows two methods for determining wear of hip joint: gravimetric and measuring using CMMs. Only the second method (measuring using a CMM) can provide information regarding the position and the geometry of the wear print. Using CMMs to determine the hip joint’s wear with no “prewear” data involves finding the initial diameter of the prosthesis and then comparing the ideal surface with the real one which was determined using the explained scanning procedure. Most times there is no accurate measurement of the prosthesis before implantation to determine the real surface of the prosthesis. The presented methodology involves scanning the sphere so that it can calculate the deviation of the scanned points compared to a theoretical circle determined using CAD or numerical methods (roundness deviation). The scanning of the prosthesis or of a sphere can be done using at least three strategies: one of them offers the possibility of scanning complete circles and the other two only allow the scanning of arcs (\({1}\over{2}\) or \({1}\over{4}\) of a complete circle’s circumference). The strategy enables the use of scan points for determine the deviation from circularity. Items that have a significant deviation from the nominal circle are the removed in a CAD program using for this purpose the roundness error map. The remaining points are used to calculate the sphere’s diameter and the center will then be taken as a reference to determine the area of the prosthesis that has wear.