| Commercially available orthopedic radial head implants are traditionally produced in quantities of standard shapes and sizes which do not fully match the irregular shape of the bone articulations. From the viewpoint of implant kinematics, wear, and fixation, an implant which more closely approximates the normal anatomy of the radial head is likely to be superior to the standard implants. This work provides a description of how reverse engineering technology can be used to replicate the geometry of the radial head and manufacture a customized implant. Reverse engineering is the process of generating accurate three-dimensional computer aided design models of free-form surfaces from measured coordinate data. The measured data is a sequence of 2D cross-sections of the bone acquired by computer tomography imagery. The surface model is generated by fitting closed contours to the edge points extracted from the individual cross-sections, and then lofting these contours. The closed contours are fitted using a Bernstein Basis Function network. It is an adaptive approach to determining a small number of control points that enables a closed Bezier curve to be reconstructed from measured points. Once the learning phase is complete, the weights of the network represent the control points of the defining polygon net used to generate the closed Bezier curves. The location of the weights are determined by a least-mean square learning algorithm. After the solid model is produced, it is used to generate a tool path for machining the implant using a computerized numerical control milling machine. The machined prototype is then inspected using a coordinate measuring machine to verify its geometry. Experiments are presented in this work in order to confirm the effectiveness of this technique for reverse engineering and manufacturing radial head replacements. |
