Cutting away part of a 3D outer surface to show some interior objects hidden beneath it can play an important role in visualization and technical illustration. Previous work includes cutaway using rules and Alpha blending employed to achieve an appearance of cutaway drawings. These methods have mainly aimed at showing the presence of the interior objects, and the shape of the hole has been considerably less important than the visibility of the interior objects. However, in some applications, it may be important for the shape of the hole to have particular properties. Our example comes from orthopaedic surgery and the shape of the aperture should represent the shape of the incision made by the surgeon when retractors are applied to it to give access to the tissue beneath. In this paper, we propose a novel approach to cutting a hole of a specific shape in a surface consisting of a 3D polygonal mesh. The user first defines the required shape on the surface; the algorithm then superimposes the shape on the surface and implements the cutaway. Jittering problems associated with the boundary generated by the above steps are resolved and experimental results demonstrate the efficiency of the algorithm.