R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

184 Medical modelling
tomography (CT) scan and imported into the software as a ‘buck’. This
means that the user can feel the surface of the skull but not alter it with
any subsequent tools. The next step was to create a piece of ‘virtual clay’
that could then be worked into the correct shape. In prosthetics terms, it
may be more appropriate to refer to this as virtual or digital wax up.
First, a sketch plane is positioned by eye over the defect and a twodimensional
perimeter drawn around it. The planes are then offset either
side of the defect and the perimeters joined to form a three-dimensional
piece of digital wax approximately the right size, as shown in Fig. 6.50.
Material removal tools were then used to ‘grind’ away material. The tools
used are similes of physical sculpting tools such as scrapers, grinding wheels,
etc. However, the size and shape of the tools can be arbitrarily altered to
suit the job in hand. A wide ‘grinding’ tool was used to work the surface
6.50 Extrusion of working material.