R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Case studies 183
extremely diffi cult to integrate human anatomy and create similar forms.
In addition, the methods used to defi ne shapes in CAD are based almost
entirely on simple mathematical geometry (straight lines, angles, arcs, etc.).
Although efforts have been made to investigate the use of CAD in cranioplasty
plate design, they have proved time consuming and only served to
highlight these limitations (6).
In contrast to engineers, prosthetists have highly developed visual and
tactile skills that allow them to handle materials to create accurate freeform
shapes. Consequently, there exists a signifi cant barrier to the application of
current engineering-based CAD software by experienced prosthetists.
Recent developments in both hardware and software have enabled
people with traditional visual and tactile sculpting skills to be able to exploit
the potential advantages of CAD. The hardware consists of a threedimensional
stylus developed from research carried out at Massachusetts
Institute of Technology. The device gives all six axes of movement via a
hand-held stylus, similar to a pen or sculpting tool. Crucially this device not
only has six axes of freedom but also incorporates tactile-feedback (7).
Thus, when moving the cursor on screen in three dimensions, when the
cursor comes to the surface of an object on screen the user feels the contact
through the stylus. The resistance can be varied to simulate materials of
different hardness and consistency.
Three-dimensional software, called FreeForm ® (SensAble Technologies
Inc., 15 Constitution Way, Woburn, MA 01801, USA), has been developed
in tandem with the tactile-feedback stylus. Unlike traditional engineeringbased
CAD systems, this software has been developed to visualise and
manipulate solid, complex, unconstrained three-dimensional shapes and
forms. The system utilises a solid three-dimensional voxel representation.
In essence, solid objects are made up of thousands of tiny cubes. The user
can then free-form these shapes with the stylus. The effect is very much
like sculpting clay in a digital environment. The software can impose tools
onto the stylus that correspond to traditional sculpting tools. Material can
be ground away, drilled, stretched, pushed or added in a manner analogous
to sculpting with clay.
Once objects are shaped, they can be integrated into other CAD packages
or used directly with computer-controlled prototyping or manufacturing
processes. Therefore, the ability to combine the existing visual and
dextrous manual skills of the prosthetist with the advantages of CAD/CAM
becomes a practical proposition.
6.10.3 An initial case study
To investigate this application, an actual case requiring a cranioplasty plate
was attempted. This involved importing a three-dimensional model of the
cranial defect. This data was derived from a three-dimensional computed