R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Case studies 195
anatomical forms and its reliance on primarily geometrically defi ned
shapes.
The research reported here aims to bring the time and labour saving
benefi ts that CAD and RP technology have shown in product design and
other areas of maxillofacial surgery and apply them to the design of facial,
soft tissue prostheses, whilst complementing the needs and existing skills
of maxillofacial prosthetists. The FreeForm ® Modeling Plus TM CAD
package (SenAble Technologies Inc., 15 Constitution Way, Woburn MA,
USA) used in this research has tools analogous to handcrafting and offers
prosthetists the ability to readily adapt their existing skills and design with
more freedom than typical engineering CAD packages. Some medical
applications of the software have already been explored, for example in the
design of cranioplasty plate patterns (9).
The research will be illustrated though a case study involving the design
of a prosthesis required in the rehabilitation of a patient with an ocular
defect (the patient had their left eye removed following cancer treatment).
The case study also describes the rationale used to select the most appropriate
RP process and how the design was manufactured using a wax material
that was incorporated into the prosthetist’s existing clinical practice.
6.11.3 Materials and methods
Data acquisition
Data from a recent CT scan was imported into Mimics (Materialise NV,
Technologielaan 15, 3001 Leuven, Belgium) and the threshold value set
to select soft tissue. Extents covering the mid-face were segmented and
converted into a high quality STL fi le that was suitable for importing into
FreeForm ® . This retained as much detail as possible but, due to the relatively
low resolution of CT data, fi ne detail such as wrinkles and dimples
were not visible.
FreeForm ® modelling
Within FreeForm ® , shapes may be designed and modifi ed arbitrarily by the
user with tools analogous to those used in handcrafting. Although the software
does not rely as heavily on mathematically defi ned and constrained
geometry as engineering CAD, accurately defi nable sculpting tools and
measuring techniques ensure precision. The computer model of the object
being worked is referred to as ‘clay’ by the software, but may be thought
of as a digital version of the wax commonly used when designing a prosthesis
pattern. The clay may be handled in a similar way as the physical
methods employed in handcrafting. Roughly defi ned shapes are gradually