R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Case studies 115
not be suitable. An overview of the principal differences between CNC
milling and RP is provided in Section 5.9.
Other RP technologies
Selective Laser Sintering (SLS ® –3D Systems Inc.) locally heats a thermoplastic
powder, which is fused by exposure to an infrared laser in a manner
similar to SL. SLS ® models do not require support structures and they are
therefore cleaned relatively easily, thus saving labour costs. An example of
the use of SLS ® in medical modelling is described by Berry et al. (25).
Laminated Object Manufacturing (LOM TM ) builds models from layers of
paper cut using a laser, which are bonded together by heating. Inexpensive
sheet materials make LOM TM very cost effective for large volume models.
However, the solid nature of the waste material means that it is not suited
to models with internal voids or cavities often encountered in human
anatomy. The SLS ® and LOM TM processes are also described in more detail
in Sections 5.5 and 5.8, respectively.
Discussion of MRP technologies
The main factors in choosing which rapid prototyping technology are most
appropriate for our clinical applications were as follows:
• dimensional accuracy of the models;
• overall cost of the model;
• availability of technology;
• model building material.
SL models are typically colourless to amber in colour, transparent and of
suffi cient accuracy to be suitable for MRP work. FDM TM models are typically
made of white acrylonitrile-butadiene-styrene (ABS) and are attractive
both in terms of appearance and material. It has been pointed out that
medical models may be dimensionally accurate to 0.62 mm +/− 0.35 mm
(26). It should be noted that the limiting factor in model accuracy is the
imaging technique rather than the RP technology employed. In general, CT
and MR typically acquire image slices, which have slice thickness of the
order of 1.0–3.0 mm, which is much greater than the limiting build resolution
of any of the RP technologies.
The potential benefi ts of exploiting RP techniques in surgical planning
have been widely acknowledged and described. The process of producing
accurate physical models directly from three-dimensional scan data of an
individual patient has proved particularly popular in head and neck reconstruction.
In addition, most of the work done to date has concentrated on
the use of three-dimensional CT data as this produces excellent images of