R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Case studies 245
micro-computed tomography, was used to perform fi nite element analysis
(FEA) on the structure of various human and animal bones. The physical
models were required to validate the results of the FEA.
The diffi culties encountered in creating physical models of these structures
arose from the nature of the structure. Not only does the highly
complex porous structure result in extremely large computer fi les but it also
presents problems of support during the build process.
6.15.3 Human sample data
The techniques of serial sectioning and micro-computed tomography (µCT)
reconstruction were used to obtain three-dimensional reconstructions of in
vitro samples of natural cancellous bone tissue. The physical size of each
sample is approximately 4 mm × 4 mm × 4 mm and their relative densities
are 9–25 %. The models were to be scaled up by an approximate factor of
ten and physically produced using RP. The considerable effort that went in
to generating the data from which these models would be made is described
elsewhere (1).
6.15.4 The use of stereolithography in the study of
cancellous bone
For this project, stereolithography (SL) was the preferred method for
several essential reasons. Firstly, it is capable of building models at an exact
layer thickness of 0.1 mm. This was desirable as the FEA was generated
from voxel data with a voxel size of 0.1 mm. Therefore, the SL model would
replicate the FEA mesh exactly, i.e. with no smoothing between the layers
or within the plane of each slice. Secondly, it is the most accurate method
available (except Solidscape machines, but this would have been extremely
slow and the models would have proved far too delicate to mechanically
test). Thirdly, although Selective Laser Sintering (SLS ® – 3D Systems Inc.,
26081 Avenue Hall, Valencia, CA 91355, USA) and three-dimensional
printing had the advantage of not requiring support structures the fi nished
models are not completely dense or suffi ciently accurate. Fused Deposition
Modelling (FDM TM – Stratasys Inc., 14950 Mantin Drive, Eden Prairie, MN
55344-2020, USA) parts can also show a small degree of porosity and are
unable to match the accuracy desired in this case. However, since these
models were built, the water-soluble supports that are now available for
FDM TM would prove extremely useful for structures such as these. Finally,
and most importantly, stereolithography could be used to generate models
from slice data rather than triangular faceted data. The intention was to
use the SLC fi le format as it resulted in dramatically smaller fi les than the
same data generated in the STL fi le format. A general description of all of