R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

130 Medical modelling
software co-ordinate system. Although this is not a major issue, it does
make control of the angles at which the entities meet more diffi cult.
Unintended displacement of the soft tissue during the CT scan results in
poor representation of the anatomy. In this example, the ear of one of the
patients had become folded over resulting in a deformed anatomical entity
(Fig. 6.13). This made positioning the contralateral ear and the implants
problematic.
6.3.5 Software problems
Initially fi le size was a concern and all of the STL fi les were produced at a
low resolution. This resulted in small STL fi les that could be handled easily
and rapidly by the software. In visual terms, all of the entities appeared to
be well represented by the low quality STL fi les. However, when attempting
one of the fi rst cases, it was found that the difference in the representation
of internal air cells in the highly pneumatised bone in the mastoid was
dramatically altered by the resolution at which the STL fi le was produced.
This led to the mistaken belief that this particular case had adequate bone
thickness when in fact the bone was unusually thin (illustrated in Fig. 6.14).
From this experience, it was decided to produce only the small amount of
bone required for the implants but at the highest possible resolution. This
resulted in only one entity having a large fi le size, which proved to be perfectly
within the capabilities of a reasonable specifi cation computer.
6.3.6 An illustrative case study
Three-dimensional CT data was used to create virtual models of the elements
necessary to plan the osseointegrated implants required to secure a
prosthetic ear. The elements consisted of the soft tissue of the head, a copy
of the remaining opposite ear, and the bone structure at the implant site.
6.13 Problems resulting from poor position during CT scanning.