R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

70 Medical modelling
most obvious reason for sectioning a model is to reduce the extent of the
model to only those areas that are needed. This reduces build time and cost.
However, it may also be used to split very large models into a number of
parts that can be accommodated within the build volume of a given RP
machine. Sectioning may also be used to gain access to trapped volumes
(internal cavities). This may be necessary to remove waste material or supports
but may be because the internal anatomy is also of interest to the
clinicians.
When sectioning models it may be advisable to incorporate a stepped or
keyed section such that the two separate parts are easily located when they
are put back together. Many RP software packages provide functions to
achieve this. An example is shown in Fig. 5.7 where a long bone has been
cut into two pieces. The keyed section helps align to the two pieces when
they are joined together.
Separating
As opposed to sectioning models at convenient locations, it may be desirable
to separate different adjacent anatomical structures. Often when preparing
data from the original scan data, different anatomical structures are
suffi ciently close to one another that the data becomes a single object. For
example, close joints can become closed, effectively creating a single object
from two distinct bones. Therefore, it may be desirable to edit the data to
separate the different anatomical structures so that they can be built individually.
This may be so that the parts can be built separately to save time
or cost but may also be because the clinicians wish to be able to articulate
the two structures.
Joining
Just as it may be desirable to separate adjacent anatomical structures, the
opposite may be true. When building separate anatomical structures, their
fi nal use should be considered. If the spatial relationship between the structures
is important, for example where bones have been fractured and displaced,
then that will have to be created physically in the resultant model.
5.7 A long bone model with a keyed section.