R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

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In some cases, this may be achieved by leaving supports in place between
the different parts. However, supports are not strong enough to achieve
this reliably. Instead, it may be necessary to create bridges that join the
separate parts together. When creating such bridges, it is important to make
them clearly artifi cial in appearance and locate them away from important
areas so that they are not confused with the anatomy. An example of a
bridge between two separate bones is shown in Fig. 5.8.
5.1.8 Basic principles of medical modelling –
trapped volumes
Compared to engineering parts, the presence of trapped volumes (internal
cavities) can pose particular problems in medical modelling. The problem
is particularly associated with stereolithography where the effect can lead
to build failure, as the resin in the trapped volume does not level correctly
(see Section 5.2 for more detail on the trapped volume effect). However,
the presence of trapped volumes can cause problems for many RP processes.
To produce the unwanted effects of a trapped volume, the cavity
does not need to be entirely closed. If the openings to such a cavity are
small enough, the effect will be as bad as a fully closed cavity. Examples of
cavities are the cranium, the sinuses in the skull and face and the marrow
space inside large bones.
The main problem with trapped volumes is removing the waste material
and/or supports from within them. Various techniques can be used to
5.8 An example of a bridged model connecting the proximal femur to
the pelvis.