R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Case studies 129
and passing through the skin to provide a rigid and fi rm fi xture for dentures,
hearing aids and prostheses (5). See medical explanatory note 8.2.1 for an
explanation of osseointegrated implants. The accuracy of the RP models
allows the depth and quality of bone to be assessed, improving the selection
of drilling sites before surgery. Although this process has dramatically
improved the accuracy and reduced the theatre time of some surgical procedures,
it incurs signifi cant time and cost to produce the anatomical model.
Whilst it does utilise RP technologies, this current route does not fully
exploit the potential advantages of computer-aided design.
To address this issue it was decided to complete as much of the planning
as possible in the virtual environment and only use RP to make small templates
that would guide the surgeon in theatre. This route would allow the
clinicians to conduct all the planning and explore many options without
damaging an expensive RP model. To be successful, the approach would
have to be simple to conduct and have low investment requirements.
6.3.3 The proposed approach
The approach would use three-dimensional computed tomography (CT)
data 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. The simple and popular STL
(6) format was chosen as the three-dimensional representation of the entities.
This format ensures easy access to a number of software options at a
reasonable cost. In this case, the software package Magics (Materialise NV,
Technologielaan 15, 3000, Leuven, Belgium) was chosen. The STL fi le
format is more fully described in Section 4.6.2. The entities were created
as STL format fi les from CT data using one of a number of specialist software
packages available for creating STL fi les from CT data (Mimics,
Materialise NV).
The STL manipulation software was used to mirror the copy of the ear
and position it in an anatomically and aesthetically appropriate location.
The software was then used to create cylinders representing the implants.
These cylinders were positioned in the preferred location by the prosthetist
observing a lateral view. Then the bone quality at the implant sites could
be assessed.
6.3.4 Scanning problems
Misalignment of the patient’s head to one side or the other means that the
optimum accuracy obtained in the axial plane during scanning is not axial
to the patient. This means that entities will not be in alignment with the