R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

142 Medical modelling
defect (5). Titanium is an inert and widely used material (6, 7), but in its
pre-formed presentation can be cumbersome for use in the orbital fl oor
and, if it has to be removed, it can present an operative challenge.
Continued development in computer-aided diagnosis and management
and construction of stereolithographic models offers unparalleled reproduction
of anatomical detail (8, 9). This technology is described in relation
to planning in trauma surgery (10) and to planning for ablative surgery
for malignancies of the head and neck (surgery to remove cancer) (11, 12).
Construction of custom-made orbital fl oor implants is possible (13, 14),
although the material of choice is debated.
We describe a simple technique for construction of custom-made titanium
orbital fl oor implants using easily available laboratory techniques
combined with stereolithography models. We estimate the implant construction
cost at around £300. This is largely accounted for by the cost of
producing the model which, depending on the height of orbital contour
required on the model, varies between £200 and £300. The making of the
implant takes about two hours of a maxillofacial technician’s time and
the medical grade titanium sheet costs only a few pounds. This compares
favourably with some of the newer alloplastic materials. The cost would
drop substantially with greater use of the technique and, when reduced
operating time is taken into account, the cost comparison is more
favourable.
6.5.3 Technique
Imaging
The detail given here is specifi c to this case; a more general overview of CT
scanning is given in Section 2.2. Scanning protocols are observed to minimise
the dose of ionising radiation to orbital tissues (15). Maximum detail
can be obtained scanning with a 0.5 mm collimation, but the 77 % increase
in dosage when compared to using a 1 mm collimation may not be justifi ed.
We use a Siemens Somatom Plus-4 Volume Zoom scanner with these
settings: 140 kV, 120 mAs, 1 mm collimation, 3.5 feed per rotation, 0.75
rotation time, giving a displayed dose of 45 mGy/100 mAs (Siemens AG,
Wittelsbacherplatz 2, D80333, Munich, Germany). Data are reconstructed
using 1 mm slices with 0.5 mm increment (50 % overlap) and smooth kernel.
Sharp reconstruction kernels normally associated with CT imaging of bony
anatomy introduce an artifi cial enhancement of the edge. If used as part of
a three-dimensional volume based on selection of specifi c Hounsfi eld
values, the enhancement artefact will be included with the bony detail, so
degrading the image. The data obtained can be used to construct sharp