R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Case studies 143
multi-plane reformats for bony detail and three-dimensional imaging for
both hard-copy imaging and for stereo viewing by the surgeons.
Model construction and stereolithography
CT scans are typically taken in the axial plane at intervals exceeding 1 mm.
This means that very thin bone lying predominantly in the axial plane may
fall between consecutive scans and, therefore, may not be present in the
data or three-dimensional model created from it. To overcome this, scans
were taken using a smooth kernel at a slice distance of 1 mm but with a
0.5 mm overlap as described above. This improves the resolution of the data
in these thin areas. The detail created is exceptionally good. The CT data
was then segmented to select the desired tissue type, compact bone, using
methods described in Sections 4.1 and 4.2.
The production of models using stereolithography is described fully in
Section 5.2. In this case, to maintain the greatest level of accuracy an epoxy
resin was chosen (RenShape ® SL5220, Huntsman Advanced Materials,
Everslaan 45, B-3078 Everberg, Belgium). This type of resin shows almost
no shrinkage during the photo-polymerisation process and can, therefore,
produce models with excellent accuracy.
Construction of the prosthesis
From stereolithography models, the orbital defect is easily seen and assessed.
The orbital defect is then fi lled with wax to reproduce a contour similar to
the opposite side and an impression is taken of both orbital cavities using
silicone putty impression material. The orbital injury side is then reproduced
by pouring a hard plaster/stone model. The defect has been fi lled
and, therefore, appears in its proposed reconstructed form. Using pressure
fl asks usually used in the construction of dentures, a layer of 0.5 mm medicalgrade
titanium is swaged onto the stone/plaster model of the orbital fl oor,
producing an exact replica of the proposed orbital fl oor and rim contour.
The titanium sheet may then be trimmed to allow suffi cient overlap and
the positioning of a fl ange to fi x the screws. The prosthesis is polished and
sterilised for use according to local protocols for titanium implants.
6.5.4 Case report
A 54-year-old man sustained a ‘blow out’ fracture of the left orbital fl oor
and presented with diplopia (double vision) and restriction of upward gaze.
Coronal plane CT scanning demonstrated the fracture (Fig. 6.23). A stereolithography
model was constructed which shows the trap door of the
fractured orbital fl oor well (Fig. 6.24). The model was then used for