R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

212 Medical modelling
down-facing to the x–y plane. The ‘sectioning’ and ‘cut’ tools were then
used to remove the exact depth of the cap. This effectively left a perfectly
fl at surface representing the top surface of the abutments. The modifi ed
STL data was then imported into FreeForm ® .
Design
As in the previous study, FreeForm ® was used to manually align the ear
taken from the CT data to the digital cast based upon the estimated aesthetic
requirements and possible substructure location. However, unlike
the fi rst case study, the data quality in this case study enabled the design
of all of the components to be attempted. Digital versions of the screws
used to attach frameworks to the abutments and cylinder components
were designed using the drawing and rotation tools in FreeForm ® . A
circular-section framework linking the two cylinders was created using the
‘add clay’ tool. As in conventional methods, this followed the thickest
section of the ear to ensure that all of the components would fi t within the
ear profi le.
Clip designs were created using the two-dimensional drawing and
‘extrude’ tools. These were copied three times and manually located
along the bar structure at key points of maximum prosthesis thickness. In
order to secure the clips into the prosthesis body and to assist application,
a substructure shell that would be bonded to the silicone was required.
This had to provide enough clearance for the clips to spring open and
closed, but provide fi rm anchorage for bonding to the silicone. Digital ‘clay’
that enclosed the framework and clips, leaving just their top features as a
point of attachment, was built up from the cast model. The ‘paint on selection’
tool was used to select and copy, then paste the raised section surrounding
the framework. The ‘create offset piece’ tool was then used to
create a shell 1.5 mm thick surrounding the clips and bar. Boolean subtraction
operations and hand carving were used to fi nalise the shell before
joining it to the clip components. The bar, clip and shell design is shown in
Figs 6.72 and 6.73.
The prosthesis profi le was thickened around the clip areas to accommodate
the shell component, and a Boolean subtraction operation used to
create a fi tting recess. The bar design was fi nalised with Boolean subtraction
operations to provide location for the screws, and smoothing operations
were applied around the joints with the cylinder features. Hemispherical
dimples were also created where cylinders located on the abutments. Fig.
6.74 shows an exploded view of the components in the FreeForm ® environment.
Each of the components was then exported as high quality STL fi les
ready for RP fabrication (see Section 4.2.6 for more information about the
STL fi le format).