RE: I beg to differ Re: Stupid STL Question

From: Mr.Le Chi Hieu (
Date: Thu Mar 21 2002 - 21:26:34 EET

Dear Mr. Larry,

> What are you creating the STL file with... that you
> want to edit with cheap software?

We use the STL files of anatomical structures
processed in Medical Image Processing software
(Mimics) for implant design.

> Most biomedical models are created from extremely
> expensive scanning equipment (MRI, X-ray, CAT, 3d
> and then processed through specialized
>software to create the STL data. Could the input
> data be processed into something other than STL.
>Are you working with a intermediate data format (STL)

> that is just introducing greater complexity?

Because the Medical Image Processing provides two
types of anatomical data format: (i) Bonny contours
(IGES, SSL, etc.) and (ii) STL solid files. Therefore,
when you start to design some things based on the
geometrical information extracted from the CT/MRI
images, we "must" use this two types of data as the
starting design database.

Because of limited capacities in manipulating the STL
files, we have to convert the STL files into the
parametric surface, concretely the NURB based ones.

Free-formed computer aided sculpting of a polygonal
mesh surface (stereolithography) can be used for
smoothing and parameterized the surface of the STL
biomodel. This sculpting method is mainly used for
sculpting and reshaping triangle surface meshes common
in surface digitizing and rapid prototyping
applications where the models are usually stored in
STL or other polygonal mesh format (e.g. DXF, OBJ); it
serves as a powerful reverse engineering and artistic
editing tools. For medical applications, it is used
for (i) Surgical simulation to show the patient what
they look like after surgery, and (ii) Editing 3D
models generated from CT/MRI scanning data . See more
information in the following paper: Alciatore, D.G.
and Wholers, T.T., “Importing and reshaping digitized
data for use in rapid prototyping: a system for
sculpting polygonal mesh surfaces”, Rapid Prototyping
Journal, Vol.2, No.1, 1996, pp13-23.

However, in the field of design and manufacturing of
implants, we used NURB free-form surface modeling
methods for modeling surfaces. Because NURB is the
standard for the representation, the design and the
data exchange of geometric information; and today,
NURB based hybrid surface/solid modeling methods are
very useful and commonly used modeling tools in the
commercial CAD/CAM software

> It seems that the data format that you need for the
> final process (RP) may not the most appropriate
> for manipulation or editing. I would look at your
> entire process and determine the best format for
> each step.

It is noted that the end-use application determines
the modeling methods to be used in the design process.
For CAD/CAM/CNC applications, points, lines, contours,
surface or solid CAD entities can be used for tool
path generation. Nevertheless, the STL files are
required and used as the interface for fabricating RP
biomodels. Moreover, the implant design method is not
only dependent on the CAM techniques used to make the
implant, it also depends on the patient geometrical
data types used as the starting design data.

Therefore, depending on the concrete applications, the
data formats are used for modeling from which the
modeling packages are selected. By this way, we did
develop some program modules that automatically solve
the problems related to the design data processing
chain; and these programs are used as the bridge (or
interface) between Medical Image Processing (MIP) and

I hope this can make clear some points for your


Le Chi Hieu

Mr.Le Chi Hieu
The Biomodeling Group, ISE.SAT.AIT
Bangkok, Thailand

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