Engineering Assisted Surgery
(Stereoscopic Lithography)

Engineering Assisted Surgery


What is Engineering Assisted Surgery?

In the 1960’s a major developments occurred in the manufacturing industry as a consequence of advances in engineering technology. These developments were largely as a result of the use of computers, which ...

automated reverse engineering technology for efficient prototype production.
standardized the quality of product.
reduced human related error in the manufacturing process.
improved the efficient use of resources and productivity.


It is therefore reasonable to examine applications of engineering technology to the Healthcare Industry, which still relies heavily on the "art and craft skills" of its clinically based workforce.

We propose to term this concept Engineering Assisted Surgery™ (EAS).



Engineering Assisted Surgery (EAS) involves:

Bringing industrial concepts of service, accurate planning, automated delivery and predictable quality to the healthcare industry.
Adoption and adaptation of efficient proactive management and IT systems, proven in the manufacturing industry.
Centralisation of Consultant Planning Teams (hub and spoke concept for delivery).
Inclusion of (bio) engineers within the clinical team.
The use of Engineering Assisted Surgery technology in clinical interventions.
A universal improvement of quality and standardisation of outcome.
More appropriate use of available resources.
Reduction in administration procedures and costs.
Reduction in medical negligence litigation through improved outcomes.
Minimising disability and promoting return to normal function.
Reduction in the cost of healthcare to the Nation.






EAS may be used in conjunction with:

the planning and facilitation of established interventions i.e. current practice
new surgical procedures
customized medical devices.


Rapid Prototyping Systems




Modelling Principles

Surface Modelling

Methods:

Initial surfaces defined over a network of curves. Creation of cross-sections along the length of the object – smoothing of surface.
Wire frame modelling to define basic geometry CAD reshaping


Solid Modelling

Creation of model by drawing a 2D cross-section and extruding or rotating to produce a solid.
Creation and fusion of primitive geometric shapes to form model.








Data Collection

Basically two forms of data are required for model building. Object geometric data is collected from scanning techniques (e.g. CT /MRI / Laser) for CAD manipulation, which produces the blueprint for model manufacture.

Model Manufacture

STL files are then generated for the chosen rapid prototyping equipment and model manufacture. These files may be emailed to the clinician for confirmation of diagnosis and clarification of abnormalities of anatomy, prior to setting thresholds for model manufacture, (see below).



STL file - complete destruction of floor of orbit
click image for link to treatment plan


Data files are manipulated using software to facilitate boundary trimming and re-intersecting edges to give an exact match of model component parts (Delcam's PowerShape). Geometric closure of the model is then required using geometrical triangulation sealing methods. Delcam’s software, TriFIX ensures that these rules are obeyed; gaps in the model are repaired and overlapping surfaces corrected.





Reverse Engineering

Reverse Engineering may be defined as "the reduplication of components for commercial production". As soon as any prototypes are created small changes may be required to improve performance or aesthetics etc. Changes may be made by re-digitising the area of interest using the reverse engineering programme CopyCAD.

The edited data is then converted into a triangular model using a reverse engineering wizard programme.

Stereolithography - SLA (Figure 1)

This was the first of the rapid prototyping technologies to be developed. The pioneers of stereolithography, 3D Systems Inc, built the first production units in 1988.

Software sections the CAD model of the desired component in to a series of adjacent 2D slices. This data is used to control a laser beam which draws each slice of the model, in turn, on the surface of a tank of resin. The resin is photosensitive and is instantaneously cured to a solid where the laser beam strikes.


Figure 1





SLA Biomodel with Customised Implant

The model is built on to a platform within the resin tank. At the start of the process the platform is positioned just below the surface of the resin. The first layer of the model, the base, is drawn by the laser to form a solid layer, typically 0.25mm thick. The platform then descends in the bath to allow new liquid resin to cover the cured layer and the next model slice is constructed above it. In this way the whole of the model is built from the base up, as the platform descends in the tank.





Micromanufacturing techniques / Vector-vector point curing

William O’Neil (Micromanufacturing Laboratory, Faculty of Engineering, University of Liverpool) has documented a resolution of >5 microns for SLA.

Process resolution for microstereolithography techniques in terms of minimum voxel size is reported as 5x5x3µm (Ikuta and Hirowatari 1993 Proc IEEE MEMS Fort Lauderdale). In the 1990’s despite introduction of a new laser with smaller spot size 0.08-0.1mm, point wise curing and liquid levelling it became apparent that manufacture of true high-resolution microparts were not possible with SLA, but was possible with other RP systems which were able to produce a resolution down to < 1 micron.

MicroTec (Duisburg, Germany) has developed a process Rapid micro Product Development using a split high resolution UV laser beams which can cure in parallel layers down to 1 µm with a minimum component size of 1x10x10 µm.

 


Laminated Object Manufacturing - LOM (Figure 2)

In this system the model is formed from successive layers of heat bonded sheet material. The sliced CAD data is used to control a laser, which cuts the perimeter (only), of each slice in the sheet material. The waste material around the slice is left in place to support the next layer of the model but to assist with subsequent removal; it is scored by the laser to form blocks. Once each layer is complete a new sheet of material, typically adhesive coated paper, is heat bonded on top of it and the laser cuts the next slice. The final model has the appearance of soft wood.

Figure 2

Figure 2



LOM is used mainly for larger bulky parts, because the paper it uses is relatively cheap compared with powders and resins used in other processes. A contraction of 2-3% occurs in the z (build) direction following the build and delamination may occur at elevated temperatures for a prolonged time.


Laser Sintering - LS (Figure 3)

This system operates in a similar way to stereolithography but in this case the photosensitive resin is replaced by a fine heat fusible powder (wax, thermoplastic etc.). A CO2 laser draws the CAD slices on the surface of the powder, locally melting and fusing it together. A new layer of powder is placed on top of the completed slice and then the next slice is drawn. The unsintered powder around the slice is left in position to provide support for the model and is removed once the building routine is complete.

Figure 3

Figure 3



Sintered nylon can be used as direct patterns for sand casting. They are more durable than SLA patterns but require more hand finishing, and need to be sealed before use. Sintered sand is a quick reliable route of producing prototype castings. Excellent internal shapes are virtually guaranteed.

Sanders Model Maker (Figures 4,5)

The ModelMaker II system produces solid 3D models with a far higher level of accuracy than other rapid prototyping systems.

The following is from Sanders Prototype Inc web site:

  • Up to 20 times more precise than some other rapid prototyping systems.


  • Build envelope: 12 x 6 x 9 in. (30.48 x 15.24 x 22.86 cm)


  • Build layer: 0.0005 in. (0.013 mm) to 0.0030 in. (0.076 mm)


  • Achievable accuracy: +/- 0.001 in.
        (0.025 mm) per inch in X, Y and Z dimensions


  • Surface finish: 32-63 micro-inches (RMS)
        Minimum feature size: 0.008 in. (0.20 mm)


  • Automatic generation of model support structure. Figure 4


  • Point and click access to job status, remaining build time, and other statistics
  •  

     

    Figure 4

    Figure 4




    EXCELLENT CASTING QUALITIES
    • Direct Casting of Models
    • Negligible co-efficient of thermal expansion (prevents casting shell ruptures)
    • Fast melt out in autoclave
    • No ash or residue contamination

    Figure 5

    Figure 5


    BUILD PRECISION

    With build layers as much as 20 times thinner, the ModelMaker II system delivers higher precision models, prototypes, and patterns than other rapid prototyping systems.

     

    3-Dimensional Printing (Figures 6,7)

    To build a part, the machine spreads a single layer of powder onto the movable bottom of the build box (steps 1,2,3). A binder is printed onto each layer of powder to form the shape of the cross-section of the model (4). Then the bottom of the build box is lowered by one layer thickness (5) and a new layer of powder is spread. This process is repeated for every layer or cross-section of the model. Upon completion, the build box is filled with powder, some of which is bonded to form the part, and some of which is loose.

    Figure 6
    Figure 6


    Description of Process:

    1. Collect Powder

    2. Spread Powder

    3. Discharge Excess Powder

    4. Print

    5. Feed Piston Up, Build Piston - Repeat



    Z-Corporation Z402 Machine (Figure 7)



    In this 3D printer, to build a part, the machine spreads a single layer of powder onto the movable bottom of the build box (steps 1-3).
    A binder is printed onto each layer of powder to form the shape of the cross-section of the model (4). Then the bottom of the build box is lowered by one layer thickness (5) and a new layer of powder is spread. This process is repeated for every layer or cross-section of the model. Upon completion, the build box is filled with powder, some of which is bonded to form the part, and some of which is loose.

    Once a build is complete, the excess powder is vacuumed away and the parts are lifted from the bed. Once removed, parts can be finished in a variety of ways to suit your needs. For a quick design review, parts can be left raw or "green". To quickly produce a more robust model, parts can be dipped in wax. For a robust model that can be sanded, finished and painted, the part can be infiltrated with ZR10 resin. The machine prints between one and two vertical inches an hour, and so in terms of speed is very fast even in comparison with other rapid prototyping systems. However, the accuracy level is compromised to achieve these speeds.

    Figure 7

    Figure 7




    Fused Deposition Modelling - FDM (Figure 8)

    Figure 8

    Figure 8



    This process employs a system very similar to an X/Y plotter. In this case the pen is replaced with an extruder head which draws each slice of the model using a fine stream of plastic (or wax). The extruded material cools very rapidly to form a solid layer. As with the other systems each layer is built on top of the preceding layer, starting at the base of the part.

    Thermojet Process (Figures 9,10)

    Figure 9

    Figure 9



    Wax parts produced in this system can be used as sacrificial patterns for investment casting. The main advantage is in the production of relatively complex castings without the need for tooling. Cost effective complex metal parts may be produced from CAD models in a relatively short period of time. Wax patterns need to be finished to a high standard. One problem with the system is the support system used which leaves undulations on all downward facing surfaces of the pattern. The supports have to be removed and surfaces cleaned by hand. This process is best suited to small numbers of complex parts that would otherwise require a significant amount of coring to accommodate undercut features.

    A major advantage with this approach is the designer does not need to add draft angles to the geometry, although radii and machining allowance may be required. Figure 10

    Figure 10
    Figure 10

     

    Solider


    Solider (Figure 11) In this process the CAD data, again in the form of slices, is used to generate a 'photocopy' of each slice on a piece of glass; the cross section of the model is represented by clear glass, the area around as black. Using the sheet of glass as a mask, a thin layer of photosensitive resin is cured instantaneously by a burst of UV light. Only the area of the model is solidified, any uncured resin is removed and replaced with wax to provide support for the next layer of model. In this way, the model is constructed from the base up, with the sheet of glass being cleaned and the next slice copied for each layer. Once the model has been completed it is heated to remove the wax.

    Figure 11

    Figure 11




    Direct Shell Production CastingTM - DSPC (Figure 12)

    Direct Shell Production Casting TM is slihtly different from the systems mentioned previously in that a cavity is produced to the dimensions of the CAD model required. The shell is built in a ceramic allowing the direct production of investment shells from CAD data. The Massachusetts Institute of Technology developed 3D printing on which the DSPC process is based. The system is licensed to Soligen Inc. USA for use in metal casting.

    A CAD file is used to define the required cavity, the system modifies this file to suit the investment casting process by introducing fillets and removing machined features such as holes. The number of castings required are then entered into the computer controlling the system and the model of the shell generated. At this point mould flow and simulation can be run.

    The process can loosely be described as a cross between SLS and FDM. A layer of alumina powder is laid down onto a cylindrical bed.

    A print jet projects a fine stream of colloidal silica in the required plan of the slice onto the powder coating. This solidifies and also adheres to the previous layer. Once the slice of shell has been drawn the machine bed lowers and the process is repeated until the entire shell has been formed. As with the SLS process the excess powder acts as a support while the shell is being produced. On completion the shell is removed from the bin and excess powder removed from within the cavity.



    Figure 12

    Figure 12

     



    ENGINEERING ASSISTED SURGERY