A method for manufacturing skeleton models using 3D scanning combined with 3D printing.

Serban, Ionel ; Rosca, Ileana ; Druga, Corneliu 等

1. INTRODUCTION

This paper addresses the issue of efficiently generating skeleton models suitable for use in computer based simulation of medical procedures. When learning medical procedures it is desirable to have a wide range of models on which to practice (Zhu et al., 2008).

Anatomical models can also be used to explain pathology and surgical procedures to patients and their next of kin. This has benefits for both the patient and the health service, as studies have shown that patients who receive pre-operative education tend to recover more quickly post-operatively, have less pain and anxiety, and are more satisfied with the outcome of their surgery (Shuldham, 1998).

2. STATE OF THE ART

Nowadays the computed tomography is considered to be a successful medical imaging method. The data obtained could be processed using MIMICS software.

The 3D handy scanners could be used as a faster method for obtaining the data processed in CAD software such as CATIA. The designed models obtained from this software, can be further on simulated in CATIA or directly manufactured using a fast prototyping technique.

The technologies used for the manufacturing of high quality medical models are, predominantly, stereo lithography and fused deposition modelling (Winder & Bibb, 2009). These techniques produce anatomical models from computer STL files.

The most commonly used rapid prototyping technique for medical applications is stereo lithography, but FDM has several potential advantages.

Most medical models to date have used stereo lithography (SL); a technique where a liquid resin is polymerized by laser light to form a solid material of the required shape. Fused deposition modeling (FDM) is a newer RP method in which a solid model is produced by controlled deposition of a molten polymer monofilament. One of the advantages of FDM, over SL, is that the model is created in a single processing step. SL models require additional cleaning and curing under ultraviolet light, which increases the time to produce a model. Furthermore, the resin is toxic and expensive. It has been suggested that the advantages of FDM make it more suitable for a hospital environment than SL. The materials available for use in an FDM machine are biodegradable. Recent models were created successfully using poly (e-caprolactone), which is a biodegradable polymer (Meakin et al., 2004).

3. HANDHELD 3D LASER SCANNER

For more than 10 years sensor manufacturers have developed laser triangulation sensors for measuring linear distance of a target from the sensor.

This linear distance can be referred to as movement, position, displacement, distance etc.

This technique has developed significantly in the last 3-5 years as the integration of digital electronics and high powered digital signal processors DSPs enable the laser to be less sensitive to target color or texture and the surrounding environment, ambient light, and temperature changes.

Laser triangulation is far better suited to step height measurements, determining the profiles of extruded products and robot positioning and control, but this measurement technique is also limited due to the spot size of a laser sensor being very small and so multiple sensors are required, which makes this technique potentially as complex and expensive as vision cameras. (*** 2000).

Laser triangulation is the principle of scanner and it is accomplished by projecting a laser line or point onto an object and then capturing its reflection with a CCD sensor located at a known distance from the laser's source. The resulting reflection angle can be interpreted to yield 3D measurements of the part ( *** 2007).

[FIGURE 1 OMITTED]

The technical specifications for the scanner, EXA 3D scanner, that was used shows a high accuracy (Table 1), which is useful considering the irregular, complex surface of the skeleton. The object that was used is an organic femur bone.

The experimental setup is composed from a scanner, a laptop which has a simple, easy to use, friendly interface that can be used by any operator.

The first step was to ensure surface is clean, then the positioning targets were placed, magnetic stickers, on the surface that is intended to be scanned (Figure 1). There should be kept a distance of 0.5 to 1 centimetres between the targets.

Afterwards, the CCD sensor is being configured so that scanner could identify the colour of the surface that is scanned. This is realized from the scanner's software by handling the scanner, in the vertical plane to the surface, until it reaches a good level of identification, visible on the laptop's screen.

The next step is to scan the positioning features, which will appear on the screen exactly as they are seen on the surface. Their role is to create the appropriate environment for an accurate identification of the surface underneath the positioning features.

Delimitating the area that will be scanned helps avoiding situations in which other objects could interfere such as hand, table or any other surface that can get in between.

After this, it is very important that the object shouldn't be moved; otherwise the entire process needs to be repeated as the scanner doesn't recognize the new positions of the features.

The surface is scanned with scanner held in a position as perpendicular as possible to the area.

The data obtained (Figure 2) is processed in the scanner's software and saved as a STL file. These generated files can be imported into inspection software and quickly manufactured.

The laser scanner is the perfect inspection tool for analyzing and reporting geometric dimensioning and tolerance.

As seen in figure 2, there are some small spots that aren't scanned on the virtual surface obtained. This is due to shiny and variable geometries of the bone. Problem discard can be performed in CATIA which is compatible with EXA scanner.

[FIGURE 2 OMITTED]

4. CAD AND SIMULATION SOFTWARE

The data obtained through the method above needs some adjustment in some areas due to the imperfections of the scanning system. This is made in a computer aided design software e.g. CATIA wherefrom it is sent to a biomechanical simulation software e.g. ANSYS.

The finite element method is applied with success due to the possibility of non contact and repetitive analysis. The von Misses stress is used to predict yielding of materials under any loading that the skeleton model is exposed to.

5. FUSED DEPOSITION MODELLING

FDM is the second most widely used rapid prototyping technology, after stereo lithography. A plastic filament is unwound from a coil and supplies material to an extrusion nozzle. The nozzle is heated to melt the plastic and has a mechanism which allows the flow of the melted plastic to be turned on and off. The nozzle is mounted to a mechanical stage which can be moved in both horizontal and vertical directions.

As the nozzle is moved over the table in the required geometry, it deposits a thin bead of extruded plastic to form each layer. The plastic hardens immediately after being squirted from the nozzle and bonds to the layer below. The entire system is contained within a chamber which is held at a temperature just below the melting point of the plastic. (*** 2008)

Rapid Prototyping is an innovative technology that has evolved within the design and manufacturing industries.

Considering the layer-thickness, 0,127 mm, of the deposition material, ABS plastic, it can be obtained models with high resolution that follow the same pattern as the physiologic skeleton.

Rapid Prototyping is revolutionary in the technology field it can be used with success in achieving any anatomical model, out of different materials as plastics, metals, nonmetals.

Model can be manufactured directly from the data acquired from the scanner in a STL file format, without using any CAD or simulation software.

6. CONCLUSION

This method is useful in the field of biomechanics, medicine where anatomical models have a great influence for their analysis and teaching. It can also be used for reconstruction of tissues or obtaining orthesis.

It offers high accuracy on the data obtained. The 3D scanner and the 3D printer have friendly interfaces and are easy to use by any operator with little knowledge.

The main difficulties with this approach are different bones positioning, surface and scan resolution and variations in the geometry of skeleton structures. A further research could be made in this direction.

7. REFERENCES

Erickson, D.M. et al. (1999). An opinion survey of reported benefits from the use of stereo lithographic models. Journal of oral and maxillofacial surgery, Vol. 57, No. 9, (September 1999) pp. (1040-1043), ISSN 0278-2391

Meakin, J.R. et al. (2004). Fused deposition models from CT scans. The British Journal of Radiology, Vol. 77, No. 918, (November 2003) pp. (504-507), ISSN 0007-1285

Shuldham, C. (1999). A review of the impact of pre-operative education on recovery from surgery. International journal of nursing studies, Vol.36, No. 2, (September 1999) pp. (171-177), ISSN 0020-7489

Winder, J. & Bibb, R. (2005). Medical Rapid Prototyping Technologies: State of the Art and Current Limitations for Application in Oral and Maxillofacial Surgery. Journal of Oral and Maxillofacial Surgery, Vol. 63, No. 7, (May 2005) pp. (1006-1015), ISSN 0278-2391

Zhu, Y et al. (2008). A Physics Based Method for Combining Multiple Anatomy Models with Application to Medical Simulation, In: Medicine Meets Virtual Reality, Westwood J.D. et al., Studies in Health Technology and Informatics, pp. (465-467), IOS Press, ISBN 978-1-58603-964-6, The Netherlands

*** (2007) http://www.3dscanco.com--3D Scanning Technical Information, Accessed on:2009-07-13

*** (2009) http://www.creaform3d.com/en/ handyscan3d/products/exascan.aspx--EXAscan Brochure, Accessed on:2009-06-10

*** (2000) http://www.engineeringtalk.com-Laser triangulation outperforms vision systems, Accesed on:2009-06-10

*** (2008) http://www.designophy.com--Rapid Prototyping: Ink Jet Printing, Accessed on:2009-07-02

Tab.1. Technical specifications. (*** 2009)

Weight 1.25 kg (2.75 lb)
Measurements 25,000 measures/s
Laser Class II (eye-safe)
Resolution in x, y, z axis 0.05 mm (0.002 in)
Accuracy Up to 40 [micro]m (0.0016 in)
ISO 20[micro]m + 0.1 L /1000
Depth of field 30 cm (12 in)