The development of a custom maxillofacial implant by means of rapid prototyping.

Dumitriu, Dan ; Drstvensek, Igor ; Hren, Natasha Ihan 等

1. INTRODUCTION

Defects in the craniofacial skeleton are of either congenital (birth defects) or developmental origin (resulting from trauma, infection, tumour, etc.). The purpose of reconstructing abnormalities is mainly cosmetic, in the idea to approximate a normal appearance, but there is also a functional point of view to it. Since they have a strong effect on the facial region, these types of alterations are highly visible, they affect the appearance, and thus the psychological state, social life, and possibility of the patient to found a family, to name a few.

Today there are modern synthetic implants like chin and mandible augmentation implants made of modern plastic materials (acrylates) available, in the shape of contoured two-piece chin implants and angular mandible augmentation implants. A good synthetic material needs to have following properties: biocompatibility, inertness, bone-similar weight or even lighter, capability to generate no artifacts on CT and MRI scans, ease of manufacturing, enough strength to resist functional stress, not expensive and low or no thermal conductivity (Zeilhofer et al.).

Three-dimensional imaging associated with rapid prototyping techniques and availability of alloplastic materials allow for the construction of an angular implant preoperatively (Bran et al.). This paper presents a technique of custom maxillofacial implant manufacturing using a titanium alloy and Selective Laser Melting (SLM), which represents state of the art in prototyping technologies .

2. PROBLEM STATEMENT

A 22 year old male patient was born with Goldenhar Syndrome, a variant of craniofacial microsomia (Goodrich et al.), which has affected the lower left half of his face. His case had been taken care of by an orthognatic surgeon from the Hospital of Ljubljana. The facial bones that have been affected by the malformation are the left zygoma, maxilla and mandible. With the exception of the mandible, the bones have been corrected to a certain extent using bone flaps from a donor site of the patient's body. The mandible has had an angular mandible augmentation implant attached to it in the past, which was made of acrylic plastics material, like the ones presented in the introduction of this paper. Soon after implant surgery the patient developed a bacterial infection at the implant site, requiring ulterior implant removal and treatment. Furthermore, the defect had been enlarged by the necessity of removing additional bone flaps from the infection site.

A solution was required to manufacture a similar implant out of a material that wouldn't allow for bacteria to develop and still successfully provide for a symmetric reconstruction of the person's appearance, while keeping the implant light enough to be functional.

3. IMPLANT MANUFACTURING

The most promising material was titanium, since it is antibacterial and strong, while a lot lighter than steel, yet heavier than bone tissue and very expensive. The problems that had to be solved were keeping a low weight and finding a method to manufacture the implant.

3.1 Development from medical data

The beginning was analyzing and processing of the data, in form of CT scans, that had already existed from previous stages of our patient's treatment. The set of CT images have then been converted into a three-dimensional digital model, using Materialise Mimics software, obtaining STL files as output, which can be manipulated and used in most RP technologies to produce real models.

Following was CAD modelling of the implant, performed with several 3D modelling and STL manipulation software packages. The idea was to split the skull in two parts in the middle, mirror the right, healthy, side over the left one and obtain the 3D model of the required implant through Boolean subtraction operations (Figure 1). However, there was a problem: due to the facial bone not being symmetric the defined mirror plane had to be different from the vertical mid-plane. Therefore, it was determined considering the certain well-defined features of the skull, like eye and nose cavities.

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After subtracting, the model was inappropriate for implanting due to medical reasons and further modelling was required using 3D software (Figure 2).

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3.2 Production of implant model

After the final inspection of the 3D model (Figure 3), there was need of intermediate real models of the skull and implant, which could be cheaper to manufacture, in order to be tested for dimensional accuracy and analysed by the surgeon, for the phase of operation planning.

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In this sense a model had been developed out of polyamide 2200 using selective laser sintering procedure (Figure 4, 5) (Drestvensek, I.).

3.3 Production of biocompatible implant

The results at the former phase looking promising, the next step would be to produce the final implant out of titanium alloy. The selected method will be state of the art SLM procedure (Wholers, T.). The weight of the implant will measure approx. 6 g, which is quite acceptable, but the compromise was providing it with thin walls (cca 0,7 mm)The powder material will be processed under normal, non-sterile conditions in order to shorten the procedure, and the implant will be sterilized by means of gas sterilization, just prior to the implanting operation.

4. CONCLUSION

The great potential of RP technologies in medical applications is shown once more by the presented case study. Custom made craniofacial implants are expensive and only justify with repairing complex-shaped defects (Binder, W. J., Kaye A.). Further improvements could be directed towards finding solutions to make such implants lighter and cheaper to manufacture.

5. REFERENCES

Binder, W. J., Kaye, A. (1994). Reconstruction of posttraumatic and congenital facial deformities with three-dimensional computer-assisted custom designed implants. Plastic and reconstructive surgery, Vol. 94, 775-785

Bran, S., et al. (2002) Reconstruction of bone defects with alloplastic biomaterials. European Cells and Materials

Drstvensek, I., (2004) Layered Technologies, ISBN 86-4350616-8, Faculty of Mechanical Engineering, Maribor, Slovenia

Goodrich et al. (1995) Craniofacial Anomalies: growth and development from a surgical perspective, Thieme, New York

Wholers, T. (2006) Wholers Report, Wholers Associates, ISBN 0-9754429-2-9, Fort Collins, Colorado, USA

Zeilhofer, H.F., et al.(1997) Moglichkeiten und Indikationsbereiche der Kohlenstoffaserverstarkten Kunststoffe zur herstellung individueller Implantate fur die Rekonstruktion des Gesichts--und Hirnschadels. Biomedizinische Technik