CAD/CAM methods in the customization of medical implants.

Dreucean, Mircea ; Tascau, Mirela Toth ; Rusu, Lucian 等

1. INTRODUCTION

The patient included in the study had a congenital skeleton defect at the level of the left mandible. The upper left limit of the mandible was missing. The pre-surgery CT reconstructed skeleton of the skull is presented in Fig. 1.

In the situation presented above, the normal procedure is to rebuild the mandible at the left condilys zone using a metal implant, a high density polyethylene implant [Dragulescu D., Popescu M. (2006)] or an auto implant extracted from the long bones of the patient. In the study the authors tried to find a solution to pre-design the implant based on the real geometry of the patient's skeleton [Saringer W, Nobauer-Huhmann I, Knosp E. (2002)].

2. GENERATION OF THE MODEL

The 3D model of the mandible was generated in Mimics after the transfer of the CT scans from the computer tomograph. This software package offers the possibility of separation the bone from the rest of the tissues, according to the density of the material. The core idea of the paper is to use the reconstructed skull as a CAD object in order to determine the exact shape of an implant which will refill the missing part of the mandible [Tardieu PB, Vrielinck L, Escolano E. (2003)]. This activity is developed prior to the surgical intervention, so that the surgeon can shape the implant before the actual implantation. This is the way of obtaining the so-called "customized implant". In the lower right corner of Fig. 2 one can see the part extracted from the 3D model and mirrored in order to be used as a template.

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After the detection of the shape of the implant on the right extremity of the mandible, this was separated using special Boolean operations and isolated as an individual part. In the next step the part was mirrored in order to fit in the congenital defect of the mandible at the left side of the face. With slight adjustments consisting of move and cut operations, the part filled the gap in the mandible and the conclusion drawn with the medical team was that the procedure of prototyping the implant as a model for the real auto implant can be started.

The process of production for the prototype of the implant was developed on a prototyping machine which works with polyethylene film. The part was produced in successive layers of thin film and compared with the shape and dimensions of the part in the model. The prototype had to be refined in several steps in order to satisfy the requests of the surgeon. In the end the part representing the model of the implant looked like in Fig. 3

3. GENERATION OF THE IMPLANT

The additive process which is the base of the rapid prototyping technology consists of the successive add of a new layer of material, with an individual contour [Yaxiong, Liu; et al. (2003)]. The layers are connected to each other in different ways, so that in the end the result is a solid part. One of the preparation stages for a model before starting the build process is the determination of the contour for each layer. The process goes automatically with the help of special software and the result is a closed contour for each section.

The Fig. 4 presents the mandible implant in the process of growth, at approximately one third of the height of the part. The generation of the prototype with the technology based on polyethylene film lasted one hour and 20 minutes.

With all the other preparation actions, the prototype was ready in about three hours from the moment when the patient left the CT room.

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Once the model of the implant was ready, the surgical process could start. The first step was the preparation of the implant from the femur bone of the patient, respecting the shape of the prototype. This was a separate surgical intervention. The implant was then prepared for implantation and stored for a short while in septic conditions until the patient was ready for a second surgical session. This second stage was much easier than a classical intervention of that type, because the implant had a proper shape and had only to be placed with very unimportant adjustments.

4. SOME ASPECTS OF THE FEA ON THE FIXATION BLADE

For the patient presented above the fixation of the implant on the rest of the mandible was achieved with a special blade made of Titanium alloy TiAlV6. The blade was taken from a set of different shapes of blades, especially designed and produced for aiding in maxilla-facial surgery.

The finite element analyze has two points of interest:

* The determination of stress and strain values in the least favorable load conditions;

* The determination of the influence of the number of fixation screws on the peak values of stress and strain.

The blade and the boundary conditions are presented in Fig. 5.

The load is considered to be developed by the pterygoidmasseter muscle [Gallas M., Fernandez R. (2004)], developing a force of 500 N in the plane of the plate. In other study there was also considered the occlusion force with a magnitude of 426.24 N. In the case of both loads acting in the same time, the value of the maximal equivalent stress is 197.62 MPa, very close to the yield limit of the material of the blade. The deformed shape of the blade is presented in Fig. 6.

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The number of fixation screws has a great influence on the value of maximal stress. The study revealed o growth of the maximal equivalent stress (MaxES) of six times when the number of fixation screws goes from 2 to 5. The situation is presented in Fig. 7.

5. CONCLUSIONS

The case study presented in this paper is based on a real case included in the records of the research centre where the authors are involved. The contribution of the authors at the final solution consisted of the shortening of the surgery cycle for such an intervention and the enhancement of the resolution of the surgical operation by the use of an implant already prepared in a separate surgical stage.

The FEA analyze of the fixation blade revealed the level of stress and strain in the fixation element and recommends the fixation of the blade with a maximum possible number of screws in order to reduce the strain and stress in the bone.

For the future, the research team is oriented towards the production of customized implants direct in metal or reinforced HDPE.

6. REFERENCES

Dragulescu D., Popescu M. (2006). The Encyclopedia of Composites. Ed. Politehnica, ISBN 973-625-272-8, Timisoara

Gallas M., Fernandez R. (2004). A three-dimensional computer model of the human mandible in two simulated standard trauma situations. Journal of Crania--Maxillofacial Surgery Volume 32, Issue 5, October 2004, page 303-307

Saringer W, Nobauer-Huhmann I, Knosp E. (2002). Cranioplasty with individual carbon fiber reinforced polymer (CFRP) medical grade implants based on CAD/CAM technique. Acta Neurochir (Wien). 2002 Nov; 144(11): page 1193-203

Tardieu PB, Vrielinck L, Escolano E. (2003). Computer-assisted implant placement. A case report: treatment of the mandible. International Journal of Oro-Maxillofacial Implants. 2003 Jul-Aug;18(4): page 599-604

Yaxiong, Liu; Dichen, Li; Bingheng, Lu; Sanhu, He; Gang, Li (2003). The customized mandible substitute based on rapid prototyping. Rapid Prototyping Journal v 9 Issue nr. 3 2003 page 167-174