Modular orthopedic implants for forearm bones based on shape memory alloys.

Tarnita, Daniela ; Tarnita, Dan ; Bizdoaca, Nicu 等

1. INTRODUCTION

Bionics or Biomechatronics is a fusion science which is interrelated with medicine, mechanics, electronics, control and computers. The finite products of research in this field are implants and prostheses for humans and animals. The roll of the implants and prostheses is to interact with muscles, the skeleton, and the nervous system to assist with or enhance motor control lost by trauma, disease, or defect. Prostheses/implants are typically used to replace parts lost by injury (traumatic) or missing from birth (congenital) or to supplement defective body parts. In addition to the standard artificial limb for every-day use, many amputees have special limbs and devices to aid with practicing sports or recreational activities.

1.1 Shape memory alloy

Shape memory alloys (SMA) constitute a group of metallic materials with the ability to recover to a previously defined length or shape when subject to an appropriate thermo-mechanical load. When there is a limitation of shape recovery, these alloys promote high restitution forces. Because of these properties, there is great technological interest in the use of SMA for different applications. Super-elastic NiTi has become a material of strategic importance as it allows specialists to overcome a wide range of technical and design issues like the miniaturization of medical devices or the increasing trend for less invasive and therefore less traumatic procedures. Essentially nitinol is an alloy containing approximately 50 at.% nickel and 50 at.% titanium. The shape memory alloy Nitinol has several advantages such as greater ductility, more recoverable motion, excellent resistance to corrosion, stable transformation temperatures, high biocompatibility, good kink resistance, less sensitivity to magnetic resonance imaging, fatigue life and the ability to be electrically heated to recover shape and this properties are presented in different papers (Pelton et al., 2000; Duering et al., 1999). The biocompatibility of these alloys is one of the important points related to their biomedical applications as orthopedic implants, cardiovascular devices, and surgical instruments. Studies about the medical applications of the shape memory alloys have been made (Friend & Morgan, 1999).

2. METHOD FOR VIRTUAL 3D BONES MODELS

The natural variability of the geometry and mechanical properties among bones from one to the other is a crucial problem that creates real difficulties in the field of biomechanical research. The dimensions, form, mechanical properties, elastic constants and physical constants of the bone are different among bones of different types and even among different bones of the same type. They depend on: age, sex, height, profession etc. The geometrical aspects of modelling the bone are dominated by the necessity of using spatial models because most of the bone elements have complicated spatial geometrical forms. The ulna and the radius are long bones of the forearm, broader proximally and narrower distally.

To obtain the cross sections of the bones, a PHILIPS AURA CT tomograph installed in the Emergency Hospital of Craiova was used. To obtain the tomography of the two bones of human forearm (radius and ulna) we used scanning schemes: for the ends of the bones the scanning operation was made at distances of 1 mm and for the medial areas at distances of 3 mm. In Figure 1 we present main images of the medial ulna, which show the changes in the shape of the bone. The obtained images were re-drawn in AutoCad over the real tomographies and the drawings were imported in SolidWorks (a parametrical CAD software), section by section, in parallel planes. The sections of the ulnar bone are presented in figure 1. Solidworks allows the yield of a solid by "unifying" the sections drawn in parallel planes. The command which makes these sections appear as a solid was Loft Shape. For the next step we used the ANSYS program for the discretisation by means of the finite elements method of the spatial structure of the ulna and radius bones (Tarnita et al, 2006).

2.1. Virtual simulations of bones stresses

In order to identify the target shape of modular implants, different studies were made (Tarnija et al., 2007). The studies identify the stresses and deformations maps developed by radius and ulna for different external statical solicitations. The geometrical models were subjected to different solicitations to allow the study of the most solicited parts of the bone which will be the likely fracture sites. In figure 2 we exemplified two simulation results, for the forearm bones: radius bone and ulna bone, important for identifying stresses distribution and fracture areas.

The clinical observations and the laboratory tests confirm the results obtained by virtual simulation.

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3. MODULAR ADAPTIVE IMPLANT

The design idea of modular adaptive implants results from the following observations:

--doctors have limited degrees of freedom in selecting the proper dimensional apparatus for bone fractures;

--the current mechanical devices used in orthopedics lose some of their mechanical characteristics after some time (especially elasticity, which should ensure a constant tension that is mandatory for the correct anatomical healing of the fractured bones);

--the process of fracture healing has a particular dynamic, which imposes the necessity of particular progressive tension or discharge to improve the recovery time, depending on the normal structure and function of the bone;

--to improve the healing process, the fractured parts have to be in permanent contact in order to ensure the proper conditions to develop bone calluses. The actual or external fixator has to be manually adjusted with respect to the main axis of the bone. Unfortunately, the degrees of freedom of current devices are limited to 3 or 4 screws;

--a minimally invasive surgery ensures protection from blood edema and improves bone recovery and vascularization of the region.

The solution to these problems is the Modular Adaptive Implant--MAI. The proper shape of MAI is related to the microscopic structure of the bone (Bizdoaca et al., 2008). As one can observe, comparing the structure of a healthy bone with that of an osteoporotic bone (figure 3), the internal architecture of the healthy bone has a regular modular structure.

A modular net, identical in structure with the bone and locally configurable in terms of tension and release, is best design solution in terms of biocompatibility (Bizdoaca et al., 2008). The identification of the mechanical solicitation of the particular bone structure leads to the concept of the practical implementation of a feasible device able to undertake the functionality of normal bones. This device will partially discharge the tensions in the fractured bones (the fractured parts still need to be tensioned to allow the formation of the callus) improving the recovery time and the healing conditions. The proposed intelligent device has a network structure, with modules made out of Nitinol, especially designed in order to ensure a rapid connection and/or extraction of one or more MAI modules. The binding of the SMA modules ensures the same function as other immobilization devices, but also respects additional conditions concerning variable tension and its discharge. Moreover, these modules allow little movement in the alignment of the fractured parts, reducing the risks of wrong orientation or additional only bones callus. We suggest the design shown in Figure 4 for the unitary SMA module structure

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The design of the SMA module ensures not only the stability of the super-elastic network and constant force requirements, but also a rapid coupling/decoupling procedure.

Doctors can use SMA modules with different internal reaction tension, but all the modules will have same shape and dimension (Figure 4). The connection with affected bones and the support for this net are similar to those of a classic external fixator, but allowing for the advantages of minimal invasive techniques. The new device leads to a simple post-operatory training program of the patient.

4. CONCLUSIONS

The study of this technique offers a feasible direction. In the future, we want to realize a different type of SMA module and to experiment with them on real bones. At the same time, the studies will be developed in the direction of numerical simulation of the complex ensemble made up of the bone and the MAI network for different functional regimes, for different weight, temperature and physical-chemical condition and especially for different types of bone fractures. A very promising direction is the design and implementation of an independent and permanent functional adaptive MAI network, which can ensure a very rapid reintegration of the patient in the social life with minimal costs of the treatment.

Acknowledgement

Research activity supported by Ministry of Education, Research and Youth, programs PNCDI2 Id_92/2007 and CEEX 259/2006

5. REFERENCES

Bizdoaca, N.; Tarnita, D.; Tarnita, D. N.; Popa, D. & Bizdoaca, E. (2008). Shape Memory Alloy based Modular Adaptive Ortophedic Impants. Proceedings of The International Conference on biomedical electronics and biomedical informatics, BEBI'08, pp 123-128, Rhodos, August, 2008.

Bizdoaca, N.; Tarnita, D.; Tarnita, D. N.& Bizdoaca, E. (2008). Application of smart materials-bionics modular adaptive implant, Proceedings of The 11th International Conference on Climbing and Walking Robots, CLAWAR'08, pp.190-197, Coimbra, Portugal, September, 2008.

Duering, T.; Pelton, A.R. & Stockel, D. (1999). An overview of nitinol medical applications. Material Science and Engineering, A273-275, pp. 149-160, ISSN 0921-5093.

Friend, C.M., & Morgan, N.B. (1999). Medical Applications for Shape Memory Alloys. Professional Engineering Publishing Ltd., UK, pp.1-16.

Pelton, A.R.; Stockel, D. & Duerig, T.W. (2000). Medical uses of nitinol, Materials Science Forum, 327-328, pp. 63-70, ISSN 0255-5476.

Tarnita, D., Popa, D., Tarnita, D. N., Grecu, D. (2006). CAD method for 3D model of the tibia bone and study of torsion and compression stress using the finite element method, Romanian Journal of Morphology and embriology, Vol. 47. No.2, (october, 2006), pp.181-186, ISSN 1220-0522.

Tarnita, D.; Negru, M.; Tarnita D.N. & Grecu, D. (2007). Stresses and Displacements of Radius Bone Using Finite Element Method. Proceedings of The 8th EUROPEAN Congress on Ortopedics, EFORT, pp.354, Firenzze, Italy, May, 2007.