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

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

1. INTRODUCTION

Applications of Shape Memory Alloys to the biomedical field have been successful because of their advantages over conventional implantable alloys, enhancing both the possibility and the execution of less invasive surgeries.

Several characteristics make NiTi extremely attractive for use in medical devices: the material has good biocompatibility, the devices can be pseudo-elastically or thermally deployed, and the material can apply a constant transformation stress over a wide range of shapes. Biocompatibility studies have shown NiTi to be a safe implant material, which is at least equally good as stainless steel or titanium alloys. Compared to stainless steel, Nitinol has the great advantage of the compatibility with the very modern radiological technique of MRI which do not pose radiation risks. (Friend & Morgan, 1999; Funakubo, 1987; Ryhanen & Kallioinen, 1999; Shabalovskaya, 1995). NiTi has been approved for use in orthodontic dental archwires, endovascular stents, vena cava filters, diagnostic and therapeutic catheters, laparoscopic instruments, intracranial aneurisms clips, bone staples, and various orthopedic implants.

To use an internal implant as a bone plate to repair a fracture, a surgeon has: to select an appropriate plate, to reduce the discontinuity, to fasten the plate to bone portions disposed on opposite sides of the fracture using suitable fasteners, (screws and/or wires), so the bone portions are fixed in position

2. MODULAR ADAPTIVE IMPLANT

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

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

--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.

--minimally invasive surgery leads to a shortening of the period necessary to recover, ensures protection and improves bone recovery and also lessens the risk of infection.

Osteosynthesis plates are attached to the bone on both sides of the fracture with bone screws, particularly in the extremities during surgery bone fractures. Healing proceeds faster if the fracture faces are under a uniform compressive stress. The implants that came in contact with the traumatized bony structures which have been analyzed in this study had a modular organization, using intelligent materials with shape memory as coupling structures between the support elements.

The proposed intelligent device is a modular bone plate with modules made of Titan and staples made of Nitinol. The Nitinol elements ensure the flexibility and elasticity of the modular structure assembled from a number of modules of the right shape and dimensions, while the identical structure of the modules ensures the attachment of the implant onto the supporting bone fragments. The attachment options differ according to the state of the fractured bone, the size of the fracture, the age and body size of the patient. The shape memory staples, in their opened shape, are placed in the special places build into the modules. Through heating, this staple tends to close, compressing the modules and determining the translation of the modules and the separated parts of bone are compressed. The fixation of the bone fracture is then achieved and an axial compression takes place. This means that the pseudo-elastic properties of the clamp allow the force on the bone surfaces in contact. The force generated by this process accelerates healing and reduces the time of bone recovery. The modules allow little movement in the alignment of the fractured parts, reducing the risks of wrong orientation or additional bones callus. After a particular stage of healing period is passed, using implant modularity, the load is gradually transferred to bone, ensuring in this manner a gradually recover of bone function. Upon cooling after fracture healing, the staples return to first shape, so that they can be easily extracted. The adaptability is related to medical possibility of doctor to made the implant to correspond to patient specifically anatomy.

3. NUMERICAL SIMULATION

To determine the tensions that the ensemble is subject to, we used special software for numerical simulations. We present an internal implant, in the case where the implant is used for consolidating a transversal dyaphiseal fracture of the humerus bone, which is a long bone of the arm. Using CT numerical bone models, the mechanical simulation of the humerus osteosynthesis is presented using Finite Element Method. For identifying the optimal design, different implants were developed and experimented. We used SolidWorks for implants designing and for the 3D virtual model of the humerus and ANSYS software for discretisation, simulation and analysis.

For the simulation of the nitinol elements behavior and for the study of their effects, we have considered the fracture placed in the dyaphiseal area. The small plates were placed both ways of the longitudinal axis of the bone, proximate under its head, following the curve and dip of the bone surface geometry. There were simulated the screws for fixing the small plates and the bone. The plates are not fixed in a initially position, they can move 2 mm. Materials: Cortical bone: isotropic, homogenous: E=17000 MPa, Poisson's Coef.=0,3; Spongious bone: isotropic, homogenous E=1800 MPa, Poisson's Coef.=0,2;Plates: isotropic, homogenous-(Titanium); Fixing screws-isotropic, homogenous-(Titanium); Holding elements: Nitinol- simulated in ANSYS using the model "shape memory alloy". The bone segments have been embedded in the extreme end. The virtual 3D model of the ensemble humerus-implant is presented in Figure 1. The numerical simulation follows 3 steps

Step 1. The upper and lower plates are fixed with screws on the bone. It simulates the mounting of head off for holding elements on the fixed plates on the bone, the holding elements having the other head already mounted in the middle plates. The temperature of all the elements and of the holding elements is 23[degrees]C. In Figure 2, the resultant displacements in plate modules are presented.

Step 2. The ends of the nitinol elements are considered mounted in plates, considering the pretension of step 1, eliminating imposed movements, and realizing the state of tension for mounting the implant. The temperature is 23[degrees]C. The Von Misses stresses in staples are presented in figure 3.

Step 3--Starting from the final state of tension obtained in step 2 we are simulating the increase of temperature for holding elements from room temperature to body temperature 36.5[degrees]C. In vitro simulation of the human humerus osteosynthesis process can be realized using a Rapid Prototyping 3D ZCorp 310 Printer system which helps us to obtain the prototyped ensemble human humerus bone-modular plate (Figure 4).

The results were validated by in vitro experiments using human cadaver bones (Figure 5).

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4. CONCLUSION

We have studied modular plates based on shape memory alloys which are implants in direct contact with traumatized bones. The 3D human humerus bone and the 3D modular plates were obtained using the CAD software SolidWorks. The process of human humerus osteosynthesis using modular adaptive plates based on shape memory alloys is numerical simulated with ANSYS software packages, following 3 steps. The results were validated by in vitro experiments using human cadaver bones. Our future work will be focused on the development, and optimisation of the modular implants.

5. ACKNOWLEDGEMENTS

This research activity was supported by Ministry of Education, Research and Innovations, Grant Ideas 92-PNCDI 2.

6. REFERENCES

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Friend, C.M. & Morgan, N.B. (1999). Medical applications for Shape Memory Alloys, Professional Engineering Publishing Ltd., UK, 1999, p.1

Funakubo, H. (1987). Shape Memory Alloys, Gordon & Bleach, New York, NY, USA, 1987

Ryhanen, M. & Kallioinen, J. (1999). Medical applications for Shape Memory Alloys, Professional Engineering Publishing Ltd., UK, 1999, p.53.

Shabalovskaya, S.A. (1995). Biological aspects of TiNi alloys surfaces. Journal de Physique IV, 5: 1199-1204, 1995

Tarnita, D., Tarnita, D.N.; s.a. (2008). Modular orthopedic implants for forearm bones based on shape memory alloys, In ISI Proceedings of The 19th Internat.DAAAM Symposium, 2008