Implementation of a CAM mechanism in a new human ankle prosthesis structure.

Copilusi, Cristian ; Dumitru, Nicolae ; Rusu, Ligia 等

1. INTRODUCTION

In the case of shank prostheses, this are fabricated from materials which posses a capacity to memorize shapes and to store energy developed in different activities.

In figure 1-A we present a shank prosthesis Venture type used in ankle disarticulations, fabricated by the College Park Industries (South-west Orthotic Centre, 2006). This posses the following characteristics: multi-axial rotations, in order to adapt at any type of terrains; energy store capacity in order to minimize the patient effort; adaptable mechanism in the sight of movements adjustment, for any amputees; fixation ability through a cup or a implant.

In figure 1-B we present the Elite Foot shank prosthesis, fabricated by Blatchford and Sons Ltd England (Blatchford prosthetics institution, 2008). This type of prosthesis posses the following characteristics: no mechanical systems; permits the ankle valgus or varus motion, due to specific form of the prosthetic foot; special design in order to take over the reaction forces from the ground contact in the concentrated points. This is an advantage because the components are individually stressed, and are fabricated from carbon fibber which posses a shape memory capacity.

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2. EXPERIMENTAL KINEMATIC HUMAN LOWER LIMB ANALISYS

The experimental research motivation was given by the impossibility to obtain these motion laws on analytical way. And so in order to obtain these motion laws we use an image acquisition system called SIMI Motion, (SIMI Reality Motion Systems GmbH, 2007) from Faculty of Physical Education and Sport, University of Craiova. The kinematic parameters were obtained through video capture and image analysis.

In order to obtain the motion laws developed at the ankle joint we analyze a human subject without locomotion disabilities (male, age 26, 1, 73 height, 65 kg weight, femur length = 401mm; tibia length = 322 mm; foot length = 210 mm).

The walking process was performed in a 2D space (figure 2). For the ankle joint we obtain: angular displacements, velocities and accelerations. As an example we present the angular displacements developed at the ankle joint's level in figure 4. Based on diagram from figure 3, we conclude that the prosthesis must respect the angular amplitude about 45[degrees] ... 55[degrees] (Buzescu & Scurtu, 1999). In the presented case this angular amplitude for the walking activity was 48 degrees.

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3. DYNAMIC MODEL ELABORATION EQUIVALENT TO HUMAN LOWER LIMB

The mathematic model for the human lower limb inverse dynamic analysis (figure 4) was elaborated by taking in account the experimental kinematic analysis (Dumitru & Nanu, 2008). With the aid of an algorithm performed in MAPLE program, we compute the connection forces from the kinematic joints of the mathematic model (Copilusi, 2009).

The relations for computing these connection forces, by considering the Lagrange multipliers are:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

The connection force component variation for the ankle joint is presented in figure 5. With this we can develop a mechanical system used in a new prosthesis design and it help us to create virtual simulations in order to validate the mechanical system proposed to use in the new prosthesis structure.

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4. THE NEW PROSTEHSIS SYSTEM DESIGN

Regarding the anterior data, the best mechanism which can be implemented on the prosthesis structure was a cam mechanism. The shock absorber was integrated in the prosthesis resistence structure and has the possibility to make some axial adjustments in order to establish the prosthesis alignement. The new ankle prosthesis design is shown in figure 6. After simulating the virtual model and validating through calculus the cam mechanism, this prosthesis was executed and adapted on an amputee (Dumitru & Margine, 2000). In figure 7 we present an aspect from the new prosthesis experimental tests.

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5. CONCLUSIONS

The cam mechanism represents the novelty element of this prosthesis. This mechanism was perfectly adapted in the prosthesis structure and respects the imposed conditions. The amplitude developed by the new prosthesis' mechanical system, which replaces the ankle joint's functions (dorsal/plantar flexion in walking activity), was 42 degrees (figure 8). This value is appropriate with the one of a human subject without locomotion disabilities (about 45[degrees] ... 55[degrees]). This confirms the prostheses improvement used in human lower limb amputations for above the knee disarticulations. On the future we want to perform other experimental tests, (stairs climbing, dancing, running, etc.) in order to validate this type of prosthesis.

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6. REFERENCES

Buzescu, A.; Scurtu L.; (1999). Anatomy and biomechanics. A.N.E.F.S. printing house, ISBN 973-8043-139-9. Bucharest

Copilusi, C.; (2009). Researches regarding some mechanical systems applicable in medicine. PhD. Thesis, Faculty of Mechanics, Craiova

Dumitru, N.; Nanu, G.; Vintila, D.; (2008). Mechanisms and mechanical transmissions. Modern and classical design techniques, didactic printing house, ISBN 978-973-312332-3, Bucharest

Dumitru, N.; Margine, A.; (2000). Modelling bases in mechanical engineering. Universitaria printing house, ISBN 973-8043-68-7. Craiova

McGeer, T.; (1990). Passive dynamic walking. International Journal of Robotics Research, vol. 9, no. 2, pp. 62-82

*** (2007) http://www.simi.com. SIMI Motion, SIMI Reality Motion Systems GmbH. Accessed on: 2007-09-18

*** (2006) http://www.southwest-ortho.com. Southwest orthotic centre. Accessed on: 2006-08-13

*** (2008) http://www.blatchford.co.uk. Blatchford prosthetics institution. Accessed on: 2008-10-25