The paper presents a protocol in customization of spinal intervertebral discs. The main stages of the protocol are: medical image measurements; conceptual design; theoretical validation of the geometry; rapid manufacturing and mechanical testing of the artificial disc. The protocol was developed for the L5-S1 human intervertebral disc but can be adapted for any of the spinal discs. An artificial disc developed in this way will fit with the anatomical requirements of the host.
1. INTRODUCTION
Degenerative disc disease represents one of the multiple causes of the low back pain (Eijkelkamp, 2002). In treating these diseases, non-surgical treatments are initially prescribed (Metfessel et al., 2005). When surgical intervention is necessary, spinal fusion or Artificial Intervertebral Disc (AID) implantation are performed. The long-term benefits of artificial disc implantation are still unknown, thus, total disc replacement with AID is considered investigational (Regence, 2009).
The development of AID is based on: anatomical knowledge, biomechanical studies, appropriate geometrical models, biomaterials and manufacturing technologies.
The surface of annulus fibrosus ([A.sub.i]) was calculated by subtracting two surfaces: one of the ellipses which circumscribe the annulus and the other one which circumscribe the nucleus (equation 1). The surface of nucleus pulposus ([A.sub.n]) was calculated using the approximate equation 2, where p = 1.6075 represents the constant of Knud Thomsen (Michon, 2008). The side surface of the analytical calculated ID was calculated using equation 3.
Comparing the measurements with the analytical calculations the differences are less than 3 [mm.sup.2] for the annulus and less than 1 [mm.sup.2] in the case of nucleus. This reduced (0.08% and 0.1% respectively) errors prove the validity of the analytical calculus.
The calculated side area, together with the values of the forces acting at this level (invasively measured by Nachemson (Panjabi & White, 2000) can be used in order to determinate the surface pressure acting on the disc.
4. CONCLUSIONS
In order to manufacture an AID, the geometry and the surface area of the natural ID must be well known. In this study, the upper surface of the natural ID was analytical approximated. The conceptual design, followed by the surface measurements confirms the validity of the approximation. In order to validate the conceptual design, the virtual 3D model was prototyped, and an AID physical model was obtained. The AID physical model is not manufactured by biocompatible materials.
The protocol can be adapted to any intervertebral disc, taking into consideration the load changes and disc size. An improved model will consist of two rigid plates and an elastomeric core interposed between them. Thus, based on the proposed protocol, other customized ID will be designed and manufactured using biocompatible materials.
5. REFERENCES
Bradford, D.S.; Berven, S.H. & Hu, S. (2009). Intervertebral Disc Replacement. A Role in the Management of Chronic Low pain Caused by Degenerative Disc Disease, Available from: http://www.spineuniverse.com/ Accessed on: 2009-05-03
Cooper, K.G. (2001). Rapid Prototyping Technology: Selection and Application, CRC Press, ISBN 978-0824702618, USA
Michon, G.P. (2008). Final Answers, Available from: http://home.att.net/~numericana/answer/ellipsoid.html Accessed on: 2009-05-03
Panjabi, M.M. & White, A.A. (2000). Biomechanics in the Musculoskeletal System, Churchill Livingstone, ISBN 0-443-06585-3, Philadelphia
Schroeder, Y.; Wilson, W.; Huyghe, J. & Baaijens, F.P.T. (2006). Osmoviscoelastic finite element model of the intervertebral disc. European Spine Journal, Vol. 15, Suppl. 3, August 2006, pp. 361-371, ISSN 1432-0932
*** (2009)
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