chapter  13
18 Pages

Human Spinal Intervertebral Disc: Optimal Structural Design Characteristics

WithDhanjoo N. Ghista, Sridhar Idapalapati, Ramakrishna Kotlanka

CONTENTS 13.1 Scope ......................................................................................................... 381 13.2 Introduction: Concept of the Intervertebral Disc

as an Optimal Structure......................................................................... 383 13.3 Disc Model Analysis: Disc Stresses, Displacements,

and Deformed Geometry....................................................................... 384 13.4 Stress Analysis of the Healthy Disc under

Compression Loading (F): Determining Disc Deformations and Stresses in Terms of F .................................................................... 388

13.5 Mechanism and Computation of Disc Deformation......................... 390 13.6 Disc Herniation, Back Pain, and Nucleotomy ................................... 391 13.7 Nucleotomized Disc Model Analysis: Geometry, Stresses,

and Displacements ................................................................................. 392 13.7.1 Stress Analysis for a Vertical Loading

on the Nucleotomized Disc..................................................... 393 13.7.2 Determination of Disc Deformation

in Nucleotomized Disc............................................................. 394 13.8 Conclusion: For the IVD to Retain Its Optimal

Structural Feature ................................................................................... 394 References ........................................................................................................... 395

What are the structural features of the spinal intervertebral disc that make it an intrinsically optimal structure? This is because it effectively contains its lateral and axial deformations, while providing the necessary flexibility to the spine. How this is achieved forms the basis of this chapter. The intervertebral disc (IVD), as illustrated in Figure 13.1, consists of the annulus

fibrosus (AF) enclosing the nucleus pulposus (NP). When the IVD is loaded in axial compression, the NP gets pressurized and transmits radial stress to the AF, which in turn gets stressed. However, the annulus fibrosus’ is a stress-stiffening solid resembling that of a hyper-elastic material. This increase in elastic modulus under loading in turn prevents the annulus fibrosus from deforming in proportion to the applied loading. In other words, as the IVD gets loaded, its deformation does not increase in the same proportion as the loading to which it is subjected. This is what makes the IVD an intrinsically optimal structure. This chapter analytically models how this is made possible in an intact

IVD under uniaxial compression. It also demonstrates that if a ruptured disc is denucleated surgically as a treatment for back pain (to prevent irritation of the spinal nerve structures by the nucleus pulposus, as it is squeezed out through the ruptured disc under compression loading), then the absence of nucleus pulposus no longer stresses the annulus fibrosus as effectively as in the case of an intact IVD. Hence, the denucleated disc in fact deforms more than the intact disc

under compression loading, and hence loses its intrinsic capacity to contain its deformation under increasing loading. This result serves as a contraindication for nucleotomy, and emphasizes that the nucleus pulposus needs to be substituted by a biocompatible gel-filled balloon, to simulate the beneficial effects of the NP. This chapter (along with the figures) is based on our paper Ghista et al.

[1], published in the International Journal of Design and Nature.*