Study of the influence of degenerative intervertebral disc changes on the deformation
behavior of the cervical spine segment in flexion
Tatyana V. Kolmakova
Citation: AIP Conference Proceedings 1783, 020095 (2016); doi: 10.1063/1.4966388 View online: http://dx.doi.org/10.1063/1.4966388
View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1783?ver=pdfcov Published by the AIP Publishing
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Study of the Influence of Degenerative Intervertebral Disc
Changes on the Deformation Behavior of the Cervical
Spine Segment in Flexion
Tatyana V. Kolmakova
Institute of Strength Physics and Materials Science SB RAS, Tomsk, 634055 Russia National Research Tomsk State University, Tomsk, 634050 Russia
Abstract. The paper describes the model of the cervical spine segment (C3-C4) and the calculation results of the
deformation behavior of the segment under degenerative changes of the intervertebral disc. The segment model was built based on the experimental literature data taking into account the presence of the cortical and cancellous bone tissue of vertebral bodies. The calculation results show that degenerative changes of the intervertebral disc cause the immobility of the C3 vertebra at flexion.
INTRODUCTION
Specialists whose professional activity is related to the development of methods for correction of human organs and structures and the creation of implants face the need to forecast the behavior of human structures under various external factors. Computer simulation allows achieving a deeper understanding of regularities of the human body functioning in health and disease, thus enhancing the development of recommendations on creation of implants replacing the whole organ or its part and effective methods for correction of the system in general.
The human spine is a complex structure that provides both mobility and stability and also protects the spinal cord [1]. The spine is divided into cervical, thoracic, lumbar and sacral regions [1]. The cervical spine moves most of all.
An intervertebral disc (IVD) separates each vertebra, except in the upper cervical spine (C1 and C2) as well as in the sacrum and coccyx, where the vertebrae are fused together [2]. The discs allow a complex movement between vertebrae without mechanical disadvantages of the opposing vertebra surfaces. They resist the spinal compression while permitting limited bending, twisting, and sliding between vertebral bodies [1]. Another function of the disc is to distribute loads applied to the spine evenly along vertebral bodies. Each disc forms a cartilaginous joint to stabilise the spine and to maintain its alignment by anchoring adjacent vertebral bodies to each other [1].
Degenerative changes of intervertebral discs are the major cause of pain in the spine and neck in those of the middle and older age. The intact IVD contains a significant amount of water. A reduction in hydrophilia of the disc reduces its height and changes its mechanical properties [3].
Alterations in intervertebral discs (IVDs) influence both themselves and other spinal structures, leading to diseases such as bulging disc, discogenic pain, and spinal stenosis [3].
The most expressed morphological changes occur in one or several IVDs. If morphological changes occur, it is required to exchange an IVD by an implant. The lack of information about changes in the stress-strain behavior of the spine in the case of its structural component degradation makes it impossible to manage it adequately and to restore degrading elements by the replacement with implants. An adequate choice of implants plays a crucial role since the improper choice of mechanical properties of implants can result in the deterioration of bone tissue and the spine in general.
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(а) (b) FIGURE 1. Geometric model of the cervical spine segment: (a) lateral view, (b) sectional lateral viewMODEL OF THE CERVICAL SPINE SEGMENT
A geometric vertebra model was built based on the literature data about experimentally obtained dimensions [4]. The simulation algorithm for the geometric vertebra model was developed and implemented in the ANSYS system with the use of the APDL language. This algorithm allows an automatic rebuilding of the model when input parameters are changed.
Figure 1 shows the geometric model of the cervical spine segment. The geometrical model includes the vertebrae С3 (1) and С4 (2), IVD (3), facet joints (4), interspinous ligament (5), vertebral arches (6), spinous processes (7), transverse processes (8), and articular processes of vertebrae (9).
The presence of the cortical and cancellous tissue in the vertebrae was taken into account. In Fig. 1b, the cancellous tissue is marked by 10, a thin layer of the cortical tissue [2] covers vertebral bodies. It is considered that vertebral arches and processes of the vertebrae fully consist of compact bone tissue. The Z axis of the coordinate system is located along the segment axis. The X axis is directed in the anteroposterior direction of the spinal segment. Materials of the cortical and cancellous tissues of the vertebral bodies, materials of the intervertebral disc, facet joints, interspinous ligament, vertebral arches, and processes of vertebrae are considered as isotropic linear elastic materials. Mechanical properties of the cervical spine segment are given in Table 1 with the relevant reference to the literature.
Degenerative changes of the IVD were simulated through a reduction of the disc height from 6 to 4.5 mm and an increase of Young’s modulus according to the previous research data [8–10], which in reality results from the water content reduction in the disc [3, 11].
STUDY OF THE DEFORMATION BEHAVIOR OF THE SEGMENT UNDER
DEGENERATIVE CHANGES IN THE INTERVERTEBRAL DISC
The deformation behavior of the model of the cervical spine segment was analyzed. The lower surface of the vertebral body of C4 was rigidly fixed. The upper surface of the C3 vertebra was loaded by the pressure 1000 N. The bending moment 7.5 Nmm [12] was applied to the central point of the upper surface of the C3 vertebral body in the negative direction of the X axis in flexion of the spine segment.
TABLE 1. Mechanical properties of structural elements of the cervical spine segment
Structural element Young’s modulus, MPa Poisson’s ratio References
Cortical bone 10000 0.3 [5]
Cancellous bone 100 0.2 [6]
Facet joints 1.5 0.3 [7]
Interspinous ligament 3.5 0.3 [8]
(а) (b)
(c) (d) FIGURE 2. Deformed shapes of the C3–C4 cervical spine: (a) h = 4.3 mm, E = 2.5 MPa, (b) h = 6 mm, E = 2.5 MPa,
(c) h = 4.3 mm, E = 98 MPa, (d) h = 6 mm, E = 98 MPa
Figure 2 shows deformed shapes of the segment with different heights (h) and Young’s modulus (E) of the intervertebral disc. The biggest bulge of the IVD is observed when its height is reduced. The disc budge is reduced with an increase of Young’s modulus.
In flexion, the C3 vertebra moves forward and down reducing the intervertebral space. Let us consider the displacement of the front point of the C3 vertebra (Fig. 3a) when loaded. Figures 3b–3d show the dependence of the displacement of the front point of the vertebra on Young's modulus of the IVD of various heights.
The results show that in flexion with the bigger degree the displacement is implemented towards the Z and X axes. The reduction in the disc height from 6 to 4 mm (1.5-fold) with its Young’s modulus 2.5 MPa is accompanied by an approximate 2-fold reduction of the displacement in all directions. The increase in Young’s modulus of the IVD from 2.5 to 50 MPa (20-fold), regardless of the IVD height, reduces the displacement of the front point of the C3 vertebra along the X axis by 20 times; along the Y axis, by 10 times; along the Z axis, by 15 times. A further increase in the modulus above 50 MPa results in the absence of the C3 vertebra displacement, regardless of the IVD height.
CONCLUSION
The conducted research allows making the following conclusions: (1) Degenerative changes of the IVD—a reduction in its height and an increase in Young’s modulus—result in the reduction of the C3 vertebra motility in flexion. (2) The displacement of the front point of the C3 vertebra decreases along the X axis by 20 times; along the
Y axis, by 10 times; along the Z axis, by 15 times, regardless of the IVD height with an increase in Young’s modulus
of the IVD from 2.5 to 50 MPa (20-fold). (3) The absence of C3 vertebra displacements regardless of the IVD height is observed with increasing modulus of elasticity above 50 MPa.
(а) (b)
(c) (d) FIGURE 3. (a) Image of the front point location on the C3 vertebral body, (b–d) dependences of the displacement
of the front point of the C3 vertebra on Young’s modulus of the IVD of various heights
ACKNOWLEDGMENTS
This research has been performed within the RFMEFI60714X0069 Project and at the financial support of the Ministry of Education and Science of the Russian Federation.
REFERENCES
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