Discussion

Two-dimensional internal displacement and strain of nondegenerate and

degenerate discs were calculated non-invasively using magnetic resonance images of the samples compressively loaded in flexion, neutral and extension. The axial compressive load of 1000 N (~1.2x body weight) corresponds to moderate physiological

stresses.(Wilke, Neef et al. 1999) T1ρ relaxation times were used as a quantitative scale of degeneration, since T1ρ relaxation times have a strong correlation to

glycosaminoglycan content, which decreases with degeneration (Figure 4-

mm) is similar to observations by Nachemson et al (0.5 - 0.8 mm); however, the values measured in this study were not dependent on degeneration, which is likely due to the initial disc height decreasing with degeneration.(Nachemson, Schultz et al. 1979) While there is little data for the change in radial bulge with degeneration, the outer annulus radial displacement measured in this study is comparable to previous experimental studies for compression and bending by Seroussi et al and Shah et al for flexion and compression: 0.5 to 1.2 mm.(Shah, Hampson et al. 1978; Seroussi, Krag et al. 1989; Heuer, Schmidt et al. 2008) The direction and magnitude of nucleus pulposus movement measured in this study is similar to observations made by Tsantrizos et al (0.85

mm).(Tsantrizos, Ito et al. 2005)

Degeneration significantly altered the radial and axial strains, especially in

bending. The average radial strain in the anterior annulus fibrosus of nondegenerate discs was compressive and became tensile with degeneration (Table 4-1). In addition, the axial strain was 2X more compressive with degeneration in many disc regions (Table 4-1), which is likely due to the degenerated nucleus being unable to effectively transfer loads and possibly buckling the annulus fibrosus tissue. The annulus fibrosus is a nonlinear, anisotropic, heterogeneous tissue with a Young’s modulus in the radial direction that is lower in tension (0.14 - 0.35 MPa) than compression (0.56 - 1.10 MPa).(Iatridis, Setton et al. 1998; Guerin and Elliott 2006) Therefore, these increases in tensile radial strain with degeneration may cause circumferential tears, and the increases in the axial compressive strain may cause radial tears.

The static compression data from this study suggests that bending from flexion to extension causes a significant translational shift in the nucleus from the posterior to the anterior, altering the radial and axial strains in the annulus. The side of bending (i.e.

posterior annulus in flexion or the anterior annulus in extension) had higher tensile radial strains and compressive axial strains, while the opposite side had lower magnitude of radial and axial strains (Figure 4-15 & 4-16). These results suggest that bending loads applied to the disc dramatically alter the internal strain environment. For example, bending of nondegenerate discs caused tensile axial strains in the annulus tissue opposite to the side of bending (Table 4-3 – extension in the anterior annulus). The alteration in the internal strain behavior could only occur through the pressurized nucleus of

nondegenerate discs, because the internal strains of degenerate discs did not follow the same behavior. Therefore, a pressurized nucleus pulposus is critical in disc load distribution.

The posterior-lateral annulus fibrosus is the most common site for disc herniation; therefore, understanding the degenerative changes is crucial to understanding what may cause failure of the tissue (e.g. radial tears). In the neutral position, the magnitude of the posterior annulus radial and axial strains were up to 4X greater than the anterior and lateral annulus fibrosus. In flexion, the nucleus pulposus moved 0.70 mm towards the posterior annulus fibrosus, and Wilke et al observed a 30% increase in the internal pressure of the disc during bending.(Wilke, Neef et al. 2001) The mechanical properties of excised tissue from the posterior annulus have been observed to be weaker than the anterior annulus along the circumferential direction, but not in the radial

direction.(Acaroglu, Iatridis et al. 1995; Fujita, Duncan et al. 1997) The structural differences between the anterior and posterior annulus fibrosus include a thinner overall thickness and more disorganized lamellae than the anterior annulus.(Cassidy, Hiltner et al. 1989) The increase in internal pressure, radial and axial strain and the translational shift in the nucleus pulposus coupled with the structural differences supports the idea that

the posterior annulus fibrosus would be more likely to have a full thickness radial tear due to an accumulation of microfractures; causing herniation of the nuclear material. Moreover, the increased tensile radial strain and compressive axial strain may lead to an increase in the structural bulge (i.e. bulging under no load) observed in more degenerate discs.

The mid-coronal and mid-sagittal planes of the disc were evaluated in the neutral position. The radial displacement of the inner and outer annulus fibrosus did not vary from the mid-sagittal and mid-coronal planes. However, this does not suggest that the disc has a uniform outward radial displacement, since the radial displacement of the anterior annulus was greater than the posterior annulus (Figure 4-23).(Seroussi, Krag et al. 1989) No differences were observed in the internal strains of the nucleus pulposus for the mid-coronal and mid-sagittal sections, which supports the idea that the nucleus pulposus is homogeneous in composition and mechanics.

The current study is subject to some limitations. Since a two-dimensional imaging sequence was used, the internal strains were analyzed only at the mid-sagittal slice and mid-coronal plane. Bending was applied using a point-load technique through the wedge instead of bending about the axis of rotation of the disc. It is possible for some anterior-posterior tissue movement along the mid-coronal plane due to the wedge

technique; therefore, the mid-coronal plane was not evaluated under bending to limit the amount of tissue movement through the imaging slice. Three-dimensional imaging of the disc would allow for a complete analysis of the internal disc mechanics under more complex loading conditions, such as shear and torsion. Furthermore, the bending was applied during the preload; therefore, the reference image already included the effect of the 5o wedge. While this technique does not allow for analysis of bending from a 0o

neutral position, it is comparable to the in vivo situation when bending to lift a heavy object where the person is already bent and applies additional loading to the spine.

This study used a non-invasive technique to evaluate the degenerative effect of the internal mechanics of the disc under physiological levels of compression and bending. The change in radial strain of the annulus fibrosus from compression to tension and increase in the axial strain magnitude suggest that the mechanisms of load distribution through the disc subcomponents are altered with degeneration, likely due to the

depressurization of the nucleus pulposus shifting more of the applied load directly to the annulus fibrosus. Moreover, the posterior annulus experiences an increase in tensile radial strain and higher compressive axial strain in extension, which may be a result of, or a cause for, radial tears and circumferential delamination.(Vernon-Roberts, Moore et al. 2007) In conclusion, this study provides insights into internal disc strains and changes with degeneration and loading position, provides data useful for validation of finite element models, and provides a technique and baseline data for evaluating surgical treatment, such as discectomy or implants. In the next two chapters, this technique will be used to further analyze the changes in internal disc strains following annulotomy and discectomy.

Chapter 5:Posterior-Lateral Annulotomy Does Not Affect the