Effect of Nucleus Pulposus Denaturation and Anulus Fibrosus Injury on the Viscoelastic Properties of Intervertebral Disc
Date Issued
2012
Date
2012
Author(s)
Wang, Mon-Chien
Abstract
Objective: To find the effect of nucleus pulposus denaturation and anulus fibrosus fatigue loading on the disc viscoelastic properties.
Background: Intervertebral disc is a viscoelastic organ composed of collagen fibers, proteoglycan and water. The major mechanical functions of discs are to resist external loading and absorb shock energy. Resistance to external loading is majorly provided by collagen fiber network and water content. Shock energy absorption is resulted from the friction force between water flow and solid tissue. The bonds between water and proteoglycan contribute to this friction force. Declination of disc function makes discs vulnerable to external stress, which in turn accelerates disc degeneration. The aging and excessive fatigue loading are recognized as major triggers of disc degeneration. Two major methods, i.e. the enzymatic denaturation and mechanical fatigue loading, are used to create disc degeneration models. The hydrolysis enzymes, such as matrix metalloproteinases (MMPs), are activated to cleave collagen fibers and proteoglycan in the aging process. Highly repeated activities or staying in vibrating environment accumulate mechanical micro-injuries within discs. Disc functions can be quantitatively represented by disc viscoelastic properties. Understanding the alteration of disc viscoelastic properties of injured discs improves the knowledge on degeneration etiology.
Methods and Materials: Thirty-two fresh porcine thoracic discs were prepared by cutting off the adjacent vertebral bodies. The soft tissue and posterior element was removed carefully. Disc specimens were equally assigned to 4 groups, i.e., intact, denatured, low-level fatigue loading, high-level fatigue loading groups. The initial disc height and dimensions were measured. The disc of intact group did not receive any forms of injury. The discs of denatured group were injected with trypsin solution (0.5 ml, 0.5 %). The discs of low- and high-level fatigue loading groups were respectively applied with a 2 hour fatigue loading at 2 magnitudes, i.e. 0.1-0.8 MPa, 0.8-1.6 MPa. The frequency of fatigue loading was 5 Hz. All specimens were soaked in the phosphate buffer saline (PBS), wrapped with gauze and then stored at -20 degree Celsius before tests. Disc viscoelastic properties were measured by dynamic mechanical analysis (DMA) techniques using a material testing apparatus (Bose ElectroForce 3510). The discs were placed in the center of a chamber mounted on the material testing apparatus. The chamber was filled with the PBS. A Teflon plate was attached to the actuator of the material testing machine to transfer cyclic loadings to the specimen. All discs were applied with 0.1-0.8 MPa sinusoidal loading at frequencies sweeping from 0.01 to 10 Hz. Stiffness, damping coefficient, storage modulus (E’), loss modulus (E’’), phase angle (δ) of discs at frequency of 0.01, 0.063, 0.1, 0.63, 1.0, 6.3, 10 Hz were analyzed according to the resultant force and deformation. One-way ANOVA was respectively performed to evaluate the effect of enzymatic denaturation and fatigue loading on the stiffness, damping coefficient, storage modulus, loss modulus and phase angle. Statistical significance was set at p<0.05.
Results: The stiffness, storage modulus and loss modulus of denatured discs were significantly lower than of intact discs. The damping coefficient and phase angle of denatured discs didn’t change significantly with intact groups when frequency was over 0.63 Hz. The stiffness of denatured discs significantly increased with increasing frequency, but the damping coefficient and phase angle were decreased. The storage modulus of denatured discs don’t increase significantly when frequency was over 0.1 Hz. However, loss modulus of denatured discs did not changed significantly with variation of frequencies. The damping coefficient, loss modulus and phase angle trends of low level fatigue discs were lower than intact discs at frequency lower than 0.01 Hz. On the other hand, the damping coefficient, loss modulus and phase angle increased with frequency when compared to the intact discs. The trend of stiffness of high level fatigue discs were higher than of intact discs, but the trend of damping coefficient, phase angle, storage modulus and loss modulus were lower than of intact one. In the low-level fatigue discs, the stiffness increased but the damping coefficient decreased with frequency, respectively. However, no significant changes for storage modulus and phase angle were found when frequency was over 0.1 Hz. The loss modulus did not change significantly with frequency. In the high-level fatigue discs, the stiffness was not significantly increased when frequency was over 0.63 Hz. The damping coefficient and phase angle significantly decreased with frequency. The storage modulus significantly increased with frequency. The loss modulus did not change significantly with frequency. The stiffness, damping coefficient, storage modulus, loss modulus and phase angle of high level fatigue loading discs were lower than low level fatigue loading discs.
Conclusion: (1) The denatured disc shows softer, weaker shock absorption characteristics, elastic modulus and viscosity in comparison with intact disc. However, the ability of shock absorption of the denatured disc did not change significantly in comparison with intact disc when frequency was over 0.63 Hz. (2) The fatigue loading caused injury in anulus fibrosus. The disc after fatigue loading was stiffer than intact one. The high level fatigue loading showed weaker shock absorber and viscosity than intact disc. The ability of shock absorption and viscosity subjected to low level fatigue loading were higher than intact, but the differences were not significant. Degenerated disc with fatigue loading showed stiffer, weaker shock absorption characteristics. The hardness, ability of absorbing shock and elasticity of high level fatigue loading were lower than of low level fatigue loading.
Subjects
Denatured nucleus pulposus
injured anulus fibrosus
viscoelastic properties
dynamic mechanical analysis
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-101-R95548040-1.pdf
Size
23.54 KB
Format
Adobe PDF
Checksum
(MD5):af54a5941f66e08ae7bfdee48be73b13