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Format:
Print
Author:
Walter, Ben
Dept./Program:
Biomedical Engineering Program
Year:
2010
Degree:
M.S.
Abstract:
Low back pain is a widespread and incapacitating disorder in American society accounting for significant. economic and productivity losses. Previous work has implicated degeneration of the intervertebral disc (IVD) as a strong contributor to low back pain of which the etiology is complex with many contributing factors including aging, genetics, and mechanical loading. Asymmetric loading induced by flexion and/or extension has been implicated as being injurious to the IVD and is thought to contribute to the degenerative changes that develop in multiple spinal pathologies, such as idiopathic scoliosis and cerebral palsy, were asymmetric loading occurs. Some small animal models have investigated how asymmetric loading affects the IVD. However, the limited information regarding how the material behavior of the annulus fibrosus (AF) and how catabolic metabolism are affected, along with limited information about the role that inflammation may play due to asymmetric loading hinders the understanding of how the altered loading contributes to the development of disease progression.
This study was designed to investigate how mechanical injury, applied through asymmetric loading, affects the material behavior of the AF, the catabolic response and the inflammatory response of the IVD in a large animal ex-vivo model. Bovine caudal intervertebral discs were assigned to either a control or wedge group (15°) and placed in organ culture for 7 days under static 0.2 MPa load. IVD structure and cellular responses were assessed through confined and unconfined compression, biochemical measurements, qRT-PCR, immunofluorescence, and western blotting. Results show that at the cellular level asymmetric loading induced cell death and an increase in caspase-3 staining compared to controls in the concave AF, as well as an up-regulation ofMMP-I, IL-l~, and IL-6 mRNA in the convex AF.
At the tissue level asymmetric loading induced an increase in both aggregate and Young's modulus and a decrease in peak stress/equilibrium stress (in confined compression) in the concave AF along with a loss of aggrecan from the tissue. On the concave side of wedged IVDs asymmetric loading caused a reduced aggregate and Young's modulus along with an increased peak stress/equilibrium stress (in both confined and unconfined) along with increased water content.
The results suggest that asymmetric loading, separate from any other disease processes, had immediate deleterious effects on both tissue and cells. Mechanical data indicates that tissue compaction occurred on the concave side of wedged IVD, this combined with the increase in cell death and caspase-3 staining on the concave side of wedged IVDs implies that tissue compaction caused the observed cell death via apoptosis. qRT-PCR results imply that a discogenic inflammatory response was induced on the convex side of wedged IVDs, which may have influenced the increase in catabolic mRNA expression. Together this study supports the theory that mechanical injury is capable of causing structural disruption leading to inflammation and apoptosis, both of which may contribute to the progression of degenerative changes within the IVD.