Contribution of disc degeneration to osteophyte formation in the cervical spine: a biomechanical investigation

Cervical spine disorders such as spondylotic radiculopathy and myelopathy a re often related to osteophyte formation. Bone remodeling experimental anal ytical studies have correlated biomechanical responses such as stress and s train energy density to the formation of bony outgrowth. Using these respon ses of the spinal components, the present study was conducted to investigat e the basis for the occurrence of disc-related pathological conditions. An anatomically accurate and validated intact finite element model of the C4-C 5-C6 cervical spine was used to simulate progressive disc degeneration at t he C5-C6 level. Slight degeneration included an alteration of material prop erties of the nucleus pulposus representing the dehydration process. Modera te degeneration included an alteration of fiber content and material proper ties of the anulus fibrosus representing the disintegrated nature of the an ulus in addition to dehydrated nucleus. Severe degeneration included decrea se in the intervertebral disc height with dehydrated nucleus and disintegra ted anulus, The intact and three degenerated models were exercised under co mpression. and the overall force-displacement response, local segmental sti ffness, anulus fiber strain, disc bulge, anulus stress, load shared by the disc and facet joints, pressure in the disc, facet and uncovertebral joints , and strain energy density and stress in the vertebral cortex were determi ned. The overall stiffness (C4-C6) increased with the severity of degenerat ion. The segmental stiffness at the degenerated level (C5-C6) increased wit h the severity of degeneration. Intervertebral disc bulge and anulus stress and strain decreased at the degenerated level. The strain energy density a nd stress in vertebral cortex increased adjacent to the degenerated disc. S pecifically. the anterior region of the cortex responded with a higher incr ease in these responses. The increased strain energy density and stress in the vertebral cortex over time may induce the remodeling process according to Wolff's law, leading to the formation of osteophytes. (C) 2001 Orthopaed ic Research Society. Published by Elsevier Science Ltd. All rights reserved .