Effects of aging and spinal degeneration on mechanical properties of lumbar supraspinous and interspinous ligaments

Abstract

Background context: The effects of aging and spinal degeneration on the mechanical properties of spinal ligaments are still unknown, although there have been several studies demonstrating those of normal spinal ligaments. Purpose: To investigate the mechanical properties of the human posterior spinal ligaments in human lumbar spine, and their relation to age and spinal degeneration parameters. Study design/setting: Destructive uniaxial tensile tests were performed on the human supraspinous and interspinous ligaments at L4–5 level. Their mechanical properties were compared with age and spinal degeneration using several imaging modalities. Patient sample: Twenty-four patients with lumbar degenerative diseases on whom posterior surgeries were performed, with the age ranging from 18 to 85 years. Outcome measures: The ultimate load and elastic stiffness as structural properties, the degree of disc degeneration, range of segmental motion, the disc height, disc space narrowing ratio and degree of facet degeneration as the parameters of spinal degeneration. Methods: Twenty-four supraspinous and interspinous ligaments at the L4–5 level were obtained from posterior surgeries of patients with lumbar degenerative disease. The mechanical tests of bone-ligament-bone complexes were performed in a uniaxial tensile fashion with a specially designed clamp device. The ultimate load and elastic stiffness were calculated as structural properties. The degree of disc degeneration, range of segmental motion, the disc height, disc space narrowing ratio and degree of facet degeneration were examined by using radiographs, computed tomography and magnetic resonance imaging. Results: The average and SD value of ultimate load, elastic stiffness, tensile strength and elastic modulus were 203±102.9 N, 60.6±36.7 N/mm, 1.2±0.6 Mpa and 3.3±2.1 Mpa, respectively. A significant negative correlation was found between age and tensile strength (p= 0.02). The specimens with facet degeneration showed lower values in tensile strength and elastic modulus than those without facet degeneration (p<0.04). However, no correlation was found between disc-related parameters and tensile strength. Conclusions: The mechanical strength of human lumbar posterior spinal ligaments decreases with age and facet degeneration, particularly in the ligament substance.