Histological investigation of rabbit ligamentum flavum with special reference to differences in spinal levels

The structure of the ligamentum flavum has yet to be fully elucidated and no studies have investigated fine structural differences at different spinal levels in any animals. The aim of the present study was to clarify structural differences in the ligamentum flavum at different spinal levels (cervical: C3/4 and C5/6; upper thoracic: T2/3; lower thoracic: T9/10; lumbar: L3/4) using light and electron microscopy of rabbit specimens. Light microscopy using resorcin-fuchsin staining revealed that the distribution of elastic fibers was diffuse in the cervical and upper thoracic regions, but was generally dense in the lower thoracic and lumbar regions. Electron microscopy demonstrated that the cervical and upper thoracic regions were rich in collagen fibers. Conversely, the lower thoracic and lumbar regions were rich in elastic fibers. Quantitative image analyses displayed thick elastic fibers in the lower thoracic and lumbar regions, with high area ratios. Radiographic examinations revealed that ranges of motion were large at the cervical region, but small at the lower thoracic and lumbar regions. These findings suggest that structure of the ligamentum flavum varies at different spinal levels with respect to differences in motion.