[Strain distribution in the L1 vertebra under axial load and load transmitting behavior between the vertebral body and posterior elements].

This study investigated the stress distribution in the L1 vertebra under a vertical axial load. Five normal spinal units (T12-L2) were obtained from embalmed human cadavers, to measure the surface strain in the L1 vertebra. Stress screening using a brittle-coat was prepared beforehand, then fourteen sites on the lateral half of the surface on the L1 vertebra were carefully chosen to install rosette strain gauges. A vertical axial load was applied up to 1471 N in two degrees of flexion, in the neutral position, and in two degrees of extension of the intact spine, and in the neutral facetectomized spine. The strain at each site was measured with strain gauges, then the tensile, compressive and shear strain values were calculated. Cracks in the brittle-coat showed the strain concentration in the base of the pedicles and rims on the superior vertebral body. Analysis of the data by strain gauge showed that the amount of strain at the rims on the superior vertebral body was greater than that at the inferior rims in the three positions of the intact spines, with a statistical significance. These strains led to a burst in the superior vertebral body rims. As for the posterior elements, the inner surface of the laminas received a tensile strain only, while the principal strain on the outer surface of the laminas was a compressive strain, in the directions parallel to the laminas. These strains led to a transformation in the posterior elements, leading to widening in the interpedicular distance and a fracture in the inner surface of the lamina. These results showed the first lumbar vertebra was easily mechanically injured by a Denis type B burst fracture. Also this study demonstrated that the pedicles were the pathway for load transmission.