Denis conceptualized the spine as three longitudinal columns, namely:

(1) Anterior column, including the anterior longitudinal ligament, the anterior two-thirds of the vertebral body, and the intervertebral disc;

(2) Middle column, consisting of the posterior one-third of the vertebral body and intervertebral disc, and the posterior longitudinal ligament;

(3) Posterior column, composed of the vertebral arch, lamina, associated structures, ligamentum flavum, interspinous, and supraspinous ligaments.

In 1984, Ferguson refined Denis's three-column classification concept, suggesting that the anterior two-thirds of the vertebral body and intervertebral disc belong to the anterior column, and the posterior one-third belongs to the middle column. This is the widely accepted three-column classification concept today, where injuries to the middle column are considered unstable fractures.