The lumbar interspinous ligaments in humans: anatomical study and review of the literature.

In textbooks of human anatomy and atlases of spinal surgery, the lumbar interspinous ligaments are described and illustrated in various and often conflicting ways. Thirty-four lumbar vertebral columns (19 males, 15 females, range 17-92 years old), sampled at autopsy, were studied by dissection and macroscopic analysis. The orientation of the fibre bundles was evaluated in the sagittal plane by tracing a reference line parallel to the cranial border of the spinous process, and the degree of bundle tilting with reference to this line was measured in the ventral, middle and dorsal parts of the ligament. Histological and immunohistochemical (anti-S100) studies of the ligaments, on sagittal, frontal, and transversal planes, were also carried out. In vivo radiological validation was performed in 25 patients by MR and CT imaging. Macroscopically, the interspinous ligaments are consistently composed of bundles of fibres arranged in a characteristic pattern. Together, they present an oblique orientation from anterior to posterior in a caudal-cranial direction. In their ventral part the bundles form a slight curve with a posterior-inferior concavity. In the middle part, the bundles are thicker and arranged in an italic S shape with a mean degree of tilting of 52 +/- 14, 21 +/- 12 and 50 +/- 17 in the anterior, intermediate and posterior subzones of the L2-L3 ligaments and of 32 +/- 14, 11 +/- 9 and 18 +/- 11 in the anterior, intermediate and posterior subzones of the L4-L5 level. The dorsal part consists of obliquely ascending bundles of fibres mostly converging in the supraspinous ligament. Histologically, these ligaments are mainly composed of collagen fibres, whereas the elastic fibres are ubiquitous, although mostly concentrated in the ventral part, which is closely linked to the yellow ligament. The interspinous ligament is well supplied by small blood vessels and sensory nerves, the latter particularly in its dorsal part and on its lateral surfaces. According to collected data, the structure of the interspinous ligaments reflects their function, i.e., the italic S-shaped course of the fibrous bundles represents an available reserve of length in a poorly elastic ligament.