Recent research on the ultrastructure of the human spinal dural sac and its contents has enhanced our understanding of the microstructure of the dura mater, arachnoid layer, trabecular arachnoid, pia mater, and nerve root cuffs. This chapter reviews new and traditional concepts regarding these structures and discusses their possible clinical implications. The distribution of epidural fat and its possible role in disposition and kinetics of neuraxial injections are also discussed.
The dural sac surrounds the spinal cord inside the vertebral column. It separates the epidural space from the subarachnoid space, ending at the second sacral vertebra. In an idealized shape, the dural sac is cylindrical with its thickness varying from about 1 mm in the cervical region and becoming gradually thinner as it descends (Figures 6–1 and 6–2). In the lumbar region, the thickness of the dural sac reaches 0.3 mm,1 although measurements taken from anteroposterior or lateral may vary somewhat even at the same vertebral level. The dura mater is the most external layer of the dural sac and is responsible for 90% of its total thickness. This fibrous structure, although permeable, somewhat provides mechanical protection to the spinal cord and its neural elements. The internal 10% of the dural sac is formed by the arachnoid layer, which is a cellular lamina that adds little extra mechanical resistance.2
Human dural sac. (Reproduced with permission from Reina MA, López García A, de Andrés JA, et al: Thickness variation of the dural sac. Rev Esp Anestesiol Reanim. 1999 Oct;46(8):344-349.)
Human dural sac and end spinal cord. (Reproduced with permission from Reina MA, Pulido P, López A. El saco dural humano: Rev Arg Anestesiol. 2007; 65:167-184.)
Dura mater comprises approximately 80 concentric laminas3,4,5 (Figure 6–3). Each dural lamina has a thickness of approximately 5 μm and consists of thinner laminas containing mostly collagen fibers (Figures 6–4 and 6–5). The collagen fibers are oriented in different directions but always within the concentric plane of the dural lamina; hence, they do not cross between the laminas. Each collagen fiber has a smooth surface and measures approximately 0.1 μm (Figure 6–6). The elastic fibers are fewer, measuring 2 μm in diameter, and have a rougher surface than that of the collagen fibers6 (Figure 6–7).
Thickness of the dural sac. Scanning electron microscopy. Magnification ×300. (Reproduced with permission from Reina MA, Dittmann M, López A, et al: New perspectives in the microscopic structure of human dura mater in the dorso lumbar region. Reg Anesth. 1997 Mar-Apr;22(2):161-166.)