Vertebral Column Anatomy
Seven cervical, 12 thoracic, 5 lumbar, 5 fused sacral vertebra and the coccyx compose the vertebral column. The vertebral column has four characteristic curvatures: the anterior convexity of the sacrum, the lumbar lordosis, the thoracic kyphosis, and the cervical lordosis. In the supine patient, the lumbar spine has its highest point at L4 and the thoracic spine has its lowest point at T4. In the lumbar area, the spinous processes project directly posteriorly whereas in the thoracic area, the spinous processes project posteriorly and more inferiorly until they reach their steepest downward angulation at the midthoracic level where they overlap with the lamina of the vertebra immediately inferior. This overlap can make the midline approach to the epidural space difficult or impossible at the T5-T9 levels. At higher thoracic levels, the spinous processes become level again to become nearly horizontal at C7. The spinal canal is enclosed by the vertebral bodies anteriorly, the pedicles laterally, and the ligamenta flava and the laminae posteriorly. The canal ends superiorly in the foramen magnum and inferiorly in the sacral hiatus.
The supraspinous ligament runs along the tips of the spinous processes and blends with the ligamentum nuchae at its superior end. In elderly individuals and in persons who engage in heavy physical activity, the ligament can become ossified, making a midline approach to the epidural space impossible. The interspinous ligament stretches vertically from the inferior border of each spinous process to the superior border of the spinous process below, except in the cervical spine, where it is absent. Dorsally, the interspinous ligament blends with the supraspinous ligament. Ventrally, it fuses with the ligamenta flava and the laminae. The laminae slope posteriorly and inferiorly so that their ventral surfaces are in close contact with the dura. The ligamentum flavum is a tough elastic ligament that attaches to the ventral surface of the superior lamina and the dorsal surface of the inferior lamina. Laterally, the ligament thins as it blends with the joint capsule of the articular processes. Within the spinal canal, the posterior longitudinal ligament runs along the dorsal surface of the vertebral bodies and is adherent to the anterior dura. The anterior longitudinal ligament joins the vertebral bodies along their anterior surface.
The Dura Mater
The dura mater is a tough fibrous membrane that envelopes the arachnoid mater, cerebrospinal fluid, pia mater, spinal nerves, spinal cord and brain. Within the cranium, the dura is composed of an outer endosteal component that lies against the bone of the cranium and an inner meningeal component. These two layers are tightly adherent except where they divide to form the venous sinuses. At the foramen magnum, the endosteal layer divides from the meningeal layer and lines the spinal canal as the endosteum of the vertebral bodies. The meningeal layer continues caudally as the dural sac, and ends at the S2 level in adults. The attachment of the meningeal dura to the endosteal dural at the foramen magnum anatomically isolates the cranial vault from the epidural space of the spinal canal.
The Arachnoid Mater
The arachnoid mater is a thin metabolically active membrane that loosely adheres to the dural sac and contains the brain and spinal cord bathed in CSF. Between the arachnoid and the dura lies the subdural space, a potential space through which local anesthetics can distribute via a misplaced spinal needle or epidural catheter. Connective tissue trabeculae extend from the arachnoid to the pial surface of the spinal cord to secure the cord in the CSF. Arachnoid granulations ranging from microscopic to 3 mm in diameter cluster around the nerve roots in the dural cuff region. These granulations emerge through the dura and press into surrounding veins and epidural fat. By transcellular vacuolar transport, the granulations clear the CSF of foreign particulate material, likely by emptying directly into the epidural venous plexus or into the epidural connective tissue, for subsequent removal by lymphatic drainage.
The Pia Mater and Spinal Cord
The pia mater is a thin highly vascular membrane composed of flat epithelial cells and tightly adherent to the spinal cord. A long filamentous extension of the pia, the filum terminale, pierces the caudal end of the dural sac and blends with the periosteum of the coccyx to secure the spinal cord within the sac. The spinal cord ends at the L1-2 level in adults. The spinal roots continue caudally to the intervertebral foramina of the lower lumbar and sacral levels as the cauda equina.