Anatomy of the epidural space

There are 24 individual vertebrae: seven cervical, 12 thoracic and five lumbar. The five (fused) sacral vertebrae and the coccyx (made up of 3–5 rudimentary vertebrae) are not always classed as being a part of the vertebral column.Vertebral anatomyvaries according to each level. The atlas and the axis are highly atypical and the first ‘recognizably normal’ vertebra isC3.Atlas andaxis anatomyare relevant to anaesthetists in that the odontoid process (dens) of the axis should be closely applied (2–3 mm) to the anterior arch of the atlas in flexion and extension (the gap is filled with cartilage). Rheumatoid arthritis and trauma may affect this relationship and the ability to recognize the relevant anatomy on lateral radiographs is important. From C3 downwards, the vertebrae (although they vary) have a recognizable anterior body, posterolateral pedicles (Latin for ‘little feet’), transverse processes and posterior laminae (‘thin layers’), which fuse to form the spinous processes. The spinal canal enclosed within these structures is also known as the epidural space, apart from the central portion occupied by the dural sac and its contents. The dura mater contains the arachnoid mater. Between the arachnoid and the pia mater, which is applied to the spinal cord, is cerebrospinal fluid. As the vertebral column grows, it leaves behind the spinal cord so that by adulthood the cord ends at the lower border of L1 (although this can vary by one vertebra). The dural sac generally ends at the lower border of S2 below which it continues as the filum terminale, a structure clearly and frequently seen with spinal endoscopy. The dural sac contains the anterior and posterior spinal nerve roots, collectively know as the cauda equina.