Standard texts agree that:
1. The cephalad limit of the extradural space is at the foramen magnum;
2. the inferior limit is the sacro-coccygeal membrane;
3. anteriorly, the posterior longitudinal ligaments separate the extradural space from the bodies of the vertebrae and the intervertebral discs;
4. posteriorly, lie the laminae of the vertebrae and the ligamenta flava; and
5. laterally, the pedicles and intervertebral foraminae (Figure E.2).
The extradural space contains the dural sac, nerve roots, blood vessels, lymphatics and connective and fatty areolar tissue.
Because of the potential effect of the geometry of the epidural space on technique and spread of drugs, interest has centred on
1. the amount and distribution of fat in extradural space,
2. the presence or absence of a median dorsal fold of the dura mater,
3. the presence of connective tissue bands or septa, and
4. the detail of the ligamenta flava.
This information is absent or inaccurate in older texts derived almost entirely from cadaveric dissection - which is much subject to artefact. Unfortunately, most modern methods of investigation also induce some degree of artefact, for example, by opening the epidural space during surgery (1), by introducing air for epiduroscopy (2, 3), or by injecting radiological contrast (4) or resin (5) into the epidural space. Cryomicrotome sectioning (6) is less disruptive of the epidural contents and magnetic resonance imaging (7) is probably the only technique not to introduce artefact.
From these studies it appears that the extradural space is divided into metamerically repeating segments, being little more than a potential space posteriorly at the level of each lamina and widening to a 4 to 6mm space opposite the ligamenta flava. Still controversial is the existence of a dorsal median fold of the dura mater (plica mediana dorsalis) and connective tissue bands (dorso-lateral extensions) with those advocating its presence being opposed by those insisting it is an artefact. Whichever is correct, epidurally injected drugs sometimes behave as if there is a variable degree of epidural space compartmentalisation, and, from a clinical point of view, the question of whether the anatomical details are artefactual or not is largely academic.
References:
1. Luyendijk W. The plica mediana dorsalis of the dura mater and its relation to lumbar peridurography (canalography). Neuroradiology 1976; 11:147-149.
2. Blomberg R. The dorsomedian connective tissue band in the lumbar epidural space of humans: an anatomical study using epiduroscopy in autopsy cases. Anesthesia and Analgesia 1986;65:747-752.
3. Blomberg RG, Olsson SS. The lumbar epidural space in patients examined with epiduroscopy. Anesthesia and Analgesia. 1989; 68:157-160.
4. Savolaine E R, Pandya J B, Greenblatt SH, Conover S R. Anatomy of the human lumbar epidural space: new insights using CT-epidurography. Anesthesiology 1988;68:217-220.
5. Harrison G R, Parkin I G, Shah J L. Resin injection studies of the lumbar extradural space. British Journal of Anaesthesia 1985; 57:333-336.
6. Hogan Q H. Lumbar epidural anatomy. A new look by cryomicrotome section. Anesthesiology. 1991; 75:767-775.
7. Westbrook JL, Renowden SA, Carrie LES. Study of the anatomy of the extradural region using magnetic resonance imaging. British Journal of Anaesthesia 1993;71:495-498.