Anatomy of the lumbar epidural region using magnetic resonance imaging: a study of dimensions and a comparison of two postures.

Many techniques used to investigate the epidural region may alter the anatomy. Magnetic resonance imaging (MRI) has been introduced as a non-invasive diagnostic technique. The aim of this study was to investigate the anatomy of the lumbar epidural region using MRI, studying the morphology, the dimensions and the modification that may result from a change of position. Ten young, healthy female volunteers were studied to obtain results relevant to obstetric anesthesia. The following measurements were made: anterior epidural space (AES); posterior epidural space (PES); ligamenta flava; distance between the PES and the skin (S-ES); and interspace distance (ISD). All these distances were measured with the subject supine and the lumbar spine either in a neutral or a non-lordotic (flexed) position. The S-ES ranged from 2.7 to 8.1 cm. This distance was greater in the lower than in the upper lumbar segments. No differences were observed in AES, PES and ligamenta flava between the neutral and flexed positions. Flexion of the spine did not affect the S-ES but increased the ISD (P < 0.05). The S-ES at any lumbar segment or interspace and the thickness of ligamenta flava at the L2/3 interspace correlated significantly with body weight (respectively, P < 0.01 and P < 0.02). In conclusion, MRI may clearly reveal the anatomy of the epidural region. The circumferentially and metamerically segmented compartments of the epidural space were clearly noted and measured. Ligamenta flava seem to be thinner in younger than in older subjects. This may partly explain a reduced loss of resistance sometimes perceived in obstetric patients.