[Thickness variation of the dural sac].

Objective: To measure carefully the thickness of the dural sac and evaluate possible variation in recently removed human specimens that had not yet undergone postmortem change. The thickness of the dural membrane is of interest because of its function as a barrier during diffusion processes and during closure of spinal lesions.

Material And Methods: After receiving the consent of our hospital's ethics committee and the family of the deceased, and immediately after extraction of organs for transplantation, we removed the dural sac and nerves contained therein from the cadaver of 56-year-old patient diagnosed of brain death. The membrane was dissected and 240 measurements of thickness were made over the entire surface of the sample. A micrometer was used, controlled through a surgical microscope. To analyze variations in thickness, the specimen was divided into 48 zones.

Results: The dural sac open on its anterior side was treated as a rectangular membrane measuring 130 x 54 mm. Mean thickness of the sample was 0.322 mm. Mean thickness of anterior zones was 0.353 mm, with no significant differences among them. Posterior zones measured a mean 0.295 mm with significant differences among them (p < 0.001). Up to the second lumbar root, anterior and posterior zone thicknesses presented no significant differences. However, after the space between the second and third lumbar roots, the posterior side was significantly thinner. Where the first, second and third lumbar roots emerged, we measured thicknesses of 0.315, 0.361 and 0.322 mm, with no significant differences among anterior, posterior and side zones on any level. At the fourth lumbar root and in the spaces of the dura mater between the emergence of the first and second, the second and third, the third and fourth and fourth and fifth lumbar roots, we observed significant differences. The measurements were 0.298 mm (p < 0.01); 0.348 mm (p < 0.01); 0.337 mm (p < 0.001), 0.306 mm (p < 0.01); 0.289 mm (p < 0.001), respectively.

Conclusion: Possible inter- or intra-individual variation in dural sac thickness is an unpredictable variable affecting the management of dural lesions. The data we report on thickness allow for future objective assessment of the maximum sizes recommended for the lateral orifices of bevelled, pencil-point needles in order to avoid straddling the membrane when subarachnoid anesthesia is given. The data also contributes to the study of substance diffusion through this membrane.