The ligamentum flavum, of which there are only a few studies in the literature, has several features discussed in this work. On the macroscopic level, it has a metameric arrangement: it has two layers, superficial and deep, whose fibers are opposite, and it has close relations with the tendons of attachment of some spinal erector muscles. On the microscopic level, its structure is unique for a ligament, because of a predominance of elastic fibers, because of its intrinsic innervation at each level of the spine, and because this innervation grows poorer with increasing degeneration. The ligamentum flavum constitutes an active ligament, with an essential biomechanical role. Its injury is probably not without consequences, and therefore there are many technical problems about the surgical interlaminar approach.

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The vascular and nervous structures and their relations with the spinal nerve roots were examined in the 2nd, 3rd and 4th posterior sacral foramina in relation to percutaneous needle insertion for neuromodulation. A foraminal branch provided by the lateral sacral artery to each foramen entered the inferior lateral quadrant of each foramen, adjacent to the nerve root medially. Facing the posterior sacral aperture and around the sacral nerves, there was no venous plexus. A venous plexus was sometimes present near the median line, and always around the proximal part of the spinal ganglion. The sacral nerve roots, especially the 3rd, had a long extradural course in the foramen, presenting a potential risk of nerve lesions during procedures involving needle insertion.Key words : Sacral nerves ; vesical dysfunction ; neuromodulation. Electrical stimulation of sacral nerve roots can modulate neural reflex behaviour with respect to the lower urinary tract in patients with urinary symptoms refractory to conventional treatment after carrying out peripheral nerve evaluation in order to establish the integrity of the sacral nerves (Schmidt et al. 1989 ;Thon et al. 1991) and employing stimulation of different modalities (Ohlsson et al. 1989). In addition, electrical stimulation can be used for the management of pain, particularly perineal pain, with meticulous titration of pulse width to allow more selective stimulation of motor and\or sensory fibres (Barolat, 1991).Only limited literature exists on the anatomy of the posterior sacral foramina and the relationships between the nervous and vascular structures that they transmit. This has highlighted some potential risk factors associated with needle insertion. We have therefore undertaken an anatomical study to establish the nature of the vascular and nervous structures and their relations in the posterior sacral foramina as they may influence percutaneous needle insertion for neuromodulation.Correspondence to Dr Dominique Liguoro, Service de Neurochirurgie-Ouest, Ho# pital Pellegrin, 1 Place Ame! lie Raba-Le! on, 33076 Bordeaux Ce! dex, France.   An anatomical study was undertaken on 6 human cadavers, 3 male and 3 female, with a mean age of 75 y. There was no previous history of spinal injury, congenital defects or gross bony pathology. Within 48 h of death, the cadavers were injected and embalmed with 10 % formalin. Eosin was added to the infusion into the arterial system to improve contrast during dissection and photography. The cadavers were placed in a prone position. A midline posterior incision was made from L4 to the coccyx. The L5 spinous process and the posterior face of the sacrum were cleared of all soft tissues. The insertions of the gluteus maximus muscle and its superficial fascia were cut off so that the posterior sacral foramina became apparent. The dissection was then continued under an operating microscope (Zeiss OPMI6). The 2nd, 3rd and 4th posterior sacral foramina were dissected following which bone drilling of the ...

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