ANATOMY OF THE EXTRADURAL COMPARTMENTS OF THE LUMBAR SPINAL CANAL: Peridural Membrane and Circumneural Sheath

The development of newer and more accurate methods of identifying the structures within the spinal canal has given a much better understanding of the soft tissue structures that lie between the dura and the surrounding bone of the vertebral canal. One anatomic structure of special importance, but seldom spoken of, is the peridural membrane. Although it was mentioned in the writings of Fick14 as early as 1904, it was Dommissee12,13 who first described it accurately and who named it the peridural membrane. It appears to be a homologue of the periosteum. There is no periosteum at any point on the bone where this membrane is located . Spine surgeons have frequently noticed that, after removing a nuclear fragment that has extruded and crept upward or downward and has come to lie over the vertebral body, one sees only rough periosteum-free bone where the fragment had been lying. Premembranous hematomas may develop because a small fragment of nuclear material has extruded out at the rim of the vertebra, anterior to the attachment of the PLL, and in tearing loose may have caused bleeding. This fragment of nuclear material may creep upward or downward over the vertebral body. There may be a pool of blood around it. Hematomas are usually on one side or the other but can lie midline, because the bony ridge down the center of the posterior surface of the vertebral body is not always complete. In other words, there are gaps in it , and it is through these gaps or by actually lifting the deep PLL-membrane complex that the nuclear material or blood can pass, giving a midline mass seen on MR imaging. The plexus of veins in the body of the peridural membrane and penetrating it posteriorly in many places is part of Batson's plexus. They were first described by Willis38 in 1663. In recent years Dommissee13 and Crock8 have made major contributions to our understanding of these vessels. It is interesting to note that if a degenerated disk is injected with a mixture of barium, dye, and gelatin and radiographs are taken, it is seen that, in most cases, Batson's veins have filled. If the disk is not degenerated and does not take more than a few milliliters of this mixture, then Baxton's veins do not fill. It is very likely that these veins fill whenever we do diskography on anything beyond a moderately degenerated disk but we do not see them on the fluoroscope because the contrast is not dense enough and by the time we get around to taking permanent radiographs, the contrast has long since disappeared into the venous circulation. In the cadaver, the blood is not flowing so the gelatin with barium has time to set in veins. Interestingly, if contrast is injected into vertebral body by simply driving a needle through the cortex of the vertebral body, Batson's veins fill with contrast in the same way. The sheath surrounding the spinal nerves out lateral to the foramina was not well known until Kikuchi19 described it in 1982. He gave a very accurate description of it and called it the epiradicular sheath. The problem with calling it by this name is that it surrounds a true mixed peripheral nerve and not a radicule (root). Because of this, we propose the name circumneural sheath. It should be noted that there already is an epineural sheath, which is the outermost covering of peripheral nerves. There is also a perineural sheath and an endoneurium. These names have been used for nearly 100 years. The segment of spinal nerve commencing at the dura and ending just beyond the ganglion is commonly called a nerve root. It actually consists of two nerve roots, a motor and sensory, inside a sleeve of dura. At a point just beyond the ganglion the two roots blend together and become a typical mixed peripheral nerve. At this same point the dural sleeve becomes adherent to the nerve and henceforth is called the epineurium. For this 1- to 2- cm segment of spinal nerve from the dura to just beyond the ganglion we propose the name dural root sleeve. After all, it is a sleeve, it is made of dura, and it covers two nerve roots.