Radiographs of the lumbar spine should be carefully reviewed preoperatively to determine whether the patient has spinal dysraphism such as such spinal bifida occulta, or otherwise large interlaminar spaces. Iatrogenic durotomy can occur in these patients during the process of elevating the paraspinal muscles off the posterior bony elements, with either the electrocautery or the periosteal elevator. Similar care is also necessary in cases of prior laminectomy. Ensuring that the tip of your instalment can feel bone will avoid inadvertent entry into the canal and injury to the dura or nerves.

Bony resection for a laminectomy or laminotomy is generally carried out with Kerrison rongeurs or motorized burrs. Residual bony spikes can lacerate the dura and should be smoothed out with a rongeur or curette. The foot plate of the Kerrison rongeur should be placed parallel to the thecal sac or nerve root to prevent inadvertent laceration. In addition, the foot plate must be inserted flush with the undersurface of the bone to minimize the risk of trapping a fold of dura within the jaws. Cottonoid pledgets are generally inserted between the dura and the overlying structures to avoid inadvertent dural tears. In patients with high-grade lumbar stenosis, insertion of large Kerrison rongeurs or cottonoid pledgets can further compromise the spinal canal, leading to neurologic deficits. In these cases, a burr is used to thin the laminar bone and curettes can then be used to remove the remaining bone. This avoids insertion of instruments into the canal, further compromising an already narrowed spinal canal. Leaving the ligamentum flavum and epidural fat layers intact during the initial phases of laminectomy can in some cases provide an additional layer of protection during the initial laminectomy. These tissues can later be easily separated from the dura and removed.

With chronic and severe lumbar spinal stenosis, the dura is occasionally adherent to the overlying soft tissue or bone. This must be recognized early to minimize the likelihood of durotomy. The adhesions are released with angled curettes or soft tissue dissectors prior to bone or ligamentum flavum removal. Movement of the dura during bone or ligamentum removal should raise suspicion for such adhesions. In some cases, it may be preferable to leave a small section of adherent ligamentum flavum attached to the dura, as long as this does not mechanically compress the thecal sac.

The next stage of the decompression generally focuses on the lateral canal, and attention is paid at this step to avoid nerve root or dural injury. Care should be exercised to avoid excessive or prolonged retraction of the thecal sac, to minimize the risk of a “battered root syndrome” or other neurologic deficit. There should be no attempt at retraction of the spinal cord or conus medullaris in the upper lumbar spine. The nerve root will occasionally resemble a disc when it is stretched taut by an underlying disc herniation. Prior to incising what appears to be annulus, the lateral edge of the nerve root and thecal sac must be clearly identified. If difficulty is encountered in locating the nerve root, more lamina is resected until the medial wall of the pedicle is located. The exiting nerve root is noted along the inferomedial aspect of the corresponding pedicle. Congenital variations in nerve root anatomy can lead to a missing nerve root at a level, and occur with an incidence of 2% to 14% in lumbar spine surgery (7).

When an intertransverse fusion is planned, the dissection will extend ventrally past the facet joint to expose the transverse processes. It is important during this dissection to identify and avoid violation of the intertransverse membrane. The ventral rami of the nerve roots lie anterior to this membrane after exiting the neuroforamen. Blunt elevation of the muscles off the membrane, combined with electrocautery of residual muscle fibers directly on the bone of the transverse process, allows safe exposure of the area.

Substantial blood loss can occur with posterior lumbar approaches if proper surgical technique is not adhered to. Subperiosteal elevation of the paraspinal musculature minimizes laceration of large intramuscular vessels. During mobilization of the thecal sac and nerve roots, significant bleeding occasionally occurs from the epidural venous plexus. In these cases, bleeding should be controlled with thrombotic agents or bipolar cautery. Unipolar cautery should be avoided in the canal. Ensuring that the abdomen is free during positioning of these patients can reduce the back pressure within these veins and reduce intraoperative bleeding. Careful exposure of the region between the pars interarticularis and facet joint will allow early identification and cauterization of the segmental artery just lateral to the pars. These segmental arteries are branches from the aorta, fourth lumbar artery, middle sacral artery, internal iliac artery, and the iliolumbar artery. The arteries travel dorsally in between the facet joint and just lateral to the pars interarticularis (Fig. 9.2).

Injury to large vessels anterior to the lumbar spinal column can occur during discectomy from inadvertent passage of instruments beyond the anterior longitudinal ligament (8,9). Visceral injuries occur rarely with the posterior approach. Injuries to the bowel, pancreas, and ureters have been reported with violation of the anterior longitudinal ligament during lumbar discectomies (10). Entry into the retroperitoneum can occur if dissection is carried anterior to the intertransverse membrane.