A tubular retractor system such as the MetRx system (Medtronic Sofamor Danek, Memphis, TN) is required. Systems should include all of the necessary instruments for retractor placement such as the dilators, retractors, and retractor arm, but also provide extra-long, anti-glare-coated instruments including a variety and sizes of curettes, disk punches, and rongeurs. If utilizing the conventional microscope, bayonet-shaped instruments and an angled drill attachment are also helpful.

While the tubular retractor can be used with an operating microscope or a 30° angled endoscope, it is our belief that endoscopic diskectomy provides better visualization of the surgical anatomy over that provided by conventional microscopy. The coupler provided with the MetRx kit allows for the use of a variety of different endoscopes with either the 16 or 18 mm working channel. Lateral fluoroscopic guidance is required for localization and during the dilation phase and electrophysiological monitoring is not routinely utilized.

This procedure can be safely performed under general, local, or spinal anesthesia [17, 18]. If being performed under general anesthesia, non-depolarizing neuromuscular blockers are avoided to allow for improved feedback from nerve root manipulation during surgery. Any position/frame/bed combination that allows for a free-hanging abdomen is acceptable so pressure is not transmitted to the epidural veins and causes bothersome bleeding during surgery (Fig. 35.2). The surgeon should stand on the same side as the patient’s pathology and a rail attachment placed on the opposite level for the table-mounted arm; typically, this is aligned with the patient’s hip. Video monitor for the endoscope is also placed across the table so the surgeon can face it directly. If a microscope is to be used in lieu of endoscopy, the microscope base can be brought in from the side opposite to the C-arm base. In either case, it is usually easiest to position the C-arm monitor at the foot of the operative table.

The whole lumbar area is prepped and draped. The fluoroscopy C-arm is also draped in sterile fashion in an under-table position to enable quick radiological confirmation throughout the case. Localization of the level of interest is performed under lateral fluoroscopy counting cranially from the sacrum, noting vertebrae with transitional morphology. Specific adjustments may be made for migrated fragments. When the radiopaque marker is in the correct location, incision is planned 1.5 cm off midline on the side of the intended approach.

The operative site is then infiltrated with local anesthetic, while the assistant secures the clamp and retractor arm to the operative table (Fig. 35.3). An 11-blade scalpel is used to make a small 20-mm incision exposing the subcutaneous fat. A stainless steel guide pin or Kirschner wire is passed through the stab incision and soft tissues on to the underlying bone. Fluoroscopy should be used to confirm location of the guide pin during such passage to avoid penetrating the interlaminar space and causing a dural leak, especially if positioned on a Wilson frame. The safest approach is to direct the guide pin perpendicular to the entry point, so that the pin will contact the zygapophysial joint or even the transverse process rather than the lamina. Once the pin is docked on the zygapophysial joint, the first dilator is passed over the pin and removed; with the dilator, it is safe to angle medial and dock at the spinous process-lamina junction, directly over the disk space (Fig. 35.3). The remaining dilators are then passed sequentially and lastly, the working channel. These devices are introduced with a twisting motion so the dorsal lumbar fascia and underlying musculature are split along their fibers rather than torn. It is important to utilize fluoroscopy during this stage to confirm the trajectory and that the dilators are resting against the bone; this minimizes the amount of tissue that needs to be removed later. A working channel of 16 or 18 mm may be utilized for endoscopic applications and up to 22 mm for use with the microscope. In either case, it is connected to the table-mounted arm and secured in position, and the dilators are removed from its interior. Following a final fluoroscopic position check, the endoscope is attached to the working channel. Should the working tube be repositioned during the procedure, it is also recommended that fluoroscopy be used: even a seemly small adjustment may prove enough to aim at the wrong disk space.

Monopolar cautery and pituitary rongeurs are used to clear the remaining soft tissue off the lamina and inferior articular process, taking care to start the dissection over solid bone laterally. All muscle tissue should be detached completely along the circumference of the tube prior to attempting removal of the muscle (Fig. 35.4). Failure to do so can result in excessive bleeding and pulling more muscle into the field beneath the edges of the working channel. The “cut” setting on the cautery appears to be more effective than “coagulation” during initial exposure. Once the muscle has been cleared, a straight curette is used to identify and clear any residual soft tissue from the caudal edge of the lamina. The surgeon’s view should be centered on the spinous process-lamina junction and the tubular retractor may need to be readjusted at this point. Straight and up-angled curettes are used to detach the ligamentum flavum medially from the overlying lamina and then work laterally under the lamina. Fluoroscopy should be used to confirm the location under the lamina dorsal to the desired disk space. Once an adequate plane has been established, 3 and 4 mm Kerrison rongeurs should be used in a rostral direction to perform a laminotomy. The up-angled curette is routinely utilized to confirm separation between the ligamentum flavum and lamina, in order to prevent injury to the underlying dura and nerve roots. In general, the laminotomy should extend the length of the neural foramen, from pedicle to pedicle; the rostral insertion of the ligamentum flavum normally marks the cranial extent and exposed dura is seen adjacent to it. At the junction of the lamina and facet complex, it is usually necessary to use a high-speed drill to perform a medial resection of the inferior articular process until an up-angled curette can be passed easily into the neural foramen. It can be useful at this point to aim the tube slightly lateral and to utilize fluoroscopy with a probe or up-angled curette in the foramen to confirm the extent of the laminotomy and foraminotomy. With these maneuvers, a medial foraminotomy is performed, which can be a stand-alone procedure or a routine, initial part of every paramedian diskectomy. Bony bleeding is easily addressed by the use of bone wax. In most cases, the bony opening exposes the ligament superficial to the shoulder and lateral margin of the traversing nerve root that exits at the foramen one level caudally.

Attention is then directed toward removal of the ligamentum flavum. A blunt, delicate dissector is used to elevate the rostral end of the flavum from the dura. If another opening has been made elsewhere, it can also be used. A curved curette is then used to establish a plane between the dura and ligament and can be used to detach the ligament from the bone along the edges of the laminotomy. Right-angle rather than 60-degree-angled punches are useful for removal of the ligamentum. The ligament can be detached laterally from the superior articular process with an up-angled curette. In cases of disk herniation, the nerve root may be distorted by the disk herniation and it may be necessary to carefully resect more of the medial zygapophysial joint in order to adequately visualize the shoulder of the nerve root. Once the shoulder of the root is exposed, gentle medial retraction usually reveals the disk fragment in extruded cases or the protruding annulus with a contained fragment. This step may be difficult if a tethering fragment is present at the axilla; careful dissection at the axilla and initial partial removal of a fragment there may be required. A combination nerve root retractor and sucker is used to retract the thecal sac medially, or a gentle, blunt-tip suction may be utilized. Extruded fragments are removed with a variety of disk punches. In case an extruded disk fragment is expected but not initially visualized, a spatula or blunt hook can be used to explore the ventral aspect of the dura and the level should be checked with fluoroscopy.

If the nucleus fragment is still retained under the posterior longitudinal ligament and annulus, once the root is retracted, epidural veins may be coagulated with the bipolar forceps in a low setting. Coagulation should be minimized around the root, as well as manipulation through the axilla. The annulus is incised and nucleus fragment removed with a disk forceps. It is our routine to not aggressively explore the interspace and remove contained fragment but just the disk material that is loose and removed easily. If the 30° endoscope is used for the procedure, the endoscope can be directed medially to help with decompression of more medially located disks, a maneuver not possible with the operating microscope. When dealing with calcified annulus or large osteophytes adjacent to the disk, a down-angled curette can be utilized to fracture the osteophyte into the annular opening and decompress the root. At the end of the decompression, a ball tip probe should be passed into the foramen to confirm adequacy.

Hemostasis is achieved while minimizing coagulation; simple tamponade with a number of different agents may be performed. We do not advocate closing the annular defect. The retractor arm is unlocked and the tube is removed under direct vision: bipolar cautery is used to coagulate more significant muscle arteries. Closure is performed in the usual fashion with fascial and subcuticular suture layers.

A tubular MIS approach is particularly useful in the management of foraminal or “far lateral” disk herniations as it obviates a very large muscular dissection. In this situation, the incision is made 5–6 cm lateral to midline rather than the 1.5 cm used for paramedian herniations. Dilation is performed as described above. Desired site for docking of the initial dilator is at the caudal edge of the medial transverse process or, alternatively, the lateral margin of the pars interarticularis. The monopolar cautery cannot be utilized in this setting as the root is exposed: a straight curette is utilized to detach the muscle fibers from the transverse process and sweep it in a lateral-caudal direction. Once the lateral limits of the neural foramen are defined, a lateral foraminotomy can be performed with the high-speed drill and Kerrison punches. The exiting nerve root can be identified at the cranial part of the foramen just caudal to the foramen. It is then retracted cranially, thus exposing the disk and diskectomy proceeds in the usual fashion. A greater or smaller degree of foraminotomy may be performed as desired based on the amount of disk expected to be removed (e.g., soft disk herniation versus calcified disk bulge with adjacent osteophytes).