Minimally Invasive Surgery and Navigation

Hemilaminotomy for Foraminotomy, Microdiscectomy

This is one of the most well described and familiar MISS techniques and is perfectly suited for precisely targeted foraminal and/or nerve root decompression. This technique is best suited for revision spine surgeries that involve persistent symptoms as a result of failed initial decompressive surgery, new foraminal stenosis or disc herniation adjacent to a previously fused segment, or recurrence of a disc herniation without evidence of instability. Limited level III evidence suggests that although both primary and revision minimally invasive microdiscectomies are associated with equivalent patient-reported clinical outcomes, revision cases are associated with a modestly increased operative time, length of stay, and immediate postoperative narcotic use.

One further important consideration is the incidence of incidental durotomy in revision minimally invasive decompressive procedures. Interestingly, although the literature for open discectomy procedures suggests a substantially increased risk of incidental durotomy in revision procedures, more recent studies on MIS microdiscectomy show equivalent rates between primary and revision procedures. This may perhaps be related to smaller working corridors, involving less disruption of scar tissue, and may stand to position MISS procedures favorably when considering a surgical plan.

Although multiple retractors are available for minimally invasive access, the critical portion of the case remains identical. Here we will describe the use of the Taylor and tubular retractor systems for this approach. The benefits of the Taylor retractor include ease of use, speed of exposure, and the ability to make significant trajectory adjustments once already exposed and docked on the facet. The main downside is the need for subperiosteal dissection, which for some surgeons would result in exclusion from what is considered MISS altogether. The patient is positioned prone on a Wilson frame to facilitate opening of the interlaminar space and allow free suspension of the abdomen. The surgical technique involves first taking a localizing lateral fluoroscopy image with a spinal needle in line with the disc space of interest. A midline incision the width of the Taylor retractor is made, centered upon the previously localized point. Making sure to preserve the supraspinous and interspinous ligaments, a limited subperiosteal dissection is performed and the paraspinal muscles are swept over the facet joint of interest. The Taylor retractor is inserted and docked to the lateral facet and secured to the operating table with a long piece of cotton gauze ( Fig. 11.1A ). A second localizing lateral fluoroscopy shot is taken to confirm appropriate placement of the retractor with the tip coaxial to the disc space of interest (see Fig. 11.1B ). The microscope is then brought into the field and important anatomical landmarks are identified (i.e., the spino-laminar junction, inferior border of the lamina, pars, and medial facet). For the indication of adjacent segment stenosis, the surgical anatomy will in all likelihood be normal and the procedure proceeds in the typical fashion. As in open surgery, for revision cases involving persistent or recurrence of foraminal stenosis or disc herniation, it is critical to identify normal dura by extending the laminotomy superiorly and then following this normal plane into the scarred area ( Box 11.2 ). This technique will minimize the risk of inadvertent nerve injury or durotomy.

Tubular access retractors result in less soft tissue trauma owing to a muscle spreading technique rather than subperiosteal dissection. Downsides of the tubular approach include the smaller working corridor, increased radiation exposure, a less flexible trajectory compared with the other retractors, and a substantial learning curve. That being said, tubular revision of a standard midline approach microdiscectomy has the benefit of a more lateral trajectory that avoids the midline scar tissue and theoretically may reduce the difficulty of the procedure.

Patient positioning is the same as described earlier. For the tubular approach, lateral fluoroscopy is used to target the intervertebral disc space using either a spinal needle or the initial dilator. The trajectory should exactly bisect the disc space, but at the same time special care must be taken to ensure the trajectory is perpendicular to the ground to ensure optimal surgical ergonomics. Centered on this point, a skin incision just longer than the preselected final dilator diameter (ranging 16 19 mm) is made 1.5 cm off midline, ipsilateral to the pathology of interest. The fascia is sharply incised and the initial dilator is inserted and docked at the junction of the inferior lamina and the upslope of the spinous process of the rostral vertebra. The appropriate trajectory is verified under lateral fluoroscopy and larger dilators are inserted sequentially until the final desired working diameter is reached. A final tubular retractor of the appropriate length and diameter is inserted and secured to the flexible mounting arm. Final anteroposterior (AP) and lateral fluoroscopy shots should be taken to ensure appropriate position, and the microscope is brought into the field ( Fig. 11.2 ). Any lingering muscle in the operating field is removed and the anatomy is carefully delineated, paying attention to the upslope of the spinous process and the inferior edge of the lamina. From here the procedure is performed in the standard fashion with considerations as described (see p.***).

Unilateral Laminotomy for Bilateral Decompression

The gold standard for the treatment of symptomatic lumbar stenosis is the midline laminectomy with medial facetectomies. This is a highly effective method to treat central, lateral recess, and foraminal stenosis bilaterally. This procedure, however, involves significant musculoligamentous disruption via bilateral subperiosteal dissection and complete disconnection of the posterior tension band that may result in segmental instability. The unilateral laminotomy for bilateral decompression or “ipsi-contra” approach was developed to achieve the same goals of lumbar laminectomy, while maintaining segmental stability. Level I evidence supports the clinical effectiveness of the ipsi-contra approach, and a long-term retrospective cohort study, with 6-years’ follow-up, documented a need for secondary fusion in only 4.5% cases. This finding supports the purported mechanical advantage of this technique.

With regard to its applicability in revision lumbar surgery, the ipsi-contra procedure may be performed in any case where a previous hemilaminotomy for decompression has been performed, as a type of “extended decompression,” if significant central and/or contralateral stenosis is present. It may also be used in rostral adjacent segment stenosis, without clear evidence of instability, in patients with a previous fusion to avoid a more extensive surgery and minimize the risk of further destabilization.

The exposure is identical to that described in the previous section (see p.***) and may be completed with either the Taylor or tubular retractor systems. We prefer the Taylor retractor because it does not require redirection to reach the contralateral lateral recess. The laminotomy proceeds in the typical fashion for an index case, extending from the inferior aspect of the lamina to the insertion of the ligamentum flavum rostrally, and as far laterally as the medial facet. For a level with a previous laminotomy, the laminotomy should be extended rostrally until normal anatomy is discovered to establish a normal tissue plane. The laminectomy is then extended medially using a combination of the matchstick burr and Kerrison rongeurs to undercut the spinous process. Typically, a cleft in the ligamentum flavum can be identified in the midline. The laminectomy proceeds toward the contralateral side and once the contralateral facet joint is reached the laminectomy is complete ( Fig. 11.3 ). Care should be taken to leave the ligamentum flavum intact, functioning as a protective barrier for the underlying dura. The contralateral foraminotomy may then be performed and the ligamentum removed with a Kerrison rongeur to complete the central and lateral recess decompression. In revision cases where a flavectomy has already been performed, any remaining ligamentum should be identified and a plane established between it and the bone. If the anatomy is too distorted or significant adhesions are present, the procedure may need to be aborted.