Interlaminar Surgical Approach for Percutaneous Endoscopic Laser Annuloplasty/Nucleoplasty

7 Interlaminar Surgical Approach for Percutaneous Endoscopic Laser Annuloplasty/Nucleoplasty

Akarawit Asawasaksakul, Alfonso García, Ketan Deshpande, and Gun Choi

7.1 Introduction

Apart from the transforaminal approach for percutaneous endoscopic laser annuloplasty/nucleoplasty, currently the interlaminar approach is one of the most popular approaches for percutaneous decompression procedures. In 2006, Choi et al 1 first reported the successful endoscopic removal of an L5–S1 disk herniation using this approach. In 2008, Ruetten et al² compared endoscopic diskectomy with microdiskectomy and found comparable clinical results but less tissue trauma with the endoscopic technique. Recently, many authors have also reported the application of this approach and successful outcomes.^3,4,5,6

7.2 Anatomical Considerations

Thorough knowledge of anatomy is an essential requirement in every surgical specialty.

• Endoscopic spine surgery is a target-oriented surgery that relies on a precise mental projection of a pathologic lesion and its relations to surrounding bony landmarks.

• Unlike in microsurgery, in endoscopic surgery the surgeon does not have the liberty of visually identifying the bony landmarks and then going through them to find the neurologic structures.

• The surgeon must know the relation of the various neural structures to the surrounding bone and must rely heavily on fluoroscopic guidance to insert the needle at the exact target point identified on the preoperative plan while avoiding all the important anatomical structures lying in between.

• Another important aspect of endoscopic spine surgery is familiarity with the endoscopic appearance of the various anatomical structures.

This chapter describes percutaneous interlaminar endoscopic diskectomy at the L5–S1 level in three sections:

• Discussion of the unique anatomical features of the L5–S1 segment that make it amenable to the interlaminar approach.

• Description of the methods used to project various important anatomical structures (as defined on CT and MRI) onto the radiographs and to make the preoperative plan.

• Description of the endoscopic appearances of various anatomical structures seen during the procedure so the reader can have a fair idea of what to expect before embarking upon the procedure.

The interlaminar approach applies not only to the L5–S1 level, but also to other levels.

7.3 Unique Features of the L5–S1 Anatomy

The L5–S1 level has unique anatomical features that apply to interlaminar endoscopic diskectomy:

• Most of the lumbar disks have a laminar overhang, meaning that the lamina of the upper vertebra extends inferiorly so that the disk space lies at a level relatively superior to the lower margin of the lamina. However, the laminar overhang over the disk space decreases from the upper lumbar to the lower lumbar levels. At the L5–S1 level, the cephalocaudal distance between the lower margin of the L5 lamina and the upper margin of the L5–S1 disk space varies from 3.0 to 8.5 mm. This is the smallest laminar overhang of the lumbar levels.

• The small laminar overhang at the L5–S1 level creates a relatively larger interlaminar space.

• The inferior margins of the upper lamina lie at a level relatively posterior to that of the superior margins of the lower lamina in the whole of the lumbar spine. This difference is seen more clearly at the L5–S1 level than at the other levels.

• In combination with a wider interlaminar space and a negligible laminar overhang, this arrangement creates a trapezoidal configuration that allows more working space for the outer cannula and its manipulation during the procedure, especially if the initial needle trajectory is kept at a 5 to 10° caudocranial angle.

• The maximum interlaminar width, as defined by the distance measured between the most inferomedial aspects of the inferior facets, is also greater at the L5–S1 space than at the upper levels (Fig. 7.1).

• The average interlaminar width at L5–S1 is 31 mm (range 21 to 40 mm), as compared to an average width of 23.5 mm at the L4–L5 level, due to the relatively wider laminae of L5.⁷

• The wider interlaminar width provides for easy passage of the working cannula (Fig. 7.1).

• The S1 nerve root has a relatively cephalad exit from the thecal sac compared with the upper lumbar levels. The S1 nerve root exits from the thecal sac at the level of the L5–S1 disk space or above it. In their cadaver analysis of the origin of lumbar spinal roots in relation to the intervertebral disk, Suh et al reported that the S1 nerve root originated above the level of the L5–S1 disc in 75% of the subjects and at the level of the disk in 25%, but never below the level of the disk.^8,9,10,11

Fig. 7.1 Illustration of the maximum interlaminar width at the L5–S1 level.

Fig. 7.2 Illustration of the S1 nerve root, which takes off at a relatively smaller angle from the thecal sac.

Fig. 7.3 Illustration of an axillary disk herniation, which can displace the S1 nerve root far into the subarticular region, creating a potential space between the thecal sac and the nerve root.

Fig. 7.4 Illustration of a shoulder disk herniation at the L5–S1 level, which pushes the S1 nerve root medially, toward the thecal sac.

• The S1 nerve root average take-off angle from the thecal sac is 17.9 ± 5.8 degrees. Although this angle is relatively less than that at the upper lumbar levels, an L5–S1 disk herniation is more likely to be axillary because of the cephalad exit of the S1 nerve root in front of the L5–S1 disk space (Fig. 7.2).^10,11

• Axillary disk herniation can also displace the S1 nerve root far into the subarticular region, creating a potential space between the thecal sac and the nerve root. This artificial space created by the pathologic lesion can be gainfully exploited for carrying out a safe interlaminar endoscopic diskectomy (Fig. 7.3).

• Shoulder disk herniation at the L5–S1 level is relatively uncommon; in this case, the herniated disk pushes the S1 nerve root medially, toward the thecal sac, and the needle can be targeted directly over the hernia mass lying over the supermodel aspect of the pedicle (Fig. 7.4).

• The ligamentum flavum is a 2- to 6-mm thick, yellow structure that spans the interlaminar space. It is an active ligament that has an essential biomechanical role. It also acts as a protective barrier for the thecal sac, and any injury to it is probably not without consequences.¹²

• Peridural fibrosis is the direct consequence of intrusion into the spinal canal with a break in the ligamentum flavum because peridural fibrosis occurs due to fibroblasts derived from overlying detached muscle that have gained access to the spinal canal.^12,13,14

• Although the ligamentum flavum is thinnest at the L5–S1 level, it is the only major protective barrier for the neural structures at this level because of the minimal laminar overhang. Therefore, preservation of the integrity and continuity of the ligamentum flavum at the L5–S1 level is most important.

• During L5–S1 percutaneous interlaminar endoscopic diskectomy, splitting the fibers of the ligamentum flavum longitudinally and then widening the hole by the passage of sequential dilators creates an opening. Upon withdrawal of the working cannula and endoscope, the opening in the ligamentum flavum closes spontaneously and restores the continuity of the protective barrier.