Lumbar Decompression and Instrumented Fusion Techniques

Clayton Haldeman

Daniel Resnick

Introduction

The goal of spinal decompression is to relieve pressure on the nerves and thereby prevent the progression of neurologic deficit, relieve pain, and in some cases, improve function. If degenerative changes, trauma, or iatrogenic maneuvers have led to spinal instability as well, fusion may be required in addition to decompression. Nerve compression can be due to multiple causes including spinal stenosis resulting in a narrowing of the central canal, foraminal stenosis resulting in narrowing of the neural foramen as the nerve root exits, or lateral recess stenosis causing impingement on the traversing nerve root preparing to exit the canal. Patients with lumbar stenosis commonly present with neurogenic claudication or radicular leg pain. The overwhelming majority of stenosis is due to degenerative changes and occurs secondary to encroachment of the canal by bony structures such as facet hypertrophy or osteophytes or by soft tissue structures such as disks, hypertrophied ligamentum flavum, or synovial cysts. Spondylolisthesis, the forward slipping of one vertebra relative to another, itself can cause stenosis or it can exacerbate pre-existing narrowing and worsen symptoms. Spinal stenosis is the most common reason for lumbar spine surgery in patients older than 65. Recent trials, such as SPORT, have shown that patients with lumber spinal stenosis and lumbar degenerative spondylolisthesis who are treated with surgery fair significantly better in terms of pain relief and improvement in function than those treated without surgery. This chapter will explore the various surgical techniques for lumbar decompression and instrumented fusion.

Lumbar Decompression

Lumbar Microdiscectomy

Lumbar microdiscectomy, also known as lumbar discectomy, is generally indicated for patients who are symptomatic, either from pain or neurologic deficit, from a herniated disk who have failed nonoperative treatment. Patients who undergo uncomplicated procedures ambulate the same day and often go home that evening or the next morning.

Procedures may be performed under local, regional, or general anesthesia depending upon patient and surgeon preference. The patient is positioned prone on a Wilson frame, Jackson table, or in a kneeling position on an Andrews table to decompress the abdomen and to open up the interlaminar space for easier exposure. The abdomen should be allowed to hang freely thereby reducing intra-abdominal pressure and epidural bleeding. The correct operative level is localized using radiographs after a spinal needle is placed under sterile conditions. It is convenient to prep an area much larger than the planned incision so it can be extended if necessary.

A small paramedian incision is made on the side of the disk herniation and taken down to the lumbar fascia. The fascia is opened a few millimeters off the midline and the medial fascia is elevated with a forceps. As the ligament is lifted off the spinous process, the paraspinal muscles can be detached from the spinous process exposing the yellow fat plane. Finger dissection allows exposure of the lamina without any muscle dissection. A thin bladed self-retaining retractor is used for soft tissue dissection and a microscope is brought into play to aid with visualization for the remainder of the case.

A radio-opaque marker is inserted beneath the caudal laminar edge and a confirmatory radiograph is obtained. In the author’s experience, deviation from the intended level as defined by the initial radiograph can occur in 10% to 15% of cases and bony resection should not begin until the level has been confirmed with bone visible. The medial edge of the facet is the extent of the lateral exposure and the facet capsule should not be disrupted. Typically, the inferior portion of the lamina of the level above (e.g., L5 for a disk protrusion at L5–S1) is drilled or resected with Kerrison rongeurs to create the hemilaminotomy. The ligamentum flavum, whose cephalad insertion is onto the anterior surface of the midpoint of the superior lamina and who’s caudal insertion is onto the superior portion of the lamina of the level below, is now visualized and can be opened with a microcurrette. The ligamentum is removed piecemeal with a 2-mm Kerrison rongeur.

With the thecal sac exposed, the lateral aspect of the shoulder of the nerve root is retracted medially to expose the disk herniation. A cruciate incision is made with an 11 blade through what is left of the annulus to enter the disk, which may herniate out through the defect in large protrusions. Micropituitary rongeurs and back angled currettes are used in combination to deliver the remainder of the disk. A pediatric feeding tube or disk space irrigator may be passed into the space for irrigation of any loose, remaining fragments. One last inspection is performed by passing a probe under the dura to ensure there is no residual compression.

Closure is facilitated by the 3- to 5-mm wide strip of fascia left attached to the spinous process during opening. We find the use of a small diameter semicircular urologic suture to be useful in closing the fascia though the small skin opening. Once the fascia is closed, a few 3-0 subcutaneous sutures and either a subcuticular closure or glue closure for the skin is performed.

Durotomy and resultant cerebrospinal fluid (CSF) leak have been reported to be as high as 5% to 8% in some series. However, in the senior author’s experience, for patients undergoing first time lumbar microdiscectomy, the durotomy rate is 0.2% (1/499). If durotomy does occur, attempt at primary closure should be made with a fine (5-0 or 6-0) suture. If direct repair fails or is not possible, a piece of muscle or fascia along with a fibrin sealant may be used. There is less than 1% chance of nerve root injury, vascular injury, or wound infection with lumbar discectomy.

When done for the proper indications, surgery for herniated lumbar disk has been shown to be beneficial. A number of studies have shown that 65% to 90% of patients get good to excellent outcomes compared to about 35% for those treated nonoperatively up to 2 years after the onset. Additionally, discectomy has been shown to be cost effective with approximately $20,600 per QALY gained.

Lumbar Laminectomy and Foraminotomy

Lumbar laminectomy is the most common spinal procedure performed in the United States. The goal is neural decompression through removal of the lamina and spinous process (Fig. 32.1). Medial facetectomy or foraminotomy can be added to the procedure when lateral recess or foraminal stenosis is also present.

The patient is placed prone on a Jackson table. The correct level should be identified with spinal needles prior to incision if a limited decompression is planned. In a multilevel procedure, radiographic confirmation of the level may be delayed until exposure of the first encountered spinous process. In any case, bone removal should not commence until radiographic confirmation of level has occurred. A midline incision is carried down to the deep fascia. The fat is reflected off and the fascia is incised just lateral to the spinous process so subperiosteal dissection can be carried down to the lamina. Electrocautery or periosteal elevator and sponge can be used to dissect the paraspinal muscles laterally until the medial aspect of the facet joint is exposed. This is done bilaterally when complete laminectomy is performed, but can be done unilaterally for a hemilaminotomy where the primary pathology is foraminal stenosis. Care should be taken not to disturb the joint capsule.

Rongeurs are then used to remove the spinous process along with the supraspinous and interspinous ligaments. A high-speed drill is then used to thin the lamina. The central portion of the laminectomy can be completed with a bone scalpel or with Kerrison punches. Resection of the ligamentum flavum can be completed using a Kerrison as well. Extension of the decompression to the lateral gutters can be carried out by undercutting the medial edge of the facet. The superior articulating process of the caudal vertebrae forms the roof of the lateral recess and the ligamentum flavum attaches to its medial portion. To ensure adequate decompression of the lateral recess, the resection should continue out laterally until the ligamentum flavum is released.

As progress is made, epidural bleeding may be encountered due to decompression of the epidural veins, which can be controlled with gelfoam. Foraminotomies are accomplished by following the traversing nerve root down along its path, using a Kerrison to widen above and below the pedicle. When the shoulder is well visualized, and when a ball tipped probe or angled curette can pass along the trajectory of the nerve, the foramen can be deemed well decompressed.

The SPORT trial examined over 600 patients (278 randomized and 356 observational) who had neurogenic claudication or radicular leg pain for at least 12 weeks duration. At both 2 and 4 years follow-up, patients who underwent surgery showed significantly more improvement in bodily pain, oswestry disability index, and physical function compared to those who did not undergo surgery.

Figure 32.1 Lumbar decompression. Sagittal (A) and axial (B) views of lumbar spine in an 82-year-old man with an 18-month history of worsening neurogenic claudication. He presents with an anthropoid gate and is unable to walk without a cart or walker for more than 50 yards. His pain is relieved by bending or sitting and he can push a grocery cart “all day” as long as he has something to lean on. His NRS pain score averages 5/10 and his Oswestry disability index is 50/100. He has a normal alignment of the spine and was treated with an L2–L5 decompressive laminectomy. At 6 weeks, he reported the ability to walk one mile and 0/10 pain. At 3 months he reported no functional disability. He will be reassessed for his 1-year follow-up when he returns from Florida this spring.

Lumbar Fusion

When spinal instability is present following nerve root compression, a fusion is indicated (Fig. 32.2). This is most commonly seen in patients with degenerative spondylolisthesis and spinal stenosis. Most fusions are now supplemented with some sort of internal fixation device as rigid pedicle screw fixation and/or interbody fusion is associated with higher fusion rates than noninstrumented posterolateral fusion. Other advantages often cited of instrumented fusion over noninstrumented fusion are lack of a need for postoperative bracing, earlier mobilization, reduced postoperative pain, and reduced hospital length of stay.

It is unclear if increased fusion rates correspond to improved functional outcomes, however. The decision to use instrumentation or interbody techniques should be based on the particular anatomy and demands of the patient. Younger patients with greater disk space height, mobility at the pathologic level, and higher demand occupations likely benefit from more aggressive surgical strategies. Older patients, those with “stable” spondylolisthesis, those with collapsed disk spaces, and those with sedentary lifestyles may benefit from decompression alone or noninstrumented fusion strategies.

Noninstrumented Fusion

Exposure and decompression are performed as described above. If instrumentation is not being used, it is not advisable to resect the entire facet as this can lead to immediate postoperative instability. The exposure should be carried out to reveal the entirety of the facet and the transverse processes. A high-speed burr is used to decorticate the pars, the dorsal and lateral portion of the facet joint, and the dorsum of the transverse processes. Locally harvested autograft is usually sufficient and is cleaned and morcelized. If additional graft is required, iliac crest may be harvested or a bone graft extender may be used. There is some suggestion in the literature that allograft may not fuse as quickly or completely. Once the graft is placed over the entirety of the decorticated area, the paraspinous muscles are replaced over the graft and the wound is closed in layers.

Lumbar Pedicle Screw Placement

In patients selected for instrumentation, decompression may be more aggressive than in those undergoing noninstrumented fusion. The instrumentation will provide immediate stability and therefore is useful in cases where preoperative instability exists or in cases where complete
facet resection is planned. The junction of the transverse process and the lateral aspect of the superior facet is the entry point for pedicle screw placement in the lumbar spine. Individual patient anatomy varies and intraoperative imaging is very useful in determining appropriate angles in the sagittal plane. On average, the L4 pedicle is angled 0 degrees in the rostral–caudal direction, or said another way, perpendicular to a straight line running from the patient’s head to his toe. Due to lordosis, L5 pedicle has 5 to 10 degrees caudal angulation, and the L3 pedicle has 5 to 10 degrees rostral angulation. Intraoperative fluoroscopy or CT guidance can be used to exactly define the sagittal trajectory, or if decompression has taken place, the trajectory of the pedicle can be felt with a probe. In the coronal plane, the pedicles tend to “medialize” as they march caudally. L1 is typically angled 5 degrees medially, and each subsequent pedicle is angled an additional 5 degrees (Table 32.1). Screws should be sized so that the diameter is 70% to 80% of the pedicle and the length should be 60% to 80% of the vertebral body. Preoperative measurements are confirmed with intraoperative radiographs.

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