Surgical Treatment of Far Lateral Lumbar Disk Herniation

In 1932 Schmorl published a manuscript in which he described common pathological changes of the lumbar intervertebral disk as found at autopsy. He famously described that in ~ 38% of cadavers, there was a herniation of the nucleus pulposus through the end plate of the adjacent vertebral body, which is now referred to as a Schmorl’s node. He also described finding posterior protrusions of the nucleus pulposus beneath the posterior longitudinal ligament in ~ 15% of the spines he examined. Although Schmorl believed that these posterior intervertebral disk herniations were rarely symptomatic, Mixter and Barr published a series of 19 patients and concluded that intervertebral disk rupture is in reality a common cause of lumbar nerve root compression and sciatic pain ¹ ². Although these early landmark papers provided the first insight into the clinical importance of paracentral lumbar intervertebral disk herniations, foraminal and extraforaminal (far lateral) disk herniations remained an elusive diagnosis until the 1970s because they did not appear on routine myelography. In 1974 Abdullah et al first described the clinical syndrome of far lateral disk herniation in a series of 24 patients who had lumbar disk herniations that were either beneath or beyond the facet complex. Abdullah determined that the clinical syndrome of these disk herniations—much like their paracentral counterparts—included unilateral radicular pain or sensory loss and variable reduction in the patellar reflex. Low back pain and signs of lumbar nerve root traction, such as a positive Lasègue’s straight-leg-raise test, were absent. The authors also observed that bending toward the side of the lesion reliably resulted in reproduction of radicular pain in 83% of patients with far lateral disk herniations. ²^, ³ Subsequently, in the 1980s, with the advent of high-resolution computed tomography (CT), increasing reports of foraminal and extraforaminal disk herniations made their way into the literature. These studies estimated the incidence of far lateral disk herniations to range from 1 to 11.7% of all disk herniations. ²^, ³^, ⁴^, ⁵^, ⁶^, ⁷

When nonoperative treatment strategies are unsuccessful in patients with intractable symptoms attributable to these lesions, excision of the disk herniation is recommended. Numerous surgical approaches have been described in the literature, and in this chapter we describe several of the more commonly accepted techniques. More specifically, we describe a midline standard interlaminar approach, which can be combined with resection of the facet or the pars interarticularis, a direct extraforaminal paraspinal muscle-splitting approach, and also minimally invasive percutaneous and endoscopic techniques. These techniques may be modified or used in combination, depending on the patient’s symptoms, anatomy, and medical comorbidities. For example, patients with spinal stenosis or nerve root compression within the spinal canal may benefit from combined approaches where the surgeon works both medial and lateral to the foramen. Also, preservation of the facet complex may be more important in patients with degenerative spondylolisthesis at the level of the far lateral disk herniation to minimize the risk of iatrogenic instability. These topics are discussed in more detail ( ▶ Fig. 44.1).

Fig. 44.1 Axial cut of high-resolution computed tomography demonstrating a left-sided extraforaminal disk herniation at L5–S1. Myelography in this patient was normal because the disk protrusion is too far lateral to affect the opaque column.(Reproduced with permission. ⁴)

44.2 Anatomy and Nomenclature

In previous publications, the nomenclature used to describe far lateral disk herniations has been inconsistent, which has sometimes led to confusion. For our purposes, we classify disk herniations from medial to lateral as either one of four types: central, paracentral, foraminal, and extraforaminal. The paracentral disk herniation is the most common, and the extraforaminal disk herniation is also referred to as a far lateral herniation. Large disk herniations can occupy two or more adjacent locations, and this may influence the surgical approach. The critical landmark in this system is the facet. Its importance derives from its role in maintaining stability. In addition, the presence of the pedicle prevents the typical caudal migration of disk fragments in the foraminal or extraforaminal locations. ³

Far lateral disk herniations are known to occur more frequently in slightly older patients, with an average age range from 44 to 57. ³ They also tend to affect the upper lumbar spine more frequently than their paracentral counterparts, with more than 75% of far lateral disk herniations seen at L3–4 or L4–5 and only 17% seen at the lumbosacral junction. ³ Far lateral disk herniations are also more likely to be sequestered fragments, and their development may be related to disk degeneration in combination with a greater degree of facet joint asymmetry ⁸ ( ▶ Fig. 44.2).

Fig. 44.2 Diagram illustrating the relationship of a far lateral disk herniation at L4–5 to the thecal sac and the exiting L4 nerve root.(Reproduced with permission. ²)

44.3 Patient Selection

As with routine lumbar disk herniations, it is important for patients to receive nonoperative therapy for at least 6 weeks before surgery unless there is a significant neurologic deficit. Nonoperative therapy for patients with far lateral disk herniation includes anti-inflammatory medications, analgesics, muscle relaxants, nerve root injections, and physical therapy. Unfortunately, the success rate of both surgical and nonsurgical care for far lateral disks is lower than for paracentral disks. ⁹ This is a reflection of the exquisite sensitivity of the dorsal root ganglia, which is often directly compressed by a far lateral disk herniaton. ⁷ One study estimated that nonoperative measures are successful in 71% of patients with far lateral disk herniations. ¹⁰ Epstein found that that excellent or good outcomes were achieved in only 68 to 79% of patients. ¹¹

44.4 Preoperative Preparation and Operative Procedure

44.4.1 Standard Interlaminar Approach in Combination with Medial Facetectomy, Complete Facetectomy, or Removal of the Pars Interarticularis

Many far lateral disks occupy the extraforaminal location with some extension into the foramen. Some authors have recommended approaching these lesions with an extension of the standard lateral laminotomy and medial facetectomy used for paracentral disks. Although this approach does not give direct access to the far lateral space, the tail end of an extruded fragment might be found within the foramen. By gently pulling on this tail, the extraforaminal fragment could possibly be extracted. ¹¹ In addition, using this approach, the surgeon can safely internally decompress the disk working ventral to the anulus, thereby avoiding injury to the exiting nerve root. The nerve root may also be decompressed by opening the dorsal side of the foramen with a Kerrison rongeur without placing the exiting root in jeopardy. ¹² This may be accomplished without compromising the structural integrity of the facet by standing on the side of the patient opposite the disk herniation and reaching under the facet joint with the Kerrison. This technique is more easily accomplished via an open approach than it would be with the use of a minimally invasive tubular retractor because a tubular retractor would have to be moved to the contralateral side requiring a second incision ( ▶ Fig. 44.3).

Fig. 44.3 Illustration of the contralateral approach for decompression of the neural foramen (black arrow) compared with facetectomy (dashed arrow).(Reproduced with permission. ¹²)

In some cases, however, a medial facetectomy alone does not provide adequate exposure for disk removal, and in these situations, it may be necessary to perform a complete facetectomy to gain access to the disk herniation. Performing a complete facetectomy introduces a risk of iatrogenic instability; however, several large series have found that the likelihood of instability requiring fusion is only 1.67 to 2.4% after complete or partial unilateral facetectomy. ¹³^, ¹⁴ Nonetheless, some authors still recommend fusing in all patients who require a complete unilateral facetectomy. ¹⁵ At the minimum, the surgeon should consider performing a primary instrumented fusion after unilateral facetectomy in patients with degenerative spondylolisthesis at the same level, as those patients may be more likely to develop mechanical instability. ¹¹^, ¹⁶^, ¹⁷ For example, Bridwell et al published a series of 44 patients with both spinal stenosis and degenerative spondylolisthesis in 1993. They found that performing a posterior decompression without fusion led to progression of spondylolisthesis in 44% of patients compared with only 4% of patients who underwent instrumented fusion. They also found that spondylolisthesis progression was associated with worse clinical outcomes. ¹⁶ Although the patients described Bridwell et al underwent facet-preserving surgeries, their study illustrates that maintaining the structural integrity of the posterior elements is critical to preserving mechanical stability in patients with degenerative spondylolisthesis ( ▶ Fig. 44.4).

Fig. 44.4 Illustration of a right-sided hemilaminectomy and L5–S1 complete facetectomy for resection of a right L5–S1 far lateral limbus fracture.(Reproduced with permission. ¹¹)

Some authors have also chosen to perform a foraminotomy by resecting the lateral aspect of the pars interarticularis. ⁷ To perform this, a midline incision is made, and the paraspinal muscles are dissected off the spinous process and posterior elements to the facet joints. The lateral margin of the foramen is defined using an up-angled curet, and the lateral aspect of the pars interarticularis is subsequently removed using a high-speed drill. If this procedure is performed in conjunction with a laminotomy, the surgeon must be cautious to leave a strut of bone connecting the lamina to the inferior articulating process. ⁷ The bone removal should extend superiorly to the pedicle above the foramen. Palpating the medial border of this pedicle will give the surgeon a better understanding of the location of the spinal canal. The most lateral aspect of the ligamentum flavum is then removed with a Kerrison rongeur, exposing the nerve root, dorsal root ganglion, and far lateral disk herniation. The nerve root is then protected, the posterior longitudinal ligament is opened, and a diskectomy is performed in the usual manner using nerve hooks, curets, and pituitary rongeurs. When the surgeon is satisfied with the decompression, a standard closure is performed ⁷ ( ▶ Fig. 44.5).

Fig. 44.5 Illustration of the boney resection of the pars interarticularis required to expose the far lateral space.(Reproduced with permission. ⁷)

44.4.2 Paraspinal Muscle-Splitting Approach

Far lateral disk herniations may be approached more directly by splitting the sacrospinalis muscle group and following an intermuscular cleavage plane between the multifidus and longissimus to the posterolateral aspect of the spinal column as described by Wiltse et al in 1968. ¹⁸^, ¹⁹ To perform this operation, the patient is first positioned prone on a frame that allows the abdomen to swing freely. For cases that require a central decompression in addition to the excision of the far lateral disk herniation, a Wilson frame may be preferable to open the interlaminar space. If no central decompression is required, an open Jackson frame will work equally well. The surgical site is then prepared and draped in a routine manner, and a 3-cm-long paramedian incision is made 2 cm lateral to the midline centered over the disk herniation. It may be desirable to create a midline incision for cosmetic reasons, especially if the patient already has a midline scar from prior spinal surgeries. In this case, the skin incision is carried down to the thoracolumbar fascia, and the skin is retraced laterally so that the fascial opening can be created in the correct location 2 cm from midline. The posterior layer of the thoracolumbar fascia is then opened with Metzenbaum scissors, and a natural cleavage plane between the multifidus and longissimus muscle is located. It may be easiest to locate this plane at the L4–5 level because below this the muscle fibers of the multifidus travel laterally to connect with the heavy combined fascia of the longissimus and iliocostalis lumborum muscles. ¹⁹ A finger is then used to bluntly dissect this plane, and the surgeon should be able to feel the lumbar facet joints and transverse processes (or sacral ala in the case of L5–S1 disk herniations) above and below the disk herniation. A self-retaining retractor, such as a standard anterior cervical retractor or Gelpi retractor, is then inserted into the wound. If an anterior cervical retractor is used, the shorter blade should be placed over the facet joint, and the longer blade should be placed in the plane between the intertransverse ligament and the longissimus muscle. ²⁰ This procedure may also be performed with the use of a table-mounted minimally invasive tubular retractor. If this type of retractor system is used, the smallest dilator should be docked at the junction of the pars and the transverse process. When using a tubular dilator, the use of an operating microscope or endoscope is required. ²¹^, ²² For an L4–5 far lateral disk herniation, the lateral aspect of the superior articulating process, medial portion of the transverse process, and pars of L4 are denuded with monopolar cautery, releasing the attachment of the multifidus muscle. Care should be taken to preserve the facet capsule, and monopolar cautery should be used only dorsal to the transverse processes to prevent iatrogenic injury to the spinal nerve root as it exits the neural foramen. The posterior lumbar artery and accompanying veins may be exposed in the ventral portion of the surgical field. ²⁰^, ²³ These vessels may be coagulated with bipolar cautery, if needed, although exposure caudal to the posterior lumbar artery may not be necessary. A localizing film should be taken at this stage to confirm the correct level. A forward-angle curet is then used to define the dorsal aspect of the neural foramen and palpate the pedicle of the level below. A high-speed drill is then used to remove 2 to 3 mm of the lateral aspect of the pars articularis. A match-head bit may be used because it produces significantly less heat than a diamond bur. If the patient has hypertrophic facets, some amount of the facet complex below, in this case the L5 superior articulating process, will have to be removed to gain access to the pars. Once the lateral few millimeters of the pars have been removed, the lateral extent of the ligamentum flavum, called the falciform ligament, comes into view. There is a free plane underneath it, and it can be removed using a Kerrison punch. After the falciform ligament is removed, the dorsal root ganglion and nerve root, which may be displaced posteriorly and laterally, come into view. The pedicle above—in this case, L4—can be palpated with a nerve hook to facilitate orientation while the root is dissected free of soft tissue. Once that has been accomplished, the root can be tracked distally to the disk herniation. The identification of the nerve root at the foramen is not strictly necessary because the compression from the extraforaminal disk herniation is further lateral. Without knowing where the root is located, however, the dissection from the facet to the disk is tedious, and excessive manipulation of the dorsal root ganglion might result in painful postoperative dysesthesias. ³ Therefore, the extra bit of work to identify the nerve as it exits the foramen can add to both the safety and speed of the operation. An alternative method for finding the spinal nerve is to identify the posterior primary ramus as it passes through the intertransverse membrane during exposure. Following this structure proximally should enable the surgeon to locate and protect the nerve and dorsal root ganglion. ²⁴ Once the disk herniation has been exposed, it is removed with standard techniques. If the disk herniation is broad, the surgeon may need to perform a partial laminectomy to approach the ruptured disk from both sides of the pedicle. Opening the spinal canal also enables the surgeon to probe the neural foramen inside and out to ensure that all disk fragments have been removed. ²⁵ After hemostasis is obtained, the fascia, subcutaneous tissue, and skin are closed in the usual fashion. ¹⁹^, ²⁶

This approach is more challenging at L5–S1 than at the levels above as a result of the increased distance from the skin to the target and the orientation of the paraspinal muscles, which becomes more oblique rather than parallel to the spinal column. In addition, the superior portion of the sacral ala many times must be removed for adequate exposure. ¹⁹ Fortunately, far lateral disks at the lumbosacral junction occur less frequently ² ( ▶ Fig. 44.6, ▶ Fig. 44.7).

Fig. 44.6 The paraspinal approach for resection of a far lateral disk herniation. In this case, Gelpie retractors are being used to retract the multifidus medially and the longissimus laterally.(Reproduced with permission ¹⁸.)

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