Far-Lateral Microdiscectomy

Lee A. Tan, Carter S. Gerard, Mick J. Perez-Cruet, and Richard G. Fessler

Abstract

Far-lateral lumbar disc herniations are much less common than paramedian disc herniations and they are often missed. They are estimated to account for about 2 to 12% of all symptomatic lumbar disc herniations. They are characterized by the presence of herniated disc fragments in the foraminal or extraforaminal space, which causes impingement of exiting nerve roots at the same level. This feature is in contrast to the more common paramedian disc herniations, in which the descending nerve roots are often affected and the exiting nerve roots are spared. Minimally invasive approach for far-lateral disc herniations can achieve excellent clinical outcome with minimal soft tissue and bone resection, decreased blood loss, quicker recovery, and shorter hospital stay. In this chapter, the minimally invasive technique is described in detail along with a case illustration and various clinical pearls.

Keywords: far-lateral, disc herniation, microdiscectomy, lumbar spine, minimally invasive

34.1 Introduction

Far-lateral lumbar disc herniations (FLLDHs) were first described by Abdullah et al in 1974 ¹ and are estimated to account for 2 to 12% of all symptomatic lumbar disc herniations.² They are characterized by the presence of herniated disc fragments in the foraminal or extraforaminal space, which causes impingement of exiting nerve roots at the same level ( Fig. 34.1). This feature is in contrast to the more common paramedian disc herniations, in which the descending nerve roots are often affected and the exiting nerve roots are spared. Patients with FLLDHs often present with severe, acute onset radicular leg pain, which is thought to be due to direct compression of the dorsal root ganglion by the herniated disc fragment.³ The pain is often described by patient as excruciating, severe, lancinating, and burning pain that is most likely due in part to compression of the dorsal root ganglion. The peak incidence of FLLDHs is in the sixth decade of life.⁴ They tend to occur at more proximal levels compared to paramedian disc herniations, with L4–L5 being the most common comprising 30 to 60% of cases.⁵ MRI can often reveal the herniated disc fragments in the foraminal or extraforaminal space ( Fig. 34.2), whereas these lesions can often be missed on CT or myelography.²

Surgical management of FLLDHs can be technically challenging due to the difficulty in gaining access to the foraminal and extraforaminal space, largely due to the important biomechanical roles the adjacent bony structures play in maintaining spinal stability. Traditional surgical approaches include midline approach, paramedian muscle-splitting or Wiltse’s approach, as well as the combined approach. The use of percutaneous approaches to FLLDH has also been described previously,⁶^,⁷ but the efficacy of these procedures has been questioned.⁸^,⁹

The advantages of the traditional midline approach include the familiar local anatomy by most spine surgeons and good visualization of the foraminal and extraforaminal space after removing the facet and/or pars interarticularis.¹^,¹⁰^,¹¹^,¹² However, varying degrees of bony resection required for exposure using this approach can often lead to destabilization of the spinal segment.¹³^,¹⁴ The Wiltse’s approach¹⁵^,¹⁶ offers a more direct approach to the neuroforamen that spares the facet. However, this muscle-splitting approach is less familiar to many spine surgeons and it lacks the usual anatomical landmarks known from traditional midline approaches. Therefore, the paramedian approach has not gained wide acceptance. The combined approach allows the surgeon to work both medially and laterally to the foramen, therefore making the removal of the pars and facet unnecessary.¹³^,¹⁴ However, the combined approach requires extensive stripping and lateral retraction of the paraspinal muscles, which equates to larger incisions and more soft-tissue injuries. The paraspinal muscular dysfunction and disability resulting from midline posterior spinal exposures have been well documented by various authors.¹⁷^,¹⁸^,¹⁹^,²⁰^,²¹^,²²

Fig. 34.1 Presence of herniated disc fragments in the foraminal space, which causes impingement of exiting nerve roots at the same level.

Fig. 34.2 (a) Axial and (b) sagittal views demonstrating the herniated disc fragments in the foraminal space with severe foraminal stenosis.

With the advancement in minimally invasive surgery (MIS) in recent years, MIS techniques have been utilized to treat FLLDHs with excellent results.²^,²³^,²⁴ The MIS approach, utilizing either endoscope or microscope, provides direct access to the foraminal and extraforaminal space with little need for soft-tissue/muscle dissection, little bony removal, and minimal blood loss, and it does not destabilize the spine. We prefer the endoscope because the 30-degree viewing angle of the lens allows one to view beyond the confines of the tubular retractor. However, the microscope allows for excellent three-dimensional visualization and may be more readily adopted by surgeons. This muscle-splitting technique, which was initially developed to approach standard lumbar disc herniations, is also ideal for application to far-lateral disc herniations.⁹

34.2 Indications and Contraindications

As a general principle for any surgical procedure, proper patient selection is paramount to a good clinical outcome. The management scheme for FLLDHs is similar to paramedian disc herniations. Symptomatic patients with corresponding MRI findings should undergo a trial of conservative treatment including nonsteroidal anti-inflammatory drugs (NSAIDs), oral steroids, physical therapy, and epidural steroid injection.²⁵ Patients with persistent and intractable radicular pain or progressive weakness after a course of conservative management should be considered for microdiscectomy.⁵^,²⁶ Patients with significant stenosis, degenerative spondylolisthesis, and concurrent FLLDHs should be considered for laminectomy, full facetectomy, discectomy, and interbody fusion.²³ Good or excellent clinical outcomes after the surgical management of FLLDH range from 68 to 100%.²⁷^,²⁸^,²⁹ The MIS technique has been demonstrated to have comparable clinical outcomes with open approaches, with the added benefits of reduced blood loss, less soft-tissue injury, faster recovery, and shorter hospital stay.²^,²³

34.2.1 Preoperative Planning

Far-lateral disc herniations often present more severe radicular symptoms than back pain. The symptom onset is usually acute. There are often absent knee-jerk reflex, dermatomal sensory loss, positive straight leg raise test, positive femoral stretch test, and reproduction of pain and/or paresthesia by lateral bending to the side of the lesion.⁵^,²⁶ MRI is the imaging modality of choice; CT and myelography can often give false-negative results which can occur in as much as 25% of cases.²⁶ Flexion–extension X-rays can also be helpful to rule out instability at the level in question.

34.3 Surgical Technique

34.3.1 Patient Positioning

The patient is brought into the operating room and placed in the prone position after undergoing general endotracheal anesthesia. Wilson’s frame can be used to place the spine in flexion, therefore widening the interlaminar space. A C-arm fluoroscope is draped into the sterile field for lateral visualization during the approach.

34.3.2 Technique

After the operative field has been prepared and draped, a tubular retractor holder consisting of an articulating arm is attached to the operating table rail ipsilateral to the disc herniation. The midline is identified and a 2-cm incision is made 4 to 5 cm lateral to the midline at the ipsilateral side of disc herniation. The target area for docking the tubular retractor is at the junction of the transverse process and pars interarticularis of the vertebra rostral to the disc herniation (e.g., the L4 vertebra for a far-lateral disc herniation at L4–L5). The preoperative MRI images can be helpful in measuring the approximate distance of the incision from the midline and planning the trajectory of the tubular dilators.

The appropriate spinal level is localized and confirmed using fluoroscope. A Kirschner wire (K-wire) is inserted through the incision and the lumbodorsal fascia, directed toward the junction of the transverse process and pedicle of the superior vertebra ( Fig. 34.3a). The initial cannulated dilator is then placed over the wire through the lumbodorsal fascia and then the K-wire is withdrawn ( Fig. 34.3b). The initial dilator is docked on the base of the cephalad transverse process at its junction with the pars. Medial–lateral orientation is determined by palpating the bony landmarks with the tip of the dilator. Anteroposterior (AP) X-rays can be used to confirm the medial–lateral placement of the tube if necessary.

Sequential dilators are passed, followed by placement of the tubular retractor ( Fig. 34.3c–f). Particular care must be taken to stay docked on bone during the insertion of the dilators and the tubular retractor. The dilators are then removed and the tubular retractor system is connected to the articulating arm holder. It is important to maintain firm downward pressure on the tubular retractor while it is connected to the articulated arm and when the arm is tightened. This maneuver prevents soft tissue from flowing underneath the edges of the tubular retractor.