Far-Lateral Lumbar Discectomies

33 Far-Lateral Lumbar Discectomies

Eric H. Elowitz

Summary

Herniated lumbar discs are a major cause of pain and disability frequently affecting younger age patients. Although the most common location of herniated discs is within the spinal canal, in either the paracentral or central space, some disc herniations are more lateral and occur in the foraminal or extraforaminal region. These disc herniations can present with severe pain, often with a dysesthetic quality due to compression of the dorsal root ganglion. Spine surgeons are generally very comfortable with the interlaminar surgical approach for paracentral disc herniations. To approach the far-lateral disc herniations, an extraforaminal approach is the most direct method and avoids an iatrogenic destabilization of the spine. The minimally invasive tubular extraforaminal approach, as described, involves docking of the dilators and tubular working channel on the lateral pars. The pars and inferior facet are drilled exposing the ligamentum flavum. The ligament is opened at the most rostral region adjacent to the upper pedicle exposing the Kambin’s triangle region. This minimally invasive approach takes advantage of the anatomy of the pars interarticularis and ligamentum flavum and can be performed in a safe and reproducible manner.

Keywords: extraforaminal far-lateral herniated disc herniated lumbar disc microdiscectomy

33.1 Introduction

Herniated lumbar discs are a major source of radicular symptoms. Approximately, 90% of disc herniations occur within the spinal canal affecting the paracentral and central compartments, typically compressing the traversing nerve root or thecal sac.1 A distinct minority of disc herniations and pathologies occur within the neural foramen or extraforaminal space.¹^,² These herniations can compress the exiting nerve root, frequently at the level of the dorsal root ganglion.³ Mechanical nerve root compression caused by these “far-lateral” disc herniations can cause extreme pain and disability for patients. The anatomy of this region was described by Parviz Kambin,⁴ and is delineated by the exiting nerve root, superior end plate of the caudal vertebral body, and the traversing nerve root or thecal sac. This “Kambin’s triangle” is a critical anatomic area of the spine and a source of clinical pathology.

Lumbar microdiscectomies are a common surgery on the spine and are often performed in younger age patients.⁵ The interlaminar anatomy and approach are well known to spinal surgeons. The use of minimally invasive tubular retractors have become quite commonplace for these disc pathologies.⁶^,⁷ However, far-lateral pathologies pose specific challenges due to the anatomy and less frequent need for approach. Traditional open approaches to the foraminal and extraforaminal regions require either a midline or paramedian incision. These open exposures can be quite extensive in order to reach laterally beyond the vertebral pars interarticularis and facet joint in order to obtain adequate visualization of the far-lateral herniated disc.⁸^,⁹^,¹⁰ The adaptation of minimally invasive approaches using tubular retractor systems is particularly well suited to approaching Kambin’s triangle and the extraforaminal space.¹¹^,¹²

In order to safely perform a minimally invasive far-lateral microdiscectomy, knowledge of the neural and bony anatomy is required. Fluoroscopic assistance is necessary for placement of the tubular retractor in order to ensure appropriate visualization and exposure of the exiting nerve root and herniated disc. The minimal bony drilling required does not result in joint destabilization. In general, the minimally invasive tubular approach has multiple advantages over the more traditional open approach to the extraforaminal region.¹³^,¹⁴^,¹⁵^,¹⁶ There is minimal tissue trauma, decreased surgical site infections, early mobilization, and short hospital stays. As this approach is generally not as familiar to spinal surgeons, a review of this anatomically based approach is useful.

33.2 Indications

The indications for a minimally invasive far-lateral microdiscectomy are the same as a more conventional approach. When evaluating patients for surgery, the majority of patients will have significant radicular pain. The pain distribution should correlate with the magnetic resonance imaging (MRI) findings of disc herniation. The exiting nerve roots are affected by pathology in Kambin’s triangle and extraforaminal space such that the L4 nerve root is symptomatic due to a far-lateral herniation at L4–L5 and the L3 nerve root at L3–L4. Patients may experience a combination of pain, numbness, or weakness. Dysesthetic pain syndromes, often with allodynia, are also more frequent with pathology in this location due to involvement of the dorsal root ganglion. Some patients may complain of a burning sensation, similar to a sunburn, and find any fabric or touch against the skin very distressing. Motor weakness is also a possibility and a thorough neurologic examination, including reflexes, should be performed in all patients.

In most patients with herniated discs, a conservative, nonsurgical approach is generally recommended initially.⁵ Treatment options include physical therapy, transforaminal steroid injections, and nonsteroidal anti-inflammatory medications. In patients with herniated discs and persistent radicular symptoms despite adequate conservative measures, surgery has been shown to be beneficial.¹⁷ The presence of motor weakness may prompt surgery on an earlier basis.¹⁸

Once the decision for a surgical microdiscectomy has been made, the choice of approach to the spine is critical. In patients with far-lateral disc herniations, the extraforaminal approach to Kambin’s triangle is most direct and expedient.⁴^,¹¹ Alternatively, some patients will have disc herniations within the neural foramen directly underlying the pars interarticularis and either a standard interlaminar or extraforaminal approach can be considered. In foraminal discs compressing the exiting nerve root, it is the author’s preference to utilize the extraforaminal approach as this requires less bone removal and has a lower risk of pars injury and potential destabilization.

In operative cases, review of the lumbar MRI scan is critical in determining the nature of the surgical approach; T2-weighted MRI is especially helpful in this regard. Both the sagittal and axial images need to be considered. On the lateral sagittal images, particular attention should be focused on evaluating the neural foramina and potential compression of the exiting nerve root. In normal, nonpathologic levels, the neural foramen should be well visualized with the nerve root and surrounding fat. However, with foraminal pathology, the fat plane can be obliterated and the nerve root compressed superiorly against the pedicle. Axial imaging is also helpful and can also demonstrate the exact location of the disc herniation and extent of foraminal and/or extraforaminal involvement.

33.3 Contraindications

A thorough understanding of the anatomy of the Kambin’s triangle region is necessary for a safe far-lateral minimally invasive microdiscectomy. An evaluation of the MRI scans is critical prior to surgery. Some patients, particularly older individuals, will have significant facet arthropathy, which can limit the exposure to the extraforaminal space. Additional drilling of the lower vertebra superior articulating process may be necessary in these patients and can provide particular surgical challenges with the minimally invasive approach. Additionally, prior extraforaminal surgery can distort the tissue planes with scar tissue and epidural fibrosis increasing the risk of nerve injury or cerebrospinal fluid (CSF) leakage. It is the author’s experience that prior midline surgery, such as a laminectomy, is not a contraindication to the minimally invasive tubular extraforaminal approach. Indeed, this is an excellent option for patients who have had prior midline open operations and present with far-lateral disc herniations.

33.4 Preoperative Planning

As in all surgeries, preoperative planning is critical to a successful outcome. A correlation between the patient’s symptoms and MRI findings is necessary in order to determine the symptomatic pathology and disc level (Fig. 33.1). However, in the majority of patients, the link between the herniated disc and radicular symptoms is clear-cut. Although not generally required, an electromyography (EMG)/nerve conduction study can be helpful in less straightforward patient presentations. Intraoperative three-dimensional navigation can also be helpful if available.

Fig. 33.1 Magnetic resonance image of a right L4/L5 far-lateral herniated disc, T2-weighted. (a) Axial slice; (b) sagittal slice with obliteration of the foramen.

In patients where the disc herniation is either foraminal or far-lateral, the extraforaminal approach is well suited for nerve decompression. There are occasional patients who may have both a paracentral as well as a far-lateral disc herniation and may require an “in and out” technique utilizing both an interlaminar and an extraforaminal approach; this would require two separate docking points, but still can be performed using minimally invasive techniques. An evaluation of the facet pathology preoperatively can also be helpful as it will alert the surgeon to the degree of bone drilling required.

33.5 Patient Positioning

Once in the operating room, successful general endotracheal anesthesia is given. Prophylactic antibiotics are administered within 1 hour of incision. The procedure is performed in the prone position. The use of a classic knee-chest position or other positioning frames do afford the benefit of significant hip flexion, which may help open the intervertebral space. The arms are placed upward on arm boards and the face and eyes are carefully padded (Fig. 33.2). Sequential compression boots are used during the procedure.