5 Minimally Invasive Far Lateral Microdiscectomy

Over the years, I have found that it is the facility that I developed with transforaminal approaches that taught me the far lateral microdiscectomy technique. The transforaminal approaches helped familiarize my mind with the unique topography of the lateral lumbar spine. As I became more comfortable exposing the pars interarticularis and decompressing the exiting nerve root in TLIFs, decompression of the exiting nerve root without removal of the pars became a more feasible concept for my mind to grasp. Working under the microscope case after case, I gained an appreciation for the way that the pars interarticularis blended into the transverse process and the superior articular process (▶ Fig. 5.1). Alongside the comfort I had with the exposure, I applied the principles of the medial microdiscectomy to the far lateral microdiscectomy. In particular, I sought to preserve as much of the native spine as possible while safely exposing and mobilizing the neural elements to decompress them.

One of the challenges I encountered was that far lateral disc herniations were much less common than medial disc herniations in my practice. I found the ratio of medial microdiscectomies to far lateral microdiscectomies over 10 years of practice was 30:1. I thought the scarcity of these cases would limit my ability to rapidly gain a facility with this technique. I soon realized that the paucity of far lateral disc extrusions would not affect my ability to acquire proficiency with this operation. The skill set for this operation originates from the transforaminal approach more so than the medial microdiscectomy. Developing that transforaminal skill set provided me with the foundation to achieve a greater understanding of the subtleties of the anatomy of the far lateral recess and readily translate that knowledge to the far lateral microdiscectomy. This procedure is effectively a miniature and lateral form of the transforaminal approach. The goal of the procedure is to identify the exiting root and remove the far lateral disc herniation while preserving as much of the lateral bony anatomy as possible. This chapter describes how to do just that. I begin by discussing the remarkable history of radiculopathy caused by a far lateral disc herniation, before describing in detail the anatomy of the foramen and the lateral lumbar spine. Finally, I present the operative technique with representative case presentations. Although this procedure builds on the familiarity of the transforaminal approach, it has its unique subtleties and nuances, which I describe in detail in the upcoming pages to help you navigate those unfamiliar waters so far off the shore.

5.2 Far Lateral Microdiscectomy: A Perspective

When it comes to the far lateral microdiscectomy with a lateral extraforaminal approach, I can no longer fathom an open operation that begins in the midline. I recall the two traditional open Wiltse approaches I performed during my residency, one for a schwannoma and the other for a far lateral disc extrusion. Both of these exposures were extensive, bloody, and disorienting. I recall the degree of discomfort that each of these patients had in the aftermath of the operation, and I cannot help but think of the wise words written by Caspar four decades ago regarding the ratio of a surgical target to a surgical exposure. Even my colleagues who question whether there is a benefit to a minimally invasive microdiscectomy for a paracentral disc herniation readily concede that the best manner to address pathology in the far lateral recess of the lumbar spine is with a minimally invasive approach. Docking a minimal access port on the far lateral aspect of the spine immediately over the affected exiting root is perhaps the most efficient and effective manner to decompress a nerve root flattened by a far lateral disc herniation. The exposure of the surgical target may be reliably achieved through a 16-mm access port and results in a favorable Caspar ratio of nearly 1:1. In comparison, a traditional Wiltse approach, where the exposure would be several times the size of the surgical target, becomes difficult to conceive. Nevertheless, every operation has its own story of evolution, which reflects the challenges that faced surgeons attempting to treat patients suffering from radiculopathies. Buried within that history are kernels of insight that provide perspective for your study of the far lateral microdiscectomy. With that statement in mind, I feel that the next few pages are an invaluable element of understanding the far lateral microdiscectomy.

5.3 Historical Perspective

What a diagnostic conundrum far lateral disc herniations must have been at the dawn of our specialty. In the early 20th century, the rudimentary diagnostic imaging with intrathecal Lipiodol (Guerbet, Villepinte, France) or air as contrast media provided surgeons a limited view of the thecal sac, nerve roots, and potential sights of compression relative to the bony anatomy. Such imaging offered a line of sight limited to neural elements in the midline and just off the midline. In light of these limitations and the potential morbidity of the diagnostic imaging itself, the emphasis was on the localization of the nerve root compression by clinical examination. A patient who had classic L4 radiculopathy would undergo an L3–4 midline microdiscectomy by a well-intended surgeon whose clinical examination unequivocally demonstrated an L4 nerve root compression syndrome. That empty feeling created by retracting the nerve root without extrusion of a disc fragment revealing itself is a feeling only a spine surgeon knows. I can sympathetically envision that early spine surgeon taking time to reassess whatever primitive imaging they had, perhaps asking the operating room nurse to read aloud the clinical note to ensure the correct laterality. All the while, that surgeon was correct in his clinical assessment. The L4 nerve root was indeed compressed, but not as it traversed the L3–4 disc space. The compression was in the far lateral recess as the nerve root exited beneath the pedicle. A far lateral disc herniation had unfurled from the L4–5 segment and compressed the exiting L4 nerve root. The surgeon’s assessment of the nerve root compression syndrome was correct. The surgeon was just looking in the wrong place. However, in the early 20th century, there was no imaging capability to unveil that blind spot.

Walter Dandy’s1 mention of “concealed discs” in his 1942 publication regarding the advancements in the management of disc herniations may have been the first acknowledgment of the existence of far lateral disc herniations, although certainly not recognized as such at the time. Dandy, along with his early pioneering colleagues, identified a group of patients who presented with classic nerve root compression syndromes, yet had negative findings at the time of surgery. Consistent with our current knowledge of far lateral disc herniations, the incidence was in the vicinity of 10%.^1,2,3 With our current technology, it is almost impossible to imagine venturing into the operating room with nothing more than a clinical examination and 5 mL of intrathecal ethiodized oil (Lipiodol) injected before an X-ray of the lumbar spine. At the time, surgeons looked where they could see, and they could see compression only in the vicinity of the central canal. Contrast media in the thecal sac could reveal compression of the midline and structures in the lateral recess. No imaging at that time shed light on the far lateral recess; therefore, surgeons did not venture into that vicinity, at least at first.

The simple law of percentages suggests that a number of those segments exposed based on the clinical examination and limited radiographic imaging may have indeed harbored a disc herniation that resided in the far lateral recess. In 1949, Echols and Rehfeldt2 suggested the possibility of foraminal compression by a far lateral disc herniation. In their thoughtful analysis of 32 patients who had negative findings at the time of surgery, these surgeons concluded that in the event of a negative exploration, the nerve root should be explored outside of the foramen. Of course, that particular approach would require a facetectomy and the potential need for fusion.

It was not until 1971, when Macnab³ wrote an article entitled “Negative disc exploration,” that the first formal description of extraforaminal disc extrusions entered the literature. Macnab described a patient with a clear L5 nerve root compression syndrome who had complete relief with a selective nerve root block at L5. An L4–5 microdiscectomy failed to reveal the expected disc herniation, but confident in his diagnosis, he ventured into the foramen after removing the entire facet and found “the root engulfed and almost buried in a diffuse lateral bulge of the disc.” Macnab³ appropriately labeled the location of the far lateral disc herniation the “hidden zone.” The sacrifice of the facet required a posterolateral fusion, but the operation relieved the patient’s radiculopathy. ▶ Fig. 5.2³ from Macnab’s study captures the essence of the anatomical circumstance beautifully.

Despite the recognition of this clinical entity, diagnostic challenges remained in the era before magnetic resonance imaging (MRI) and computed tomography (CT). In 1974, Abdullah and colleagues⁴ published a compelling argument for using discography for the diagnosis of far lateral disc herniations. Despite the absence of a surgical technique section in their study, they appear to be the first who recommended a simple discectomy instead of a complete facetectomy.

The introduction of CT vastly facilitated the recognition and diagnosis of far lateral disc herniation, and as a result, several technique papers began to populate the literature. A 1988 publication, again by Abdullah and colleagues,⁵ details the operative technique, which they applied to 138 cases. Not surprisingly, the recommended approach was medial to lateral, an approach that builds on the familiarity of the midline structures and extends laterally ( ▶ Fig. 5.3).⁵ In subsequent years, authors began to explore lateral to medial approaches. Surgical series describing the various surgical approaches began to populate the literature. The medial facetectomy, trans-pars interarticularis and extraforaminal and intertransverse process approaches were all reported for the management of far lateral disc herniation.^6,7,8

Wiltse and Spencer,⁹ Maroon et al¹⁰ and Jane et al¹¹ were among the first to recommend a lateral to medial approach for the management of far lateral disc herniations via an extraforaminal inter-transverse process approach. One look at the illustrations from their technique brings immediately to mind the statement made by Caspar ¹² regarding the ratio of exposure to the surgical target ( ▶ Fig. 5.4).^10,12 With the surgical target defined as the area from the pedicle to the disc space, the surgical exposure required with conventional retractors far exceeds the 10 × 6 mm of the surgical target as determined by Reulen and colleagues¹³ in their anatomical study of the lateral approach, which had a very unfavorable Caspar ratio.

By 1997, Foley and Smith¹⁴ had already introduced the use of minimal access ports for the management of paramedian disc herniations. Applying that same technique and those access ports to the lateral spine would be the natural solution to reconcile the unfavorable Caspar ratio for the exposure needed to treat a far lateral disc herniation. Foley and colleagues¹⁵ turned their access ports to the far lateral recess and 2 short years after their initial manuscript, they published their series on the minimally invasive management of far lateral disc herniations. They demonstrated the efficacy and, especially, the efficiency of a minimally invasive far lateral approach. Since that publication, surgeons have populated the literature with several series applying minimally invasive techniques to the treatment of far lateral disc herniation. The use of the minimal access port to approach the lateral spine for removal of a far lateral disc herniation through an extraforaminal approach is the focus of this chapter.

5.4 Anatomical Basis of the Minimally Invasive Approach

The key to the far lateral microdiscectomy is the pedicle that corresponds to the compressed nerve root. Identification of that pedicle brings into focus the orientation, confirms the boundaries of the foramen, helps identify the nerve root safely and unveils the disc extrusion. For this operation, the pedicle is the North Star in the absence of the shoreline. In symptomatic far lateral disc herniations, the disc herniation typically displaces the nerve root upward against the pedicle ( ▶ Fig. 5.1). In the lumbar spine, the nerve root exits below its like-numbered pedicle (i.e., the L4 nerve root exits below the L4 pedicle). Therefore, one must find the pedicle first and sift through a sea of epidural fat and engorged crimson veins after resecting the lateral aspect of the ligamentum flavum to identify the symptomatic nerve root. The final element of the exposure is to identify the disc extrusion and the actual disc space. A thoughtful examination of the anatomical basis of the minimally invasive approach provides the framework for understanding, learning and applying this technique.

The ideal diameter for the access port in the far lateral microdiscectomy is 16 mm. That diameter provides a metric to compare to the anatomy at depth relative to the diameter of the exposure. Knowledge of the various anatomical distances will help reconstruct the anatomy of the spine at depth in your mind’s eye. An aptitude for mental reconstruction of the spinal anatomy is helpful in all cases, but this mental ability is especially advantageous in the far lateral microdiscectomy. In the absence of the midline structures, the potential for disorientation through such a limited field of view remains high. The anatomical studies by Reulen et al¹³ and Schlesinger et al¹⁶ fill the void of our limited experience in the lateral spine and are essential reading for this procedure.

If one were to deconstruct the anatomy of the lumbar spine in the lateral and AP projections, one could examine the far lateral microdiscectomy from the inside out ( ▶ Fig. 5.5). In particular, we could begin with the anatomic measurements of the neural foramen of the affected root.

The distance between the pedicles above and below a nerve root determines the rostrocaudal dimension of the neural foramen ( ▶ Fig. 5.6). Logically, the distance between the pedicles becomes the first measurement we need to examine to establish the anatomical basis for the far lateral microdiscectomy. That interpedicular distance, as measured from the bottom of the rostral pedicle to the top of the caudal pedicle, is seldom greater than 18 mm (range, 14–20 mm from caudal to rostral). Although this distance is inherently linked to the disc space height, because of lordosis, the level has a bigger impact than one might think (i.e., it is less at L5–S1 and more at L3–4). Intuitively, we know that the interpedicular distance is greater in the upper segments of the spine and decreases as we proceed to the sacrum. For instance, we know that we will likely need a 40- or 45-mm rod to connect the L3 pedicle to the L4 pedicle, whereas at L5–S1, a 30-mm rod will be more than adequate. Various anatomical studies have confirmed that intuition. ▶ Fig. 5.7 illustrates the foraminal height for the different segments of the lumbar spine. It is important to note that ▶ Fig. 5.6 is an idealized illustration without degeneration of the disc space incorporated into the measurement. Any collapse of the disc directly affects the foraminal height to some extent.

Reulen and colleagues were among the first to precisely define the operative window as the lateral aspect of the pars interarticularis (the isthmus), the inferior aspect of the pedicle, the transverse process, and the superior aspect of the facet joint. At segment L1–2, L2–3 and L3–4, they found this distance to be approximately 10 mm (range, 5–16 mm). At L4–5, this distance decreased to 7.9 mm (range, 3–14 mm), and at L5–S1, the operative window was a paltry 5.1 mm (range, 0–11 mm; ▶ Fig. 5.8).¹³ The L5–S1 far lateral disc herniation is therefore almost a separate entity that requires careful consideration before endeavoring to treat it with a minimally invasive procedure.

The disc space of the segment where the far lateral disc herniations originate is between the pedicles and closest to the caudal pedicle by only a few millimeters. The anatomical studies of Reulen and colleagues show that distance from the rostral pedicle, the North Star of this operation, to the disc space is seldom more than 10 mm. This distance is essential to keep in mind so that once you have identified the pedicle, you have a sense of how far it is to the disc space below.

The anterior aspect of the foramen is primarily made up of the posterior aspects of the rostral vertebral body and disc space. There is only a modest contribution from the caudal vertebral body, as shown in ▶ Fig. 5.5. Consequently, the exiting nerve root is in the direct path of a lateral disc herniation. Far lateral disc herniations can therefore result in severe compromise of the foramen and compression of the nerve root. The posterior aspect of the foramen is made up of the pars interarticularis and the superior articular process ( ▶ Fig. 5.5). The anteroposterior (AP) dimension of the neural foramen is a meager 8 to 10 mm, which is not much room for a disc extrusion and exiting nerve root to coexist in that space.

It is equally important to recognize the path of the exiting root relative to the surface anatomy of the spine. Although we know that the nerve root exits beneath its like-numbered pedicle, we cannot see the pedicle from the surface anatomy. The pars interarticularis, on the other hand, can be readily visualized on the surface anatomy of the lateral spine. The nerve root traverses immediately beneath the rostral aspect of the pars interarticularis as it blends into the transverse process and pedicle. ▶ Fig. 5.9 demonstrates how the exiting root leaves the neuroforamen at the level of the rostral pars interarticularis just beneath its like-numbered pedicle. Thus, the pars interarticularis becomes a valuable anatomical landmark on the surface of the spine to target for the exposure. Both Reulen et al¹³ and Schlesinger et al¹⁶ have elegantly described this anatomy in their studies of the lateral spine for extraforaminal approaches. Again, I emphasize the importance of reading these classic articles alongside this chapter to achieve an understanding of the anatomy required for the procedure. The distance from the midline to the limited field of view requires mastery of the anatomy and a sophisticated capacity to readily reconstruct the lateral anatomy of the spine at depth in your mind’s eye, with only limited glimpses of the bony structures.

From these anatomical measurements, it becomes evident that a 16-mm diameter encompasses the rostrocaudal exposure needed to reveal the affected nerve root and the disc space ( ▶ Fig. 5.9).

5.6 Anatomical Basis of the Distance from Midline for the Incision

The distance from the midline to the lateral aspect of the pars is seldom more than 20 mm in the upper segments of L1–2 and L2–3, scarcely more than 25 mm at the L3–4 and L4–5 segments and up to 30 mm at the L5–S1 segment ( ▶ Fig. 5.8).¹³ Therefore, an incision of 25 to 30 mm lateral to the midline for the upper segments and 35 to 40 mm lateral to the midline for the lower segments allows for a converging trajectory onto the lateral pars interarticularis and the neural foramen. With the center of the diameter at the junction of the lateral pars and superior facet, it becomes readily apparent how all the requisite anatomy for a far lateral microdiscectomy comes into view with a well-positioned 16-mm diameter access port ( ▶ Fig. 5.10). Such a diameter encompasses the entire operative window defined by Reulen and colleagues.¹³

The measurements of the foramen and the position of the pars interarticularis relative to midline establish the anatomical basis for the minimal access port and offer guidance for selection of the diameter of the minimal access port. The 16-mm diameter access port assures a favorable Caspar ratio of almost 1:1, which is in step with the guiding principles of minimally invasive spine surgery. A firm grasp of the knowledge of these dimensions and measurements will increase your confidence as you peer through the microscope at the anatomy at the bottom of the access port. Early in my experience, I routinely used 18-mm diameter ports, thinking that a wider exposure was of greater value. As my experience with this approach grew, I found the value of 16-mm diameter ports. A smaller diameter was easier to secure into the extraforaminal corridor and less disruptive of the lateral musculature and facet. The larger diameters were more of a liability than an asset. Based on all the measurements of the neural foramen, the precise placement of a 16-mm access port over the lateral aspect of the pars interarticularis provides all the necessary exposure needed for the procedure.

The pars interarticularis is the first beacon to identify so that you remain oriented to the anatomy. From there, you will locate the pedicle that will guide you safely into the lumbar neural foramen so that you may safely and efficiently identify and then decompress the exiting nerve root by removing the far lateral disc extrusion.

5.7 Operating Room Setup

There is no difference between the operating room set up for a paramedian microdiscectomy and a far lateral microdiscectomy ( ▶ Fig. 5.11). The Jackson table and Wilson frame are equally as valuable for a far lateral discectomy as they are for a paramedian discectomy. The microscope stands sterilely draped and ready on the symptomatic side of the prone patient while the fluoroscope remains initially parked at the knees of the patient so that the scrub technician can drape it into the field. The operating room nurse secures the clamp for the table-mounted arm at the base of the Wilson frame to prevent any delay in securing the minimal access port after the operation begins.

Related posts:

Minimally Invasive Decompressions for Metastatic Spinal Disease Minimally Invasive Posterior Cervical Laminectomy Minimally Invasive Resection of Intradural Extramedullary Lesions within the Thoracic Spine Minimally Invasive Lateral Transpsoas Interbody Lumbar Fusion Anterior Cervical Discectomy with Arthroplasty or Fusion Minimally Invasive Transforaminal Lumbar Interbody Fusion

Stay updated, free articles. Join our Telegram channel

Join