3 Minimally Invasive Lumbar Laminectomy

Abstract

Keywords: facetectomy, laminectomy, ligamentum flavum, lumbar stenosis, minimally invasive

Based on over 45 years’ experience in the surgical treatment of lumbar disc disease, it is recommended that the following operations be eliminated: the simple discectomy, which may cure the sciatica but not the back pain; the decompressive laminectomy, which leaves the patient with painful instability and nerve-root scarring; and chemonucleolysis, which does not provide permanent relief of either low-back or leg pain. The PLIF [posterior lumbar interbody fusion] technique is the answer to treatment of diseases of the lumbar spine and may be the operation of the future.

Dr. Ralph Bingham Cloward 1

3.1 Introduction

Ralph Bingham Cloward certainly maintained a passionate and absolute opinion regarding the lumbar laminectomy. These words come from a surgeon who, over the course of a brilliant career, saw patients come in and out of his clinic after microdiscectomies, laminectomies, and posterior lumbar interbody fusions (PLIFs). Since Cloward spent his career on the island of Hawaii, his patients had few other places to go, and Cloward had no place to hide. As extreme as Cloward’s position may seem in our modern era of spine surgery, given his decades of experience and the historical context of the time, it is difficult to discount his statement altogether. Perhaps we can even find some kernel of truth in his conclusion.

I can envision Dr. Cloward walking into that clinic room to examine a postoperative laminectomy patient wearing an expression of pain, anxiety, and dissatisfaction that we surgeons have all seen. He would stand there shaking his head while biting his lip as the patient described their painful instability originating only from their back without symptoms into the legs. “I should have fused this patient,” he would likely have whispered beneath his breath. (How many times have I thought and mumbled the same?) I can also see Cloward knock on that same clinic door before peeking in to see a smiling patient who had undergone his PLIF procedure. As the PLIF patient thanked him and praised him, he would sigh with the satisfaction that only a spine surgeon knows. After years of this pattern repeating itself, the origins of the statement that begins this chapter become obvious.

One of the first spine surgeries I saw as a medical student at Georgetown University was a lumbar laminectomy. I was aghast. Looking at the immensity of the retractors, the length of the incision and the amount of blood loss, I wondered how this surgery could be of benefit to any patient. I remember the first step performed by the chief resident was using the Horsley Stille bone cutter to remove a pair of spinous processes. After interruption of the posterior tension band, the laminae were removed bilaterally to the edge of the facets. With each phase of the operation, the chief resident deepened the retractors and clicked them open wider and wider. I could not register in my mind that such an extensive wide-open approach was the best approach. After the case concluded, I distinctly remember asking the chief resident how a patient could possibly improve after such a procedure. I received a shrug and a muted response. I had not yet read Cloward’s statement on the lumbar laminectomy, but if I had, I likely would have agreed with him.

In 1966, Love described the technique for the “decompressive laminectomy” as the removal of the spinous process and bilateral laminae.² In more specific terms, it can be defined by a midline approach to the spine, subperiosteal dissection of the paraspinous muscles and multifidus off the spinous process, laminae, and facets. The exposure is followed by complete resection of the spinous process, bilateral lamina, and medial facets. The very nature of this procedure interrupts the posterior intraspinous process ligament. That posterior tension band assists the body in maintaining lumbar lordosis and its loss has the potential to contribute to the flattening of our backs. With such a procedure, we lose the band that strings the bow.³ When Cloward made his statement regarding the laminectomy, Love’s description of the operation is likely what he had in mind. Since context is everything, it is important to note that Love’s description of the laminectomy occurred at a time when localization in the operating room was accomplished at times by counting up from the sacrum and preoperative imaging was a rudimentary form of myelography.

Despite the preservation of the majority of the facet joint, the elimination of the posterior tension band explains how such a procedure has the potential to be destabilizing. Cloward was not alone in his opinion of the lumbar laminectomy. Over the years, spine surgeons have made the same intuitive observation about their patients regarding the lumbar laminectomy.^4,5 Few would argue that interruption of the posterior tension band, the devascularization of the paraspinous muscles along with the complete removal of the bony anatomy, does not have the capacity to affect the stability of the decompressed segment. More recent biomechanical studies have validated these intuitive concerns. ^3,6

Over the decades, surgeons must have looked at the task at hand, the anatomy they were dealing with alongside the consequences of the current techniques and thought to themselves, “There must be a better way.” For Cloward, it was decompression with stabilization of the spine. Admittedly, this technique was developed in an era without full appreciation of either the impact of adjacent segment degeneration or the importance of segmental lumbar lordosis. But it certainly addressed the issue of stability.

Another approach arose that offered the same extent of decompression with minimal disruption of the native spine. Surgeons began to look at a lumbar segment in three dimensions instead of two. With this perspective, suddenly the spinous process and the lamina were no longer obstacles to the central canal ( ▶ Fig. 3.1). A common theme arose among the innovative surgeons who developed an approach that allowed for the preservation of the midline elements. That theme resonates in perfect pitch with modern-day minimally invasive principles. It was this rational approach to decompress without sacrifice of the midline ligamentous or bony anatomy that became the foundation of what has become the minimally invasive laminectomy.

Fig. 3.1 The lumbar spine in three dimensions. Illustration of the true target of a lumbar laminectomy: the geometric center of the lumbar canal. There are several corridors that allow access to the central canal. A direct posterior approach will mandate complete resection of the spinous process and subsequently the lamina in order to decompress the neural elements. By beginning off the midline and angling onto the spine with a trajectory of 15 to 25 degrees, as illustrated in this figure, the same target may be reached without having to sacrifice the spinous process and both laminae.

3.2 Historical Perspective

Long before the development of modern minimally invasive techniques, surgeons sought a method to decompress the lumbar spine without sacrificing the midline elements. It was those lumbar laminectomy patients who experienced successful relief of neurogenic claudication and radiculopathy but developed new-onset back pain and radiographic evidence of instability that undoubtedly weighed heavily on the minds of those surgeons. The question was whether a successful decompression of the neural elements could be accomplished any other way?

Surgeons had made the astute observation that it is seldom the bony elements that result in central canal stenosis, but rather, the hypertrophied ligamentum flavum and facet arthropathy that cause compression of the neural elements. One thing was certain, the spinous processes and interspinous ligaments were completely innocent. Nevertheless, they became the collateral casualties of the classic wide midline decompression. Comments such as “the removal of bone is extensive for what is essentially a segmental disease confined largely to the posterolateral elements” began to surface in the literature regarding the traditional midline laminectomy.⁷ Surgeons began asking why a satisfactory decompression could not be accomplished if the midline structures were not considered obstacles to the central canal.

Paul Lin⁸ was one of the first surgeons to attempt to answer this question when he published his 1982 technical note on the internal decompression of the canal for the treatment of lumbar stenosis. Removal of the medial aspect of the inferior and superior articular processes, the inferior aspect of the rostral lamina, and the superior aspect of the caudal lamina made possible the decompression of the neural elements with preservation of the lamina, spinous processes, and posterior tension band ( ▶ Fig. 3.2).⁸ In 1988, Young and colleagues⁷ published a similar technique of “multilevel subarticular fenestrations as an alternative to wide laminectomy” that confirmed the efficacy of Lin’s technique in a series of 32 patients with up to 5 years of follow-up. In 1990, Aryanpur and Ducker⁹ published another series of 32 patients using the same technique adding further evidence to the efficacy of this technique ( ▶ Fig. 3.3).⁹

Fig. 3.2 Decompression of the central canal without sacrifice of the midline elements. Illustrations from Lin’s⁸ description of internal decompression of the central canal for management of lumbar stenosis. (a) Mesial facetectomy. (b) Superior facetectomy. (Reproduced with permission from Lin PM. Internal decompression for multiple levels of lumbar spinal stenosis: a technical note. Neurosurgery. 1982; 11(4):546549.)

Fig. 3.3 Illustration from Aryanpur and Ducker⁹ demonstrating the technique of multilevel medial facetectomies with preservation of the midline elements. (Reproduced with permission from Aryanpur J, Ducker T. Multilevel lumbar laminotomies: an alternative to laminectomy in the treatment of lumbar stenosis. Neurosurgery. 1990; 26(3):429432; discussion 433.)

Lin 8 made the following astute and prescient observation in his commentary on Aryanpur and Ducker’s paper: “It is our experience when only the angled rongeur is being used, that adequate bony decompression of the foramen can only be accomplished if the operating surgeon stands on the opposite side of the lesion.” That simple statement was prophetic. It captured the essence of the contralateral decompression achieved in current minimally invasive techniques. In 1995, Poletti¹⁰ advanced this concept further with what appears to be the first formal description of a unilateral laminotomy with bilateral “ligamentectomy” to decompress the central canal ( ▶ Fig. 3.4).¹⁰ In his patients, Poletti maintained the integrity of the posterior tension band, spinous process, and contralateral lamina. He rationalized that doing so would minimize any instability and muscle devascularization that would occur with a traditional midline approach. Through this unilateral approach, Poletti was able to completely resect the ligamentum flavum that was causing the compression by undercutting the spinous process and the contralateral lamina.

Fig. 3.4 Illustration from Dr. Poletti’s 1995 publication¹⁰ before the advent of minimally invasive spine techniques. Remarkably, this illustration augurs the technique that minimally invasive surgeons would eventually use to perform minimally invasive lumbar laminectomies through minimal access ports. (Reproduced with permission from Poletti CE. Central lumbar stenosis caused by ligamentum flavum: unilateral laminotomy for bilateral ligamentectomy. Neurosurgery. 1995; 37(2):343-347.)

Despite the efficacy of the unilateral approach that was demonstrated through the middle and late 1990s, the midline laminectomy remained the mainstay in treatment for lumbar stenosis. It was the approach I was taught in my residency and to a large part, it continues to be taught to this day.

Parallel with the desire to maintain the native structures of the spine in the treatment of lumbar stenosis, there has been an increasing interest in minimally invasive approaches to the spine. The paramedian transmuscular approaches developed by surgeons seeking a minimally invasive procedure to access the spine would find a perfect marriage with a technique that intended to preserve the posterior tension band and as much of the midline bony elements as possible. The synthesis of these two techniques popularized by Foley and Fessler and written about by Khoo and Fessler 11 would eventually lead us to the minimally invasive laminectomy.

3.3 The Minimally Invasive Conversion

I emphasized in Chapter 2, Minimally Invasive Microdiscectomy, that there are more similarities than differences between the open and minimally invasive microdiscectomy. The approach, the bone work and the procedure are virtually identical between the two; it is the access to the requisite anatomy that is the only real difference between these two techniques. It is those very similarities that make that operation the ideal starting point for minimally invasive surgery on the spine. In this chapter, I acknowledge that the minimally invasive laminectomy is more of a departure from the midline laminectomy. The bone work is different, the exposure is different and the incision is different. Nevertheless, the decompression is the same. The advantage that you have in developing a skill set with the minimally invasive microdiscectomy is that those same skills readily translate to the minimally invasive laminectomy. From that standpoint, the minimally invasive laminectomy is the logical next chapter in this Primer.

There are key differences between the microdiscectomy and laminectomy that are worthwhile to highlight. Unlike the incision for the minimally invasive microdiscectomy, the incision for a minimally invasive laminectomy is slightly more off the midline and has a greater angle of convergence onto the spine as seen in ▶ Fig. 3.1 at the beginning of this chapter. The combination of increased angle and greater distance from the midline has the potential to disorient the surgeon’s mind. After all, the midline is the basis of our orientation. A return to the basic principles of sounding the lamina, spinous process and facet that I introduced in Chapter 2 will help you reconstruct the anatomy at depth. These components will keep your mind oriented. In turn, the minimally invasive laminectomy becomes yet another building block that will become the foundation for minimally invasive lumbar fusions.

3.4 Patient Selection

The focus of this Primer is the surgical technique. However, it would be difficult not to make a comment regarding patient selection for the minimally invasive laminectomy, given some of the unique elements of this procedure. Neurogenic claudication caused by single-level lumbar stenosis is perhaps the most common condition that I treat in my practice. Over the years, I have observed that this entity seldom occurs at L5–S1 or higher than L2–3 in the absence of previous surgery. A review of my last 100 or so cases demonstrates that the vast majority of the surgeries I have performed have been at L4–5 or L3–4, or both. Remarkably, the vast majority have been single-level procedures.

The ideal candidate for a minimally invasive lumbar laminectomy has single-level stenosis, typically at L3–4 or L4–5. The subjective complaints of the patient may have a unilateral radicular component, but their history is primarily one of neurogenic claudication. A back pain component may be present, but without advanced disc degeneration, the goal of surgery is to decompress the neural elements and alleviate symptoms of neurogenic claudication. Back pain that resolves after the operation, I attribute to the inability to adequately distinguish the back pain from symptoms of neurogenic claudication.

Magnetic resonance imaging (MRI) should demonstrate the predominance of thickening in the ligamentum flavum over any disc protrusion or bony encroachment of the canal on the axial images. The amount of facet arthropathy and disc degeneration is relevant especially in the context of spondylolisthesis. Several aspects shown on the MRI in ▶ Fig. 3.5 make this patient an ideal candidate for a minimally invasive lumbar laminectomy.

Fig. 3.5 Lumbar stenosis in a patient with neurogenic claudication. T2-weighted magnetic resonance imaging of the lumbar spine. (a) Sagittal view demonstrates single-level lumbar stenosis at L4–5. Note the preservation of the disc height at L4–5 and the relative preservation of lumbar lordosis. (b) Axial view demonstrates primarily ligamentum flavum hypertrophy with a mild facet arthropathy.

First, the patient has single-level compression at L4–5 with no evidence of spondylolisthesis at that segment. The flexion-extension studies (not shown) do not demonstrate abnormal motion. The MRI demonstrates relative preservation of lumbar lordosis. There is no significant mismatch between the pelvic incidence and the lumbar lordosis.

Second, despite the advanced disc degeneration present at L5–S1, there is no significant degeneration of the disc space at L4–5. In fact, the disc height is well preserved at L4–5. From a clinical standpoint, this patient reports a history of primarily neurogenic claudication symptoms in the legs and mild left-sided L5 radiculopathy.

Finally, the axial MRI demonstrates a mild degree of facet arthropathy with most of the compression due to the thickened ligamentum flavum. Once the ligamentum flavum has been cleared and a medial facetectomy has been performed, the canal and the lateral recess will be well decompressed. Collectively, these clinical and radiographic aspects make this patient an ideal candidate for a minimally invasive lumbar decompression.

Characteristics in the imaging that would make the patient a less-than-ideal candidate for a minimally invasive laminectomy would include advanced degeneration, loss of lumbar lordosis and disc collapse causing neuroforaminal compromise of the L4 nerve root. In those circumstances, the restoration of the disc height and restoration of lumbar lordosis may be as important as the decompression of the neural elements.

Next, consider a patient with lumbar stenosis secondary to a grade I spondylolisthesis at L4–5 and advanced facet arthropathy ( ▶ Fig. 3.6). The decision-making process is slightly different for a minimally invasive approach than for a traditional midline open approach. In this patient, the absence of motion on flexion-extension studies and the preservation of the posterior tension band and the lamina with a minimally invasive approach allows for more stability than with a traditional midline approach, in which the posterior tension band and the lamina would be removed.^3,12 Furthermore, the disc height is well maintained, thus precluding the need to restore foraminal height. The extensive facet arthropathy requires removal of a substantial portion of the facet to achieve an adequate decompression. In this case, a minimally invasive decompression may be considered, with the patient monitored over time to check for any progressive instability.

Fig. 3.6 Magnetic resonance imaging (MRI) of the lumbar spine of a patient with spondylolisthesis. (a) Sagittal T2-weighted MRI demonstrates grade I spondylolisthesis at L4–5 with well-preserved disc height. (b) Axial T2-weighted MRI demonstrates severe central canal stenosis secondary to facet arthropathy. There was no evidence of instability on flexion-extension radiographs (not shown).

Finally, consider a patient with multiple levels of stenosis. ▶ Fig. 3.7 represents the inherent limitation of a minimally invasive laminectomy. The patient has four levels of stenosis. Working through a minimal access port at four different levels is possible, but far from efficient. At some point, simultaneous exposure of all the levels that require decompression will offer greater efficiency than securing a minimal access port at four different levels. I believe that crossover in efficiency occurs after more than two levels are involved. Although attempting to decompress the worst segment or perhaps the two worst segments is an option, such extensive lumbar stenosis argues for a more comprehensive solution as perhaps the best course of action. However, a traditional midline exposure does not mandate sacrifice of the spinous process and the posterior tension band. Applying the techniques described by Lin,⁸ Aryanpur and Ducker,⁹ and Poletti¹⁰ remains a viable alternative even in the setting of a midline approach for multilevel stenosis.

Fig. 3.7 Multiple levels of lumbar stenosis. Sagittal T2-weighted magnetic resonance image of the lumbar spine demonstrating four levels of lumbar stenosis. This patient would be a poor candidate for a minimally invasive decompression.

3.5 Anatomical Basis

From a conceptual standpoint, the operative anatomy in the axial plane may be considered a triangle formed by the two arms of the lamina and the disc space ( ▶ Fig. 3.8).² The main goal of the laminectomy, whether open or minimally invasive, is to decompress the thecal sac and the traversing nerve roots by removing the thickened ligamentum flavum and the compressive components of the hypertrophied facets. In a traditional midline laminectomy, these goals are accomplished by approaching and removing the apex of the triangle (the spinous process) along with the two limbs of the triangle (bilateral laminectomies and medial facets). In such an approach, the decompression is accomplished from the outside in ( ▶ Fig. 3.9).

Fig. 3.8 (a) Axial T2-weighted magnetic resonance imaging of the lumbar spine demonstrating severe central canal stenosis at the L4–5 segment. The limbs of the lamina and the disc space form a triangle in the axial plane of the compression of the thecal sac. In a traditional midline approach, both limbs of the triangle would be removed, as described by Love,2 thereby achieving decompression from the outside in. The minimally invasive technique approaches one side of this triangle and decompresses from the inside out. (b) Illustration of the thickened ligamentum flavum resulting in the compression of the thecal sac. The magenta triangle demarcates the borders of the canal: the lamina, medial facets and disc space. The spinous process clearly does not result in compression of the neural elements. Despite this, it is the first structure surgeons remove in a traditional midline lumbar laminectomy.

Fig. 3.9 Postoperative image of a traditional midline laminectomy. (a) Preoperative axial T2-weighted magnetic resonance imaging (MRI) demonstrating mild compression of the neural elements by the ligamentum flavum. (b) Postoperative axial T2-weighted MRI in a patient who underwent a traditional midline laminectomy. Note the removal of the spinous process, the lamina and the medial facets.

With the minimally invasive technique, the surgeon approaches one face of the triangle and removes most of that side to gain access to the inside of the triangle. Since most of the compression is due to the ligamentum flavum, the surgeon may accomplish the majority of the decompression by essentially excavating the ligamentum flavum from within this triangle. Thus, in the minimally invasive approach, the decompression of the thecal sac is accomplished from the inside out ( ▶ Fig. 3.10).¹⁰

Fig. 3.10 A traditional midline open versus minimally invasive lumbar laminectomy. (a) Illustration demonstrating the approach for a midline lumbar laminectomy. The first structure encountered is the spinous process. After removal of the spinous process, the laminae are removed bilaterally followed by the medial facets. With access to the central canal, the ligamentum flavum is resected and the neural elements decompressed. In this circumstance the decompression would be performed from the outside in. (b) Illustration of a minimally invasive approach for a lumbar laminectomy. A unilateral approach to the lamina, similar to what was described by Poletti 10 in 1995. In this approach, the spinous process would be undercut, the medial facet removed and one side of the lamina removed. Access to the central canal in this manner allows for resection of the ligamentum flavum. In this circumstance, the operation would be performed from the inside out. (c) Illustration of a postoperative laminectomy performed through a traditional midline approach. The neural elements have been decompressed from the outside in. (d) Illustration of a postoperative minimally invasive laminectomy with preservation of the posterior midline structures. The decompression achieved is the same.

Effectively accomplishing a minimally invasive decompression requires access to the entire swath of ligamentum flavum within the canal. Thus, exposure of the rostral and caudal insertion points of the ligamentum flavum is necessary. Such access allows for the detachment and removal of the entire ligamentum flavum, thereby decompressing the whole segment. Accordingly, the operative exposure should include the insertion points of the ligamentum flavum, which are at the rostral aspects of the superior lamina and the inferior lamina of the segment. Since the theme of a minimally invasive decompression is from the inside out and the focus is the ligamentum flavum, you would do well to study the anatomy of the ligamentum flavum from the inside out. Reviewing an anatomical illustration with a perspective from the underside of the ligamentum flavum is a useful exercise. It can help you consider the anatomy as if you were lying down on the floor of the spinal canal looking outward at the targets you need for a minimally invasive decompression. Carefully studying the anatomy from this perspective provides your mind with the unique insight to achieve the goals of excavating the segment from the inside out ( ▶ Fig. 3.11).

Fig. 3.11 (a) Illustration of viewing the ligamentum flavum from within the canal. (b) If you were to consider lying down in the canal looking up at the ligamentum flavum then your field of view would encompass viewing the ligamentum flavum from the underside of the laminae. The insertion points of the ligamentum flavum are clearly evident on the underside of the rostral and caudal laminae. These insertion points (highlighted in the illustration) are the targets for the exposure needed to accomplish complete decompression of the segment. Note the shadow of the facets bilaterally behind the curtain of ligamentum flavum. Ipsilateral and contralateral medial facetectomies may still be performed from a unilateral approach.

Exposure to the insertion points of the ligamentum flavum in and of itself allows the piecemeal resection of the ligamentum flavum with Kerrison rongeurs and pituitary rongeurs; but this type of exposure also lends itself to a more simplified method of decompression. Conceptually, the release of the rostral and caudal insertion points of the ligamentum flavum makes possible the en bloc removal of the ligamentum flavum. A study of the precise anatomical distance from the insertions will facilitate this technique. As noted in Chapter 2, Minimally Invasive Microdiscectomy, there is great value in knowing with certainty the specific anatomical measurements relevant to the procedure. For the minimally invasive lumbar laminectomy, the distance from the insertion point to the insertion point of the ligamentum flavum is perhaps the most valuable anatomical measurement. The MRI shown in ▶ Fig. 3.12 demonstrates the insertion points of the ligamentum flavum at L4–5. Depending on the disc height, that distance is typically around 25 mm. That is the approximate distance you must cover in a minimally invasive exposure to achieve a segmental decompression. Reulen et al further corroborate the 25-mm approximation in their analysis of laminar height.¹³ The analysis of 31 cadaveric lumbar spines found that the range of laminar heights for L3, L4, and L5 were 23 mm (20–32 mm), 21 mm (16–29 mm), and 17 mm (14–22 mm), respectively ( ▶ Fig. 3.12d–f). Laminar height in the lumbar spine is yet another valuable measurement to keep in mind as a minimally invasive spine surgeon.

Fig. 3.12 Sagittal T2-weighted magnetic resonance imaging (MRI) demonstrating that (a) the distance from the rostral insertion point of the ligamentum flavum to the caudal insertion point is a distance of approximately 25 mm. (b) Sagittal T2-weighted MRI demonstrates the insertion points (arrows) that are the target for resection in a minimally invasive lumbar laminectomy. (c) An illustration of the distance from rostral insertion to caudal insertion of the ligamentum flavum into the lamina of a segment. The distance from insertion point to insertion point is seldom more than 25 mm. Posterior view of (d) L3, (e) L4 and (f) L5 laminae showing laminar measurements that further demonstrate the anatomic basis for a minimally invasive approach.

It is equally valuable to appreciate the dimensions of the lumbar canal to further establish the rationale for a minimally invasive approach for a lumbar laminectomy.▶ Fig. 3.13¹⁴ demonstrates the dimensions of the canal as determined by Panjabi et al¹⁴ at the various levels of L1 to L5. Recognizing that the width of the lumbar canal ranges from 23 to 27 mm provides the context for a minimal access port measuring 16 to 18 mm in diameter. Such an access port can readily cover that dimension when repositioned at the various angles.

Fig. 3.13 An illustration demonstrating the varying depths and widths of the lumbar canal. The canal is widest at L5 with a measurement of 27.1 mm as reported by Panjabi and colleagues.14 Access to the central canal through a unilateral hemilaminectomy measuring 16–18 mm will reliably provide access to the entire central canal. Abbreviations: SCD, spinal canal diameter; SCW, spinal canal width.

An analysis of the axial MRIs alongside the measurements in ▶ Fig. 3.13 demonstrates the extent of the medial to lateral exposure required to accomplish the goals of the operation. It is helpful to keep in mind that the intrapedicular distance is approximately 2 cm and that the length of the lamina (one side of the triangle) is approximately 18 mm ( ▶ Fig. 3.14).

Fig. 3.14 Axial T2-weighted magnetic resonance imaging of the lumbar spine demonstrating that one limb of the triangle measures approximately 18 mm.

Looking at these various measurements collectively, the anatomical basis of the minimally invasive laminectomy begins to unveil itself. We have defined the rostrocaudal and lateral dimensions of exposures to be approximately 25 mm. It becomes conceivable how a minimal access port secured onto one of the laminae at an angle parallel to the adjacent side can provide adequate access to the entire central canal to accomplish the decompression ( ▶ Fig. 3.15). A diameter of 16 to 18 mm provides adequate exposure of the lamina for decompression in the axial plane of compression. However, a 16- or 18-mm aperture will not provide the necessary rostrocaudal exposure to reach the insertion points of the ligamentum flavum on the lamina. As demonstrated in ▶ Fig. 3.12, up to 25 mm of exposure may be necessary. Expandable minimal access ports can simultaneously encompass all the requisite anatomy within a single field of view but will result in greater disruption to the paraspinal musculature and will bring more muscle tissue into the operative field. Furthermore, the wide exposure offered by expandable minimal access ports shifts the Caspar ratio disadvantageously. Another viable option is simply to change the trajectory of the minimal access port to reach the limits of the exposure ( ▶ Fig. 3.16). Such a technique allows for an expansive exposure with no further disruption of the paraspinal musculature. I prefer using this technique to cover the 25 mm required to reach the insertions of the ligamentum flavum. Returning to the concept of the Caspar ratio, where the goal is to achieve a ratio of surgical target to surgical exposure as close to 1 as possible, we find that the minimally invasive laminectomy is the one circumstance where the ratio actually exceeds 1: A surgical target of 25 mm is accomplished through an incision 18 mm in length using an access port 16 mm in diameter ( ▶ Fig. 3.16).

Fig. 3.15 A conceptual drawing of a minimal access port anchored onto one limb of the triangle. The trajectory of the access port in the axial plane should be parallel to the adjacent side of the lamina.

Fig. 3.16 Artist’s illustration showing three concentric circles of exposure that indicate how subtle repositioning of the minimal access port allows sequential access to the insertion points of the ligamentum flavum along with the contralateral and ipsilateral canal.

As you proceed with the operation and reposition the minimal access port, there will be overlap with the initial field of view. That overlap allows for both a continuous exposure of the initial field of view and for completion of the remaining 7 to 10 mm of exposure. With these anatomical measurements as our guide, we have established the anatomical basis for the use of a 16- to 18-mm fixed-diameter minimal access port.

3.6 Operating Room Setup

As with a microdiscectomy, I prefer patients to be positioned on a Jackson table with a Wilson frame. The Jackson table facilitates insertion and extraction of fluoroscopy. Jackson tables that have the capacity to rotate are ideal, as they optimize the ergonomics for the surgeon. Standard operating tables with a Wilson frame will work as well but will require some complex navigation of the fluoroscopic unit around the base of the table.

I always place the fluoroscopic unit into position before prepping and draping the patient so as not to interrupt the flow of the operation after it begins. The scrub technician drapes the microscope and positions it opposite the image intensifier of the fluoroscope ( ▶ Fig. 3.17). Patients typically have symptoms that are more prominent in one leg. By convention, I will dock the access port on the lamina of the more symptomatic side indicating to my team where to position the microscope within the operating room ( ▶ Fig. 3.18). In patients who have purely neurogenic claudication, the laterality of approach is by surgeon preference. Regardless of the side of the approach, the decompression is bilateral. The objective is to decompress the entire central canal.

Fig. 3.17 Illustration of the operating room setup, which is identical to the setup for a minimally invasive microdiscectomy. (a) Bird’s-eye view of a patient undergoing an L4–5 laminectomy with a right-sided approach. In this image, the surgeon is confirming the level and planning the incision. The patient is positioned on a Jackson table atop a Wilson frame. The fluoroscope is in position at the start of case with the image intensifier on the opposite side of the surgeon and the microscope at the ready on the same side as the surgeon. Having all components in position optimizes the efficiency of the operation. (b) Lateral view of the operating room once the minimal access port has been secured. The fluoroscope transitions out as the microscope rolls into position.

Fig. 3.18 Intraoperative photograph demonstrating operating room setup. This patient has predominantly left-sided symptoms. The draped microscope stands on the symptomatic side. The image intensifier component of the fluoroscope remains opposite the microscope. The microscope also stands at the ready on the side of the approach draped to optimize the flow of the operation.

I position the patient prone atop the Wilson frame, which is then fully expanded and then palpate the bony landmarks of the anterior superior iliac spine and the intraspinous process space. The L4–5 level is approximated in this manner and marked. That mark now becomes my reference point. I then mark the incision relative to my presumptive L4–5 mark. If I am to operate on L3–4, then I mark one interspinous process space up. If I am to operate on L5–S1, then I mark one level down and so on. After I have approximated the level, I palpate the spinous processes and mark them to establish the midline. I plan the incision to be 18 to 20 mm in length and 20 to 25 mm lateral to midline, which translates to a finger breadth lateral to the spinous process. That distance is slightly more lateral than the distance for a microdiscectomy. A slightly more lateral starting point will allow for the necessary angle of convergence to accomplish a midline and contralateral decompression. By contradistinction, in a microdiscectomy, only the ipsilateral aspect of the thecal sac and traversing nerve root need to be visualized. A more medially based incision allows for that straighter trajectory ( ▶ Fig. 3.19).

Fig. 3.19 Trajectories for microdiscectomy and laminectomy. (a) Illustration of the straighter trajectory for microdiscectomy to be immediately over the traversing root and lateral aspect of the thecal sac. For such a trajectory, an incision 1.5 cm from the midline is optimal. (b) Illustration demonstrating a greater angle of convergence onto the lamina to access the entire canal. Planning an incision 20–25 mm lateral to the spinous process will optimize that trajectory. (c) Intraoperative photo of a planned incision for an L4–5 laminectomy, with superimposition of the spinal anatomy on the patient.

3.7 Localization

After the incision has been prepped and draped with the fluoroscopic unit in position at the operative level, I pass a 20-gauge spinal needle onto the lamina. Similar to localization in a microdiscectomy, care should be taken to ensure that the trajectory onto the spine is divergent from the interlaminar space, so as to minimize, if not eliminate, the risk of dural puncture. As the needle descends down its path, the tip should make contact with the lamina or facet almost immediately. If the passage of the needle seems inordinately long, then I reassess my trajectory and even obtain a fluoroscopic image. Because of the divergent trajectory, it is conceivable, although highly unlikely, that with an extreme angle the needle may pass lateral to the spine altogether. At times, securing the needle onto the spine will require what seems to be an excessively deep placement in patients with an elevated body mass index. Additional fluoroscopy can make such needle placement a more tolerable experience for the surgeon.

After I have confirmed the level, I adjust the position of the spinal needle in the rostral or caudal direction so that it is pointed to the rostral insertion of the ligamentum flavum ( ▶ Fig. 3.20). I infiltrate the incision with a combination of bupivacaine and lidocaine with epinephrine as I withdraw the spinal needle and then infiltrate more local anesthetic into the superficial incision with a hypodermic needle. When I have re-marked the optimal location for the incision, the operation is ready to begin. Throughout the entire incision planning process, I maintain the fluoroscope in position to optimize the flow of the surgery. After all, securing the minimal access port is only moments away and moving the fluoroscopy unit back and forth only results in inefficiency.

Fig. 3.20 Lateral fluoroscopic images showing localization of the level and planning the incision. (a) Confirmation of the L4–5 segment with a 20-gauge spinal needle. Note that the needle is pointing higher than the disc space and more to the rostral insertion of the ligamentum flavum. Planning the incision in the axial plane of the rostral insertion of the ligamentum flavum will allow immediate identification and release of that insertion once the lamina has been removed. With the level confirmed, the stylet is removed and the incision track infiltrated with local anesthetic. (b) A mental reconstruction of the anatomy at L4–5. Superimposed on the fluoroscopy image is the neural anatomical structures and the ligamentum flavum from rostral insertion to caudal insertion. The spinal needle points to the rostral insertion. In this image, it becomes readily evident that a 16- or 18-mm access point will encompass the rostral insertion.