2 Minimally Invasive Microdiscectomy

I am constantly reminded by my colleagues who routinely perform open spine surgery that there is little, if any, difference between their open incision for a microdiscectomy and my minimally invasive incision. There is truth to that statement. After all, prospective studies examining minimally invasive and open microdiscectomies have demonstrated equivalence in clinical outcomes. 1,2​ That observation would suggest that the presumed benefits that I will present in this chapter may not have enough magnitude to be captured clinically. The circular argument begins again, and critics of minimally invasive surgery would question the point of investing the time and energy in developing a skill set that does not demonstrate a difference in clinical outcomes. I will not argue that the differences are in fact there and perhaps not adequately captured by the outcome measures in the current literature. Instead, I would emphasize that the microdiscectomy is the gateway procedure for the larger breadth of minimally invasive spine procedures. The gap in clinical outcomes between minimally invasive and traditional midline approaches widens considerably when the minimally invasive procedure is a lumbar fusion for a mobile grade 1 spondylolisthesis or a thoracic decompression of a metastatic lesion causing spinal cord compression.

The goal of beginning with the microdiscectomy is to develop familiarity with bayoneted instruments, which you will need to deftly maneuver through a constrained corridor. Equally important is the precise placement of the minimal access port in the ideal position and trajectory for completion of the procedure. The minimally invasive microdiscectomy begins the process of teaching the mind how to begin to reconstruct the anatomy at depth from a paraspinal approach and how to stay oriented during a minimally invasive procedure without the spinous process or the entire lamina as reference points. Thus, applying minimally invasive techniques to the microdiscectomy builds the foundation for those minimally invasive procedures that will unequivocally demonstrate clinical benefit to patients.

2.3 The Muscle–Retractor Interface: A Historical Perspective

The true benefit of minimally invasive surgery is not the length of the incision but the decreased pressure on the interface of the muscle and the retractor. In traditional midline approaches for microdiscectomies, the self-retaining retractor stays in position because of the force exerted simultaneously on the spinous process and muscle. As the retractor opens, the pressure increases on the muscle–retractor interface. Increasing the exposure and stabilizing the retractor requires pressure on the muscle to reach a certain point so that the retractor will not move. For instance, when using a McCulloch retractor, the hook engages the spinous process and allows the retractor blade to generate an asymmetrical force against the skin and paraspinal muscles. It is that anchor against the spinous process that displaces the skin and muscle, thus providing the exposure for the procedure. The inevitable consequence of that displacement is that blood flow to the paraspinal muscles plummets ( ▶ Fig. 2.1). ³

In open midline exposures of the spine, the more the retractor is opened, the greater the exposure but the more compromised the blood flow to the skin and muscle. Exposure and blood flow are diametrically opposed to one another. The sustained compromise in blood flow from the muscle–retractor interface over time can add to the postoperative discomfort experienced by patients and may contribute to the atrophy observed on postoperative magnetic resonance imaging (MRI) years after surgery. The potential effect that sustained retraction has on the blood flow to the skin edge may also contribute to the differences in infection rates between minimally invasive approaches and their open equivalents. 4

The muscle–retractor interface has been of interest to spine surgeons for decades. The spine surgery literature is replete with concerns about the downstream consequences of muscle retraction in spine surgery long before the rise of modern minimally invasive spine surgery. That body of literature reads almost as if it were a collective plea for a better way. Kawaguchi and colleagues ³ wrote perhaps one of the finest articles that illustrates the immediate effects of blood flow by traditional midline retractors. A carefully thought-out porcine laminectomy model elegantly demonstrated the effect of retractors on blood flow and offered the clearest explanation of the atrophy that occurs after lumbar laminectomies ( ▶ Fig. 2.2). ³ What is most remarkable is that their article was published well before Foley and Smith ⁵ described a paraspinal transmuscular approach with a table-mounted access port, which collectively addressed the blood flow at the muscle–retractor interface and the skin edge.

In the minimally invasive application, the minimal access port maintains its position not by the pressure generated at the muscle–retractor interface but instead by a table-mounted arm ( ▶ Fig. 2.3). ⁵ Herein lies the main advantage of the minimally invasive approach: The pressure is relieved from the paraspinal muscles and transferred to the table-mounted arm. Unlike an open exposure, the minimally invasive approach exerts no significant force on one side of the skin and muscle. The stability of the access port is dependent on the table-mounted arm, not on the pressure generated by the muscle–retractor interface ( ▶ Fig. 2.4). As a result, there is no significant decrease in blood flow to the muscle or the skin. The putative benefits are less postoperative discomfort and less paraspinal muscle atrophy. The exceptionally low infection rate reported for minimally invasive approaches further supports the optimization of blood flow at the skin and muscle–retractor interface. ⁴

Equally important is a cylindrical-shaped minimal access port. Conceptually, a cylinder is the most efficient shape for distributing the forces of pressure to the entire circumference of an incision and muscle. Distribution of the forces equally to the skin and muscle prevents the asymmetric compression of one side of the incision. Thus, whatever forces exist at the muscle–retractor interface are equally distributed by the cylindrical shape of the access port ( ▶ Fig. 2.4).

Caspar 6 was the one who actually introduced the concept of a cylinder in lieu of a self-retaining retractor in his 1977 chapter on advancements in microdiscectomy. He had observed that the ratio of exposure to the surgical target was far too great in spine surgeries. Caspar’s objective was to decrease the ratio of the requisite anatomy to the surgical exposure. To that end, he developed a speculum-type retractor that could be placed precisely over the requisite anatomy ( ▶ Fig. 2.5). ⁶

Although Caspar ⁶ appears to have been the first to describe the use of a cylindrical speculum to minimize tissue trauma, the focus of his chapter was the use of the operating microscope to optimize visualization, which he and Yaşargil ⁷ both championed. Caspar’s prescient development of a focused cylindrical port, which is strikingly like modern-day access ports, seems in large part lost to the pages of history. Caspar began his remarkably insightful manuscript with the statement:

Caspar’s observation that the exposure is disproportionately large compared to the requisite anatomy demonstrates his awareness of the need to increase the efficiency of the surgical exposure. The Caspar ratio, which I introduced in the introduction to the Primer, precisely defines the surgical efficiency as the ratio of the surgical target to the surgical exposure. Striving for a Caspar ratio of 1 is a central tenet of minimally invasive spine surgery. To remedy the perceived shortcomings of the lumbar herniation operations performed in his day, Caspar 6 set forth the following criteria:

1. 
   

   Precise monosegmental access.

   
   
2. 
   

   Minimal lesion in the approach to the actual area of surgery.

   
   
3. 
   

   Better visual clarity (use of the operating microscope) in depth and thus more gentle manipulation of the nerve root and dural sac.

These criteria ring as true now as they did when Caspar wrote them in 1977. Collectively, adherence to these measures set a trajectory that could lead only to the current minimally invasive techniques we have today. These techniques are the inevitable outcome of fully realizing and adhering to Caspar’s criteria.

Caspar’s concept of a cylindrical speculum resolves several aspects of the muscle–retractor interface evenly distributing the forces to the skin and muscle. The concept was an important advancement but fell short because of the inability to maintain the stability of the speculum. My conversations with surgeons who attempted procedures through the speculum indicate that the lack of stability may have been what limited the wide adoption of the Caspar concept. Twenty years would pass before the stability of a cylindrical access port would be resolved with the introduction of the table-mounted arm by Foley and Smith. 5 Displacing the forces away from the incision and onto a table-mounted arm further minimizes the muscle–retractor interface. Foley and Smith refined the approach by shifting the incision completely off the midline, thereby eliminating the subperiosteal dissection and instead using a paraspinal transmuscular approach to the spine. Although endoscopic visualization was initially used, the microscope has become a more practical form of visualization today. The microdiscectomy performed through a paraspinal transmuscular minimal access port, secured to a table-mounted minimal access port and visualized through a microscope, is the technique I describe in this chapter. ^5,8

2.4 Requisite Anatomy

The advice I give my residents and fellows when we are performing a minimally invasive microdiscectomy together for the first time is for them to not reinvent the wheel. In other words, they should not approach a minimally invasive microdiscectomy as if it were a distinct procedure from its open counterpart. By the time residents and fellows do a rotation with me, they have already gathered some significant experience with the microdiscectomy. They have assisted in dozens of open microdiscectomies and have likely performed several virtually on their own. Their familiarity with the anatomy at depth enables them to successfully perform an open procedure—or at least to recognize how the exposure should look. These residents and fellows have reached a point in their training where reconstruction of the anatomy from memory is ripe for development. So, the first thing I emphasize to these residents is to make the exposure through the minimal access port exactly what they would expect to see if the operation was a midline open approach. It is important to recognize that the exposure in a minimally invasive operation requires, by its very nature, an element of recall memory. If you cannot see all the requisite bony landmarks that you would see in an open procedure when the minimal access port is in position, then stop and reassess. No good will come from taking a drill to anatomy about which you are uncertain.

The first logical question when considering a minimally invasive approach for a microdiscectomy is: What is the requisite anatomy for any microdiscectomy, open or minimally invasive? Compare the traditional midline microdiscectomy exposure to a minimally invasive microdiscectomy exposure with the requisite anatomy for the procedure ( ▶ Fig. 2.6) as defined by Williams ⁹ and corroborated by Maroon. ¹⁰

In ▶ Fig. 2.6 ⁹ the yellow highlighted area on the right of the lumbar spine represents the requisite anatomy for a microdiscectomy. Regardless of the technique you use, the lateral aspect of the thecal sac, the traversing root and the disc space must be visualized to enable you to perform the procedure. Note that the midline is not necessary for the procedure itself. ▶ Fig. 2.6c demonstrates the superimposed exposures of an open approach and a minimally invasive approach.

In an open procedure, the 1-inch or so incision is made either on the midline or slightly off the midline in the direction of the symptomatic nerve root. The medial aspect of the spinous process is exposed to allow for a subperiosteal dissection of the paraspinal muscle to unveil the lamina. The medial facet is exposed with caution so as not to interrupt the facet capsule. Typically, a Williams retractor, a McCulloch retractor or an equivalent anchors into the spinous process and is used to retract the muscle over the lamina to the medial facet after it has been exposed through the dissection.

The only difference between a minimally invasive exposure and an open exposure should be the absence of the medial aspect of the spinous process and the medial lamina, neither of which is actually required for the operation. There are no neuroanatomical structures needed for the procedures beneath the spinous process and medial lamina relevant to this procedure. Their exposure is the inevitable consequence of a midline approach. It is worth mentioning that neither the spinous process nor the medial lamina was part of the exposure in the microdiscectomy that Caspar described in 1977. ⁶ Other than that, the exposures should be identical. As a minimally invasive surgeon, you must adopt the mentality that a microdiscectomy with a minimal access port is simply another way to perform the same operation that otherwise would have been done with a midline incision and conventional retractors.

2.5 Efficiency of the Exposure

As demonstrated in ▶ Fig. 2.7, a magnified version of ▶ Fig. 2.6c, the minimum requisite anatomy established by Williams ⁹ is the inferolateral aspect of the rostral lamina, the medial facet and the superior and lateral aspects of the caudal lamina. A well-positioned 16-mm-diameter ( ▶ Fig. 2.7b) access port readily encompasses all the requisite anatomy. A midline approach with a conventional retractor does not expose more of the requisite anatomy; it exposes anatomy not necessary for the operation ( ▶ Fig. 2.7c). Remember, there is no need to try to reinvent the wheel. It is a mistake to attempt to make the operation any different from the one with which you are already familiar. The goal is to leverage the experience and skill set you have acquired from open microdiscectomies and apply them to a minimally invasive approach so that you can become familiar with the instruments and the subtleties that an altered trajectory has on the appearance of the surface anatomy.

A thoughtful examination of ▶ Fig. 2.7c demonstrates that the midline incision with a conventional lumbar spine retractor is actually outside what is necessary to perform the operation. By its very nature, this approach requires a more rostrocaudal extension of the incision to displace the muscle and skin laterally enough to expose the requisite anatomy. The exposure is more a consequence of a midline incision needing to be long enough to allow enough exposure to reach the medial facet. The result is a triangular-shaped exposure, where the rostral and caudal limbs of the triangle are a consequence of the exposure but are not needed at all for the operation. It is important to note that a midline approach does not offer any more exposure of the requisite anatomy necessary for the operation than a well-positioned minimal access port.

The most efficient shape for exposure of the requisite anatomy is circular, as Caspar 6 championed in 1977. If the efficiency of an exposure was to be measured by the ratio of requisite anatomy to exposed anatomy, then a well-positioned cylindrical access port would demonstrate a greater efficiency than the triangular-shaped conventional midline exposure. Although the difference may be slight for a microdiscectomy, the efficiency of exposures becomes more pronounced the more lateral the requisite lies. The far lateral microdiscectomy is a perfect illustration of this concept.

2.6 Anatomical Basis

As residents and fellows begin to perform these procedures, I can sense their concern regarding the amount of exposure necessary for the procedure and whether they will be able to achieve enough exposure through the minimal access port. So, a word or two is warranted about the anatomical basis of the operation within the context of the various diameters of the access port. It is tremendously valuable to have a few anatomical measurements at your fingertips as you begin to explore minimally invasive microdiscectomies. The quote from the ancient Panchatantra that opened this chapter rings especially true here: “Knowledge is the true organ of sight.” In this case, your knowledge of the anatomy will help orient your mind and increase your certainty of the anatomy at depth. You need not see everything to know where everything is. You do, however, need to possess the knowledge of the anatomy to have the “true organ of sight” in a minimally invasive approach. That knowledge goes beyond your recognition of the anatomy. Your goal must be complete recall of the anatomy from your memory and the aptitude to reconstruct the anatomy at depth.

Keep in mind that there are more constants than variables when it comes to the lumbar spine. While disc heights may vary even within the same patient, depending on the degree of degeneration, the diameter of the canal and the distances between the pedicles are remarkably constant. ▶ Fig. 2.8 ¹¹ and ▶ Fig. 2.9 illustrate the various measurements of canal diameter, interpedicular distance, intrapedicular distance and foraminal height. ¹¹ Having a general sense of these measurements instills confidence when you are peering down a 16-mm-diameter access port and wondering about what is all the way down at the bottom.

As determined by Panjabi et al, 11 the intrapedicular distance (i.e., the distance between the pedicles of the same vertebral body) is on average 24 mm (range, 23–27 mm). Knowledge of this measurement indicates that the distance from the spinous process (the midline) to the pedicle is seldom more than 12 mm. The interpedicular distance (i.e., the distance between adjacent pedicles) will be inherently tied to the disc space and the level. The distance is less in patients with advanced collapse of the disc space than in healthy disc spaces. At L5–S1, where lordosis is greatest, thereby bringing the pedicles closest together, the interpedicular distance has the smallest value (typically about 28 mm), while at L3–4 where there is less segmental lordosis, the distance can increase to as much as 35 mm. The final measurement is the disc height. Although disc heights are directly related to the degree of degeneration, the height, even in a healthy disc, is seldom more than 14 mm. Collectively, these measurements point the way to the anatomical basis for a minimally invasive microdiscectomy. ▶ Fig. 2.9 demonstrates the various distances from the junction of the base of the spinous process and lamina to the medial facet and over the top of the disc space. A 16-mm diameter offers ample access to the neural elements and the bony anatomy to enable you to readily perform the operation.

At the same time, a poorly placed access port makes the operation virtually impossible. When all you have is a 16-mm-diameter view, the precise placement of that diameter is imperative. How does one ensure that the requisite anatomy at depth will be there when the cautery has cleared off the soft tissue over the top of the lamina? The answer is threefold: preoperative planning, fluoroscopic guidance and sounding the anatomy.

Much in the same way we can place a ventricular catheter into the third ventricle from a point 11 cm back from the glabella and 2 cm over from the midline just as easily as we can from a point 10 cm back and 3 cm over, the incision used for a minimally invasive microdiscectomy can vary. Surgeons have recommended 1, 2 and 3 cm from the midline. Any of these distances may work because the exposed anatomy will be more a function of the trajectory. However, I prefer an incision 1.5 cm off the midline, as recommended by Foley and Smith, 5 for reasons I will expound on later. Some surgeons advocate the benefits of a transverse incision, while others see no reason not to start directly over the relevant anatomy. Again, it is not so much the incision or the location; it is what you do with that incision to accomplish the requisite exposure.

2.6.1 Anatomical Basis for Minimal Access Diameters

At this point, it is germane to discuss the diameter of the minimal access port. Of the various diameters available, 18 mm is an ideal one with which to start. Although diameters up to 22 mm are available, the measurements on ▶ Fig. 2.9 demonstrate that there is no anatomical basis for such a large diameter for a microdiscectomy. If anything, diameters larger than 18 mm may be more of a liability than an asset. Such a diameter can disrupt structures that lay outside the requisite anatomy for the operation. The illustration of the lumbar spine in an axial view shown in ▶ Fig. 2.9 demonstrates that the distance from the lateral aspect of the spinous process to the medial facet at the L3, L4, and L5 levels is no more than 16 mm.

The distance from the center of the disc space to the medial aspect of the pedicle is no more than 18 mm at L3, L4, and L5, as demonstrated in ▶ Fig. 2.10.

The measurements listed in ▶ Fig. 2.9 and ▶ Fig. 2.10 show how the necessary exposure for a microdiscectomy—open or minimally invasive—need not expose more than 18 mm lateral to the spinous process. In fact, more than 18 mm may actually be a liability, because it may expose the facet capsule. Furthermore, with these measurements in mind, one can argue that a well-positioned 16-mm, or even a 14-mm, minimal access port provides ample exposure of the requisite anatomy from a mediolateral standpoint. Ideal placement and trajectory are paramount for these diameters, as every millimeter of exposure will be indispensable for the operation.

From a rostrocaudal standpoint, the distance from just above the disc space into the foramen measures approximately 16 mm. The illustration of the lumbar spine in the coronal projection in ▶ Fig. 2.11 demonstrates that 16 mm allows the lateral aspect of the nerve root at S1 to be followed from the medial facet into the foramen.

These rostrocaudal and mediolateral measurements establish the anatomical basis for the diameters used in minimally invasive microdiscectomies. It quickly becomes evident from these measurements that an 18-mm-diameter port offers a generous exposure. With experience, a 16-mm-diameter port or even a 14-mm-diameter port will become a feasible option ( ▶ Fig. 2.12).

2.7 Operating Room Setup

Always set up your operating room in a manner that will optimize the flow of the operation and minimize idle time when the patient is under anesthesia. Idle time can occur at transition points, such as while waiting for the microscope or securing the table-mounted arm. To prevent delays, the scrub technician, operating room nurse and radiology technologist should all know their roles to efficiently get you to the point of making an incision, docking the access port and working under the operating microscope.

For the operating room setup, I position the microscope on the side of the prone patient’s symptoms and the fluoroscope opposite the side of the radiculopathy. The scrub technician begins to drape the microscope as the patient is being anesthetized. Consequently, by the time the minimal access port is in position, there is no delay in transitioning to the microscope in the surgical field ( ▶ Fig. 2.13).

The operating room nurse secures the table mount and the clamp that anchors the table-mounted arm to the bed. These steps are taken before draping but immediately after positioning the patient ( ▶ Fig. 2.14).

The radiology technologist brings in the fluoroscopy unit and places it at the level of the patient’s knees to facilitate draping ( ▶ Fig. 2.15). The smaller X-ray tube is on the side of the surgeon, whereas the bulky image intensifier is on the asymptomatic side opposite the surgeon. Such a configuration facilitates draping the fluoroscope into the surgical field and eventually allows for efficient removal of the fluoroscope and transition to the operating microscope. There is no need for preoperative fluoroscopic images, consistent with the focus on minimizing radiation exposure. Preoperative fluoroscopy will not eliminate the need to obtain images as you dock the access port; so, it is best to reserve those images for the actual procedure.

After placing the sterile drapes on the patient and the fluoroscope, the scrub technician, with the assistance of the operating room nurse, secures the sterile arm to the table-mounted bracket before bringing in the Mayo stand. That proactive maneuver eliminates delays once dilatation of the incision is complete and the minimal access port is ready to be secured. All these steps are part of the unspoken routine for the minimally invasive ensemble, which emphasizes the importance of assembling such a team ( ▶ Fig. 2.16).

2.8 Patient Positioning

I prefer to use a Wilson frame atop a flattop Jackson table for minimally invasive microdiscectomy operations. The Jackson table allows for unencumbered passage of the fluoroscope up and down the underside of the table. With the patient in position, I crank open the Wilson frame to its fully expanded curvature. The arc created by a fully expanded Wilson frame opens the interlaminar space and thereby limits the bone work necessary to access the lateral aspect of the canal, mobilize the nerve root, and remove the herniated disc. The Wilson frame also indicates the ideal position for the clamp that will secure the table-mounted arm. I place this clamp at the base of the Wilson frame opposite the side of the symptoms ( ▶ Fig. 2.13 and ▶ Fig. 2.15). This particular position of the clamp creates an ideal configuration for the table-mounted arm to readily secure the minimal access port in a low-profile manner. For disc herniations at L2–3 and L1–2, this clamp should be moved up to the midportion of the Wilson frame. As mentioned previously, securing the clamp should be part of the unspoken ritual of the smoothly functioning minimally invasive ensemble.

A Jackson table is not absolutely necessary, but it is preferable because it facilitates positioning of the fluoroscopic unit and allows its movement to the foot or the head of the bed. Attempting to navigate around the base of a standard operating room table can be more challenging and disruptive to the flow of the operation.

2.9 Surgical Technique

2.9.1 Conceptualizing the Position of the Minimal Access Port

A well-placed minimal access port will allow the surgeon to begin operating immediately after bringing in the microscope, without concern that the port is not ideally positioned over the requisite anatomy. Ensuring that the exposure at the bottom of the port will be identical to what could be accomplished with a midline approach before peering down the operating microscope requires adherence to certain principles. The first principle is to ensure that the operating corridor is precisely in the axial plane of the disc herniation. Such positioning is achieved by maintaining the minimal access port completely parallel to the disc space. The first step in achieving that position entails precisely planning the incision ( ▶ Fig. 2.17).

Related posts:

Minimally Invasive Decompressions for Metastatic Spinal Disease Minimally Invasive Posterior Cervical Laminectomy Minimally Invasive Far Lateral Microdiscectomy Minimally Invasive Lateral Transpsoas Interbody Lumbar Fusion Anterior Cervical Discectomy with Arthroplasty or Fusion Minimally Invasive Transforaminal Lumbar Interbody Fusion

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