8 Minimally Invasive Posterior Cervical Laminectomy

Despite the desire by many, including me, to address multiple levels of cervical stenosis with a minimally invasive approach, no such procedure has reliably achieved all the goals that can be so readily accomplished by a definitive midline open operation. One reason is that minimally invasive approaches are trajectory dependent. In the lumbar spine, an instrumented decompression and fusion may be accomplished from the same general trajectory. Pedicle screws converge onto the spine 15 to 25 degrees from the point of the incision, and the decompression requires convergence of 20 to 30 degrees in the same direction. For decompression and instrumentation of the cervical spine, these trajectories are divergent. The trajectory for placement of a lateral mass screw is entirely the opposite of the trajectory needed for a central decompression. Over the years, the divergent trajectory conundrum for instrumentation and decompression in the cervical spine has been an insurmountable Gordian knot for minimally invasive spine surgeons to unravel (▶ Fig. 8.1).

In the end, it is difficult to conceive a minimally invasive procedure that can offer both trajectories for decompression and instrumentation from one incision and thereby accomplish the goals of the operation in a circumstance that requires multiple levels of decompression. Even if we were to consider laminoplasty, a multilevel minimally invasive laminoplasty would have its own set of challenges. Finally, the nature and complexity of the posterior cervical musculature create a difficult circumstance for any multilevel minimally invasive solution. Although it pains me to write these words, the reality is that the anatomy of the posterior cervical spine is a circumstance where the clinical utility of a minimally invasive approach for posterior multilevel cervical stenosis with our current techniques is limited, at least at the time of this writing. Perhaps, in the years to come, the Gordian knot will be disentangled by a reader of this Primer.

On the other hand, a midline incision with wide exposure of the cervical lamina and lateral masses affords the surgeon the greatest capacity to safely decompress and, if need be, instrument the spine. Therefore, it would make little sense in a patient with a congenitally narrow canal and multiple levels of stenosis due to posterior compression from C3 to C6 with myelomalacia to attempt a minimally invasive procedure. It is not the incapacity to perform such a procedure. Rather, as mentioned above, the unique anatomical elements of the posterior cervical spine do not allow such a procedure to play to the strengths of a minimally invasive approach.

At times, patients present specifically requesting a minimally invasive procedure (▶ Fig. 8.2). Other times, elderly patients with comorbidities present needing a comprehensive operation with a well-intended referring physician recommending minimally invasive surgery. In both of these circumstances, it is essential to be mindful of the goals that need to be accomplished by the operation and determine whether those goals can be met through a focused minimally invasive approach. If not, proceeding with a minimally invasive option would be ill-advised and likely lead to incomplete relief, potential complications and the need for more surgery. Taking the time to counsel patients who have an understandable preference for a minimally invasive motion-preserving approach is an investment in your clinical time and the patient’s overall well-being. In the circumstance of the elderly patient with comorbidities, keep in mind that if that particular patient is not healthy enough for the appropriate operation, then they certainly are not healthy enough for the wrong one.

On occasion, cervical stenosis with dorsal compression of the spinal cord occurs focally at one segment. Such circumstances present an opportunity to apply the minimally invasive approach. Much in the same way that a minimally invasive approach is more of a liability than an asset for multilevel cervical exposures, the tables are completely reversed for a traditional midline exposure for focal compression. The strength of a midline approach is the capacity to widely expose multiple levels of the spine for both decompression and possible instrumentation. However, for focal dorsal compression of the spinal cord, that wide exposure now becomes more of a liability than an asset. Just as I would never consider a minimally invasive approach for a patient with multiple levels of cervical stenosis, it would be difficult for me to conceive of a midline approach for such a focal single-level entity in the cervical spine. Open posterior exposures themselves come at the cost of considerable disruption and devascularization of the posterior musculature.

Furthermore, the very nature of a midline approach results in disruption of the posterior tension band and the potential for postoperative progressive kyphosis. The morbidity and discomfort introduce the question of whether or not such exposure is truly needed for a single level of stenosis. After all, the majority of the exposure is the unavoidable consequence of having to reach the anatomy with conventional retractors. Therein lies the weakness of a midline approach: the need for such an extensive subperiosteal dissection of the muscle insertions off of the spinous process and lamina and the sacrifice of the posterior tension band to see a limited amount of the anatomy. Couple this with the use of self-retaining retractors that compromise the blood flow to the muscles and the skin, and it would be difficult to argue an open approach for single-level compression. As already identified throughout this Primer, open procedures do not utilize the entire field of exposure. The fact of the matter is that surgeons work through a fraction of an open exposure to accomplish the goals of the operation (▶ Fig. 8.3).

Throughout this Primer, a theme that continues to arise is what I have referred to as the Caspar ratio, which is the ratio of the surgical target to the surgical exposure. From that standpoint, minimally invasive approaches are highly efficient techniques for the simple reason that almost every millimeter of exposure is utilized to perform a procedure. On the other hand, midline open procedures, by their very nature, are not as efficient. In the occasional circumstance where there is focal dorsal compression of the cervical spinal cord, a wide exposure is not only unnecessary but also becomes burdensome. I believe that Mr. Chris Lynch captured the essence of minimally invasive posterior cervical surgery versus its open equivalent for a single-level entity by making the statement that began this chapter, “Sometimes seeing everything just gets in the way.” The minimally invasive cervical laminectomy is the embodiment of that statement and a perfect illustration of the Caspar ratio (▶ Fig. 8.4).

What I hope to convince the reader of in this chapter is that for focal dorsal compression of the cervical spinal cord, a wide exposure through a traditional midline approach is not only cumbersome, but also unnecessary. Since only a fraction of the open exposure is used to accomplish the goals of the operation, a focused exposure of the affected level is not only preferable but also ideal. Although focal single-level dorsal compression of the spinal cord may be rare, I suggest to the reader that the one approach that readily accomplishes the goals of decompression most effectively and efficiently is a minimally invasive one.

8.2 A Unique Clinical Circumstance

As mentioned in the Introduction, cervical stenosis with only dorsal compression tends to be a multilevel entity, and decompression of a single level or two is seldom the clinical circumstance for any one patient. Nonetheless, these circumstances do occur. The cause may be a degenerative, neoplastic, hemorrhagic or an infectious process. All of these scenarios are presented in the case illustrations at the end of this chapter. When such a patient presents, a mind prepared with a minimally invasive perspective sees a straightforward approach that precludes the need for exposure of the spinous process, disruption of the posterior tension band and devascularization of the paraspinal muscles. Louis Pasteur’s adage rings especially true in this circumstance, “Fortune favors the prepared mind.” The management of these focal lesions with minimally invasive approaches is more straightforward for the surgeon and better tolerated by the patient than its open counterpart.

Analogous to the lumbar spine, where the microdiscectomy serves as the platform from which to step into the lumbar laminectomy, the prerequisite for the minimally invasive posterior cervical laminectomy is mastery of the posterior cervical foraminotomy described in Chapter 7. Once you have established facility with the exposure and poise with the approach, you will confidently be able to turn the trajectory of the access port from the foramen of the cervical nerve root onto the lamina to decompress the cervical spinal cord.

8.3 Cervical Interlaminar Measurements

When I first began turning my dilators medially onto the lamina for minimally invasive cervical laminectomies, the greatest concern I had was the inadvertent passage of the initial dilator through the interlaminar space and into the spinal canal. After all, a minimally invasive posterior cervical laminectomy was distinct from a posterior cervical foraminotomy. The final target for the field of view was immediately over the central canal where the interlaminar spaces widened. For a posterior cervical foraminotomy, the target was over the nerve root and, therefore, lateral to the canal where the lamina converged. I found that passing dilators onto the lateral cervical spine was less harrowing for me than dilating over the central canal itself. Although there is always a concern for the potential passage of a dilator into the canal, the trajectory for a posterior cervical foraminotomy places the dilator in an area of the cervical spine where the laminae converge to form the articulation of the facets. The convergence of the lamina decreases the interlaminar space and thereby the theoretical risk of passage of a dilator into the canal (▶ Fig. 8.5).

Dilating with a medial trajectory onto the cervical lamina, on the other hand, has always been more irksome for me. As the tip of the dilator ventures from the lamina–facet junction toward the midline, in preparation for a midline decompression, the laminae diverge, thus creating the interlaminar space. Passage of a dilator into the canal all of sudden becomes a possibility. Intuitively, I reasoned that the interlaminar space only increases when proceeding from the rostral levels to the caudal ones. That interlaminar space increases even more when a patient is positioned in flexion, but by how much?

To allay my concerns and increase my understanding of the anatomy, I once again turned to the literature to review the various anthropometric studies that reported the dimensions of the cervical spine. In particular, I sought the measurements of the cervical interlaminar spaces and the effect that flexion had on the measurement. I felt that a greater understanding of the interlaminar measurements would make the dilatation process for a posterior cervical laminectomy less vexing. Furthermore, the knowledge would allow me to establish the theoretical risk of passage into the canal by juxtaposing the diameters of the dilators I was using to the cervical interlaminar dimensions. Understanding the interlaminar dimensions relative to the dilator diameter would replace concern with confidence as I dilated onto the cervical lamina. However, as I sifted through the various publications, I found a glaring hole in the reported measurements; there were no reported measurements for the cervical interlaminar spaces.

The various published studies reported on the dimensions within each cervical vertebral body but not the relation of one lamina relative to another. The common denominator in these publications was that they were written before the rise of minimally invasive techniques. In an open approach, knowledge of the interlaminar dimension is not essential. Dissection proceeds from the spinous process to the lamina with direct visualization. Direct visualization of the bony anatomy allows the interlaminar space to be identified and safely avoided. However, in a minimally invasive approach, knowledge of the interlaminar measurements has an increased relevance. A dilator needs to find its way directly onto the lamina without visualization of the midline structures. Equally important, the dilator needs to exert downward pressure onto the lamina to optimize the interface of the minimal access port with the lamina and prevent muscle creep. All the while, the interlaminar space remains unseen and thus susceptible to a potential passage of the initial dilator if not optimally positioned immediately over the lamina. I recognized that the risk of passage into the canal would be proportional to the size of the dilator relative to the interlaminar space. I knew the diameters of the various dilators; what I needed to find out was the dimensions of the interlaminar space.

Using eight cervical cadaveric specimens, my biomechanics team and I measured the distance between the cervical lamina in the neutral, flexed and extended positions of the cervical spine. The interlaminar distances that we measured are reported in ▶ Table 8.1.

Using these values, I generated a visual interpretation of the effect of flexion and extension on the interlaminar space (▶ Fig. 8.6).

I then collected the various diameters of various commercially available dilators and placed them into ▶ Table 8.2. Juxtaposing these two tables provided me with the theoretical risk of passage into the canal during the dilatation process.

There were several observations that I made looking at the data in ▶ Table 8.1 . The first observation is that a Kirschner wire, with a diameter of 0.9 to 1.5 mm, would have the greatest risk of passage into the canal. Conventional wisdom among minimally invasive circles had already convinced me to forgo the use of this wire in any minimally invasive approach years ago. The knowledge of the interlaminar measurements only served to reinforce that principle. The second observation was that in certain positions of the cervical spine, specifically flexion, the first dilator, and in some instances the second (depending on the system), could enter the canal. The interlaminar distances range from 3.8 to 6.6 mm in the neutral position. In flexion, the interlaminar distance increases by several millimeters, up to 7.68 at C6–7, one of the most common levels requiring surgical intervention. ¹ Therefore, the unthinkable consequence of passing a dilator through the interlaminar space was a theoretical possibility when considering the interlaminar space relative to the diameter of the first dilator (▶ Fig. 8.7).

The cervical interlaminar measurements in ▶ Table 8.1  create a crucial framework for the dilatation phase of a posterior cervical laminectomy. The first dilator needs to begin its trajectory over the top of the foramen, similar to that of a posterior cervical foraminotomy. Placing that first dilator where the laminae converge, rather than diverge, decreases, if not eliminates, the risk of passage into the spinal canal. It is important to note that, based on the measurements in ▶ Table 8.1 , the diameter of the first dilator in all of the commercially available systems has the theoretical risk of passage into the canal when the neck is flexed. The sequence of dilatation, therefore, proceeds from the top of the foramen with the smallest diameter dilator, where the interlaminar distance is narrowest. With each sequential dilator, the angle may converge increasingly onto the lamina. By the third dilator, the risk of passage into the canal becomes almost an anatomical impossibility and downward pressure against the lamina may be safely applied to optimize the lamina–access-port interface.

Before I measured the cervical interlaminar space, I passed the dilators onto the lamina with hesitation and avoided downward pressure against the lamina out of concern for passage into the canal, which resulted in suboptimal exposure and excessive muscle creep. The outcome was a suboptimal interface of the minimal access port with the lamina, and the need arose for excessive muscle resection to reach the cervical lamina. The additional muscle resection resulted in unnecessary postoperative discomfort for the patient.

I found that once I had studied the dimensions of the cervical interlaminar space relative to the diameters of the dilators, I was confidently able to establish an ideal corridor onto the requisite lamina. In this circumstance, knowledge is the true organ of sight, as quoted in the beginning of Chapter 2. Even though I could not see the interlaminar space, the knowledge of the interlaminar measurements relative to the diameter of the dilators transformed the dilatation phase of the operation. The certainty of the exact diameters of the dilators relative to the cervical interlaminar measurements is the first step in establishing this confidence. Mastery of the measurements and diameters in ▶ Table 8.1  and ▶ Table 8.2  and integrating them into the dilation process allow your mind to reconstruct the anatomy at depth. Knowledge of these values allows you to safely navigate the unexposed and unseen interlaminar space and dilate onto the cervical lamina for a minimally invasive cervical laminectomy.

8.4 Anatomical Basis

Understanding the interlaminar distance enables one to have confidence in the placement of the dilators onto the lamina but does not establish the anatomical basis for the operation. The anatomical basis for a minimally invasive cervical laminectomy becomes readily clear when the dimensions of the cervical canal and the lamina are examined. Panjabi and colleagues measured these dimensions in 12 fresh autopsy specimens (8 males and 4 females).² The mean measurements reported in Panjabi and colleagues’ paper truly provide an understanding of the cervical dimensions in the average patient (▶ Table 8.3 ).

The two most important dimensions for a minimally invasive cervical laminectomy are spinal canal width (SCW) and spinal canal depth. ▶ Table 8.3  provides these dimensions for each cervical vertebra, and ▶ Fig. 8.8 provides a visual interpretation of that data.

The average SCW among the most commonly operated lamina (C5, C6 and C7) is approximately 25 mm. The hemilamina of the cervical spine from lateral mass to the spinous process measures approximately 13 mm throughout the cervical spine. With that number in mind, it becomes readily conceivable how a 14-mm field of view that converges onto the cervical lamina allows for access to the entire width of the canal in the three positions demonstrated in ▶ Fig. 8.9.

8.5 Operating Room Setup

The operating room setup is identical to the posterior cervical foraminotomy (▶ Fig. 8.10). As mentioned in Chapter 7, the fluoroscope and microscope are positioned to optimize the transitions. I position the fluoroscope opposite the side of the approach and the microscope on the same side of the approach. I prepare my colleagues in anesthesia for a crowded space at the head of the bed (▶ Fig. 8.11). Additional room is needed at the head for both the anesthesiologist to have access to the airway and for me to have access to the posterior cervical spine, which requires the operating table to be shifted further away from the anesthesia circuit and more into the middle of the room. As meddlesome as this seems initially, I have found that my anesthesia colleagues welcome this warmly over having the patient in the seated position. Similar to the posterior cervical foraminotomy, the clamp for the table-mounted arm resides at the level of the elbow opposite the side of the approach.

Related posts:

Minimally Invasive Decompressions for Metastatic Spinal Disease Minimally Invasive Far Lateral Microdiscectomy 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

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