4 Minimally Invasive Transforaminal Lumbar Interbody Fusion
Keywords: facet, interbody fusion, Kambin triangle, pars interarticularis, pedicle screw, spondylolisthesis, transforaminal corridor, transverse process
The eyes will see only what the mind is prepared to comprehend.
Henri Bergson
4.1 Introduction
The previous two chapters focused on the decompression of the neural elements with minimally invasive techniques. This chapter continues to build on that skillset and introduces instrumentation of the spine through minimally invasive approaches. It is the experience of minimally invasive microdiscectomies and laminectomies that creates a natural transition from decompression to instrumentation of the spine through a paramedian approach beginning off the midline. The familiarity of the anatomy that you have acquired through a minimally invasive perspective has laid a foundation for the techniques that I describe in this chapter. It is important to recognize that the transforaminal approach is more lateral, and the angles involved are more acute than those in the microdiscectomy or laminectomy ( ▶ Fig. 4.1). For that reason, I will once again invoke the principle that the farther off the midline the working channel resides, the more potential there is for disorientation with the anatomy at depth. If you are cognizant of the potential cause of disorientation, you will be able to work to eliminate it. As the distance from the midline and the angles of convergence continues to increase, you must use the most recognizable and familiar structure that is off the midline as a beacon for orientation: the facet joint.
Fig. 4.1 Illustration demonstrating the distance off the midline and the angle of convergence at L4–5 for a microdiscectomy, laminectomy and transforaminal lumbar interbody fusion. (a) The position of a minimal access port for a microdiscectomy at 1.5 cm off the midline with an angle of convergence of 5 degrees. (b) The position of a minimal access port for a laminectomy at 2.0-2.5 cm off the midline with an angle of convergence of 10 to 15 degrees. (c) The position of an expandable minimal access port 4.0 cm off the midline with an angle of convergence of 25 degrees encompassing the facet joint.
In the minimally invasive transforaminal lumbar interbody fusion (TLIF), the facet joint is the North Star that will establish your bearings ( ▶ Fig. 4.2). The facet guides you to the entry points for your pedicle screws, helps establish the boundaries for decompression, and offers transforaminal access to the disc space. When the exposure at depth is complete, the anatomy should be as equally obvious as it is if the approach were through a traditional midline approach. It has been the minimally invasive decompressions, both microdiscectomies and laminectomies, that have prepared your mind to comprehend the anatomy from this lateral and angled vantage point. The three-dimensional anatomical knowledge of the spine that your mind can now re-create at depth is what will allow you to do more through 28 mm of exposure than you otherwise would through an open exposure twice that size. As Henri Bergson eloquently stated, “the eyes will see only what the mind is prepared to comprehend.”
Fig. 4.2 The North Star of the minimally invasive transforaminal lumbar interbody fusion: the facet joint. The blue fiducial indicates the docking point for the first dilator. Pedicle screw entry points (marked with red fiducials) are only millimeters away from the blue fiducial as are the osteotomy cuts (dotted lines). The transforaminal access into the disc space is relative to this joint.
4.2 Minimally Invasive TLIF: A Heterogeneous Entity
Before describing the minimally invasive TLIF in this chapter, it is important to acknowledge that there is no universally accepted minimally invasive transforaminal approach. Over the years of its development, surgeons have combined several existing technologies that include percutaneous placement of pedicle screws with minimal access approaches, unilateral fixation, facet fixation, and combinations thereof. Fluoroscopy and a variety of forms of image guidance systems have been the mainstay of imaging for the procedure. A PubMed search including the terms “minimally invasive transforaminal lumbar interbody fusion” and “technique” since 2005 generated over 100 references. However, despite the multiple permutations of this procedure, when one distills the various forms of the minimally invasive TLIF from these references, three main techniques arise:
Percutaneous placement of pedicle screws and decompression with interbody placement through a fixed tubular retractor as described by Fessler and Foley.1,2
Use of expandable minimal access retractors for bilateral direct exposure of the bony anatomy and placement of pedicle screws, decompression and interbody placement as described by Mummaneni and Rodts.3
A hybrid technique of 1 and 2: Percutaneous pedicle screw placement on one side of the anatomy and use of an expandable minimal access port for pedicle screw placement decompression and interbody placement on the other.
Over the years, I have employed all the above-mentioned TLIF permutations and have settled on one approach. My journey to that one particular approach was a philosophical one. Throughout this book, I have emphasized that the procedures performed with a minimally invasive approach should be indistinguishable from the same procedure performed with an open approach. Applying that criterion implies that the decompression performed through a minimal access port would need to be indistinguishable from its open equivalent. The same would apply to the actual fusion construct, which includes the interbody and posterolateral arthrodesis. I was also mindful of the various criticisms of the minimally invasive TLIF, specifically that the procedure takes far too long, exposes the surgeons to too much ionizing radiation because of the reliance on fluoroscopy, does not adequately restore segmental lordosis, and does not allow for adequate central decompression. As I evolved my surgical technique, I incorporated these criticisms of the minimally invasive TLIF along with my philosophical standard that whatever procedure I performed in a minimally invasive manner should be indistinguishable from that of the open procedure. The procedure that I believe allowed me to meet all of these criteria while simultaneously addressing the perceived shortcomings of minimally invasive surgery (MIS) was a TLIF performed through direct bilateral exposure of the requisite anatomical structures using expandable minimal access ports.
4.3 Rationale: Percutaneous versus Direct Placement of Pedicle Screws
There is nothing more familiar to a spine surgeon than directly looking upon the junction of the pars interarticularis, transverse process and inferior lateral aspect of the lumbar facet to envision the entry point for a pedicle screw in the lumbar spine ( ▶ Fig. 4.3). The direct exposure of these anatomical landmarks allows for anatomical certainty that a fluoroscopic image or even a computer-generated navigation image cannot supplant. Medial to these landmarks are the lamina and the spinous process, which house the compressed thecal sac and nerve roots. While I am exposing entry points for the pedicle screws, I am simultaneously exposing the requisite anatomy for a decompression and transforaminal access to the disc space. Thus, two elements of the operation occur concurrently, moving the whole operation forward.
Fig. 4.3 The anatomy of the lumbar pedicle. Illustration demonstrating the pedicle screw entry point for L5 (a) and L4 (b) at the junction of the pars interarticularis, transverse process and facet denoted by the red fiducial. (c) The trajectory for an L5 pedicle screw converging at 25 degrees into the pedicle.
I secure the expandable minimal access ports over the top of the facets on either side and begin the exposure of the pedicle screw entry points. No further fluoroscopy is immediately necessary. In fact, the fluoroscope may be moved away from the operative field until the exposure is complete. I identify the four pedicle screw entry points with direct visualization of the anatomical landmarks within the first 15 minutes of the operation through two well-positioned access ports. With the exposure complete, the fluoroscope is rolled back into the operative field so that I can confirm entry points and trajectories into the pedicles. I drill the entry points, probe through the cancellous bone with a pedicle probe and ensure the integrity of the cortical wall of the pedicle with a ball-tipped probe. I tap the pedicle, determine the width and length of the pedicle and place the screws with only a few lateral fluoroscopic images. On average, there will be no more than four or five fluoroscopic images obtained per instrumented pedicle.
Percutaneous placement of pedicle screws, on the other hand, is either a fluoroscopically driven or image-guided process. The absence of direct visualization of the bony anatomy precludes the ability to use the skill set that we all developed in open surgery. It eliminates our tactile feel of the pedicle that allows us to ensure the integrity of the pedicle and replaces it with anteroposterior (AP) and lateral fluoroscopic images of a Jamshidi needle advancing a K-wire. The absence of direct visualization of the bony anatomy mandates the need for additional fluoroscopic images to guide the placement of the instruments, thereby increasing the radiation exposure to the patient, surgeon and operative team. I readily concede that use of image guidance nullifies this point; however, the absence of tactile feedback remains an issue, at least for me.
The lack of access to the facets and transverse processes is another concern I have with percutaneous techniques. The inability to perform a posterolateral fusion, facetectomy or Smith–Petersen osteotomy is a considerable limitation. With a paramedian incision and direct visualization of the pedicle screw entry points, the transverse processes come fully into view, allowing the drill to thoroughly decorticate and then heap a bounteous amount of morselized autograft, allograft or both onto them to achieve a posterolateral arthrodesis. Paramedian exposures of the transverse process are vastly superior to a midline approach for access to the posterolateral space. I am routinely able to drill the entire transverse process with direct visualization all the way to its lateral tip. An equivalent exposure in an open midline approach would be quite a feat requiring extensive dissection and added length to what is already a sizeable incision.
Finally, the importance of achieving segmental lordosis in transforaminal approaches cannot be overemphasized. One of the weaknesses identified in the literature with transforaminal approaches is the limitation to restore lordosis.4 In patients with either iatrogenic or degenerative flat back, restoration of segmental lordosis is a vital objective of the operation. A percutaneous approach to the pedicles creates an inherent limitation to achieve segmental lordosis because the contralateral facet remains entirely unexposed and intact. The absence of bilateral facetectomies or the capacity to perform a Smith–Petersen osteotomy on the side contralateral to the transforaminal corridor limits the degree of compression you can achieve and thereby limits the restoration of lumbar lordosis.5,6,7
The distinct advantage of an expandable minimal access port is that it permits simultaneous exposure of the entire facet, lamina and pars interarticularis with the port in the same position that was used for the instrumentation. A complete facetectomy on the side of the transforaminal approach and a Smith–Petersen osteotomy on the opposite side, as recommended by Shaffrey and colleagues,8 optimize the restoration of the lordosis that would be difficult to achieve when one facet remains unexposed.
The advantages of percutaneous technology surpass minimal access direct visualization approaches in multilevel operations, specifically for three or more levels. Although the minimal access approaches described in this chapter become untenable after two levels, the percutaneous techniques offer the possibility for the least disruption of the native spine in the stabilization of multiple levels. In a five-level fusion, direct visualization through a paramedian approach no longer plays to the strength of that technique. Percutaneously placed pedicle screws in that circumstance offer a distinct advantage, and it is always wise to play to the strengths of an individual technique.
4.4 Rationale: Decompression—Fixed Tube versus Expandable Retractor
For guidance regarding the type of minimal access port to use, I look no further than the anatomy. In my estimation, the decompression in any TLIF, whether minimally invasive or open, should include a pedicle-to-beyond-pedicle decompression. The entire thecal sac should be decompressed along with the exiting and traversing roots at the operative segment. As seen in Chapter 2 and shown in ▶ Fig. 4.4, the interpedicular distances range from 36 mm at L3–4 to 28 mm at L5–S1. Therefore, to achieve a pedicle-to-beyond-pedicle decompression, one would need exposure from the inferior aspect of the rostral pedicle to the superior aspect of the caudal pedicle. In most circumstances in the lumbosacral spine, this would require 26 to 32 mm of exposure. The minimal access port that allows for such an exposure all at once is the logical one to use.
Fig. 4.4 Illustration of the interpedicular distances in the lumbar spine. This illustration from Chapter 2, Minimally Invasive Microdiscetomy, is especially relevant not only for the anatomical basis of the minimally invasive transforaminal lumbar interbody fusion but also for the selection of the minimal access port. The access port that can simultaneously encompass the relevant anatomy of an entire segment is the logical access port to employ for the procedure.
Early in my experience, I used a 22-mm fixed access port for decompression and interbody placement. I could readily decompress the traversing root but had difficulty exposing and visualizing the exiting root within the same field of view. Decompression of the entire thecal sac was possible by angling the tube medially, but I found myself making multiple adjustments to fully expose the anatomy that required decompression. When the decompression was completed, I was unable to visualize all the anatomy that I had decompressed within one field of view. Regardless of the trajectory of the port, some element of the anatomy was always outside of that field of view. Although this limited visualization is acceptable for a simple decompression, the inability to visualize all the anatomy, specifically the exiting nerve root, when placing an interbody through a transforaminal approach, presented some potential hazards to that exiting root.
Placement of the interbody spacer through a fixed access port became another issue. Such a feat was technically difficult because of the constraints of a fixed diameter. I found that the access port dictated the geometry of the interbody spacer that I would place, specifically the straight spacers designed for posterior lumbar interbody fusions (PLIFs), instead of those with curved geometry, which better occupy the apophyseal ring in the anterior aspect of the disc space. Finally, visualization during placement of the interbody was limited. I was unable to feel comfortable blindly retracting the nerve root while simultaneously securing the interbody into position. For these reasons, I began to explore the role of an expandable minimal access port through which to perform the decompression and secure the interbody spacer.
I also found that I was better able to visualize the base of the spinous process with a well-placed mediolateral retractor and thereby achieve a midline and contralateral decompression. The exposure that I could achieve with an expandable minimal access port allowed for a pedicle-to-beyond-pedicle decompression of the thecal sac, along with the decompression of both the exiting and traversing nerve roots. The exposure further allowed me to use a curved interbody geometry without compromising my visualization of the neural elements. Finally, I felt much more comfortable retracting the traversing nerve root under direct visualization while securing the interbody into position.
In the final analysis, the rationale for the technique that I describe in this chapter evolved to optimize visualization and decompression of the neural elements and facilitate placement of the interbody while minimizing the need for fluoroscopy for placement of pedicle screws on the ipsilateral side. On the contralateral side, an expandable minimal access port allows for placement of pedicle screws with minimal fluoroscopy and offers access to the transverse processes for a posterolateral fusion and for a facetectomy or Smith–Petersen osteotomy to restore segmental lordosis. Collectively, I feel that employment of a combination of these techniques increases the efficiency of the operation and is more consistent with performing these procedures in ambulatory surgical centers, where computer-assisted navigation may be cost prohibitive. But most importantly, achieving all these objectives optimizes the long-term outcomes.7
4.5 Requisite Anatomy
The requisite anatomy for a single-level TLIF that allows for transforaminal access on one side, a posterior column osteotomy and a posterior lateral fusion on the other, is demonstrated in ▶ Fig. 4.5. Whether you are performing the procedure through a traditional midline approach or a minimally invasive one, the requisite anatomy includes access to the pedicle screw entry points, pars interarticularis, facets, lamina and transverse processes of a segment. Notably absent from the list of the requisite anatomy are the spinous processes, whose exposure in a midline approach is the inevitable consequence of the location of the incision. Access to the pedicles employing percutaneous techniques slightly narrows that exposure but comes with the inherent limitations detailed above. For a two-level TLIF, the exposure includes another set of pedicles, laminae, and facets. I believe that a three-level minimally invasive TLIF resides outside of the strengths of MIS with direct visualization as described in this chapter. Multiple levels of interbody and pedicle instrumentation cross into a realm where percutaneous technology now offers greater advantages than disadvantages.
Fig. 4.5 Requisite anatomy for a (a) single-level and (b) two-level transforaminal lumbar interbody fusion. Whether performed minimally invasively or through a traditional, midline open approach, the pedicles, facets, medial aspect of the lamina and the pars interarticularis are all needed for the operation. In both of these illustrations, the requisite anatomy is demarcated in red, and the pedicles are demarcated in blue.
4.6 Anatomical Basis
The anatomical basis for the minimally invasive TLIF is determined by the angle of convergence into the pedicle. In Section 4.4, Rationale: Decompression: Fixed Tube Versus Expandable Retractor, I touched on the anatomical basis for using an expandable minimal access port for minimally invasive TLIFs, but not the anatomical basis for applying minimally invasive techniques for the TLIF itself. However, one glance at the requisite anatomy highlighted in ▶ Fig. 4.5 begins to build that argument for me. After all, ▶ Fig. 4.5 demonstrates that all the requisite anatomy is lateral, not medial. Beginning in the midline will require a considerably longer incision not only for exposure of the requisite anatomy but also to attain the converging angles into the pedicle. The optimal angle into the pedicle is from lateral to medial as seen in ▶ Fig. 4.6. From that standpoint, the most direct and efficient access to a segment would be from a paramedian starting point that converges onto the epicenter of the requisite anatomy, which is the facet. From there, the surgeon has ready access to the pedicles that are only millimeters away. Finally, the trajectory into the pedicle would be parallel to the trajectory of the access port. It has been my experience with a midline approach that I am waging a battle against the skin and muscle to reach the lateral aspect of the exposure to accomplish the same angle.
Fig. 4.6 The anatomical basis of a minimally invasive transforaminal lumbar interbody fusion. In this posterior view of the spine, the optimal angle into each pedicle from L3 to S1 is demonstrated on the left side of the spine illustration. A lateral to medial converging angle into the pedicle is most effectively accomplished with a paramedian incision converging onto the facet instead of beginning in the midline and attempting to accomplish that medially converging angle by working in an increasingly lateral direction. The right side of the spine has the posterior elements removed to demonstrate the interpedicular distances in the lumbosacral spine. A midline incision mandates a longer exposure to reach the lateral aspects of the requisite anatomy. In a minimally invasive approach centered over the requisite anatomy, the incision is determined by the interpedicular distance. When the surgeon is untethered from a midline approach, the exposure may be focused and defined by the anatomy itself instead of the constraints of the exposure.
Once the surgeon is untethered from the midline and instead working directly over the requisite anatomy, the anatomy determines the extent of the exposure and the length of the incision. ▶ Fig. 4.6 illustrates the interpedicular distance at the various segments from L3 to S1. Limiting the exposure to what the anatomy dictates adheres to the principle set forth by Caspar regarding the ratio of the surgical target to the surgical exposure. A 25-mm incision at L5–S1 on either side of the midline allows for the 26- to 28-mm exposure needed for the operation. A 28-mm incision at L4–5 and a 32- to 35-mm incision at L3–4 accomplish the same. The surgical target defined by the requisite anatomy in ▶ Fig. 4.5 and the exposure determined by ▶ Fig. 4.6 result in a favorable Caspar ratio.
4.7 Preoperative Considerations
In addition to a clinical history and neurologic examination, magnetic resonance imaging (MRI) and AP and lateral radiographs, along with flexion–extension radiographs, are essential. The MRI unveils the level or levels of compression of the neural elements, which should correlate with the patient’s neurological examination findings and subjective complaints, both of which guide surgical decision making and the extent of the surgery. MRIs also demonstrate alignment and allow for grading of spondylolisthesis.
I have found that most patients present to the clinic with their MRIs, but a set of static and dynamic radiographs is seldom provided. In the preoperative surgical planning phase, radiographs are equally as important as the MRIs. Flexion and extension studies are helpful in determining the degree of stability of the segment. Extension studies are particularly helpful in determining how much reduction of spondylolisthesis will be obtainable by positioning. The AP and lateral radiographs are predictive of the type of imaging that can be obtained with fluoroscopy in surgery. It is valuable to appreciate a severe coronal imbalance before surgery, so that necessary adjustments can be made to the fluoroscope and incision. ▶ Fig. 4.7 illustrates a case where the preoperative imaging prompted adjustment of the fluoroscope for surgery. In this patient, a severe leftward coronal imbalance on an AP preoperative radiograph prompted a preoperative AP fluoroscopic image. That image defined the angle through the disc space and determined the optimal location for the incisions over the requisite anatomy.
Fig. 4.7 The importance of preoperative radiographs. (a) Preoperative radiograph of a patient who presented with a left L4 radiculopathy secondary to a severe coronal imbalance. This preoperative radiograph prompted placement of a Steinman pin over the top of the segment to plan the incision and guide the position and the wag of the fluoroscope as seen in this (b) photograph and (c) fluoroscopic image. Recognizing the extent of the coronal imbalance, which is not as apparent on magnetic resonance imaging, facilitated planning the incision in a manner that would optimize placement of the pedicle screws as seen in the (d) fluoroscopic image and correction of the coronal imbalance as seen in the (e) postoperative anteroposterior radiograph.
Any concern for degenerative scoliosis on AP or lateral imaging should prompt standing 36-inch scoliosis radiographs. It is important to recognize that there is an inherent limitation to the amount of lumbar lordosis that may be restored in a single-level minimally invasive TLIF. It has been my experience that 12 degrees of lordosis is at the upper threshold that I can reliably achieve per segment after an ipsilateral complete facetectomy and contralateral Smith–Petersen osteotomy. A significant mismatch in lumbar lordosis and pelvic incidence warrants careful consideration during operative planning. Patients invariably request a minimally invasive solution to their symptoms, but the anatomical circumstance of their degeneration may reside outside the realm of a single minimally invasive approach. The surgeon must recognize these circumstances and define the surgical objectives that need to be achieved to bring the spine back into balance. I always remind the patient who is fixated on a minimally invasive approach that if they think they are too old for the right operation, then they are far too old for the wrong one ( ▶ Fig. 4.8).
Fig. 4.8 The limitations of a single-level minimally invasive transforaminal lumbar interbody fusion. (a) Anteroposterior and lateral radiographs in a patient who presented requesting a minimally invasive operation. The patient had undergone three previous operations over the past decade. (b) The pelvic incidence and lumbar lordosis mismatch, degenerative scoliosis, positive sagittal vertical axis (SVA) and multiple levels of spondylolisthesis required the correction of too many parameters and were outside of what could be accomplished with a minimally invasive single-level or two-level operation. (c) A more comprehensive surgical plan was offered that would correct these various parameters. Minimally invasive techniques such as transpsoas approaches at L2–3 and L3–4 were used as part of the operative strategy. However, a (d) traditional midline approach was needed to allow for multiple levels of Smith–Petersen osteotomies and a transforaminal access to L4–5 to correct the lumbar lordosis and SVA. Clearly defining the objectives that need to be met with the surgery and then deciding whether those objectives can be met with a minimally invasive option is paramount.
4.8 Operating Room Setup
I prefer to use a Jackson table with the ability to rotate to perform these operations. The Jackson table accomplishes two objectives: it prevents flattening of the back and decreases blood loss. Having the abdomen freely hanging decreases the intra-abdominal pressure and thereby decreases central venous pressure ( ▶ Fig. 4.9). In my experience, a patient on a Jackson table tends to have less venous bleeding than a patient on a Wilson frame. In fact, I have had greater blood loss from a microdiscectomy on a Wilson frame when tangling with engorged epidural veins near the pedicle than from a TLIF on a Jackson table. Allowing the abdomen to freely hang allows for a greater capacity to restore lumbar lordosis. Placing the hips in slight hyperextension also captures more lumbar lordosis. As mentioned in Chapter 3 on lumbar laminectomy, the capacity to rotate the patients away from the surgeon allows for an ergonomically sound position to decompress the contralateral recess.
The radiology technologist positions the fluoroscope immediately after positioning the patient with the image intensifier opposite the side of the transforaminal access and parks it at the level of the patient’s knees. In this manner, the image intensifier is also opposite the side of the microscope. In the absence of any severe coronal imbalance or degenerative scoliosis seen on preoperative plain radiographs, I defer obtaining any preoperative fluoroscopic images. The operating room team places the microscope on the side of the transforaminal access and the scrub technician drapes it as the patient is anesthetized ( ▶ Fig. 4.10).
Fig. 4.10 Schematic operating room set up for a minimally invasive transforaminal lumbar interbody fusion. (a) Illustration demonstrating the patient positioned on a Jackson table. The microscope is positioned on the symptomatic side of the patient, and the base of the fluoroscope is positioned opposite the microscope. The surgeon performs phase I (pedicle screw placement) and phase III (interbody placement) of the operation with the fluoroscope in position. (b) Phase II is performed under the operating microscope with the fluoroscope rolled to the head of the bed, ready to roll back into position for phase III.
4.9 Operative Technique: The Three Phases of the Operation
Phase I entails planning the incision, docking the minimal access ports, exposing the pedicle screw entry points, and securing the pedicle screws into position. I perform this part of the procedure with loupes and a headlight. Phase II entails the laminectomy, facetectomy and decompression of the neural elements, along with the discectomy performed under the operating microscope. If a Smith–Petersen osteotomy is planned for the contralateral side, I perform it during the second phase. Finally phase III, performed under loupes and a headlight, entails final preparation of the end plates, identifying the interbody height using a series of trials, rotating the interbody spacer into position, and securing the rods with compression and closure. Each phase will be reviewed in depth in the next sections (Video 4.1).
4.9.1 Phase I: Incision, Docking Minimal Access Ports and Pedicle Screw Placement
After positioning the patient on a Jackson table, I palpate the anterior superior iliac spine to approximate the L4–5 level. I mark the presumptive level, along with the spinous processes above and below, which helps me establish the midline. As described in Chapters 2 and 3, if the level is L2–3, L3–4, L4–5 or L5–S1, I mark what I believe to be the appropriate interspinous process space according to my initial approximation of L4–5. Regarding the length of the incision, I mark 10 mm down from the interspinous process space and 15 to 20 mm up. The rationale is that the interspinous space is indicative of the disc space and the caudal pedicle is closer to the level of the disc space than the rostral pedicle, as seen in ▶ Fig. 4.11. Another glance at ▶ Fig. 4.4 and ▶ Fig. 4.6 reminds us that at L5–S1, the interpedicular distance is seldom more than 28 mm, thereby allowing for a smaller incision of about 25 mm. The pedicles may be reliably accessed with a 28-mm incision at L4–5, where the interpedicular distance increases. At L3–4 and L2–3, the interpedicular distance can be up to 36 mm, mandating a slightly longer incision. I will make these incisions 30 to 35 mm in length at those levels.
The lateral placement of the incision is based on the level operated upon and the body habitus of the patient. At the lower segments of L4–5 and L5–S1, depending on the size and girth of the patient, I plan two incisions on either side of midline approximately 3.5 cm from the spinous process for leaner patients and up to 4.0 cm for larger patients (body mass index [BMI] > 35). That distance off the midline optimizes the trajectory into the pedicle, which must be up to 25 degrees for L5 and up to 30 degrees for S1. For the upper segments of L3–4 and L2–3, I plan the incision 3.0 to 3.5 cm on either side of the midline, again with the rationale that not only is the intrapedicular distance smaller, bringing the facets closer together, but the angle of convergence into the pedicle is also less, about 15 to 20 degrees ( ▶ Fig. 4.12).
Fig. 4.12 Rationale for the distance from midline of the incision. The distance from the midline is inherently tied to the angle of convergence into the pedicle. (a) The larger angles at L5 and S1 mandate a more lateral starting point to ensure the trajectory of the access port is parallel or nearly parallel to the pedicle angle. Therefore, at L4–5 and L5–S1, 4.0 cm is preferred, whereas 3.5 cm is preferred in patients with a lower body mass index. (b) The smaller angles and smaller intrapedicular distance make an incision of 3.0 to 3.5 cm the ideal distance from the midline. Capturing the angle that leads into the pedicle vastly facilitates instrumentation of the pedicle.
The fluoroscope remains parked at the patient’s knees as I plan, measure and mark the incisions ( ▶ Fig. 4.13). I then prep and drape the patient and include the fluoroscope in the field to have it ready for an immediate image and thereby optimize the workflow of the operation. In a straightforward degenerative case, I am reluctant to obtain a fluoroscopic image before beginning the operation. A preoperative image will not preclude needing to take the same images once the operation has begun. The time invested in evaluating the preoperative AP and lateral radiographs pays immediate dividends at this point. If the AP images demonstrate scoliosis or a significant coronal imbalance, as seen in ▶ Fig. 4.7, then it is worthwhile to obtain an AP image immediately after positioning the patient and prior to prepping and draping. That preoperative image helps guide the position of the C-arm wag for the ideal lateral image, and I incorporate any coronal imbalance into the incision planning. But in the absence of coronal imbalance, the first image awaits the passage of the spinal needle.
Fig. 4.13 Incision planning for L4–5 minimally invasive transforaminal lumbar interbody fusions. (a) Photograph demonstrating the planned incision. The interspinous process space has been marked in the midline (note the incision is no longer than the incision used for a previous midline microdiscectomy). (b) At L4–5, two incisions 28 mm in length are marked 4 cm lateral to the midline. (c) Artist’s depiction of spine superimposed on photograph of proposed skin incision demonstrating the proximity of the requisite anatomy relative to the incisions (different patient).
Similar to the lumbar microdiscectomy and lumbar laminectomy procedures, I use an infiltration and incision planning set for the MIS TLIF. However, this set is specific for the TLIF. There are two spinal needles, one an 18 gauge and the other a 20 gauge to distinguish between the two on a lateral fluoroscopic image, along with two syringes of local anesthetic and two hypodermic needles ( ▶ Fig. 4.14). I first pass the 20-gauge spinal needle into the marked incision at the 1-cm mark on one side and dock onto the facet. At 3.5 cm (for low BMI) to 4.0 cm (high BMI) lateral to midline, the risk of a dural puncture is low unless an extreme angle is taken medially toward the interlaminar space. A slight converging angle, typically no more than 15 to 20 degrees, reliably secures the spinal needle onto the facet. The unmistakable tactile sense of the metal of the spinal needle encountering the bone of the facet prompts the first lateral fluoroscopic image, not only to confirm the operative segment but also to provide an ideal trajectory into the disc space ( ▶ Fig. 4.15). If needed, I reposition the spinal needle to optimize the incision. It is at this point where any adjustments with regard to the wag of the fluoroscope need to be made to optimize visualization of the pedicles.
Fig. 4.14 Infiltration and incision-planning set for a minimally invasive transforaminal lumbar interbody fusion. The contents include two syringes filled with local anesthetic, two spinal needles (18 and 20 gauge so that they can be distinguished from each other on a lateral fluoroscopic image), two hypodermic needles to infiltrate the skin and superficial muscles, a marking pen and a ruler in case the incision needs to be adjusted and remarked. Two Ray-Tec sponges (Johnson & Johnson, New Brunswick, NJ) and a foam needle counter for the four needles used in this process. This set may be handed to the surgeon to confirm and optimize the incision as the scrub technician passes the wires for cautery and the tubing for suction. In that manner, the surgeon moves the operation forward, confirming and remarking the incision while the scrub technician passes suction tubing, cautery and bipolar wires to be connected.
Fig. 4.15 Intraoperative photograph of spinal needle localization. (a) A lateral view of two spinal needles in position at the L4–5 segment docked onto the facets. (b) View from the head of the bed demonstrating the convergence of 15 to 20 degrees onto the spine.
I confirm the ideal entry point and trajectory over the disc space and onto the facet and remark the incision again 10 mm down from the spinal needle entry point and 15 mm above for L5–S1 (total length: 25 mm), 18 mm for L4–5 (total length: 28 mm), 20 mm for L3–4 and 22 mm for L2–3 (total length: 32 mm) (Fig. 4.11). I place an 18-gauge spinal needle on the opposite side to confirm the incision and the trajectory for that incision. The larger gauge needle is readily distinguishable from the 20-gauge spinal needle, so there is no confusion regarding which needle is confirming which incision ( ▶ Fig. 4.16).
Fig. 4.16 Confirming the planned incision for an L4–5 minimally invasive transforaminal lumbar interbody fusion in the management of a grade I L4–5 spondylolisthesis. Lateral fluoroscopic images demonstrating a 20-gauge spinal needle passed through the 1-cm line of the incision and docked onto the facet. (a) The spinal needle has a suboptimal trajectory, indicating the need to adjust the incision lower so that a trajectory completely parallel to the disc space can be achieved with the access port. (b) The position of the 20-gauge needle was adjusted before obtaining the second image, and the second spinal needle (an 18-gauge needle) was docked onto the facet on the opposite side and in need of adjustment as well. (c) The 18-gauge needle was adjusted before the final fluoroscopic image, which shows both needles at an optimal trajectory to position the access port to instrument and decompress the spine. The incision is re-marked according to these confirmed entry points into the skin.
I remove the stylets from the spinal needles and infuse lidocaine with epinephrine mixed with bupivacaine as I slowly remove the needles. The infusion anesthetizes the future path of the dilators and the access port while the epinephrine mitigates paraspinal muscle bleeding along that same path. Once the spinal needles are removed, I use a hypodermic needle to infiltrate the skin and paraspinal muscles. I make the two incisions with a No.15 blade and dissect onto the lumbosacral fascia with cautery. I have the radiology technologist roll the fluoroscope to just above the hips of the patient for the exposure of the pedicle screw entry points. It will only be 15 minutes or so before the fluoroscope is needed again, so I ask the radiology technologist to remain in the room.
The fascial opening determines the trajectory that I will have for pedicle screw and interbody placement. Before opening the thoracolumbar fascia, I reorient my mind with regard to the midline. As with lumbar microdiscectomies and laminectomies, palpating the spinous process from within the incision provides you with a sense of where the midline resides. At times, despite adequate surgical planning, what was marked as the midline may not necessarily be the exact midline. In patients with a BMI greater than 35, marking the midline from the level of the skin is especially challenging. Thus, confirming the midline by palpation of the spinous process is essential to ensure an ideal fascial opening immediately over the facet joint and optimal trajectory into the pedicle.
A poorly placed fascial opening will not allow for the ideal trajectory needed for ideal pedicle screw placement and has the potential to make a case long and frustrating. Furthermore, the trajectory needs to afford access to the confluence of the lamina and base of the spinous process for an optimal decompression and placement of the interbody. The ideal trajectory will be from lateral to medial onto the facet. If the skin incision turns out to be too medial and you find yourself less than 2 cm from the spinous process, you will need to make a fascial opening lateral to where the skin incision was made to provide the ideal trajectory for pedicle screw placement. If, when palpating the spinous process, you appreciate that the midline is a good distance away, the fascial opening will need to be medial to your skin incision. The skin is more accommodating than the thoracolumbar fascia for these adjustments. Under ideal circumstances, the fascial opening is slightly medial to the skin incision, creating a lateral to medial converging trajectory onto the facet and into the pedicles.
I make the fascial opening with cautery, just as I would in traditional open surgery. The fascial opening needs to be approximately 10% longer than the skin incision. Keep in mind that the skin incision for an L4–5 minimally invasive TLIF is only 28 mm, but the distance between pedicles may be as much as 32 to 34 mm. Therefore, a slightly larger fascial opening will allow angling of the blades in a rostral-caudal direction. Failure to open the fascia adequately will result in a struggle to identify either the rostral or caudal pedicle screw entry points or both.
4.9.2 Docking the Minimal Access Ports
The facet of the segment to be operated upon is the target of the dilatation, since it is the central reference point for the entire procedure. I guide the first dilator onto the facet with my index finger, anchor it firmly in place and obtain a confirmatory fluoroscopic image ( ▶ Fig. 4.17). I use a converging trajectory onto the spine from a mediolateral perspective at this point. It is crucial to set the appropriate angle when docking the dilators to prevent struggling with the blades of the access port when instrumenting the spine. If I secure an access port into position with a converging angle of only 10 degrees, it will be difficult, if not impossible, to achieve an angle of 25 degrees for the pedicle screw. Angulation of the minimal access port that does not match the angulation of the pedicle will entail wrangling with the blades of the access port and the pedicle screwdriver. To prevent this mismatch, I invest the requisite time to carefully set the trajectory of the minimal access port.
As demonstrated in ▶ Fig. 4.6, each pedicle of the lumbar spine has an ideal angle of convergence. I strive to match that angle with the minimal access port. The ideal angle at L5–S1 is 30 degrees; at L4–5, 25 degrees; at L3–4, 20 degrees; and at L2–3, 15 degrees. Setting the access port along the ideal angle of convergence facilitates pedicle screw placement, decompression and interbody spacer placement.
I determine the sagittal trajectory with a fluoroscopic image to ensure that I capture a trajectory parallel to the disc space. The position of the first dilator should correlate precisely with the 1-cm line marking of the planned skin incision ( ▶ Fig. 4.17). I set the ideal trajectory, firmly anchor the access port against the facet and dilate the surgical corridor up to 22 mm. Provided that I maintain the same trajectory, there is no need to obtain further fluoroscopic images, fulfilling our prerequisite to minimize radiation exposure. As I dilate to the larger diameters, my hands receive the tactile sense of the dilators swallowing the facet. The concept is that of a cylinder over the top of a sphere ( ▶ Fig. 4.18).
The precise placement of the larger diameter dilators facilitates stabilization of the final few dilators. The diameter of the dilator now exceeds the diameter of the dome of the facet. The number on the side of the last dilator determines the length of the minimal access blades needed for the access port. The scrub technician assembles the expandable minimal access port with the appropriate blade length. I slip the minimal access port over the dilators and onto the facet. An additional fluoroscopic image at this point ensures that the access port is completely parallel to the disc space. Once I have captured the ideal position and trajectory, I secure the minimal access port onto the table-mounted frame with downward pressure to mitigate muscle creep ( ▶ Fig. 4.19).
Fig. 4.19 Securing the minimal access ports onto the facets. (a) Lateral fluoroscopic image demonstrating a minimal access port in position with the dilators still in place. A trajectory parallel to the disc space is captured in the sagittal plane. Note that the entry point for the caudal pedicle screw is already within the field of view. The entry point for the rostral pedicle screw is only millimeters away. (b) Intraoperative photograph demonstrating the converging lateral to medial angle onto the facet.
4.9.3 Exposure
Suboptimal trajectories of the access port and inadequate exposure of the anatomy are the root causes of difficulty that may arise during the placement of pedicle screws. Very early in my experience I recognized that the exposure of the pedicle screw entry points was technically easier in a paramedian minimally invasive approach than in an exposure that begins in the midline. After all, the access port is immediately over the relevant anatomy, which places the entry points in my direct line of sight and offers me an optimal trajectory onto the pedicle. In contrast, a midline approach presents a constant struggle to reach the lateral margins of the spine and capture a converging angle. Minimizing the extent of muscle creep is the most important operative nuance to keep in mind to optimize the exposure. Constant downward pressure on the minimal access port against the facet is essential when anchoring the retractor to the table mount. Throughout the exposure, should muscle creep begin to occur, reseating the access port again with downward pressure may improve the exposure. I remove the dilators, and if I have proficiently anchored the expandable minimal access port into position over the facet, I will be looking at the facet and its capsule. On a good day, there should be little, if any, muscle creeping around the perimeter of the port.
Early in my experience, the mistake I made was opening the minimal access port too soon. Opening the blades of the access port immediately after removing the dilators will almost assuredly result in muscle creep that will obstruct your view of the relevant anatomy for the rest of the procedure. I quickly learned that it is essential to keep the access port closed until the entire 22-mm diameter within the blades is devoid of muscle and soft tissue.
Similar to the microdiscectomy exposure, I divide the area of exposure for the TLIF into four quadrants, representing the sequence of the exposure, from the lateral safe zones to the more potentially perilous medial zones. The process of exposing the facet begins in a sequential fashion quadrant by quadrant. ▶ Fig. 4.20 demonstrates the exposure through a minimal access port.
Before proceeding further with the technique, it is worthwhile to comment on the area beneath the learning curve regarding exposure and pedicle screw placement. I distinctly remember my first few cases with a paramedian minimally invasive approach where I worked with the greatest trepidation in the medial zones. Exposures for those early cases were painstakingly slow. My uncertainty with the anatomy slowed me down for fear of an errant pass with the cautery into the canal. As I gained experience, I worked with greater certainty. I incorporated the tactile feedback that dilating and securing the access port gave me. I secured the access port with downward pressure to minimize muscle creep. I began to see the anatomy at depth in my mind before exposing it with cautery. I began the process of transitioning my mind from recognition memory of the spinal anatomy to recall memory of the anatomy at depth. My mind adjusted to the angle of the exposure, and soon I was reconstructing the anatomy at depth, filling the holes of visual input offered by a wide exposure with the tactile feel, fluoroscopic images, and direct visualization of the target anatomy. Soon, operative times declined from painstakingly slow to highly efficient. After 50 cases, I was consistently placing four pedicle screws within 30 minutes of making the incision. Relying more on tactile feedback and direct visualization than on fluoroscopic images, I filled in the void of the midline that I could not see to maintain my orientation. Overcoming the obstacles of limited exposure and orientation are the elements of the learning curve with which you must wrestle in your mind to achieve efficiency.
The exposure begins in the safest quadrant away from the neural elements and proceeds caudal to the pedicle screw entry point, where the transverse process and lateral facet can be unmistakably visualized. With a good sense of the facet, the medial boundaries may be exposed and the interlaminar space may be avoided.
As soon as I have exposed the entire facet of the segment, the access port may be opened in the rostral and caudal direction. You will find that with the muscle swept back to the perimeter of the access port blades, there is less creep as you open the access port. It is essential to remember the distances from pedicle to pedicle at the various segments of the lumbar spine when opening the access port ( ▶ Fig. 4.4 and ▶ Fig. 4.6). The temptation is to open the access port far wider than the anatomy dictates. The inevitable result is a wall of muscle collapsing down into your exposure. For instance, at L4–5, the distance from the L4 pedicle to L5 pedicle is 28 to 32 mm. The more collapsed the disc space, the closer the pedicles will be to one another. Since the diameter of an unopened minimal access port is 22 mm, the port need not be opened more than 6 to 8 mm to reach the pedicle. Remember, you still have the capacity to angle the blades, which provide several more millimeters of rostral and caudal exposure.
If I expand the access port too widely, muscle invariably creeps into the exposure and obstructs my view of the anatomy. Equally important, an exposure with excessive muscle creep will cause greater postoperative discomfort for the patient. Thus, I emphasize opening the access port only to the amount necessary to expose the pedicle screw entry points. Every effort should be made not to exceed the interpedicular distance. I typically open the expandable access port only enough to fit the blades of a mediolateral retractor. That distance is typically no more than 5 mm.
With the entire facet exposed, my next objective is to expose all the pedicle screw entry points. For the sake of being systematic, the sequence of exposure is caudal to rostral. In a single-level fusion at L4–5, for instance, the L5 transverse process is only millimeters away from the inferior-lateral aspect of the exposed L4–5 facet. Use of a suction retractor is helpful to pull the muscle tissue laterally and complete the exposure of the entire transverse process (of L5 in this example) with cautery ( ▶ Fig. 4.21). Mediolateral blades added to the access port widen the medial and lateral exposure. I completely expose the caudal transverse process from top to bottom. Based on anatomical landmarks, the pedicle corresponds with the middle of the transverse process. A few bursts of cautery on the superior-most aspect of the pars interarticular of L5 reveal that landmark. The exposed field now includes the L5 transverse process, the superior aspect of the pars interarticularis of L5 and the entire L4–5 facet joint. You are now looking at the first pedicle screw entry point ( ▶ Fig. 4.21).
Fig. 4.21 Illustration of the caudal pedicle screw entry point from within the minimal access port in an L4–5 transforaminal lumbar interbody fusion. The suction retractor holds the soft tissue away and reveals the transverse process. When the L4–5 facet is exposed in its entirety, the superior aspect of the L5 pars interarticularis should be readily visible. Unequivocal exposure of the pars interarticularis, transverse process and facet reliably unveils the pedicle screw entry point. With the exposure seen in this illustration, the pedicle screw entry point (demarcated with the magenta fiducial) may be confirmed with nothing more than a single fluoroscopic image.
I return to the L4–5 facet, the central point of my field of view, and work in the rostral direction. I follow the inferior articular process of L4, as it blends into the pars interarticularis of L4 but stop shy of the L3–4 facet joint. Although the inferolateral aspect of the rostral facet (the L3–4 facet for the L4 pedicle in this case) needs to be exposed for pedicle screw insertion, care should be taken to not disrupt the facet capsule, thereby minimizing the risk of iatrogenic degeneration at that level. The key to preventing inadvertent cautery of the facet capsule is staying lateral. I cannot emphasize this point enough. So instead, I connect the lines of what I can clearly see with my eye (the pars interarticularis of L4) to what I can see only with my mind (the L4 transverse process). I use that mental reconstruction of the anatomy to “leap” onto the transverse process and avoid the L3–4 facet altogether. I accomplish this leap by using a probing suction tip to confirm the location of the L4 transverse process through a thin layer of muscle fibers. The unmistakable sensation of the tip of a metal suction encountering bone confirms the transverse process precisely where my mind has reconstructed it. I use cautery to expose the entire L4 transverse process, and then I nudge medially toward the L3–4 facet without disrupting the facet capsule. Remember, the distinct advantage of the direct visualization of the pedicle screw entry points over percutaneous techniques is the elimination of facet capsule disruption. Leaping from the pars interarticularis to the transverse process ensures that I am playing to the strength of this technique and preserving the rostral facet capsule. I can feel the lateral aspect of the L3–4 facet by nudging the tip of the suction up against the lateral aspect of the facet, which is all that I need to confirm the pedicle screw entry point of L4. Both pedicle screw entry points are now in my field of view ( ▶ Fig. 4.22).
Fig. 4.22 Illustration of the rostral pedicle screw entry point (L4) from within the minimal access port in an L4–5 transforaminal lumbar interbody fusion. After exposure of the caudal pedicle screw entry point, the lamina and pars interarticularis are exposed. Instead of exposing the rostral facet, the focus becomes the rostral transverse process. It is imperative that the capsule of the rostral facet joint be kept intact.
It is a worthwhile mental exercise to go through the difference in sequence between a traditional midline open exposure and a minimally invasive one. A midline open approach provides the spinous process to guide you to the lamina, the facet, the pars interarticularis, and ultimately to the transverse process, all in sequential anatomical fashion. The midline structures are the basis of orientation. A minimally invasive approach has the facet of the operative segment as the center of the field of view. The facet is the first structure that you identify and is the basis of your orientation. All of the relevant structures are only millimeters in each direction: the lamina, the pars interarticularis and the transverse process. Mastery of the anatomical measurements and the topography of the inferior articular process merging into the pars interarticularis allows you to confidently sweep away the soft tissue to complete the exposure for the placement of pedicle screws and decompression. The inability to see the midline structures is no obstacle once you possess the anatomical certainty of the lateral aspect of the lumbar spine. It is this knowledge that is the true organ of sight.
4.9.4 Pedicle Screw Placement
The technique that I describe below for pedicle screw placement capitalizes on the direct visualization of the anatomy to minimize the amount of fluoroscopy needed to safely instrument the spine. The principles espoused by Lenke and colleagues9,10 in their free-hand technique are all incorporated into the technique that I describe below. This technique harnesses the advantage of having the minimal access ports to use as a frame of reference to align the trajectory for a pedicle probe, tap or pedicle screw. Using the minimal access ports in this manner helps decrease your exposure to ionizing radiation. I avoid AP imaging and instead carefully assess my angulation throughout the process of probing, tapping and placement of the pedicle screw. Furthermore, I use electrophysiological stimulation at every step of the process to minimize the risk of missing a pedicle breach.
A minimally invasive procedure should not be a license to increase the use of fluoroscopy. Instead, I encourage you to develop a mentality that it is exactly the opposite. You should consider that a direct paramedian exposure of the entry points and minimal access ports offering a trajectory in line with the pedicle decreases the need for fluoroscopy.
It is only after I have exposed all the pedicle screw entry points that I bring the fluoroscope back into the field to its previous mark. Under ideal circumstances with two surgeons operating, the exposures described earlier should be done simultaneously and take no more than 15 minutes to complete. If I am operating by myself, I complete the exposures on both sides before beginning with instrumentation. When I have completed the exposure, the fluoroscope rolls back into position, and I place the drill at the junction of the pars interarticularis, midtransverse process and inferior lateral facet. I confirm the proposed entry point with a single lateral fluoroscopic image. The ideal position for the entry point is in the upper half of the pedicle ( ▶ Fig. 4.23). Depending on the amount of facet arthropathy, the lateral aspect of the facet may need to be drilled to adequately expose the entry point. I use a drill with a minimally invasive attachment to create a breach through the cortical bone and unveil the blush of cancellous pedicle bone.
Fig. 4.23 Confirming the pedicle screw entry point. Lateral fluoroscopic image demonstrating confirmation of the entry point. The tip of the drill is positioned at the junction of the pars interarticularis, transverse process and facet. With the entry point clearly visualized, no anteroposterior fluoroscopic image is needed. Although the pedicles of L5 appear to be in perfect alignment, the pedicles of L4 are not. The image shows a double shadow through the pedicle and requires adjusting the wag of the fluoroscope before instrumenting the L4 pedicles.
With regard to the operative sequence, I prefer to start at the caudal pedicle and then line up the pedicle entry points for ease of rod placement. However, it is not unusual for the fluoroscope position to be ideal for a lateral image of the pedicles of one vertebral body but not for the other. Under those circumstances, I begin with whichever level has an ideal lateral fluoroscopic image with the pedicles lined up, regardless of whether it is the rostral or caudal pedicle ( ▶ Fig. 4.23). In the case of an L4–5 TLIF, where there is no coronal imbalance, I drill the entry points for both pedicle screws before probing the pedicles. I place the tip of the drill at the junction of the pars interarticularis and the midtransverse process of L5 while nudging into the inferior and lateral aspect of the L4–5 facet. A fluoroscopic image confirms the ideal entry point, and the pilot holes are drilled ( ▶ Fig. 4.23). I repeat the same process for the L4 pedicle and obtain another fluoroscopic image to confirm the L4 entry point. I then explore the cortical breach with the pedicle probe in search of cancellous bone. Depending on the pedicle being probed, the medial-lateral angulation will vary. For a sacral pedicle trajectory, the angle may be as much as 25 to 30 degrees, whereas for an L3 pedicle trajectory, it may be as little as 5 to 10 degrees.
The term I use to describe the indisputable sensation of a pedicle probe displacing cancellous bone as it advances into the pedicle is “pedicular.” When the probe advances with more wiggle than push, delivering a soft haptic sense of crunching cancellous bone to the palm of your hand, you are experiencing the definition of this neologism. Pedicular advancement of the probe provides you with the unmistakable sensation that you are within the cortical walls of the pedicle and advancing in the right direction along the ideal trajectory ( ▶ Fig. 4.24a). The probe should advance with little resistance. If you encounter stiff resistance, it is likely that the tip of the pedicle probe is abutting the unforgiving cortical bone of the pedicle. Should this occur, pause and reassess. Forcing the pedicle probe against resistance is a recipe for a breach. Never ask for a mallet.
Fig. 4.24 Probing the pedicle. (a) Illustration demonstrates probing of the pedicle with an appropriate trajectory. The unmistakable tactile feedback of the tip of the pedicle probe displacing cancellous bone is a distinct tactile feel compared to encountering the stiff resistance of the cortical wall of the pedicle. (b) Illustration demonstrates a potential pitfall in probing the pedicle. Encountering stiff resistance when probing the pedicle may be an indication that the tip of the pedicle probe is against the cortical bone of the pedicle. Failing to make an adjustment and instead continuing in that trajectory is a recipe for a breach in the pedicle. Awareness of the pedicular probing sensation and assessment of any resistance allows for adjustment of the trajectory to find the cancellous bone again.
The breaches I have caused have come after encountering significant resistance where at first there was none. Convinced I knew the anatomy of the pedicle, I continued to push the probe down the errant path and forced my way through that resistance ( ▶ Fig. 4.24b). I erroneously reasoned that I had found the cancellous bone of the pedicle again because, after some initial resistance, the probe began to pass easily again. When I stimulated the probe to 20 mA, my jaw dropped as my patient’s leg began to fire compound motor action potentials with a sickening rhythmicity. In actuality, I had forced the tip of the pedicle probe through the cortical wall of the pedicle. With the cortical wall breached, I no longer met resistance and the probe advanced once again. For this reason, if you encounter stiff resistance that suddenly gives way, stop. Remove the pedicle probe and check the integrity of the pedicle with a ball-tipped probe. Depending on the entry point, the trajectory, and the shape of the pedicle, the breach may have been medial or lateral. Palpating the pedicle with a ball-tipped probe invariably revealed the breach and confirmed the error in trajectory.
Now that my technique has evolved from the countless miscalculations I have made over the years, any resistance that I encounter gives me pause. I may check an additional lateral fluoroscopic image, reassess my trajectory and entry point, or stimulate the probe (using electrophysiological monitoring) up to 20 mA to see if it generates a compound motor action potential. If I am still unable to probe into the pedicle after reassessing the entry point and my angle, I obtain an AP fluoroscopic image and visualize the pedicle from another view. Regardless, experience has taught me to never force a pedicle probe past significant resistance. Instead, I adjust the angle until the tip of the probe finds the welcoming cancellous bone.
When I have passed the pedicle probe a distance of 30 mm, I obtain an additional fluoroscopic image to ensure an optimal trajectory and to stimulate the probe to 20 mA to ensure that there is no generation of a compound motor action potential. A positive response under 10 mA may be indicative of a breach within the pedicle. Any response under 20 mA automatically prompts an AP image and potentially an “owl’s eye” view (an angled view down the pedicle). The positive stimulation is indicative that there is something that I am not appreciating with the anatomy of the pedicle and that I need additional information to proceed.9,10 If there is no compound motor action potential at 20 mA, I continue instrumenting the pedicle ( ▶ Fig. 4.25b).
I strive for a trajectory parallel to the end plate. I can still adjust the trajectory as I pass the probe an additional 10 mm to a total distance of 40 mm if the anatomy allows it. I take note of the position of the probe within the minimal access port and use this as a frame of reference for the trajectory of the tap and pedicle screw.
Fig. 4.25 Fluoroscopy sequence of an L3–4 transforaminal lumbar interbody fusion with severe coronal imbalance. (a) Lateral fluoroscopic image demonstrating the drill confirming the pedicle screw entry point. In this circumstance, the wag of the fluoroscope provided an ideal lateral view through the pedicles of L3 but not L4 (note the double shadow of the L4 pedicle). The L3 pedicle was instrumented first, and then the wag of the fluoroscope was adjusted for the L4 pedicles. (b) Lateral fluoroscopic image demonstrating an advancing pedicle probe with a medial angle. Once in position at 30 mm, the pedicle probe is stimulated to rule out a medial breach. The absence of a compound motor action potential does not necessarily rule out a lateral breach. (c) A ball-tipped probe ensures the integrity of all five walls of the pedicle. Note the drill confirming the entry on the contralateral side by the second surgeon working on that side. (d) Knowledge of the length of the tap helps determine the length of the screw when the threads are buried on a lateral fluoroscopic image. In this lateral fluoroscopic image, the tap with the threads buried measures 37.5 mm. Review of the distance to the anterior part of the vertebral body prompted placement of a 45-mm pedicle screw. (Note the pedicle probe advancing in the contralateral pedicle.) (e) Placement of the L3 pedicle screw on the left with the pedicle screw already in position on the right. The pedicle screw is parallel to the end plate and reaches 80% of the depth of the vertebral body.
In the absence of any irritation of the traversing nerve root, I use a ball-tipped probe to ensure the integrity of the pedicle. I do not assume an intact pedicle because of the absence of a compound motor action potential. Lenke and colleagues emphasize this principle in perhaps the most comprehensive analysis in the neurosurgical literature of pedicle screws placed with electrophysiological monitoring.9,10 The ball-tipped probe should be used to confirm all five sides of the pedicle. The “bottom” aspect of the probed pedicle, which represents the anterior aspect of the vertebral body or the floor, is always the first boundary that I check. The absence of a bottom typically suggests a lateral breach, which is the result of inadequate medialization of the trajectory. If the floor of the probed hole is intact, I proceed to confirm the medial, lateral, superior and inferior walls of the pedicle.
I use a pedicle tap with the diameter one size smaller than the proposed width of the pedicle screw as determined in my preoperative planning; that is, if I am planning to place a 7.5-mm screw, I will use 6.5-mm tap. I pass the pedicle tap along the same trajectory as the pedicle probe, with an initial fluoroscopic image obtained to ensure the trajectory within the pedicle and a subsequent fluoroscopic image taken when the threads of the tap are buried. The minimal access port can serve as a frame of reference for the various instruments passing into the pedicle. ▶ Fig. 4.26 demonstrates the relative position of the pedicle probe, tap and pedicle screwdriver. In the sequence of those photos, the pedicle probe, tap and pedicle screwdriver are all in the same position relative to the access port. Keeping a mental image of the position of these instruments relative to the access port ensures the same trajectory and has more potential value than any additional fluoroscopic image.
Fig. 4.26 Intraoperative photos demonstrating (a) the pedicle probe, (b) the tap and (c) the pedicle screw driver in the same position relative to the minimal access port. Registering in your mind the position of the instruments within the frame of reference of the retractor helps to decrease the need for fluoroscopy and minimizes radiation exposure to the patient, the surgeon and the operating room staff.
I stimulate the tap once again, and in the absence of a compound action potential, I probe the tapped hole again with a ball-tipped probe to ensure the integrity of the pedicle. If I am working on the side of the transforaminal approach, I secure the pedicle screw into position again using the frame of reference of the access port and one final fluoroscopic image to set the trajectory. If on the contralateral side, I decorticate the transverse processes and pack graft material onto them before I secure the pedicle screw. I have found that I can decorticate the transverse process and place morselized graft for a posterolateral fusion more proficiently without the pedicle screw in position.
Throughout the placement of the caudal pedicle screw on one side, if a second surgeon is operating, they perform a mirror operation on the contralateral side simultaneously, which has several advantages ( ▶ Fig. 4.27). First, it saves fluoroscopic radiation exposure. Second, it allows the pedicle screws to be aligned one with another within the vertebral body and gives the appearance of one pedicle screw on a lateral image, which is an element of craftsmanship. Finally, it allows both surgeons to confirm the degree of convergence by having both taps or both pedicle probes in place at the same time.
Fig. 4.27 Simultaneous instrumentation of the pedicles. Intraoperative photographs demonstrating simultaneous probing of the pedicles at L5 (a) as well as probing the pedicle of L4 on the right while (b) tapping the pedicle of L4 on the left.