Minimally Invasive Posterior Lumbar Fusion Techniques

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CHAPTER 58 Minimally Invasive Posterior Lumbar Fusion Techniques

Chadi Tannoury, MD, Amar A. Patel, BS, D. Greg Anderson, MD

Lumbar fusion is a reliable treatment option for a wide variety of spinal pathologies resulting in spinal column instability and/or spinal-related pain. Various spinal fusion techniques are the subject of ongoing clinical investigations with goals to improve surgical technique, graft biomaterials, and implant designs in order to achieve a stable, symptom-free spinal column with the least chance of patient morbidity.

Despite these advances, the morbidity of spinal fusion surgery remains significant. The standard posterior midline exposure is notorious for paraspinal muscle stripping and denervation leading to significant postoperative scar formation. The limitations of this approach for spinal fusion have been well documented, especially regarding a prolonged recovery period and muscle damage that may affect a patient following surgery.¹^–⁵

In recent years, less invasive surgical approaches have been developed to minimize damage to the paraspinal soft tissues during surgical exposure. These “minimally invasive” surgical approaches are becoming more popular because they offer the surgeon a method to achieve the goals of spinal surgery while minimizing some of the perioperative morbidity inherent to the classic posterior approach.^6,⁷

Minimally invasive spinal surgery (MISS) is a rapidly evolving field that is supported by a number of technologic innovations. These include the operative microscope, C-arm fluoroscopy, tubular retractor systems, cannulated pedicle screws, and for some, image guidance systems. The basic hand instruments used during a minimally invasive spinal procedure are similar to those used during a traditional spinal case but are often longer and bayoneted to improve visualization through a tubular retraction system. A high-speed burr or drill with a long and thin shaft is useful in decorticating or thinning the bony elements of the spine. To be successful with MISS, a surgeon must be familiar with the microscopic anatomy of the spine. He or she must gain the skills necessary to work safely and efficiently despite a limited field of view and become facile with the use of MISS equipment. This set of skills can only be gained with experience, and thus a significant but definable learning curve should be anticipated by surgeons interested in becoming proficient in MISS techniques. The length and slope of the learning curve will also depend on the individual surgeon’s skill set and prior spinal experience. The learning process is best accomplished in a slow, step-wise fashion, mastering basic skills with simple cases before attempting to approach the more challenging spinal pathologies in a minimally invasive surgical fashion. This chapter provides an overview to the field of MISS as it applies to lumbar fusion techniques for common conditions of the lumbar spine.

Principles of Minimally Invasive Spinal Surgery

All minimally invasive spinal procedures, despite the type and the location, have the common goal of correcting the underlying spinal pathology while avoiding excessive damage to the paraspinal soft tissue envelope. As with other spine procedures, an MISS procedure begins with the careful analysis of the preoperative imaging studies to precisely localize the spinal pathology. Before making a surgical incision, preoperative fluoroscopy is used to localize the involved spinal segments and plan the skin incision. During the surgical approach, the paraspinal muscles are split rather than cut or resected using serial tubular dilators to create a surgical corridor between the skin incision and the spine (Fig. 58–1). Only necessary portions of the vertebral columns are exposed, and excessive use of electrocautery or vigorous retractor pressures should be avoided.

FIGURE 58–1 Serial tubular dilators (A, individual; B, assembled) are used to gain access to the spine through muscle-splitting rather than cutting through the paraspinal muscles.

The surgeon must understand the muscular anatomy of the paraspinal region to design the optimal approach for an MISS procedure. There are two distinct muscular compartments: the multifidus compartment, which overlies the midline spinal structures, and the lateral compartment, which overlies the transverse processes (Fig. 58–2). The multifidus muscle surrounds the spinous processes, lamina, and facet joints. The multifidus muscles receive its nerve and blood supply from the medial branches of the dorsal rami and segmental vessels, respectively, which course from the intervertebral foramen along the base of the transverse process and enter the lateral margin of the muscle in the region of the pars intra-articularis. Care should be taken to avoid rupturing the lateral attachments of the multifidus muscle, which would disrupt the nerve and blood supply to the multifidus muscle, leading to atrophy and scar formation in the substance of the muscle. The lateral muscle compartment contains the longitudinally oriented muscles of the erector spinae group. The lateral compartment overlies the transverse processes and includes the entry site for pedicle screw insertion at the base of the transverse process. The lateral compartment is traversed whenever a posterolateral onlay fusion is performed.

FIGURE 58–2 Deep paraspinal muscle compartment: The multifidus muscle (C) overlies the midline, and the lateral compartment (A, B) contains the longissimus and iliocostalis muscles, overlying the transverse processes.

When approaching the spinal canal, laminae, or facet joint, a trans-multifidus compartment approach is required. When placing percutaneous pedicle screws or performing a posterolateral onlay fusion, a translateral compartment approach is necessary. To operate within a particular compartment, the fascia over that compartment should be opened and then the muscular tissues dilated or split to reach the spinal structures of interest. When moving from compartment to compartment, the surgeon should never transgress the fascial barrier between the compartments because this would disrupt the nerve and blood supply to the multifidus muscle. Instead, the retractor should be withdrawn and the fascia should be opened over the other compartment followed by a muscle-splitting approach to the spinal contents of the compartment. The same skin incision can generally be used for two separate fascial incisions used to access the compartments.

When discussing a minimally invasive surgical approach with a patient, the surgeon should ensure that the patient understands the available surgical options (both MISS and open surgery) for treating the spinal condition. In addition, the surgeon should include in the surgical consent process the possibility that the less invasive procedure may need to be converted to a larger, open approach to achieve the ultimate goals of surgery. Both the surgeon and patient should remember that correction of the spinal pathology is the most important issue, whereas the approach and incisional size are lesser considerations.

Surgical Setup for a Posterior Fusion Procedure

Setup and Imaging

Following placement of surgical monitoring equipment and the induction of general anesthesia, the patient should be positioned prone on a radiolucent spinal frame (Fig. 58–3). The abdomen should be free of compression, and free access to the lumbar region for fluoroscopy should be confirmed. The preoperative imaging studies should be available in the room with the operative plan clearly marked. The surgeon should ensure the availability of the proper implants and instruments prior to commencing with the operative procedure.

FIGURE 58–3 The patient is positioned prone on a radiolucent spinal frame.

After a sterile skin preparation and draping, the C-arm mobile fluoroscopy unit is used to demarcate the location of bony landmarks, which are drawn on the skin. A critical step is to ensure that the skin incisions are localized in an optimal position to allow access to the underlying spinal pathology. In some cases, it may be useful to introduce a spinal needle along the proposed trajectory of the surgical incision and check the position of the needle on both anteroposterior (AP) and lateral fluoroscopic views to ensure an optimal path to the pathology is achieved.

Surgical Incisions and Approach

The number and length of skin incisions should correspond to the surgical plan, which must be more “thought out” compared with a traditional open surgery. A single skin incision may be used during different phases of the surgical procedure to reach different areas of the spine. For instance, one incision may initially be used to decompress the neural elements (multifidus compartment). Subsequently, the same skin incision may be used to perform a posterolateral fusion and place pedicle screw instrumentation (lateral compartment). Although a single skin incision is used, separate fascial incisions should be used to reach each individual compartment.

When working through perimedian incisions, two distinct fascial layers are encountered. The superficial layer corresponds to the thoracodorsal fascia, while the deeper layer is a thin fascia that overlies the muscle of the compartment. Both fascial incisions should be a little longer than the corresponding skin incision because this will allow the subsequently placed tubular retractors to be maneuvered and angulated freely as needed to reach the various areas of the spine necessary to perform the operation. The muscles of the compartment can be split with the surgeon’s digit or with an instrument such as a Cobb elevator. It is often helpful to palpate bony landmarks such as the facet joint or transverse processes to assist with placement of the initial instruments through the skin and muscle portal to the vertebral column.

When operating through a tubular retractor, the smallest dilator is then docked at the appropriate bony site and serial dilation is used to expand the operative corridor. Care should be taken into bringing each subsequent dilator in contact with the bony elements. The correct length of the tubular retractor can then be selected, inserted, and secured using an operating table–mounted retractor holder. Once the tubular retractor is in place, the position of the retractor should be verified using fluoroscopy (Fig. 58–4).

FIGURE 58–4 Following insertion of the tubular dilator system (A), the tubular retractor is secured in place using an operating table mounted rigid retractor holder (B). Fluoroscopy is used to verify correct positioning of the tubular retractor (C).

Spinal Decompression

The topic of spinal decompression is substantial and exceeds the goals of this chapter; however, a few points deserve mention. The authors prefer to perform the decompression first when performing both a decompression and fusion of the lumbar spine. This allows the surgeon to obtain local autogenous bone from the decompression site that may be used for the spine fusion and also exposes important bony landmarks such as the pedicle, which is useful in subsequent stages of the procedure. The decompression is done by entering and traversing the multifidus compartment. Thus it is useful to perform a facet fusion during this phase of the procedure, before exiting the multifidus compartment.

Posterior Interbody and Transforaminal Interbody Fusion

When performing a posterior or transforaminal lumbar interbody fusion (PLIF and TLIF) via a minimally invasive approach, it is important to align the tubular retractor collinear with the disc space on the lateral view (Fig. 58–5). When performing a TLIF procedure, the tubular retractor must be aligned with enough lateral to medial angulation to allow the surgeon to reach the contralateral side of the disc space for preparation of an adequate fusion bed (Fig. 58–6). During the exposure, adequate facet joint must be removed to minimize retraction of the neural elements and provide working access to the disc space.⁸

FIGURE 58–5 Lateral fluoroscopic view shows the tubular retractor positioned in a proper alignment (i.e., collinear with the disc space).

FIGURE 58–6 Angulation of the tubular retractor system allows access to the contralateral side of the disc space and therefore preparation of the fusion bed.

The detrimental effects of over-retraction of the neural elements with the PLIF procedure have been well documented in the literature.⁹ Facet removals for a PLIF or TLIF can be achieved with either osteotomes or a high-speed burr. It is helpful to skeletonize the upper and medial portions of the caudal pedicle (e.g., L5 pedicle for a L4-5 TLIF) to gain adequate access to the disc space and allow safe retraction/protection of the dural/neural elements.

Once the disc space has been adequately exposed, the posterolateral annulus is incised with a scalpel and the posterior margin of the disc is removed. The posterior “lip” of the vertebral body should be resected so that the opening is flush with the most concaved portions of the disc space. Disc material and the cartilaginous endplates are thoroughly débrided from the interbody space using curettes, shavers, and/or pituitary rongeurs until the interspace is clean, leaving only intact bony endplates to support the interbody cage. If the disc space is collapsed, the endplates should be dilated to restore the foraminal height and improve the sagittal contour of the spine.

After disc space preparation, the interspace should be packed with autogenous bone graft or an adequate fusion substrate. An interbody fusion cage, of appropriate size, is selected and packed with the graft material, before impacting the cage into the disc space. The optimal position of the cage is toward the anterior portion of the disc space.^10,¹¹ This produces better reconstruction of the sagittal contour of the spine and allows ample bone graft material to be packed around and behind the cage.

Instrumentation, most commonly with pedicle screws, is a standard component of both the modern PLIF and TLIF procedures. Following the insertion of pedicle screws and rods, compression of the interbody construct is performed to restore the lumbar lordosis and ensure compressive loading of the interbody grafts.

Posterolateral Fusion (Intertransverse Onlay Fusion)

From the traditional midline approach, access to the intertransverse region for onlay fusion requires complete stripping of the paraspinal muscles to the tips of the transverse processes, an act that causes destruction, or at least disruption, of the multifidus muscle and significant postoperative scarring.⁵ Using the paraspinal muscle-splitting approach (Wiltse approach), exposure of the intertransverse region is simple to achieve without major muscle stripping. This provides direct access to the intertransverse region for fusion.

The skin incision for a paraspinal approach for intertransverse fusion is made at least 3.5 to 4 cm lateral to the midline. The fascia is divided in line with the skin incision, and the paraspinal muscles are split in line with their fibers to expose the transverse processes. For fusion purposes, the entire transverse process at both levels should be exposed. Either a tubular retractor (preferably an expandable tubular retractor) or side-to-side (e.g., McCullough retractor) retractor can be used to visualize the intertransverse interval. The authors prefer to use an expandable tubular retractor, which allows both transverse processes to be simultaneously exposed (Fig. 58–7).

FIGURE 58–7 The use of an expandable tubular retractor allows simultaneous exposure of both transverse processes.

Once the intertransverse region has been exposed, the soft tissues are meticulously cleaned away from the transverse processes and intertransverse membrane. The transverse processes are decorticated using a high-speed burr and then the interval is packed with autogenous bone graft or a suitable graft material. When withdrawing the retractors, care should be taken to not displace the graft materials from the fusion bed.

Facet Fusion

Fusion of the facet joints is useful as an adjunct to interbody or intertransverse fusion but has not been well accepted as a stand-alone fusion due to the relatively small surface area of the facet joints. However, the facet joint offers a number of theoretical advantages as a fusion site including the ease of access to the joint, the small gap across which the fusion must heal, and the compression of the fusion site that is achieved during normal upright posture of the patient. In addition, a facet fusion is a quick, simple, and low-morbidity procedure.

To perform a facet fusion, the retractor should be docked on the facet, which resides in the lateral portion of the multifidus compartment (see Fig. 58–2). If decompression of the spinal canal is required, facet fusion can easily be performed during the exposure through the multifidus compartment. Once the facet is exposed, the capsule is removed with electrocautery and the articular surfaces of the inferior and superior articular processes are identified. A high-speed burr is used to decorticate the facet joint along its entire length, and the joint space is packed with fragments of autogenous bone or a suitable bone substitute.

In some cases osteophytic bone material may overlie the true facet joint, and this should be removed to expose the native joint surfaces. The surgeon should be cognizant of the normal anatomy of the facet joint with the superior articular process lying lateral and deep to the inferior articular process. The specific topography of the facet joint can also be defined preoperatively by analyzing imaging studies (magnetic resonance imaging [MRI] or computed tomography [CT]).

Instrumentation of the Spine

Pedicle Screw Instrumentation

Pedicle screw instrumentation has emerged as the most common form of internal fixation used for thoracolumbar arthrodesis. Pedicle screws offer numerous advantages compared with hooks or wires, which are less rigid. Pedicle screws can be used when posterior spinal elements are deficient due to prior surgery, and they provide rigid segmental immobilization, thereby minimizing the need for postoperative brace immobilization. Because of the three-column support provided by the transpedicular fixation, these implants are effectively used in various complex spinal pathologies including deformities, which require corrective forces to be employed.¹²

With the advent of cannulated pedicle screws systems, these implants can be placed through the same skin incisions used for the decompression or fusion portions of the spinal operation. Our preference is to place instrumentation as the final stage of surgery so that the bulk of the implants will not physically interfere with other stages of the operation.

Some surgeons prefer to use noncannulated pedicle screws, placed with direct visualization of the spinal anatomy, using an expandable tubular retractor system. This approach is best used for short procedures (one to two levels) in the lower lumbar spine, where the natural spinal lordosis brings the trajectory of the pedicles into close proximity. In such a situation, it is not difficult to place pedicle screws at adjacent levels using a small, paramedian incision and appropriate expandable retractor system. Placement of pedicle screws in an MISS fashion offers significant advantages compared with traditional pedicle screw instrumentation, which requires full exposure of the spine and major paraspinal muscle stripping.

Cannulated Pedicle Screw Insertion

The first step in placing cannulated pedicle screws involves obtaining a true AP image of each vertebra to be instrumented (Fig. 58–8). Because of the natural sagittal contour of the spine, the C-arm must be angulated to the specific sagittal profile of each individual vertebra in order to obtain the true AP view. It is helpful to have the radiology technician mark the exact angle of the C-arm where the true AP image can be obtained to assist rapid return to the proper image (Fig. 58–9). A properly aligned AP C-arm image will demonstrate the superior vertebral endplate as a single, dense line, and the pedicles will be localized just below the upper endplate. Correct rotation of the vertebrae is ensured when the spinous process shadow is centered between the pedicles. The true AP view is most useful when cannulating the pedicle during pedicle screw insertion.

FIGURE 58–8 True anteroposterior imaging of each vertebra to be instrumented is crucial for safe pedicle screw placement.

FIGURE 58–9 Marking of the exact C-arm angle where the true AP image can be obtained helps the radiology technician and assists rapid return to the proper image.

True lateral fluoroscopic images are also used during pedicle screw instrumentation, particularly during assembly of the construct (Fig. 58–10). The true lateral image will demonstrate the superior endplate as a single, dense line. The pedicles will be superimposed. The posterior cortex of the vertebral body should also appear to be a single radiopaque line, confirming that no rotation of the vertebra is present. The true lateral view is useful during pedicle tapping, placement of pedicle screws, and assembly of the construct. In cases where scoliosis is present, the C-arm may need to be angled (i.e., “wig-waged”) to obtain a true lateral view of each vertebra.

FIGURE 58–10 True lateral images are also necessary for proper pedicle screw instrumentation.

When performing percutaneous instrumentation, obtaining properly aligned C-arm images are, by far, the most important step in the procedure. Thus it cannot be overemphasized that good images should be obtained before attempting to implant percutaneous pedicle screws. If adequate C-arm images cannot be obtained due to severe osteopenia, obesity, intra-abdominal contrast, or any other reason, placement of percutaneous pedicle screw implants should not be attempted.

After obtaining a true AP fluoroscopic image of a given level, a K-wire should be aligned over the skin of the back so that it appears to bisect the pedicles (Fig. 58–11). Next, a horizontal line is drawn along the skin using the K-wire (Fig. 58–12). This step should be repeated using a true AP image for each of the vertebrae in the construct. Vertical lines are then drawn (using a K-wire placed over the skin of the back) along the lateral pedicle shadow (Fig. 58–13). Skin incisions for percutaneous pedicle screw insertion should be placed about 1 cm lateral to the vertical line (Fig. 58–14).

FIGURE 58–11 Marking the skin (horizontally): With fluoroscopic assistance, a K-wire is placed horizontally over the skin of the back bisecting the pedicles of the level to be addressed.

FIGURE 58–12 The skin is marked using the K-wire to draw a horizontal line.

FIGURE 58–13 A, Marking the skin (vertically): Under fluoroscopy, the K-wire is vertically placed over the skin of the back and positioned along the lateral pedicle shadow. B, The skin is then marked using the K-wire as a reference to draw a vertical line.

FIGURE 58–14 Skin incisions, for percutaneous pedicle screws instrumentation, are placed 1 cm lateral to the marked vertical lines.

Once the skin and fascia have been divided, the surgeon can digitally palpate the transverse process of the vertebra whose pedicles are to be cannulated. A Jamshidi needle is then placed at the base of the transverse process (at the junction of the transverse process and superior articular process), and a true AP image is obtained (Fig. 58–15). The goal is to position the tip of the needle directly over the lateral margin of the pedicle shadow (at the 3 o’clock and 9 o’clock positions) on the true AP view (Fig. 58–16). The tip of the needle should be adjusted until the tip of the needle lies directly at the lateral boarder of the pedicle. The needle shaft is then aligned parallel to the endplate (or transverse process) on the AP image, which ensures a needle trajectory parallel to central axis of the pedicle (Fig. 58–17). The shaft of the needle should also be held with a lateral to medial trajectory of approximately 10 to 15 degrees, depending on the level to approximate the normal divergence of the pedicles anatomically. Then, the needle is tapped gently a few times to seat the needle tip into the bone and ensure that slippage of the needle tip does not occur as the needle is driven through the pedicle. A final true AP image is checked to be sure that the needle is properly positioned and aligned.

FIGURE 58–15 The Jamshidi needle is placed, with fluoroscopic assistance, at the base of the transverse process.

FIGURE 58–16 True AP view illustrates the recommended positioning of the needle tip over the lateral margin of the pedicle shadow at the 3 o’clock and 9 o’clock positions.

FIGURE 58–17 Fluoroscopy is valuable in localizing the needle tip at the lateral border of the pedicle (A and B) and the needle shaft parallel to the endplate (C).

Next, a line is drawn on the shaft of the Jamshidi needle, 20 mm above the skin edge (Fig. 58–18). Because the average length of the pedicle is 20 mm from the starting point, this line is used to determine the depth of the needle tip as it is driven through the pedicle. With the Jamshidi needle properly aligned, the needle is tapped with a mallet to drive the needle through the bone of the central pedicle. When this line on the needle shaft reaches the skin edge, the needle tip has traversed the pedicle isthmus and is at approximately the depth of the base of the pedicle. At this point another true AP fluoroscopic image is obtained to ensure that the needle tip lies well within the pedicle shadow, no more than three fourths of the distance (from lateral to medial) across the pedicle (Fig. 58–19). This true AP image should be critically analyzed, and if the needle tip is in proper position, then it is deemed acceptable for pedicle screw insertion.

FIGURE 58–18 The Jamshidi needle is marked 20 mm above the skin edge, an estimate of the depth of the pedicle.

FIGURE 58–19 On true AP fluoroscopy, the needle tip should be located within the pedicle shadow, no more than three fourths of the distance across the pedicle.

Next, the Jamshidi needle is driven 5 to 10 mm deeper into the vertebral body and a guidewire is inserted through the Jamshidi needle, into the cancellous bone of the vertebral body. The surgeon should feel “crunchy,” cancellous bone at the base of the needle and will generally be able to insert the guidewire 10 to 15 mm beyond the needle tip into the vertebral body with manual pressure. If the bone is too hard for manual insertion, a Kocher clamp can be placed on the guidewire 10 mm above the top of the Jamshidi needle and tapped with a mallet to achieve positioning of the guidewire into the vertebral body. The same procedure is repeated for all the pedicles in the surgical construct. It is the author’s preference to cannulate all pedicles in the construct using AP fluoroscopy before adjusting the C-arm into the lateral position.

Once all of the pedicles in the construct have been cannulated and guidewires have been placed, the C-arm is adjusted to obtain true lateral images of the spine (Fig. 58–20). The position of the guidewires on the lateral fluoroscopic view is verified before proceeding with pedicle preparation. Pedicle preparation and pedicle screws placement are then carried starting from one end of the construct in an “assembly line” fashion. Each pedicle is tapped using a cannulated tap. The authors prefer to stimulate the tap using stimulus-evoked EMG to ensure no low voltage activity is present. If so, it might indicate a breech of the pedicle. Then cannulated pedicle screws are placed over the guidewires at each level and threaded into the pedicles. The pedicle screws are adjusted in height as needed to maintain polyaxial motion of the screw crowns and to achieve a smooth contour of the screws at adjacent levels (necessary for rod seating). It is also the author’s preference to stimulate each pedicle screw, after insertion with stimulus-evoked EMG, using an insulated port over the screws (Fig. 58–21).

FIGURE 58–20 True lateral fluoroscopic image showing the guidewires properly placed within the pedicles.

FIGURE 58–21 Following their insertion, pedicle screws are tested with stimulus-evoked electromyography, using an insulated port.

Once are the screws are positioned, the proper rod length is measured and rods are inserted through the screw extensions and into the screw crowns. The details of rod insertion differ slightly among manufacturers of cannulated screw systems. The surgeon should be familiar with the details of the specific system selected. After the rods are placed, screw caps are inserted into each screw to capture the rod. Compression or distraction of the construct can be performed as needed, followed by final tightening of the construct. At the conclusion of the procedure, AP and lateral imaging of the entire construct should be obtained (Fig. 58–22).

FIGURE 58–22 Anteroposterior (A) and lateral (B) fluoroscopic views of the construct at the conclusion of the procedure.

Technical Tips

A few technical points are worth mentioning. First, the advancement of the Jamshidi needle across the pedicle should proceed smoothly with light to moderate taps of the mallet. If the surgeon encounters very hard bone, it generally indicates that the needle tip is displaced medially into the facet joint (the needle tip is striking the hard cortical surface of the superior articular facet). In this instance, the surgeon should withdraw the needle tip and begin with a slightly more lateral starting point to prevent the needle tip from slipping into the facet joint. Another useful tip is to consider the en face view if the AP view fails to clearly show the outline of the pedicle (this is most commonly a concern at the L5 level). To obtain an en face view, start with a true AP view and then angulate the C-arm 10 to 15 degrees in the axial plane to line up the beam with the pedicle axis (Fig. 58–23). Using the en face view, the center of the pedicle should be targeted, keeping the shaft of the needle in line with the C-arm beam. Another useful tip concerns making minor adjustments to a cannulated pedicle screw trajectory. In such a case, the pedicle can be tapped (with a cannulated tap) to the base of the pedicle and then, leaving the tap in place, the guidewire can be withdrawn into the tap, allowing the trajectory of the tap to be adjusted as desired with the assistance of fluoroscopy. Once the new trajectory is achieved, the guidewire is reinserted into the vertebral body along the new trajectory. Finally, stimulus-evoked EMG testing of the taps and screws has proven to be a useful adjuvant to the placement of percutaneous pedicle screws. Any low voltage activity (<8 mV) should alert the surgeon to pursue additional measures to ensure correct placement of the implant.

FIGURE 58–23 En face view: obtained through angulating the C-arm 10 to 15 degrees in the axial plane (A) to line up the beam with the pedicle axis (B).

Direct Pedicle Screw Insertion via a Paramedian Approach

Some surgeons prefer direct insertion of pedicle screws, using an expandable tubular retractor system to directly visualize the anatomic landmarks for pedicle screw insertion. This approach is especially useful when performing a minimally invasive TLIF procedure. In this technique, an expandable tubular retractor is positioned to visualize the junction between the base of the transverse process and the superior articular process. Any overlying soft tissue is cleared away using electrocautery to expose the bony landmarks. A starting hole for the pedicle screw insertion is then made using the normal anatomic landmarks as with traditional open pedicle screw insertion. C-arm fluoroscopy can be used to document the location of the starting point and trajectory for the pedicle cannulation. A “gear shift” or similar type blunt instrument is inserted into the starting hole and passed through the pedicle, using fluoroscopic guidance as needed. The walls of the pedicle are palpated to ensure the absence of a breach. Next, the site is tapped and the walls of the pedicle are again palpated for integrity. Pedicle screws are inserted using a technique analogous to open pedicle screw placement. Electrical testing of the screw can be used if desired, followed by the introduction of a rod and caps.

Facet Screw Instrumentation

Although pedicle screws are the “work horse” for most spinal fixation strategies, facet screws offer certain advantages in selected cases. Facet screws are quick and relatively easy to place. They are generally less expensive, compared with pedicle screw implants, and yet offer comparable initial stiffness for short constructs.¹³ In certain clinical situations such as following anterior lumbar interbody fusion, facet screw instrumentation has been shown to produce favorable clinical results.^14,¹⁵

Benini and Magerl ¹⁶ described a technique using large-fragment (4.5-mm) cortical bone screws to perform a translaminar fixation of the facet joint. The screw path begins at the base of the spinous process on one side and is then advanced across the contralateral lamina and facet joint (Fig. 58–24). Two screws are placed to immobilize the facet joints bilaterally using a miniopen or percutaneous technique.^17,¹⁸

FIGURE 58–24 Facet screw instrumentation (A): The screw path begins at the base of the spinous process on one side and then advances across the contralateral lamina and facet joint (B).

To insert translaminar facet screws using the miniopen technique, a midline incision is performed and the spinous processes, bilateral laminae, and facet joints are exposed. The facet joint capsules may be removed and the joint decorticated and packed with bone graft to promote local fusion following instrumentation. When placing translaminar facet screws, pilot holes are made on each side of the spinous process in line with the anticipated screw trajectories. The pilot holes should be slightly staggered to prevent the two screws from contacting one another as they cross through the base of the spinous process.

Next, the trajectory for each screw is defined, using fluoroscopy if desired. Some surgeons prefer to make a small laminotomy and palpate the medial wall of the pedicle as a landmark. This ensures direct visualization of the dura during intralaminar drilling for the screw path. Additionally, decompression of the lateral recess using a fenestration technique can be performed as needed.

Next, a line connecting the midfacet (or medial boarder of the pedicle) and the pilot hole is marked on the skin. This trajectory can be extended superiorly and laterally to the midline incision. A small, percutaneous incision is made along this line, about 10 to 12 cm from the midline such that the drill trajectory will be in line with the contralateral lamina. A drill guide is inserted into the percutaneous incision and advanced into the midline exposure (Fig. 58–25). The drill is inserted and seated into the pilot hole at the base of the spinous process. The drill is adjusted as needed so that the drill will traverse the lamina and then facet joint. As the drill is advanced, the surgeon should feel uniform resistance until the facet joint has been breeched. A momentary change in resistance may be noted as the facet joint space is traversed, but the cortical bone of the superior articular process will then be encountered. After drilling, the length of the screw path is measured. Then, a 4.5-mm, fully threaded cortical screw is placed to secure the position of the facet joint. A similar, percutaneous technique has been described, relying only on fluoroscopic images to ensure adequate placement of the translaminar facet screw implants.¹⁵

FIGURE 58–25 Facet screw instrumentation: Various instruments (e.g., drill guides, depth gauge, screws) are used during percutaneous facet screws placement.

Patient Selection

Patient selection remains the most crucial outcome variable for any spinal procedure.¹⁹ The same selection criteria that have been shown to produce success in traditional spinal surgery apply to patient selection for an MISS approach. In addition, patients being considered for an MISS approach have some additional selection criteria that should be considered.²⁰

In the early experience of an individual surgeon, only simple cases should be considered for an MISS approach. Experience can be gained working with MISS equipment such as tubular retractors and microscopes by performing straightforward cases (e.g., microdiscectomy). As a surgeon gains experience with minimally invasive spinal procedures, progressively more complex cases may be tackled in a minimally invasive fashion. Other aspects should be considered as well including the size of the patient (obese patents will be more difficult), severity of the pathologies, and history of any prior spinal surgery.