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Pedicle screw fixation has become a widely accepted method for spinal stabilization. This technique provides rigid, three-column stability and can be used in the surgical management of a wide range of spinal disorders. One of the disadvantages of traditional pedicle screw placement, however, is that it requires extensive soft tissue dissection to expose the anatomical landmarks and achieve the proper trajectory for screw insertion. The tissue trauma that occurs during the surgical exposure can be considerable and is at least partially responsible for the significant cost and lengthy hospital stays associated with instrumented lumbar fusion.1 In addition, the morbidity associated with these procedures has become an increasing concern for many surgeons. In part, this morbidity is related to the significant iatrogenic muscle and soft tissue injury that occurs during routine lumbar fusion exposures.

Multiple authors have documented the harmful effects of the extensive muscle dissection and retraction that normally occur during lumbar procedures.^2–8 Kawaguchi et al^2,3 analyzed the effects of retractor blade pressure on the paraspinous muscles during lumbar surgery. They determined that elevated serum levels of creatine phosphokinase MM isoenzyme, an indicator of muscle injury, is directly related to the retraction pressure and duration. These findings support the work by Gejo et al,⁴ who examined postoperative MRIs and trunk muscle strength in 80 patients who previously had lumbar surgery. They concluded that the damage to the lumbar musculature was directly related to the time of retraction during surgery. Furthermore, the incidence of low back pain was significantly increased in patients who had long muscle retraction times. Styf and Willen⁵ determined that retractor blades may actually increase intramuscular pressure to levels of ischemia. Mayer et al⁶ evaluated trunk muscle strength in patients who had previous lumbar surgery and found that patients who had undergone fusion procedures were significantly weaker than those who had undergone diskectomy. Rantanen et al⁷ concluded that patients with poor outcomes after lumbar surgery were more likely too have persistent pathologic changes in their paraspinal muscles.

In this chapter we will describe a technique and instrumentation designed by the senior author (KTF) for minimally invasive posterior fixation of the lumbar spine by using percutaneous screws and rods (Sextant; Medtronic Sofamor Danek, Memphis, TN). Paraspinous tissue trauma is greatly minimized without sacrificing the quality of the spinal fixation. Although percutaneous lumbar pedicle screw insertion has previously been reported, a minimally invasive approach to inserting a longitudinal connector for these screws has proven more challenging. The Sextant system allows for the straightforward placement of lumbar pedicle screws and rods through percutaneous stab wounds. The screws and rods are placed in an anatomical position similar to that achieved by an analogous open surgical approach.

Minimally Invasive Lumbar Interbody Fusion

A tubular retractor system was first developed for microdiskectomy in 1994 by Foley and Smith9; its basic concept is the foundation on which several contemporary approaches to minimally invasive posterior lumbar fusion are based. The system consists of a series of concentric dilators and thin-walled tubular retractors of variable length. The spine is accessed via serial dilation of the natural cleavage plane between muscle fascicles, instead of a more traumatic muscle-stripping approach. The use of a tubular retractor, rather than blades, allows the retractor itself to be thin-walled (0.9 mm), even when the wound is quite deep. In addition, unlike blades, the tube circumferentially defines a surgical corridor through the paraspinous tissues. This helps prevent muscle from intruding into the exposure. All of the midline supporting musculoligamentous structures are left intact with this technique. An appropriately sized working channel is created that permits spinal decompression and fusion. Surgery can be performed using the operating microscope, loupes, an endoscope, or a combination of techniques, depending on the preference of the surgeon. The tubular retractor approach can be utilized for minimally invasive lumbar fusion via posterolateral onlay posterolateral interbody fusion (PLIF) or transforaminal lumbar interbody fusion (TLIF).

Sextant Percutaneous Pedicle Screw and Rod Fixation

This section will describe the technique for Sextant percutaneous pedicle screw and rod fixation. Of course, the percutaneous instrumentation is always performed in conjunction with fusion. The fusion may be done anteriorly (standard, mini-open, or laparoscopic anterior lumbar interbody fusion, ALIF). If so, then the patient is merely turned to the prone position after the ALIF (same operative setting), and the percutaneous screws and rods are inserted. Each screw is placed through a 14 to 15 mm incision. Because of the lumbar lordosis, the small incisions for screw placement at L5–S1 typically overlap; thus, both screws are placed through the same 1-inch (2.56 cm) incision. If the patient requires a concomitant posterior decompression, or if there is no indication (or a contraindication) for an ALIF, posterior and posterolateral approaches to fusion are performed in conjunction with Sextant fixation. We typically perform these in minimally invasive fashion using a 22 mm diameter tubular retractor. This tubular retractor, which derives from the microendoscopic diskectomy technique, can be inserted through a 1-inch (2.56 cm) paramedian incision and relies on the same muscle-splitting principle that is used with the Sextant system. We have used this approach for instrumented PLIF and TLIF. Once the fusion has been performed through the tubular retractor, the retractor is removed, and the Sextant instrumentation is inserted through the same 1-inch (2.56 cm) incision. Single-level posterior and posterolateral fusion and pedicle screw instrumentation can therefore be performed through bilateral, paramedian 1-inch (2.56 cm) incisions. Sextant fixation has also been successfully applied to two-level fusion and fixation.

Patient Position

• The patient is in the prone position.
  
• A radiolucent table and padded bolsters support the clavicle, anterior chest wall, and iliac crest.
  
• The hips are neutral or slightly extended, maintaining or improving the lumbar lordosis.
  
• The upper extremities are padded and placed laterally on arm boards with less than 90 degrees of shoulder abduction to prevent brachial plexus injury.
  
• Avoid compression along the medial side of the elbow to prevent ulnar nerve injury.
  
• The head should rest gently on a donut or foam cutout that allows a neutral neck position and prevents any pressure points from developing on the face.
  
• Prior to prepping and draping the patient, a C-arm fluoroscopy unit is used to ensure that the appropriate spinal anatomy is visible.
  
• On anteroposterior images the pedicles should be oval-shaped and have crisp, dark outer cortical margins. The position of the spinous process should appear to bisect the interval between the two pedicles symmetrically at each level of interest.
  
• The lateral images should reveal sharp vertebral body end plates and a single pedicle.
  
• The fluoroscope is draped into the operative field in preparation for surgery.

PLIF Using the METRx and Tangent Technique

• Operative interspace is determined with the fluoroscope and a 22-gauge spinal needle (Fig. 23–1).
  
• Two 1-inch (2.56 cm) incisions are made at this level, ~25 mm to either side of the midline, and carried only into the subcutaneous tissue.
  
• The METRx Microdiskectomy Surgical Technique (Medtronic Sofamor Danek) instruments are used.
  
• A guidewire is inserted through the small incision and penetrates the underlying fascia.
  
• A cannulated soft tissue dilator is passed over the guidewire, directed toward the inferior aspect of the superior lamina.
  
• Once the dilator penetrates the fascia, the guidewire is removed, and the dilator is advanced to the lamina.
  
• Sequentially larger dilators are passed over the first dilator down to the lamina.
  
• Markings on the sides of the dilators indicate the depth from the skin surface. A 22 mm tubular retractor of appropriate length is chosen and advanced over the final dilator
  
• After the retractor has been locked in position using the articulated, table-mounted retractor arm, the dilators are removed.
  
• The underlying anatomy is visualized with the operating microscope, an endoscope, or surgical loupes.
  
• Residual soft tissue is cleared from the laminar surface, exposing the lamina and the ligamentum flavum.
  
• A second tubular retractor is placed in an identical fashion through the contralateral incision.
  
• Laminotomies are performed bilaterally through the tubular retractors, using rongeurs and/or a high-speed drill. The ligamentum flavum is removed, exposing the dural sac and the traversing nerve root.
  
• Bilateral diskectomies are then performed, using the Tangent Posterior Impacted Instrument Set (Medtronic Sofamor Danek).
  
• Interspace height is restored using sequentially larger interbody distractors inserted via the tubular retractors. The final distractor is left in place on the contralateral side.
  
• The appropriate-sized box chisel is then used to mortise the end plates on the ipsilateral side. The box chisel is removed, the interspace is packed with morcellized autograft bone, and a Tangent machined allograft is impacted into the interspace (Fig. 23–2).
  
• On the contralateral side, the distractor is removed, the interspace is mortised, and a second Tangent allograft is inserted along with additional autograft bone.
  
• The tubular retractors are then removed, and segmental fixation with the Sextant pedicle screw and rod system is performed.

FIGURE 23–1 PLIF using the METRx and Tangent system.

FIGURE 23–2 Placement of the Tangent interbody bone graft as seen through a METRx tubular retractor system.

Sextant Screw and Rod Placement Technique

The Sextant screws are inserted through the same 1-inch (2.56 cm) incisions that were used for the tubular retractors. The pedicles are localized with anteroposterior and lateral fluoroscopy. The FluoroNav Virtual Fluoroscopy System (Medtronic Sofamor Danek) can be utilized as an alternative.

• An 11-gauge bone biopsy needle is inserted until its tip reaches the junction of the facet and transverse process (Fig. 23–3A).
  
• Lateral fluoroscopy should show the needle at the top of the pedicle cylinder, aligned with and bisecting the pedicle. Anteroposterior fluoroscopy should show the tip to be located at the lateral margin of the pedicle cylinder (Fig. 23–3B).
  
• Using a medially directed trajectory, the needle is then carefully advanced into the pedicle by tapping the base of the needle with a mallet.
  
• The needle will pass through the cancellous bone toward the base of the pedicle on the lateral image (Fig. 23–3C).
  
• As the needle reaches the pedicle-vertebral body junction, the tip should be positioned in the center of the pedicle on the anteroposterior image.
  
• If the needle reaches the center of the pedicle on the anteroposterior image when first entering the pedicle, the trajectory is too medial, and there is a significant risk of entering the spinal canal.
  
• Once the needle has safely entered the vertebral body, the inner trocar is removed, and the K-wire is placed into the needle. A power driver is used to pass the wire through the pedicle under serial anteroposterior and lateral fluoroscopic guidance (Fig. 23–4).
  
• After the K-wire has been positioned, the sequential dilators are placed, and the pedicle is tapped with the cannulated tap (Figs. 23–5, 23–6).
  
• Review of the preoperative CT or MRI will allow for selection of proper screw diameter; screw length can be chosen based on the calibration markings on the tap.
  
• Prior to placing the Sextant screws, the screw extenders must be attached. The screw extenders have inner and outer sleeves.
  
• A lock plug is placed in the inner sleeve by inserting the cap of the plug into the distal end of the sleeve. The cap of the lock plug is held within the sleeve, allowing the threaded end to hang freely (Fig. 23–7A).
  
• The inner sleeve is then placed into the outer screw extender and left at its most upward position.
  
• There are two positions for the inner sleeve within the outer screw extender. The first allows the threads of the lock plug to engage the saddle of the multiaxial Sextant screw but leaves an opening in the saddle for rod insertion. The second allows the lock plug to be driven into the final position, locking the rod to the Sextant screw (Fig. 23–7B).
  
• The saddle of a cannulated, multiaxial Sextant screw is placed in the distal end of the assembled screw extender, and the plug driver is used to engage the lock plug, thus attaching the Sextant screw to the extender.
  
• The assembly is then examined to ensure that the set screw is in the correct position, and the Sextant screw is firmly attached to the extender. A rod can also be passed immediately below the set screw into the multiaxial screw head saddle to make certain of proper assembly.
  
• The cannulated screwdriver is passed into the proximal screw extender; its tip passes through the set screw and engages the Sextant screw head.
  
• The entire screw-extender assembly is placed over the K-wire, and the screw is inserted into the pedicle under fluoroscopic guidance (Fig. 23–8).
  
• The K-wire is removed once the screw has traversed the pedicle to prevent inadvertent advancement.
  
• The exact procedure is repeated for the pedicle screw on the same side at the adjacent level.
  
• The proximal portion of each screw extender has a flat surface; one of these surfaces has an extrusion, and the other has a matching receptacle. The screw extenders are rotated so that their flat surfaces are flush and the extrusion from the first surface fits in the receptacle of the other (Fig. 23–9).
  
• The maneuver of aligning the extender surfaces external to the patient also aligns the screw saddles beneath the paraspinous muscle, readying them for rod insertion.
  
• Once the surfaces are flush against one another, the rod inserter is connected to the two screw-extender assemblies (Fig. 23–10).
  
• The rod connects to the screw-extender-inserter assembly in such a fashion that the rod is geometrically constrained to pass along an arc that intersects the openings in the screw saddles (Fig. 23–11).
  
• Prior to passing the rod, a trocar tip is attached to the rod inserter. A small stab wound is made where the trocar intersects the skin, and the rod inserter creates a pathway through the fascia and muscle to the first screw head under fluoroscopic guidance.
  
• The pathway can either be made rostral or caudal to the levels of fusion.
  
• One can also adjust the sagittal trajectory of the rod insertion pathway by slightly advancing one screw-extender assembly and backing out the other. If bony obstruction is encountered along the lateral aspects of the facet complexes, the screw-extender-inserter assembly can be rotated laterally (remaining connected) to allow for a more lateral rod trajectory.
  
• A rod template is attached to the screw extenders and determines the appropriate length rod. The trocar is then replaced by the curvilinear rod, and the rod is passed through the multiaxial screw saddles.
  
• Multiplanar fluoroscopic views are obtained to verify that the rod is properly positioned.
  
• Compressive or distractive forces can be applied to the construct prior to the final tightening.
  
• The inner extender sleeves are then advanced to their final position, allowing the lock plugs to be able to engage the rod. The lock plugs should tighten within 1.5 rotations.
  
• If the lock plugs need to be loosened for any reason, it is wise to consider removing the screw-extender assembly after first replacing a K-wire through it. The screw and extender can be properly assembled under direct vision and then simply reinserted over the K-wire. If one loosens the lock plug with the screw and extender in the patient, one risks inadvertently disassembling the extender from the screw (remember that the lock plug holds the screw to the extender).
  
• Final tightening is performed with the plug driver. The rod inserter serves as the countertorque device. The lock plug heads shear off and are retained within the inner sleeves, detaching the extenders from the Sextant screws.
  
• The rod is detached from the inserter. The extenderinserter assembly is then removed from the field. A percutaneous rod and screw construct is left in place in a traditional anatomical position (Fig. 23–12).
  
• The entire procedure is repeated on the opposite side, after which the wounds are irrigated and closed in layered fashion. We prefer to use an absorbable, subcuticular suture and Steri-strips for cosmetic purposes (Fig. 23–13).

FIGURE 23–7 Screw extender assembly. (A) Inner screw driver placed within screw extender shaft. (B) Engagement of the locking screw and screwdriver on the polyaxial screw head.

Sixty-three patients have undergone percutaneous pedicle screw and rod insertion with the Sextant system at our institution since March 2000. Thirty-nine of the patients have been followed for at least 12 months. Twenty-two of the patients were male, and 17 were female. Their ages ranged from 23 to 80 years, with a mean of 46 years. The diagnoses were isthmic spondylolisthesis in 17 patients (11 grade I, 5 grade II, and 1 grade III), degenerative spondylolisthesis (stenosis) in 15 patients, degenerative disk disease in five patients, and one trauma; the remaining two patients suffered from symptomatic nonunion related to previous failed fusion. Thirty-seven patients had single-level fusions, and two had two-level fusions. Twenty-five patients underwent concomitant ALIF, 12 underwent minimally invasive PLIF or TLIF, one underwent a minimally invasive retroperitoneal approach, and one had a minimally invasive posterolateral fusion. The instrumented levels were L5–S1 in 19 patients, L4–L5 in 16 patients, L3–L4 in one patient, and L2–L3 in one patient. Two patients underwent twolevel fusion and fixation (one L3–L5 and another L4–S1).

FIGURE 23–10 Once the surfaces are flush against one another, the rod inserter is connected to the two screw-extender assemblies. (A) Artist’s depiction. (B) Intraoperative picture.

FIGURE 23–12 Postoperative anteroposterior (A) and lateral (B) x-rays after a METRx-Tangent-Sextant percutaneous PLIF.

Percutaneous lumbar fixation was designed, in part, to minimize the paravertebral muscle injury that occurs with conventional open procedures. Magerl10 first reported the use of percutaneous pedicle screws combined with an external fixator in 1982. The most obvious limitation of this technique was the risk of infection, not to mention the discomfort of an external appliance. Mathews and Long¹¹ described the use of percutaneous pedicle screws with longitudinal connectors placed under direct vision in the suprafascial, subcutaneous space. This superficial instrumentation was uncomfortable to the patient and associated with a significant nonunion rate as well, perhaps secondary to the long lever arms of the hardware.

The Sextant system allows for placement of percutaneous screws and rods through paramedian stab incisions. The conventional anatomical position of the construct avoids the instrumentation-related discomfort that was associated with earlier versions of percutaneous fusion. The geometrically constrained arc produced by the Sextant apparatus simplifies the connection of the percutaneous rods and screws.

There are several distinct advantages of the Sextant system compared with standard open lumbar pedicle fixation. The paraspinous muscles are bluntly separated rather than stripped from their attachments and are minimally retracted using a sequential dilation technique, as described by Foley and Smith⁹ for microendoscopic diskectomy. This results in significantly less intraoperative blood loss, less iatrogenic muscle injury, and less postoperative pain. Patients are therefore able to ambulate and mobilize much more quickly, resulting in a decreased rate of perioperative complications, shorter hospital stays, and decreased cost.¹² From a technical perspective, it is also easier to achieve the desired lateral to medial pedicle screw trajectory because there is not a wall of soft tissue that limits the angulation of the instruments (as can be encountered in the open surgery). This is particularly helpful in obese patients because more extensive exposure and retraction can be avoided. Operative time is also significantly lessened; it takes only 1 hour for the surgeon to place four screws and two rods.

The Sextant system is an emerging component in the rapidly developing field of minimally invasive spine surgery. It is an important advancement and serves as a complement to other newly established minimally invasive fusion techniques for ALIF, PLIF, TLIF, and posterolateral onlay fusion. As the technology continues to evolve, the indications for Sextant will certainly expand from primarily degenerative disease to include multilevel fusions for spinal disorders due to trauma and neoplastic conditions. The clinical utility of Sextant appears promising because our early experience suggests that the system is able to achieve the same clinical results as conventional open procedures while significantly reducing the exposure-related morbidity.

REFERENCES

1. Thomsen K, Christensen FB, Eiskjaer SP, et al. The effect of pedicle screw instrumentation on functional outcome and fusion rates in posterolateral lumbar spinal fusion: a prospective, randomized clinical study. Spine. 1997;22:2813–2822.

2. Kawaguchi Y, Matsui H, Tsuji H. Back muscle injury after posterior lumbar spine surgery: a histologic and enzymatic analysis. Spine. 1996;21:941–944.

3. Kawaguchi Y, Matsui H, Tsuji H. Back muscle injury after posterior lumbar spine surgery, II: Histologic and histochemical analyses in humans. Spine. 1994;19:2598–2602.

4. Gejo R, Matsui H, Kawaguchi Y, et al. Serial changes in trunk muscle performance after posterior lumbar surgery. Spine. 1999;24: 1023–1028.

5. Styf JR, Willen J. The effects of external compression by three different retractors on pressure in the erector spine muscles during and after posterior lumbar spine surgery in humans. Spine. 1998;23:354–358.

6. Mayer TG, Vanharanta H, Gatchel RJ. Comparison of CT scan muscle measurements and isokinetic trunk strength in postoperative patients. Spine. 1989;14:33–36.

7. Rantanen J, Hurme M, Falck B, et al. The lumbar multifidus muscle five years after surgery for a lumbar intervertebral disc herniation. Spine. 1993;18:568–574.

8. Sihvonen T, Herno A, Paljiarvi L, Airaksinen O, Partanen J, Tapaninaho A. Local denervation atrophy of paraspinal muscles in postoperative failed back syndrome. Spine. 1993;18:575–581.

9. Foley KT, Smith MM. Microendoscopic discectomy. Tech Neurosurg 1997;3:301–307.

10. Magerl F. External skeletal fixation of the lower thoracic and the lumbar spine. In Uhthoff HK, Stahl E, eds. Current Concepts of External Fixation of Fractures. New York: Springer-Verlag; 1982: 353–366.

11. Mathews HH, Long BH. Endoscopy assisted percutaneous anterior interbody fusion with subcutaneous suprafascial internal fixation: evolution, techniques and surgical considerations. Orthop Int Ed. 1995;3:496–500.

12. Foley KT, Gupta SK, Justis JR, Sherman MC. Percutaneous pedicle screw fixation of the lumbar spine. Neurosurg Focus. 2001;10: 1–8.