Percutaneous Placement of Lumbar Pedicle Screws: Indications and Techniques

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Chapter 167 Percutaneous Placement of Lumbar Pedicle Screws

Indications and Techniques

Introduction

The use of pedicle screws for spinal stabilization has become increasingly popular worldwide. Pedicle screw systems engage all three columns of the spine and can resist motion in all planes. Several studies suggest that pedicle screw fixation is a safe and effective treatment for many spinal disorders.1,2 Standard techniques for pedicle screw placement, however, require extensive tissue dissection to expose entry points and to provide a lateromedial orientation for optimal screw trajectory. Open pedicle fixation and spinal fusion have been associated with extensive blood loss, lengthy hospital stays, and significant cost.3 Minimally invasive placement of pedicle screws can potentially address these issues without compromising the accuracy of placement.4

The use of percutaneous lumbar pedicle screws and an interconnecting subfascial rod was first described in 20015 and later adopted by others.6,7 The spinal implants used for percutaneous pedicle screw fixation are essentially the same as those used in conventional open techniques. To preserve soft tissue and allow for safe and effective insertion of these implants into the lumbar spine, certain principles need to be followed. For example, the pedicle screws are commonly placed over K-wires. These allow the surgeon to gain access to a pedicle pilot hole with minimal, if any, direct visualization. Screw extenders, which allow for percutaneous manipulation and localization of the screws once they have been inserted and cannot be seen directly by the surgeon, also provide a means for screw alignment so that an interconnecting rod can be successfully joined to the screws in a minimally invasive fashion. This chapter describes techniques for percutaneous lumbar pedicle screw fixation and the present related outcome data.

Indications and Preoperative Assessment

The indications for percutaneous lumbar pedicle screw fixation are essentially the same as those for conventional open fixation. These include degenerative conditions, such as spondylolisthesis, and traumatic conditions, such as certain burst fractures. Of course, the percutaneous fixation needs to be performed in conjunction with spinal fusion. Although certain fractures can fuse spontaneously when treated with percutaneous pedicle fixation (e.g., Chance fractures), the surgeon usually needs to perform a fusion in conjunction with the fixation. A full description of minimally invasive spinal fusion, which can be performed in a variety of fashions (posterolateral onlay, facet, and various interbody techniques), is beyond the scope of this chapter.

Preoperatively, it is important to ensure that the pedicles are of sufficient caliber to accommodate percutaneous pedicle screws. Typically, these screws are available in the same diameters and lengths as conventional pedicle screws. Pedicle diameters can be measured using a preoperatively obtained magnetic resonance image (MRI). Occasionally, such as for a spondylolisthesis patient, the pedicles are a bit dysmorphic and can be better measured with a computerized tomographic (CT) study. The patient’s body habitus should also be assessed preoperatively. Although percutaneous techniques can be quite successful when used in larger patients, a morbidly obese patient can have enough subcutaneous fat tissue to “swallow” a screw extender. It is the senior author’s practice to have the patient lie in the prone position on the examination table in the clinic. If the tips of the lumbar spinous processes can be palpated by the examiner (even with significant manual pressure), the patient is a potential candidate for percutaneous pedicle fixation.

Operative Techniques

Initial Incision and Pedicle Identification

Fluoroscopic images are obtained in the AP and lateral planes to ensure that the pedicles can be adequately visualized. If necessary, oblique (or “owl’s-eye”) views can be obtained as well. It is important that “true” AP and lateral views are obtained because even a minor misalignment in imaging may lead to wayward screw placement. To ensure a true AP view, the spinous process must lie precisely in the center of the interpedicular space. To obtain a true lateral view, the pedicles should overlie one another and the end plates should be linear (not elliptical). If the patient is scoliotic, the C-arm should be angled to achieve these views. Furthermore, to avoid parallax, center the target anatomy on the fluoroscopic screen. It is therefore important to check the fluoroscopic views before the skin is prepared and the patient is draped. This allows for manipulation and repositioning of the patient, bed, and fluoroscopic C-arm without compromising the sterile field. Additionally, the use of a radiolucent table (e.g., Jackson table) and, if used, a radiolucent positioning device (e.g., gel chest rolls) is important. A small incision is made approximately 4 to 5 cm off the midline, depending on the size of the patient (Fig. 167-1A). For a thin patient, an incision 4 cm off the midline is utilized. A more lateral incision is made for an obese patient. The principle is that a lateromedial trajectory is desired; the object is to maximize the bone purchase of the pedicle screw while avoiding the facet complex. A larger patient, with more soft tissue dorsal to the spine, requires an incision that is farther from the midline to achieve the same trajectory as that for a thinner patient. A 22-gauge spinal needle can be used to localize the position of the incision prior to cutting the skin (Fig. 167-1B). The incision length is dependent on the diameter of screw extenders but is approximately 1.5 cm for a single screw and 2.5 cm for two screws.

Pedicle Screw Placement

Thorough knowledge of pedicle anatomy and the sagittal and axial angulation of the individual pedicles is mandatory for safe percutaneous screw placement.8 These angles are best judged using preoperative CT or MRI of the lumbar region. The pedicle can be navigated using a conventional C-arm fluoroscope that is alternated between AP, lateral, and oblique views. One must obtain multiple sequential views of the pedicle probe in at least two planes as it is advanced down the pedicle.1,8 It is important to keep these trajectories in mind to ensure the accuracy of percutaneous screw placement. The ideal pedicle entry point is at the base of the transverse process as it joins the superior articular process; this location allows for the lateromedial trajectory of screws and minimizes the anatomic disturbance of the facet joint. One advantage of percutaneous screw placement over the conventional open technique is that it is much easier to achieve the required medial angulation because extensive soft-tissue and muscle retraction is avoided. A Jamshidi-type needle is used to create a pilot hole under fluoroscopic guidance (Fig. 167-1C and D). The needle is advanced through the pedicle into the vertebral body. Fluoroscopic images of three positions of the Jamshidi needle should be obtained: the initial starting point, the middle position within the pedicle, and the position at the pedicle-vertebral body junction. For the initial starting point, the Jamshidi needle should be coaxial with the pedicle on the lateral fluoroscopic image. The starting AP image should reveal the tip of the needle to be at the lateral border of the pedicle (Fig. 167-2A and B). When lateral fluoroscopy shows the tip of the needle has been advanced to the pedicle-vertebral body junction, the AP image should reveal that the Jamshidi needle tip is just lateral to the medial cortical wall of the pedicle, with approximately 4 mm of space to spare (Fig. 167-2C and D). This ensures that there is adequate room for the subsequent placement of a screw (with a radius of approximately 2.75 to 3.75 mm), thus minimizing the potential for medial wall breach and nerve root irritation. The value of obtaining AP and lateral fluoroscopic views of the needle when it has been advanced approximately one half the way down the pedicle is that course adjustments can still be made (Fig. 167-2E

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