Percutaneous Lumbar Pedicle Screws Indications Technique Results

Gerald E. Rodts, Jr.

Department of Neurosurgery, Emory University, Atlanta, Ga., USA

Lumbar pedicle fixation has proven to be a safe, effective means of lumbar stabilization. Prospective, randomized trials and community clinical experience using lumbar instrumentation have demonstrated an increased fusion rate. Lumbar instrumentation has also improved the ability to mobilize patients after surgery and perhaps brace them for shorter periods of time or not at all. To date, techniques of lumbar pedicle screw placement have involved a single midline or bilateral paramedian incision. Some degree of muscular tendon detachment from the bone and muscle retraction has been necessary. Without question, this is a source of postoperative pain and subsequent muscle atrophy and scar formation.

Until recently, techniques of percutaneous lumbar fixation have involved external instrumentation or epifascial techniques [1-3]. Described herein is a new technique to allow for direct subfascial lumbar pedicle fixation, rod insertion, distraction/compression, and final tightening, all with percutaneous entry points. Compared to open techniques, the presumed benefit of the percutaneous technique is avoidance of muscular tendon detachment, elimination of large-scale muscle retraction, diminished postoperative pain, less blood loss, and improved cosmetic result. In the future, improved minimally invasive fusion techniques and the use of bone morphogenetic protein may increase the applicability of percutaneous techniques.

Indications

The indications for percutaneous lumbar pedicle fixation initially are limited to a few specific clinical conditions. Currently, percutaneous lumbar pedicle fixation may be most useful when a surgeon wishes to create a posterior tension band. As posterior endoscopic or other minimally invasive techniques progress, percutaneous fixation may be combined with simultaneous laminec-tomy, discectomy, interbody or posterolateral fusion.

One indication for percutaneous posterior fixation is following a laparoscopic or open (retroperitoneal or transperitoneal) anterior lumbar interbody fusion (ALIF). Many surgeons do not rely on a stand-alone ALIF for the treatment of a mobile (unstable) lumbar spondylolisthesis. Whether threaded titanium cylinders, vertical carbon or titanium or ceramic cages, allograft femoral ring wedges or threaded allograft bone dowels are used for the ALIF, the placement of posterior instrumentation in the setting of a mobile spondylolisthesis provides greater biomechanical stability. Furthermore, studies have shown a higher rate of pseudoarthrosis following stand-alone ALIF using allograft bone. With posterior fixation, the rate of fusion following ALIF with bone-only is improved.

Another consideration for the use of posterior percutaneous fixation is in the case of a pseudoarthrosis following previous stand-alone ALIF. If a nonunion is present yet the surgeon is satisfied with the structural integrity of the previously implanted bone graft, posterior instrumentation ultimately can help to achieve a successful arthrodesis. An example would be a case of previous femoral ring ALIF with an intact graft, absence of subsidence or vertebral body lysis, but no evidence of bone union on plain radiographs or CT with reconstructions.

Percutaneous pedicle fixation is also a reasonable alternative to open placement of instrumentation in the setting of a pseudoarthrosis following previous posterolateral fusion. In this setting, decortication of the previous fusion mass and placement of bone graft can be done through the same portals used for percutaneous screw placement.

More recently, advances in less invasive techniques using tubular retractors and blunt dissection through muscle have allowed for lumbar bone decompression (laminectomy, laminotomy, medical facetectomy, discectomy), posterolat-eral fusion, and interbody fusion. Percutaneous pedicle screw fixation using the same or different stab incisions may be a beneficial adjunct to these newer approaches to decompression and arthrodesis.

Materials and Techniques

For percutaneous pedicle screw fixation, radiographic image guidance is essential. Several options may be considered. Placing the instrumentation using plain radiographs is not recommended. A single fluoroscopic C-arm can be alternated between anterior-posterior (AP), oblique or 'owl's eye', and straight lateral views. The owl's eye view allows a coaxial view down the barrel of the pedicle. Performing percutaneous screws with a single fluoroscopic

C-arm is possible; however, it is tedious and significant time must be devoted to changing sterile drapes and moving the C-arm into different positions.

Simultaneous biplanar fluoroscopy offers immediate feedback in two planes. Ergonomically, it is challenging to operate inside of and around two C-arms placed for lateral and AP or lateral and oblique views. Simultaneous dual views allow for instant feedback when an instrument trajectory is altered manually by the surgeon in one plane. One limitation of biplanar fluoroscopy in percutaneous procedures, however, is the difficulty in seeing the tip of an instrument at the level of the skin surface. This is particularly true in patients with a large body habitus where there is a great distance between skin surface and the spine.

Computer-assisted, virtual fluoroscopic systems offer a tremendous advantage. We have had extensive experience using the FluoroNav™ virtual fluoroscopic system (Medtronic Sofamor Danek, Memphis, Tenn., USA). With this technology, a stereotactic reference arc with light-emitting diodes is attached rigidly to the patient's spine through a small stab incision. The arc can also be attached to the ileum using a screw. Routine lateral, oblique or owl's eye, and AP images are obtained. The images are obtained with an overlying calibration array of 'fiducials' attached to the C-arm. Images are then transferred automatically from the C-arm monitor to the Stealth™ or ION™ (intraoperative navigation) computer monitor. The images are automatically 'registered' and ready for navigation. Left and right oblique images are recommended for each level to be instrumented. Up to four images can be displayed and monitored simultaneously on the same screen. Digitized instruments with light-emitting diodes are then recognized by the camera and their location in the room is thereby known by the computer. In short, a surgeon can navigate on two-dimensional images as if the C-arm were operating 'live' continuously. No further irradiation is necessary. Virtual cartoons or representations of the surgical instruments move real time on the monitor as the surgeon moves them in surgery.

Perhaps the most powerful component of the virtual fluoroscopic system is the ability to virtually 'extend' the tips of the various instruments from the skin surface down to the spine. Thus, when an instrument is held on the skin surface, the tip is extended down to recognizable radiographic landmarks that represent the entry point for pedicle fixation. This is not possible with conventional fluoroscopy or radiography unless a wire is actually passed through the skin and soft tissues to simulate a trajectory. The lateral view identifies the center of the pedicle and appropriate trajectory in the sagittal plane. The owl's eye or oblique view gives the surgeon feedback in the axial plane. Straight AP images represent the coronal plane. One helpful technique is to extend the tip of the virtual instrument on the computer screen to the point where the pedicle enters the posterior aspect of the vertebral body (fig. 1, 2). In the sagittal view, therefore, the instrument would have virtually probed the entire length of the pedicle in the AP direction. When one then looks simultaneously at the oblique view, the very tip of the extended instrument should be within the cortical confines (margins) of the pedicle. If the tip is medial, medial wall perforation may be possible. In short, the surgeon's brain is able to imagine three-dimensional trajectories based on multiple two-dimensional fluoroscopic images. Once the desired skin entry point is identified, a small stab incision is made (approximately 10-12mm) (fig. 3).

In open placement of lumbar pedicle screws, slight variations of the same technique are used to identify the entry point for pedicle fixation. It is recommended that the proximal or medial-most aspect of the transverse process be considered for the entry point. This is as opposed to performing a partial inferior-medial facetectomy for a more medial entry point. For obvious reasons, one would favor a technique for percutaneous placement that does not

Fig. 1. Virtual representation of probe extending through skin down to the level of vertebral body (AP and lateral views) (photo courtesy of Kevin Foley, MD).

Percutaneous Pedicle Screws

Fig. 1. Virtual representation of probe extending through skin down to the level of vertebral body (AP and lateral views) (photo courtesy of Kevin Foley, MD).

Fig. 2. Schematic drawing of extending virtual tip of probe beyond length of pedicle to help stay within confines of cortex.

Fig. 3. Reference arc and digitized probe just beneath skin surface.

require bone removal. Another advantage of using the medial transverse process is that the surgeon can palpate with a wire or other image-guided probe the superior and inferior edges of the transverse process at the same time that visual feedback is given from the AP virtual fluoroscopic view. One can then feel the slight groove where the transverse process meets the superior facet process.

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