Instrumented Lumbar Fusion

Andrew J. Schoenfeld, MD

Christopher M. Bono, MD

Dr. Schoenfeld or an immediate family member serves as a board member, owner, officer, or committee member of the American Academy of Orthopaedic Surgeons. Dr. Bono or an immediate family member has received nonincome support (such as equipment or services), commercially derived honoraria, or other non-research-related funding (such as paid travel) from Harvard Clinical Research Institute and Intrinsic Therapeutics and serves as a board member, owner, officer, or committee member of the American Academy of Orthopaedic Surgeons, the International Society for the Advancement of Spinal Surgery, and the North American Spine Society.

Disclaimer: Some authors are employees of the U.S. federal government and the United States Army. The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or reflecting the views of William Beaumont Army Medical Center, the Department of Defense, or the United States government.

PATIENT SELECTION

Transpedicular (pedicle) screws gained wide popularity in the United States through the efforts of Steffee et al,¹ and since the late 1980s, their indications and use have expanded substantially.²^,³^,⁴ Pedicle screws are anchored within the corticocancellous core of the vertebral pedicle. This anatomic region offers the strongest point of fixation within the spine and affords pedicle screws a biomechanical advantage over other instrumentation techniques such as hooks, wires, or anterior vertebral body screws.²^,⁵^,⁶^,⁷ Several investigations have shown that the use of pedicle screw fixation enhances arthrodesis rates in the lumbar spine.²^,³^,⁴ It is an important component of more modern surgical techniques, such as transforaminal lumbar interbody fusion, all-posterior correction of idiopathic scoliosis, minimally invasive treatment of fractures, and posterior dynamic stabilization.

PREOPERATIVE IMAGING

Ideally, patients who are selected for a lumbar fusion procedure should have plain radiographs and an MRI or CT scan. At minimum, MRI or CT is required to assess the pedicle dimensions. Plain radiographs are most useful to assess spinal alignment. Rotational deformities must be appreciated because these can affect the degree of medialization of the screw path. Hyper- or hypolordosis must also be noted because this will affect the sagittal alignment of the screw path. In the lumbar spine, the optimal starting point for a pedicle screw can be identified by the intersection of two orthogonal lines: a horizontal line that bisects the transverse process, and a vertical line along the medial aspect of the pars interarticularis (Figure 1). These anatomic landmarks should be visualized on the AP view preoperatively.

More precise preoperative planning can be achieved using axial MRI or CT images. Factors that may prevent the safe insertion of pedicle screws, such as dysplastic or absent pedicles, aberrant nerve roots, or dural ectasia, are best appreciated in these images. Axial images should be routinely used to measure the pedicle diameters and approximate screw lengths at the proposed instrumented levels (Figure 2, A). Screw length can be measured preoperatively by measuring from the posterior aspect of the superior articular process to a desired depth within the vertebral body (Figure 2, B). The smallest transverse width of the pedicle should be used to determine pedicle screw diameter. The pullout strength of the pedicle screw is dependent on the interface between the cortical bone of the pedicle and the screw threads.⁶^,⁷ Therefore, it is not advisable to undersize the screws. Screws with a slightly larger width may be used as “rescue” screws in the setting of a compromised screw tract. Careful, slow insertion will allow the cortical walls of the tract to accommodate screws with
a moderately larger width. There is no good evidence, however, that supports the routine use of larger width pedicle screws in conventional tracts that have not been otherwise compromised. Approximate screw sizes for each vertebral level and side are transcribed on a preoperative template paper that can be brought to surgery.

FIGURE 1 The appropriate starting point for pedicle screw insertion can be identified on an AP radiograph. Line A extends along the medial aspect of the pars interarticularis; line B bisects the transverse process. The intersection of these two lines indicates an appropriate starting point for screw insertion.

VIDEO 106.1 Lumbar Laminectomy. Howard S. An, MD; Dino Samartzis, BS; Ashok Biyani, MD (4 min)

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Video 106.1

FIGURE 2 Preoperative images used to plan pedicle screw placement. A, Axial MRI shows the templating used to determine the appropriate size of pedicle screws. The widths (lines A and B) and lengths (lines C and D) are determined for pedicle screws bilaterally. B, Axial CT scan shows how the accessory process (arrow) can be used as an anatomic landmark for the screw insertion site.

PROCEDURE

Room Setup/Patient Positioning

Proper positioning of the patient is critical. The patient should be placed prone on a well-padded radiolucent table appropriate for spinal surgery. We prefer a Jackson four-poster surgical frame with a chest pad and supports for the iliac crest and thighs. The abdomen should hang free to reduce intra-abdominal pressure, thereby decompressing the Batson plexus and minimizing epidural bleeding during a decompression procedure. Additional padding should be placed under the knees and along the anterior legs to avoid pressure points. The shoulders should be abducted and the elbows flexed no more than 90° on well-padded arm boards. The anterior shoulder should be properly bolstered if needed to prevent brachial plexus stretch. A head positioner should maintain the neck in a neutral position, avoid pressure around the eyes, and allow access to the endotracheal tube.

Once proper positioning has been ensured, all wires, lines, and catheters should be secured to the frame of the operating table. This allows the C-arm to move about the patient with less risk of inadvertently dislodging a critical monitor or access mechanism. The operating table should be placed in the center of the room, with the anesthesia station located at the head. The surgeon and assistant stand on opposite sides of the patient. The fluoroscopic imaging system can be placed in an appropriate position where the operating surgeon can easily see the images. The preoperative template paper may be secured to the image viewer for ease of consultation.

Before the patient is prepared and draped, we make sure that adequate fluoroscopic views can be obtained. Minor adjustment of the patient’s rotation on the table can yield a better AP view of the surgical levels. In addition, the lateral view can be used to mark the levels of the pedicles to be instrumented and thus help determine the incision size.

Special Instruments/Equipment/Implants

Pedicle screws are inserted by first preparing a tract (Figure 3). Starter awls, taps, and depth gauges can be used during this process and are usually provided in the pedicle screw manufacturer’s instrumentation set. With the screw in place, a rod is secured to each with a “blocker” or set screw. The set screws are final tightened using the manufacturer’s torque-limiting driver, which avoids undertightening or stripping of the set screw.

Some surgeons find it useful to have a burr available for instrumentation cases. A larger (5-mm) burr can be used to define the lateral edge of arthritic facet joints to appropriately visualize the starting point for the pedicle screws. A larger burr is also effective for removing the posterior cortical bone at the insertion site to better visualize the cancellous bone of the pedicle. A large burr is also the most common tool used to decorticate bone in the area of the fusion bed.

If desired, electrophysiologic monitoring can be used to assess nerve root activity during the procedure and test pedicle screws for aberrant placement.⁸^,⁹ Triggered electromyography (EMG) performed by stimulating the pedicle screw after it has been inserted yields a threshold at which the adjacent lumbar nerve root is stimulated. High thresholds, typically greater than 10 mA, are needed if the screw is contained in bone and are thus indicative of a well-placed screw.⁴^,⁸^,⁹ Moderate thresholds (5 to 9 mA) suggest that a breach of the pedicle wall may be present but the screw is not contacting the nerve; low thresholds (<5 mA) suggest that the screw is in direct contact with the nerve. If there is a discrepancy in the threshold reading, the EMG probe can be used to directly stimulate a nerve root to obtain a baseline reading for comparison.⁴

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