Instrumented Lumbar Fusion



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.

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







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.


Feb 2, 2020 | Posted by in ORTHOPEDIC | Comments Off on Instrumented Lumbar Fusion

Full access? Get Clinical Tree

Get Clinical Tree app for offline access