Navigating Preexisting Lumbar Instrumentation in MIS Revision Surgeries



Navigating Preexisting Lumbar Instrumentation in MIS Revision Surgeries


Courtney Pendleton

George M. Ghobrial

James S. Harrop



The use of intraoperative navigation in spinal surgery has increased in tandem with the last several decades as the cost of this technology declined. Both two-dimensional screw navigation utilizing fluoroscopy and three-dimensional (3-D) navigation achieved by intraoperative imaging obtained against a digital reference array (DRA) have been shown to limit variability in pedicle screw placement when compared to the freehand technique, particularly in the thoracic spine. Some cost-effectiveness data suggest that centers performing just over 250 instrumented fusions annually might be able to justify the use of 3-D navigation over fluoroscopy financially through limiting reoperation for malpositioned hardware.1

The prevalence of symptomatic adjacent segment lumbar disease at levels above or below prior fusion constructs has been reported as 8.5% of all patients in a multiyear meta-analysis,2 and symptomatic patients may require further operative intervention. Additionally, symptomatic malpositioned pedicle screws, graft subsidence, hardware failure, and proximal junctional kyphosis are only a few of the many indications for hardware revision.

Standard open revision techniques entail a subperiosteal exposure of the posterior elements and can be laborious due to the frequent need to debride scar tissue, calcifications, and fusion mass to obtain sufficient exposure and remove preexisting rods or screws. This leads to increased postoperative pain, blood loss, operative duration, and infection risk.

Minimally invasive surgical (MIS) techniques may be able to provide an adequate operative corridor while minimizing tissue dissection and retraction, reducing intraoperative blood loss, and contributing to a reduction in postoperative pain and overall hospital length of stay. Use of intraoperative navigation through a variety of modalities provides information regarding preexisting bony anatomy and instrumentation without requiring a traditional open exposure.


MIS TECHNIQUE


Relevant Anatomy

In patients with prior lumbar surgeries, particularly those with prior open approaches, the anatomy may be distorted or complicated by scar tissue. The use of a minimally invasive approach allows for the avoidance of previously operated regions, and provides a more straightforward soft tissue dissection plane. Prior decompressions alter the bony anatomy, and may complicate docking of the Kirschner wire (K-wire) and retractor system. Patient anatomy may also be altered by iatrogenic scoliosis, proximal junctional kyphosis, prior trauma, or bony changes from prior infection or tumors. Computed tomography scans and magnetic resonance imaging allow surgeons to identify anatomic changes from prior operations, and are essential for preoperative planning in these cases. In selecting patients for MIS procedures, body habitus is often a limiting factor, and the availability of longer instruments and retractors should be considered as part of the preoperative planning.


Patient Positioning

The procedure is performed under total intravenous anesthesia such that continuous electromyographic recordings (EMGs) may be monitored intraoperatively where irritation of a nerve
root will yield a measurable motor response distally where intramuscular needle electrodes are placed and secured for intraoperative monitoring. Stimulus-evoked EMG is another modality of neurophysiologic monitoring where a voltage is applied to each lumbar pedicle screw and the amplitude of the distal current is measured. As expected, medial wall pedicle breaches should decrease the resistance to this applied voltage, resulting in a relatively higher distal current. This will therefore lower the threshold voltage at which a motor response can be obtained from an electrical stimulus to the screw.

The patient is positioned prone on a Jackson frame with chest and iliac crest support pads in position. Sterile preparation is completed in the usual fashion, and the patient is draped widely to ensure access to the intended operative levels and the posterior iliac spine if needed for anchoring of the DRA. Typically, image guidance provides relative accuracy at up to a distance of five spinal segments from the DRA, and long constructs greater than five levels may need repositioning of the DRA for further intraoperative imaging.


Surgical Approaches

For placement of additional instrumentation, using the tracking tool, the level of interest is identified, and a trajectory overlying the pedicle is selected. A 2 to 3 cm skin incision is made and a navigated Jamshidi needle is introduced and docked at the junction of the transverse process and facet. It is advanced through the pedicle along the navigated trajectory. A K-wire is introduced and advanced beyond the tip of the Jamshidi needle, which is then removed. The K-wire is secured by an operative assistant to ensure it does not migrate or back out, while serial dilators are passed to obtain the operative corridor. When the surrounding soft tissue is adequately retracted, hemostasis is obtained, and a retractor of the final desired size has been placed, navigation is used to confirm the trajectory of the proposed pedicle screw. The pedicle is then tapped over the K-wire using a cannulated instrument. After cannulation, the wire is removed and the pedicle screw is placed. As described above, the screw is stimulated to ensure no pedicle breach occurred.

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Oct 7, 2018 | Posted by in RHEUMATOLOGY | Comments Off on Navigating Preexisting Lumbar Instrumentation in MIS Revision Surgeries
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