Ross R. Moquin

Single-stage Treatment of Pyogenic Spinal Infection With Titanium Mesh Cages

Study Design Single institution retrospective review. Objectives To report a series of pyogenic spinal infections treated with single-stage debridement and reconstruction with titanium mesh cages. Summary of Background Data Various studies have reported surgical results of pyogenic spinal osteomyelitis with anterior debridement, strut grafting and fusion, including delayed posterior spinal instrumentation. Additionally, various authors have recommended against the use of instrumentation because of the concern about glycocalyx formation on the metal and chronic infection. At our institution, we routinely treat chronic vertebral osteomyelitis with single-stage debridement, reconstruction with a titanium mesh cage filled with allograft chips and demineralized bone matrix, and posterior pedicle screw instrumentation. To our knowledge, this is the largest single series reporting single-stage debridement and instrumentation of pyogenic spinal infection with titanium mesh cages and posterior instrumentation. Materials and Methods We retrospectively reviewed the patient records and radiographs of 21 consecutive patients (average age 49.3 years, range 23 to 80 years) with pyogenic vertebral osteomyelitis, all treated with titanium mesh cages. Average follow-up was 44 months (range, 25 to 70 months). Spinal levels included 6 thoracic, 4 thoracolumbar, 9 lumbar, and 2 lumbosacral (L5-S1) lesions. All patients had preoperative serum evaluation, which usually included blood cultures, complete blood count, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP), in addition to plain radiographs and magnetic resonance imaging. A positive needle biopsy was available in only 2/7 patients (29%), and overall, preoperative pathogen identification was available in only 7/21 patients (33%). All patients were treated postoperatively with a minimum of 6 weeks of intravenous antibiotics, with a specific antibiotic regimen directed toward the postoperative pathogen when identified (17/21 cases). Extensive radiographic evaluation was also performed. Results ESR and CRP were routinely elevated (18/20 and 11/17 cases respectively), whereas the white blood count was elevated in only 8 out of 21 cases (38%). The average duration of symptoms to diagnosis was approximately 13.6 weeks (range 3 weeks to 10 months). The indications for surgery included neurologic compromise, significant vertebral body destruction with loss of sagittal alignment, failure of medical treatment, and/or epidural abscess. All patients had resolution of infection, as noted by normalization of the ESR and CRP. Further, 16 out of 21 patients also had a significant reduction of pain. There were no deaths or new postoperative neurologic compromise. The most common pathogen was Staphylococcus aureus. Two patients required a second surgery (posterior irrigation and debridement) during the same admission for persistent wound drainage. Radiographically, the average segmental kyphosis (or loss of lordosis) was 11.5 degrees (range, 0 to 24 degrees) preoperatively, and +0.8 degrees (range, –3 to +5 degrees) at latest postoperative follow-up. There was an average of 2.2 mm cage settling (range, 0 to 5 mm) on latest follow-up. There were no instrumentation failures, signs of chronic infection, or rejection. Conclusions Titanium mesh cages present a viable option for single-stage anterior surgical debridement and reconstruction of vertebral osteomyelitis, without evidence of chronic infection or rejection. When used in conjunction with pedicle screw instrumentation, there is minimal cage settling without loss of sagittal alignment.

Pulse-train stimulation for detecting medial malpositioning of thoracic pedicle screws.

Study Design. Thoracic pedicle screw location and the current needed to stimulate adjacent neural tissue was evaluated using brief, high-frequency pulse trains and monitoring electromyography (EMG) from muscles in the lower limbs. Objective. To establish a safe and reliable method for detecting medial malpositioning of pedicle screws placed in the thoracic spine during instrumentation and fusion. Summary of Background Data. Neurophysiologic studies for testing thoracic pedicle screw placement used single-pulse stimulation and monitored EMG from thoracic-innervated muscles. We propose that with this approach, stimulation fails to activate lower motor neurons innervated by spinal cord axons, such that medial malplacement of screws will go largely undetected. Methods. EMG was monitored from multiple lower-limb muscles. Pedicle tracks were created free-hand, using a curved pedicle finder. A ball-tipped probe—insulated along its shaft—was used to palpate the walls of the pedicle tracks. During probing, constant-current, high-frequency 4-pulse stimulus trains were delivered through the ball tip, and the minimum current (i.e., threshold) needed to evoke EMG was determined for each pedicle track. The threshold current for stimulation through each screw was also determined. Postoperative serial computed tomography scans of all implanted thoracic and L1 screws were rated with respect to screw position and the pedicle wall. Results. A total of 116 screws were implanted in 7 subjects. Two pedicle tracks were redirected during surgery because of particularly low thresholds to stimulation. Definite medial defects were found in 19 screws, 18 of which were detected by the experimental technique. For these screws, the average threshold to probe stimulation of their associated pedicle tracks was 7.9 ± 4.6 mA, much lower than current thresholds for less medially placed pedicle tracks. Stimulation of these screws resulted in high thresholds (19.8 ± 5.3 mA) when a response was evoked at all; stimulating 8 of these 19 medially malpositioned screws failed to elicit any lower-limb EMG at considerably higher (25 or 30 mA) stimulus intensities. Conclusion. This preliminary study supports the hypothesis that high-frequency stimulus pulse trains areeffective at detecting defects in the medial wall of pedicles in the thoracic spine during instrumentation, thereby improving on techniques using single-pulse stimulus protocols.

Stabilizing properties of the halo apparatus

Study Design. A cadaveric cervical spine specimen fixed between a fiberglass torso and a plastic skull was used as a model to determine the effect of halo structural parameters on motion at a lesion simulated at C5-C6. In a second part, nine commercially available halo devices were compared. Objectives. To define the contributions of the various components of the halo apparatus to reducing motion in an injured cervical spine and to compare the stability offered by a sample of commercially available halo devices. Controversy exists concerning the ability of the halo apparatus to stabilize the injured cervical spine. Summary of Background Data. The halo apparatus has been shown to be the most effective nonsurgical method for stabilizing the fractured spine. Nonetheless, several clinical studies have demonstrated that unacceptably large motions can occur at the injured spinal segment stabilized with a halo apparatus. Methods. Each cadaveric cervical spine was mounted onto a fiberglass torso and a rigid plastic skull was attached to the base of the occiput. A posterior ligamentous lesion was created between C5 and C6. The halo ring was fitted to the skull and a vest to the torso. Loads were applied to the skull in flexion, extension, and lateral bending, and relative angulation between C5 and C6 was measured with electroinclinometers. In the first part, the effect of parameters such as vest tightness, vest-thorax friction, vest deformation, and connecting bar rigidity on spinal angulation were measured using one vest. In the second part, the stability offered by each of nine commercially available halo devices was compared. Results. Increasing chest strap tightness and decreasing vest deformation reduced angulation at the spinal lesion. Once connecting bar joints were tightened to 25% of their recommended torque, increased tightening or adding additional bars had no effect on rigidity. Although specific vests permitted significantly greater motion in specific directions, no vest allowed greater angulation consistently in all loading planes. Conclusions. Increasing vest tightness, decreasing the deformability of the vest, and ensuring a good fit can reduce motion in the fractured spine. Most commercially available halo vests provide similar mechanical stability to the injured cervical spine.

Neuromonitoring with pulse-train stimulation for implantation of thoracic pedicle screws: a blinded and randomized clinical study. Part 1. Methods and alarm criteria

OBJECT Reports of the accuracy of existing neuromonitoring methods for detecting or preventing medial malpositioning of thoracic pedicle screws have varied widely in their claimed effectiveness. The object of this study was to develop, test, and validate a novel neuromonitoring method for preventing medial malpositioning of pedicle screws in the thoracic spine during surgery. METHODS This is a prospective, blinded and randomized study using a novel combination of input (4-pulse stimulus trains delivered within the pedicle track) and output (evoked electromyography from leg muscles) to detect pedicle track trajectories that-once implanted with a screw-would cause that screw to breach the pedicles medial wall and encroach upon the spinal canal. For comparison, the authors also used screw stimulation as an input and evoked electromyogram from intercostal and abdominal muscles as output measures. Intraoperative electrophysiological findings were compared with postoperative CT scans by multiple reviewers blinded to patient identity or intraoperative findings. RESULTS Data were collected from 71 patients, in whom 802 screws were implanted between the T-1 and L-1 vertebral levels. A total of 32 screws ended up with screw threads encroaching on the spinal canal by at least 2 mm. Pulse-train stimulation within the pedicle track using a ball-tipped probe and electromyography from lower limb muscles correctly predicted all 32 (100%) of these medially malpositioned screws. The combination of pedicle track stimulation and electromyogram response from leg muscles proved to be far more effective in predicting these medially malpositioned screws than was direct screw stimulation and any of the target muscles (intercostal, abdominal, or lower limb muscles) we monitored. Based on receiver operating characteristic analysis, the combination of 10-mA (lower alarm) and 15-mA stimulation intensities proved most effective for detection of pedicle tracks that ultimately gave rise to medially malpositioned screws. Additional results pertaining to the impact of feedback of these test results on surgical decision making are provided in the companion report. CONCLUSIONS This novel neuromonitoring approach accurately predicts medially malpositioned thoracic screws. The approach could be readily implemented within any surgical program that is already using contemporary neuromonitoring methods that include transcranial stimulation for monitoring motor evoked potentials.

Intraoperative neuromonitoring: Can the results of direct stimulation of titanium-alloy pedicle screws in the thoracic spine be trusted?

Objective: Intraoperative neuromonitoring of thoracic-level pedicle screw implantation for detecting breaches in the pedicle cortex has adopted methods originally developed in the early 1990s for stainless steel (SS) alloy screws used at lumbosacral levels. In our recent attempts to monitor thoracic-level pedicle screw placement, we were surprised to find that these widely used stimulation parameters were largely ineffectual when stimulating directly through titanium alloy (Ti-alloy) pedicle screws. The objectives of this study, then, were twofold: (1) to report the number of episodes in which intraoperative neuromonitoring of thoracic screw position failed to detect a medially directed breach (or malplacement) in a previously described and limited sample set; and (2) to compare the frequency-specific impedance of a sample of Ti-alloy pedicle screws to comparably sized screws made of SS alloys. We predicted that Ti-alloy screws would demonstrate impairment in conduction properties that could help explain the difficulties we, and others, have recently experienced with neuromonitoring of thoracic pedicle screw placement. Methods: Based on threshold values for train-of-four stimulation of spinal motor pathways, we quantified the incidence of medial breaches of thoracic-level pedicles in a small cohort of subjects. We also evaluated the conductive properties of Ti-alloy pedicle screws and compared these with SS screws. Eleven pedicle screws were examined using energy-dispersive x-ray spectroscopy to identify their alloys, after which DC resistance and AC impedance for each screw was measured. Furthermore, a subset of five screws was used to investigate the current delivery under dynamic testing conditions. Results: Postoperative computed tomography of 6 subjects revealed 10 instances of significant medial screw malpositioning, out of a total of 88 screws placed. In each of these 10 instances, direct stimulation of thoracic pedicle screws at intensities considered in the literature to be clinically significant (i.e., ⩽11 mA) failed to predict these medial pedicle breaches, yet each breach was reliably identified with low-intensity stimulation applied via a ball-tipped probe. For in vitro studies, most screws made of titanium alloys had higher resistance and impedance at tested frequencies compared with their SS counterparts. Moreover, there was widespread variability in conduction properties between Ti-alloy screws, whereas SS screws behaved in a more homogeneous manner. Conclusions: When compared with screws made of SS, most Ti-alloy pedicle screws behaved more like semiconductors, showing conduction properties that were highly frequency dependent. These properties likely contributed to the difficulties we encountered in interpreting thoracic screw placements based on stimulus-evoked electromyography from direct screw stimulation.

Is in vivo manual palpation for thoracic pedicle screw instrumentation reliable

OBJECT Previous reports on the accuracy of manual palpation for thoracic pedicle screw placement have been restricted to cadaveric studies. Authors of the present novel study assessed the accuracy of manual palpation for the detection of medial and lateral pedicle breaches during thoracic spine surgery in living adult humans. METHODS Pedicle tracks were created freehand and manually palpated using a ball-tipped probe. Postoperative CT scans of all implanted thoracic and L-1 screws were evaluated with respect to screw position and the pedicle wall. RESULTS Five hundred twenty-five pedicle track/screw placements were compared. There were 21 pedicles with medial breaches measuring ≥ 2 mm. The surgeon correctly identified only 4 of these pedicle tracks as having a medial breach. The surgeon correctly identified 17 of 128 pedicles with a significant (≥ 2 mm) lateral breach. One hundred two screw placements had no measurable breach in any direction (medial, lateral, or foraminal). The surgeon correctly identified 98% of these ideally placed screws. CONCLUSIONS In this real-time study of thoracic pedicle screw placement, the accuracy of manual palpation for detecting medial or lateral breaches that were ≥ 2 mm was disturbingly low. These findings are consistent with those in recent cadaveric evaluations of palpation accuracy and point to the critical need for more reliable alternative methods to assess pedicle integrity during the placement of thoracic pedicle screws for spine instrumentation surgery.