Evaluation of intrapedicular screw position using intraoperative evoked electromyography

Anterior Column Realignment: Adult Sagittal Deformity Treatment Through Minimally Invasive Surgery

This article focuses on the treatment of sagittal spinal deformity using a minimally invasive technique, anterior column realignment. Traditional methods to address sagittal spine deformity have been associated with high morbidity, long operative times, and excessive blood loss. This technique uses a minimally invasive lateral retroperitoneal approach to release the anterior longitudinal ligament and apply a hyperlordotic implant for interbody fusion to restore lumbar lordosis and sagittal alignment.

Intraoperative Triggered Electromyography for Pedicle Screw Placement Under Spinal Anesthesia: A Preliminary Report

BACKGROUND

Triggered electromyography (tEMG) is an intraoperative neuromonitoring technique used to assess pedicle screw placement during instrumented fusion procedures. Although spinal anesthesia is a safe alternative to general anesthesia in patients undergoing lumbar fusion, its use may potentially block conduction of triggered action potentials or may require higher threshold currents to elicit myotomal responses when using tEMG. Given the broad utilization of tEMG for confirmation of pedicle screw placement, adoption of spinal anesthesia may be hindered by limited studies of its use alongside tEMG.

OBJECTIVE

To investigate whether spinal anesthesia affects the efficacy of tEMG, we compare the baseline spinal nerve thresholds during lumbar fusion procedures under general vs spinal anesthesia.

METHODS

Twenty-three consecutive patients (12 general and 11 spinal) undergoing single-level transforaminal lumbar interbody fusion were included in the study. Baseline nerve threshold was determined through direct stimulation of the spinal nerve using tEMG.

RESULTS

Baseline spinal nerve threshold did not differ between the general and spinal anesthesia cohorts (3.25 ± 1.14 vs 3.64 ± 2.16 mA, respectively; P = .949). General and spinal anesthesia cohorts did not differ by age, body mass index, American Society of Anesthesiologists score status, or surgical indication.

CONCLUSION

We report that tEMG for pedicle screw placement can be safely and effectively used in procedures under spinal anesthesia. The baseline nerve threshold required to illicit a myotomal response did not differ between patients under general or spinal anesthesia. This preliminary finding suggests that spinal anesthetic blockade does not contraindicate the use of tEMG for neuromonitoring during pedicle screw placement.

A Modified Method of Triggered Electromyography Monitoring in Minimally Invasive Spine Surgery: Comparison to Conventional Techniques and Correlation with Body Mass Index

PURPOSE

Conventional triggered electromyography (EMG) in percutaneous pedicle screw (PPS) systems may be unreliable due to the interaction between the insertion apparatus and patient's soft tissue. Our aim was 1) to describe a modified technique of triggered EMG monitoring using insulated Kirschner wire (K-wires), 2) to compare EMG potentials with conventional techniques, and 3) to demonstrate the relationship between patient body mass index (BMI) and triggered EMG potentials.

METHODS

This was a prospective cross-sectional study of 50 patients undergoing minimally invasive PPS placement. Triggered EMG measurements using K-wires before and after insulation were compared. The difference between EMG measurements before and after insulation was correlated with patient BMI.

RESULTS

A total of 50 patients, 22 females and 28 males, underwent triggered EMG testing using K-wires prior to final PPS placement in the thoracic and lumbosacral spine for a total of 472 triggered EMG measurements. When compared to standard triggered EMG monitoring, insulated triggered EMG monitoring demonstrated an average 55.4% decrease in EMG values (P < 0.001). Increasing BMI correlated to increasing % decrease in EMG values (r-coefficient, 0.376; P < 0.01).

CONCLUSIONS

We describe a cost-effective, efficient, and reliable technique for triggered EMG during PPS placement which may help ensure accurate screw placement and minimize potentially devastating complications.

Effects of anode position on pedicle screw testing during lumbosacral spinal fusion surgery

OF BACKGROUND DATA

Pedicle screws are commonly placed with lumbar/lumbosacral fusions. Triggered electromyography (tEMG), which employs the application of electrical current between the screw and a complementary anode to determine thresholds of conduction, may be utilized to confirm the safe placement of such implants. While previous research has established clinical thresholds associated with safe screw placement, there is variability in clinical practice of anode placement which could lead to unreliable measurements.

PURPOSE

To determine the variance in pedicle screw stimulation thresholds when using four unique anode locations (ipsilateral/contralateral and paraspinal/gluteal relative to tested pedicle screws).

STUDY DESIGN

Prospective cohort study. Tertiary medical center.

PATIENT SAMPLE

Twenty patients undergoing lumbar/lumbosacral fusion with pedicle screws using tEMG OUTCOME MEASURES: tEMG stimulation return values are used to assess varied anode locations and reproducibility based on anode placement.

METHODS

Measurements were assessed across node placement in ipsilateral/contralateral and paraspinal/gluteal locations relative to the screw being assessed. R² coefficients of correlation were determined, and variances were compared with F-tests.

RESULTS

A total of 94 lumbosacral pedicle screws from 20 patients were assessed. Repeatability was verified using two stimulations at each location for a subset of the screws with an R² of 0.96. Comparisons between the four anode locations demonstrated R² values ranging from 0.76 to 0.87. F-tests comparing thresholds between each anode site demonstrated all groups not to be statistically different.

CONCLUSION

The current study, a first-of-its-kind formal evaluation of anode location for pedicle screw tEMG testing, demonstrated very strong repeatability and strong correlation with different locations of anode placement. These results suggest that there is no need to change the side of the anode for testing of left versus right screws, further supporting that placing an anode electrode into gluteal muscle is sufficient and will avoid a sharp ground needle in the surgical field.

Intraoperative Electromyography in Awake Minimally Invasive Transforaminal Lumbar Interbody Fusion: A Case Study on Nerve Activation Under the Effects of Local Anesthesia

SUMMARY

With the versatility of lumbar spine surgery continually expanding, intraoperative electromyography (EMG) has become a common adjunct used to reduce risk of nerve injury and postoperative neurologic deficit. EMG monitoring has been deemed particularly useful in the minimally invasive transforaminal lumbar interbody fusion. A more recent evolution of the minimally invasive transforaminal lumbar interbody fusion entails complete percutaneous access to the disc through Kambin's triangle, followed by a percutaneous lumbar interbody fusion. Given the lack of direct visualization of nervous structures with percutaneous entrance into the disc, there is risk of injury to surrounding nervous structures with this approach. In effort to reduce risk of nerve injury, application of triggered EMG to gauge proximity of nervous tissue has been evaluated. Recently, patients presenting with contraindications or concerns for general anesthesia have been offered the alternative to undergo their procedure with spinal anesthesia, allowing them to remain awake. Spinal anesthesia entails intrathecal administration of local anesthetic, which mechanistically acts to reduce overall excitability of surrounding neural structures. However, nerve activation under conditions of local anesthetic is relatively unknown, and the ability of triggered EMG monitoring to reliably detect nerve proximity becomes questionable. This case report demonstrates nerve activation at thresholds comparable with those seen under general anesthesia. Although this has sparked interest in the possibility that local anesthetic may not remarkably affect nerve excitability as measured by triggered EMG activation, further investigation is recommended to reliably apply triggered EMG monitoring in awake spine surgery.

Threshold-based Monitoring of Compound Muscle Action Potentials for Percutaneous Pedicle Screw Placement in the Lumbosacral Spine: Can We Rely on Stimulation of the Uninsulated Screw to Provide a Valid Safety Warning?

STUDY DESIGN

A prospective analysis.

OBJECTIVE

To test if threshold-based monitoring of compound muscle action potentials (CMAPs) by stimulating the screw loaded to uninsulated extender sleeve provides a valid safety warning for percutaneous pedicle screw (PPS) placements in the lumbosacral spine.

SUMMARY OF BACKGROUND DATA

Utility of the CMAP monitoring to PPS procedures remains controversial.

METHODS

A series of 202 patients underwent a total of 1664 lumbosacral PPS placements under CMAP monitoring without fluoroscopic guidance. The monitoring consisted of stimulating the PPS assembled to uninsulated extender sleeve and recording CMAPs from the vastus medialis, biceps femoris, tibialis anterior, and medial gastrocnemius. Automated steps of a threshold hunting algorithm using 0.2-ms duration pulses of increasing intensities delivered at 2/s allowed quick determination of a minimum stimulation current to evoke >100-μV amplitude CMAPs.

RESULTS

At L2 through S1 spines, postoperative CT scans identified 51 medial or inferior pedicle wall breaches of 1536 screws (3.3%) without neurologic complications. The receiver operating characteristic curve analysis determined the critical cutoff threshold value of 27 mA (74% sensitivity and 95% specificity) for predicting 35 breaches of 627 screws (5.6%) at L2 and L3, and of 17 mA (100% sensitivity and 98% specificity) for 16 of 909 (1.8%) at L4 through S1. While advancing the screw, three breaches (5.9%) showed a particularly low threshold of ≤6-mA, allowing the surgeon to immediately redirect the screw and retest the new trajectory as safe.

CONCLUSION

Screw stimulation with threshold hunting algorithm has a distinct advantage over the time-consuming insulated pilot hole stimulation, allowing an uninterrupted flow of the surgery. The present findings have documented practical usefulness and reliability of CMAP monitoring using direct stimulation of the PPS assembled to uninsulated extender sleeve.

Triggered Electromyography is a Useful Intraoperative Adjunct to Predict Postoperative Neurological Deficit Following Lumbar Pedicle Screw Instrumentation

STUDY DESIGN

Systematic review and meta-analysis.

OBJECTIVES

Malposition of pedicle screws during instrumentation in the lumbar spine is associated with complications secondary to spinal cord or nerve root injury. Intraoperative triggered electromyographic monitoring (t-EMG) may be used during instrumentation for early detection of malposition. The association between lumbar pedicle screws stimulated at low EMG thresholds and postoperative neurological deficits, however, remains unknown. The purpose of this study is to assess whether a low threshold t-EMG response to lumbar pedicle screw stimulation can serve as a predictive tool for postoperative neurological deficit.

METHODS

The present study is a meta-analysis of the literature from PubMed, Web of Science, and Embase identifying prospective/retrospective studies with outcomes of patients who underwent lumbar spinal fusion with t-EMG testing.

RESULTS

The total study cohort consisted of 2,236 patients and the total postoperative neurological deficit rate was 3.04%. 10.78% of the patients incurred at least 1 pedicle screw that was stimulated below the respective EMG alarm threshold intraoperatively. The incidence of postoperative neurological deficits in patients with a lumbar pedicle screw stimulated below EMG alarm threshold during placement was 13.28%, while only 1.80% in the patients without. The pooled DOR was 10.14. Sensitivity was 49% while specificity was 88%.

CONCLUSIONS

Electrically activated lumbar pedicle screws resulting in low t-EMG alarm thresholds are highly specific but weakly sensitive for new postoperative neurological deficits. Patients with new postoperative neurological deficits after lumbar spine surgery were 10 times more likely to have had a lumbar pedicle screw stimulated at a low EMG threshold.

Application of different thresholds for instrumentation device testing in minimally invasive lumbosacral spine fixation

The main aim of this study was evaluating the reliability of stimulus-evoked electromyography (using different thresholds for stimulation of the instrumentation devices) for minimally invasive pedicle screw placement in the lumbosacral spine. A threshold of 5 mA was applied for the pedicle access needle. 7 mA was applied for the tapscrew and pedicle screw stimulation. The existence of threshold differences between vertebral levels was also assessed. All patients underwent postoperative computed tomography (CT) to determine the accuracy of pedicle screw placement. A total of 172 percutaneous pedicle screws were placed in 52 patients. 94.1% of screws were placed at L4, L5 and S1 vertebral levels. No statistically significant differences existed in thresholds of the pedicle access needles, tapscrews and pedicle screws between vertebral levels. In four instances, the pedicle access needle stimulation had a threshold of 5 mA (no breaches were associated). In the rest of occasions, the pedicle access needles had stimulation thresholds above 5 mA. In all instances, tapscrew and pedicle screw thresholds were above 7 mA; the tapscrews and pedicle screws had significantly greater thresholds than the pedicle access needles. No statistically significant differences existed in thresholds between tapscrews and pedicle screws. Postoperative CT imaging revealed one lateral pedicle violation. Both breach rate and false negative rate were 0.5%. No false positive cases were observed. No patients experienced postoperative pedicle screw-related neurologic deficits. A threshold of 5 mA for the pedicle access needle stimulation seems to be safe. Greater than 7 mA should be used for the tapscrew and pedicle screw stimulation.

Triggered EMG (T-EMG) Values of Pedicle Screws with a Powered Screwdriver vs A Standard Probe in Adolescent Idiopathic Scoliosis Do Not Agree: A Prospective Validation Study

Triggered electromyography (t-EMG) is a common technique used during spinal instrumentation in surgery for adolescent idiopathic scoliosis. This study tests the validity of t-EMG values obtained with a standard ball-tipped probe after completion of screw placement versus t-EMG values obtained during screw insertion with a powered screwdriver. t-EMG values were collected for screws spanning T7-L5 using both a standard probe and a powered screwdriver. A power analysis determined that a sample size of 300 screws would provide enough precision to estimate limits of agreement within ±2 mA. A monopolar constant current stimulation technique (0.2 ms duration and 3.11 Hz stimulation rate) was used at each level. EMG was acquired with placement of bipolar pairs of subdermal needle electrodes. A Bland-Altman plot was used to assess agreement between threshold readings from the two techniques. Twenty-nine patients were enrolled in this study with 305 screws. t-EMG values measured using a powered screwdriver were on average 1 mA lower than values from a standard probe. When readings less than or equal to 20 mA were considered, the limits of agreement were approximately 4 and 7 mA overall. In total, 28/305 (9%) screws were removed and reinserted, 9/305 (3%) screws were redirected, and 3/305 (1%) screws were aborted based on t-EMG readings. Despite a small overall difference in t-EMG value between the standard probe and screwdriver, there was still large variability in agreement between the two techniques. t-EMG values obtained with a powered screwdriver during screw insertion are not interchangeable with values measured by a probe.

Focal hole versus screw stimulation to prevent false negative results in detecting pedicle breaches during spinal instrumentation

OBJECTIVE

We describe a stimulus-evoked EMG approach to minimize false negative results in detecting pedicle breaches during lumbosacral spinal instrumentation.

METHODS

In 36 patients receiving 176 lumbosacral pedicle screws, EMG threshold to nerve root activation was determined using a focal probe inserted into the pilot hole at a depth, customized to the individual patients, suitable to position the stimulating tip at the point closest to the tested nerve root. Threshold to screw stimulation was also determined.

RESULTS

Mean EMG thresholds in 161 correctly fashioned pedicle instrumentations were 7.5 mA ± 2.46 after focal hole stimulation and 21.8 mA ± 6.8 after screw stimulation. Direct comparison between both thresholds in individual pedicles showed that screw stimulation was always biased by an unpredictable leakage of the stimulating current ranging from 10 to 90%. False negative results were never observed with hole stimulation but this was not true with screw stimulation.

CONCLUSIONS

Focal hole stimulation, unlike screw stimulation, approaches absolute EMG threshold as shown by the lower normal limit (2.6 mA; p < 0.05) that borders the upper limit of threshold to direct activation of the exposed root.

SIGNIFICANCE

The technique provides an early warning of a possible pedicle breakthrough before insertion of the more harmful, larger and threaded screw.

Intraoperative Neurophysiological Monitoring : A Review of Techniques Used for Brain Tumor Surgery in Children

Intraoperative monitoring (IOM) utilizes electrophysiological techniques as a surrogate test and evaluation of nervous function while a patient is under general anesthesia. They are increasingly used for procedures, both surgical and endovascular, to avoid injury during an operation, examine neurological tissue to guide the surgery, or to test electrophysiological function to allow for more complete resection or corrections. The application of IOM during pediatric brain tumor resections encompasses a unique set of technical issues. First, obtaining stable and reliable responses in children of different ages requires detailed understanding of normal ageadjusted brain-spine development. Neurophysiology, anatomy, and anthropometry of children are different from those of adults. Second, monitoring of the brain may include risk to eloquent functions and cranial nerve functions that are difficult with the usual neurophysiological techniques. Third, interpretation of signal change requires unique sets of normative values specific for children of that age. Fourth, tumor resection involves multiple considerations including defining tumor type, size, location, pathophysiology that might require maximal removal of lesion or minimal intervention. IOM techniques can be divided into monitoring and mapping. Mapping involves identification of specific neural structures to avoid or minimize injury. Monitoring is continuous acquisition of neural signals to determine the integrity of the full longitudinal path of the neural system of interest. Motor evoked potentials and somatosensory evoked potentials are representative methodologies for monitoring. Free-running electromyography is also used to monitor irritation or damage to the motor nerves in the lower motor neuron level : cranial nerves, roots, and peripheral nerves. For the surgery of infratentorial tumors, in addition to free-running electromyography of the bulbar muscles, brainstem auditory evoked potentials or corticobulbar motor evoked potentials could be combined to prevent injury of the cranial nerves or nucleus. IOM for cerebral tumors can adopt direct cortical stimulation or direct subcortical stimulation to map the corticospinal pathways in the vicinity of lesion. IOM is a diagnostic as well as interventional tool for neurosurgery. To prove clinical evidence of it is not simple. Randomized controlled prospective studies may not be possible due to ethical reasons. However, prospective longitudinal studies confirming prognostic value of IOM are available. Furthermore, oncological outcome has also been shown to be superior in some brain tumors, with IOM. New methodologies of IOM are being developed and clinically applied. This review establishes a composite view of techniques used today, noting differences between adult and pediatric monitoring.

Evaluation of robot-guided minimally invasive implantation of 2067 pedicle screws

Objective Recent studies have investigated the role of spinal image guidance for pedicle screw placement. Many authors have observed an elevated placement accuracy and overall improvement of outcome measures. This study assessed a bi-institutional experience following introduction of the Renaissance miniature robot for spinal image guidance in Europe. Methods The medical records and radiographs of all patients who underwent robot-guided implantation of spinal instrumentation using the novel system (between October 2011 and March 2015 in Mainz and February 2014 and February 2016 in Regensburg) were reviewed to determine the efficacy and safety of the newly introduced robotic system. Screw position accuracy, complications, exposure durations to intraoperative radiation, and reoperation rate were assessed. Results Of the 413 surgeries that used robotic guidance, 406 were via a minimally invasive approach. In 7 cases the surgeon switched to conventional screw placement, using a midline approach, due to referencing problems. A total of 2067 screws were implanted using robotic guidance, and 1857 screws were evaluated by postoperative CT. Of the 1857 screws, 1799 (96.9%) were classified as having an acceptable or good position, whereas 38 screws (2%) showed deviations of 3-6 mm and 20 screws (1.1%) had deviations > 6 mm. Nine misplaced screws, implanted in 7 patients, required revision surgery, yielding a screw revision rate of 0.48% of the screws and 7 of 406 (1.7%) of the patients. The mean ± SD per-patient intraoperative fluoroscopy exposure was 114.4 (± 72.5) seconds for 5.1 screws on average and any further procedure required. Perioperative and direct postoperative complications included hemorrhage (2 patients, 0.49%) and wound infections necessitating surgical revision (20 patients, 4.9%). Conclusions The hexapod miniature robotic device proved to be a safe and robust instrument in all situations, including those in which patients were treated on an emergency basis. Placement accuracy was high; peri- and early postoperative complication rates were found to be lower than rates published in other series of percutaneous screw placement techniques. Intraoperative radiation exposure was found to be comparable to published values for other minimally invasive and conventional approaches.

Perils of intraoperative neurophysiological monitoring: analysis of "false-negative" results in spine surgeries

BACKGROUND CONTEXT

Although some authors have published case reports describing false negatives in intraoperative neurophysiological monitoring (IONM), a systematic review of causes of false-negative IONM results is lacking.

PURPOSE

The objective of this study was to analyze false-negative IONM findings in spine surgery.

STUDY DESIGN

This is a retrospective cohort analysis.

PATIENT SAMPLE

A cohort of 109 patients with new postoperative neurologic deficits was analyzed for possible false-negative IONM reporting.

OUTCOME MEASURES

The causes of false-negative IONM reporting were determined.

MATERIALS AND METHODS

From a cohort of 62,038 monitored spine surgeries, 109 consecutive patients with new postoperative neurologic deficits were reviewed for IONM alarms.

RESULTS

Intraoperative neurophysiological monitoring alarms occurred in 87 of 109 surgeries. Nineteen patients with new postoperative neurologic deficits did not have an IONM alarm and surgeons were not warned. In addition, three patients had no interpretable IONM baseline data and no alarms were possible for the duration of the surgery. Therefore, 22 patients were included in the study. The absence of IONM alarms during these 22 surgeries had different origins: "true" false negatives where no waveform changes meeting the alarm criteria occurred despite the appropriate IONM (7); a postoperative development of a deficit (6); failure to monitor the pathway, which became injured (5); the absence of interpretable IONM baseline data which precluded any alarm (3); and technical IONM application issues (1).

CONCLUSIONS

Overall, the rate of IONM method failing to predict the patient's outcome was very low (0.04%, 22/62,038). Minimizing false negatives requires the application of a proper IONM technique with the limitations of each modality considered in their selection and interpretation. Multimodality IONM provides the most inclusive information, and although it might be impractical to monitor every neural structure that can be at risk, a thorough preoperative consideration of available IONM modalities is important. Delayed development of postoperative deficits cannot be predicted by IONM. Absent baseline IONM data should be treated as an alarm when inconsistent with the patient's preoperative neurologic status. Alarm criteria for IONM may need to be refined for specific procedures and deserves continued study.

ELECTROMYOGRAPHY AND INSTRUMENTATION IN PATIENTS WITH IDIOPATHIC SCOLIOSIS

ABSTRACT Objective: The objective of this study is to relate the use of intraoperative electromyography with surgical time, proper placement of screws, type of curve and time spent per screw in idiopathic scoliosis correction surgery in a group of surgeons from Belo Horizonte. This study used the database of protocol evaluation of patients operated in the service, and separately analyzed the results of motor and somatosensory potentials. Methods: Retrospective study of 80 patients undergoing surgery for correction of idiopathic scoliosis between December 2008 and January 2015. A single group of Belo Horizonte spine surgeons performed the intraoperative electromyographic (EMG) monitoring. EMG was performed with stimulation of pedicle screws in patients undergoing instrumentation with pedicle screws as fixation elements. Results: The sample consisted of 85% females (mean age 17 years) and 37.5% of cases had classification type 1AN of Lenke. Of the total surgical cases, 60% had EMG changes. Of the total cases analyzed, 66.3% were true positives for the result. Conclusion: Intraoperative monitoring with EMG is a very important tool for the surgical treatment of patients with scoliosis undergoing instrumentation with pedicle screws. It enables to check if the screw is located on the correct path, helping to decrease the error rate and providing corrections to the surgical approach through a change of strategies. Moreover, it contributes to decrease the time to screw positioning and the total surgical time.

The use of intraoperative triggered electromyography to detect misplaced pedicle screws: a systematic review and meta-analysis

OBJECT

Insertion of instruments or implants into the spine carries a risk for injury to neural tissue. Triggered electromyography (tEMG) is an intraoperative neuromonitoring technique that involves electrical stimulation of a tool or screw and subsequent measurement of muscle action potentials from myotomes innervated by nerve roots near the stimulated instrument. The authors of this study sought to determine the ability of tEMG to detect misplaced pedicle screws (PSs).

METHODS

The authors searched the US National Library of Medicine, the Web of Science Core Collection database, and the Cochrane Central Register of Controlled Trials for PS studies. A meta-analysis of these studies was performed on a per-screw basis to determine the ability of tEMG to detect misplaced PSs. Sensitivity, specificity, and receiver operating characteristic (ROC) area under the curve (AUC) were calculated overall and in subgroups.

RESULTS

Twenty-six studies were included in the systematic review. The authors analyzed 18 studies in which tEMG was used during PS placement in the meta-analysis, representing data from 2932 patients and 15,065 screws. The overall sensitivity of tEMG for detecting misplaced PSs was 0.78, and the specificity was 0.94. The overall ROC AUC was 0.96. A tEMG current threshold of 10–12 mA (ROC AUC 0.99) and a pulse duration of 300 µsec (ROC AUC 0.97) provided the most accurate testing parameters for detecting misplaced screws. Screws most accurately conducted EMG signals (ROC AUC 0.98).

CONCLUSIONS

Triggered electromyography has very high specificity but only fair sensitivity for detecting malpositioned PSs.

Electrical stimulation threshold in chronically compressed lumbar nerve roots: Observational study

OBJECTIVES

Intraoperative neuromonitoring (IONM) is a common practice in spinal surgery, mostly during pedicle screw placement. However, there is not enough information about the factors that can interfere with IONM data. One of these factors may be existing damage of the nerve root whose function must be preserved. The main purpose of the present study is to evaluate the effect of chronic compression in lumbar nerve roots in terms of stimulation thresholds during direct nerve stimulation.

PATIENTS AND METHODS

Direct electrical stimulation was performed in 201 lumbar nerve roots during lumbar spinal procedures under general anaesthesia in 80 patients with different lumbar spinal pathologies. Clinical and radiological data were reviewed in order to establish the presence of chronic compression.

RESULTS

Chronically compressed nerve roots showed a higher stimulation threshold than non compressed nerve roots (11.93 mA vs. 4.33 mA). This difference was confirmed with intra-subject comparison (paired sample t test, p=0.012). No other clinical factors were associated with this higher stimulation threshold in lumbar nerve roots.

CONCLUSION

A higher stimulation threshold is present in compressed lumbar nerve roots than non compressed roots. This needs to be taken into consideration during pedicle screw placement, where intraoperative neurophysiological monitoring is being used.

Utility of Intraoperative Neuromonitoring during Minimally Invasive Fusion of the Sacroiliac Joint

Study Design. Retrospective case series. Objective. To document the clinical utility of intraoperative neuromonitoring during minimally invasive surgical sacroiliac joint fusion for patients diagnosed with sacroiliac joint dysfunction (as a direct result of sacroiliac joint disruptions or degenerative sacroiliitis) and determine stimulated electromyography thresholds reflective of favorable implant position. Summary of Background Data. Intraoperative neuromonitoring is a well-accepted adjunct to minimally invasive pedicle screw placement. The utility of intraoperative neuromonitoring during minimally invasive surgical sacroiliac joint fusion using a series of triangular, titanium porous plasma coated implants has not been evaluated. Methods. A medical chart review of consecutive patients treated with minimally invasive surgical sacroiliac joint fusion was undertaken at a single center. Baseline patient demographics and medical history, intraoperative electromyography thresholds, and perioperative adverse events were collected after obtaining IRB approval. Results. 111 implants were placed in 37 patients. Sensitivity of EMG was 80% and specificity was 97%. Intraoperative neuromonitoring potentially avoided neurologic sequelae as a result of improper positioning in 7% of implants. Conclusions. The results of this study suggest that intraoperative neuromonitoring may be a useful adjunct to minimally invasive surgical sacroiliac joint fusion in avoiding nerve injury during implant placement.

Overview of Intraoperative Neurophysiological Monitoring During Spine Surgery

Intraoperative neurophysiologic monitoring has had major advances in the past few decades. During spine surgery, the use of multimodality monitoring enables us to assess the integrity of the spinal cord, nerve roots, and peripheral nerves. The authors present a practical approach to the current modalities in use during spine surgery, including somatosensory evoked potentials, motor evoked potentials, spinal D-waves, and free-run and triggered electromyography. Understanding the complementary nature of these modalities will help tailor monitoring to a particular procedure to minimize postoperative neurologic deficit during spine surgery.

C-OnSite ® for intraoperative 3D control of pedicular screw positions

BACKGROUND

Two-dimensional image guidance and navigation can help to reduce the number of misplaced pedicle screws, but do not completely prevent misplacement. This experimental, retrospective, non-inferiority study was designed to evaluate and compare the efficacy of a novel 3D imaging technique versus conventional postoperative CT-scan, for intra-operative determination of pedicle screw position accuracy.

METHODS

The capacity of C-OnSite® to intraoperatively assess screw placement was evaluated in 28 clinical cases and 23 deliberately misplaced screws in a cadaver model, and compared to placement accuracy determined by standard CT. The position of each implant, as viewed by both modalities, was graded by three neurosurgeons, one orthopaedic-surgeon and one radiologist. The intermodal variance determined the difference between CT- and C-OnSite® results for each observer, while the inter-observer variance measured the difference between ratings of the same modality by different observers.

RESULTS

C-OnSite® successfully assessed 120/138 screws (25/28 cases). Mean procedural fluoroscopy time was 132 ± 51 s, and 40 ± 16s per C-OnSite® scan. The average inter-modality variance was ,15 % with mismatches >1° between C-OnSite® and the gold-standard imaging technique in only 2 % of the comparisons. Average inter-observer variances were about similar (12 % for CT and 18 % for C-OnSite®), with deviations of >1° reaching 1 % for CT and 3 % for C-OnSite®. Individual variances between experienced only observers differed even less.

CONCLUSIONS

C-OnSite® is a feasible, reliable and intuitive means of intraoperatively visualizing pedicle screw positions and might render the majority of postoperative CTs superfluous. C-OnSite® might help avoid re-operations for screw re-positioning.

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 pedicle's 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.

Solid and hollow pedicle screws affect the electrical resistance: A potential source of error with stimulus-evoked electromyography

BACKGROUND

Although stimulus evoked electromyography (EMG) is commonly used to confirm the accuracy of pedicle screw placement. There are no studies to differentiate between solid screws and hollow screws to the electrical resistance of pedicle screws. We speculate that the electrical resistance of the solid and hollow pedicle screws may be different and then a potential source of error with stimulus-evoked EMG may happen.

MATERIALS AND METHODS

Resistance measurements were obtained from 12 pedicle screw varieties (6 screws of each manufacturer) across the screw shank based on known constant current and measured voltage. The voltage was measured 5 times at each site.

RESULTS

Resistance of all solid screws ranged from 0.084 Ω to 0.151 Ω (mean =0.118 ± 0.024 Ω) and hollow screws ranged from 0.148 Ω to 0.402 Ω (mean = 0.285 ± 0.081 Ω). There was a significant difference of resistance between the solid screws and hollow screws (P < 0.05). The screw with the largest diameter no matter solid screws or hollow screws had lower resistance than screws with other diameters. No matter in solid screws group or hollow screws group, there were significant differences (P < 0.05) between the 5.0 mm screws and 6.0 mm screws, 6.0 mm screws and 7.0 mm screws, 5.0 mm screws and 7.0 mm screws, 4.5 mm screws and 5.5 mm screws, 5.5 mm screws and 6.5 mm screws, 4.5 mm screws and 6.5 mm screws. The resistance of hollow screws was much larger than the solid screws in the same diameter group (P < 0.05).

CONCLUSIONS

Hollow pedicle screws have the potential for high electrical resistance compared to the solid pedicle screws and therefore may affect the EMG response during stimulus-evoked EMG testing in pedicle screw fixation especially in minimally invasive percutaneous pedical screw fixation surgery.

Improving safety in spinal deformity surgery: advances in navigation and neurologic monitoring

INTRODUCTION

The treatment of spinal deformities has rapidly changed during the past decade. The advent of new surgical techniques, particularly thoracic pedicle screws and spinal osteotomies, allow more aggressive deformity correction, and require an increased focus on safety.

MATERIALS AND METHODS

Review of the navigation systems and neuromonitoring techniques currently available.

CONCLUSION

Navigation systems today are where intraoperative neuromonitoring was 20 years ago: new, under investigation, not widely accepted, with concerns for cost, safety and efficiency. Navigation enhances the accuracy of pedicle screws placement in deformed spines, reducing the rate of misplaced screws and potential complications. With further use and investigation, navigation, like neuromonitoring, will soon become standard at major spine centers throughout the world.

Ability of electromyographic monitoring to determine the presence of malpositioned pedicle screws in the lumbosacral spine: analysis of 2450 consecutively placed screws

Object

Pedicle screws provide efficient stabilization along all 3 columns of the spine, but they can be technically demanding to place, with malposition rates ranging from 5% to 10%. Intraoperative electromyographic (EMG) monitoring has the capacity to objectively identify a screw breaching the medial pedicle cortex that is in proximity to a nerve root. The purpose of this study is to describe and evaluate the authors' 7-year institutional experience with intraoperative EMG monitoring during placement of lumbar pedicle screws and to determine the clinical utility of intraoperative EMG monitoring.

Methods

The authors retrospectively studied 2450 consecutive lumbar pedicle screws placed in 418 patients from June 2002 through June 2009. All screws were inserted using a free-hand technique and anatomical landmarks, stimulated at 10.0 mA, and evaluated with CT scanning within 48 hours postoperatively. Medial pedicle screw breach was defined as having greater than 25% of the screw diameter extend outside of the pedicle, as confirmed on CT scanning or intraoperatively by a positive EMG response indicating a medial breach. The sensitivity and specificity of intraoperative EMG monitoring in detecting the presence of a medial screw breach was evaluated based on the following definitions: 1) true positive (a positive response to EMG stimulation confirmed as a breach intraoperatively or on postoperative CT scans); 2) false positive (positive response to EMG stimulation confirmed as a correctly positioned screw on postoperative CT scans); 3) true negative (no response to EMG stimulation confirmed as a correctly positioned screw on postoperative CT scans); or 4) false negative (no response to EMG stimulation but confirmed as a breach on postoperative CT scans).

Results

One hundred fifteen pedicle screws (4.7%) showed positive stimulation during intraoperative EMG monitoring. At stimulation thresholds less than 5.0, 5.0–8.0, and > 8.0 mA, the specificity of a positive response was 99.9%, 97.9%, and 95.9%, respectively. The sensitivity of a positive response at these thresholds was only 43.4%, 69.6%, and 69.6%, respectively. At a threshold less than 5.0 mA, 91% of screws with a positive EMG response were confirmed as true medial breaches. However, at thresholds of 5.0–8.0 mA or greater than 8.0 mA, a positive EMG response was associated with 89% and 100% false positives (no breaches), respectively.

Conclusions

When using intraoperative EMG monitoring, a positive response at screw stimulation thresholds less than 5.0 mA was highly specific for a medial pedicle screw breach but was poorly sensitive. A positive response to stimulation thresholds greater 5.0 mA was associated with a very high rate of false positives. The authors' experience suggests that pedicle screws showing positive stimulation below 5.0 mA warrants intraoperative investigation for malpositioning while responses at higher thresholds are less reliable at accurately representing a medial breach.

Triggered electromyography for placement of thoracic pedicle screws: is it reliable?

Reliable electromyography (EMG) thresholds for detecting medial breaches in the thoracic spine are lacking, and there is a paucity of reports evaluating this modality in patients with adolescent idiopathic scoliosis (AIS). This retrospective analysis evaluates the ability of triggered EMG to detect medial breaches with thoracic pedicle screws in patients with AIS. We reviewed 50 patients (937 pedicle screws) undergoing posterior spinal fusion (PSF) with intraoperative EMG testing. Postoperative CT scans were used for breach identification, and EMG values were analyzed. There were 47 medial breaches noted with a mean threshold stimulus of 10.2 mA (milliamperes). Only 8/47 breaches stimulated at 2-6 mA. Thirteen of the forty-seven screws tested at an EMG value ≤6 mA and/or a decrease of ≥65% compared with intraosseously placed screws. The sensitivity and positive predictive value for EMG was 0.28 and 0.21. A subanalysis of T10-T12 screws identified six of seven medial breaches. Using guidelines from the current literature, EMG does not appear to be reliable in detecting medial breaches from T2 to T9 but may have some utility from T10 to T12.

Dynamically evoked, discrete-threshold electromyography in the extreme lateral interbody fusion approach

OBJECT

because the psoas muscle, which contains nerves of the lumbar plexus, is traversed during the extreme lateral interbody fusion (XLIF) approach, appropriate nerve monitoring is needed to avoid nerve injury during surgery and prevent approach-related neural deficit. This study was performed to assess the effectiveness of dynamically evoked electromyography (EMG) to detect and prevent neural injury during the XLIF approach.

METHODS

one hundred two patients undergoing XLIF at L3-4 and/or L4-5 were enrolled in a prospective, multicenter, nonrandomized clinical study. The EMG threshold values for each of the 3 successive dilators were recorded at the surface of the psoas muscle, mid-psoas, and on the spine. At each location, the dilators were rotated 360°, taking recordings immediately posterior, superior, anterior, and inferior. For each dilator, the authors noted the rotational position (the angle in degrees) at which the lowest threshold was found. Findings of pre- and postoperative neurological examinations were also recorded.

RESULTS

nerves were identified within proximity of the dilators (alert-level EMG feedback) in 55.7% of all cases during the XLIF approach. Although nerves were more commonly identified in the posterior margin (63%), there was significant variability in the location of nerves identified. Despite the fact that the posterior half of the disc space was targeted in 90% of cases, no significant long-lasting neural deficits were identified in any case; 27.5% experienced new iliopsoas/hip flexion weakness and 17.6% experienced new postoperative upper medial thigh sensory loss. Transient motor deficits were identified in 3 patients (2.9%), and all had resolved by the 6-month follow-up visit.

CONCLUSIONS

the ability to identify and report a discrete, real-time EMG threshold during the transpsoas approach helps to avoid nerve injury and is required for the safe performance of the XLIF procedure. Additionally, nerve location is variable, thus reinforcing the need for real-time directional and proximity information.

Intraoperative electrophysiological monitoring in spine surgery

STUDY DESIGN

Review of the literature with analysis of pooled data.

OBJECTIVE

To assess common intraoperative neuromonitoring (IOM) changes that occur during the course of spinal surgery, potential causes of change, and determine appropriate responses. Further, there will be discussion of appropriate application of IOM, and medical legal aspects. The structured literature review will answer the following questions: What are the various IOM methods currently available for spinal surgery? What are the sensitivities and specificities of each modality for neural element injury? How are the changes in each modality best interpreted? What is the appropriate response to indicated changes? Recommendations will be made as to the interpretation and appropriate response to IOM changes.

SUMMARY OF BACKGROUND DATA

Total number of abstracts identified and reviewed was 187. Full review was performed on 18 articles.

METHODS

The MEDLINE database was queried using the search terms IOM, spinal surgery, SSEP, wake-up test, MEP, spontaneous and triggered electromyography alone and in various combinations. Abstracts were identified and reviewed. Individual case reports were excluded. Detailed information and data from appropriate articles were assessed and compiled.

RESULTS

Ability to achieve IOM baseline data varied from 70% to 98% for somatosensory-evoked potentials (SSEP) and 66% to 100% for motor-evoked potentials (MEP) in absence of neural axis abnormality. Multimodality intraoperative neuromonitoring (MIOM) provided false negatives in 0% to 0.79% of cases, whereas isolated SSEP monitoring alone provided false negative in 0.063% to 2.7% of cases. MIOM provided false positive warning in 0.6% to 1.38% of cases.

CONCLUSION

As spine surgery, and patient comorbidity, becomes increasingly complex, IOM permits more aggressive deformity correction and tumor resection. Combination of SSEP and MEP monitoring provides assessment of entire spinal cord functionality in real time. Spontaneous and triggered electromyography add assessment of nerve roots. The wake-up test can continue to serve as a supplement when needed. MIOM may prove useful in preservation of neurologic function where an alteration of approach is possible. IOM is a valuable tool for optimization of outcome in complex spinal surgery.

Brachial plexus injury following spinal surgery

OBJECT

In the present study, the authors identified the etiology, precipitating factors, and outcomes of perioperative brachial plexus injuries following spine surgery.

METHODS

We reviewed all the available literature regarding postoperative/perioperative brachial plexus injuries, with special concern for the patient's position during surgery, duration of surgery, the procedure performed, neurological outcome, and prognosis. We also reviewed the utility of intraoperative electrophysiological monitoring for prevention of these complications.

RESULTS

Patient malpositioning during surgery is the main determining factor for the development of postoperative brachial plexus injury. Recovery occurs in the majority of cases but may require weeks to months of therapy after initial presentation.

CONCLUSION

Brachial plexus injuries are an increasingly recognized complication following spinal surgery. Proper attention to patient positioning with the use of intraoperative electrophysiological monitoring techniques could minimize injury.

Intraosseous ultrasonography to determine the accuracy of drill hole positioning prior to the placement of pedicle screws: an experimental study

Object

Dorsal fixation with rods and pedicle screws (PSs) is the most frequently used surgery to correct traumatic and degenerative instabilities of the human spine. Prior to screw placement, screw holes are drilled along the vertebral pedicles. Despite the use of a variety of techniques, misplacement of screw holes, and consequently of the PSs, is a common problem. The authors investigated the usefulness of an intraspinal, intraosseous ultrasonography technique to determine the accuracy of drill hole positioning.

Methods

An endovascular ultrasound transducer was used for the intraluminal scanning of bore holes in trabecular bovine bone, 12 pedicle drill holes in cadaveric human spine, and 4 pedicle drill holes in a patient undergoing thoracic spondylodesis. Seven of the experimental bore holes in the cadaveric spine were placed optimally (that is, inside the pedicle) and 5 were placed suboptimally (breaching the medial or lateral cortical surface of the pedicle). Computed tomography scans were obtained in the patient and cadaveric specimen after the procedure.

Results

The image quality achieved in examinations of native bovine bone tissue, the formalin-fixed human spine specimen, and human vertebrae in vivo was equal. The authors endosonographically identified correct intrapedicular and intravertebral positions as well as poor (cortex breached) placement of drill holes.

Conclusions

Intraosseous ultrasonography is a promising technique for the investigation of PS holes prior to screw implantation, and may add to the safety of PS placement.

Recording diffusion responses from contralateral intercostal muscles after stimulus-triggered electromyography: refining a tool for the assessment of thoracic pedicle screw placement in an experimental porcine model

STUDY DESIGN

A new stimulus-triggered electromyography (EMG) test for detecting stimulus diffusion to contralateral intercostal muscles during thoracic pedicle screw placement was assessed in a porcine model.

OBJECTIVE

To determine if electromyographic thresholds in the intercostal muscles of both sides of the thorax could discriminate thoracic pedicle screw malpositions with and without neural contact at different aspects of the spinal cord and nerve roots.

SUMMARY OF BACKGROUND DATA

There is controversy about the value of triggered EMG stimulation for aiding precise insertion of thoracic pedicle screws. A universally validated threshold that confirms screw malposition has not been established. Diffusion of EMG responses to the contralateral intercostal muscles has not previously been investigated.

METHODS

Nine domestic pigs weighing 60 to 75 kg had 108 pedicle screws placed bilaterally in the thoracic spine from T8-T13. Before spine instrumentation, neural structures were stimulated in 4 animals under direct vision at different anatomic locations from T9-T12. Recording electrodes were placed over the right and left intercostal muscles. Increasing stimulus intensity was applied until muscle response was detected at the contralateral side (EMG diffusion phenomenon). After this first experiment, the thoracic spine was instrumented in all 9 animals. Screws were placed in the pedicle in different positions, the anatomic intrapedicular location and within the spinal canal, with and without contact with the neural elements.

RESULTS

Response thresholds to direct nerve root stimulation were significantly lower than those obtained by stimulation of the dorsal aspect of the spinal cord (0.44 +/- 0.22 mA vs. 1.38 +/- 0.71 mA, P < 0.01). However, a 14-fold stimulation intensity (6.50 +/- 0.29 mA) was necessary to obtain diffusion of the EMG response to the opposite (left) side if the right nerve root was stimulated. A 2-fold increment (3.17 +/- 0.93 mA) elicited diffusion of the EMG responses to the contralateral side when stimulation was applied to the dorsal aspect of the spinal cord. EMG recordings of the 108 stimulated screws showed a significant decrease in the EMG response when the screw was in contact with the spinal cord (2.72 +/- 1.48 mA; P < 0.01) compared with that found when the pedicle track was intact (mean: 5.01 +/- 1.89 mA). Screws violating the medial wall of the pedicle, but not touching neural tissues, responded to slightly lower intensities than well-positioned screws, but this was not statistically significant (3.91 +/- 1.39 mA vs. 4.89 +/- 1.30 mA, P > 0.05).

CONCLUSION

Stimulus-triggered EMG can identify screws that violate the medial pedicle wall if they are in contact with neural tissues. EMG thresholds could not discriminate screws that violated the medial pedicle wall without neural contact from screws with accurate intraosseous placement. However, recording EMG potentials at the contralateral intercostal muscles (stimulus diffusion phenomenon) proved to be a reliable method for identifying the neural structures at risk.

Comparação entre o teste de despertar e a monitoração neurofisiológica intra-operatória com potencial evocado somato-sensitivo nas cirurgias de escoliose

OBJETIVO: comparar a incidência de lesões neurológicas em cirurgias corretivas de curvas rígidas de escoliose idiopática, utilizando-se somente o teste do despertar e utilizando-se a monitoração neurofisiológica intra-operatória por meio dos Potenciais Evocados Somato-Sensitivo (PESS). MÉTODOS: foram realizadas 111 cirurgias para correção de escoliose idiopática, com curvas rígidas por meio de instrumentação e artrodese pela via posterior, no período de janeiro de 1985 a maio de 2001. Os procedimentos foram divididos em dois grupos, sendo 80 pacientes operados sem a monitoração intra-operatória, utilizando somente o teste do despertar, no período de janeiro de 1985 a janeiro de 1998. A partir de maio de 1998, as cirurgias passaram a ser monitoradas com potencial evocado somato-sensitivo (PESS), para prevenção de lesão neurológica, formando um segundo grupo de 31 pacientes. RESULTADOS: no primeiro grupo, um paciente teve lesão neurológica irreversível e quatro pacientes tiveram lesão neurológica reversível. Entre os 31 pacientes do segundo grupo, com monitoração neurofisiológica sensitiva durante a cirurgia, oito apresentaram lesão neurológica reversível constatada no intra-operatório, sem nenhuma repercussão clínica após o procedimento. CONCLUSÃO: os resultados apresentados sugerem a eficácia da monitoração intra-operatória, com o potencial evocado somato-sensitivo, para a prevenção de lesões neurológicas, nas cirurgias corretivas de curvas rígidas na escoliose idiopática.

A finite element model of electrode placement during stimulus evoked electromyographic monitoring of iliosacral screw insertion

Pelvic ring fractures that occur as a result of substantial orthopedic trauma are frequently repaired using iliosacral screws to stabilize the fracture. Stimulus evoked electromyography, using pulsed current stimuli provided through the drill bit cathode, has been advocated to prevent nerve root injury during iliosacral screw insertion. Our objective was to examine the effects of anode location, drill bit position, and anatomical structure on the nerve monitoring technique. A three-dimensional finite element model was constructed from computed tomography data to evaluate the effectiveness of five anode locations at four stations of drill bit insertion. Results indicate that the anode location should be at the midline or on the side contralateral to drill bit insertion. Locating the anode at other positions, such that the nerve root is outside of the primary electromagnetic field, leads to an attenuated electromyographic response that will ultimately lead to the failure of the monitoring technique.

Correlation between low triggered electromyographic thresholds and lumbar pedicle screw malposition: analysis of 4857 screws

STUDY DESIGN

A retrospective analysis of 1078 spinal surgical procedures with lumbar pedicle screw placement at a single institution.

OBJECTIVE

Based on previously established normative values, triggered electromyographic stimulation (TrgEMG) was re-examined to evaluate its efficacy in determining screw malposition.

SUMMARY OF BACKGROUND DATA

Threshold values for confirmation of intraosseous placement of pedicle screws with EMG stimulation is controversial.

METHODS

TrgEMG threshold values for 4857 pedicle screws placed from L2 to S1 from 1996 to 2005 were analyzed. An ascending method of constant current stimulation was applied to each pedicle screw to obtain a compound muscle action potential (CMAP) from lower extremity myotomes. Previously determined threshold value normative data from a published clinical series of 233 screws were as follows: 0 to 4 mA, high likelihood of pedicle wall breach; 4 to 8 mA, possible pedicle wall breach; >8 mA, no pedicle wall defect.

RESULTS

A total of 7.74% (376 of 4857) of all screws tested had threshold values <8.0 mA. A total of 19.1% (72 of 376) of these were <4.0 mA: 54% (39 of 72) were repositioned (26) or removed (13) while the remaining 33 screws were left in place following repalpation. A total of 80.9% (304 of 376) had thresholds between 4 and 8 mA: 17.4% (53) were repositioned (38) or removed (15). Nine screws had thresholds of <or=2.8 mA and were either repositioned or removed following confirmation of a medial wall breach. A total of 74.5% (280 of 376) of all screws with thresholds <8.0 mA were verified as correctly placed by repalpation/radiography and therefore left in place.

CONCLUSION

The probability of a medial breach pedicle screw detected by triggered EMG stimulation increases with decreasing triggered EMG thresholds: 0.31% for >8.0 mA, 17.4% for 4.0 to 8.0 mA, 54.2% for <4.0 mA, and 100% for <2.8 mA. At 2.8 mA, triggered EMG has a specificity of 100%, with sensitivity of 8.4%; at 4.0 mA, specificity of 99% and sensitivity of 36%; and at 8.0 mA, 94% specificity and 86% sensitivity. TrgEMG is an adjunct technique and should always be used in conjunction with palpation and radiography to optimize safe pedicle screw placement.

Neurologic complications of lumbar pedicle subtraction osteotomy: a 10-year assessment

STUDY DESIGN

Clinical, radiographic, and outcomes assessment focusing on neurologic complications in patients undergoing pedicle subtraction osteotomy (PSO). Clinical data were collected prospectively. Radiographic analysis was performed retrospectively.

OBJECTIVE

To evaluate intraoperative and postoperative neurologic deficits following lumbar PSOs in order to determine risk factors, treatment strategies, and patient outcome.

SUMMARY OF BACKGROUND DATA

Although technically demanding, PSOs have been increasingly used to restore lumbar lordosis and correct sagittal deformity. Although some reports have commented on various complications of the procedure, to our knowledge, there have been no studies focusing on neurologic complications of the osteotomy.

METHODS

An analysis of 108 consecutive patients with an average age of 54.8 +/- 14.0 years and treated with a lumbar PSO at 1 institution over a 10-year period (1995-2005) was performed. Medical records, radiographs, and neuromonitoring data were analyzed. Clinical outcome was assessed using the Oswestry Disability Index and the Scoliosis Research Society (SRS)-24 instruments.

RESULTS

A total of 108 PSOs were performed. Following surgery, lumbar lordosis increased from -17.1 degrees +/- 19.3 degrees to -49.3 degrees +/- 14.7 degrees (P < 0.000), and sagittal balance improved from 131 +/- 73 mm to 23 +/- 48 mm (P < 0.000). Intraoperative and postoperative deficits (defined as motor loss of 2 grades or more or loss of bowel/bladder control) were seen in 12 patients (11.1%) and were permanent in 3 patients (2.8%). With time motor function improved by 1 grade in 2 patients and all 3 were able to ambulate. Intraoperative neuromonitoring did not detect the deficits. In 9 patients, additional surgical intervention consisted of central enlargement and further decompression. Deficits were thought to be due to a combination of subluxation, residual dorsal impingement, and dural buckling.

CONCLUSION

Intraoperative or postoperative neurologic deficits are relatively common following a PSO; however, in a majority of cases, deficits are not likely to be permanent.

Diathermy testing: a novel method with electric knife stimulation to avoid nerve injuries during lumbar pedicle screw placement

✓ The purpose of this retrospective study was to demonstrate the utility of diathermy in avoiding nerve injuries due to misplacement of lumbar pedicle screws (PSs).

The authors used diathermy to assess whether a screw deviated from the pedicle by observing synchronous leg movements caused by intermittently touching an electric knife to the pedicular instrument. Diathermy was performed in 259 cases in which 1301 PSs had been placed. Leg movements were observed in 36 cases, and the sensitivity of diathermy was 85.7%, with a specificity of 99.5%. No neurological complications associated with the placement of PSs were observed after adding diathermy testing to conventional methods.

Diathermy testing may be a way to avoid nerve injuries during lumbar PS placement.

Intraoperative electromyography monitoring in minimally invasive transforaminal lumbar interbody fusion

Object

Minimally invasive transforaminal lumbar interbody fusion (TLIF) is an increasingly popular method for achieving lumbar decompression and fusion. The procedure is technically more demanding than open fusion, with correspondingly more theoretical risk of complication. The authors describe the use of intraoperative electromyography (EMG) as an adjunct to surgery to reduce the risk of complications.

Methods

Between August 2005 and April 2006, 25 consecutive patients underwent minimally invasive TLIF in which a total of 105 pedicle screws were placed. Intraoperative EMG was performed and included passive recordings during decompression and interbody graft placement, as well as active recording during the placement of the pedicle access needle and testing of the pedicle tap. A uniform protocol for active monitoring was used, with the pedicle access needle set at 7 mA. To assess hardware placement, all patients underwent postoperative radiography and 20 underwent postoperative computed tomography (CT) scanning.

In no patient did the authors observe significant EMG activation during decompression. In five cases, intermittent nerve root firing was noted after the interbody graft was placed, but this did not correlate with any postoperative deficits. Using the active stimulation protocol, 76.2% of screw placements required one or more changes to the trajectory of the pedicle access needle. With successful placement of the pedicle access needle, in all 105 screws, the pedicle tap nerve root stimulation threshold was greater than 15 mA. Postoperative radiography was performed in all patients and CT scanning was performed in 20 patients (with 85 screws being placed). Postoperative imaging revealed only three cases of pedicle breach. In all cases, the breach was at the lateral wall of the pedicle and not thought to be clinically relevant.

Conclusions

A continuous stimulation pedicle access needle alerts the surgeon to incorrect medial trajectories and may lead to safer pedicle cannulation. As a result of electrophysiological feedback, the pedicle access needle trajectory was altered in 76.2% of the reported cases. The use of the authors’ protocol resulted in a 0% incidence of clinically relevant malpositioned hardware and a low overall neurological complication rate. Intraoperative nerve root monitoring is a useful adjunct to minimally invasive TLIF.

Automated intraoperative EMG testing during percutaneous pedicle screw placement

BACKGROUND

EMG screw testing has been shown to be sensitive and reliable in open spinal instrumentation cases. However, there is little evidence to show its applicability to percutaneous screw placement.

PURPOSE

To demonstrate the utility of EMG testing in percutaneous techniques, where lack of direct visualization poses an added risk to nerve injury.

STUDY DESIGN

Summary of intraoperative EMG results during percutaneous pedicle screw placement.

METHODS

Percutaneous pedicle screws were placed in twenty patients (22 levels, 88 pedicles). The initial fluoroscopically-guided k-wires and the subsequent taps were insulated and stimulated via an automated EMG system. Low threshold values prompted repositioning of the pedicle trajectory.

RESULTS

Four (5%) k-wires induced EMG thresholds less than 10mA, prompting repositioning. One was repositioned without improvement, but with improvement upon tapping. One k-wire with very low threshold (3mA) was repositioned with an improved result (13mA). In 78 pedicles (89%) the tap threshold was greater than the k-wire.

CONCLUSIONS

EMG testing helps to identify suboptimal screw trajectories, allowing for early adjustment and confirmation of improved placement. Tapping often improved thresholds, perhaps by compressing the bone and creating a denser, more insulative pedicle wall. EMG testing may improve the safety of percutaneous screw techniques, where the pedicle cannot be visually inspected.

Intraoperative Electrophysiologic Monitoring: Considerations for Complex Spinal Surgery

Intraoperative neurophysiologic monitoring techniques have evolved as the complexity of spinal surgery has increased and the limitations of individual modalities have become apparent. Current monitoring strategies include a combination of techniques directed toward detecting changes in sensory, motor, and nerve root function. Close coordination and communication between the monitoring personnel, surgeon, and anesthesiologist is essential to effective intraoperative monitoring.

Intraoperative Applications Of The H-Reflex And F-Response: A Tutorial

Traditional intraoperative monitoring of spinal cord function involves the use of three techniques: 1. Orthodromic ascending somatosensory evoked potentials (SSEPs) and 2. antIDromic descending neurogenic somatosensory evoked potentials (DNSSEPs) monitor long-tract sensory function. SSEPs and DNSSEPs do not monitor interneuronal gray matter function. 3. Transcranial motor evoked potentials (TMEPs) monitor descending long-tract motor function and measure interneuronal gray matter function by activating motor neurons. TMEPs activate from 4-5% of the motor neuron pool. When using TMEPs 95-96% of the motor spinal cord systems activating the motor neurons are not monitored. Our ability to interact with our environment involves not only intact sensation and strength, but also complex coordinated motor behavior. Complex coordinated motor behavior is controlled by groups of electrically-coupled spinal cord central pattern generators (CPGs). The components of CPGs are: descending and propriospinal systems, peripheral input, and segmental interneurons. The point-of-control is the level of excitation of interneurons, which is determined by the integrated activity of the other components. Spinal cord injury (SCI) changes segmental reflex gain by uncoupling these components. Changes in gain are detected by recordings from muscles. SSEPs, DNSSEPs and TMEPs provIDe limited information about the status of CPGs. H-reflexes measure the function of from 20-100% of the motor neuron pool. F-responses measure the function of from 1-5% of the motor neuron pool. H-reflexes and F-responses provIDe information about the degree of coupling between CPG components. Recording H-reflexes and F-responses together with SSEPs and TMEPs not only monitors spinal cord long-tract function, but also provIDes a multiple-systems approach that monitors those spinal cord systems that are responsible for the control of complex coordinated motor behavior. The objective of this paper is to describe how H-reflexes and F-responses can be used to monitor complex coordinated motor behavior.