JBJS | Intraoperative Monitoring with Stimulus-Evoked Electromyography during Placement of Iliosacral Screws. An Initial Clinical Study*

The Journal of Bone & Joint Surgery, Volume 80, Issue 4

Articles | April 01, 1998

Intraoperative Monitoring with Stimulus-Evoked Electromyography during Placement of Iliosacral Screws. An Initial Clinical Study*

BERTON R. MOED, M.D.†; B. K. AHMAD, M.D.‡; JOSEPH G. CRAIG, M.D.‡; GARY P. JACOBSON, PH.D.‡; MARK J. ANDERS, M.D.§, DETROIT, MICHIGAN

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Investigation performed at the Division of Orthopaedic Traumatology, Department of Orthopaedic Surgery, Henry Ford Hospital, Detroit

The Journal of Bone & Joint Surgery. 1998; 80:537-46

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Abstract

A consecutive series of twenty-seven patients who had thirty acute unstable (type-C) fractures of the pelvic ring was studied prospectively to evaluate the use of stimulus-evoked electromyography to decrease the risk of iatrogenic nerve-root injury during the insertion of iliosacral screws. A prerequisite for inclusion in the study was a normal neurological status preoperatively; somatosensory evoked potentials were monitored to further document the neurological status both before and after insertion of the screw or screws. A total of fifty-one iliosacral screws were inserted, and a current threshold of more than eight milliamperes was selected as the level that indicated that the drill-bit was a safe distance from the nerve root. Four of the fifty-one screws were redirected because of information obtained with stimulus-evoked electromyography.Postoperatively, all patients had a normal neurological status. Computerized tomography, although not accurate for detailed measurements, demonstrated that all of the screws were in a safe, intraosseous position. Monitoring with stimulus-evoked electromyography appears to provide reliable data and may decrease the risk of iatrogenic injury to the nerve roots during operations on the pelvic ring.

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

Fixation with iliosacral screws, a recommended method for the operative stabilization of acute disruptions of the pelvic ring^5,14,15,23, has emerged as the treatment of choice for posterior fractures of the pelvic ring because of its biomechanical superiority^5,9,14,15,23. Whether the screw is inserted percutaneously in conjunction with closed reduction or after open reduction, the optimum intraosseous course of the screw brings it close to the fifth lumbar and sacral nerve roots as well as the spinal canal^9,26. Therefore, one of the potential hazards of this technique is that misdirection of the guide-wire, drill-bit, or screw will result in injury of the nerve root or compromise of the spinal canal. Indeed, the prevalence of iatrogenic neural injury has been reported to range from 1 per cent (one of eighty-three cases in seventy-six patients)¹⁵ to 10 per cent (three of thirty screws in twenty-one patients)²⁸.

Techniques such as monitoring of somatosensory evoked potentials and continuous electromyography have been used in an attempt to minimize the risk of iatrogenic neural injury^8,28. However, monitoring of somatosensory evoked potentials can demonstrate abnormal signals only after injury to the nerve has altered a previously normal signal, and continuous electromyography can do so only after a neural injury has generated an abnormal spontaneous burst of neurotonic activity. Thus, with either nerve-monitoring system, the signal is associated with neural injury caused by traction, crushing, laceration, or thermal damage. The injury may be minor and reversible, such as that caused by a guide-pin pressing on or tenting the nerve, or it may be major and permanent, such as a laceration or a crush injury¹⁷.

Stimulus-evoked electromyography has been used successfully during operations on the facial nerve and the recurrent laryngeal nerve^2,7,13,19,22 and, recently, during pedicle-screw fixation of the lumbosacral spine in both animals and humans^{1,3,4,11,12,25}. The methodology differs somewhat depending on the specific application. In an operation on the facial nerve or the recurrent laryngeal nerve, the objective is to locate but not injure the nerve; in a spinal operation, the goal is to avoid contact with the nerve (without localization). However, in all procedures, the technique for monitoring has been essentially identical: the electrodes for electromyographic monitoring are placed in the muscle or muscles innervated by the nerve in question and the potentially injurious dissecting instrument or implant serves as the stimulating electrode^2-4,12,19,22. The purpose of monitoring is to warn the surgeon that the implant or instrument is close to the nerve.

Theoretically, monitoring with stimulus-evoked electromyography could alert the surgeon to impending contact with a neural structure, so that the direction of a wire, drill-bit, or screw can be altered to avoid neural injury. We first studied the potential applicability of this type of monitoring during operations on the pelvic ring in a canine model¹⁸. The methodology in that feasibility study was similar to the one used by Calancie et al.⁴ for insertion of lumbosacral pedicle screws in an animal model, but we incorporated their recommended³ change from constant-voltage stimulation to constant-current stimulation. The findings of our pilot study of an animal model¹⁸ were similar to those of Calancie et al.⁴ and served as a stimulus to implement a clinical trial that was based on the clinical method described by those authors³. In the current report, we present our initial experience with the clinical use of this technique for monitoring during the insertion of iliosacral screws to stabilize acute type-C disruptions²⁷ of the pelvic ring.

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

†Department of Orthopaedic Surgery, University Health Center, 4201 St. Antoine Boulevard, Suite 7C, Detroit, Michigan 48201.

‡Departments of Neurology (B. K. A.), Radiology (J. G. C.), and Audiology (G. P. J.), Henry Ford Hospital, 2799 West Grand Boulevard, Detroit, Michigan 48202.

§Erie County Medical Center, 462 Grider Street, Buffalo, New York 14215.

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

†Department of Orthopaedic Surgery, University Health Center, 4201 St. Antoine Boulevard, Suite 7C, Detroit, Michigan 48201.

‡Departments of Neurology (B. K. A.), Radiology (J. G. C.), and Audiology (G. P. J.), Henry Ford Hospital, 2799 West Grand Boulevard, Detroit, Michigan 48202.

§Erie County Medical Center, 462 Grider Street, Buffalo, New York 14215.

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TABLE I SUMMARY OF RESULTS

*As measured on computerized tomography scans. †The drill-bit was redirected because the current threshold decreased to eight milliamperes or less (the lowest current threshold is given in parentheses). ‡Greater than twenty milliamperes was recorded by the monitoring personnel.

Case	Side of Injury	Screw	Location of Screw	Final Current Threshold (mA)	Distance between Screw and Nearest Nerve Root* (mm)
1	R	1	S1	18.0	3
	R	2	S2	22.0	8
2	L	3	S1	>50	5
	L	4	S2	24.3	6
3	R	5	S1	38.0	8
	R	6	S1	19.0 (8.0)†	3
4	L	7	S2	43.0	5
	R	8	S1	9.4	3
	R	9	S1	11.8	5
5	R	10	S1	12.4	5
	R	11	S1	22.2	6
6	L	12	S1	50.0	5
7	R	13	S1	40.0	5
8	R	14	S1	28.6	15
	R	15	S1	14.0	2
9	L	16	S1	40.0	10
	L	17	S1	45.5 (7.9)†	8
10	R	18	S1	12.6	5
	R	19	S1	19.6	5
11	L	20	S1	22.0	5
	L	21	S2	19.0	5
12	L	22	S1	40.8	8
13	R	23	S1	25.0	5
	R	24	S1	40.0	7
14	L	25	S1	>20‡	6
	L	26	S1	>20‡	4
15	L	27	S1	40.0	10
	L	28	S1	40.0	5
16	R	29	S1	21.0 (8.0)†	10
	R	30	S2	50.0	5
17	L	31	S1	48.0	5
	L	32	S2	50.0	5
	R	33	S1	31.0	5
18	L	34	S1	11.2	2
	L	35	S2	20.0	8
19	R	36	S1	50.0	3
	R	37	S2	30.2	2
20	L	38	S1	35.3	4
21	L	39	S1	20.0	3
22	R	40	S1	46.5	5
	R	41	S2	11.8	2
23	L	42	S1	50.0	5
	L	43	S2	38.0	3
24	L	44	S1	31.2	10
	L	45	S1	48.2	5
	R	46	S1	28.6	8
25	R	47	S2	7.5 (7.0)†	2
26	L	48	S1	17.0	5
	L	49	S2	22.0	5
27	L	50	S1	12.6	2
	L	51	S2	26.3	3

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TABLE I SUMMARY OF RESULTS

Case	Side of Injury	Screw	Location of Screw	Final Current Threshold (mA)	Distance between Screw and Nearest Nerve Root* (mm)
1	R	1	S1	18.0	3
	R	2	S2	22.0	8
2	L	3	S1	>50	5
	L	4	S2	24.3	6
3	R	5	S1	38.0	8
	R	6	S1	19.0 (8.0)†	3
4	L	7	S2	43.0	5
	R	8	S1	9.4	3
	R	9	S1	11.8	5
5	R	10	S1	12.4	5
	R	11	S1	22.2	6
6	L	12	S1	50.0	5
7	R	13	S1	40.0	5
8	R	14	S1	28.6	15
	R	15	S1	14.0	2
9	L	16	S1	40.0	10
	L	17	S1	45.5 (7.9)†	8
10	R	18	S1	12.6	5
	R	19	S1	19.6	5
11	L	20	S1	22.0	5
	L	21	S2	19.0	5
12	L	22	S1	40.8	8
13	R	23	S1	25.0	5
	R	24	S1	40.0	7
14	L	25	S1	>20‡	6
	L	26	S1	>20‡	4
15	L	27	S1	40.0	10
	L	28	S1	40.0	5
16	R	29	S1	21.0 (8.0)†	10
	R	30	S2	50.0	5
17	L	31	S1	48.0	5
	L	32	S2	50.0	5
	R	33	S1	31.0	5
18	L	34	S1	11.2	2
	L	35	S2	20.0	8
19	R	36	S1	50.0	3
	R	37	S2	30.2	2
20	L	38	S1	35.3	4
21	L	39	S1	20.0	3
22	R	40	S1	46.5	5
	R	41	S2	11.8	2
23	L	42	S1	50.0	5
	L	43	S2	38.0	3
24	L	44	S1	31.2	10
	L	45	S1	48.2	5
	R	46	S1	28.6	8
25	R	47	S2	7.5 (7.0)†	2
26	L	48	S1	17.0	5
	L	49	S2	22.0	5
27	L	50	S1	12.6	2
	L	51	S2	26.3	3

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Figs. 1-A through 1-H: Case 25, a forty-year-old man who sustained multiple injuries, including a type-C disruption²⁷ of the pelvic ring, in a motor-vehicle accident. After initial stabilization at a local hospital, the patient was transferred to our institution for additional medical care and definitive operative treatment of the pelvic injury. Fig. 1-A: Anteroposterior radiograph of the pelvis, made after an external fixator was applied at the referring hospital. Of note is an abnormal, dysplastic²⁴ first sacral segment.

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Fig. 1-B Representative computerized tomography scan, made at the second sacral level after application of the external fixator, demonstrating disruption of the right sacroiliac joint as well as some impaction injury of the sacral ala.

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Fig. 1-C: Initial intraoperative anteroposterior (top) and outlet (bottom) fluoroscopic images of the posterior part of the pelvic ring, made with the patient supine on a radiolucent operating-room table before reduction and fixation. The abnormal, dysplastic first sacral segment on the side of the injury (right) can again be seen.

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Fig. 1-D: Inlet (top) and outlet (bottom) fluoroscopic images made after alignment and partial posterior reduction (achieved with closed manipulation combined with open reduction and internal fixation of the disrupted pubic symphysis) followed by insertion of a 2.8-millimeter drill-bit toward the body of the second sacral vertebra. Fluoroscopically, the drill-bit appeared to be in a satisfactory location and in line with the targeted area in the body of the second sacral vertebra. However, as the drill-bit was advanced to this point, the current threshold on stimulus-evoked electromyography decreased rapidly from 8.5 to 7.0 milliamperes. No spontaneous electromyographic activity was recorded.

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Fig. 1-E Inlet (top) and outlet (bottom) fluoroscopic images made after the drill-bit was backed out and used as a guide for insertion of a second bit (x), beginning in a more superior and anterior location and continuing into the center of the body of the second sacral vertebra. The current threshold on stimulus-evoked electromyography remained greater than eight milliamperes throughout the course of the second drill-bit. The current threshold was 8.5 milliamperes when these images were made.

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Fig. 1-F Inlet (top) and outlet (bottom) fluoroscopic images made after insertion of the screw. The final current threshold on stimulus-evoked electromyography was 7.5 milliamperes.

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Fig. 1-G Anteroposterior radiograph made postoperatively, showing the reduction and fixation. The patient had a completely normal neurological status (unchanged from the preoperative findings).

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Fig. 1-H Computerized tomography scan (at a level similar to that in Fig. 1-B) made postoperatively, showing the anatomical reduction of the dislocation of the sacroiliac joint and the location of the screw in the body of the second sacral vertebra. The screw was noted to be in a completely intraosseous position, but according to the measurement made on the scan it was only two millimeters from the nerve root of the second sacral vertebra. If the course of the original drill-bit (Figs. 1-D and 1-E) had been continued, the final position would have been inferior and posterior to that shown here and the nerve root probably would have been injured.

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Fig. 2 Case 1. Postoperative computerized tomography scan (at the level of the body of the first sacral vertebra) showing the screw to be at least three millimeters away from the nearest (first sacral) nerve root.

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

In January 1995, we established a protocol for monitoring with stimulus-evoked electromyography during iliosacral-screw fixation of an unstable posterior sacroiliac complex, which is pathognomonic for a type-C injury²⁷ of the pelvic ring. The criteria for inclusion in the study consisted of an acute type-C (unstable) pelvic injury that was amenable to fixation with iliosacral screws, either alone or in combination with a tension-band plate⁹; a normal peripheral-nerve status preoperatively, as determined with a detailed physical examination; and normal function of the lumbosacral plexus before insertion of the screws, as documented with monitoring of somatosensory evoked potentials. Any preexisting neural abnormality alters the response to a current stimulus²⁵. Therefore, patients who had evidence of a neural injury were excluded from the study.

Fifty-two patients who had a type-C pelvic injury²⁷ were seen over the ensuing twenty-four months. Twenty-seven patients who had a total of thirty posterior pelvic injuries (three had a bilateral fracture) met the inclusion criteria and were consecutively evaluated with intraoperative monitoring with stimulus-evoked electromyography. Twenty patients were male and seven were female. The average age was thirty-six years (range, fifteen to sixty-four years). There were nineteen dislocations or fracture-dislocations through the sacroiliac joint and eleven displaced fractures through the sacrum (two in zone I and nine in zone II, as described by Denis et al.).

Preoperative skeletal traction was used routinely in order to minimize cephalad migration of the unstable hemipelvis. The traction was removed after induction of general anesthesia with the patient in the hospital bed. General anesthesia was induced with use of narcotic agents, short-acting muscle relaxants, and isoflurane (1.0 per cent or less)^3,11, under conditions appropriate for the monitoring procedures used in this study. The leads then were placed for monitoring of somatosensory evoked potentials and for stimulus-evoked electromyography. To monitor somatosensory evoked potentials, stainless-steel disposable electroencephalographic needle electrodes were inserted into the subcutaneous tissue of the scalp and neck. The active electrodes were placed on the midline of the scalp, two centimeters behind the vertex (Cz' according to the international 10—20 electroencephalography electrode placement system) and posteriorly over the seventh cervical vertebra. The reference electrode was placed on the scalp, in the midline of the frontal area (Fz). The stimulating electrodes were placed over the tibial nerve, just posterior to the medial malleolus, and over the peroneal nerve at the fibular head in both lower extremities. For electromyographic monitoring, bipolar needle recording electrodes were positioned at the motor points for both the gastrocnemius and the tibialis anterior muscle.

The patient then was positioned, on a radiolucent operating-room table, either supine, for closed reduction and percutaneous fixation (sixteen patients), or prone, for open reduction and internal fixation (eleven patients). Baseline somatosensory evoked potentials were recorded after the patient had been placed in the appropriate position and were monitored throughout the operative procedure. An abnormal change in the somatosensory evoked potential was defined as an increase in latency of at least 10 per cent or a decrease in amplitude of more than 50 per cent^17,21. The Viking IV System (Nicolet Biomedical Instruments, Madison, Wisconsin) was used both for monitoring of somatosensory evoked potentials and for stimulus-evoked electromyography. All intraoperative nerve-monitoring was performed under the direction of either a neurologist (B. K. A.) or a neurophysiologist (G. P. J.).

After the fracture had been reduced and a normal recording of somatosensory evoked potentials had been obtained, the iliosacral screws were inserted under image-intensification control^{5,14-16,23,24}. The cannulated-screw system included a 2.8-millimeter drill-bit (7.3-millimeter system; Synthes, Paoli, Pennsylvania) or a 3.2-millimeter drill-bit (6.5-millimeter system; Howmedica, Rutherford, New Jersey). Constant-current stimulus-evoked electromyography^3,12,18 was incorporated into the operative technique, with the drill-bit acting as an electrode. Monopolar, monophasic square-wave stimuli of 0.2-millisecond duration were delivered at three hertz with the cathode (stimulating) lead attached to the drill-bit. The anode (reference) lead was an electroencephalographic needle that was placed in the subcutaneous tissue in the midline. Monitoring needle electrodes were placed in the gastrocnemius muscle and the tibialis anterior muscle. This particular setup for stimulation and recording (the wave form, the type and duration of the stimulus, and so on) directly followed the method and recommendations of Calancie et al.³. As noted previously, monitoring in this way (observing for a myogenic response) precludes neuromuscular blockade. To evaluate for neuromuscular blockade intraoperatively, a nerve stimulator was applied over the ulnar nerve. The stimulator produces a train of four stimuli that deliver pulses of twenty to eighty milliamperes¹⁰. Neuromuscular blockade was kept at a minimum during stimulus-evoked electromyography, as indicated by a response of three or four twitches to the train of four stimuli¹¹. A searching current of fifty milliamperes initially was applied to the drill-bit. A current threshold of more than eight milliamperes to evoke the electromyographic response was selected, on the basis of previous findings^3,11,12,18, to provide a safe distance between the drill-bit and the nerve root. A current threshold of eight milliamperes or less was an indication to redirect the drill-bit.

Once the drill-bit was in a satisfactory position, the cannulated screw was inserted over the drill-bit, again while monitoring with stimulus-evoked electromyography (the cathode lead was connected to the shaft of the screwdriver). Once the screw was fully seated, a final current threshold was recorded; 6.5 milliamperes was selected as the absolute minimum acceptable level for final placement of the screw^3,12,18. The process was repeated for additional screws as necessary (Table I). A final run of somatosensory evoked potentials was recorded after completion of the iliosacral-screw fixation. For the last fourteen patients in the series, the overall monitoring regimen included observation to detect spontaneous electromyographic activity because we had become aware of a report supporting such observation²⁸. This addition required no modification of the technique for monitoring other than the need to note any spontaneous neurotonic burst activity.

After the operative procedure, the patients were asked about subjective symptoms, such as numbness, abnormal sensations, and muscular weakness, and were given a detailed physical examination to evaluate neurological function. The examination, which was performed independently by two of us (B. R. M. and B. K. A.), included the use of pinprick and light touch for dermatomal sensory evaluation, testing of toe-joint position sense for evaluation of proprioception, and assessment of gross motor strength.

Computerized axial tomography (model 9800 or High Speed Advantage scanner; General Electric Medical Systems, Milwaukee, Wisconsin) was performed postoperatively, with three-millimeter contiguous sections. The distance from the tip of the screw to the nearest portion of the nerve-root periphery or the margin of the neural canal (if the nerve root could not be clearly visualized) was measured independently by two of us (B. R. M. and J. G. C.), with one of us blinded to the clinical status of the patient. When the interobserver difference was two millimeters or more, the discrepancy was resolved by a radiologist who was not otherwise involved in the study. When the interobserver difference was less than two millimeters, the shorter distance was recorded. All distances were measured in the x-y plane. The spatial resolution in the x-y plane for the computerized tomography scanners used in this study was less than 0.87 millimeter (the 9800 scanner) or 0.45 millimeter (the High Speed Advantage scanner), according to data obtained from the Annual Compliance Reports, Division of Physics, Department of Diagnostic Radiology, Henry Ford Hospital.

Results

Introduction | Materials and Methods | Results | Discussion | References

Fifty-one iliosacral screws were used to fix thirty unstable injuries of the posterior part of the pelvic ring in twenty-seven patients (Table I). The drill-bit had to be redirected during insertion of four of the screws because stimulus-evoked electromyography demonstrated a current threshold of eight milliamperes or less (Table I and Figs. 1-A, 1-B, 1-C, 1-D, 1-E, 1-F, 1-G and 1-H). In most patients, there was a small (less than two-milliampere) difference or no difference between the final current threshold after insertion of the screw and the threshold during use of the drill-bit. However, a decrease of more than two milliamperes (8.8, 3.7, and 5.1 milliamperes) was observed in three patients (Case 19, screw 37; Case 20; and Case 22, screw 41). A spontaneous burst of electromyographic activity was observed during insertion of the drill-bit for the second screw in one patient (Case 27). Fluoroscopy revealed that the drill-bit had just crossed the sacroiliac joint and was not in proximity to a neural structure and that the osseous reduction had not changed. The electromyographic current threshold was greater than forty milliamperes at that time, and the somatosensory evoked potentials were normal. Therefore, the spontaneous burst of electromyographic activity was disregarded.

The final recordings of somatosensory evoked potentials were normal in all except one patient (Case 15). In this patient, the amplitude of the somatosensory evoked potential at the completion of the operative procedure was decreased to 40 per cent of the baseline value. On the postoperative physical examination, all of the patients (including Cases 15 and 27) had normal neurological findings (unchanged from the preoperative status).

The position of the screws was evaluated on postoperative computerized tomography scans by one of us (J. G. C.) who was blinded to the current-threshold data as well as by the operating surgeon (B. R. M.). Forty-five of the fifty-one screws were at least three millimeters away from the nearest nerve root, and six were two millimeters away (Table I and Figs. 1-H and 2). There was an interobserver difference of two millimeters for three screws, which was resolved by the intervention of the third observer (a radiologist). The Pearson correlation coefficient was 0.24, indicating a poor linear relationship, for the association between the proximity of the screw to the nearest nerve root, as measured on computerized tomography scans, and the final current threshold. However, it was very clear that all of the screws were in a completely intraosseous location.

No patient had a problem related to the anesthetic technique. However, reversal of any residual neuromuscular blockade during the portion of the procedure in which the iliosacral screw was inserted required careful attention to ensure an adequate level of anesthesia, which was maintained with the administration of narcotics as well as low levels of isoflurane. There were no instances of recall.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

The insertion of iliosacral screws is challenging because the screws must follow a narrow path to remain intraosseous and because of the proximity of important structures in the surrounding soft tissues²⁶. A change in the angulation of the surgeon's hand of as few as 4 degrees can cause a potentially disastrous alteration in the trajectory of the screw²⁶. As the course of these screws cannot be visualized directly, the use of image-intensification fluoroscopy with multiple views has become the standard method for determining their position^{9,14,15,23,24}. However, accurate placement of these screws with use of this method relies on obtaining anatomical reduction and on the radiographic imaging of osseous landmarks that occasionally may be obscured or misinterpreted^20,24,28.

Continuous electromyography and monitoring of somatosensory evoked potentials have been advocated to avoid injury to the nerve root during fixation of pelvic fractures^8,28. These monitoring procedures provide information by indicating a change from a normal to an abnormal condition. The usefulness of monitoring of somatosensory evoked potentials is limited by the need for a rapid response to an abnormal signal and by the fact that the sensory, not the motor, pathway is being monitored¹⁷. Monitoring with continuous electromyography relies on observation of any spontaneous burst of neurotonic activity, which indicates that there has been some perturbation or injury to the nerve²⁸.

Therefore, when either of these monitoring techniques is used, information regarding an acute untoward neurological event is after the fact. The hope is that the loss of signal (somatosensory evoked potentials) or the abnormal signal (electromyography) has been caused by a reversible injury, such as stretching of the nerve by retractors or minor nerve-root compression⁸. However, a laceration or a crush injury of the neural structures by a guide-pin, drill-bit, or screw cannot be prevented with use of either of these after the fact monitoring techniques. The fact that two patients (Cases 15 and 27) had false-positive findings, one on monitoring of somatosensory evoked potentials and one on continuous electromyography, is cause for additional concern regarding the intraoperative reliability of these monitoring techniques.

Intraoperative monitoring with stimulus-evoked electromyography is of interest because it can provide information regarding the proximity of the neural structure being monitored. In operations involving the facial nerve and the recurrent laryngeal nerve, in which the objective is to locate but not injure a neural structure that is obscured by abnormal anatomy, intraoperative monitoring with stimulus-evoked electromyography has proved useful for indicating that an instrument or implant is getting too close to the nerve at risk^2,7,13,19,22. This early warning has allowed surgeons to change the course of the instrument to avoid contact with and injury to the nerve. There were early indications that, with modification, this technique could be helpful in avoiding neural injury during the placement of pedicle screws in the lumbosacral spine^{1,3,4,11,12,25}. With use of the method of Calancie et al.^3,4 and Lenke et al. for monitoring during the insertion of pedicle screws, we sought first to establish the feasibility of this technique for operations on the pelvis in an appropriate animal model¹⁸ and then to apply it in the clinical setting.

A critical aspect of monitoring with stimulus-evoked electromyography is the selection of an appropriate current threshold as the trigger to redirect the instrument or implant. In our animal-model study¹⁸, four 2.5-millimeter-diameter wires were inserted percutaneously, with use of image-intensification fluoroscopy, into the first sacral body of the hemipelvis in five dogs in an attempt to injure the first sacral nerve root. A searching current of twenty milliamperes initially was applied to each wire, and the monitoring electrodes were placed in the gastrocnemius muscle. The current threshold required to evoke an electromyographic response was recorded for each wire. The actual location of each wire was determined by anatomical dissection. The current threshold was found to be directly related to the proximity of the wire to the nerve root (r = 0.94, p < 0.001)¹⁸. Evaluation of the 95 per cent confidence interval of that relationship revealed that a current threshold of more than eight milliamperes ensured that there was at least five millimeters between the wire and the nerve root¹⁸. This calculation, in conjunction with the known diameter of the implants and the expected linearity of the relationship, was used to select the value of more than eight milliamperes as the criterion for safe placement of the implant in humans.

In the current report, we present the results of our initial clinical trial. We studied patients who did not have preexisting or post-traumatic neurological injury for two main reasons. First, the specific pattern of response of an abnormally functioning nerve to a current stimulus is unknown. Therefore, any preoperative abnormality such as preexisting neuropathy or neural injury caused by the traumatic injury may alter the response to a given current stimulus^1,25. This could have rendered any data nearly incomprehensible. Second, a study population that was made up entirely of patients who were neurologically intact simplified the postoperative evaluation.

We chose to use an initial searching current of fifty milliamperes in the present study, rather than the twenty milliamperes used in our canine model¹⁸, because of the greater size of the human pelvis. We believed that fifty milliamperes, which is within the range of forty³ to eighty¹¹ milliamperes that has been used in previous clinical studies, was necessary to guarantee an electromyographic response and thereby ensure adequate functioning of the monitoring apparatus. There was only one instance (Case 2, screw 3) in which more than fifty milliamperes was necessary to evoke an electromyographic response.

The applicability to the clinical setting of the data from our pilot canine-model study¹⁸ and from the studies of animals by others^4,11 is open for debate. However, it is important to recognize that the findings of all of these investigations of animals are consistent with the current thresholds recorded, with use of a similar if not completely identical monitoring system, during insertion of pedicle screws in humans^3,11,12. Data derived during insertion of pedicle screws in humans, which included direct visualization of the nerve root and of the position of the screw (something that is not possible during procedures for injuries of the pelvic ring), support the use of a current threshold of eight milliamperes as indicating a complete intraosseous location of the implant and a current threshold of six milliamperes or less as indicating a risk of neural injury^3,11,12. Therefore, previous experience in animals as well as humans substantially supports the use of the current thresholds that were employed in the present study.

All of the patients in the present series had a normal neurological status postoperatively, as determined by two independent examiners. The computerized tomography scans also clearly demonstrated a totally intraosseous location for all of the screws. Had compromise of the neural canal occurred it would have been seen on the scans, as we used three-millimeter contiguous sections to evaluate the position of screws with a diameter of at least 6.5 millimeters. However, despite the expectation that all of the screws would be at least three millimeters from any neural structure, six screws were only two millimeters away (Table I). In addition, there did not seem to be any consistent relationship between the current threshold and the distance (as indicated by a Pearson correlation coefficient of 0.24) as there had been in our canine model, in which direct measurements were made on anatomical specimens¹⁸. The most likely explanation for this discrepancy is the limited accuracy of such measurements on two-dimensional computerized tomography scans. In our animal model, the measurements made on computerized tomography scans were accurate only to within five millimeters¹⁸. Additional study is needed to improve the accuracy of measuring these distances.

Although intraoperative nerve-monitoring can be expensive ($410 per hour at our institution), the addition of stimulus-evoked electromyography to a system for monitoring somatosensory evoked potentials or continuous electromyography involves no cost and is easily implemented. Overall, our findings are in accord with those from our canine-model study¹⁸ and the clinical studies of placement of pedicle screws^1,3,11,12. It could be argued that the information provided by stimulus-evoked electromyography prevented neural injury in four of our twenty-seven patients. This monitoring technique appears to provide reliable data and should decrease the risk of iatrogenic injury of nerve roots during operations on the pelvic ring.

NOTE: The authors thank Suzanne Havstad, M.S., Department of Biostatistics and Research Epidemiology, Henry Ford Health Sciences Center, for assistance in providing the statistical analysis.

References

Introduction | Materials and Methods | Results | Discussion | References

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Fig. 1-F Inlet (top) and outlet (bottom) fluoroscopic images made after insertion of the screw. The final current threshold on stimulus-evoked electromyography was 7.5 milliamperes.

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Fig. 1-G Anteroposterior radiograph made postoperatively, showing the reduction and fixation. The patient had a completely normal neurological status (unchanged from the preoperative findings).

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TABLE I SUMMARY OF RESULTS

Case	Side of Injury	Screw	Location of Screw	Final Current Threshold (mA)	Distance between Screw and Nearest Nerve Root* (mm)
1	R	1	S1	18.0	3
	R	2	S2	22.0	8
2	L	3	S1	>50	5
	L	4	S2	24.3	6
3	R	5	S1	38.0	8
	R	6	S1	19.0 (8.0)†	3
4	L	7	S2	43.0	5
	R	8	S1	9.4	3
	R	9	S1	11.8	5
5	R	10	S1	12.4	5
	R	11	S1	22.2	6
6	L	12	S1	50.0	5
7	R	13	S1	40.0	5
8	R	14	S1	28.6	15
	R	15	S1	14.0	2
9	L	16	S1	40.0	10
	L	17	S1	45.5 (7.9)†	8
10	R	18	S1	12.6	5
	R	19	S1	19.6	5
11	L	20	S1	22.0	5
	L	21	S2	19.0	5
12	L	22	S1	40.8	8
13	R	23	S1	25.0	5
	R	24	S1	40.0	7
14	L	25	S1	>20‡	6
	L	26	S1	>20‡	4
15	L	27	S1	40.0	10
	L	28	S1	40.0	5
16	R	29	S1	21.0 (8.0)†	10
	R	30	S2	50.0	5
17	L	31	S1	48.0	5
	L	32	S2	50.0	5
	R	33	S1	31.0	5
18	L	34	S1	11.2	2
	L	35	S2	20.0	8
19	R	36	S1	50.0	3
	R	37	S2	30.2	2
20	L	38	S1	35.3	4
21	L	39	S1	20.0	3
22	R	40	S1	46.5	5
	R	41	S2	11.8	2
23	L	42	S1	50.0	5
	L	43	S2	38.0	3
24	L	44	S1	31.2	10
	L	45	S1	48.2	5
	R	46	S1	28.6	8
25	R	47	S2	7.5 (7.0)†	2
26	L	48	S1	17.0	5
	L	49	S2	22.0	5
27	L	50	S1	12.6	2
	L	51	S2	26.3	3

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TABLE I SUMMARY OF RESULTS

Case	Side of Injury	Screw	Location of Screw	Final Current Threshold (mA)	Distance between Screw and Nearest Nerve Root* (mm)
1	R	1	S1	18.0	3
	R	2	S2	22.0	8
2	L	3	S1	>50	5
	L	4	S2	24.3	6
3	R	5	S1	38.0	8
	R	6	S1	19.0 (8.0)†	3
4	L	7	S2	43.0	5
	R	8	S1	9.4	3
	R	9	S1	11.8	5
5	R	10	S1	12.4	5
	R	11	S1	22.2	6
6	L	12	S1	50.0	5
7	R	13	S1	40.0	5
8	R	14	S1	28.6	15
	R	15	S1	14.0	2
9	L	16	S1	40.0	10
	L	17	S1	45.5 (7.9)†	8
10	R	18	S1	12.6	5
	R	19	S1	19.6	5
11	L	20	S1	22.0	5
	L	21	S2	19.0	5
12	L	22	S1	40.8	8
13	R	23	S1	25.0	5
	R	24	S1	40.0	7
14	L	25	S1	>20‡	6
	L	26	S1	>20‡	4
15	L	27	S1	40.0	10
	L	28	S1	40.0	5
16	R	29	S1	21.0 (8.0)†	10
	R	30	S2	50.0	5
17	L	31	S1	48.0	5
	L	32	S2	50.0	5
	R	33	S1	31.0	5
18	L	34	S1	11.2	2
	L	35	S2	20.0	8
19	R	36	S1	50.0	3
	R	37	S2	30.2	2
20	L	38	S1	35.3	4
21	L	39	S1	20.0	3
22	R	40	S1	46.5	5
	R	41	S2	11.8	2
23	L	42	S1	50.0	5
	L	43	S2	38.0	3
24	L	44	S1	31.2	10
	L	45	S1	48.2	5
	R	46	S1	28.6	8
25	R	47	S2	7.5 (7.0)†	2
26	L	48	S1	17.0	5
	L	49	S2	22.0	5
27	L	50	S1	12.6	2
	L	51	S2	26.3	3