Intraoperative stimulus-evoked electromyographic monitoring for placement of iliosacral implants: an animal model

An anatomical update on the morphologic variations of S1 and S2

Although percutaneous fixation with iliosacral screws has been shown to be a safe and reproducible method for sacroiliac dislocation and sacral fractures, it is a technically demanding technique, and one of its contraindications is sacral anatomical variations and dysmorphism. The incidence and pattern of S1 and S2 anatomical variations were evaluated in 61 patients (35 women and 26 men) using magnetic resonance imaging of the sacrum in an attempt to explore the possible existence of groups of individuals in whom percutaneous sacroiliac fixation is difficult due to local anatomy. S1 and S2 dimensions in both the transverse and coronal planes were recorded and evaluated. In each individual, S1 and S2 dimensions both in the coronal and transverse planes were proportional, with S2 dimensions being 80% of those of S1 on average. Patients were separated into 4 groups based on the S1 and S2 body size and the asymmetry of dimensions in the transverse and coronal planes. In 48 patients (78.6%), dimensions in both planes were symmetrical despite the varying size of the S1 and S2 body. In 2 patients (3.3%) there was a combination of large transverse plane and small coronal plane dimensions, with large S1 and S2 body size. In 9 patients (14.8%), coronal plane dimensions were disproportionately smaller compared to those of the transverse plane, with a varying size of S1 and S2 body making effective sacroiliac screw insertion a difficult task. Thus, a preoperative imaging study, preferably computed tomography scan, of S1 and S2 body size and coronal plane dimensions and an intraoperative fluoroscopic control of S1 and S2 dimensions on the coronal plane are suggested for safe sacroiliac screw fixation.

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.

Percutaneous fixation of the pelvic ring: an update

With the development of systems of trauma care the management of pelvic disruption has evolved and has become increasingly refined. The goal is to achieve an anatomical reduction and stable fixation of the fracture. This requires adequate visualisation for reduction of the fracture and the placement of fixation. Despite the advances in surgical approach and technique, the functional outcomes do not always produce the desired result. New methods of percutaneous treatment in conjunction with innovative computer-based imaging have evolved in an attempt to overcome the existing difficulties. This paper presents an overview of the technical aspects of percutaneous surgery of the pelvis and acetabulum.

Electrode placement during stimulus evoked electromyographic monitoring of iliosacral screw insertion: a finite element model

Stimulus evoked electromyography is currently used during surgery to prevent nerve root injury during iliosacral screw insertion. In order to evaluate the importance of anode location in this procedure, a three dimensional finite element model was constructed from computed tomography data to evaluate the effectiveness of five electrode locations. Three anode locations (CASIS, dorsal midline and ventral midline) performed well, including the one most commonly used clinically (ventral midline). The dorsal midline, ventral midline, and CASIS anode locations provide data on the proximity to the neural structure in the superior, inferior and contralateral directions respectively that were attenuated at other anode locations. Modification of the technique to include monitoring of multiple anode locations could further reduce the incidence of injury.

S2 iliosacral screw fixation for disruptions of the posterior pelvic ring: a report of 49 cases

OBJECTIVES

The purpose of this study was to evaluate the clinical safety and efficacy of using S2 iliosacral screws for pelvic fracture fixation.

DESIGN

Retrospective analysis of a treatment protocol in a consecutive patient series.

SETTING

Level 1 trauma center.

PATIENTS

Between 1996 and 2001, 49 patients were treated with S2 iliosacral screws. There were 9 bilateral injuries with a total of 53 S2 screws inserted. Patients ranged in age from 14 to 71 years. Follow-up averaged 19 months (range 6 months to 6 years). Preoperative and postoperative radiographic evaluation included anteroposterior, inlet and outlet pelvic x-rays and two-dimensional computerized tomography (CT) with 3-mm slice thickness. Candidates for S2 screw fixation required adequate space on CT, which was defined as a minimum of 1 cm between foramina on 3 sequential preoperative CT slices in conjunction with inadequate available space in S1.

INTERVENTION

S2 iliosacral screw fixation of disruptions of the posterior pelvic ring.

MAIN OUTCOME MEASUREMENTS

Intraoperative iatrogenic nerve root injuries, postoperative screw position, and maintenance of the fixation construct under physiologic load.

RESULTS

There were no intraoperative iatrogenic nerve injuries. However, postoperative loss of reduction requiring revision surgery occurred in 2 patients with osteopenia. One was associated with injury to the S1 nerve root, which had full return of function within 1 year. Satisfactory screw position was documented on postoperative CT in all cases.

CONCLUSIONS

S2 iliosacral screw fixation is a safe and effective technique. However, it should be used with caution in patients with suspected pelvic osteopenia. Furthermore, any S2 screw with questionable purchase should be removed and an alternative fixation method selected.

The significance of anode location for stimulus-evoked electromyography during iliosacral screw placement

OBJECTIVES

To determine the effect of anode location on the current threshold required to provoke an electromyograph response during stimulus-evoked electromyography for iliosacral screw placement.

DESIGN

Prospective cohort.

SETTING

Level I trauma center.

PATIENTS

Nineteen consecutive patients with 23 unstable posterior pelvic ring injuries treated with iliosacral screws.

INTERVENTION

Iliosacral screws were inserted percutaneously over guidewires. Twenty-seven screws were inserted, all into the first sacral vertebrae. The guidewire was used as the cathode for constant-current, stimulus-evoked electromyography for all data collection. Stimulus-evoked electromyographs were obtained with the guidewire at four different stations: at the sacroiliac joint (station I), at the first sacral neuroforamen (station II), in the body of the sacrum (station III), and when the iliosacral screw was in final position over the guidewire (station IV).

MAIN OUTCOME MEASURE

Stimulus-evoked electromyographs were obtained with the anode at four different locations for each of the implant stations. Location A had the anode adjacent to the percutaneous insertion site of the guidewire, location B at the ipsilateral anterior superior iliac spine, location C at the midline, and location D at the contralateral anterior superior iliac spine.

RESULTS

Moving the anode from midline (location C) toward the entry point of the guidewire increased the current threshold required to provoke an EMG response as much as 67.1% (p < 0.05). Moving the anode from midline to the contralateral anterior superior iliac spine decreased thresholds as much as 3.4% (p > 0.05). In one case, anode placement close to the guidewire insertion site (locations A and B) failed to identify a potentially dangerous implant because current thresholds were >8 mA. With the anode at the midline, current thresholds were <8 mA, indicating unsafe guidewire position leading to redirection of the guidewire.

CONCLUSION

The physical location of the anode during stimulus-evoked electromyography monitoring for iliosacral screw placement significantly changes the current thresholds required to provoke an electromyograph response. Current thresholds required to stimulate nerves increase as the anode is moved toward the stimulating electrode. Anode placement ipsilateral to the stimulating electrode may provide a false indication of safe guidewire placement. We recommend anode location at or beyond the midline for stimulus-evoked electromyography monitoring during iliosacral screw placement.

Staged reconstruction of pelvic ring disruption: differences in morbidity, mortality, radiologic results, and functional outcomes between B1, B2/B3, and C-type lesions

OBJECTIVE

To analyze injury pattern, surgical therapy, radiologic results, and functional outcome in unstable B-type and C-type pelvic ring fractures.

DESIGN

Retrospective study.

SETTING

Level I University Trauma Center.

PATIENTS

Two-hundred-twenty-two consecutive patients, admitted during a nine-year period with unstable B-type (n = 100) and C-type (n = 122) pelvic ring injuries, of whom 122 (61.3 percent of surviving patients) were eligible for evaluation with a minimum follow-up of one year.

INTERVENTIONS

Staged reconstruction dependent upon injury pattern. Emergency external compression of the pelvic ring in case of hemodynamic instability. Management of associated lesions. Secondary open reduction and internal fracture fixation.

MAIN OUTCOME MEASURES

Assessment of perioperative and postoperative mortality and morbidity depending on fracture pattern. Fifty-five B-type and sixty-seven C-type lesions were evaluated clinically and radiologically an average of 21.6 months after trauma.

RESULTS

Perioperative mortality was 5 percent in B-type and 15 percent in C-type fractures. External fixation was part of the definitive treatment in 52 percent of B-type and in 38 percent of C-type lesions. Planned secondary operative procedures were performed in 15 percent of B-type and in 26.2 percent of C-type fractures. Radiologic results were anatomic in 93.5 percent of B1, 75 percent of B2/B3, and 62.7 percent of C-type lesions. Functional results were excellent or good in 74 percent of the B1, 92 percent of the B2/B3, and in 71 percent of the C-type fractures.

CONCLUSIONS

Unstable pelvic ring fractures require a staged approach. Mortality is higher in C-type than in B-type lesions. Functional outcome is worse in C-type than in B-type lesions. Within the B-type group, B1 lesions have a worse functional end result than B2/B3 fractures. These findings are not only related to the stability and symmetry of the pelvic ring, as seen in the radiologic picture, but also depend on the severity and amount of damage to the soft tissues around the pelvis.

Inadvertent sacral bar transfixation of the cauda equina

We present a case of inadvertent sacral bar transfixation of the cauda equina as a complication of posterior pelvic internal fixation. The "safe zone" for sacral bar placement is narrow, especially in the presence of fracture displacement. This complication can be avoided by achieving fracture reduction before internal fixation. Intraoperative fluoroscopy, including inlet and lateral sacral views, should be used in addition to tactile guidance to confirm the reduction. Even after prolonged compression, hardware removal under direct visualization can alleviate radicular symptoms with less effect on motor deficit.

Evaluation of intraoperative nerve-monitoring during insertion of an iliosacral implant in an animal model

BACKGROUND

The use of continuous electromyographic and somatosensory-evoked-potential monitoring systems has been advocated to assist in avoiding nerve-root injury during operations on the pelvic ring. More recently, it was suggested that stimulus-evoked electromyographic monitoring may further decrease the risk of iatrogenic nerve-root injury during posterior pelvic fixation by enabling the surgeon to determine the actual distance of an implant from a nerve root. The purpose of the current study was to evaluate the relative efficacy of these three methods of monitoring for minimizing the risk of neural injury during the placement of iliosacral implants.

METHODS

While the function of the first sacral nerve root was monitored with the use of stimulus-evoked electromyographic, continuous electromyographic, and somatosensory-evoked-potential monitoring techniques, a 2.0-millimeter stainless-steel Kirschner wire was progressively inserted, guided by a high-speed computerized tomographic scanner, into the first sacral body of seventeen hemipelves in nine dogs. The end point was contact with the nerve as demonstrated by the computerized tomographic images. It was expected that this end point would be heralded by a burst of spontaneous electromyographic activity and an abnormal somatosensory-evoked-potential signal. Anatomical dissection at the completion of the study documented the final position of the Kirschner wire.

RESULTS

Anatomical dissection demonstrated compression or penetration of the nerve root in sixteen of the seventeen specimens. A spontaneous burst of electromyographic activity was not recorded for any specimen on continuous electromyographic monitoring; this finding was significantly different from what had been expected (p<0.001). Because of technical problems, somatosensory evoked potentials could be recorded for only twelve hemipelves that had nerve-root compression or penetration, and abnormal somatosensory evoked potentials were recorded for only one of the twelve; this finding was significantly different from what had been expected (p<0.001). A total of 113 stimulus-evoked electromyographic data points were obtained. The correlation coefficient for the relationship between the current threshold recorded with stimulus-evoked electromyographic monitoring and the distance of the wire from the nerve was 0.801 (p<0.001). The actual measured current thresholds were of an observed proportion not different from what had been expected (p = 0.48).

CONCLUSIONS

Continuous electromyographic and somatosensory-evoked-potential monitoring techniques failed to indicate contact with the nerve root reliably in this animal model. However, stimulus-evoked electromyographic monitoring consistently provided reliable information indicating the proximity of the implant to the nerve root.

Intraoperative monitoring with stimulus-evoked electromyography during placement of iliosacral screws. An initial clinical study

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.