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

Repeated L5 Nerve Root Compromise Detected with Motor Evoked Potentials (MEP), but Not Electromyography (EMG): A Case Report

We report a case where neuromonitoring, using motor evoked potentials (MEP), detected an intraoperative L5 nerve root deficit during a lumbosacral decompression and instrumented fusion procedure. Critically, the MEP changes were not preceded nor accompanied by any significant spontaneous electromyography (sEMG) activity. Presumptive L5 innervated muscles, including tibialis anterior (TA), extensor hallucis longus (EHL) and gluteus maximus, were targets for nerve root surveillance using combined MEP and sEMG techniques. During a high-grade spondylolisthesis correction procedure, attempts to align a left-sided rod resulted in repeated loss and recovery cycles of MEP from the TA and EHL. No accompanying EMG alerts were associated with any of the MEP changes nor were MEP variations seen from muscles innervated above and below L5. After several attempts, the rod alignment was achieved, but significant MEP signal decrement (72% decrease) remained from the EHL. Postoperatively, the patient experienced significant foot drop on the left side that recovered over a period of 3 months. This case contributes to a growing body of evidence that exclusive reliance on sEMG for spinal nerve root scrutiny can be unreliable and MEP may provide more dependable data on nerve root patency.

Evaluation of Partial Cut-out of Sacroiliac Screws From the Sacral Ala Slope via Pelvic Inlet and Outlet View

STUDY DESIGN

An anatomic and radiographic study of placement of sacroiliac screws.

OBJECTIVE

The aim of this study was to quantitatively assess the risk of partial cut-out of sacroiliac screws from the sacral ala slope via inlet and outlet view.

SUMMARY OF BACKGROUND DATA

The partial cut-out of sacroiliac screws from the superior surface of sacral ala can jeopardize the L5 nerve root, which is difficult to identify on the pelvic inlet and outlet views.

METHODS

Computed tomography images of 60 patients without pelvic ring deformity or injury were used to measure the width (on inlet view) and height (on outlet view) of the sacral ala. The angle of the sacral ala slope was measured on lateral view. According to the measured parameters, the theoretical safe trajectories of screw placement were calculated using inverse trigonometric functions. Under fluoroscopic guidance, a sacroiliac screw was placed close to the midline on both inlet and outlet views, including posterosuperior, posteroinferior, anterosuperior, and anteroinferior regions to the midline. The incidence of screw partial cut-out from the superior surface of sacral ala was identified.

RESULTS

The measured widths and heights of the sacral alas were 28.1 ± 2.8 and 29.8 ± 3.1 mm, respectively. The average angle between the superior aspect of the S1 vertebral body and the superior aspect of the sacral ala was 37.2 ± 2.5 degrees. The rate of partial cut-out of the screws from the superior surface of sacral ala slope was 12.5% (5/40) in posterosuperior, 0% (0/40) in posteroinferior, 70% (28/40) in anterosuperior, and 20% (8/40) in anteroinferior.

CONCLUSION

To avoid the risk of partial cut-out from sacroiliac screw placement, more precise description should be added to the conventional description: the sacroiliac screws should be placed at the inferior half portion on outlet view and at the posterior half portion on inlet view.

LEVEL OF EVIDENCE

4.

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.

Sacroiliac secure corridor: analysis for safe insertion of iliosacral screws

Objective

Posterior pelvic lesions, especially of the sacral-iliac joint, have high mortality and morbidity risks. Definitive fixation is necessary for the joint stabilization, and one option is the sacral percutaneous pinning with screws. Proximity to important structures to this region brings risks to the fixation procedure; therefore, it is important to know the tridimensional anatomy of the pelvis posterior region. Deviations of the surgeon's hand of four degrees may target the screws to those structures; dimorphisms of the upper sacrum and a poor lesion reduction may redound in a screw malpositioning. This study is aimed to evaluate the dimensions of a safe surgical corridor for safe sacroiliac screw insertion and relations with age and sex of the patients.

Method

One hundred randomly selected pelvis CTs of patients with no pelvic diseases, seen at a tertiary care teaching Hospital. Measurements were made by computer and the safest area for screw insertion was calculated by two methods. The results were expressed in mm (not in degrees), in order to be a further surgical reference.

Results

There was a significant size difference in the analyzed sacral vertebra, differing on a wider size in men than in women. There was no significant statistical difference between vertebral size and age. By both methods, a safe area for screw insertion could be defined.

Conclusion

Age does not influence the width of the surgical corridor. The surgeon has a safe corridor considered narrower when inserting screws in a female pelvis than when in a male one. However, as the smallest vertebra found (feminine) was considered for statics, it was concluded that this corridor is 20 mm wide in any direction, taking as a reference the centrum of the vertebra.

Iliosacral screw placement: are uniplanar changes realistic based on standard fluoroscopic imaging?

BACKGROUND

With the aim of improving the understanding of iliosacral screw placement, two hypotheses were tested: (1) standard intraoperative inlet and outlet images are not based on orthogonal coordinates, and (2) therefore making starting point and aim changes by moving perpendicular to the c-arm beam will displace the guide wire on the other intraoperative radiographic view.

METHODS

This is a prospective case series with review of intraoperative data from consecutive patients treated at a University Level I trauma center. The study group included ten consecutive patients with nondysmorphic upper sacral segments and unstable posterior pelvic ring injuries that required surgical treatment. Posterior surgical stabilization included iliosacral screw placement using a standardized three- view technique in the supine position. The main outcome measurement included the angles from the perpendicular required to achieve what have been considered the ideal inlet and outlet views intraoperatively. The angle arc for each patient created by the recorded angles was then determined.

RESULTS

The average sagittal plane tilt required to achieve the ideal inlet view was 25 degrees (range, 21-33 degrees). The average sagittal plane tilt required to achieve the ideal outlet view was 42 degrees (range, 30-50 degrees). The average arc between the ideal inlet and outlet views was 67 degrees (range, 62-76 degrees). These views never created an orthogonal system.

CONCLUSION

We commonly work in orthogonal systems. Within these systems, it is possible to make a uniplanar correction by moving perpendicular to one plane or radiographic view. The ideal views to image the safe zone for iliosacral screw placement do not create an orthogonal system. When this average angle arc is placed on a graphic model of the pelvis, it becomes clear that the plane of the radiographic beam of the ideal inlet view is collinear with the anterior aspect of the upper two sacral bodies. The outlet view is oblique to the upper sacral bodies. Surgeons must keep this in mind when using fluoroscopic views to insert iliosacral screws.

Locked transsacral screw fixation of bilateral injuries of the posterior pelvic ring: initial clinical series

OBJECTIVES

The purpose of this study was to evaluate the clinical safety and efficacy of using a cannulated transsacral screw having a novel locking capability for pelvic fracture fixation.

DESIGN

Retrospective analysis of a treatment protocol, patient series.

SETTING

Level I trauma center.

PATIENTS

Beginning in 2001, 10 patients with bilateral injury to the posterior pelvic ring were treated using a cannulated transsacral screw having a novel locking capability. Patients ranged in age from 21 to 64 years. Follow-up averaged 2 years (range, 1-5 years). Preoperative and postoperative radiographic evaluation included anteroposterior, inlet and outlet pelvic x-rays, and two-dimensional computerized tomography with 3-mm slice thickness. Candidates for this fixation required adequate space estimated on computerized tomography across either the first or second sacral body.

INTERVENTION

Locked transsacral screw fixation of bilateral injuries of the posterior pelvic ring consisting of a long 7.0-mm cannulated screw inserted over a washer from the near ilium, across one sacroiliac joint, through the body of the sacrum, and across the other sacroiliac joint, exiting the far iliac cortex. A self-locking nut was placed on the distal end of the screw.

MAIN OUTCOME MEASUREMENTS

Intraoperative iatrogenic nerve root injuries, postoperative screw position, and maintenance of the fixation construct until fracture healing.

RESULTS

There were no iatrogenic nerve injuries. Satisfactory screw position was documented on the postoperative computerized tomography in all cases. Fixation failure did not occur and satisfactory pelvic ring position was maintained in all cases.

CONCLUSIONS

Locked transsacral screw fixation is a safe and effective technique that should be added to our surgical armamentarium. Indications include bilateral posterior injury as well as any situation in which routine transsacral screw fixation might otherwise be considered such as the presence of pelvic osteopenia or insufficient space for a second point of posterior fixation.

Radiographic quantification and analysis of dysmorphic upper sacral osseous anatomy and associated iliosacral screw insertions

OBJECTIVE

To quantify upper sacral dysmorphic osseous anatomy and assess its impact on second sacral segment iliosacral screw insertion.

DESIGN

Retrospective evaluation of a prospective trauma database.

SETTING

Regional Level I trauma center.

PATIENTS

Twenty-four patients with unstable posterior pelvic ring disruptions and sacral dysmorphism were evaluated radiographically and second segment (S2) screws were placed using a standard technique.

MAIN OUTCOME MEASUREMENTS

The sacral osseous pathway limits were measured using preoperative pelvic computed tomography at the upper and second sacral segments. The S2 screw location relative to the sacral nerve root tunnels and the maximum possible screw lengths for both S1 and S2 screws were evaluated with postoperative pelvic computed tomography. The S2 screw positions were graded as intraosseous, juxtaforaminal, or extruded. Preoperative and postoperative peripheral neurologic examinations were documented.

RESULTS

The dysmorphic S1 width available for screw insertion averaged 13.2 mm. The S2 pathway width averaged 15.2 mm. The maximum potential screw length for the dysmorphic S1 averaged 100.8 mm and for S2 measured 151.9 mm. Twenty of 24 patients with S2 screws were intraosseous and in four patients were juxtaforaminal. There were no extruded screws. There were no neurologic injuries.

CONCLUSIONS

Dysmorphic S1 segments are anatomically competent for routine screw fixation. The S2 segment provides a larger osseous site for screw insertion than S1 in dysmorphic sacrums. Significantly longer screws are possible in S2 compared with the dysmorphic S1 segment. S2 iliosacral screws can be safely and accurately accomplished using a standard technique in patients with unstable posterior pelvic ring disruptions and sacral dysmorphism. Safe screw insertions avoid iatrogenic nerve root injuries.

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.

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.