Can triggered electromyograph thresholds predict safe thoracic pedicle screw placement?
METHODS
A prospective clinical study of thoracic pedicle screws monitored with triggered electromyographic testing.


OBJECTIVE
To evaluate the sensitivity of recording rectus abdominis triggered electromyographs to assess thoracic screw placement.


BACKGROUND
Triggered electromyographic testing from lower extremity myotomes has identified medially placed lumbar pedicle screws. Higher thresholds indicate intraosseous placement because of increased resistance to current flow. Lower thresholds correspond to compromised pedicles with potential for nerve impingement. No clinical study has correlated an identical technique with rectus muscle recordings, which are innervated from T6 to T12.


METHODS
A total of 677 thoracic screws were placed in 92 consecutive patients. Screws placed from T6 and T12 were evaluated using an ascending method of stimulation until a compound muscle action potential was obtained from the rectus abdominis. Threshold values were compared both in absolute terms and also in relation to other intrapatient values.


RESULTS
Screws were separated into three groups: Group A (n = 650 screws) had thresholds >6.0 mA and intraosseus placement. Group B (n = 21) had thresholds <6.0 mA but an intact medial pedicle border on reexamination and radiographic confirmation. Group C (n = 6) had thresholds <6.0 mA and medial wall perforations confirmed by tactile and/or visual inspection. Thus, 3.9% (27 of 677) of all screws had thresholds <6.0 mA. Only 22% (6 of 27) had medial perforation. Group B screws averaged a 54% decrease from the mean as compared with a 69% decrease for Group C screws (P = 0.0160). There were no postoperative neurologic deficits or radicular chest wall complaints.


CONCLUSIONS
To assess thoracic pedicle screw placement, triggered electromyographic thresholds <6.0 mA, coupled with values 60-65% decreased from the mean of all other thresholds in a given patient, should alert the surgeon to suspect a medial pedicle wall breach.
Raynor
Barry L
BL
Department of Orthopaedic Surgery, Washington University Medical Center, BJC Health Systems, St. Louis, Missouri 63110, USA.
Lenke
Lawrence G
LG
Kim
Yongjung
Y
Hanson
Darrell S
DS
Wilson-Holden
Tracy J
TJ
Bridwell
Keith H
KH
Padberg
Anne M
AM
D000200:Action Potentials / Q000502:physiology; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001863:Bone Screws / Q000009:adverse effects*; D002648:Child; D004558:Electric Stimulation / Q000379:methods; D004576:Electromyography / Q000379:methods*; D006801:Humans; D008875:Middle Aged; D016343:Monitoring, Intraoperative / Q000379:methods; D011183:Postoperative Complications / Q000209:etiology / Q000517:prevention & control; D011237:Predictive Value of Tests; D011446:Prospective Studies; D017568:Rectus Abdominis / Q000294:innervation / Q000502:physiology; D012680:Sensitivity and Specificity; D012684:Sensory Thresholds / Q000502:physiology; D013119:Spinal Cord Injuries / Q000209:etiology / Q000517:prevention & control*; D013122:Spinal Diseases / Q000601:surgery; D013123:Spinal Fusion / Q000009:adverse effects* / Q000295:instrumentation / Q000379:methods; D013904:Thoracic Vertebrae / Q000294:innervation / Q000601:surgery*