Triggered Electromyographic Threshold for Accuracy of Thoracic Pedicle Screw Placement in a Porcine Model
SJ Lewis, MD,
LG Lenke, MD,
BL Raynor, J Long, DVM,
KH Bridwell, MD,
KD Riew, MD,
AM Padberg, MS
Washington University School of Medicine,
Department of Orthopaedic Surgery, St. Louis, MO, USA
INTRODUCTION:
The use of pedicle screws is increasing in the thoracic spine.
Misplaced thoracic pedicle screws may have significant implications
if the spinal cord is injured. Triggered electromyographic (EMG)
stimulation has been a valuable aid in determining appropriate
placement of lumbar pedicle screws (Spine 1995;20:1585). The
purpose of this study was to determine if established criteria
used to assess pedicle screw placement in the lumbar spine were
applicable to pedicle screws inserted in the thoracic spine in
an animal model.
METHODS:
Four 120–150 lbs domestic pigs had 45 pedicle screws placed
bilaterally in the thoracic spine. Pedicle screws were first
inserted entirely in the pedicle (Group A). The medial pedicle
wall was then removed and the screw was placed with no osseous
bridge between the screw and the neural tissue, however, with
no contact to the neural tissue (Group B). Finally, the screws
were placed medial to the pedicle with purposeful contact to
the nerve root and the spinal cord (Group C). Pairs of ½”
subdermal needle electrodes were placed in the rectus abdominus
and intercostal muscles for each instrumented spinal level. An
ascending method of stimulation using constant current (mA) was
applied to each screw to obtain a compound muscle action potential
(CMAP). Threshold intensity and muscle(s) responding were recorded
for each trial.
RESULTS:
In 38 of the 45 screws, there was a relatively consistent decrease
in the triggered EMG response from Group A (mean 4.62±0.25
mA) to Group C (mean 2.08±0.18 mA) screws (p<0.05).
There was little difference in the response obtained from Group
A to Group B (mean 4.17±0.28 mA) screws (p>0.05). In
Group B screws, 15 of 38 (40%) screws had thresholds greater
than their respective A screw. In the case of 7 screws, the nerve
root was felt to be injured by the technique of breaking the
medial pedicle wall. In these screws, the responses obtained
from the Group B (mean 6.32±1.21 mA) and C screws (mean
6.36±0.68 mA) were significantly greater than the responses
obtained in Group A (mean 3.27±0.65 mA, p<0.05), suggesting
that a greater threshold was required to obtain a response from
the injured root.
DISCUSSION:
There was a mean 53.4 ± 3.8 % decrease in the EMG threshold
for the screws with neural contact compared to the screws properly
placed in the pedicle, however, the individual threshold for
each screw was variable. Five of the 38 (13%) properly placed
screws had thresholds below 3.0mA, with all but one of these
values being higher than the corresponding C screw. There were
five C screws (13%) placed in direct contact with the dura that
scored greater than 3.0mA, with all of these thresholds lower
than their respective A screw. Similarly, 28 of 38 B screws (74%)
had thresholds greater than 3.0mA. Even though in an individual
pedicle there was a consistent decrease between the A and C screws,
we could not determine a cut–off trigger EMG level that
would consistently differentiate properly from improperly placed
pedicle screws. Furthermore, this method could not differentiate
screws clearly in the pedicle with screws with medial pedicle
wall breakthrough.
CONCLUSIONS:
We do not feel this animal model of root recordings to be reproducible
in identifying misplaced thoracic pedicle screws. A more direct
method of spinal cord monitoring must be established to provide
the surgeon with early warning of the potential of neural injury
in the placement of thoracic pedicle screws.