Pedicle Screw Placement
Different concerns for cervical, thoracic, and lumbar placed screws;
Concern for spinal cord protection warrants the use of SEP and MEP in cervical and thoracic regions;
EMG may be useful in thoracic levels but loss of a single thoracic root has minor consequences.
EMG definitely indicated in cervical areas; paralysis of upper extremity muscles and/or diaphragm devastating
lumbar - avoid radiculopathy, no spinal cord at this level L2 in most adults, worst case is pain or weakness in the distribution of a single root
EMG in lumbosacral screws; drill, probe, screw itself; detects misplaced screws better than x-ray; published criteria range from 6-10mA
EMG in appropriate muscles for each segmental level
thoracic - avoid damage to spinal cord and artery of Adamciewicz; worst case lower extremity paralysis, loss of bowel and bladder function; radiculopathy of lower concern
EMG in appropriate muscles for each segmental level
cervical - avoid damage to spinal cord and cervical roots; worst case quadripilegia; the higher you go the lower you can fail
EMG in appropriate muscles for each segmental level
Stimulation
directly stimulate roots to confirm the system, thresholds should be low ~1mA; radiculopathy will increase thresholds
stimulate via drill probe and/or screw; monopolar, cathode to screw, before any linking
Thresholds; greater than 10/12mA is probably okay; 6-10mA is questionable/surgeon discretion, < 6mA probably should be replaced; bone wax to insulate
Stimulation thresholds for intact pedicles may be lower, due to thinner bone even with perfect placement
can use the same electrodes for tcMEP stimulation
Stimulation of pedicle screw
probe on the screw itself, not the adjustable cap
threshold for good screw may be slightly lower than gearshift, due to thinner bone where the screw threads cut
warning criterion; < 6mA suspect, 6-10 mA possible suspect, >11 probably okay
Good placement versus bad placement of screws/hardware; insertion within pedicle; medial and lateral breaches
False positives and false negatives; research shows 81.9% moderate accuracy
thresholds < 6mA likely means medial pedicle wall breach, should alert surgeon to suspect
note that the stimulus duration = 300 us
Examples of false negatives;
image shows medial breach but threshold was 19mA
image shows anterior breach but threshold was 22mA
note there are no neural structures anterior to vertebral body; aorta and vena cava but they don't stimulate
Examples of true positives;
lateral breach observed; EMG threshold was 11mA
no breach observed; EMG threshold was 19mA
Examples of well placed screws
Examples of misplaced screws
Concern for spinal cord protection warrants the use of SEP and MEP in cervical and thoracic regions;
EMG may be useful in thoracic levels but loss of a single thoracic root has minor consequences.
EMG definitely indicated in cervical areas; paralysis of upper extremity muscles and/or diaphragm devastating
lumbar - avoid radiculopathy, no spinal cord at this level L2 in most adults, worst case is pain or weakness in the distribution of a single root
EMG in lumbosacral screws; drill, probe, screw itself; detects misplaced screws better than x-ray; published criteria range from 6-10mA
EMG in appropriate muscles for each segmental level
- L1 - Iliopsoas
- L2-4 quadriceps and adductors
- L5 - tibialis anterior
- S1 - hamstrings, gastrocnemius
- S2 - Intrinsic foot muscles
- S3 - anal sphincter
- larger pedicles
- SEP probably uses due to cross-innervation of multiple roots; spinal cord ends at L2
- MEPs give specificity
- pediatric patients and/or tethered cord: may have more than just roots in lumbar region
thoracic - avoid damage to spinal cord and artery of Adamciewicz; worst case lower extremity paralysis, loss of bowel and bladder function; radiculopathy of lower concern
EMG in appropriate muscles for each segmental level
- T1 - flexor carpi ulnaris, instrinsic muscles of the hand, thenar/hypothenar
- T2-6 - upper and lower intercostals
- T6-12 - abdominals/rectus abdominis; overlap of information, move as a pack; a single pair should suffice; easier in non obese patients
- smaller pedicles
- place needles subdermally, be careful of a pneuomothorax
- separate pairs for left and right side; upper and lower intercostals
- protect the cord!
- SEP, MEP, EMG all indicated
- high rate of false positives
- anterior breaches may have significant clinical consequences but may not be detected by EMG due to the distance to nerves
cervical - avoid damage to spinal cord and cervical roots; worst case quadripilegia; the higher you go the lower you can fail
EMG in appropriate muscles for each segmental level
- XI, C1-4 - Spinal Accesory/Trapezius/deep muscles of the neck which are not easily accessible
- C5,C6 - Deltoid, Biceps
- C7 - triceps, flexor carpi ulnaris, extensor carpi radialis
- C8 - abductor pollicis brevis, flexor digiti minimi; intrinsic muscles of the forearm and hand
- smaller pedicles
- protect the cord!
- SEP, MEP, EMG all indicated
- stealth guidance recommended for cervical levels
- spinal accessory nerve arises between dorsal and ventral roots from C1-5, up through the foramen magnum and out the jugular foramen to the trapezius and sternocleidomastoid
- C3-4; may be able to record from the diaphragm at the Xiphoid; phrenic nerve
- EMG activity at the phrenic nerve; evoked response from stimulation; can possibly elicit with transcranial stimulation
- can overlap needle electrodes, so placement is not critical; trapezius, deltoid, biceps/triceps, fcu/ecr, thenar/hypothenar
- warning thresholds; usually warn of sustained activity or large spikes in EMG activity; surgeon preference
- stealth guidance is often recommended for the placement of cervical pedicle screws; high risk if the pedicle diameter is < 4.5mm
- stealth; reference fixed to spinous process, system registered with surface landmarks; place probe on the surface of the pedicle, orient until trajectory is within the pedicle; can even image trajectory while drilling
- probing the hole with the gear shift; calibrated to indicate depth, correlate with the thickness of the vertebra on scan; determine appropriate length of screw; BRAINLAB
Stimulation
directly stimulate roots to confirm the system, thresholds should be low ~1mA; radiculopathy will increase thresholds
stimulate via drill probe and/or screw; monopolar, cathode to screw, before any linking
Thresholds; greater than 10/12mA is probably okay; 6-10mA is questionable/surgeon discretion, < 6mA probably should be replaced; bone wax to insulate
Stimulation thresholds for intact pedicles may be lower, due to thinner bone even with perfect placement
can use the same electrodes for tcMEP stimulation
Stimulation of pedicle screw
probe on the screw itself, not the adjustable cap
threshold for good screw may be slightly lower than gearshift, due to thinner bone where the screw threads cut
warning criterion; < 6mA suspect, 6-10 mA possible suspect, >11 probably okay
Good placement versus bad placement of screws/hardware; insertion within pedicle; medial and lateral breaches
False positives and false negatives; research shows 81.9% moderate accuracy
thresholds < 6mA likely means medial pedicle wall breach, should alert surgeon to suspect
note that the stimulus duration = 300 us
Examples of false negatives;
image shows medial breach but threshold was 19mA
image shows anterior breach but threshold was 22mA
note there are no neural structures anterior to vertebral body; aorta and vena cava but they don't stimulate
Examples of true positives;
lateral breach observed; EMG threshold was 11mA
no breach observed; EMG threshold was 19mA
Examples of well placed screws
Examples of misplaced screws