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4 Spinal Imaging and Diagnostic Tests
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Fi g . 4 . 8 ( a – c ) Sa g it tal m ag ne t ic re s o n a n ce im ag in g d e m on st rat in g t yp e 1 Mo d ic ch a n g es. (Re pro -
duced from Imhof H, et al. Spinal Imaging. Direct Diagnosis in Radiology Series. Stuttgart, Germany:
Ge o rg Th ie me Ve rlag ; 2 0 0 8 : Figs . 3 .4 and 3 .5, wit h p e rm iss io n .)
Fi g . 4 . 9 Sag it t al m ag n e t ic reso n an ce im ag in g d e mo n strating type 2 Modic changes. (Reproduced from
Uh le nb ro ck D. MR Im ag in g of t h e Sp ine a n d Sp in a l
Co rd . St ut t ga r t, Ge rm a ny: Ge org Th ie m e Ve rla g ; 2 0 04 :
Fig . 4 .3 1 , w it h p e rm issio n .)
4 Spinal Imaging and Diagnostic Tests 77
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Fi g . 4 . 1 0 ( a , b ) Sa g it tal m ag n e t ic re so n a n ce im ag ing d e mo n st rat ing t yp e 3 Mod ic ch a n g es. (Re pro -
duced from Uhlenbrock D. MR Imaging of the Spine and Spinal Cord. Stuttgart, Germany: Georg
Th ie m e Verla g ; 2 0 0 4 : Fig . 4 .3 3 , w it h p e rm issio n .)
4.2 Electrodiagnostic Tests
I. Ele ct rom yog r a ph y (EMG) a n d n er ve con du ct ion st u d ies (NCSs).
A. EMG/ NCS on ly e va lu at es t he m ot o r t r a ct s o f t h e n e r ve r o o t .
1. Radiculopathy may also involve motor, sensory, and autonomic bers of the
nerve root.
B. Co m p o u n d m u s c le a ct io n p o t e n t ia l in p er ip h e r al n e r ve s w ill s h o w r e d u c e d
amplitude in proportion to the amount of axonal degeneration that occurs after
a compressive nerve root lesion.
C. Co m p o u n d m u scl e a ct io n p ot e n t ia l in p er ip h e ra l n e r ve s is m o r e m a rke d w h en
multiple roots are involved, as in lumbar spinal stenosis.
D. Ner ve con du ction velocit y or latency sh ou ld not be a ected by a focal proxim al
lesion, such as in radicu lopathy.
E. Th e go ld s t an d a r d fo r e le ct r od ia g n osis o f r a d icu lo p at h y is n e e d le EMG.
1. The earliest EMG nding in acute radiculopathy is a decrease in the number
of motor unit potentials seen on recruitment.
2. An increase in the number of polyphasic motor unit potentials may be seen
early.
3. Prolonged H re ex latency for C7 or S1 roots and reduced number of F waves
in weak muscles may be obser ved after several days of radiculopathy.
4. Spontaneous motor activity, brillations, and positive F waves are the
hallmarks of acute radiculopathy.
5. Large, long-duration, polyphasic potentials indicate reinnervation.
6. As the radiculopathy resolves, polyphasic potentials tend to reduce in the
number of phases, but the motor unit potentials may remain larger and
be of longer duration than the normal motor unit potentials of uninvolved
muscles.
78
4 Spinal Imaging and Diagnostic Tests
F. In d i c a t i o n s fo r EM G / N CS :
1. Clinical ndings suggest other neurological disorders, such as anterior horn
cell disease, ner ve entrapm ent syndrom e, cer vical sten osis, am ong others.
2. Imaging studies and clinical ndings do not correlate well in patients with
suspected radiculopathy.
3. In cases where neurological progression or deterioration must be
documented.
II. Som at o se n sor y evoke d p ot en t ia ls (SSEPs) an d m ot o r evoke d p ote n t ia ls .
A. Ass e ss s e n so r y t ra c t s fr om a p er ip h e ra l n e r ve t o t h e p ost e r io r colu m n o f t h e
spinal cord (Fig. 4.11).
B. Mo st co m m o n ly u se d a s a n in t ra op er at iv e m o n it o r in g t e c h n iq u e t o p ro t e ct t h e
spinal cord during surgery.
C. De r m a t om a l SSEPs c a n b e u se d t o m o n it or n er ve r o o t fu n ct io n d u r in g su rge r y.
D. Mot or evoke d pot e n tials assess t h e m ot or p ath way in th e spin al cord,
particularly during anterior procedures of the spine that may jeopardize the
anterior part of the spinal cord.
Fi g . 4 . 1 1 Co r t ic a l s o m at o se nso ry-e vo ke d p ot en t ia l fro m t h e p ost erio r co lu m n s . (Re p ro d uce d fr o m
Devlin VJ, Schwart z DM. Int ra o p e ra t ive n e u ro p hysio lo gic m on it o r y d u rin g spinal su rg e r y. J Am Acad
Ort hop Surg 2007;15(9):549–560, with permission.)
4 Spinal Imaging and Diagnostic Tests 79
Sugg este d Reading
Be rq u i s t TH. Im ag i n g o f t h e p ost o p e r a t ive sp in e . Ra d io l Cli n No r t h Am 2 0 0 6 ; 4 4 (3 ):
407–418
de Graaf I, Prak A, Bierma-Zeinstra S, Thomas S, Peul W, Koes B. Diagnosis of lumbar
spinal stenosis: a system atic review of the accuracy of diagnostic tests. Spine 2006;
31(10):1168–1176
Haughton V. Imagin g in tervertebral disc degen eration. J Bone Join t Surg Am 2006 ;
88(Sup pl 2):15– 20
Ja y a k u m a r P, Nn a d i C, Sa ifu d d in A, M a c s w e e n e y E, Ca s e y A. Dy n a m i c d e ge n e r at ive l u m -
bar spondylolisthesis: diagnosis with axial loaded magnetic resonance imaging. Spine
2006;31(10):E298–E301
5 Intraoperative Neuromonitoring
5.1 General Considerations
I. In t r o d u ct ion .
A. Co n t e m p o ra r y in t r a o p e r at ive n e u ro p hys io logic a l m o n it o r in g (IONM ) o f t h e
spinal tracts stem s from advancem ents made in the 1970s.
B. Pr io r t o m o d er n m od alit ie s, t h e St ag n a r a w a k e - u p t e s t w a s t h e go ld s t a n d a rd
for t h e a ssess m en t of global m ot o r fu n ct ion .
1. Characterized by awakening the patient to assess motor function.
2. Patient was awakened after critical points during the procedure (e.g.,
implant placem ent, cu rve reduction ).
a. Limited applications outside of scoliosis correction.
3. Signi cant limitations of the wake-up test.
a. Poor patient cooperation.
b. Lack of real-time monitoring.
c. False-negatives.
d. Venous air embolism.
e. Inability to detect delayed neurological insults.
f. In abilit y t o t ake exp e d it iou s cor r e ct ive m easu re s.
C. So m at o s e n s o r y e vo k e d p ot en t ia l ( SSEP) m o n i t or in g w as t h e r st a dva n ce m e n t
in IONM.
1. Sensory dorsal column–medial lemniscus pathway.
D. Recen t advan ces in clu de the assessm en t of t ranscran ial m otor evoked p ote n tials
(tcMEPs) and electromyography (EMG).
1. Enable real-time monitoring of the motor tracts and nerve roots and limit
signal interference from anesthesia.
E. Nu m er o u s va r ia b le s co n fo u n d IONM.
1. Anesthesia (most common).
2. Body temperature.
3. Degree of neural development (age, neuromuscular comorbidities, delayed
development).
4. Mean arterial pressures.
5. Medications.
6. Length of procedure.
80
7. Personnel expertise.
II. Som at o s e n sor y evoke d p ote n t ia ls (SSEPs):
A. Elicit e d by t h e st im u la t ion o f a p e r ip he r a l n er ve , t yp ic a lly e it h er t he p ost e r io r
tibial, peroneal, ulnar, or median nerves.
B. Th e s ig n a l t r ave r s es t h e d o r sa l co lu m n m e d ia l le m n is cu s p a t h w ay in t h e s p in a l
cord to the brain .
C. Th e a sc e n d in g s ign a l is c a p t u r e d b y a n a m p li e r p la c e d o n t h e h e a d , w h ic h t h en
records cortical SSEPs.
5 Intraoperative Neuromonitoring 81
1. Subcortical responses can also be tracked by placing the ampli er electrodes
on the anterior or posterior neck.
2. Subcortical SSEPs are thought to be less inhibited by the e ects of anesthesia
and the degree of neural development.
D. Am p lit u de redu ction of > 50% or late n cy o f m ore t h an 10% of b aselin e is cau se
for co n ce rn .
E. Sign i c a n t lim it at io n s o f SSEPs:
1. Not true real-time monitoring, because the potentials must summate when
they are recorded.
2. Lack of motor track monitoring.
3. Signi cantly a ected by neuroanesthesia.
III. Tr a n scr a n ial m ot o r evoke d p ot en t ials:
A. Re co r d t h e d es ce n d in g m ot o r co r t icosp in al t ra ct s .
B. Ch a r a c t e r ize d b y t r a n sc r a n ia l s t im u la t io n .
C. A r e co r d in g ele ct ro d e is p la ce d in t h e s u bd u ra l s p a c e o f t h e sp in e, o r a
subderm al electrode can be used to record the peripheral musculature.
1. Descending stimulation can summate temporally or spatially to trigger
skeletal muscle action, thereby producing compound muscle action
potentials (CMAPs).
2. Epidural monitoring is characterized by monitoring the D-wave, which
represents direct stim ulation of the corticospinal neurons.
D. Myoge n ic MEPs (m MEPs):
1. Assess the nerve roots and peripheral nerves.
E. A lo s s o f MEPs o r a s u d d en d e cr e a se in 7 5% o f a m p lit u d e is ch ar a ct e r ist ic o f
neural insult.
IV. Elect rom yograp hy:
A. Pla ce m e n t o f co n ce n t r ic n ee d le e le ct ro d e s in t o t h e e x t r e m it y m u sc u la t u re
to record amplitude, frequency, duration, and shape of the motor unit action
potential.
1. Assesses spinal nerve root insults due to stretch or direct injury.
2. Records changes in the pattern of motor unit action potentials as a result of
nerve root injury/irritation.
3. Provides real-time information to the surgeon to reverse any noxious stimuli
to the nerve roots.
a. False negatives are often encountered if nerve root injury results from
vascular injury or prolonged stretch inju r y, which m ay have a delayed
presentation.
B. Tr ig g e r e d EM G ( t EMG) .
1. Used during implant placement with transpedicular screw xation.
2. A current is applied to the pedicle screw head after placement, and the
CM APs o f t h e co r re sp o n d in g n e r ve r oo t m u sc u la t u re a re r e co r d e d .
3. Assess the signal intensity required to depolarize the musculature:
a. With pedicle wall breach, the stimulus magnitude is signi cantly lower
than control values, because there is less impedance of current ow to
the neural elements.
b. Less than 10 milliamps may be indicative of a medial wall breach.
4. Continuous EMG is used while the surgeon is traversing the psoas muscle in
an e ort to prevent lumbosacral plexus injury.
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5 Intraoperative Neuromonitoring
V. A n e s t h e t i c e e c t s :
A. Ne e d t o s t r ike a b a la n ce t o p ro v id e ad e qu at e a n e s t h e sia w h ile e n s u rin g re lia b le
and meaningful IONM.
B. In h a la t io n al a ge n t s , in clu d i n g s e vo u ra n e , iso u ra n e , a n d d es u r a n e, a re o ft e n
implicated for producing latency in SSEPs.
C. To t a l in t r ave n ou s a n e s t h e s ia (TIVA) i s o ft e n u s e d in co nju n ct io n w it h IONM t o
prevent suppression of cortical responses.
D. Neu rom u scu lar blockin g agen t s in t e r fere w it h t he acqu isit ion a n d reliabilit y of
MEPs and EMG recordings.
1. There is considerable controversy regarding the use of partial
neuromuscular blocking agents in conjunction with IONM.
VI. Mu lt im od al m on it or in g w it h SSEPs a n d MEPs is t yp ica lly u se d a t m ost ce n t e r s t o
account for limitations of each modality.
VII. Cor re ct ive m eas u r e s:
A. If a n in t r a o p e r at ive n eu rom o n it o r in g a le r t ar is es , t h e r st s t ep s in vo lve
hemodynamic stabilization.
1. Maintain MAPs at least around 60 mm Hg and temporarily raise pressures as
an initial step.
2. Check the patient’s body temperature.
3. Check for technical issues.
a. Patient positioning.
b. Monitoring equipment malfunction.
c. Drug infusion dosing.
d. Lines.
B. Re ve rs e p re vio u s s u r gica l m a n e uve r (i m p la n t p la ce m e n t , c u r ve c o r r ec t io n , r od
placement, distraction).
C. If n on e o f t h e co r r e ct ive m e a s u re s r e so lve s t h e is su e, a n in t r a op e r a t ive w ake - u p
test should be considered.
D. Th ere is considera ble con troversy regard ing w het her t he case should b e
discontinued if neuropotentials cannot be restored.
Sugg este d Reading
Cla r k AJ, Zi e w ac z JE, Sa fa e e M , e t a l. In t r a o p e r at ive n e u ro m o n it o r in g w i t h M EPs a n d p r e -
diction of postoperative neurological de cits in patients undergoing surgery for cervical and cer vicoth oracic m yelopat hy. Neu rosurg Focus 201 3;35(1):E7
Fe h li n gs M G, Br od ke DS, No r ve ll DC, De t t o r i JR. Th e e vid e n ce fo r in t ra o p e r a t ive n e u r o -
physiological monitoring in spine surgery: does it make a di erence? Spine 2010;
35(9, Su pp l):S37 –S46
Mich ael KW, Bod e n SD. Int raop erative neurom on itor ing in spin e su rge r y. Con tem p Spin e
Surg 20 12:13
Zie w a c z JE, Be r ve n SH, Mu m m a n e n i VP, et al. Th e d es ig n , d eve lo p m en t , a n d im p le m en -
tation of a checklist for intraoperative neuromonitoring changes. Neurosurg Focus
2012;33(5):E11
6 Biomechanics of the Spine and
Spinal Instrume ntatio n
6.1 General Considerations
I. In t r o d u ct ion .
A. Fu n c t ion al sp in al u n it .
1. Intervertebral disk.
2. Adjacent vertebral bodies.
3. Facet joint complex.
B. Sp in al s t ab ilit y.
1. Under physiological loading, there is neither abnormal strain nor excessive
motion in the functional spinal unit.
2. This stability is maintained by the bony and ligamentous components of the
function al spin al unit, muscular tension , abd om inal an d thoracic pressu res,
and rib cage support.
C. Sa g it t a l b ala n ce .
1. De ned and maintained by the cervical lordosis, thoracic kyphosis, lumbar
lordosis, and pelvic tilt.
2. On a standing lateral plain lm radiograph, the weight-bearing axis or
plumb line should cross C1, C7, T10, and S2.
3. Modulation of back extension by the paraspinal muscles can help center the
weight-bearing axis over the pelvis and feet.
4. Additional maintenance of the sagittal balance is achieved by retroversion of
the pelvis (pelvic tilt).
II. Kin em at ics.
A. Ce r vic a l sp in e.
1. Occipitoatlantal joint (occiput–C1).
a. Thirteen degrees exion/extension.
(1) Head nod.
b. Eight degrees lateral bending.
c. Four degrees axial rotation.
d. Coupled motion.
(1) Occipitoatlantal extension with chin-out maneuver.
2. Atlantoaxial joint (C1–C2) (Fig. 6.1).
a. Ap proxim ately 45° of a xial rotat ion .
b. Ten degrees exion/extension.
c. No lateral exion.
3. Subaxial cervical spine.
a. Flexion/extension.
(1) Greater mobility in the sagittal plane due to the orientation of the
facet joint s (4 5° h orizon t a l plan e).
(a) C2–C3 (8 °).
83
84
6 Biom ech anics of the Spine and Spinal In strum entat ion
Fi g . 6 . 1 Po we r’s rat io (BC/ DA) > 1 sign i es an t e rior occipitoat lan t al inst ab ilit y. The d ist ance be t we e n
the basion and the spinolaminar line of C1 is divided by the distance between the posterior margin of
the foramen magnum (opisthion) and the posterior margin of the anterior arch of C1.
(b) C3 –C4 (13°).
(c) C4–C5 (1 2°).
(d) C5–C6 (1 7°).
(e ) C6–C7 (1 6°).
(f) C7–T1 (9°).
b. Lateral bending.
(1) Sixty degrees coupled with rotation.
(a) The spinous processes rotate toward the convexity.
c. Axial rotation.
(1) Fifty percent of cervical rotation takes place in the subaxial cervical
spine.
B. Th o r a cic s p i n e .
1. The ribs and steep orientation of the facets limit range of motion (ROM).
a. Flexion/extension.
(1) Seventy- ve degrees com bined sagittal m otion.
(2) Flexion is greater than extension.
(3) Flexion increases caudally.
b. Axial rotation.
(1) Seventy degrees axial rotation.
(2) Rotation decreases caudally.
c. Lateral bending.
(1) Seventy degrees lateral bending.
2. More exion/extension and lateral bending motion is present in the lower
vertebral segm en t s, but there is less rot at ion.
6 Biom echanics of t he Spine and Spinal Inst rum e nt ation 85
3. Some degree of rotation accompanies lateral bending.
a. Spinous processes rotate toward the convexity in the upper thoracic
region.
b. In the middle to lower thoracic region, the direction of coupling is not
consistent.
C. Lu m b a r s p in e .
1. Flexion/extension.
a. Eighty– ve degrees combined exion/extension ROM.
b. Flexion is greater than extension.
c. Motion is greater caudally.
2. Lateral bending.
a. Thirty degrees ROM.
3. Axial rotation.
a. Sagittal orientation of the facets limits rotation.
b. Rotation is least at L5–S1.
III. Bio m ech a n ics of sp in al in st abilit y, o r t h o sis, an d in st r u m en t at ion .
A. Occip u t– ce r vica l s p in e (Table 6.1).
1. C2 fractures.
a. Odontoid fractures produce C1–C2 instability.
b. C2 pedicle or the hangman’s fracture (traumatic spondylolisthesis of C2)
(Fig. 6.2):
(1) Unstable in exion.
B. Fix a t io n o f t h e u p p er ce r vic al s p i n e .
1. Posterior methods.
a. C1 lateral mass screw/C2 pedicle screw (Fig. 6.3 and Fig. 6.4).
(1) Strongest biom echanical xation.
2. Anterior odontoid screws (Fig. 6.5).
a. Biomechanically stronger with two screws.
(1) One screw may be adequate clinically.
3. C2 translaminar screw.
a. Used if the posterior elements are intact.
C. Bio m e ch an ics o f t h e low e r ce r v ic a l sp in e.
1. White and Panjabi checklist for clinical instability (Table 6.2).
a. Anatomical components.
(1) Anterior stabilit y.
(a) Annulus brosus.
(b) Anterior longit udinal ligam ent.
(c) Vertebral body.
(2) Posterior stability.
(a) Posterior longitudinal ligament.
(b) Facet joint and capsules.
(c) Lam ina and interspinous ligam ents.
2. Ligam en tous d isr u p t ion of > 3.5 m m or 11° in d icates in stab ilit y (Fig. 6.6).
3. Bony disruption:
a. Vertical compression and compressive exion injuries.
(1) The status of the posterior colum n in uences the overall stabilit y.
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