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58 I Patient Evaluation and Spine Overview
Electrodiagnostic Tests
I. Electromyography (EMG) and nerve conduction studies (NCS)
A. EMG/NCS only evaluate the motor tracts of the nerve root.
1. Radiculopathy may also involve motor, sensory, and autonomic fibers
of the nerve root.
B. Compound muscle action potential in peripheral nerves will show re-
duced amplitude in proportion to the amount of axonal degeneration that
occurs after a compressive nerve root lesion.
C. Compound muscle action potential in peripheral nerves is more marked
when multiple roots are involved as in lumbar spinal stenosis.
D. Nerve conduction velocity or latency should not be affected by a focal
proximal lesion such as in radiculopathy.
E. The gold standard for electrodiagnosis of radiculopathy is needle EMG.
1. The earliest EMG finding 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 reflex latency for C7 or S1 roots and reduced number of
F waves in weak muscles may be observed after several days of
radiculopathy.
4. Spontaneous motor activity, fibrillations, and positive F waves are the
hallmark 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 un-
involved muscles.
F. Indications for EMG/NCS
1. Clinical findings suggestive of other neurological disorders such as an-
terior horn cell disease, nerve entrapment syndrome, cervical stenosis,
etc.
2. Imaging studies and clinical findings do not correlate well in patients
with suspected radiculopathy.
3. In cases where neurological progression or deterioration must be doc-
umented
II. Somatosensory evoked potentials (SSEPs) and motor evoked potentials
A. Assess sensory tracts from a peripheral nerve to the posterior column of
the spinal cord
B. Most commonly used as an intraoperative monitoring technique to pro-
tect the spinal cord during surgery
C. Dermatomal SSEPs can be used to monitor nerve root function during sur-
gery.
D. Motor evoked potentials assess the motor pathway in the spinal cord, par-
ticularly during anterior procedures of the spine that may jeopardize the
anterior part of the spinal cord.
◆
Chapter 3 Spinal Imaging and Diagnostic Tests 59
Suggested Reading
Berquist TH. Imaging of the postoperative spine. Radiol Clin North Am 2006;44:407–418
de Graaf I, Prak A, Bierma-Zeinstra S, et al. Diagnosis of lumbar spinal stenosis: a systematic review of
the accuracy of diagnostic tests. Spine 2006;31:1168–1176
Haughton V. Imaging intervertebral disk degeneration. J Bone Joint Surg Am 2006;88(Suppl 2):15–20
Jayakumar P, Nnadi C, Saifuddin A, et al. Dynamic degenerative lumbar spondylolisthesis: diagnosis with
axial loaded magnetic resonance imaging. Spine 2006;31:E298–E303
4
Biomechanics of the Spine and
Spinal Instrumentation
I. General considerations
A. Functional spinal unit
1. Intervertebral disk
2. Adjacent vertebral bodies
3. Facet joint complex
B. Spinal stability
1. Under physiological loading, there is neither abnormal strain nor ex-
cessive motion in the functional spinal unit in which neurological
structures are protected.
2. Maintained by muscular tension, abdominal and thoracic pressures,
and rib cage support in addition to the function spinal unit
C. Sagittal balance
1. Maintained by cervical lordosis, thoracic kyphosis, lumbar lordosis,
and sacral kyphosis
2. Weight–bearing axis crosses C1, C7, T10, and S2.
II. Kinematics
A. Cervical spine
1. Occipitoatlantal joint (occiput–C1)
a. Thirteen degrees flexion/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)
a. About 45 degrees axial rotation
b. Ten degrees flexion/extension
c. No lateral flexion
60
Chapter 4 Biomechanics of the Spine and Spinal Instrumentation 61
3. Subaxial cervical spine
a. Flexion/extension
(1) Greater mobility in the sagittal plane is due to the orientation
of the facet joints in 45–degree horizontal plane.
(a) C2–C3 (8 degrees)
(b) C3–C4 (13 degrees)
(c) C4–C5 (12 degrees)
(d) C5–C6 (17 degrees)
(e) C6–C7 (16 degrees)
( f ) C7–T1 (9 degrees)
b. Lateral bending
(1) Sixty degrees coupled with rotation
(a) Spinous process is rotated toward the convexity.
c. Axial rotation
(1) Fifty percent of rotation takes place in the subaxial cervical spine.
B. Thoracic spine
1. The ribs and steep orientation of the facets limit range of motion
(ROM).
a. Flexion/extension
(1) Seventy–five degrees combined sagittal motion
(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 flexion/extension and lateral bending motion is present in the
lower vertebral segments, but rotation is less.
3. Some degree of rotation accompanies lateral bending.
a. Spinous process rotates toward the convexity in the upper thoracic
region.
b. In the middle to lower thoracic region, the direction of coupling is
not consistent.
C. Lumbar spine
1. Flexion/extension
a. Eighty–five degrees combined flexion/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. Biomechanics of spinal instability and instrumentation
A. Occiput–cervical spine
1. Occiput–C1 instability
62 I Patient Evaluation and Spine Overview
a. Distance from the tip of the dens to basion of the occiput
(1) Normal is 4 to 5 mm.
(2) Greater than 1 mm translation on flexion–extension is abnormal.
(3) Power’s ratio is used to determine anterior atlanto–occipital
dislocation (Fig. 4–1).
(a) Distance between the basion and the spinolaminar line of
C1 divided by the distance between the posterior margin of
the foramen magnum (opisthion) and the posterior margin
of the anterior arch of C1
(i) A ratio 1 signifies anterior atlanto–occipital instability.
2. Basilar invagination
a. McGregor’s line
(1) Greater than 4.5 mm odontoid projection above the foramen
magnum
b. Ranawat’s C1–C2 index
(1) Less than 13 mm is abnormal.
c. Redlund–Johnell O–C2 index
(1) Abnormal distance
(a) Less than 33 mm (men)
(b) Less than 27 mm (women)
3. Atlantoaxial (C1–C2) instability
a. Transverse ligament is essential for stability
(1) Atlantodens interval (ADI)
(a) ADI 3 mm indicates rupture of the transverse ligament.
(b) Greater than 5 mm of ADI indicates rupture of the trans-
verse and alar ligament.
Figure 4–1 Power’s ratio (BC/DA)1 signifies anterior occipitoatlantal instability. The distance
between the basion and the spinolaminar line of C1 is divided by the distance between the poste-
rior margin of the foramen magnum (opisthion) and the posterior margin of the anterior arch of
C1. (From An HS. Principles and Techniques of Spine Surgery. Baltimore: Williams and Wilkins,
1998. Reprinted with permission.)
Chapter 4 Biomechanics of the Spine and Spinal Instrumentation 63
(c) Greater than 4.5 mm is abnormal in children.
(2) Space available for the cord (SAC)
(a) Greater than 10 mm of ADI or SAC 14 mm impinges on
the spinal cord.
b. Atlas fractures with 6.9 mm lateral displacement indicates rup-
ture of the transverse ligament.
4. C2 fractures
a. Odontoid fractures produce C1–C2 instability.
b. C2 pedicle or the Hangman’s fracture (traumatic spondylolisthesis
of C2)
(1) Unstable in flexion
B. Fixation of the upper cervical spine
1. Posterior methods
a. Sublaminar wiring
(1) Gallie wiring is weaker than Brooks technique.
(a) Particularly in rotatory and anterior translatory motions
(Fig. 4–2)
b. Transarticular screw technique (Magerl)
(1) Stronger fixation especially in rotation (Fig. 4–3)
c. Occipitocervical plates or Luque rods are stronger than wires.
d. C1 lateral mass screw/C2 pedicle screw (Figs. 4–4, 4–5)
(1) Strongest biomechanical fixation
2. Anterior odontoid screws (Fig. 4–6)
a. Biomechanically stronger with two screws
(1) One screw may be adequate clinically.
C. Biomechanics of the lower cervical spine
1. White and Panjabi checklist for clinical instability
a. Anatomic components
(1) Anterior stability
(a) Anulus fibrosus
(b) Anterior longitudinal ligament
(c) Vertebral body
(2) Posterior stability
(a) Posterior longitudinal ligament
(b) Facet joint and capsules
(c) Lamina and interspinous ligaments
b. Clinical checklist
(1) A total of 5 points or more is considered unstable.
(a) Disruption of anterior elements: 2
(b) Disruption of posterior elements: 2
(c) Relative sagittal plane translation greater than 3.5 mm: 2
(d) Relative sagittal plane rotation greater than 11 degrees: 2
(e) Positive stretch test: 2
(f ) Cord damage: 2
(g) Root damage: 1
(h) Abnormal disk narrowing: 1
(i) Dangerous loading anticipated: 1
2. Ligamentous disruption of
3.5 mm or 11 degrees indicates instability.
64 I Patient Evaluation and Spine Overview
Figure 4–2 Bohlman triple–wiring method of posterior occipitocervical fusion. (A) Wires are passed
through the occipital protuberance, C1, and C2. (B)Bone grafts are obtained from the outer table of
the iliac crest. (C) Wires are passed through drill holes in the grafts. (D) The grafts are secured in place.
Figure 4–3
Figure 4–3 (Continued) Magerl’s transarticu-
lar technique of C1–C2 screw fixation and fu-
sion. Screws enter at the inferior portion of C2
and pass through the facet joints in the lateral
masses of C1.
Figure 4–4 Schematic depicting the starting points and trajectory for a C1 lateral mass screw
(Harm’s technique) and a C2 pedicle screw.
Chapter 4 Biomechanics of the Spine and Spinal Instrumentation 65
66 I Patient Evaluation and Spine Overview
Figure 4–5 (A,B) Anteroposterior/lateral radiograph demonstrating a C1–C2 posterior cervical
fusion accomplished via a C1 lateral mass screw and C2 pedicle screw.
A
B
Chapter 4 Biomechanics of the Spine and Spinal Instrumentation 67
Figure 4–6 Open mouth (A) and lateral radiograph (B) demonstrating anterior odontoid screw
fixation for a C2 dens fracture.
3. Bony disruption
a. Vertical compression and compressive flexion injuries
(1) Status of the posterior column influences the overall stability
(a) Laminectomy or facetectomy
(i) Laminectomy
* Eighteen percent loss of stability
* Laminectomy at C2 or C7 causes greater instability
(ii) Laminectomy plus facetectomy
* Sixty percent loss of stability
* Partial bilateral facetectomy (greater than 50%)
causes instability
D. Cervical fusion
1. Anterior cervical fusion
a. Disruption of all anterior ligaments reduces strength by 52%.
(1) Anterior interbody fusion restores stability to 100% of normal in
flexion.
(2) Fifty–five percent restoration in extension (Smith–Robinson–type
graft)
(3) Bone mineral density affects the compressive strength of the
graft.
(4) Anterior interbody fusion plus plating adds strength in extension.