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68 I Patient Evaluation and Spine Overview
2. Posterior cervical instrumentation
a. Interspinous wiring
(1) Thirty–three percent of normal in flexion
(a) Strength varies with different techniques
b. Facet wiring and graft
(1) Fifty–five percent normal in flexion
c. Interfacet wiring
(1) Eighty–eight percent normal in flexion
d. Posterior lateral mass screw/rod (Figs. 4–7, 4–8)
(1) Strongest in both flexion (92%) and extension (60%)
e. Pedicle screw
(1) High incidence of medial wall violation
3. Done with either fluoroscopic imaging and/or laminoforaminotomies
4. Cervical orthosis
a. Cervical collars
(1) Soft collar
(a) Used only for comfort
(b) Does not maintain stability
(2) Philadelphia (Miami–J) collars
(a) Thirty percent of normal flexion/extension allowed
(b) Ineffective in controlling rotation and lateral bending
Figure 4–7 Comparison schematic of Magerl, Anderson, and An methods of lateral mass screw
orientation.
Chapter 4 Biomechanics of the Spine and Spinal Instrumentation 69
Figure 4–8 Close–up diagram of the An technique of lateral mass screw placement.
(3) Four–poster–type orthosis
(a) Good for controlling flexion/extension at the midcervical
level (20% of normal motion is allowed)
(4) Cervicothoracic–style orthoses
(a) Sterno-occipital-mandibular immobilized (SOMI) brace
(i) Good for controlling upper cervical spine flexion (C2–C5)
(ii) Does not restrict extension effectively
(b) Rigid cervicothoracic braces (“Yale type”)
(i) Good for controlling flexion/extension
(ii) Controls rotation slightly
(iii) Controls bending by only 50%
(5) Halo devices
(a) Best device for restriction of all planes of motion
(b) Particularly for the upper cervical spine
(c) Cannot maintain distractive force (Fig. 4–9)
E. Thoracic and thoracolumbar spine instrumentation
1. Supporting structures
a. Anterior
(1) Anterior and posterior longitudinal ligaments
(2) Intervertebral disk
(3) Vertebral body
b. Posterior
(1) Ligamentum flavum
(2) Facet joints
70 I Patient Evaluation and Spine Overview
(3) Pedicle
(4) Costovertebral–transverse complex
2. Thoracic spine is mechanically stiffer and less mobile than thoracolum-
bar junction.
a. Stress concentration due to change in stiffness at the thoracolum-
bar junction
3. Thoracic or thoracolumbar trauma
a. Spine stability
(1) Three–column concept of Denis (Fig. 4–10)
(a) If two or more columns are disrupted, the spine is consid-
ered to be unstable.
Figure 4–9 Halo application. Depicting the safe zone for anterior halo pin placement. Anterior
pins should be placed below the equator of the skull, lateral to the supraorbital nerve with the
eyes closed. Closing the eyes ensures that when the halo is placed the patient will be able to
blink normally.
Chapter 4 Biomechanics of the Spine and Spinal Instrumentation 71
(b) If the middle column is disrupted, the spine is regarded as
unstable.
(i) Middle–column disruption is less important above T8
because of the stability provided by the rib cage.
(c) Anterior column
(i) Anterior longitudinal ligament
(ii) Anterior anulus fibrosis
(iii) Anterior half of the vertebral body
(d) Middle column
(i) Posterior longitudinal ligament
(ii) Posterior anulus fibrosis
(iii) Posterior half of the vertebral body
(e) Posterior column
(i) Pedicle
(ii) Facet joints
(iii) Lamina
Figure 4–10 Three–column classification of Denis.
72 I Patient Evaluation and Spine Overview
(iv) Spinous processes
(v) Interspinous and supraspinous ligaments
b. Compression fractures
(1) Anterior column failure
(2) End plates are weaker than the intervertebral disks.
(a) Displacement of the nucleus pulposus into the vertebral
body may occur.
(3) Bony fractures are more common in older osteoporotic patients.
c. Burst fractures
(1) Anterior and middle column failure
(2) Neurological injuries are common secondary to retropulsion of
the middle column.
d. Fracture/dislocation
(1) Three–column failure
(2) Shear/translational, flexion/distraction, or flexion–rotation
injuries
(3) Posterior stabilization is required.
4. Instrumentation for thoracolumbar fractures
a. Purpose
(1) Early mobilization
(2) Prevent late deformity and pain
(3) Indirect decompression by distraction and extension in trau-
matic injuries
(4) Temporary stabilization until fusion matures increases healing
rates.
b. Harrington rods
(1) Correction by distraction and hyperextension
(2) Relies on tension of the anterior longitudinal ligament
(3) Effective for resisting axial loads but weak in resisting torsion
c. Luque rods
(1) Better rotational stability
(2) Not effective for resisting axial loading
d. Transpedicular instrumentation (Fig. 4–11)
(1) Provides the stiffest construct with shortest segment fusion
(2) The gold standard for thoracolumbar fracture management
e. Anterior fusion
(1) Primary treatment in fractures with neurological deficit
(2) Adjunct treatment after posterior instrumentation
(3) Anterior plate construct is equal to a construct with an anterior
strut graft plus posterior transpedicular instrumentation
5. Spinal deformities in the thoracolumbar spine
a. Scoliosis
(1) Lateral flexion with rotation of the spinous process toward the
concavity of the spine
(2) Hypokyphosis and posterior wedging of the vertebral body is
frequently seen
(3) Correction
(a) Distraction on the concave side produces thoracic kyphosis
Chapter 4 Biomechanics of the Spine and Spinal Instrumentation 73
(b) Compression on the convex side reduces kyphosis
(c) Bending and translation
(i) Cantilever bending and segmental fixation correct the
deformity in the coronal and sagittal plane.
(ii) Rotation–derotation also corrects the deformity in
coronal and sagittal planes by shifting the regions of
the spine en bloc.
b. Kyphosis
(1) Anterior column fails with compression
(2) Posterior column fails with tension
(3) Deformity increases moment arm, further increasing defor-
mity.
(4) Eccentric loading affects cartilaginous growth.
(a) Compression decreases growth anteriorly.
(b) Tension increases growth posteriorly.
(5) Various instrumentations are used to correct kyphotic deformi-
ties.
(a) Mild and flexible curves can be corrected by posterior com-
pression rods.
(b) Greater curves should be approached by combined anterior
and posterior fusion and instrumentation.
(i) Posterior instrumentation is applied with cantilever
bending and compression forces.
Figure 4–11 Thoracic pedicle screw starting points.
74 I Patient Evaluation and Spine Overview
F. Lumbar and lumbosacral spine
1. Stability (Fig. 4–12)
a. Anterior stability is maintained by
(1) Anterior longitudinal ligament
(2) Vertebral body
(3) Anulus fibrosus
b. Posterior stability
(1) Facet joint
c. The role of the muscles, including erector spinae, abdominal mus-
cles, and psoas are important in overall stability.
2. Lower lumbar and lumbosacral spine
a. Degenerative disk and facet diseases
(1) Intervertebral disk
(a) Anulus fibrosis
(i) Collagen fibers arranged in an oblique direction
(ii) Provides axial loading stability and 40 to 50% torsional
stability
(b) Nucleus pulposus
(i) Gel–like core acts as a ball bearing, changing center of
rotation.
(2) Intradiscal pressure
(a) Load on the disk is about twice the body weight when
sitting.
(b) Thirty percent lower disk pressure when standing
(c) Fifty percent lower disk pressure when lying on the side
(d) Eighty to 90% lower disk pressure when lying supine com-
pared with sitting
(3) Disk degeneration
(a) Shifts the instant center of rotation posteriorly
(b) Increases stress to the facet joint
(c) Disk degeneration affects the motion of the functional
spinal unit.
(i) Early disk degeneration with radial tears of the anulus
fibrosus decreases stiffness in flexion, lateral bending,
and rotation.
(ii) Advanced disk degeneration with loss of disk height
and osteophytes increases the stiffness.
(4) Facet joints
(a) Provide torsional stability
(b) Support less than 20% of load with weight bearing
b. Spondylolisthesis
(1) Abrupt change in stiffness across the lumbosacral junction
(2) Pars interarticularis is strong but susceptible to fatigue frac-
tures, especially with extension injuries.
(3) Shear stress at the pars interarticularis
(a) Physiological flexion contracture of the hip and secondary
hyperlordosis create pincerlike effect from the superior ar-
ticular process of S1 and inferior articular process of L4.
Chapter 4 Biomechanics of the Spine and Spinal Instrumentation 75
G. Biomechanics of transpedicular instrumentation
1. Anatomy
a. The pedicle is a cylinder of cortical bone.
(a) The horizontal diameter from T9 to L5 increases from 7 mm to
1.5 cm.
(2) Vertical diameter is ~1.5 cm.
(3) Inner diameter is less than 80% of outer diameter.
Figure 4–12 White and Panjabi’s criteria for lumbar segmental instability.
76 I Patient Evaluation and Spine Overview
(a) Diameter of pedicle screw should be smaller than the inner
diameter.
(4) Pedicle depth is approximately 45 to 50 mm from entrance
point to the anterior vertebral margin.
b. Entrance points and directions (Figure 4–11)
2. Pedicle screw design and biomechanics
a. Structural characteristics
(1) The most significant factor for pullout strength is the outer
(major) diameter.
(2) Deeper threads increase the pullout strength.
(a) Bending strength diminishes due to smaller minor diame-
ter (Fig. 4–13).
(b) Pullout strength is also increased by the depth of penetration.
(c) Not significantly affected by the shape of thread
3. Transverse connectors
a. Important for less rigid systems
b. Important if triangulation technique is to be utilized
c. Important in the osteoporotic spine
d. Improves torsional stability of the construct
H. Sacral fixation
1. Types
a. Galveston technique
b. Iliosacral screw
c. Sacral screws
d. Intrasacral rods (Jackson)
e. Dunn–McCarthy rods (through S1 foramen)
2. Screws are generally better than hooks in the sacrum.
a. Sacral fixation with a single sacral screw has a high failure rate
(pullout).
b. S1 screw
(1) Anteromedial direction toward the sacral promontory below
the superior sacral end plate
(a) Safest and biomechanically acceptable
c. S2 screw
(1) Weak but may enhance overall stability
(2) Screw is directed 30 to 40 degrees laterally
(a) Avoid penetrance of the anterior cortex if the screw is
directed laterally to avoid injuries to the iliac vein, lum-
bosacral trunk, and sigmoid colon.
I. Instrumentation rigidity and stiffness
1. A rigid implant construct increases fusion mass.
2. May also cause device–related osteoporosis (stress shielding)
a. Stress shielding by rigid instrumentation is ~15%.
b. Benefit of rigid instrumentation outweighs stress–shielding
phenomenon
Chapter 4 Biomechanics of the Spine and Spinal Instrumentation 77
Suggested Reading
Crawford NR. Biomechanics of lumbar arthroplasty. Neurosurg Clin N Am 2005;16:595–602
Durbhakula MM, Ghiselli G. Cervical total disc replacement, part I: rationale, biomechanics, and implant
types. Orthop Clin North Am 2005;36:349–354
Kanayama M, Hashimoto T, Shigenobu K. Rationale, biomechanics, and surgical indications for Graf liga-
mentoplasty. Orthop Clin North Am 2005;36:373–377
Kumar N, Judith MR, Kumar A, et al. Analysis of stress distribution in lumbar interbody fusion. Spine
2005;30:1731–1735
McLain RF. The biomechanics of long versus short fixation for thoracolumbar spine fractures. Spine
2006;31:S70–S79
Smucker JD, Sasso RC. The evolution of spinal instrumentation for the management of occipital cervical
and cervicothoracic junctional injuries. Spine 2006;31:S44–S52
Figure 4–13 Lateral radiograph of the lumbar spine. Note that the sacral screw has broken
because fusion has not occurred. Pedicle screws will eventually fail with cyclical loading if a solid
arthrodesis is not obtained.