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opposite directions at approximately a 30 degree angle from
the vertebral endplates. Their orientation provides a greater
resistance to torsion than distraction, compression, or
bending.
25. A 19-year-old male awakens from a good night’s
rest and takes a ten-hour hike through the woods.
How has the water content of his intervertebral discs
changed from awakening compared to at the con-
clusion of his hike?
a) Water content of the discs is unchanged
b) Water content of the discs is decreased along with
the intr adiscal oncotic pressure
c) Water content of the discs is increased along with
the intr adiscal oncotic pressure
d) Water content of the discs is decreased while
intradiscal oncotic pressure is increased
e) Water content of the discs is increased while
intradiscal oncotic pressure is decreased
Response (D) The pressure within the intervertebral disc
with upright, weight-bearing posture is much greater than in
supine position. An increase in intradiscal pressure with
weight bearing causes fluid to migrate out of the disc space.
This leads to an increase in the concentration of micro
molecules and oncotic pressure within the disc space, low-
ering the water content but increasing the absorptive
capacity of the disc [6].
26. Which of the following correctly describes the effect
of traction on intradiscal pressure, intradiscal water
content, and disc height?
a) Traction causes a decrease in intradiscal pressure,
intradiscal water content, and disc height
b) Traction causes an increase in intradiscal pressure,
decrease in intradiscal water conten t, and decrease in
disc height
c) Traction causes a decrease in intradiscal pressure,
increase in intradiscal water content, and increase in
disc height
d) Traction causes no change in intradiscal pressure,
intradiscal water content, or disc height
e) Traction causes an increase in intradiscal pressure,
decrease in intradiscal water content, and increase in
disc height
Response (C) Traction, as opposed to weight bearing,
provides a mechanism by which the intradiscal pressure can
be reduced, thereby permitting in increase in disc water
content and an increase in disc height.
27. In which direction is the annulus fibrosus weakest
and least abundant?
a) Dorsally
b) Ventrally
c) Laterally
d) Equally strong and abundant in all directions
e) Direction of highest abundance and strength is
gender-dependent
Response (A) The annulus fibrosus is weaker and less
abundant dorsally than ventrally or laterally, a feature which
contributes to the locality of disc herniations [2].
28. What pressure changes account for the vacuum disc
phenomenon?
a) Negative pressure within the central disc interspace
as a result of severe degenerative changes and
increased peripheral loading
b) Increased central disc pressures as a result of severe
degenerative changes and decreased peripheral
loading
c) Increased central disc pressures as a result of severe
degenerative changes and increased peripheral
loading
d) Uniform high pressures across the disc and periph-
eral annulus fibrosus
e) Erosion of the disc through the superior vertebral
endplate
Response (A) As the spine ages and degenerative pro-
cesses take place, the desiccated central nucleus pulposus
can no longer bear axial loads as in youth. Therefore, axial
loads are borne more peripherally by the fibrocartilaginous
equivalent of the nucleus pulposus, and the central disc
pressure is diminished. With severe degenerative changes,
pressures within the disc space can drop below zero with
resultant gas formation giving rise to the vacuum disc
phenomenon.
29. Which region of the spine is typically spared from
spinal canal encroachment by osteophyte
formation?
a) Cervical
b) Thoracic
240 L. A. Silveira et al.
c) Lumbar
d) Cervical and thoracic
e) Cervical and lumbar
Response (B) Osteophyte formation typically takes place
along the concavity of a spinal curve, coinciding with the
direction in which the annulus fibrosus bulges and elevates
the periosteum of adjacent vertebral bodies at Sharpey’s
fiber insertion sites. The natural lordosis of the cervical and
lumbar spine result in a dorsally oriented concavity of spinal
curvature, with osteophyte formation therefore occurring
toward the spinal canal in these regions. The thoracic spine,
by contrast, is typically spared from spinal canal encroach-
ment by osteophyte formation because of its natural
kyphosis and concordant ventrally oriented concavity of
spinal curvature [2].
30. How might intervertebral disc height loss lead to
torsional instability?
a) Leads to annular fibers becoming increasingly taut
b) Leads to annular fibers becoming increasing lax
c) Induces annular fiber overgrowth
d) Causes direct tearing of annular fibers
e) Leads to detachment of annulus fibrosis at Sharpey’s
fibers insertion sites
Response (B) The 30-degree orientation of the annular
fibers with respect to the disc space is optimal for resisting
torsional forces, but only if the annular fibers remain taut.
Loss of intervertebral disc height can lead to increased laxity
in the annular fibers and thereby lead to torsional instability.
31. In a region with kyphotic posture, which pathologic
fracture morphology might be expected in an
osteoporotic patient?
a) Wedge Compression fracture
b) Burst fracture
c) Chance Fracture
d) Tear-drop fracture
e) Clay-shoveler’s fracture
Response (A) In a region with kyphotic posture, a ventral
wedge compression fracture might be expected. Kyphosis
predisposes the spine to ventral vertebral body collapse
because the moment arm applied to the spine at the apex of
kyphosis extends ventrally and is longer than in a region
without significant kyphosi s.
32. Which of the following best defines a “moment arm”
as it relates to spinal biomechanics?
a) The orthogonal distance between the force vector
and the instantaneous access of rotation of the body
on which the force acts
b) The distance between two sequential intervertebral
discs
c) The distance from the middle of the vertebral body
to the tip of the spinous process
d) The distance between two force vectors acting on a
body
e) the distance between a kyphot ic apex and the most
proximal lordotic apex
Response (A) The moment arm refers to the orthogonal
distance between a force vector and the instantaneous axis of
rotation (IAR) of the body on which the force acts. For
practical purposes, the moment arm is the lever length upon
which a force vector acts.
33. Which of the following is the term applied to
describe the axis upon which each vertebral segment
rotates at any given moment in time?
a) Instantaneous Axis of Rotation
b) Torsional focus
c) Central Spin point
d) Rotatory tangent point
e) Radial Fixed setting
Response (A) The instantaneous axis of rotation refers to
the center of the coordinate system for each vertebral motion
segment. It is the axis upon which each vertebral segment
rotates at any given moment.
34. How many degrees of freedom exist about each
instantaneous axis of rotation (IAR)?
a) 2
b) 4
c) 6
d) 8
e) 10
11 Biomechanics of the Spine 241
Response (C) There are 6 degrees of freedom about each
IAR, with both rotation and translation possible about each
of the three Cartesian axes (x, y, and z axes).
35. Which of the following accurately describes the
relationship between rod diameter and its moment
of inerti a for a rod of uniform density?
a) Moment of inertia is inversely proportional to rod
diameter squared
b) Moment of inertia is proportional to rod diameter
squared
c) Moment of inertia is directly proportional to rod
diameter
d) Moment of inertia is proportional to rod diameter
to the fourth power
e) Moment of inertia is inversely proportional to rod
diameter to the fourth power
Response (D) For a rod of uniform density, the moment of
inertia is proportional to rod diameter to the fourth power.
36. Which of the following accurately describes the
relationship between core screw diameter and its
section modulus?
a) Section modulus is inversely proportional to core
screw diameter
b) Section modulus is proportional to core screw
diameter squared
c) Section modulus is directly proportional to core
screw diameter
d) Section modulus is proportional to core screw
diameter to the fourth power
e) Section modulus is proportional to core screw
diameter cubed
Response (D) A screw’s section modulus (Z) is an indi-
cator of its strength and is proportional to core screw
diameter cubed.
37. Which of the following joints is an amphiarthrodial
joint?
a) Intervertebral disc joint
b) Facet joints
c) Interphalangeal joints
d) Occipital condylar joints
e) Hip joint
Response (A) The intervertebral disc joint is an
amphiarthrodial joint, meaning there is no synovial
membrane. The osteophytes of spondylosis are associated
with degeneration of this amphiarthrodial joint, differentiat-
ing this degenerative process from the seropositive inflam-
matory arthritis that classically involve synovial membranes
of diart hrodial joints such as the facet joints.
38. In the event of a disc herniation, the application of
an axial load, lateral bending, and flexion causes the
nucleus pulposus to migrate in which direction?
a) In the direction of the annulus fibrosis under the
most tension
b) In the direction of the annulus fibrosis under the
least tension
c) Ventrally
d) In no particular direction; it generally remains
stationary
e) Toward the superior vertebral body endplate
Response (A) The described loading pattern causes
increased tension in the weakest portion of the annulu s
fibrosis and migration of the nucleus pulposus toward this
portion of the annulus fibrosis, away from the region of
asymmetrically increased intradiscal pressure. This loading
pattern, therefore, may result in a characteristic dorsal
paramedian disc herniation.
39. Which of the following represents a healthy,
non-pathologic vertebral body morphology and
cervical spine posture?
a) lordotic posture, ventral vertebral body
heights > dorsal vertebral body heights
b) lordotic posture, ventral vertebral body
heights < dorsal vertebral body heights
c) kyphotic posture, ventral vertebral body
heights = dorsal vertebral body heights
d) kyphotic posture, ventral vertebral body
heights < dorsal vertebral body heights
e) kyphotic posture, ventral vertebral body
heights > dorsal vertebral body heights
Response (A) In the non-pathologic cervical spine, dorsal
vertebral body height is typically less than ventral vertebral
body height contributing to a normal lordotic curvature.
40. A 32 year old woman is nine months pregnant with a
baby weighing an estimated 50 N (11.2 pounds), and
complains of significant back pain. Why must the
erector spinae muscles exert a greater downward force
than that exerted by the weight of the baby in utero,
for the mother to stand upright and not fall over?
242 L. A. Silveira et al.
a) The erector spinae muscles are located a signifi-
cantly shorter distance from the instantaneous
axis of rotation of the spine than the baby in utero
b) The erector spinae muscles are located a signifi-
cantly longer distance from the instantaneous axis
of rotat ion of the spine than the baby in utero
c) The erector spinae muscles have come to physi-
ologically anticipate increased weight of the baby
and exert a greater force preemptively
d) The erector spinae muscles always exert a fixed
downward force independent of the weight of the
baby in utero
e) The erector spinae muscles need not exert any
greater downward force as a result of the ventral
downward force of the baby in utero
Response (A) If the woman described is standing upright
and not in motion, a dorsal downward application of force
must be applied, in this case by the erector spinae muscles,
that are located a short distance from the instantaneous axis
of rotation (IAR) of the spine (*5 cm) compared to the
baby in utero (*25 cm). Given that the dorsal and ventral
bending moments must be equal with respect to the IAR, the
erector spinae must generate a greater downward force than
the weight of the baby in utero because the erect or spinae
have a shorter moment arm (D = 5 cm) compared to the
baby in utero (D = 25 cm). Bending moment is equal to
force distance from IAR, or force moment arm. In this
theoretical scenario, if the erector spinae muscles are the
only countering force, they would have to exert a downward
force 5 that of the weight of the baby.
41. How is the bending moment application related to
the moment arm length?
a) The bending moment is proportional to moment
arm length cubed
b) The bending moment is inversely proportional to
moment arm length
c) The bending moment is directly proportional to
moment arm length
d) The bending moment is inversely proportional to
moment arm length cubed
e) The bending moment is proportional to moment
arm length squared
Response (C) The bending moment (M) is synonymous
with torque and is the product of an applied force and the
length of the moment arm through which it acts. The
moment arm refers to the orthogonal distance between a
force vector and the instantaneous axis of rotation (IAR) of
the body on which the force acts.
42. Newton’s laws are highly relevant to spinal biome-
chanics. Which law states that for every action, there
is an equal (in magnitu de) but opposite (in directi on)
reaction?
a) Newton ’ s First Law
b) Newton ’ s Second Law
c) Newton ’ s Third Law
d) Newton ’ s Fifth Law
e) Newton ’ s Sixth Law
Response (C) Newton’s Third Law states that for every
action, there is an equal but opposite reaction.
43. Which of the following correctly characterizes the
orientation of the lumbar facet joints and their rel-
ative ability to resist certain motions?
a) The lumbar facets are oriented in the coronal
plane, most significantly limiting translation
b) The lumbar facets are oriented in the coronal
plane, most signi
ficantly limiting flexion
c) The lumbar facets are oriented in the coronal
plane, most significantly limiting axial rotation
d) The lumbar facets are oriented in the sagittal
plane, most significantly limiting rotation
e) The lumbar facet s are oriented in the sagittal
plane, most significantly limiting translation
Response (D) The lumbar facets are oriented in the sagittal
plane, substantially limiting rotation but with minimal ability
to resist flexion or translation.
44. Which of the following equations correctly defines
the bending moment (M) with relation to force
(F) applied to a lever arm and length of the lever
arm (D)?
a) M = F/D
b) M = F D
c) M = F D
2
d) M = F
2
D
e) M = F/D
2
11 Biomechanics of the Spine 243
Response (B) The bending moment (M) is equal to the
product of the force applied to a lever arm (F) and the length
of the lever arm (D).
45. Which of the following is the best refers to severe
spinal pain that worsened with increased loading of
the spine and alleviated with unloading?
a) Mechanical axial spine pain
b) Myofascial pain
c) Inflammatory axial spine pain
d) Chronic pain
e) Anesthes ia dolorosa
Response (A) Mechanical axial spine pain is best described
as severe spinal pain that is worsened with increased loading
of the spine and alleviated with unloading. Myofascial pain
refers to pain caused by paraspinous muscle tenderness.
Inflammatory axial spine pain is characterized by early
morning back stiffness that lessens throughout the day after
movement. Chronic pain is pain that is not well character-
ized or consistent.
46. If a force vector passes precisely through the
instantaneous access of rotation of a vertebral body
such that the vertebral body is stressed to failure by
the axial force, which fracture morphology results?
a) Ventral wedge compression fracture
b) Burst fracture
c) Dorsal wedge compression fracture
d) Hangman’ s fracture
e) Lateral wedge compression fracture
Response (B) A pure axial force directly through a spinal
segment’s instantaneous axis of rotation results in a burst
fracture.
47. If a force vector passes precisely through the
instantaneous access of rotation of a vertebral body
such that the vertebral body is stressed to failure by
the axial force resulting in a burst fracture, how long
is the moment arm of the force vector?
a) at least half the width of the vertebral body
b) length is zero
c) the exact width of the vertebral body
d) Equal to the width of the vertebral body squared
e) Equal to the height of the vertebral body
Response (B) The moment arm length of this force vector
is zero, as the perpendicular distance from the force vector to
the inst antaneous axis of rotation is zero.
48. A 62 year old man crashes his vintage automobile
into a tree, and is taken to the ED immediately fol-
lowing extrication which involves cutting of an old
lap belt that cannot be dis-engaged on the scene.
After imaging is obtained, he is diagnosed with a
chance fracture. Which of the following mechanisms
accounts for the occurrence of this fracture?
a) Flexion-distraction
b) Extensi on
c) Translation
d) Rotation-distraction
e) Axial compression
Response (A) A chance fracture the result of a
flexion-distraction injury with the above clinical history
detailing the most common circumstances including a
deceleration accident with a motorist wearing a lap belt but
no chest/shoulder restraints. In such a scenario, there is
restriction of pelvic and lumbosacral movement with unre-
stricted distraction and forward flexion of the spine superior
to that point.
49. Which of the following forces most characteristically
result in cervical unilateral facet dislocation?
a) Flexion plus rotation
b) Extensi on
c) Pure rotation
d) Axial loading
e) Pure translation
Response (A) A flexion moment combined with a rota-
tional component leads to a unilateral facet dislocation. By
contrast, a pure flexion moment more commonly results in
bilateral facet dislocation.
50. Which spinal region is the least likely to have neural
injury after a trauma?
a) Atlanto-Axial region
b) Cervical spine
c) Thoracic spine
d) Lumbar spine
e) Occipital Cervical junction
244 L. A. Silveira et al.
Response (D) Of the listed regions, the lumbar spine is
least likely to have neural injury following a trauma.
51. Which of the following ligaments provides the most
resistance to flexion?
a) Anteri or longitudinal ligament
b) Posterior longitudinal ligament
c) Interspinous ligament
d) Ligamentum flavum
e) Capsular ligament
Response (C) Of the listed ligaments, the interspinous
ligament provides the most resistance to flexion, as it has the
longest perpendicular moment arm. The large distance
between the instantaneous axis of rotation and the attachment
to the spinous process allows for this substantial resistance.
52. Which of the following correctly describes the
motion of the atlantoaxial facets during axial head
rotation?
a) The C1 inferior facet s glides posteriorly over the
superior C2 facet on the side to which the head is
turned.
b) The C1 inferior facet glides anteriorly over the
superior C2 facet on the side to which the head is
turned
c) The C1 inferior facet and C2 superior facet do not
move in relation to one another during head
turning
d) The C1 inferior facet glides posteriorly over the
superior C2 facet on the side opposite head
turning
e) The atlantoaxial facets joints are not real joints, as
the C1 inferior facet is rigidly fused to the C2
superior facet
Response (A) During axial head rotation, the C1 inferior
facet glides posteriorly over the C2 superior facet on the side
to which the head is turned. The C1 inferior facet glides
anteriorly over the C2 superior facet on the contralateral side
to which the head is turned [7].
53. In a normally aligned lordotic cervical spine, which
of the following accurately describes the axial load
bearing distribution between the facets and the
intervertebral discs for the sub-axial cervical levels?
a) The facet joints bear about 1/3 of the axial load,
while the remaining 2/3 of the axial load is
transmitted via the intervertebral discs
b) The facet joints bear about 2/3 of the axial load,
while the remaining 1/3 of the axial load is
transmitted via the intervertebral discs
c) The facet joints bear about 1/2 of the axial load,
while the remaining 1/2 of the axial load is
transmitted via the intervertebral discs
d) The facet joints bear the entirety of the axial load,
with none transmitted through the intervertebral
discs
e) The facet joints bear none of the axial load, as it is
entirely transmitted through the intervertebral
discs
Response (B) For a normally aligned, lordotic cervical
spine, the subaxial facet joints bear about 2/3 of the axial
load. The remaining 1/3 of the axial load is transmitted
through the intervertebral discs.
54. How does the magnitude of the bending moment
(M) imparted by the axial load of the cranium on the
cervical spine change with cervical kyphosis?
a) The magnitude increases because the moment arm
length (L) increases with cervical kyphosis
b) The magnitude decreases because the moment arm
length (L) increases with cervical kyphosis
c) The magnitude increases because the moment arm
length (L) decreases with cervical kyphosis
d) The magnitude of the bending moment increases but
the moment arm length (L) stays the same with
cervical kyphosis
e) The magnitude of the bending moment stays the
same despite but the moment arm length (L) in-
creasing with cervical kyphosis
Response (A) With a cervical kyphotic deformity, the
cranium’s center of mass is shifted anteriorly with respect to
the cervical spine instantaneous axis of rotation, increasing
the moment arm length (L) and therefore increasing the
magnitude of the bending moment [7]. Bending moment
magnitude (M) is equal to force (F) x moment arm length
(L).
55. How does a cervical kyphotic deformity impact the
portion of axial load that the intervertebral discs
bear as compared to the facet joints in the subaxial
cervical spine?
a) Cervical kyphosis does not alter the portion of the
axial load that is transmitted through the inter-
vertebral discs, but alters the portion of axial load
borne by the facets
11 Biomechanics of the Spine 245
b) The portion of axial load transmitted through the
intervertebral discs decreases with cervical
kyphosis, and the portion borne by the facets
increases
c) The portion of axial load transmitted through the
intervertebral discs increases with cervical
kyphosis, and the portion borne by the facets
decreases
d) The portion of axial load transmitted through the
intervertebral discs decreases with cervical
kyphosis, and the portion borne by the facets
decreases as well
e) Cervical kyphosis does not alter the portion of the
axial load that is transmitted through the inter-
vertebral discs or the portion of axial load borne
by the facets
Response (C) When a cervical kyphotic deformity is pre-
sent, the axial load is shifted anteriorly such that a greater
portion is transmitted through the intervertebral discs such
that cervical disc degeneration is potentially accelerated.
56. A 28-year-old woman involved in a high-speed
motor vehicle collision presents to the emergency
department with neck pain and the CT cervical
spine below (Fig. 11.2) depicting a unilateral facet
dislocation. Which of following mechanisms most
commonly accounts for this unilateral dislocation?
a) Extension
b) Axial compressi on
c) Flexion plus rotation
d) Pure flexion
e) Lateral bending
Response (C) A flexion moment combined with a rota-
tional component results in a unilateral facet dislocation such
as the unilateral C6-7 jumped facet depicted. A true flexion
moment without the rotational component most commonly
results in bilateral facet dislocation.
57. Which of the following is the best description of the
mechanism of injury of a judicial hangman’s
fracture?
a) A hyperextension not associated with a distraction that
is commonly associated with a C2-C3 dislocation
b) A distraction in combination with a capital hyperex-
tension resulting from a rostrally oriented force vector
c) A small axial load and extension
d) A violent extension and distraction which results in a
separation of the rostral and caudal parts of the ventral
C1 ring
e) Pure axial compression
Response (B) A judicial hangman’s fracture can appear as
a distraction in combination with a capital hyperextension. It
can also result from a rostrally oriented force vector.
A hangman’s fracture can appear as a hyperextension not
associated with a distraction and can be commonly associ-
ated with a C2-C3 dislocation. A small axial load and
extension is more commonly associated with a Type 1 C2
Body Fracture with C2-C3 Extension-Subluxation. A violent
extension and distraction that results in a separation of the
rostral and caudal parts of the ventral C1 ring is more
characteristic of a horizontal C1 fracture.
58. Which of the following can be most appropriately
treated with an odontoid screw?
a) A horizontal Type II dens fracture
b) A Type III dens fracture
c) A diagonally oriented type II dens fracture
d) A comminuted Type II dens fracture
e) A chronic Type II dens fracture in which a fibrous
nonunion has developed
Response (A) For some patients with favorable anatomy,
an odontoid screw provides an effective and appropr iate
option for treating a type II horizontally oriented dens
fracture. Type III dens fract ures are not typically treated
Fig. 11.2 Parasagittal bone CT cut through the facet joints in the
cervical spine
246 L. A. Silveira et al.
surgically. Old fractures in which a fibrous nonunion has
developed are associated with worse arthrodesis rates.
Diagonal fractures lend themselves to angulation and
translation of bone fragments during compression. Com-
minuted fractures generally do not permit adequate com-
pressive effect. Comminution, in tandem with motion, leads
to increased rates of arthrodesis failure.
59. Which of the following screw characteristics has the
greatest effect on resisting screw pullout?
a) Outer diameter of the screw
b) Inner diameter
c) Screw head diameter
d) Screw pitch
e) Screw length
Response (A) Of the screw characteristics listed, increased
outer diameter has the greatest effect on resisting screw
pullout. Screw pitch was found to be related to screw dis-
placement before failure.
60. What is the thread pitch of a screw and what are the
implications of a larger thread pitch?
a) Thread pitch refers to the distance between
threads of a screw.
b) Thread pitch refers to the distance between the
screw tulip and tip.
c) Thread pitch refers to the difference between the
outer and inner diameter of a screw.
d) Thread pitch refers to the diameter of the screw
neck compared to the screw head
e) Thread pitch refers to rate at which a conical
screw diameter decreases from proximal shaft to
tip
Response (A) Thread pitch refers to the distance between
threads of a screw. The larger the pitch, the more bone is
grabbed with each rotation of the screw.
61. Which of the following factors is most significantly
increased for vs. cylindrical screws?
a) Insertional torque
b) Fatigue resistance
c) Strength
d) corrosion resistance
e) Osteoi ntegration
Response (A) Conical screws have greater insertional tor-
que than cylindrical screws. Corrosion resistance and
osteointegration are material properties not related to screw
configuration [8].
62. Which of the following correctly describes the effect
of inner screw diameter and outer screw diameter
on screw strength and screw resistance to pullout?
a) Increased outer diameter contributes most signif-
icantly to screw strength while increased inner
diameter contributes most to increased pullout
resistance.
b) Increased outer diameter contributes most sig-
nificantly to increased pullout resistance while
increased inner diameter contributes most to
increased screw strength.
c) Inner and outer diameter contribute equall y to
screw strength.
d) Inner and outer diameter contribute equally to
pullout resistance
e) Outer diameter contributes more significantly to
increased pullout resistance and screw strength
than inner diameter.
Response (B) Increased outer screw diameter contributes
more significantly to increased pullout resistance while
increased inner diameter contributes more significantly to
increased screw strength.
63. What effect does triangulation of screws such that
screw heads converge toward each other in the axial
plane of the lumbar spine have?
a) Paired screws in 30 degree convergence decrease
pullout strength compared to parallel screws.
b) Paired screws in 30 degree convergence increase
pullout strength compared to parallel screws.
c) Screw convergence has no effect on pullout strength
d) Paired screws in 30 degree convergence decrease
pullout strength compared to parallel screws but
increase rates of boney fusion
e) Paired screws in 30 degree convergence decrease
pullout strength compared to parallel screws but
improve immediate post-operative pain scores
Response (B) Paired screws at 30 degree convergence
increased pullout strength by 28.6% over parallel screws in
the lumbar spine [9].
64. The dorsal and ventral heights of a given vertebral
body vary most significantly at which of the fol-
lowing spinal levels?
11 Biomechanics of the Spine 247
a) C6
b) T2
c) T12
d) T6
e) L5
Response (E) The dorsal and ventral vertebral heights vary
most significantly from one another at the lower lumbar
levels, specifically at L4 and L5.
65. Which of the following is true for screw failure in
healthy vs osteoporotic bone?
a) There is no appreciable difference in rates of
screw failure in healthy vs osteoporotic bone
b) Screw failure is more often due to screw migra-
tion than screw bending in healthy bone
c) Screw failure is more often due to screw bending
than screw migration in osteoporotic bone
d) Screw failure is more often due to screw bending
than screw migration in healthy bone
e) Screw s are not susceptible to failure by bending
or migration in healthy bone
Response (D) Screw failure more often due to screw
bending in healthy bone, but more often due to screw
migration in osteoporotic bone [10].
66. Increased insertional torque is most closely corre-
lated with which of the following?
a) Increased bone mineral density
b) Bone tapping
c) Speed of screw insertion
d) Patient positioning
e) Decreased bone mineral density
Response (A) Increased insertional torque has been found
to correlate with increased bone mineral density [11].
67. The angular moment of force required to advance a
screw into bone is referred to as which of the
following?
a) Insertional vice
b) Insertional torque
c) Grip strength
d) Arm of resistance
e) Tactile feedback
Response (B) The insertional torque of the screw is pri-
marily generated by the shearing force and friction in the
bone–screw interface. It can be defined as an angular moment
of the force required to advance a screw into bone [11].
68. Which of the following is true regarding prelordosed
rods vs intraoperarively bent rods?
a) Pre-lordosed rods have a lower fatigue strength
than rods bent intra-operatively with a French rod
bender
b) Pre-lordosed rods have a higher fatigue strength
than rods bent intra-operatively with a French rod
bender
c) Pre-lordosed rods have the same fatigue strength
as rods bent intra-operatively with a French rod
bender
d) Pre-lordosed rods have the lower fatigue strength
as rods bent intra-operatively with pliers
e) Pre-lordosed rods have a higher fatigue strength
than unbent straight rods
Response (B) Pre-lordosed rods have a higher fatigue
strength than rods bent intra-operatively due to the stress
relieving effect achieved during the annealing process.
Maximum fatigue strength with lordosed rods can be
achieved using pre-lordosed rods.
69. Which of the following materials is most notch
sensitive?
a) Titanium
b) cobalt-chromium
c) stainless steel
d) Titanium-alloy
e) No difference between metals.
Response (A) Pure titanium is most notch sensitive.
Therefore, a reduction in fatigue strength due to rod bending
is expected.
70. Which of the following refers to the extent to which
sensitivity of a material to fracture is increased by
cracks or scratches?
a) Splitting sensitivity
b) Ductility
c) Rigidity
d) Coarseness
e) Notching sensiti vity
248 L. A. Silveira et al.
Response (E) Notching sensitivity refers to the extent to
which sensitivity of a material to fracture is increased by
cracks or scratches.
71. Pelvic incidence is equal to which of the following?
a) Sacral slope + pelvic tilt
b) Sacral slope − pelvic tilt
c) (Sacral slope + pelvic tilt)/2
d) 2*(sacral slope + pelvic tilt)
e) 2(pelvic tilt) − sacral slope
Response (A) Pelvic incidence is a constant and specific
parameter equal to sacral slope + pelvic tilt.
72. Spondyloptosis is equivalent to which Myerding
classification grade of spondylolisthesis?
a) 1
b) 2
c) 3
d) 4
e) 5
Response (E) Spondyloptosis is a grade 5 spondylolisthesis
in concordance with the Myerding grading system, wherein
grades are broken down into quartiles with total translation
of one vertebral body over another or greater than 100%
spondylolisthesis equivalent to a grade 5 spondylolisthesis.
73. What effect does tapping osteoporotic bone have on
screw pullout strength?
a) Increased pullout strength by reducing volume of
bone left over for screw placement
b) Decreases pullout strength and reduces volume of
left over bone for screw placement
c) Increased pullout strength by aerating bone prior to
screw placement
d) No effect on pullout strength
e) Increases pullout strength by increasing the internal
shear area of the threads
Response (B) Removal of bone by tapping enlarges the
screw hole volume and thereby decreases the shear area of
bone captured between screw threads [12].
74. What effect does PMMA cement augmentation have
on pullout strength in the spinal fixation of the
osteoporotic patient?
a) Increases pullout strength
b) Decreases pullout strength and reduces volume of
left over bone for screw placement
c) Decreases pullout strength by aerating bone prior to
screw placement
d) No effect on pullout strength
e) Decreases pullout strength by increasing the internal
shear area of the threads
Response (A) PMMA cement augmentation can signifi-
cantly increase screw pullout strength in the spinal fixation
of the osteoporotic patient.
75. Which of the following terms refers to the ability of a
material to be plastically deformed without
fracture?
a) Cross reactivity
b) Ductility
c) Stiffness
d) Rigidity
e) Neutrality
Response (B) Ductility refers to the ability of a material to
be plastically deformed without fracture.
76. Which of the following characterizes a material that
displays elastic behavior right up to failure and
displays no plastic behavior?
a) Brittle
b) Ductile
c) Malleable
d) Serpentine
e) Neutral
Response (A) A brittle material displays elastic behavior
right up to the point of failure. Unlike a ductile material, a
brittle material display no elastic behavior. When subjected
to stress, a brittle material fractures with little elastic defor-
mation and no plastic deformation.
77. Which of the following terms describes the failure of
a material with repetitive loading at stress levels
below the ultimate tensile strength?
a) Fatigue failure
b) Ultimate failure
c) Interval collapse
d) Delayed decay
e) Temporal decline
11 Biomechanics of the Spine 249