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Chapter 6 Biomechanics of the Spinal Motion Segment 93
A
Intervertebral
Thoracic kyphosis
Transverse
process
Superior articular
Cervical lordosis
7 Cervical
vertebrae
SECTION
I
FIG. 6.2 Lumbar vertebra and its posterior elements. (Modied from Marras
WS. The Working Back: A Systems View. Hoboken, NJ: John Wiley & Sons;
2008.)
cancellous bone. is type of bone is less dense and more
elastic than cortical bone. Cancellous bone forms the interior
scaolding of the structure and helps the bone to maintain its
shape despite compressive forces. is structure is composed
of bundles of short and parallel strands of bone fused together.
form a protective channel, or tunnel, for the spinal cord and
nerves (see Fig. 6.1B). e biomechanical role of the posterior
elements is to control the position of the vertebral bodies.
ese elements provide attachment points for muscles to
control the position of the vertebra and supply lever arms to
provide the system with mechanical advantage. In addition,
these structures control motion and provide mechanical
“stops” to prevent excessive movement of the vertebral body.
Vertebral foramen
Nerve root
disc
Vertebra
Spinal cord
Functional spinal unit
FIG. 6.1 (A) Arrangement of the vertebral bones and spinal curves and (B) a functional spinal unit or spinal
motion segment. (Modied from Marras WS. The Working Back: A Systems View. Hoboken, NJ: John Wiley & Sons;
2008.)
Pedicle
process
process
Spinous process
Inferior articular
Laminae
e bony structures that constitute the posterior elements
B
12 Thoracic
vertebrae
Lumbar lordosis
5 Lumbar
vertebrae
Sacrum
Coccyx
Sacral kyphosis
A signicant portion of the mechanical load is borne by the
posterior elements, relieving the disc of excessive loading.
As shown in Fig. 6.2, toward the top of the posterior surface
of each vertebra are pedicles. e pedicles provide a robust
support structure (a type of pillar) to transmit force between
the posterior elements and the vertebral body. Projecting out
from each pedicle are the lamina structures that come together
at the midline of the body and form a neural arch. is arch
is a strong structure that provides protection to the spinal cord
in the form of a channel (vertebral foramen).
Emanating out from the junction of the two laminae at the
midline of the body is a bony protrusion called the spinous
process. Projecting laterally on each side of the structure at the
junction of the pedicle and the laminae is another bony
structure called the transverse process. ese processes provide
muscle attachment surfaces and mechanical advantage for
control of the spinal column.
Two sets of articulating surfaces are also present in the
posterior elements. Projecting out from each of the cephalic
lateral corners of the lamina is a bony extension called the
superior articular process. A portion of this surface is covered
by articular cartilage. Emanating from the caudal lateral
corner of the lamina on each side are the inferior articular
processes. e superior articular process from the lower vertebra interacts with the inferior articular process of the vertebra
above it to form a synovial joint known as the zygapophyseal
joint. is joint is also referred to as the facet joint. e inclination of the facet joint changes from the cervical spine to the
thoracic spine to the lumbar spine. is joint is dened as a
plane surface in the cervical and thoracic joints, but becomes
94 BASIC SCIENCE
fibrosus
A
Nucleus
AF
a curved surface in the lumbar spine. In the lumbar spine, the
inferior facets are convex, whereas the superior facets have a
concave shape. In addition, the angle of these surfaces relative
to the sagittal plane changes (increases) as one moves down
the lumbar spine. e dierences in orientation of these facet
joints restrict movement in dierent planes of motion. ey
serve an important function in that they permit certain
motions and limit other motions of the spine. ey can be
thought of as the guidance system of the spine.
Collectively, the posterior elements can provide a signicant
load path for the forces running through the spinal column.
Approximately one-third of a spinal load is carried through
the posterior elements in the upright posture. e nature of
the load transmission can be altered when spine degeneration
occurs by altering the vector of force and magnitude of force
transmitted through these posterior elements. is load path
can be disengaged, however, when the spine is in a exed
posture, and the load can be entirely passed through the disc.
Disc
e vertebral bodies are connected by discs that serve several
biomechanical purposes. First, the discs act as shock absorbers
between the vertebrae, absorbing a portion of the mechanical
forces transmitted through the spine. Second, they can transmit a portion of the mechanical load between vertebrae. ird,
the discs are able to permit and govern motion between the
vertebral bodies. Functionally, the discs are intended to
provide a separation between consecutive vertebrae. is
separation provides space between vertebrae so that the vertebral bodies can independently change their orientation and
execute bending movements. With this arrangement, a pliable
and deformable spinal structure is possible.
e disc consists of two distinct portions, each of which
is associated with a distinct mechanical function. e outer
portion of the disc, called the anulus brosus (AF), consists of
alternating layers of bers that are oriented at a 60- to 65-degree
angle relative to the vertical. e AF consists of about 10 to 20
concentric, circumferential sheets of collagen called lamellae
that are nestled together around the periphery of the disc (Fig.
6.3). e lamellae are sti and can withstand signicant com-
pression loading. Given the collagenous nature of these lamellae, they are pliable and can also permit bending of the spinal
column. If the structure were to buckle, however, it would lose
its stiness and would be unable to support compression. e
second portion of the disc—the nucleus pulposus (NP)—is
designed to overcome this potential problem.
Within the AF is a gelatinous core, the just-mentioned
nucleus pulposus (see Fig. 6.3). When compressed, this core
expands radially and places the AF in tension, providing stiness. e integrity of the system changes throughout the day.
e disc absorbs water while one is recumbent, which makes
the system stier than when one is upright. Conversely, when
one is upright, water is squeezed out of the disc, and the
structure becomes more lax.
Finally, the endplate is located at the intersection of the
disc and the vertebral body. e endplates are composed of
cartilage and cover the superior and inferior portions of the
Anulus
Intervertebral
disc
Nucleus
pulposus
Endplate
Posterior
AF
Anterior
pulposus
B
FIG. 6.3 (A) Disc, vertebral endplate, and vertebral body. (B) Construction
of intervertebral disc. AF, anulus brosus. (Modied from Marras WS. The
Working Back: A Systems View. Hoboken, NJ: John Wiley & Sons; 2008; and
Bogduk N. Clinical Anatomy of the Lumbar Spine and Sacrum, ed 4.
Edinburgh: Elsevier; 2005.)
FIG. 6.4 Ligaments of the spine. (From White AA III, Panjabi MM. Clinical
Biomechanics of the Spine, ed 2. Philadelphia: JB Lippincott; 1990.)
disc. ese structures bind the disc bers to the vertebral
bones and play a signicant role in disc nutritional transport.
Spinal Ligaments
e spinal ligaments play a signicant role from a biomechanical standpoint. Ligaments are most eective in supporting loads in the direction in which their bers run. ey
support loads under tension and can buckle under compression. ese structures can store energy and act much like a
rubber band in that they can provide resistance to loads by
developing tension.
e ligaments play three roles in biomechanics. First, they
permit motion and help orient the vertebrae without muscle
recruitment. Second, ligaments protect the spinal cord by
restricting spinal motion segment movement to within specic
ranges. ird, they absorb energy and protect the spinal cord
during rapid motions.
e spinal ligaments are shown in Fig. 6.4. e arrangement of these structures provides support for the spine in
dierent dimensions of loading. Because support is oered in
Chapter 6 Biomechanics of the Spinal Motion Segment 95
Sagittal
Horizontal
the dierent directions of motion, these structures provide
stability when the spinal system is intact.
Coordinate System and Force/Movement
Denitions
A biomechanical assessment of the spine is concerned with the
analysis of movements and forces developing within the spine
as it is exposed to activities of daily living (ADLs) and other
work or environmental conditions. Movements or motions
are compared with the natural limits of movement, and forces
imposed on a tissue (also called tissue loading) are compared
with tissue tolerances (magnitude of load at which damage
occurs). To describe movement and force transmission
through tissue accurately, it is necessary to describe precisely
direction of movement and direction and magnitude of the
force application on the tissue. Direction is dened relative to
a coordinate system or reference frame. e central (global)
coordinate system of the body is shown in Fig. 6.5. e origin
or center of this coordinate system is located at the base of
the spine. Fig. 6.5 describes the coordinate system (used in
this chapter) as a traditional three-dimensional cartesian
coordinate system with three mutually perpendicular axes
oriented with a vertical z axis. Some references have adopted
the International Society of Biomechanics (ISB) coordinate
convention, in which the y axis is dened as the vertical axis.
All movements of the spine are described relative to the
origin of the central coordinate system. Flexion and extension
are typically described in the sagittal plane, lateral bending
occurs in the coronal plane, and twisting occurs along the
horizontal or transverse plane. Most activities are combinations of movements in these planes.
Coronal
plane
plane
x
FIG. 6.5 Central or global coordinate system for the body.
plane
z
y
Within the spinal motion segment or functional spinal
unit, a local coordinate system can also be dened. e con-
vention that denes this local coordinate system is shown in
Fig. 6.6. Movement of the spinal motion segments is dened
relative to the subjacent vertebrae. Movements of the motion
segment can be either translations (indicating straight line
movements in any direction) or rotations (indicating movement around a point, as when bending).
Fig. 6.6 indicates that forces and moments (torques) can
develop along each dimension of the reference frame. Forces
along the z dimension are either compression or tension
depending on whether they compress the spinal motions
segment or pull on the tissues. ese are typically the forces
of concern when liing an object in the sagittal plane. Two
types of shear forces are also of concern when evaluating
the biomechanics of the spine. Anteroposterior shear force
describes the forward or backward force in the y axis that
can result from pushing or pulling activities. e lateral shear
forces refer to the sideways forces acting along the x axis and
represent the forces that develop in the spinal motion segment
when pushing an object to the side of the body.
Compression of the disc causes pressure within the NP
in all directions; this pressure places the AF under tension.
As shown in Fig. 6.7, the nucleus pressure can lead to deformation near the center of the endplate with this form of
loading.
Fig. 6.8 illustrates how shear, torsion, and tension inuence
the bers of the anulus. Shear forces tense the bers in the
direction of movement and relax the bers in the opposite
direction. Similarly, torsion or twisting tenses the bers that
are lengthened by the movement and relaxes the remaining
bers. is dierential of force among the bers is believed to
result in tissue damage. Finally, lengthening of the spine places
the bers under tension. is action increases the force on all
the bers regardless of their orientation.
Bending moments refer to forces acting around an axis, as
seen in Fig. 6.6. e curved arrows in this gure show the
direction in which moments act around a spinal segment. A
bending moment can be dened around the x axis, resulting in a movement in the sagittal plane (forward bending
moment), or it can be dened around the y axis, indicating
a sideways or lateral bend. In either of these situations, the
moment or torque around the central axis denes the loading
of the segment. Twisting of the spine can result when forces
are applied around the z axis of the spine. is situation results
in what is typically referred to as a torsional moment.
e forces and moments can be dened around each vertebra along the spine, resulting in a very large number of forces
and moments and numerous degrees of freedom. For practical
purposes, the forces and moment are typically dened in most
situations around one particular vertebra or disc (e.g., L5–S1)
depending on the purpose of the study.
Movements between vertebral bodies can also be coupled.
Coupling refers to the motion relationship of one vertebra
around an axis relative to another vertebra around a dierent
axis. In other words, coupling refers to the motion in dierent
planes that occurs simultaneously. e spine can bend forward
and twist at the same time—this is a coupled motion.
SECTION
I
96 BASIC SCIENCE
AB
AB
FIG. 6.6 (A) Spinal motion segment planes and directions of motion and (B) biomechanical coordinate system
and direction of forces and moments. Motions and forces are described relative to this coordinate system.
(Reproduced with permission from Bogduk N. Clinical Anatomy of the Lumbar Spine and Sacrum. 4th ed.
Edinburgh: Elsevier, 2005.)
Endplate load
Deformation
FIG. 6.7 (A) Compression of disc leading to increased pressure in the disc
nucleus. (B) Increased nucleus pressure causes endplate loading and
deformation. (From White AA III, Panjabi MM. Clinical Biomechanics of the
Spine, ed 2. Philadelphia: JB Lippincott; 1990.)
e amount of displacement between the neutral position
of the vertebra and the point at which resistance to physiologic
motion is experienced is referred to as a neutral zone.2 Neutral
zones can be dened for translational and rotational movements. e neutral zone can be described for each of 6 degrees
of freedom.
Tissue Load Characteristics
e forces represented in Fig. 6.6 dene the direction of load
application and the magnitude of the force. e nature and
temporal characteristics of the loading situation also dene
the probability that the load application will result in tissue
damage. It is believed that tissue damage can result from
several dierent “types” of trauma to the tissue. Each type of
trauma is believed to be associated with very dierent tolerance levels. First, acute trauma is the most familiar type of
loading. Acute trauma refers to a single application of force
that exceeds the tolerance level of the tissue. is would be the
case if a large load was imposed on the spinal motion segment
and a rupture of the disc occurred. In this case, the magnitude
of the force applied in a particular direction would far exceed
the tissue strength of the disc, resulting in a rupture.
Another well-recognized mechanism of tissue disruption
involves repeated cumulative loading of the tissues. With
cumulative trauma, moderate repetitive loads are applied to
the tissues; this repeated loading is believed to weaken the
structure so that the tolerance of the tissue is reduced. Although
moderate loading can cause the tissues to strengthen and
adapt to load, repetitive loading without proper rest (adaptation) time can cause degeneration of the tissues. Repetitive
application of force to a structure is believed to cause microtrauma, which weakens the structure and leads to failure at
lower levels than would be expected with an acute trauma to
the tissue.
A third type of biomechanical trauma (instability) has
received much attention in the literature.
3–8
Stability is the
ability of a system to respond to a perturbation and reestablish
a state of equilibrium.2 Instability of the spine refers to the
abnormal displacement of the spine under physiologic loading.
Chapter 6 Biomechanics of the Spinal Motion Segment 97
ABC
FIG. 6.8 The eects of shear (A), torsion (B), and tension (C) on the bers of the anulus brosus. (From Adams
MA, Bogduk N, Burton AK, et al. The Biomechanics of Back Pain, ed 2. Edinburgh: Elsevier; 2013.)
SECTION
I
e abnormal displacement can occur in translation or rotation, but most likely would be some combination of these two
types of motions. ese abnormal motions are oen small in
magnitude, but the displacement may be enough to stimulate
pain in sensitive tissue. Stability is signicant because it is
oen the initiator of tissue damage when the system is out of
alignment or when the musculoskeletal system overcompensates for a perturbation.2 When the supporting musculature
cannot oer adequate stability to a joint (owing to improper
muscle recruitment, fatigue, structure laxity, or weakness), the
structure may move abnormally and result in sudden and
unexpected force applications on a tissue. is type of trauma
is similar to the acute trauma pathway but is initiated by a
miscalculation of the muscle recruitment pattern. Instability
can also be secondary to trauma, developing over time in cases
of degeneration and cancer.
Mechanical Degeneration: Tissues at Risk
Many tissues in the spinal motion segment can be inuenced
by structure loading. ese tissues include bones, discs, ligaments, tendons, and nerves. Tissue loading can result in a
disruption of tissue integrity. Bones can be cracked or broken,
disc endplates can sustain microfractures, the disc can bulge
or rupture, muscle can experience ber tears, and blood ow
to the tissues can be disrupted. All of these events are believed
to be capable of initiating a sequence or cascade of events
leading to back pain. e tolerance of many of these structures
within the spine is reviewed in detail in this chapter.
Clinicians are beginning to understand that low back disorders can occur before tissue damage. Biochemical studies
have shown that these types of tissue insults can result in an
upregulation of proinammatory cytokines. is upregulation
may result in tissue inammation at much lower levels of load
than would occur under normal conditions. is inammation makes nociceptive tissues more sensitive to pain and may
initiate back pain.
Much attention in spine biomechanics and clinical care
has been focused on the intervertebral disc because disc
9
disruption has been associated with pain. Over the past several
decades, clinicians have also begun to understand how spine
loading can initiate the degeneration process within the disc.
To appreciate this process, the system behavior of the disc,
vertebral body, and endplate must be considered in response
to cumulative trauma. e disc receives no direct blood supply
for nourishment. It relies heavily on nutrient ow and diusion
from surrounding vascularized tissue for disc viability. e
nourishment is transported from the vertebral body through
the endplate to the disc. e endplate is very thin (about 1 mm
thick) and facilitates nutrient transport to the disc.
When endplate loading exceeds its tolerance limit,
microfractures can occur in the structure. Microfracture of
the endplate itself usually does not initiate pain because few
pain receptors reside within the disc and endplate. Repeated
microfracture of this vertebral endplate can lead to the formation of scar tissue and calcication, which can interfere with
nutrient ow to the disc bers. Because scar tissue is thicker
and denser than endplate tissue, the scar tissue interferes with
nutrient delivery to the disc. is reduced nutrient ow can
lead to atrophy and weakening of the disc bers and disc
degeneration. Because the disc has relatively few nociceptors
except at the outer layers, this degenerative process is usually
not noticed by the individual until the disc is weakened to the
point at which bulging or rupture occurs, and surrounding
tissues that are rich in nociceptors are stimulated. Fig. 6.9
illustrates this sequence of events that are believed to lead
to disc degeneration and potential tissue damage, such as
herniation.
9
e literature also provides some evidence that excessive
motion within the spinal segment can lead to degeneration.
Excessive motion at a joint is believed to increase the cumulative trauma on the spinal structures and potentially initiate
either tissue degeneration or an upregulation of proinammatory cytokines. is has become apparent in studies that have
examined the degeneration of segments adjacent to spinal
fusions.10 If two spinal levels are fused, trunk motion usually
results in exacerbated movement, especially at the facet joints
within spinal levels adjacent to the fusion. One study noted
98 BASIC SCIENCE
n
A
Excessive or highly
repetitive forces
Endplate microfracture
Scar tissue
Reduced nutrients
Degeneration
(anulus fibrosus)
FIG. 6.9 (A) Sequence of events associated with cumulative or repeated trauma leading to disc degeneration.
(B) Herniated disc showing disruptions to the anulus brosus. (B, Courtesy Ehud Mendel.)
B
hypertrophic degenerative arthritis of the facet joints in
motion segments adjacent to a fusion typically following a
symptom-free period (8.5 years, on average).10 Another study
found signicant evidence of degeneration at levels adjacent
to a fusion with the rate of symptomatic degeneration at the
adjacent segment warranting either decompression or
arthrodesis to be 16.4% at 5 years aer fusion and 36.1% at 10
years aer the surgery.11 In addition, more recent studies
examining articial discs have reported facet arthrosis.12 Facet
load forces have been shown to depend on articial disc placement and the subsequent load transferred to the facets.
13
e application of damaging compressive forces on the
vertebral body can result in several dierent types of failures
of vertebrae. e failure characteristics have been described in
the literature14 and are shown graphically in Fig. 6.10. is
gure indicates that seven types of failures are typically seen
as a result of compression. ese consist of stellate fracture,
step fracture, intrusion fracture (with Schmorl’s nodes),
depression of the endplate, Y-shaped fracture, edge fracture,
and transverse fracture.
Many of these fractures suggest weakness of the endplate.
is weakness is a result of the thinness of the endplate necessary for nutrient transport to the disc. ese fractures are
believed to result from the NP of the adjacent disc bulging into
the vertebra.15 Clinically, vertebral body fractures that occur
purely from axial compression are classied as type A based
on the AOSpine classication system.16 Fig. 6.11 shows four
common subtypes of type A fractures.
In Vitro Spine Biomechanics
Motion Characteristics (Kinematics) of the Spinal Motion Segments
e typical ranges of motion (ROMs) associated with cervical,
thoracic, and lumbar motion segments have been well
described in the literature4 and are summarized in Table 6.1.
A graphic estimate of spinal segment ROM associated with the
entire spine is presented in Fig. 6.12.2 Table 6.1 shows the vast
dierences in motion capacity for the various vertebrae as a
Tissue disruptio
function of the spine region and the vertebral level. Each
region of the spine allows or limits motion in a particular
motion direction compared with other regions of the spine.
is information shows that, in the sagittal plane, the most
ROM occurs in the cervical spine followed by the lumbar
spine. Laterally directed motions, although much smaller in
magnitude than motions in the sagittal plane, occur freely in
the cervical spine, with much less movement available in the
thoracic and lumbar spine. Finally, very little axial rotation is
possible in the lumbar spine, with most motion occurring in
the thoracic vertebrae except for C1–C2.
Collectively, the body of work described in Table 6.1 and
Fig. 6.12 represents the summary of expected movement
characteristics derived in vitro. To the extent that in vitro
characteristics are indicative of in vivo characteristics, they
can provide a baseline for movement expectations for the
various vertebrae along the spinal column.
It is also possible that abnormal movement of the motion
segment can indicate disc damage. Studies have also shown
that tears in the AF change the movement characteristics of
the motion segments. Specically, tears in the anulus increase
the amount of motion in the motion segment when torque is
applied to the segment.
17
Axis of Rotation
To understand and describe better how motion occurs among
vertebrae, an axis (or center) of rotation is oen dened.
When bones move relative to one another in a single plane,
there is a point around which the object rotates. If a hypothetical line is extended from the constant point within a vertebra,
the point at which these two lines meet when the vertebra
moves between two dierent positions is called the instanta-
neous axis of rotation. is concept can be extended to threedimensional space; however, identifying the axis of rotation
becomes more complex. Understanding of the axis of rotation
helps one understand how kinematics are altered because of
degeneration or surgical intervention. Identication of this
point also has implications for how forces are transmitted
through the spine.
Chapter 6 Biomechanics of the Spinal Motion Segment 99
SECTION
I
FIG. 6.10 Seven types of fractures identied by Brinkmann and colleagues.14 (From Adams MA, Bogduk N,
Burton AK, et al. The Biomechanics of Back Pain, ed 2. Edinburgh: Elsevier; 2013.)
Relative movement of a vertebra can be divided into translational movement (sliding motions) and rotational movement.
During physiologic movements, the components of compression force and bending moment acting on the spine vary, along
with the translational and bending movements. is action
results in a varying axis of rotation position. e axis of rotation
is dened as a “locus,” or path, that the axis of rotation takes.
18
During sagittal and frontal plane motions, the axis of rotation in the cervical spine is believed to be located in the
anterior portion of the subjacent vertebra.2 Coupling also
occurs with cervical motions, however. In the thoracic spine,
loads applied during exion and extension motions result in
an axis of rotation located at the inferior endplate of the lower
vertebra. is axis of rotation moves farther down the vertebra
when posterior shear force occurs during extension motions.2
During exion and extension motions, the axis of rotation
occurs in the superior endplate of the inferior vertebra of the
spinal motion segment.
During sagittal plane bending, the axis of rotation varies
according to whether forward or backward bending is occurring. Because much of the exion and extension in the sagittal
plane occurs in the lumbar spine, much of the interest in the
axis of rotation has also been focused on the lumbar spine.
e superior vertebra translates anteriorly and posteriorly
relative to the inferior vertebra as the vertebral body rotates
around the nucleus. Aer degeneration of the disc, the axis of
rotation can change dramatically,19 resulting in marked
changes in spine loading. Under these degenerative conditions, the axis of rotation has been reported to migrate toward
the zygapophyseal joint during extension motions.20 During
exion, the axis of rotation seems to move and is dependent
on coupling patterns during the exion movement.
During lateral motions, the axis of rotation in the lumbar
spine lies at the opposite side of the disc from the direction of
motion. In other words, when bending to the right, the le
side of the disc is where the axis of rotation is located.
2
100 BASIC SCIENCE
No posterior wall involvement
Subtype A1
Wedge or impaction fractures
Subtype A3
Incomplete burst fractures
Subtype A2
Split or pincer-type impaction fractures
Posterior wall involvement
Subtype A4
Complete burst fractures
FIG. 6.11 Type A compression fractures based on the AOSpine Classication. (From Reinhold M, Audige L,
Schnake KJ, et al. AO spine injury classication system: a revision proposal for the thoracic and lumbar spine.
Eur Spine J. 2013;22[10]:2184-2201.)
e axis of rotation for axial (torsion) movements has been
dicult to locate. is axis of rotation is believed to lie within
the posterior AF when exposed to torque.21 Even small axial
motion can create compression at one facet surface and tension
at the opposite facet surface.5 With disc degeneration, the axis
of rotation becomes far less apparent, however, in the lumbar
spine.4 Under degenerative conditions, the locus of the axis of
rotation has been reported to be signicantly spread out over
an extended area.
21
Collectively, the literature has described the locations of the
axis of rotation for various “normal” motions. It is apparent,
however, that these axes change dramatically with degeneration, and should be factored in when considering load bearing
through the spine and motion proles.
Motion Coupling
A signicant amount of coupling has been observed along the
spinal column. Coupling is a function of the geometric characteristics of specic vertebrae, limitations in tissue properties
of the disc and ligaments, and spine curvature. Movements are
considered coupled when one motion is accompanied by
motion in a dierent plane.2 e motion in the primary, or
intended, plane of movement is referred to as the main motion;
the accompanying motions are referred to as coupled motions.
Because coupling can have profound implications on the
transmission of forces through the spine, it is important that
the nature of coupling in the dierent regions of the spine be
understood. From a clinical perspective, coupling is important
in understanding the impact of various pathologies, such as
scoliosis and dierent types of spine trauma. In addition, an
appreciation for coupling is important for understanding the
impact of surgical interventions, such as the impact of fusion.
Coupling is most common in the cervical and lumbar
spine, but can also occur in the thoracic spine. Coupling in
the cervical and lumbar spine involves axial rotation coupled
with lateral bending. Lumbar motion can involve crosscoupling in all three rotation directions. Motions in the lumbar
spine are rarely unaccompanied by coupled movements.
Coupled motions of the lumbar spine vary as a function of the
spine level and a function of spine posture.
2
Coupling patterns within the spine dier depending on the
region of the spine. e cervical spine exhibits a striking
degree of coupling in that lateral bending of the head is
TABLE 6.1 Limits and Representative Values of Ranges of Rotation for Cervical, Thoracic, and Lumbar Spine
Combined
One side
One side
5° 10° 15° 20° 25° 5° 10° 15° 5° 10° 15° 35° 40°
Interspace
COMBINED FLEXION-EXTENSION
(± Y-AXIS ROTATION)
Limits of Ranges
(Degrees)
Representative
Angle (Degrees)
ONE SIDE LATERAL BENDING
(X-AXIS ROTATION)
Limits of Ranges
(Degrees)
Representative
Angle (Degrees)
ONE SIDE AXIAL ROTATION
(Z-AXIS ROTATION)
Limits of Ranges
(Degrees)
Representative
Angle (Degrees)
C0–C1 25 5 5
C1–C2 20 5 40
Middle
C2–C3 5–16 10 11–20 10 0–10 3
C3–C4 7–26 15 9–15 11 3–10 7
C4–C5 13–29 20 0–16 11 1–12 7
Lower
C5–C6 13–29 20 0–16 8 2–12 7
C6–C7 6–26 17 0–17 7 2–10 6
C7-T1 4–7 9 0–17 4 0–7 2
T1–T2 3–5 4 5 5 14 9
T2–T3 3–5 4 5–7 6 4–12 8
T3–T4 2–5 4 3–7 5 5–11 8
T4–T5 2–5 4 5–6 6 5–11 8
T5–T6 3–5 4 5–6 6 5–11 8
T6–T7 2–7 5 6 6 4–11 7
T7–T8 3–8 6 3–8 6 4–11 7
T8–T9 3–8 6 4–7 6 6–7 6
T9–T10 3–8 6 4–7 6 3–5 4
T10–T11 4–14
9 3–10 7 2–3 2
T11–T12 6–20 12 4–13 9 2–3 2
T12–L1 6–20 12 5–10 8 2–3 2
L1–L2 5–16 12 3–8 6 1–3 2
L2–L3 8–18 14 3–10 6 1–3 2
L3–L4 6–17 15 4–12 8 1–3 2
L4–L5 9–21 16 3–9 6 1–3 2
L5–S1 10–24 17 2–6 3 0–2 1
From White AA III, Panjabi MM. Clinical Biomechanics of the Spine, ed 2. Philadelphia: JB Lippincott; 1990.
C
E
R
V
I
C
A
L
C0–C1
C2–C3
C4–C5
C6–C7
flexion/extension
(± y-axis rotation)
lateral bending
(x-axis rotation)
T1–T2
T
T3–T4
H
O
T5–T6
R
A
T7–T8
C
I
T9–T10
C
T11–T12
L
L1–L2
U
M
L3–L4
B
A
L5–S1
R
axial rotation
(z-axis rotation)
FIG. 6.12 Composite estimate of representative values for ranges of motion at dierent levels of the spine in
sagittal, lateral, and transverse planes of the body. (From White AA III, Panjabi MM. Clinical Biomechanics of the
Spine, ed 2. Philadelphia: JB Lippincott; 1990.)
102 BASIC SCIENCE
TABLE 6.2 Coupled Motions of the Lumbar Spine
AXIAL ROTATION, DEGREES
Primary Movement
and Level
Right Rotation
L1
L2
L3
L4
L5
Left Rotation
L1 1
L2 1
L3 2 0 to 1 0
L4 2 0 to 1 0
L5 0
Right Lateral Flexion
L1 0
L2 1
L3 1
L4 1 0 to 1 0
L5 0
Left Lateral Flexion
L1 0
L2
L3
L4
L5
Mean Range Mean Range Mean Range
−1 −2 to 1
−1 −2 to 1
−1 −3 to 1
−1 −2 to 1
−1 −2 to 1
−1 −3 to 1 −3 −4 to −1
−1 −4 to 1 −2 −4 to 3
−1 −4 to 1 −1 −4 to 2
−2 −3 to 1
(+ TO LEFT)
−1 to 1
−1 to 1
−2 to 1
−3 to 1 −2 −5 to 1 −5 −8 to −
−1 to 1 −1 −3 to 1 −5 −8 to −4
−1 to 1 −1 −3 to 1 −5 −11 to 2
−1 to 1
−2 to 1 −2 −9 to 0
FLEXION-EXTENSION, DEGREES
(+ FLEXION)
0
0
0
0
0
0
0
0
2
0
LATERAL FLEXION, DEGREES
(+ TO LEFT)
−3 to 3
−2 to 2
−2 to 2
−9 to 6
−5 to 3 −2 −7 to 0
−4 to 4 −3 −7 to −1
−4 to 4 −3 −5 to 0
−3 to 2 −3 −6 to 0
−7 to 2 −2 −5 to 1
−5 to 3
−1 to 4 −3 −5 to 1
−3 to 8
−5 to 5 −3 −6 to 1
3
4 1 to 9
3 1 to 6
1
1 0 to 2
0
6 4 to 10
6 2 to 10
6
3
−1 to 5
−3 to 3
−2 to 3
−3 to 8
−3 to 6
2
(From Adams MA, Bogduk N, Burton AK, et al. The Biomechanics of Back Pain, ed 2. Edinburgh: Elsevier, 2006.)
accompanied by signicant amounts of cervical rotation. is
is evident by observing the position of the spinous processes
as lateral bending occurs. When lateral bend to the le occurs,
the spinous processes point to the right; when lateral bending
to the right occurs, the spinous processes go to the le. It is
generally thought that the angle of incline of the facet joints
in the sagittal plane increases from the head toward the lower
spine.2 Generally, the average ratio of the coupled lateral
bending compared with axial rotation is 0.51.
22
e coupling of lateral bending and spine rotation can also
occur in the thoracic spine. As with the cervical spine, lateral
bending is coupled with axial rotation in such a way that the
spinous process moves toward the convexity of the lateral
curve. e vertebrae in the upper portion of the thoracic spine
have motions that are strongly coupled, but not to the same
degree as in the cervical spine. In the middle segments of the
thoracic spine, the coupling motions are far less apparent.
Coupled motions in this portion of the thoracic spine are
coupling pattern of the lumbar spine seems to be lateral
bending coupled with axial rotation (Table 6.2).24 In this case,
the spinous process moves in the same direction as lateral
bending. is is exactly opposite to the pattern in the cervical
and upper thoracic spine. One group of researchers25 reported,
however, that coupling at L5–S1 occurs in a fashion similar to
that of the lower cervical spine and opposite to that of the rest
of the lumbar spine.
In vivo studies of the lumbar spine have shown the importance of muscular involvement in determining coupling patterns of the lumbar spine.25 In vitro studies have reported that
lateral bending motion was coupled with exion motions
between L1 and L3, whereas in vivo studies reported that
lateral motions are coupled with extension movements in
these vertebrae. In addition, biomechanical analyses have
shown that coupling in the lumbar spine can be inuenced by
posture of the spine.
can also play an important role in coupling patterns.
inconsistent and can result in rotations opposite of those in
the upper thoracic spine. Coupling patterns in the lower
portion of the thoracic spine are weak. Although the patterns
of coupling between axial rotation and lateral bending have
been described in the literature, most likely owing to a desire
to understand scoliosis, Panjabi and colleagues23 have shown
that coupling can occur in all 6 degrees of freedom.
Coupling patterns in the lumbar spine seem to dier from
those of the cervical and thoracic spine. e most dominant
Neutral Zone Limits
As discussed earlier, the neutral zone is important for understanding when tissues rst experience resistance to movement.
Low intersegmental resistance to motion can be an indication
of biomechanical problems. e neutral zones for the dierent
planes of motion have been extensively described by Panjabi
and colleagues.
25,26
One would expect that muscle control
27–29
Table 6.3 shows estimates for the neutral