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33 Biomechanical Principles of Spine Stabilization
a
55
d
50
45
b
40
35
30
443
c
Fig. 33.2 Facet joint orientation. The relative coronal
plane orientation in the cervical region (a), the intermediate orientation in the thoracic region (b), and the relative
sagittal orientation in the lumbar region (c). The facet
joint orientation changes substantially in the lumbar
25
Facet Joint Angle (with respect to midline)
20
L1-2
load-bearing structures of the spine (e.g., intervertebral discs) fail as well.
Spinal ligaments also provide important stabilization. Each ligament confers differing strength,
but together they act as a tension band along the
length of the spine to resist translational forces.
This tension band effect is derived from the overall tensile strength of the ligaments. Lastly, a
destabilizing force can result from an imbalance
in the aggregate musculature which will accentuate strain on the other stabilizing components of
the spine. The most important musculature is the
paraspinal muscles and its multiple attachments
spanning several segments.
Biomechanical Physical Principles and Kinematics
Any force applied to the spine can be deconstructed into three component vectors that exist
within a three-dimensional Cartesian coordinate
L2-3
region; here the facet joint angle (with respect to midline)
is depicted versus spinal level (d) (Fig. 1.6 in Biomechanics
of spine stabilization, Benzel E, ed. Printed with permission from Thieme Medical Publishing)
L3-4
Spinal Level
L4-5
L5-S1
system and have fixed orientations. Each force
can act either directly on the spine or as a lever
arm that can rotate around an instantaneous axis
of rotation (IAR) (i.e., a moment arm), which
creates a bending moment when a force is applied
perpendicularly. To rephrase, the effect of the
force on the spine is relative to an IAR that acts as
a fulcrum and is dependent on where the force is
applied. The IAR is not a singular, permanent
entity or property of the spine; rather, the IAR is
dynamic, changing with every spinal segment
over the time interval of a movement (i.e., the
IAR migrates with motion).
The healthy spine allows physiological movement in the ventral-dorsal, right-left, and cranialcaudal axes with either a translational or rotational
component resulting in six distinct potential
movements that are referred to as degrees of freedom (Fig.
33.3). Physiologic range of motion is
contextual and depends on the spinal region (i.e.,
cervical versus lumbar) and is dependent on the
orientation and properties of structural components.
444
A.Y. Chan et al.
Fig. 33.3 The six fundamental segmental movements, or
types of deformation, of the spine along or about the IAR
are (1) rotation or translation about the long axis of the
spine (a), (2) rotation or translation about the coronal axis
of the spine (b), (3) rotation or translation about the sagittal axis of the spine (c), (4) translation along the long axis
Therefore, normal physiologic motion of one
region of the spine can be considered pathologic
in another.
External forces can change the physical characteristics of the spine. In theory, the magnitude
of strain (i.e., the deforming force) of an ideal
object is directly proportional to the stress
applied to it (i.e., Hooke’s Law). Biological tissues can deviate to make the relationship
between strain and stress segmented and nonlinear; this can be described by the load-deformation curve (Fig. 33.4). First, there exists a neutral
zone where there is high flexibility at low levels
of stress which is essential for normal physiologic motion. Second, there is a point where
enough stress is applied to the tissue to cause
permanent distortion known as the “elastic
limit.” If additional stress is applied beyond the
elastic limit, it causes a disproportionate amount
of strain that eventually leads to failure of the
tissue. Also important to these concepts is section modulus and moment of inertia.
The section modulus is an indicator of the
object’s strength and therefore reflects the ability
to resist failure (flexion or yield point of the tissue), whereas the moment of inertia portrays
of the spine (a), (5) translation along the coronal axis of
the spine (b), and (6) translation along the sagittal axis of
the spine (c) (Fig. 6.1 in Biomechanics of spine stabilization, Benzel E, ed. Printed with permission from Thieme
Medical Publishing)
Fig. 33.4 A typical load-deformation curve depicting the
neutral and elastic zones (deformation or strain versus
load or stress) (Fig. 1.20 in Biomechanics of spine stabilization, Benzel E, ed. Printed with permission from
Thieme Medical Publishing)
stiffness against angular rotation around a rotational axis (i.e., torque) and thus measures the
object’s distribution of mass around its center.
33 Biomechanical Principles of Spine Stabilization
445
Spinal Stability Versus Instability
Clinical stability of the spine is equated to the
ability of the spine to limit patterns of
displacement under physiologic loads to prevent
debilitating deformation or pain [
stabilization is to create an architecture of the
vertebral column to allow fusion or healing (e.g.,
percutaneous screw fusionless constructs) to
occur and to protect the neural elements. This
stability is maintained by an active subsystem
(i.e., the musculature), a passive subsystem (i.e.,
the vertebral column), and a neural-derived
component.
Destabilization of the spine occurs when the
spine is unable to resist loads or abnormal spinal
movements. Instability should be thought of as a
spectrum that ranges from “stable” to “grossly
unstable” rather than an all-or-nothing phenomenon. Thus, defining a standard cutoff for “excessive” is difficult and may vary based on many
factors including bone integrity, structural anatomy, forces applied, etc.
Furthermore, instability can be categorized as
acute or chronic. Acute instability is described as
being either overt or limited and associated with
conditions such as trauma, iatrogenia, infection,
or malignancy. Overt instability is when the spine
loses integrity in both the ventral and dorsal elements, resulting in loss of sufficient support during physiologic activities. In other words,
circumferentially vertebral column integrity is
lost which prevents the ability to resist sudden
development of a spinal deformity. Overt instability should almost always be treated surgically.
In contrast, limited instability is integrity lost
in only the ventral or dorsal component of the
vertebral column. It should be noted that limited
instability usually confers enough support to
allow most physiologic motions and that it is
often treated without surgical intervention.
Overt and occasionally limited acute instability can progress to a chronic form if left
untreated, but such can also be the result of
degenerative changes without an inciting acute
event (e.g., infection or trauma). Chronic instability can be categorized, including glacial instability or dysfunctional segment motion. Glacial
5]. The goal of
instability is when the instability progresses
slowly and steadily, likened to the movement of
a glacier. Dysfunctional segment motion may
contribute to the pain experienced without progression of instability but lacks precise consensus definitions.
Spinal Column Pathology
The spine can undergo pathological changes due
to a variety of factors. For example, the spine
degenerates over the individual’s life due to combinations of genetics, health, and life events.
Degeneration is an expected part of aging. The
end result is the spine becomes less flexible with
a lower range of motion as it ages [6–9]. The cervical spine in particular may be most vulnerable
to these changes because it exhibits the highest
degree of motion and complexity in certain
aspects of its anatomy [6]. Another example of
potential pathology is infection. Vertebral osteomyelitis can have devastating consequences like
paralysis or death [10] and thus must be managed
quickly and appropriately. Conversely, the intervertebral disc space can become infected and can
present as a complication following surgery [11],
though it has been shown to be spontaneous as
well [12]. Further, the pathogen responsible for
hematogenous pyogenic infections is often
Staphylococcus aureus [13]. Ultimately, patho-
logical changes in general can affect the biomechanical properties of the spine.
Spinal Alignment
The alignment of the spine may change due to
age or pathology which leads to an altered stress
distribution within the apophyseal joints and
intervertebral discs. The spine has a conformation that maximizes tolerance of concentric and
eccentric loads, allowing for flexibility of physiologic motions. Specifically, the cervical and
lumbar regions are lordotic in curvature and the
thoracic curvature is kyphotic; the curves are,
ideally, of equal summative magnitude, which
results in a balanced distribution that allows for
446
A.Y. Chan et al.
bipedal upright posture. When lumbar lordosis
decreases (or thoracic kyphosis increases), the
moment arm lengthens for each vertebral segment which results in a greater bending moment
when a force acts on the spine. Deformities in the
coronal plane (e.g., scoliosis) occur by the same
mechanism.
Spinal Fusions
Stabilization of the spine is ultimately achieved
by bony fusion. A spinal implant will fail, eventually, unless bony fusion occurs prior to fatigue
of the implant (Fig. 33.5). The structural integrities of the implant and affected bone have contrasting courses: the implant is strongest
immediately and gradually weakens over time
(i.e., implant failure), while the bone is weakest
initially and strengthens over time (i.e., arthrodesis begets fusion). Thus, a proverbial “race”
exists between implant failure and fusion of the
bone, and bone graft should be utilized in most
instances of internal fixation [1].
maximizes axial loading resistance and thus stabilizes the torso. Moreover, the optimal graft is
also placed within its neutral axis or the location
that is displaced the least during flexion and
extension. The position of the ventral graft at the
IAR in the sagittal plane and neutral axis is most
optimal if the dorsal spinal stability is not intact.
However, if the dorsal elements are intact, an
interbody graft may be positioned more ventrally,
as the axial loading would be evenly distributed
between the graft and the dorsal elements [5].
Lastly, even distribution of axial loads by placing
a ventral interbody graft prevents kyphotic deformation in the region of the fusion [14].
Ideally, the consistency and integrity of a ventral bone graft should be similar to that of the vertebral bodies. This prevents graft penetration into
the adjacent vertebral body or nonunion to occur
from either a too strong or too weak interbody
bone graft, respectively (Fig. 33.6). Specifically,
the vertebral body end plate is weaker toward the
center and strongest in the periphery; thus, interbody devices should account for this strength
gradient and concentrate loading on the periphery to produce the best outcomes.
Ventral Fusion
The position of where a ventral bone graft is
placed matters, especially in the sagittal plane. A
ventral interbody graft has the advantage of lying
in the weight-bearing region of the spine and is
usually at the IAR within the sagittal plane. This
Fig. 33.5 After surgery,
the relationship between
bone fusion acquisition
and spinal implant
integrity changes over
time (Fig. 10.1 in
Biomechanics of spine
stabilization, Benzel E,
ed. Printed with
permission from Thieme
Medical Publishing)
Structural Integrity
of Implant and Fusion
Surgery
Posterior Fusion
In contrast to ventral fusions, posterior fusions do
not contribute as much to axial load resistance
which is intuitive since the ventral anatomy of the
spine provides almost all of the resistance to axial
Bone Fusion
Component
Spinal Implant
Component
Time
33 Biomechanical Principles of Spine Stabilization
447
Fig. 33.6 The importance of matching the integrity of the
bone graft bed (the vertebral body) and that of the bone
graft with ventral interbody fusions cannot be overemphasized. If a bone graft that is denser than the vertebral body
is used, the tendency of the graft to “knife” its way through
the vertebral body (piston) is significant (a and b).
Conversely, if the bone graft is less dense and weaker than
loading. Therefore, ventral fusions with higher
compression on the graft confers a faster healing
rate [15] compared to the fusion rate for a posterior fusion which is under tensile forces which
have a less stimulatory effects on osteoblast function. However, the graft resists flexion well due to
a flexion-resisting moment arm created by the
distance between the graft and the IAR.
Fusion with Bone Graft Alone
One of the decisions made in spinal fusions is
whether an implant is necessary. In some circumstances, the bone graft can act alone as a spinal
instrument by providing structural support without deformation in response to applied forces.
Allograft or autograft structural bone implants
are stiff and resist unidirectional forces (mainly
axial loading) immediately postoperatively, but
they are contingent on intact tension bands (i.e.,
adequate ligament integrity or supplemental
instrumentation). Furthermore, the integrity of
bone grafts is affected by the ratio of cortical to
medullary bone; the strength of the bone increases
as the cortex to medulla ratio rises [16]. However,
stand-alone ventral grafts may provide some
translational resistance if the graft is placed in
adequately carved mortise with proper depth.
the vertebral body, the bone graft may fail (c). Therefore,
a bone graft that is of similar density, integrity, and modulus of elasticity to the vertebral body is optimal. It is neither the weakest nor the strongest link in the “stability
linkage system” (Fig. 10.5 in Biomechanics of spine stabilization, Benzel E, ed. Printed with permission from
Thieme Medical Publishing)
Still, the translational resistance is relatively
weak even with a properly crafted mortise, and
thus stand-alone grafts between vertebral bodies
may not resist translational and rotational forces
adequately.
Principles of Construct Design
Many spinal constructs provide stability by functioning as a tension band that has adequate
strength to convert tension into a compressive
force and provide resistance to bending moments.
Specifically, a construct attached above and
below a spinal segment will convert the tension
into a compressive force that acts on the segment.
This principle implies the spinal segment can
withstand additional compressive force. The
resulting compressive force may help encourage
fusion. A posterior single-level fixation with cervical hook plates or posterior wire fixation is an
example of a tension band.
When a weight-bearing component of the ver-
temporarily (e.g., burst fracture), a construct can
be used to span the entire length of the damaged
spinal segment to provide support, as well as
maintain alignment and proper length. This is
known as bridge fixation and it allows for load
448
A.Y. Chan et al.
sharing. The placement of dorsal pedicle screws
and rods to treat a burst fracture is an example.
An implant can act as a buttress for a weak
point and ideally is implemented on the side of
the load application where the spine requires
additional support for stability. A specific example is an anterior cervical locking plate system
because it prevents axial deformity while
providing some resistance against shearing or
compressing.
In a cantilever system, the moment arm can be
either fixed or non-fixed. When the moment arm
is fixed, it is perpendicular to the screw, but when
the moment arm is not fixed, the screw will experience a three-point moment force that is greatest
at the fulcrum. In either case, the screw will fracture where the force is greatest.
Construct Failure
A construct fails when it stops providing the support necessary to maintain stability. A construct
undergoes millions of loading cycles and failures
can occur when inappropriate constructs are
used. The amount or frequency of stress may
have been underestimated, the construct poorly
designed, or improper patient selection may have
occurred.
Construct failure depends on both the intrinsic
material property of the device and the amount of
exposure to stress. Instrumentation will fail when
the ratio of the applied bending moment and section modulus is highest which is the maximum
stress that can be applied whether this arises from
an instantaneous or cyclic overload. “Fatigue
failure” describes the breakage of a construct following repeated bouts of sustained excessive
force. Technical aspects of the procedure can also
contribute to failure. For example, manipulating
the shape of instrumentation (e.g., contouring or
bending a rod or plate) can create structural
weakness by altering the point where stress is
concentrated.
Finally, significant vulnerability of the construct occurs at the implant attachment points.
The bone may not be sturdy enough to resist the
loading forces of the construct causing screws to
loosen or pull out of the bone. The screwhead
may even fracture from the screw shaft if excess
force is applied. Multi-segmental constructs with
long and rigid fixed moment arms may load the
caudal screws more than the cranial ones and
thus are associated with a high rate of failure of
the caudal screws.
Avoiding Iatrogenic Spinal Destabilization
Unintentional destabilization during exposure and
decompression of the spine, regardless of
approach, is important as this may impact ultimate
outcomes. Preserving load-resisting structures
such as facet joints, interspinous ligaments, and
muscular attachments minimize the risk of destabilization during a dorsal decompression.
Resecting roughly one-third to one-half of the
facet joint is tolerated without development of
instability, though removing any of the facet joint
may transfer forces to other areas of the spine
(e.g., annulus and longitudinal ligaments). This
may accelerate degeneration over time [17].
Further, avoiding excessive resection of the pars
interarticularis can help preserve lumbar facet
integrity during laminectomy. Lastly, the interspinous ligament is relatively weak but should be preserved during dorsal decompression if possible
because its long moment arm stabilizes the spine.
Some parts of the ventral spine may be sacrificed, at least partially. In a corpectomy, for
example, the amount of bone spared in the ventral part of the VB is directly related to the
strength it can provide. For instance, removing
the middle and/or dorsal part of the VB may not
result in instability if the ventral part remains
intact. Ligamentous disruption can also reduce
the intrinsic stability of the spine. The anterior
longitudinal ligament (ALL) and the posterior
longitudinal ligament strength vary throughout
the spine (Fig. 33.7). These ligaments are typically removed in certain procedures, but if it is
unnecessary to remove a ligament, then it should
be kept in place.
33 Biomechanical Principles of Spine Stabilization
449
Fig. 33.7 Failure
strength of spinal
ligaments versus spinal
regional (ALL anterior
longitudinal ligament,
PLL posterior
longitudinal ligament,
LF ligamentum flavum,
CL capsular ligament,
ISL, interspinous
ligament) (Fig. 1.17 in
Biomechanics of spine
stabilization, Benzel E,
ed. Printed with
permission from Thieme
Medical Publishing)
500
450
400
350
300
250
200
Failure Strength of
150
Spinal Ligaments (newtons)
100
50
0
ALL PLL LF CL ISL
Biomechanics of Non-fusion Implants
There are currently three major categories of
non-fusion implants: nuclear implants, total disc
replacement (TDR), and posterior stabilization
devices.
Nuclear Implants
Nuclear implants replace an injured nucleus
pulposus, theoretically restoring viscoelastic disc
function, proper tension within the annulus fibrosus, and thus biomechanically relevant loadbearing capabilities [18]. The VB in contact with
the nuclear implant undergoes an adaptive
remodeling that is probably caused by a shift in
load concentration. To maximize the outcome,
the material of the implant should be relatively
pliable and have a high area of contact with the
flanking VBs. Nuclear implants are still relatively
new and additional research is needed to determine how they are best utilized. Their failure
mode has been expulsion through entry point.
ThoracicMid to Lower Cervical
Spinal Region
Lumbar
Total Disc Replacement (TDR)
For optimal function, total disc replacement
should restore normal kinematics to the functional spinal unit. This will minimize stress on
the implant and on adjacent load-bearing structures. If the replacement disc is not ideal, then
loading in the anteroposterior or lateral translational direction could transfer to the facet joints
which may accelerate their deterioration.
Biomechanical constraints of the TDR design
that restrict motion in these directions will ameliorate some of this concern and put more of the
load on the implant and the implant-bone inter-
18].
face [
Ideally, a TDR will have a similar IAR as the
original disc, meaning that a relatively posterior
IAR will result in a better range of motion since
this accurately mimics the normal physiologic
IAR [
19]. Additionally, the ratio of rotational to
translational movement with a disc replacement
is governed by the radius of curvature which is
determined by the distance between the IAR and
the surface of the implant. Essentially, a smaller
radius confers more rotational movement, while
a larger radius confers more translational.
450
A.Y. Chan et al.
Posterior Stabilization Devices
Posterior stabilization devices that are currently
available restrict specific motions, alter load
transferring, and unload the disc and/or facets.
Normally these devices maintain a normal or
slight focal kyphosis of the facet joints because it
transfers the load from the anterior part of the
disc to the posterior annulus as well as the facets.
Another result is that the IAR is altered, causing
the posterior portion of the disc to become more
of a fulcrum between the tensile forces derived
from the disc and the compressive forces that
exist within the disc itself.
Technical Pearls
• The end plates of vertebral bodies are weakest in the center; optimize placement of interbody devices and constructs by focusing the
load of the spinal column on the edges
instead.
• Whether the bone graft fits into the mortise is
crucial. The surgeon should take care to properly fashion the mortise and the bone graft to
ensure a tight fit.
• The amount of surface contact between the
graft and the vertebral body determines how
much the graft will subside; i.e., the more
surface contact will result in less subsidence
because the loading is more evenly
distributed.
Conclusion
Understanding the biomechanical principles of
the spine is crucial to properly treating spine
pathology that alters its normal properties. This
chapter briefly describes the more basic principles that should be considered and contextualized when surgical treatment is appropriate.
Importantly, surgery is not always appropriate;
an old adage states that surgery can always be
done, but never undone [
lous and proper planning for every procedure is
1]. Therefore, meticu-
crucial. The biomechanical principles of the
spine lie at the foundation of this planning, and
understanding them will help in avoiding some
mistakes that cannot be “undone.”
References
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ed. New York: Thieme Medical Publishers; 2015.
2. Natarajan RN, Andersson GBJ. The influence of
lumbar disc height and cross-sectional area on the
mechanical response of the disc to physiological loading. Spine. 1999;24:1873.
3. Granata KP, Marras WS, Davis KG. Variation in
spinal load and trunk dynamics during repeated
lifting exertions. Clin Biomech (Bristol, Avon).
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4. Cappozzo A. Compressive loads in the lumbar vertebral column during normal level walking. J Orthop
Res. 1984;1:292.
5. White AA, Panjabi MM. Clinical biomechanics of
the spine. 2nd ed. Philadelphia: JB Lippincott; 1990.
p. 30–342.
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Shender B, Paskoff G. Biomechanics of the aging
spine. Biomed Sci Instrum. 2006;42:1.
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Spine. 1998;23:1957.
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Goel VK. Contribution of disc degeneration to osteophyte formation in the cervical spine: a biomechanical
investigation. J Orthop Res. 2001;19:977.
9. Ng HW, Teo EC, Zhang Q. Influence of cervical disc
degeneration after posterior surgical techniques in
combined flexion-extension—a nonlinear analytical
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10. McHenry MC, Easley KA, Locker GA. Vertebral
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from 7 Cleveland-area hospitals. Clin Infect Dis.
2002;34:1342–50.
11. Rawlings CE, Wilkins RH, Gallas HA, Goldner LJ,
Francis R. Postoperative intervertebral disc space
infection. Neurosurgery. 1983;13:371–6.
12. Friedman JA, Maher CO, Quast LM, McClelland RL,
Ebersold MJ. Spontaneous disc space infections in
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Muffoletto AJ. Hematogenous pyogenic spinal
infections and their surgical management. Spine.
2000;25:1668–79.
14 Whang PG, Wang JC. Bone graft substitutes for spinal
fusion. J Spine. 2003;3:155–65.
15 White AA, Panjabi MM, Thomas CL. The clinical
biomechanics of kyphotic deformities. Clin Orthop
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16 Mirvosky Y, Neuwirth MG. Comparison between
the outer table and intracortical methods of obtaining
autogenous bone graft from the iliac crest. Spine.
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18 Huang RC, Wright TM, Panjabi MM, Lipman
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E122–9.
Bone Grafting and Spinal Fusion Options
Zorica Buser, Andre Jakoi, Bhavesh Katbamna,
Rahul Basho, and Jeffrey C. Wang
Introduction
Low back pain and neck pain are the top contributors to high disability rates worldwide [1]
with the annual costs for spine care in the United
States averaging around $90 billion. When conservative treatments fail, spinal fusion is often a
treatment of choice for various conditions including deformity, trauma, and degenerative disc disease. Bone healing and new bone formation are
key components of spine fusion and are influenced by the local bone environment and graft
materials. Initial stability in the fusion area is
achieved with spine instrumentation, while bone
grafts provide a foundation for bone healing and
remodeling that happens over a longer period of
time. While advancements in the fusion technique
Z. Buser, PhD (*) • A. Jakoi, MD • J.C. Wang, MD
Department of Orthopaedic Surgery, Keck School of
Medicine, University of Southern California,
Los Angeles, CA 90033, USA
e-mail: zbuser@usc.edu; ajakoi22@gmail.com;
jeffrey.wang@med.usc.edu
B. Katbamna, BS
Medical College of Wisconsin, Milwaukee,
WI, USA
bhaveshkatbamna@gmail.com
e-mail:
R. Basho, MD
Department of Orthopaedic Surgery, Hannibal
Regional Hospital, Hannibal, MO, USA
rahulbasho@gmail.com
e-mail:
34
and bone biology have improved fusion success,
nonunion (pseudarthrosis) remains a main procedural complication. Studies have shown that,
depending on the approach, number of levels,
and the type of grafting material, nonunion rates
in the lumbar and cervical spine can range from
several percent for single level to up to 60% for
multilevel cervical procedures [2–4]. Patients
with non-fused or partially fused segments often
have poor clinical outcomes and require another
surgery. The need for a revision surgery and further care contributes to extensive medical
expenditures.
An ideal graft material has all three essential
characteristics: it is osteogenic (contains mature
osteoblasts and progenitor stem cells that will
drive new bone formation), osteoinductive
(growth factors facilitating stem cell recruitment
and differentiation), and osteoconductive
(mechanically stable scaffold with pore sizes that
promote neovascularization and bone ingrowth).
Grafting materials used in spine surgery can be
divided into two major groups: autografts (have
all the desired properties) and allografts (have
some of the ideal graft characteristics). Allografts
can be further stratified based on their function
when coupled with autograft: graft extenders
(reducing the amount of autograft needed), substitutes (fully replacing the autograft), or enhancers
(combined with autograft to enhance fusion).
Although allograft bone substitutes lack some of
the properties of autograft, they have demonstrated
© Springer International Publishing AG 2017
L.T. Holly, P.A. Anderson (eds.), Essentials of Spinal Stabilization,
DOI 10.1007/978-3-319-59713-3_34
453