Spinal biomechanics and Fusion basics.pptx

Spinal Biomechanics and Basics of Spinal Instrumentation
Moderated by :
Dr Vikas Chandra Jha
Dr Gaurav Verma
Dr Nitish Kumar
Dr Vivek Sinha
Presented By:-
Dr Rahul Jain
SR-3 Neurosurgery

Spinal Curvature
• Human spinal column consists of 33 vertebrae
separated into five anatomic regions. These regions
include seven cervical (C1‒C7), twelve thoracic (T1‒
T12), five lumbar (L1‒L5), five sacral (S1‒S5), and
four coccygeal bones.
• The spine develops largely in utero. The primary
curves develop during this time: kyphosis in the
thoracic and sacral regions created by created by
wedge-shaped vertebral bodies, where the anterior
height of the body is less than the posterior height.

• Through early childhood, the secondary curvatures
of the spine develop. These curves, lordosis wedge-
shaped intervertebral discs, where the anterior
height of the disc is greater than the posterior
height in the cervical and lumbar regions.
• All four of these normal curves occur in the sagittal
plane. There are no standard curves in the coronal
plane.
• The neutral positions taken by normal spinal
curvature allow for a horizontal gaze while standing
in an upright position, increased flexibility, and
shock absorbance.

• The spinal column consists of three regions: the
highly mobile, lordotic cervical segment supporting
the head; the hypomobile kyphotic, thoracic spine;
and the mobile, lordotic lumbar segment, which
bears 60% to 75% of the body’s weight.
• The spine can be divided into the following
components: vertebral bodies (VBs), intervertebral
disks, facet joints, and ligamentous structures.

Vertebra Anatomy
• contribute most of the bulk and strength of the spine.
• functions to provide stability to the spinal column,
along with support and protection for the spinal cord
and associated nerve roots.
• The compressive forces are significant in a stacked
column, and the cortical lamellae are arranged
vertically to aid in resisting these forces.
• The cancellous bone found in the inner trabeculae
allows for a compromise between strong mechanical
support and limiting vertebral weight.

• The anterior column functions to transfer
body weight to the pelvis while standing in
an erect posture.
• Owing to its function in load bearing,
vertebral bodies increase in size from the
cervical to the lumbar region.
• Dorsal elements function as a tension band
and a lever, transferring muscular
contractions of the paraspinal musculature
through the anterior and middle columns
of the spine.
• The junction of the laminae, where the
spinous processes arise, supports the
functional stability of the spine with their
ligamentous and muscular attachments.

ATLAS (C1) AND AXIS (C2)
• C1 – doesn’t have true ventral
body, articulates with occipital
condyle of cranium which is basis
for significant flexion and
extension of the head.
• The caudal facet surfaces of the
lateral mass articulate with the
superior facets of the axis.
• C2 – unique body with odontoid
process projecting cranially from
the dorsal aspect of the vertebral
body.
• Functions to provide upper
cervical rotation and stability to
the upper cervical region.

SUBAXIAL CERVICAL VERTEBRAE
(C3‒C7)
• smallest in size when
compared with all other
regions of the spine
• end plates of the vertebrae
form the uncovertebral joints
(joints of Luschka) - sites of
arthritic changes that can
cause nerve root impingement
• Facet joints are coronally
oriented, and the facet
capsule is weak, which allows
for the mobility of the cervical
spine

THORACIC VERTEBRAE
• The vertebral bodies gradually increase
in size from T1‒T12.
• The first four thoracic vertebrae
maintain some cervical features, and
the last four thoracic vertebrae possess
some lumbar features maintaining a
smooth transition between the
adjacent regions.
• The superior vertebral notch is the
cervical feature of T1, and the lumbar
features of T12 include lateral direction
of inferior articular processes.
• Because of the presence of the rib cage
the thoracic spine is the least mobile
spinal segment.

• Thoracic facets are primarily
arranged in a coronal plane which
allows for some limited flexion and
extension, and further limited
rotation.
• The “junctional” regions of the spine,
such as C7‒T1 and T12‒L1, are sites
of transition from a rigid spinal
region to one with maximal spinal
motion.
• These junctional sites are often the
sites of natural and iatrogenic
pathology.
• Kyphotic apex is typically at T8,
which also corresponds to the
narrowest cross-sectional area of the
spinal canal and the most oblique
spinous processes in the spine.

LUMBAR VERTEBRAE
• largest vertebral bodies, progressively increase in
transverse diameter than AP, when approaching the
sacrum.
• The facet joints of the lumbar spine are progressively
more sagittally oriented from L1‒L5.
• This acts to limit rotation while providing flexion and
extension. However, the L5‒S1 facet has a unique
coronal orientation to resist anteroposterior
translation.
• The L1‒L2 vertebral bodies have greater depth
dorsally, whereas the L4‒L5 vertebral bodies have
greater depth ventrally,

• These variations produce changes in intervertebral disc
height and foramen cross-sectional area, which are
functionally linked to motion during flexion and extension.
• Cadaveric studies have shown that in the L4‒L5 region
flexion results in a greater dorsal disc bulge than in the L1‒
L2 region.
• These changes can be associated with susceptibility to nerve
root impingement.

SACRUM AND COCCYX
• Five vertebrae, costal ligaments, and transverse
processes are fused to create the sacrum.
• The unique fusion of vertebrae in the sacrum
function to provide strength and stability to the
pelvis, and through articulation with the ilea at the
sacroiliac joints the weight of the body is
distributed to the pelvic girdle.
• Coccyx is the terminal portion – Tailbone.
• Function of the coccyx is to serve as a site of
attachment for pelvic muscles.

Intervertebral Discs
• This viscoelastic structure is made up
of a central nucleus pulposus,
peripheral annulus fibrosus, and
cartilaginous end plate of the disc,
which separates it from the vertebral
bodies above and below.
• Viscoelasticity is defined as a property
of a material that exhibits both
viscous and elastic characteristics
when undergoing deformation.
• There are 23 intervertebral discs
starting between C2 and C3 and
ending at L5 and S1 (no IVD between
C1-C2 and below L5-S1).

• The central nucleus pulposus is composed of
mucopolysaccharides and mucoprotein, forming a gel with
water content, ranging from 70% to 90%.
• The nucleus pulposus makes up approximately 30% to 50%
of the cross-sectional area of the disc, and it seems to lie
more posterior in the lumbar spine at the junction of the
middle and posterior thirds of the disc in the sagittal plane.
• Annulus fibrosus is made up of collagenous fibers in
concentric laminated bands, with each band oriented 90
degrees to the adjacent bands and 30 degrees to the disc
plane.
• Inner fibers of the annulus are attached to the cartilaginous
end plates, and the outermost fibers, known as Sharpey
fibers, are attached to the cortical bone of the vertebral
body.

• Because of the orientation of the fibers, the
intervertebral disc effectively resists rotational,
tensile, and shear stresses.
• With age, the water content of the nucleus
pulposus decreases, and the nucleus itself becomes
less deformable.
• This desiccation results in an overall stiffer
intervertebral disc that behaves differently under
compressive loads.

• In a disc in a younger person, the nucleus is
compressed, resulting in viscoelastic deformity at
the central part of the end plate, but when the
nucleus becomes less elastic and stiffer, the load is
transmitted through the annular fibers around the
periphery with very little deformation, leading to a
higher load at the end plate. This can result in
compression fractures, as are commonly observed
in the elderly

• When several forces act on a
solid, with a resultant net force
of zero, the solid is deformed.
• When the healthy intervertebral
disc is initially axially loaded (in
the neutral zone), the disc
undergoes a proportionally large
deformation for a given load as
the disc bulges.
• As the axial load increases, the
segment enters the elastic zone,
and the annular fibers tighten
and begin to resist and contain
the bulging nucleus, therefore, a
proportionally smaller amount of
deformation for a given load.

• Degeneration results in disc desiccation, decreased
intradiscal height, and annular fiber degeneration.
• The anterior portion of the functional spinal unit (the
disc complex) is less able to bear or resist axial loads
effectively because it has become lax, and as a result
the disc will readily deform in response to
proportionally smaller loads than in the intact disc
condition.
• This creates a large, shallow neutral zone in stress-
strain curve. It also shifts the IAR dorsally, which
increases the axial load on the facets.
• In accordance with Wolff’s law, which states that bone
remodels itself in accordance with stress placed upon
it, the facets hypertrophy. There is also an acceleration
of the degenerative process on the facet cartilage.

SPINAL LIGAMENTS
• The ligaments of the spinal column
are composed of elastin and collagen.
• There are seven major spinal
ligaments that work to stabilize the
subaxial spine in its physiological
range of motion.
• Also work to restrict the motion of
the spine to well-defined limits,
• at the same time allowing adequate
motion and fixed postures.
• Ant to post - ALL, PLL, capsular
ligaments, intertransverse process
ligaments, ligamentum flavum,
interspinous ligament, and
supraspinous ligament.

• The ALL spans the entire length of
the spinal column, extending from
the ventral border of foramen
magnum (basion), where it is known
as the anterior atlantooccipital
membrane, to the sacrum.
• The ALL spans 25% to 33% of the
ventral surface of the vertebral
bodies and intervertebral discs,
supporting the annulus fibrosus and
preventing hyperextension.
• The ALL is arranged in three layers:
the outermost layer spanning four
to five levels, the middle layer
spanning three levels and
connecting vertebral bodies and
intervertebral discs, and the
innermost layer binding adjacent
vertebral discs

• The PLL begins as the tectorial membrane at C2 and extends to
the sacrum.
• The PLL runs within the vertebral canal and flares at the level of
the intervertebral disc, where it is interwoven with the annulus
fibrosus, and narrows at the vertebral bodies, where it is loosely
attached.
• The layers of the PLL are similar to the ALL, but function to
prevent hyperflexion.
• Ligamenta flavum connect spinal laminae in a discontinuous
fashion and are intertwined with the facet joint capsule.
• The elasticity of the ligamentum flavum allows it to stretch
during flexion without limiting motion and allows it t become
taut when returning to neutral during extensions.
• As a person ages, the elastin is replaced with a higher
percentage of collagen, causing it to become less elastic, which
may lead to buckling into the spinal canal.

• The interspinous and supraspinous ligaments connect the
adjacent spinous processes.
• The interspinous ligament extends from the base to the tip
of each spinous process, and this ligament is present from
C2 toS1.
• The supraspinous ligament begins at the dorsal aspect of
C7, attaching to the tip of the spinous process, and extends
to the lumbosacral region. It is thought to be a continuation
of the ligamentum nuchae
• These ligaments use the long
moment arm of the spinous process
to provide an effective resistance to
flexion.
• A longer moment arm gives a
relatively weak ligament a mechanical
advantage so that it may contribute
more to overall spinal stability.

• Much of the stability of the
craniocervical region is provided
by the ligaments with the spinal
canal, which are ventral to the
spinal cord. These are arranged
in three layers.
• The tectorial membrane,
continuation of PLL.
• The cruciate ligament is the
middle layer and functions to
constrain ventral translation
between C1 and C2. It is a
complex ligament with both
horizontal and vertical bands.
• Odontoid ligament, or apical
ligament, is the most ventral

SPINAL MUSCULATURE
• With the majority of body weight ventral to the
vertebral body, the musculature of the back is crucial
to balancing the forces placed on the vertebral column.
• The extrinsic back muscles include the latissimus dorsi,
trapezius, rhomboids, and serratus posterior.
• Weakness of the muscle groups that support the spine
can lead to deformity or pain. The musculature plays a
significant role in spine stability.
• In the degenerating spine, toned musculature aids in
offloading the detrimental forces and disruption in load
balance arising from the degeneration.

Spinal Alignment and Balance
• Normal spinal alignment is characterized by cervical
lordosis, thoracic kyphosis, and lumbar lordosis, which
together result in sagittal balance in which the body’s
center of gravity is maintained in an axis above the
pelvis and feet.
• Because of this balance, minimal muscular exertion is
needed to oppose gravity in the erect posture.
• In adults, normal sagittal angulation is generally
defined as 30 to 40 degrees of cervi-cal lordosis, 20 to
40 degrees of thoracic kyphosis, and 20 to 45 degrees
of lumbar lordosis.

• Spinal organ concept describes
the spine as the central axial organ
of the entire body, starting from
the entire pelvis (which is
considered to be the pelvic
vertebra), continuing through the
succession of vertebral units, and
ending with the entire head
(playing the role of the cephalic
vertebra).
• For a normal population, the chain
of alignment and balance of erect
human posture (standing or
sitting) starts from the plantar
soles of the two feet, creating the
polygon of support.

• The concept of the “cone of economy” is easy to
understand; it is the mechanical consequence of the “chain
of balance.”
• Projection of the body’s center of gravity on the floor. This
point is constantly moving toward the front, back, right, or
left.
• The entire body and the spinal organ, even at rest, move
within a small “cone” of space, using a minimum of muscle
power to stand above this centre.
• This concept explains the
phenomenon of
compensation or
adaptation of the posture
(especially at the level of
the lower limbs) to try to
keep the body within this
“economical” small cone.

• Concept of compensation can be found at every level
of the chain of balance, using the maximum possible
active motion in the joints located above or below the
pathological level to maintained the global alignment
required for function.
• The chain of balance is centered at the pelvis, and
tilting the pelvis leads to compensation with lumbar
lordosis.
• Patients with lumbar kyphosis compensate with an
increased pelvic tilt.
• Patients with thoracic kyphosis compensate with
increased lumbar lordosis and cervical lordosis.
• Cervical kyphosis is compensated for by
hyperextension between the occiput and C1.

• Pelvic incidence is measured on
lateral projection x-ray by the
angle created by a line drawn
from the center of the S1
plateau to the center of femoral
head and a line orthogonal to
the center of S1 plateau.
• Generally small at birth, and
during early childhood it
increases progressively as the
child assumes erect posture and
grows.
• May change in response to
stresses at the sacroiliac joint,
including fusion of the spine,
lumbar kyphosis with aging

• Two other parameters, postural parameters because
they are related to the position of the pelvis in 3D
space.
• Sacral slope is the angle of projection of S1 plateau in
relation to the horizontal plane.
• Pelvic tilt or pelvic version is the angle between a line
that passes through the center of the femoral head (the
center of the S1 plateau) and a vertical line. Practically,
we can employ the term anteversion when the pelvis
tilts frontward and retroversion when the pelvis tilts
backward.
• Normal values of sacral slope 40 degrees, Pelvic tilt 15
degrees and pelvic incidence approximately 50 degrees.
PI = SS + PT (pelvic incidence = sacral slope + pelvic tilt)

• Small incidence angle correlates
with a small lumbar lordosis angle,
and conversely, a large incidence
angle correlates with large lumbar
lordosis.
• Most of the total lumbar lordosis is
in segments L4–S1.
• For a low incidence angle, lower
lumbar lordosis (L4/S1) is 68% and
upper lordosis (L1–L4) is 32%, but
for a high incidence angle these
values are 54% and 46%,
respectively.
• Spinal harmony stems from the
strong correlation between pelvic
incidence, lumbar lordosis, and
thoracic kyphosis.

Spine Fusion: Biology and Biomechanics
• Graft – based on origin – auto or allo or xeno or iso;
-- based on placemnt – antomically
appropriate site then orthotopic and anatomically
dissimilar site then heterotopic
-- based on composition – cortical or
cancellous or corticocancellous or osteochondral
• Cortical Bone - dense, solid mass, possesses a
volume fraction of pores less than 30% and has an
apparent density of up to about 2 g/mL. Its
compressive strength is approximately tenfold that
for a similar volume of cancellous bone.

• Cancellous bone is porous and appears as a lattice of
rods, plates, and arches individually known as
trabeculae.
• It has a greater surface area and can be readily
influenced by adjacent bone marrow cells. Because of
this structural difference, cancellous bone has a higher
metabolic activity and responds more readily to
changes in mechanical loads.
• Autogenous cortical bone is useful when structural
support is needed at the graft site. Otherwise, it is less
desirable than cancellous bone because of the absence
of robust bone marrow and, as a result, fewer
osteoprogenitor cells.
• Vascular ingrowth into cortical bone is slow. Mechanical
strength lags because incorporation takes longer.

• Autograft remains the gold standard in most fusion
applications.
• Successful spine fusion requires a sufficient area of
decorticated host bone, ample graft material, minimal
motion at the fusion site, and a rich vascular supply.
• Cancellous grafts are revascularized more rapidly and
completely than cortical grafts. The open trabecular
pattern of cancellous bone facilitates vessel ingrowth.
• Revascularization has been reported to begin within a
few hours after grafting and may be complete by 2
weeks in cancellous bone whereas takes months in
cortical bone.

Systemic factors –
• Smokers have a higher rate of pseudarthrosis than
nonsmokers. Tobacco smoke induces calcitonin
resistance, increases fracture end resorption, and
interferes with osteoblastic function.
• Chemotherapeutic agents administered in the early
postoperative period inhibit bone formation and
arthrodesis.
• NSAIDs suppress the inflammatory response and
may inhibit spine fusion.
• Osteoporosis - decreased bone density that is the
hallmark of osteoporosis makes stabilization with
instrumentation difficult in this population.

• Growth hormone, via somatomedins, exerts a
stimulatory effect on cartilage and bone formation.
• Thyroid hormone, which acts synergistically with
growth hormone, is required for somatomedin
synthesis by the liver.
• Corticosteroids detrimental to bone healing,
increasing bone resorption and decreasing bone
formation.
• Estrogens may increase bone mineralization by
increasing serum levels of parathyroid hormone
and vitamin D3.

Biomechanics of Fusion
• The type of surgical construct and choice of bone graft
should be individualized, based on the biological and
mechanical considerations.
• The fusion site during all stages of the reparative
process is highly susceptible to mechanical forces
directly related to the amount of motion.
• As healing proceeds, the amount of motion decreases
• Frequently, with excessive motion the fusion mass is
incomplete, and a pseudarthrosis develops.
• Rigid internal fixation has been demonstrated to reduce
pseudarthrosis rates in most clinical applications.

• Porosity is a dominant factor in determining the
material properties of bone. Cortical bone grafts
initially may have as little as 5% to 10% porosity,
whereas cancellous grafts may be as high as 70% to
80%.
• Cancellous grafts are incorporated by an early
appositional phase. New bone formation onto the
necrotic trabeculae of the graft tissue leads to an
early increase in graft strength.
• Cortical bone grafts first undergo osteoclastic bone
resorption, which significantly increases graft
porosity and thus decreases the graft strength.

Axis of Rotation and Spine
Fusion
• Instantaneous axis of rotation
(IAR) is defined as the axis around
which the vertebral body rotates.
• Usually passes through the
confines of the vertebral body.
With isolated destruction of
columns of the spine, the IAR
migrates to the remaining intact
structures.
• Importantly, at the IAR there is
neither compression nor tension.
The farther instrumentation or
bone graft is placed from the IAR,
the greater the leverage.

Load Sharing
• Denis introduced the three-column theory of the spine
to classify and assist with the management of
thoracolumbar spine injuries.
• The anterior and middle column carry about 80% to 90%
of the load, and the posterior column carries the rest
(10%‒20%) in the normal upright position.
• The anterior column resists compression and axial
loading, and the posterior column maintains the
tension.
• To maintain an erect posture, all forces and movements
must be balanced about the IAR.

• Deficiencies in the anterior or posterior column in the
thoracolumbar spine may lead to kyphosis.
• Kyphosis is corrected by lengthening the anterior
column or shortening the posterior column.
• If the anterior or middle column is destroyed, alignment
can be restored by a ventral structural graft and the
resulting fusion. In this situation the axial load is shared
by both anterior and posterior columns.
• Ventral instrumentation, without structural bone
grafting, usually fails. A strong structural graft is
required to resist axial loading and flexion.
• Tricortical ilium, fibula, humerus, or titanium cages
packed with autogenous graft provide excellent anterior
column support.

• In dorsolateral spine fusion, instrumentation adds to
the stability of the fusion by significant load sharing.
• As the fusion mass develops in vivo, theload-sharing
component of the instrumentation decreases. If an
adequate fusion mass does not develop, the cyclical
stresses placed on the instrumentation will lead to
hardware failure.

CANTILEVERS IN SPINAL INSTRUMENTATION
• cantilever beam is simply defined as
a beam that is rigidly supported only
at one end and carries a load.
• Each screw is a cantilever beam
supported by the vertebral body
support. Each rod is a cantilever
beam supported by a screw support.

BONE–SCREW CANTILEVER
• Shear stress resistance of a typical
screw is often much greater than
the resistance of the bone it is
embedded.
• Resistance to shear stress at the
bone–screw interface is primarily
determined by the mechanical
properties of the bone composing
the vertebra.
• Although the cancellous bone has
significantly lower yield strength,
the larger area of the bone–screw
interface mitigates this property.

• Because of the difference in yield strength of the
two types of bone, the cortical bone at the
entrance to the vertebral body, particularly in the
region of the pedicle, can be regarded as a fulcrum.
• A load is applied to the head of the screw across
the instantaneous axis of rotation at the cortical
fulcrum, creating a moment.
• The magnitude of this moment varies in accordance
with the formula: moment = force × distance (from
the fulcrum).
• With only the short screw head protruding beyond
the cortex, the applied moment may be quite low.

• Failure at the bone–screw interface occurs in
response to a bending moment if the yield strength
of either the cortical bone (fulcrum) or the
cancellous bone (downward force) behind the
fulcrum is exceeded.
• Longer screws increase resistance to a bending
moment by increasing the distance between the
cortical fulcrum and the distal tip of the screw,
increasing the moment (distance × force) of the
resistive force in the cancellous region.
• The bending resistance of the screw is related to
the area moment of inertia, determined primarily
by the minor diameter of the screw.

• The advantage of a larger-diameter screw lies not
only in the increased contact area between the
screw and the bone behind the fulcrum, but also in
the larger area moment of inertia.
• The use of a longer screw increases both the area of
bone–screw contact and the length of the lever arm
in cancellous bone behind the fulcrum, although
this effect occurs at the expense of increasing
bending moment at the screw entry point.
• The bending moment is least at the proximal and
distal ends of the screw and is inversely
proportional to the distance from the fulcrum. The
bending moment reaches its maximum at the point
of the cortical fulcrum.

• Commonly used spinal implant materials include stainless
steel, titanium, and cobalt-chrome.
• Stainless steel has the favorable properties of strength,
resistance to corrosion, and high yield strength, but is
incompatible with MRI.
• Titanium alloys provide better resistance to corrosion, less
imaging distortion during MRI but are less strong than
stainless steel.
• Titanium alloys are
considered high in
strength and fatigue
resistance. The
decreased stiffness of
titanium alloys relative to
stainless steel helps to
off-load stress at the
bone-implant interface.

• Cobalt-chrome alloys have become more popular,
especially for use in rods. The stiffness of
Cobaltchrome is higher than stainless steel and is
much higher than titanium alloys.
• MRI compatibility of cobalt-chrome is between that of
stainless steel and titanium alloys.
• Implant size also plays a role in the strength of a
spinal construct.
• Pedicle screw diameter and length may likewise be
important. Moreover, surgical techniques such as
triangulation of the screws at the end of a construct
or bicortical purchase when appropriate (such as for
C1 lateral mass screws or tricortical purchase for S1
screws) can increase pullout strength

FAILURE AND STRATEGIES TO
REDUCE THEIR INCIDENCE
• Failures of cantilever-derived instrumentation do
occur.
• The more common failure mode is loss of fixation at
the proximal or distal end of a construct through
failure of the bone–screw interface.
• A second, less common, mode is failure of the
instrumentation itself.

BONE–SCREW INTERFACE
FAILURE
• as the result of overwhelming shear stress.
• Failure occurs more commonly by either fracture of
the cortical fulcrum or more frequently due to
compaction of the cancellous bone behind the
fulcrum as a result of the application of a moment.
Strategies to Increase the Resistive Moment
• Larger-diameter screws increase the contact area
which increases the yield strength in both regions
(cortical and cancellous); an increased area moment
of inertia, also improving the bending resistance.

• Longer screws also increases the contact area between the
screw and bone, although only in the cancellous region;
increase the length of the lever arm behind the fulcrum; this
produces an increase in bending resistance proportional to
the increase in screw length.
• The use of cross-links in addition to the usual screw and rod
construct produce further resistance to axial torsion of the
construct and increased overall stiffness when used in three-
level constructs compared with two-level constructs.

Strategies to Increase the Resistive Moment
• use of additional intervening screws
(segmental fixation)
• The rostral bone–screw interface
experiences a moment proportional to the
length of the lever arm, in this case
determined primarily by the length of rod
between fixation points.
• The addition of an intervening screw also
changes the function of the other two
screws, which function as a cantilever with
the rod as the beam.
• The middle screw acts as a fulcrum, and the
remote screw acts as an anchor. Thus, these
screws are subjected to a pure pullout stress
depending on the direction of the moment.

• Modify the magnitude of the applied bending moment.
• Proper posture during the healing process helps protect the
bone–screw interface from potentially excessive moments
• Bracing, in addition to encouraging proper posture,
decreases the magnitude of the moment by way of
buttressing effect.
• Early mobilization decreases the incidence of complications
and improves clinical outcomes.
• Avoid prestressing of the
instrumentation during placement.
This situation is facilitated by careful
contouring of the rod, judicious
adjustment of screw head height,
and avoidance of unnecessary use of
rod reduction instruments
(“persuaders”).

Biomechanics and Implant
Materials: The Anterior Column
• Biomechanically, the most effective means of eliminating
motion between two vertebrae is through the disc space
rather than the facet joints, transverse processes, or spinous
processes.
• Posterolateral fusion does not completely achieve
immobilization of the motion segment despite the presence of
a solid fusion.
• Because the center of body mass lies anterior to the spine,
and the function of the spine en bloc is to act as a tension
band in the upright posture, grafts placed within the anterior
and middle columns are subject to compression under
physiological loads. Fusion, therefore, is promoted according
to Wolff’s law

• Post Interbody fusion – D sc height and lumbar
lordosis are better restored, thereby improving
overall alignment in patients with positive sagittal
balance.
• The disadvantages of including an interbody device
include additional cost, increased operative time,
risk of neurological injury because of nerve root or
dural sac retraction, and the long-term effects of
complete immobilization of a motion segment on
the adjacent lumbar levels.
• Biomechanically, any interbody implant or graft
must be capable of withstanding physiological loads
to facilitate fusion.

• Early on, autologous bicortical
iliac crest autograft was the gold
standard, but high rates of
pseudarthrosis, graft collapse,
migration, and loss of correction
were observed.
• Structural interbody cages have
thus become more popular.
• Cage provides immediate rigid
axial mechanical support and
stability postoperatively,
allowing the graft material
inside the cage, as well as that
surrounding the cage, to form a
solid biological fusion.

Biomechanics of cage
• Anterior lumbar interbody fusion (ALIF) may be achieved with a
stand-alone cage or supplemented with dorsal or ventral
instrumentation.
• Exclusively ventral approach allows for superior preparation of
the fusion surfaces while preserving the dorsal elements.
• Number of biomechanical studies have investigated the
stabilization provided by stand-alone ALIF cages versus those
supplemented with dorsal instrumentation. In general, stand-
alone interbody cages significantly reduce flexion and lateral
bending compared with native.
• Modern ALIF cages are available
in a variety of sizes with varying
degrees of lordosis to assist in
restoring normal sagittal

• Dorsally inserted transforaminal lumbar interbody
fusion (TLIF)/PLIF cages can also be contoured with a
lordotic wedge, the grafts are much smaller than ALIF
cages and therefore theoretically provide a less
substantial immediate mechanical lordotic platform.
• The sustained ability of ALIF versus TLIF/PLIF cages to
correct lordosis and restore height to provide indirect
decompression has been directly studied.
• Superior results are noted for ALIF cages, which
restored foraminal height by 18.5% and lumbar
lordosis by 6.2 degrees, compared with TLIF cages,
which decreased foraminal height by 0.4% and
decreased lumbar lordosis by 2.1 degrees.27
Nevertheless, the authors did not note any clinically
significant differences between groups.

ADJACENT-SEGMENT DEGENERATION
• Vertebral arthrodesis has been used for many years as a
treatment for the dysfunctional motion segment.
• Fusion eliminates the abnormal motion of the motion
segment, and some of the loads that were previously borne
by that segment are now shared with the adjacent intact
segments. This is believed to be the driving force behind ASD.
• In a 2016 systematic review and metaanalysis of 83 studies,
Kong et al reported that the pooled prevalence of
radiographic ASD was 28.28% after cervical spinal surgery,
whereas the prevalence of symptomatic adjacent segment
disease was 13.34%, and the pooled reoperation rate was
5.78%.

• It is clear that alignment is related to ASD. Roughly
80% of patients whose cervical segments were
fused in kyphosis develop ASD.
• Similar results were shown to be true in the lumbar
spine, where one study demonstrated that the
incidence of ASD was approximately five times
higher in patients with sagittal imbalance or vertical
sacral inclination after lumbar fusion.

BIOMECHANICS OF MOTION-
SPARING IMPLANTS
• The ideal motion-sparing implant replicates the
anatomy, motion, and mechanics of the intact,
healthy FSU.
• Functional spinal unit - smallest motion segment of
the spine that exhibits biomechanical
characteristics representative of the physiological
motion of the whole spine. Consist of two
vertebrae (including their facet joints), the
intervertebral disc, and the associated supportive
ligaments.

• Ideal intervertebral disc arthroplasty should replicate
the form and function of a healthy intervertebral disc.
The intact disc is akin to a radial tire, with a
lamellated, firm but flexible outer shell and a soft,
gelatinous core.
• This allows the disc both to accept and deform after
the application of small loads and firm up and provide
greater resistance to deformation as the load
gradually increases.
• The arthroplasty should not permanently alter the
location of the IAR of the FSU so as not to place the
dorsal ligaments and facets under undue stress.
• The implant must also be durable and able to stand up
to decades of repetitive, cyclical motion and loading.

TOTAL DISC ARTHROPLASTY
• Clinical objectives of TDA are to preserve or restore
normal biomechanical function, which includes
preservation of motion while maintaining the center of
rotation of the FSU, absorbing axial load, and
attenuating rapidly administered forces.
• Artificial discs are categorized by material, articulation,
fixation, design, and kinematics.
• They can be classified as nonarticulating, uniarticulating,
or biarticulating, as well as modular or nonmodular.
• With regard to motion, they may be constrained,
semiconstrained, or unconstrained.

Ideal Total Disc Arthroplasty
• Structure will be nearly identical to that of a normal
disc (ball-and-socket model does not replicate the
anatomy of the normal disc)
• stress-strain curve of this device will be nonlinear
and will closely replicate that of a normal disc
• end caps of the device will readily bond with the
end plates of the normal surrounding vertebrae.
• device materials will have acceptable wear, fatigue,
and failure resistance properties.

• device will be constructed in such a manner as not
to place undue stress on the surrounding intact
spinal elements.
• It will not excessively distract the disc space and
will allow for the restoration of the normal variable
IAR that moves in response to movement of the
spinal unit.
• The normal range of motion of a healthy spinal unit
will be restored by this device. It will not restrict
movement of the spinal unit in any way.
• In the unlikely case that the device fails, it should
be easily removable and possibly replaceable.

• Several biomechanical studies have analyzed ASD after cervical
TDA and ACDF.
• Rao et al. in 2005 found that intradiscal pressures and
intervertebral motion at adjacent levels were not significantly
affected after anterior cervical fusion
• Diangelo et al. in 2003 found that placement of an anterior
cervical plate significantly decreased motion across the FSU
relative to a noninstrumented control and relative to placement
of an artificial joint, causing increase motion of adjacent
segments.
• Evaluation of the FDA Investigational Device Exemption (IDE) trials
and international studies of similar trial design for single-level
trials show that a statistically significant difference favoring TDA
becomes evident at 4 years and persists at the 5- and 7-year
marks.
• Two-level trials, the difference in the rate of adjacent-segment
surgery between TDA and ACDF became significant at 7 years,
and favored TDA.

CAGE MATERIAL AND DESIGN
Features desirable in the design of an interbody cage
• Have a hollow region of sufficient size to allow packing of
bone graft
• structurally sound so that it can withstand the great forces
applied.
• should have a modulus of elasticity that is close to that of
vertebral bone to optimize fusion and avoid subsidence.
• should have ridges or teeth to resist migration or retropulsion
• should be radiolucent to allow visualization of fusion on
radiographs and should have radiopaque markers.
• If inserted from a dorsal approach (TLIF or PLIF), it should be
tapered, with a bullet-shaped tip to allow easier initial
insertion

• PEEK is a semicrystalline hydrophobic aromatic
polymer that is biomechanically similar to cortical
bone.
• PEEK is chemically inert and does not promote
cellular adhesion, bony ingrowth, or protein
absorption. PEEK implants have been shown to lead
to excellent clinical outcomes.
• PEEK cages have been shown to provide stability
similar to that of titanium cages, reduce stress at
the end plates adjacent to the cage, and increase
load transfer through the graft.

• Over time, settling of the cage into the vertebral
end plates can occur. If significant subsidence
occurs, it can lead to segmental loss of lordosis and
loss of anterior column support.
• Causes - combination of improper graft selection,
poor bone quality, insufficient bony healing, lack of
supplemental open or percutaneous fixation, and
overexuberant endplate preparation.
• Removing the bony end plate may compromise the
stability of the end plate, weaken the compressive
strength of the vertebral body, and lead to
subsidence of the interbody device as bony end
plate itself is very thin (usually <0.5-mm thick).

FACTORS AFFECTING CONSTRUCT
RIGIDITY
• Annular tension is influenced primarily by the vertical
height of the cage. Axial “oversizing” of the cage leads
to increased annular tension, which may improve the
rigidity of the construct.
• Intraoperative assessment of annular tension with trial
implants is the most reliable method of determining
cage size.
• Vertebral bone quality, or end-plate strength, is critical
to cage stability. The dorsolateral region of the end
plate, the region near the pedicle base, has the greatest
resistance to subsidence, whereas the central region
provides the least resistance.

• In performing a TLIF, in which smaller cages are used,
positioning the cage dorsolaterally in these cases
maximizes stability of the construct.
• Cages that are inserted through a dorsal approach are
typically countersunk relative to the dorsal vertebral
body by at least 3 to 4 mm, with attempt made to
cross the midsagittal plane for coronal stability.
• Cage size - larger cages have greater maximum load to
failure of the end plates than smaller cages do. Wider
implants that are supported by the periphery of the
end plate provide superior stability.
• Cage features – serrations, spikes or ridges. Cages with
end-plate spikes provided improved motion segment
rigidity in bending modes and particularly in torsion.

How much rigidity is necessary?
• Degree of micromotion that would notcompromise
biological fusion is not known.
• although small micromotion of up to 28 μm does
not affect bone ingrowth, large micromotion of
more than 150 μm can produce fibrous tissue
development at the implant–endplate interface.
• If optimal construct rigidity is not obtained,
supplemental fixation should be considered,
particularly in cases of osteopenia, end-plate
disruption, or excessive annular laxity.

Conclusion
Planning spinal surgery requires an understanding of
the biomechanical factors in play both at the
segmental level and at the level of global spinal
alignment.
Appropriate construct design and choice of implant
material requires knowledge of the patient’s bone
quality and anticipated construct demands.
Understanding spinal biomechanics can improve the
chance of successful outcome with spinal fixation
and decrease the risk of implant failure.

References
1. Youmann and Winn 8th
ed
2. Benzels Spine surgery 5th
ed

Spinal biomechanics and Fusion basics.pptx

More Related Content

What's hot

Similar to Spinal biomechanics and Fusion basics.pptx

More from Dr. Rahul Jain

Recently uploaded

Spinal biomechanics and Fusion basics.pptx

Editor's Notes