BIOMECHANICS OF
THE SPINE
Presented by: Shashwat Mishra
What is biomechanics ?
• In the context of the spine:
“Biomechanics is the study of the
consequences of application of external
force on the spine “
Motion segment
• In the biomechanical context, the spine is
treated as consisting of motion segments.
• Concept allows the laboratory study of
biomechanics of the spine in vitro
• Assuming that behaviour of spinal column
can be deduced from summing the
behaviour of motion segments is fallacious
The vertebral column: Basic
anatomy
• 33 vertebrae ( 7 cervical, 12 thoracic, 5
lumbar, 5 sacral and 4 coccygeal)
• A typical vertebra consists of a cylindrical
body and a dorsal arch
• The dorsal arch consists of pedicle, lamina,
pars interarticularis and spinous process.
• 2 primary curvatures ( thoracic and
lumbosacral kyphosis)
• 2 secondary curvatures (cervical and lumbar
lordosis)
• Curvatures maintained by variation in the
intervertebral disc heights and vertebral
body dimensions.
• Center of gravity of the spinal column
passes from the dens of the axis to the
promontory of the sacrum.
Regional characteristics of the
vertebral column
The cervical column
• Cervical vertebrae smaller
• Lamina narrow and overlap
• The pars interarticularis in the cervical
spine have been termed the lateral
masses
• The superior and inferior facets extend
from the pars
• The cervical facets from C2-3 to C6-7 are
oriented approximately at 45 degrees
with respect to the horizontal
• Coronal orientation of the facets.
Thoracic spine
• Thoracic vertebra are somewhat heart
shaped
• Uniquely, they possess costal facets at
the junction of the body and transverse
process for articulation with ribs
• Transitional features : upper thoracic (T1-
4) resemble cervical, lower (T9-12)
resemble lumbar.
• Spinous processes of T1, T2 , T11 and
T12 are horizontal
• T3, T4 and T9, T10 are oblique
• T5 –T8 spinous processes overlap
considerably and are long and vertical
• The thoracic facets are oriented along a
coronal plane
• At the thoraces-lumbar junction there is
change to assume a more sagittal
orientation
Lumbar spine
• Lumbar vertebral bodies are the largest
and typically increase in diameter as one
descends
• The bodies of L1-2 vertebra are deeper
dorsally, that of L4 -5 deeper ventrally
while L3 is transitional
• Fifth lumbar vertebra represents the
transition from lumbar to sacral spine
• L5 body is taller ventrally contributing to
the lumbosacral angle
• The lumbar articular facets are oriented
obliquely in the sagittal plane limiting axial
rotation of the spine.
Sacrum and Coccyx
• Sacrum is triangular, concave and having
a relatively smooth pelvic surface.
• Dorsal surface formed by the fusion of
costal ligaments and transverse processes
of sacral vertebral elements.
• The fused bodies are demarcated by
transverse lines that end laterally in four
pair of ventral sacral foramina.
Intervertebral discs
• 23 Interverbral discs are interposed
between the vertebral bodies.
• Most rostral- C2-3 disc and distally L5- S1
disc.
• They account for one third to one fifth of
the height of vertbral column
• Four concentrically arranged components
– Outer alternating layer of collagen fibres
forming the peripheral rim of annulus fibrosus
– Fibrocartilage component that forms a major
portion of the annulus
– Transitional region: annulus and nucleus
merge
– Central nucleus pulposus ( NP) : Mucoprotein
gel
• Age related disc changes :
– loss of water content of NP and height
– Number and size of collagen fibres decreases
• Structural organisation of the discs permits
them to tolerate compression,shear,torsion
and bending forces
• During axial loading stress causing failure,
the first component to fail is the vertebral
end plate, due to herniation of the nucleus
pulposus into the end plate.
Ligaments
• The longitudinal ligaments
– Anterior
– Posterior
• Ligamentum flavum
• Supraspinous ligament
• Interspinous ligament
• Intertransverse ligament
• Capsular ligaments
• Anterior longitudinal
– From occiput to sacrum covering a fourth to
third of the ventral circumference of vertebral
bodies.
– Consisting of three layers
• Deepest layer binds the edges of disc extending
between adjacent vertebrae.
• Middle layer binds vertebral bodies and disc over
three levels
• Superficial layer extends over four or five levels
– High collagen content preventing hyperextension and
over distraction
• Posterior longitudinal ligament
– Begins at C2 as the tectorial membrane and extends
upto sacrum
– Fibres spead out at the disc level and narrow in the
middle of vertebral body
– The ligament is much thinner over the vertebral body
than over the disc
– Multilayered, maximum thickness in thoracic region
• Ligamentum flavum
– Yellow ligament (flava= yellow)
– High elastin content , one of the most elastic
tissues in body.
– Broad paired ligaments which connect the
lamina of adjacent vertebrae
– Extend from C1-2 level to L5-S1
– Arise from ventral surface of caudal lamina
and attach to dorsal border of adjacent rostral
lamina.
– High elasticity, assume their original size once
a flexed spine straightens or extends
– Loose their elasticity with age, impinge upon
the dura when slack.
• Capsular ligaments
– Attach to vertebra adjacent to articular joints.
– They are perpendicular to the plane of facets
– Primarily prevent distraction of the joint.
• Intertransverse ligaments
– Seen only in thoracic and upper lumbar spine.
– They pass between the transverse processes and
attach to the deep muscles of the back.
• Interspinous and supraspinous ligaments
– Interspinous attach from base to tip of each spinous
process
– Supraspinous attach at the tips of spinous processes
– Ligament is weakest in cervical region and becomes
progressively stronger caudally
Spinal canal dimensions in relation to the vertebral level
• Force deformation characteristics
– Stiffness is the ability to resist deformation (∆ force/
∆ deformation)
– Flexibility is inverse of stiffness.
Spinal motion
• Degrees of freedom is a useful concept in
the description
• No of unique independent motion one
vertebra can have with respect to another.
• Six degrees of freedom
– Three translational
– Three rotational, along three axes
• Significance of facet joint orientation
– In cervical spine, facets are oriented 45
degree to horizontal, almost in the coronal
plane
– In thoracic spine, the orientation is
intermediate allowing axial rotation
– In the lumbar spine, rotation is prevented by
relatively sagittal orientation of the facets
while flexion and extension is free
• Coupling
– Defined as obligatory movements of the spine
(translations and rotations) that accompany a
primary motion
– Principal motion is defined as the motion
produced in the plane of the force
– Any associated out of phase motion is
coupled
• Instantaneous axis of rotation
– Defined as the axis perpendicular to the plane
of motion and passing through the points
within or outside the body which is static
during the motion
– Example, when opening a door, the axis of
rotation passes through the hinges
Functional biomechanics of the
spine
Spinal stability
– Paramount concept
– Ability of spine to maintain its pattern of
displacement under physiologic loads without
producing-
• Incapacitating pain
• Deformity
• Neurological deficit
Theories of spine stability
• The two column concept:
– Anterior column : vertebral body, ALL, PLL
– Posterior column : Posterior ligamentous complex
(PLC) : Interspinous, supraspinous, ligamentum
flavum and apophyseal joints
– Advanced by Holdworth
– Stressed upon the integrity of posterior ligamentous
complex in maintaining stability.
– Unstable fractures involved disrupted PLC and one
component of anterior Column.
• Three column concept:
– Better agreement with clinical observations
regarding spine stability
• Anterior column : Anterior wall of vertebral body,
ALL and anterior annulus
• Middle column : PLL, posterior annulus fibrosus,
posterior wall of vertebral body
• Posterior column : posterior ligamentous complex.
– For instability 2 out of the 3 columns must be
damaged.
• Jefferson fracture
– Diffuse axial loading of cervical spine
– Bilateral anterior and posterior arch fractures
– Biomechanically, stable till the lateral mass
displacement more than 5 mm, implying
transverse ligament disruption.
• Fracture of the dens
– Type1- avulsion injury of the dens. Stable
– Type 2- dens fractured along the base due to
flexion/ extension injury. Unstable , because
dislocation may increase
– Type 3 : produced by flexion or compression
forces or both.
Lower cervical spine
• Burst fractures
– Disruption of the body and intervertebral discs
– Direct axial loading of the spine
– Theoretically should be stable as the PLC is intact
– But, mostly associated with PLL damage and disc
injury making it unstable
– Biomechanically, only anterior decompression and
fusion is not appropriate as it disrupts viable ALL and
PLL
– Circumferential stabilization may be indicated
Thoracic spine
• Relatively narrow spinal canal
• Restriction of flexion and extension due to
articulation with fixed ribs T1-T9
• Axial rotation tolerated
• 3 degree per level flexion and extension
between T1 and T5. Increases
progressively downwards.
• Lateral bending limited in the entire extent
of the fixed ribs, increasing below that.
Thoracic spine injuries
• Mostly caused by flexion compression
forces
• The bending moment developed at the
vertebra in question is dependent on the
length of the column and distance
between the line of gravity and the
centrum
Distance
from
centrum
Gravitational load
Bending moment = Load x distance
• In flexion compression injury,
– distraction and damage of posterior ligament
complex may be more pronounced
– Thus these injuries are more unstable than
burst fractures.
– The flexion bending moment produced
naturally in the lower thoracic spine (T10-12)
by anatomic factors
• Termination of rib cage
• Normal thoracic kyphosis
• Orientation of the facets
Thank you
