SPINE BIOMECHANICS

1. Bio- Mechanics of Spine
Presented By-Debanjan Mondal
MPT(Musculoskeletal), BPT, CMT,
Ergonomist.

2. Introduction.
• Spine or Vertebral Column or Backbone is a
complex structure meant for support and a
linking structure between upper and lower
extremities.
• It provides a base of support for the head and
internal organs, a stable base for the attachment
of ligaments, bones and muscles of the
extremities, rib cage and the pelvis.
• It is a link between the upper and the lower
extremities and provide mobility for the trunk.
• It protects the Spinal cord.

4. Structure;
• The Spine comprises 33 vertebral bones and 23 Intervertebral disks.
• Cervical (7), Thoracic (12), Lumbar (5), Sacral (5), Coccygeal (4)
• The vertebrae increase in size from the cervical to lumbar region
and decrease in size from the sacral to coccygeal region.
• Vertebral column when viewed from the posterior aspect, all
regions together present a single vertical line that bisects the trunk.
• When viewed from the side, a number of curves are seen that vary
with age .
• Primary curve (Kyphotic curve)- Thoracic and Sacral, they retain the
original posterior convexity throughout life.
• Secondary curve (Lordotic curve)- Cervical and Lumbar, they
develop as anterior convexity as a result of the accommodation of
the skeleton to the upright posture.

5. Articulations
• Two main types of articulations are found in the vertebral column:
• (1)Cartilaginous joints of the symphysis type between the vertebral
bodies and the interposed disks and
• (2)Diarthrodial or Synovial joints between the zygopophyseal facets
located on the superior articular processes of one vertebra and the
zygopophyseal facets on the inferior articular processes of an
adjacent vertebra above.
• The joints between the vertebral bodies and disks are referred to as
the Intervertebral joints.
• The joints between the zygapophyseal facets are called the
Zygapophyseal (apophyseal or facet) joints.
• All the zygapophyseal joints, except for the joint between the first
two cervical vertebrae, are plane synovial joints.
• The vertebral column articulates with the ribs, with the skull and
the pelvis at the sacroiliac joints.

6. • The motion between any two vertebrae at the intervertebral joints
is extremely limited and consists of a small amount of gliding
(translation) and rotation.
• According to White and Punjabi, one vertebra can move in relation
to an adjacent vertebra in six different directions ( 3 translations
and 3 rotations) along and around three axes.
• The compound effects of these small amounts of translation and
rotation at a series of vertebrae produce a large range of motion for
the column as a whole.
• So Vertebrae has got 3 degree of freedom, permitting flexion and
extension, lateral flexion and rotation.
• However, motions in the vertebral column often are coupled
motions. Coupling- the consistent association of one motion about
an axis with another motion around a different axis. Eg. Axial
rotation is coupled with lateral flexion and forward flexion)

7. A Typical Vertebra.
• The structure of a typical vertebra consists of two major parts: an anterior
cylindrically shaped vertebral body and a posterior irregularly shaped vertebral
arch.
• Vertebral body- is composed of a block of trabecular or spongy bone, which is
covered by a layer of cortical bone. According to Wu and Chen, the stresses
induced in the cortical bone shell are always much higher than in the trabecular
bone and stresses induced in the vertebral body are always larger at the anterior
side than on the posterior side.
• The cortical covering of the superior and inferior surfaces or plateaus is thickened
around the rim where the epiphyseal plates are located and in the center by a
layer of hyaline cartilage called the cartilaginous end-plate.
• The vertical, oblique and horizontal trabecular systems correspond to the stresses
placed on the bodies. Vertical system sustain the body weight and resist
compression forces while other trabecular systems help to resist shearing forces.
• Areas of strength are the area where the trabeculae cross each other and the area
of weakness is evident in the anterior portion of the body, which is a potential site
for collapse of the vertebrae resulting compression fracture.

9. • The vertebral arch is a more complex structure than the body,
because it has many projections, including 3 nonarticular processes
and 4 articular processes.
• Nonarticular processes are 2 transverse and 1 spinous process. They
provide sites for the attachment of ligaments and muscles. The 2
transverse processes divide the arches into anterior and posterior
portions.
• The portions of the arch located anterior to the transverse
processes are called Pedicles which attach the arches to the right
and left upper posterior walls of the vertebral body.
• The portions of the arches posterior to the transverse processes
constitute the laminae. The posterior portions of the laminae that
are located between the superior and inferior articular processes
on each side are called the pars interarticularis where vertically
oriented lamina and horizontally oriented pedicle meet.

10. Summary; Vertebral Function.
Structure Function
Body Resists compressive forces.
Transmits compressive forces to vertebral end plates.
Pedicles Transmit bending forces (exerted by muscles attached to
the spinous and transverse processes) to the vertebral
bodies
Laminae Resist and transmit forces (that are transmitted from
spinous and zygapophyseal articular processes) to pedicles.
Serve as attachment sites for muscles and ligaments.
Transverse processes Serve as attachment sites for muscles and ligaments.
Spinous processes Resist compression and transmit forces to laminae. Serve
as attachment sites for ligaments and muscles.
Zygapophyseal facets Resist shear, compression, tensile and torsional forces.
Transmit forces to laminae.

11. Intervertebral Disk
• The intervertebral disks make up about 20% to 33% of the length of the Spine. (
approx. 70 cm in male, 60 cm in female)
• The size increases from the cervical to the lumbar region and disk thickness varies
from approx. 3mm in the cervical to about 9 mm in lumbar region.
• The ratio between disk thickness and vertebral body height is greatest in the
cervical and lumbar regions and least in the thoracic region. The greater the ratio,
the greater the mobility, hence the disks in the cervical and lumbar regions
contribute to the greater mobility in comparison with the thoracic region
• The disk is composed of two parts; a central portion called the nucleus pulposus
and a peripheral portion called the annulus fibrosus.
• The composition of the nucleus and annulus are similar in that they both are
composed of water, collagen, and proteoglycans (PGs). However the fluid and PG
concentrations are highest in the nucleus and lowest in the outer annulus.
Conversely collagen concentrations are highest in the outer annulus and lowest in
the nucleus pulposus.
• Maturation, aging and disease as well as normal and abnormal stresses on the disk
may affect the distribution and relative proportions of type 1 and type 2 collagen.

13. Ligaments and Joint Capsules.
• The ligamentous system of the vertebral column
is extensive and exhibits considerable regional
variability. There are 6 main ligaments associated
with the intervertebral and zygapophyseal joints.
I. Anterior longitudinal ligaments
II. Posterior longitudinal ligaments
III. ligamentum flavum
IV. Interspinous
V. Intertransverse
VI. Supraspinous ligaments

14. Anterior and Posterior Longitudinal
ligaments.
• The anterior and posterior longitudinal ligaments are associated with the
intervertebral joints.
• The anterior longitudinal ligament runs along the anterior and lateral surfaces of
the vertebral bodies from the sacrum to the second cervical vertebra. An
extension the ligament from C2 to the occiput is called the anterior atlantoaxial
ligament.
• It has got two layers that are made up of thick bundles of collagen fibers. The
fibers in the superficial layer are long and bridge several vertebrae, whereas the
deep fibers are short and run between single pairs of vertebrae.
• The ligament is well developed in the lordotic sections (cervical, lumbar) but has
little substance in the region of thoracic kyphosis. The anterior longitudinal
ligament increases in thickness and width from the lower thoracic to L5/S1. The
tensile strength of the ligament is greatest at the high cervical, lower thoracic and
lumbar regions. However when tested in axial tension, the ligament demonstrates
its greatest tensile strength (675N) in the lumbar area. The ligament is compressed
in flexion and stretched in extension and slack in the neutral position. The anterior
longitudinal ligament is reported to be twice as strong as the posterior longitudinal
ligament.

15. • The posterior longitudinal ligament runs within the vertebral canal along the
posterior surfaces of the vertebral bodies from the second cervical vertebra to the
sacrum. It consists of two layers, a superficial and a deep layer.
• Superiorly the ligament continues to the occiput becoming the tectorial
membrane at C2. in the lumbar region, the ligament narrows to a thin ribbon that
continues into the sacral canal. The ligament provides little support for the
intervertebral joints in the lumbar region.
• The posterior longitudinal ligament’s resistance to axial tension in the lumbar area
is only one sixth of that of the anterior longitudinal ligament ( 160N only).
• It is stretched in flexion where maximal strain in the ligament occur s when it is
slack in extension. However if the axis of motion moves posteriorly, as it does
when the nucleus pulposus is destroyed either experimentally or by degenerative
processes, the ligament may be stretched in extension.

16. Ligamentum Flavum
• The ligamentum flavem is a thick, elastic ligament, which is located on the
posterior surface of the vertebral canal. The fibers of the ligament run
within the canal from the second cervical vertebra to the sacrum,
connecting laminae of adjacent vertebrae.
• In the lumbar region the ligament is composed of superficial and deep
components. It attaches to adjacent laminae and contributes to the
formation of the smooth dorsal wall of the spinal canal that abuts the
dura. Most of the anterior surface of the lumbar laminae is covered by the
ligamentum flavum.
• It is the strongest in the lower thoracic and weakest in the midcervical
region. Although the highest strain in this ligament occurs during flexion
when the ligament is stretched, this ligament is under constant tension
even when the spine is in a neutral position. The ligamentous tension
creates a continuous compressive force on the disks, which causes the
intradiskal pressure to remain high. The raised pressure in the disks makes
the disks stiffer and thus more able to provide support for the spine in the
neutral position.

17. Interspinous Ligaments
• The interspinous ligaments vary from region to region. In the
cervical and upper two thirds of the thoracic region, the ligaments
connect and cover the margins of adjacent spinous processes. The
parallel fibers of the ligaments run diagonally and fill up the space
between the spinous processes.
• In the lumbar region the ligaments are particularly well developed
and attach to both the thoracolumbar fascia and the caudal fibers
of the joint capsules.
• The ligaments are slack in extension and stretched in forward
flexion when they resist the separation of the spinous processes
that accompanies flexion. The ligaments have a tensile strength of
only 24 to 185 N and thus are potentially weaker in tensile strength
than the anterior and posterior longitudinal ligaments and the
ligamentum flavum.

18. Supraspinous Ligament
• The supraspinous ligament is a strong cordlike
structure that connects the tips of the spinous
processes from the seventh cervical vertebra to L3 or
L4. the fibers of the ligament become indistinct in the
lumbar area where they merge with the thoracolumbar
fascia and insertions of the lumbar muscles.
• In the cervical region the ligament beccomes the
ligamentum nuchae. The supraspinous ligament like
the interspinous ligament is stretched in flexion and its
fibers resist separation of the spinous processes during
forward flexion. During hyperflexion the supraspinous
and the interspinous are maximally stretched and are
the first of the posterior ligaments to fail.

19. Intertransverse Ligaments
• The ligaments pass between the transverse processes
and attach to the deep muscles of the back. In the
cervical region only a few fivers of the ligaments are
found.
• The membranous fibers of the ligament form part of
the thoracoluumbar fascia.
• The ligaments are alternatively stretched and
compressed during lateral bending. The ligaments on
the right side are stretched and offer resistance during
lateral bending to the left while the ligaments on the
left side are slack and compressed during this motion
and vice versa.

20. Zygapophyseal Joint Capsule
• The zygapophyseal joint capsules assist the ligaments
in providing limitation to motion and stability for the
spine. The capsule s are strongest in the thoracolumbar
region and at he cervicothoracic junction sites where
the spinal configuration changes from a kyphotic to
lordotic curve and from a lordotic to kyphotic curve,
respectively.
• The joint capsules, like the supraspinous and
interspinous ligaments, are vulnerable to hyperflexion,
especially in the lumbar region. It provide more
restraint to forward flexion than any of the posterior
ligaments because they fail after the supraspinous and
interspinous ligaments when the spine is hyperflexed.

22. Functions of Spine.
• Stability:
• The stiffness of the vertebral column is the column’s ability to resist an applied
load. Stiffness can be represented graphically by the slope of the stress-strain
curve. The steeper the slope of the curve, the stiffer the structure.
• By applying a specified load to a motion segment (two adjacent vertebrae and the
intervening soft tissues) stiffness of that particular segment can be determined.
• The neutral zone is the ROM through which the spine can be displaced from a
neutral position to the point at which elastic deformation begins when a small load
is applied. In a stress-strain curve the neutral zone would be represented by the
toe region of the curve.
• Punjabi has suggested that the existence of a large neutral zone indicates
instability. Instability of the vertebral column can be considered as a lack of
stiffness and an unstable structure is one that is not in an optimal state of
equilibrium.
• The spine is subjected to axial compression, tension, bending, torsion and shear
stress not only during normal functional activities but also at rest which depend
upon the type, duration and rate of loading; the person’s age and posture; the
condition and properties of the various elements.

23. Axial Compression.
• Axial compression (force acting through the long axis at an 90 degree)
occurs due to the force of gravity, ground reaction forces and forces
produced by the ligaments and muscular contractions. Most compressive
force is resested by the disks and vertebral bodies, but the arches and
zygapophyseal joints share some of the load in certain postures and during
specific motions. The compressive load is transmitted from the superior
end plate to the inferior end plate through the trabecular bone of the
vertebral body and the cortical shell.
• The cancellous body contributes 25%-55% of the strengthof a lumbar
vertebra under the age of 40yrs and the cortical bone carries the
remainder.
• Depending on the posture and region of the spine, the zygapophyseal
joints carry from 0%-33% of the compression load. The spinous processes
also may share some of the load when the spine is in hyperextension.
• The nucleus pulposus acts as a ball of fluid that can be deformed by a
compression force. The pressure created in the nucleus actually is greater
than the force of the applied load.

24. When a weight is applied to the nucleus pulposus from above, the nucleus loses
height as it exhibits a swelling pressure and tries to expand outward toward the
annulus and the end plates. As the nucleus attempts to distribute the pressure in
all directions, stress is created in the annulus and central compressive loading
occurs on the vertebral end plates.
• The forces of the nucleus on the annulus and the annulus on the nucleus form an
interaction pair which provide sufficient resistance to the swelling pressure in the
nucleus to reach an maintain a state of equilibrium.
• The pressure exerted on the end plates is transmitted to the superior and inferior
vertebral bodies. The annulus fibrosus is under tensile stress and thus is able to
better resist he compressive load. The disks and trabecular bone are able to
undergo a greater amount of deformation without failure than the cartilaginous
end plates or cortical bone when subjected to axial compression. The end plates
are able to undergo the least deformation and therefore will be the first to fail/
failure under high compressive loading. The disks will be the last to fail.
• The disks are subjected to a constant load by forces that are not large enough to
cause permanent damage, the disks exhibit creep. The recovery of fluid that
returns the disk to its original state explains why a person getting up from bed is
taller in the morning than in the evening and why an astronaut returning from
weight weightlessness of space is taller on his return than on his departure.
• Running is a form of dynamic loading that decreases disk height more rapidly than
static loading. The loss of height that occurs as people grow older is due to the fact
that the nucleus loses a large proportion of its fluid-imbibing capacity with aging.

25. Bending
• Bending causes both compression and tension on the structures of the spine. In
forward flexion the anterior structures are subjected to compression; the posterior
structures are subjected to tension. The resistance offered to the tensile forces by
collagen fibers in the posterior outer annulus fibrosus, zygapophyseal joint
capsules, and posterior ligaments help to limit extremes of motion and hence
provide stability in flexion.
• The resulting deformation of supporting structures such as ligaments, joint
capsules, and intervertebral disks leads to an increase in the ROM beyond normal
limits and places the vertebral structures at risk of injury.
• In extension the posterior structures generally are either unloaded or subjected to
compression, whereas the anterior structures are subjected to tension. Resistance
to extension is provided by the anterior outer fibers of the annulus vibrosus,
zygapophyseal joint capsules, passive tension in the anterior longitudinal ligament
and by contact of the spinous processes.
• In lateral bending, the ipsilateral side of the disk is compressed; that is, in right
lateral bending the right side of the disk is compressed while the outer fibers of
the left side of the disk are stretched so the contralateral fibers and ligaments help
to provide stability.

26. Torsion
• Torsional forces are created during axial rotation that occurs as a part of
the coupled motions that take place in the spine. The torsional stiffness in
flexion and lateral bending of the upper thoracic region from T1 to T6 is
similar in stiffness, but torsional stiffness increases from T7/T8 to L3/L4.
• Tortional stiffness is provided by the outer layers of both the vertebral
bodies and intervertebral disks and by the orientation of the facets.
• The outer shell of cortical bone reinforces the trabecular bone and
provides resistance to torsion.
• When the disk is subjected to torsion, one half of the annulus fibers resist
clockwise rotations, whereas fibers oriented in the opposite direction
resist counter clockwise rotations. It has been suggested that the annulus
fibrosus may be the most effective structure in the lumbar region for
resisting torsion.
• The risk of rupture of the disk fibers is increased when torsion, heavy axial
compression and bending are combined.

27. Shear
• Shear forces act on the mid plane of the disk and
tend to cause each vertebra to undergo
translation (move anteriorly, posteriorly or from
side to side in relation to the inferior vertebra)
• In the lumbar spine the zygapophyseal joints
resist some of the shear force and the disks resist
the remainder. When the load is sustained, the
disks exhibit creep and the zygapophyseal joints
may have to resist all of the shear force.

28. Mobility
• Motions at the intervertebral and zygapophyseal joints are
interdependent. The amount of motion available is determined primarily
by the orientation of the facets. The motion that occurs between the
vertebral bodies at the intervertebral joints is similar to what occurs when
a rubbing ball is placed between two blocks of wood. The blocks ma be
tilted or rotated in any direction and may glide if the ball rolls.
• The motions of flexion and extension occur as a result of the tilting and
gliding of a superior vertebra over the inferior vertebra.
• The nucleus pulposus acts like a pivot but unlike a ball is able to underfo
greater distortion because it behaves as a fluid.
• The magnitude of motion created by the ratio of disk height to width, a
gliding motion occurs at he zygapophyseal joints as the vertebral body tilts
over the disk at the intervertebral joint.
• If the superior and inferior zygapophyseal facet surfaces of three adjacent
vertebrae lie in the sagittal plane, the motions of flexion and extension are
facilitated. If the zygapophyseal facet surfaces are placed in the frontal
plane the predominant motion that is allowed is that of later flexion.

29. Flexion
• In vertebral flexion the anterior tilting and gliding of the superior vertebra causes a
widening of the intervertebral foramen and a separation of the spinous processes.
• The amount of tilting is partly dependent on the size of the disks, tensionin the
supraspinous and interspinous ligaments resists separation of the spinous
processes and thus limits tje extent of flexion. Passive tension tn the
zygaapophyseal joint capsules, ligamentum flavum, posterior longitudinal
ligament, posterior annulus, and the back extensors also imposes controls on
excessive flexion. Tension in the posterior ligaments can be produced by
contractions of the hip extensors pulling downward on the pelvis when a person is
in the standing position.
• Tension in the thoracolumbar fascia produced by contractions of the transversus
abdominis also can limit flexion by the pull of the fascia on the spinous processes
in the lumbar area.
• The disks influence flexion because the anterior portion of the annulus fibrosus is
compressed and bulges anteriorly during flexion while the posterior portion is
stretched and resists separation of the vertebral bodies.

30. Extension
• In extension, the intervertebral foramen is narrowed and the
spinous processes move closer together. The amoutnt of motion
available in extension, in addition to being limited by the size of the
disks, is limited by bony contact of the spinous processes, passive
tension in the zygapophyseal joint capsules, anterior fibers of the
annulus, anterior trunk muscles, and the anterior longitudinal
ligament.
• There are many more ligaments that limit flexion than there are
ligaments that limit extension.
• The only ligament that limits extension is the anterior longitudinal
ligament. The numerous checks to flexion follow the pattern of
finding ligamentous checks to motion where bony limits are
minimal.
• Few ligamentous checks to extension are necessary given the
presence of numerous bony checks.

31. Lateral Flexion
• In lateral flexion, the superior vertebra tilts, rotates and
translates over the adjacent vertebra below. The annulus
fibrosus is compressed on the concavity of the curve and
stretched on the convexity of the curve. Passive tension in
the annulus fivers, intertransverse ligaments and anterior
and posterior trunk muscles on the convexity of the curve
limit lateral flexion, the rotation that accompanies lateral
flexion differs slightly from region to region because of the
orientation of the facets.
• All intervertebral and zygapophyseal joint motion that
occurs between the vertebrae from L5 to S1 adheres to the
general description that have been presented. Regional
variations in the structure, function and musculature of the
column are different in different region.

32. Regional Variations in Vertebral Str.
Part Cervical Vert. Thoracic Vert. Lumbar Vert.
Body The body is small with a
transverse diameter greater
than anterior posterior
diameter. Anterior surface of
the body is convex; posterior
surface is flat. The superior
surface of the body is saddle
shaped due to the presence
of uncinate processes on the
lateral aspects of the
superior surfaces.
The transverse and
anterior posterior
diameters of the
bodies are equal.
Anterior height is
greater than
posterior. Two
demifacets for
articulation with the
ribs are located on
the postero lateral
corners of the
vertebral plateaus.
The body is massive
a transverse
diameter greater
than the anterior
posterior diameter
and height.
Pedicles Project posterolaterally. Variable in shape
and orientation
Short and thick
Laminae Project posteromedially and
are thin and slightly curved.
Short, thick and
broad
Short and broad

33. Spinous
processes
Short, slender and extend
horizantally.have bifid tips.
T1-T10 slope
inferiorly T11 and
T12 have a triangular
shape.
Broad, thick and
extend horizantally.
Vertebral
foramen
Large and roughly triangular. Small and circular Triangular. It is larger
than the thoracic but
smaller than the
cervical.
Superior
zygapophyse
al facets
Face superiorly and
medeally.
Thin and flat and face
posteriorly,
superiorly and
laterally.
Vertical and concave
and face
posteromedially.
Support mamillary
processes on
posterior borders.
Inferior
zygapophyse
al facets
Face anteriorly and laterally. Face anteriorly,
superiorly and
medially.
Vertical, convex and
face anterolaterally.
Transverse
processes
Prossess foramen for
vertebral artery, vein and
venous plexus. Also have
agutter for spinal nerve.
Processes are large
with thickened ends.
Possess paired oval
facets for articulation
with the ribs. Show a
caudal decrease in
Processes are long,
slender and extend
horizontally. They
support accessory
processes o the
posterior inferior

34. Muscles of the Spine
• Flexors:
• Muscles that flex the trunk are located anteriorly and laterally with attachments
on the ribs, sternum and pelvis these muscles act indirectly on the vertebral
column by exerting a pull on the adjacent structures. Contractions of the flexor
muscles cause compression forces on the vertebral column.
• When the pelvis and ribs are free to move, a shortening contraction of the flexors
will pull these structures closer together and as a consequence flex the total spine
as in a sit up.
• If the ribs are fixed, a shortening contraction of the rectus abdominis muscle will
exert an upward pull on the anterior pelvis. The resulting posterior rotation of the
pelvis in the sagittal plane (posterior pelvic tilt) will flex the contiguous lumbar
spine.
• The internal and external oblique abdominal muscles turn and twist the lumbar
spine into axial rotation.
• The psoas major muscle has been described as a flexor, as a stabilizer and as an
extensor of the lumbar spine.

35. Rotators and Lateral flexors
• Rotation of the trunk is usually coupled with some degree of lateral
flexion. Anterior muscles that produce rotation and lateral flexion
are the external and internal oblique abdominals.
• Rotation of the trunk to the left requires a simultaneous contraction
of the right external oblique and the left internal oblique and vice
versa.
• Rotation and lateral flexion to the same side are a function of the
iliocostalis, longissimus, spinalis muscles, quadratus lumborum and
serratus posterior superior.
• Muscles that produce rotation to the opposite side are the
semispinalis thoracis, multifidus, rotatores and intertransversarii
thoracis.
• The lateral flexors of the trunk are the quadratus lumborum and the
iliopsoas.

36. Extensors
• Muscles that extend the vertebral column are located posteriorly.
The sacrospinalis muscles (erector spinae), consist of three
divisions. This group of muscles extends from the sacrum to the
occipital portion of the skull, attaching to the transverse and
spinous processes of all vertebrae and the angles of the ribs by
various divisions.
• Lateral division- attaches to the ribs is called the iliocostalis group
and consists of the iliocostalis cervicis, thoracis and lumborum.
• Medial division- attaches to the spinous processes and is called the
spinalis group, which includes the spinalis cervicis, capitis and
thoracis.
• Intermediate division- attaches to the transverse processes is called
the longissimus, which includes the longissimus capitus, cervicis
and thoracis.

38. Thank- You