The document discusses the anatomy and biomechanics of the lumbar spine and sacrum. It describes the typical lumbar vertebrae, intervertebral discs, spinal ligaments, muscles, and motions of the lumbar spine. It also summarizes common spinal disorders like back strain, herniated discs, spondylolysis, and scoliosis. The sacroiliac joint and sacral anatomy are also briefly covered.

1. Spine

2. Sreeraj S R
BIOMECHANICS

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Lumbar Anatomy
5 vertebrae L1-L5
5 intervertebral discs
5 pair of exiting nerve
roots
Lumbar lordosis L1-S1
ranges from 30°–80°
The apex of lumbar
lordosis L3-L4
1
2
3
4
5

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Lumbar Spine Anatomy
Typical lumbar vertebra (L2)
Body
Vertebral foramen/canal
Intervertebral foramen
Pedicle
Transverse process
Lamina
Spinous process
Facet joints
Pars interarticularis
inferior
Superior
Anterior (oblique)
A Lateral P
Posterior (oblique)
Superior
Inferiorsuperior

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Intervertebral Disc
Soft fibro-cartilaginous
cushions
Between two vertebra
Allows some motion
Serve as shock
absorbers
Total – 23 discs
¼ th of the spinal column's
length
Avascular
Nutrients diffuse through
end plates
Collagen

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Intervertebral Disc
NUCLEUS PULPOSUS
Has more water and PGs
PG are macro-molecules
Attract and retain water
Hydrophilic gel–like
matter
Resists compression
Amount of water
Activity related
Varies throughout the
day

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Intervertebral Disc
NUCLEUS PULPOSUS
Eccentrically positioned posteriorly
Young & healthy, 90% water, bound
to proteoglycans
Aging> desiccation> increase
viscosity> fissuring
Young nucleus> even distribution of
load
Old nucleus> undue concentration
on vertebral body edges
Small displacement w/ ROM, ball-
bearing like
Compressive stress predominates

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Intervertebral Disc
NUCLEUS PULPOSUS
Pascal’s law: Fluid mass within closed
container> local increase in pressure>
transmit around entire side wall
(annulus)
Nucleus pulpous imbibes water
Develops internal pressure
Pressure exerted in all directions
Lateral forces against annulus
Superiorly and inferiorly directed
forces against end plates
Increases stiffness of end plate
and annulus fibrosus

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Intervertebral Disc
ANNULUS FIBROSUS
Strong radial tire–like
structure
Series of lamellae
Concentric sheets of
collagen fibers
Connected to end plates
Orientated at various angles
Under compression
Become horizontal
Encloses nucleus pulposus

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Intradiscal Pressure
INTRADISCAL PRESSURE
Compressive loads in vivo:
500N standing, 700N sitting
Increased to 3000 to 6000N
during lifting of moderate
weights, decreases with load
closer to body
Estimate of P = 1.5X
compressive load divided by
the cross sectional area
Disk pressure is usually
uniform
Pressure lowest in supine
position
Disk usually does not fail,
but end plates fracture

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Spinal Ligaments
Anterior Longitudinal
Posterior Longitudinal
Ligamentum Flavum
Interspinous
Ligaments
Supraspinous
Ligaments
Intertransverse
Ligaments

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Lumbar Spine

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Lumbar Spine
Thoraco lumbar fascia
Stabilizing corset
Transmit load
longitudinally to the
spinous process
Ilio lumbar ligament
Stabilize 5th lumbar
vertebrae from ant.
Displacement

14. Types of motion

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Stress-Strain Curve

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The Motion Segment
Functional Spinal Unit
2 adjacent vertebrae &
intervening soft tissue
Anterior
Vertebral body
Disk
ALL, PLL
Support, absorb impact,
restrict vertical translation
Posterior
Neural arch & its processes
Facet joint

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STABILITY
The vertebral column subject to
Axial compression
Bending
Torsion
Shear

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STABILITY
Primary load-transmitting
element, 80-90%
Bone Mineral Content, Size
Osteoporosis> loss of
horizontal trabeculae
Increasing size from C to L
spine
Compressive load> pressure
higher in center of end plates
than periphery
In vivo, filled with blood>
greater strength, hydraulic
shock absorber

19. Sreeraj S R
STABILITY
POSTERIOR ELEMENTS
pedicles, lamina, facet joints,
spinous & transverse processes
Bony processes> lengthen
moment arms of muscles
Forces on processes>
transmitted to Lamina
Forces on posterior elements>
transmitted to vertebral bodies
from Pedicles
Pars Interarticularis
Large bending forces;
excessive extension
Thicker than rest of lamina
Common site of
stress/fatigue fractures>
weakens motion segment>
spondylolithesis

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STABILITY
Facet Joints
Major role in controlling motion
Resist torsion & shear, role in compression
Lumbar FSU – facets 40% torque resistance, 40% disk,
20% ligaments
Load sharing varies with flexion & extension
Seated position> decreased lumbar lordosis> increased
intradiscal pressure & decreased load-bearing of the facets
Orientation of facets
C spine - 45º transverse, parallel frontal
T spine - 60º transverse, 20º frontal
L spine - 90º transverse, 45º frontal
Capsules lax> allow gliding

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MOBILITY
Flexion-Extension
large, due to sizable
disks & lack of facet
restraint
posterior half of
disk, moves w/ flex-
ext
Lateral bending
Axial rotation

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MOBILITY
Lumbo pelvic rhythm
Coordinated
simultaneous activity of
lumbar flexion and
tilting of pelvis
LPR can increase the
range of forward flexion,
anterior pelvic tilt and
flexion of lumbar spine

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Lumbo sacral angle
Ferguson’s angle
Is formed by the fifth
lumbar vertebra and first
sacral segment
The first sacral segment ,
which inclined anteriorly
and inferiorly forms an
angle with the horizontal
35-40⁰ considered
normal

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Sacral Anatomy
The sacrum is a series of
3, 4, or 5 fused coccygeal
vertebrae
The coccyx articulates
with the inferior aspect
of the sacrum1
2
3
4
C

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SACROILIAC JOINT
A joint that connects
the spinal column with
the pelvis. The V-
shaped sacrum near the
base of the spine fits like
a wedge between the
wide wings of
the ilium (hipbone).

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SACROILIAC JOINT

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MOBILITY AND STABILITY
Poorly understood
Permits a small amount of motion
Stiff, coarse interdigitating articular surfaces
Complete ankylosis in up to 76% over age of 50
Nutation, as described by Kapandji, is the anterior inferior
motion of the sacral base.
counter- nutation as the movement of the sacral base
posteriorly and superiorly.
This nutation and counter- nutation motion of the sacrum
is a pivoting type of motion, so that when the base moves
forward, the sacral apex (inferior part of the sacrum) moves
posteriorly.

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Muscles

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Iliocostalis Lumborum
O
Common tendon origin in
sacrum, iliac crest, lumber
vertebrae
I
Lower borders ribs 6-12
N
Dorsal rami of spinal nerves
F
Bilateral
Spinal extension
Maintenance of erect posture
Stabilization of spine during
flexion
Unilateral
Lateral flexion
Ipsilateral rotation

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Longissimus Thoracis
O
Common tendon origin
in sacrum, iliac crest,
lumber vertebrae
I
T1-12 transverse
processes
N
Dorsal rami of spinal
nerves
F
Same as above

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Spinalis Thoracis
O
Common tendon origin
in sacrum, iliac crest,
lumber vertebrae
I
T3-8 spinous processes
N
Dorsal rami of spinal
nerves
F
Same as above

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Multifidus
O
Transverse processes C4-L5
Sacrum
PSIS
I
Spinous process of vert above
origin
N
Spinal nerve roots
F
Extend and lateral flexion of
vertebral column

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Quadratus Lumborum
O
Iliolumbar Ligament
Iliac crest
I
Lower border 12th rib
L1-L4 transverse processes
N
ventral branches of T12 and L1 to
L4.
F
Pelvis elevation
Trunk extension
Trunk lateral flexion
Pulls down rib 12 to fix origin of
diaphragm

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Rotatores
O
Transverse processes from
axis to sacrum
I
Laminae of vert above
N
Direct branches over spinal
nerve roots
F
Spine extension
Rotation to opposite side

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Disorders Of The Back/Spine
Back Strain/Sprain
Ankylosing Spondylitis
Cauda Equina
Herniated Nucleus Pulposus (HNP)
Spinal Stenosis
Kyphosis/Scoliosis
Low Back Pain (LBP): Spondylolysis, Spondylolisthesis

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Back Strain/Sprain
LBP is the most frequent cause of
lost work time and disability in
adults <45 years
Most symptoms of limited
duration
85% of patients
improve and returning
to work within 1 month

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Ankylosing Spondylitis
Progressive spinal flexion
deformities (may progress
to a chin-on-chest
deformity)

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Cauda Equina symdrome

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Herniated Nucleus Pulposus (HNP) of the
Lumbar Spine
Displacement of the
central area of the disc
(nucleus) resulting in
impingement on a nerve
root

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Kyphosis
Defined: abnormally
increased convexity in
the curvature of the
thoracic spine as viewed
from side
Scheuermann’s Disease
Hyperkyphosis that
does not reverse on
attempts at
hyperextension

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Scoliosis
Lateral curvature of the spine
of greater than 10 degrees,
usually thoracic or lumbar,
associated with rotation of
the vertebrae and sometimes
excessive kyphosis or lordosis
Idiopathic scoliosis
Lateral deviation and
rotation of the spine without
an identifiable cause

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Low Back Pain
Spondylolysis
Unilateral Pars defect is
the result of a fatigue
fracture from
repetitive
hyperextension

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Low Back Pain
Spondylolisthesis
Bilateral Pars Interarticularis
defect
Forward slippage of one
vertebra on another
Usually L5-S1