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Review anatomy of spine its functions. Analyze overall mechanical effects on spine during movement.
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Biomechanics of
the Spine
Vertebral Column
• Complex structure• Base of support• Link between upper and lower
extremities• Transfer load from head trunk to
the pelvis• Protects spinal cord• Stability vs. mobility
General Motion Segment
• Functional Spinal Unit– 2 adjacent vertebrae & intervening soft
tissue– Divided into 2 aspects
•Anterior aspect•Posterior aspect
General Motion Segment
Vertebra
Cortical Shell
Cancellous or Trabecular Bone
Endplate
Spinous Process
Transverse Process
PedicleFacet Joint
Annulus FibrosusNucleus Pulposus
Disc
Anterior Aspect of Spinal Column
• Vertebral body• Primary load-transmitting element, 80-
90%• 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• Weaker anterior trabeculae, Wolff’s
law
Posterior Aspect of Spinal Column
• Pedicles, lamina, facet joints, spinous & transverse processes
• Bony processes– Lengthen moment arms of muscles
• Forces on processes– Transmitted to Lamina
• Forces on posterior aspects– Transmitted to vertebral bodies from
Pedicles
Posterior Aspect of Spinal Column
• Pars Interarticularis– Large bending forces; excessive extension– Thicker than rest of lamina
• Facet Joints– Major role in controlling motion– Resist torsion & shear, role in compression– Load sharing varies with flexion &
extension– Capsules lax, allow gliding
Facet Joint• Articulation
between the superior and inferior facets
• Guide intervertebral motion through their orientation in the transverse and frontal planes
Facet Joint Capsule
• Limit motions• Strongest in thoracolumbar and
cervicothoracic regions where the curvatures change
• Resist flexion and undertake tensile loading in the superior portion with axial loading or extension.
• Resists rotation in lumbar region
Function of the Facet Joints
• Limit the range of motion in the
different regions of the spine
• To assist in load bearing, sustaining
up to 30% of the compressive load
on the spine, particularly when the
spine is in hyperextension
Facets
• Change orientation– From Cervical to
Lumbar regions
• Articular joints– Allow smooth
articulations between vertebrae
• Resist motion – Through bony
interactions– Aided by ligaments
Intervebral Foramina
• Exit for nerve root.• The size is dictated
by the disc heights and the pedicle shape.
• Decreases by 20% with extension and increases 24% with flexion
Spinal Ligaments
• Anterior Longitudinal
• Posterior Longitudinal
• Ligamentum Flavum• Interspinous
Ligaments• Supraspinous
Ligaments• Intertransverse
Ligaments
• Limit motion, provide stability/equilibrium
• Anterior longitudinal ligament– Interlinked to disks– Resists extension– 2X tensile strength of Posterior
longitudinal ligament
• Posterior longitudinal ligament– Narrow over vertebral bodies– Resists flexion
Spinal Ligaments
Spinal Ligaments• Ligamentum Flavum
– Elastic & strong– “shingled” configuration with laminae– Lengthen w/ flexion, shorten w/
extension
• Interspinous & Supraspinous– Resist flexion– Long moment arms
• Intertransverse Ligaments– Resist lateral flexion
Motion Segment Studies• 6 degrees of freedom
– Translation & Rotation– 3 orthogonal planes– Motion usually coupled
• Center of gravity – In front of 2nd sacral segment
Intervertebral Disc• Make up 20-30% of
the height of the column
• thickness varies from – 3mm in cervical region– 5mm in thoracic region – 9 mm in the lumbar
region
• Total – 23 discs
• ¼ th of the spinal column's length
Intervertebral Disc
• Ratio between the vertebral body height and the disk height will dictate the mobility between the vertebra – Highest ratio in cervical region allows
for motion– Lowest ratio in thoracic region limits
motion
• Avascular • Nutrients diffuse through end plates
Intervertebral Disc
• Spongy center – Nucleus pulposus
• Surrounded by a tougher outer fibrous ring – Anulus fibrosus
Nucleus Pulposus• Is located in the center
– Except in lumbar lies slightly posterior
• 80-90% is H2O – decreases with age
• Disc volume will reduce 20% daily (reversible) – Causes a loss of 15-25 mm of height in
the spinal column
Annulus Fibrosus
• Mostly avascular • Thickest anteriorly• Outermost 1/3 connects to vertebral
body • Outer 2/3 connect to the end plate• Collagen arranged in sheets called
lamellae (outer layers).– arranged in concentric rings -10-12 layers
that lessen in number with age and thicken
Intervertebral Disc Functions
• Movement of fluid within the nucleus– Allows vertebrae to rock back and forth– Flexibility
• Act to pad and maintain the space between the twenty-four movable vertebrae
• Act as shock absorbers• Allow extension and flexion
Theory of weight bearing
• 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
Types of Loading
• Axial Compression– Caused by gravity,
ground reaction forces, muscle contraction and ligaments reaction to tensile forces
Types of Loading
• Axial Compression– Causes tension at the annulus,
changing the angle of the fibers and increasing the stability
– Most load in anterior segment– Posterior segment can load from 0-30%
depending upon segments position
Types of Loading
• Bending– Combination of
compression, shear and tensile forces on the segment from translation
Types of Loading
• Torsion– Caused by axial
rotation and coupled motions
– Annulus fibrosus resists, 1/2 fibers CW other 1/2 CCW in a tensile manner
– facets resist depending upon the orientation
Types of Loading
• Shear– Annulus will
undergo some tensile forces depending upon direction and the fiber orientation or angle
Intradiscal Pressure
• Disk pressure is usually uniform• Pressure lowest in supine position• Compressive loads in vivo: 500N
standing, 700N sitting• Increased to 3000 to 6000N during
lifting of moderate weights, decreases with load closer to body
Mechanical Characteristics
Tensile stiffness of the disc annulus in different directionsHighest along – 150
Lowest along – the disc axis
Highest – Along normal direction of annulus fibers( 3 times stronger than that along horizontal direction)
Mechanical Characteristics
Theory of weight bearing
Spinal Discs under Various Loads
Normal Load
Additional Load
Uneven Load
Torsion Load
Spinal Movement
• Spinal movement is the combination of– Intervertebral joints– Facet joints
Back Flexion
• Superior vertebra will anterior tilt and forward gliding– Widen the intervertebral foramina 24%.– Add compressive forces on the anterior
aspect of the anterior segment – Move the nucleus pulposus posteriorly– Tensile forces will be placed on
posterior segment– Central canal is widened
Back Extension
• Superior vertebra will tilt and glide posteriorly– The intervertebral foramina narrowed
up to 20%– The central canal is narrowed– Nucleus pulposus moves anteriorly
Back Lateral Flexion
• Superior vertebra will translate, tilt and rotate over inferior in different direction– Tensile forces on convexity– Compressive forces on
concavity– Extension in ipsilateral facet– Flexion in contralateral
facet
Rotation
• Accessory motions are like lateral flexion due to same coupling in cervical and upper thoracic spine
• Exception with lower T/S and L/S in neutral coupling then opposite (in most references)
The Spinal Column
• A curved stack of 33 vertebrae structurally divided into five regions:
– Cervical region; 7 vertebrae
The Spinal Column
• A curved stack of 33 vertebrae structurally divided into five regions:
– Thoracic region;
12 vertebrae
The Spinal Column
• A curved stack of 33 vertebrae structurally divided into five regions:
– Lumbar region;
5 vertebrae
The Spinal Column
• A curved stack of 33 vertebrae structurally divided into five regions:
– Sacrum; 5 fused vertebrae
– Coccyx - 4 fused vertebrae
Spinal Curvatures
• Prior to birth “C-shaped”• There are 4 distinct curves in an
adult– Primary spinal curves– Secondary spinal curves
Primary Spinal Curves
• The thoracic and sacral curves
• Concave anteriorly
• Are present at birth
Secondary Spinal Curves
• The lumbar and cervical curves
• Concave posteriorly
• Develop from supporting the body
in an upright position after young
children begin to sit and stand
Secondary Spinal Curves• In the sagittal plane
– ‘S’ shape
• As a small child– When starts to sit– Cervical lordosis
• Toddler and adult– When starts to stand– Lumbar lordosis– Allows spring-like
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