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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