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Section 36: Spine
Biomechanics
36-1
36-2 From: Peeler
36-3 From: Iatridis
Theory of weight bearing
Theory of weight bearing
• Nucleus pulpous imbibes water
Nucleus pulpous imbibes water
• Develops internal pressure
• Pressure exerted in all directions
• Pressure exerted in all directions
– Lateral forces
• Against annulus
Against annulus
– Superiorly and inferiorly directed forces
• Against end plates
g
– Increases stiffness
• Of end plate and annulus fibrosus
36-4 From: Ziv
Nucleus Pulposus
• Nucleus Pulposus
– Eccentrically positioned posteriorly
– Young & healthy
– Young & healthy
• 50% cross-sectional
• 90% water, bound to proteoglycans
– Aging> dessication> increase viscosity> fissuring
– Pascal’s law
• Fluid mass within closed container> local increase in
• Fluid mass within closed container> local increase in
pressure> transmit around entire side wall (annulus)
• Young nucleus> even distribution of load
Old l d t ti t b l b d d
• Old nucleus> undue concentration on vertebral body edges
– Small displacement w/ ROM, ball-bearing like
– Compressive stress predominates
36-5 From: Yang
Compressive stress predominates
36-6 From: Iatridis
Theory of weight bearing (cont’d)
Theory of weight bearing (cont d)
36-7 From: Ziv
• Intradiscal Pressure
– Compressive loads in vivo: 500N standing,
700N sitting
– Increased to 3000 to 6000N during lifting of
moderate weights decreases with load closer
moderate weights, decreases with load closer
to body
– Estimate of P = 1 5X compressive load
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
f
36-8 From: Yang
fracture
Measurements of In vivo Loads
Measurements of In vivo Loads
• Needle pressure
Needle pressure
transducer
• Calibrated
– Introduced into
nucleus pulpous of
d i f ti l
cadaveric functional
unit
• Inserted in vivo in L3-
• Inserted in vivo in L3-
4 disc
36-9 From: Ziv
Shear & Tensile Characteristics
Shear & Tensile Characteristics
• In direct shear tests
In direct shear tests
– Shear stiffness in horizontal direction
• 260 N/mm2
• Spine rarely fails in pure shear
• Similarly under normal physiologic activities
– Pure tensile loading doesn’t occur
– But annulus undergoes tensile loading during
• Bending
• Axial rotation
• Extension
36-10 From: Ziv
36-11 From: Yang
36-12 From: Peeler
36-13 From: Peeler
36-14 From: Peeler
36-15 From: Peeler
36-16 From: Peeler
Low Back Pain
Low Back Pain
• Most frequent cause of activity limitation in people under
q y p p
age 45
• Direct annual cost of treating low back pain patients
– $11 4 billion in 1994
$11.4 billion in 1994
– Not including the secondary costs of lost work and activity
• Mechanical loading conditions associated with back pain
F t b di d t i ti
– Frequent bending and twisting
– Heavy physical work
– Sedentary environment
Vib ti
– Vibration
36-17 From: Iatridis
36-18 From: Peeler

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36-Slides - Spine Biomechanics.pdf

  • 4. Theory of weight bearing Theory of weight bearing • Nucleus pulpous imbibes water Nucleus pulpous imbibes water • Develops internal pressure • Pressure exerted in all directions • Pressure exerted in all directions – Lateral forces • Against annulus Against annulus – Superiorly and inferiorly directed forces • Against end plates g – Increases stiffness • Of end plate and annulus fibrosus 36-4 From: Ziv
  • 5. Nucleus Pulposus • Nucleus Pulposus – Eccentrically positioned posteriorly – Young & healthy – Young & healthy • 50% cross-sectional • 90% water, bound to proteoglycans – Aging> dessication> increase viscosity> fissuring – Pascal’s law • Fluid mass within closed container> local increase in • Fluid mass within closed container> local increase in pressure> transmit around entire side wall (annulus) • Young nucleus> even distribution of load Old l d t ti t b l b d d • Old nucleus> undue concentration on vertebral body edges – Small displacement w/ ROM, ball-bearing like – Compressive stress predominates 36-5 From: Yang Compressive stress predominates
  • 7. Theory of weight bearing (cont’d) Theory of weight bearing (cont d) 36-7 From: Ziv
  • 8. • Intradiscal Pressure – Compressive loads in vivo: 500N standing, 700N sitting – Increased to 3000 to 6000N during lifting of moderate weights decreases with load closer moderate weights, decreases with load closer to body – Estimate of P = 1 5X compressive load 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 f 36-8 From: Yang fracture
  • 9. Measurements of In vivo Loads Measurements of In vivo Loads • Needle pressure Needle pressure transducer • Calibrated – Introduced into nucleus pulpous of d i f ti l cadaveric functional unit • Inserted in vivo in L3- • Inserted in vivo in L3- 4 disc 36-9 From: Ziv
  • 10. Shear & Tensile Characteristics Shear & Tensile Characteristics • In direct shear tests In direct shear tests – Shear stiffness in horizontal direction • 260 N/mm2 • Spine rarely fails in pure shear • Similarly under normal physiologic activities – Pure tensile loading doesn’t occur – But annulus undergoes tensile loading during • Bending • Axial rotation • Extension 36-10 From: Ziv
  • 17. Low Back Pain Low Back Pain • Most frequent cause of activity limitation in people under q y p p age 45 • Direct annual cost of treating low back pain patients – $11 4 billion in 1994 $11.4 billion in 1994 – Not including the secondary costs of lost work and activity • Mechanical loading conditions associated with back pain F t b di d t i ti – Frequent bending and twisting – Heavy physical work – Sedentary environment Vib ti – Vibration 36-17 From: Iatridis