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Orthopaedic Biomechanics > Lecture 3 - Spine > Flashcards

Lecture 3 - Spine Flashcards

1
Q

Lumbar spine

A
  • L1-L5
  • carries weight of upper body (larger, broader)
  • peripheral nerves (legs, pelvis)
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2
Q

Cervical spine

A
  • C1-C7
  • more flexible
  • supports head
  • wide ROM
  • peripheral nerves (arms, shoulder, chest and diaphragm)
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3
Q

Sacral and coccygeal region

A

Sacrum:

  • triangular
  • base of spine
  • connects spine to pelvis
  • nerves to pelvic organs

Coccyx:
- few small tailbones

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4
Q

Thoracic spine

A
  • T1-T12
  • mid-back / dorsal
  • ribs attach to vertebrae
  • immobile
  • peripheral nerves (intercostal)
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5
Q

Spinal functional unit

A
  • 2 vertebrae + intervertebral disc
  • two joints between vertebrae:
    - intervertebral disc (symphysis)
    - 2 gliding facet joints
  • 6 degrees of freedom
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6
Q

Spinal curvature over the ages

A 
Babies:
- simple curvature
Small child:
- cervical lordosis
- when child starts to sit
Toddler and adult:
- when start to stand
- lumbar lordosis

‘S’ shape in sagittal plane

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7
Q

Lordosis

A

Lordosis:

- excessive inward lordotic curvature

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8
Q

Kyphosis:

A

Kyphosis:

- outward (convex) curvature

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9
Q

Scoliosis:

A

Scoliosis:

- sideways curvature - ‘S’ or ‘C’ shaped

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10
Q

Facet joints

A
  • channel and limit the range of motion

- assist in load bearing (30% compressive load)

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11
Q

Upright position - biomechanical effects

A
  • spinal compression (body weight + weight held by arms and hands)
  • COG anterior to spinal column –> constant forward bending moment on spine
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12
Q

Forces acting on the spine

A
  • body weight (curved spine - compressive and shear components)
  • tension (spinal ligaments + surrounding muscles)
  • intra-abdominal pressure
  • external loads
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13
Q

Why should you lift with your legs and not your back?

What effect does spinal rotation have on the IVDs?

A
  • spinal muscles have small moment arms w.r.t vertebral joints
  • have to generate large forces to counteract torque
  • spinal rotation –> shear stress in intervertebral discs
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14
Q

Torque

A

Rotary effect of force around an exis of rotation

= Force x perpendicular distance between forces line of action and the axis

To maintain upright position - counteracted by tension in the back extensor muscles

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15
Q

Backrests

A

Support spinal loads by supporting a portion of the weight of the trunk
- reduce the moment arm of the trunk

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16
Q

Intervertebral disc

A
  • fibro-cartilaginous cushions, avascular
  • flexibility and transmit load
Nucleus pulposus
- inner region, mostly water
Annulus fibrosus
- firm + banded outer region
Cartilaginous endplates
- thin layer of hyaline cartilage
17
Q

Annulus fibrosus

A

Structure

  • 20 layers, unidirectional lamina
  • t=0.1mm, 60deg (outer)
  • t=0.4mm, 45deg (inner)
  • 70% water, PG and collagen (I outer, II inner)

Function

  • resist tensile strains
  • E = 25MPa (circumferential) and 0.5MPa (radial)
  • Viscoelastic behaviour - shear mod increases with load

Mechanical behaviour

  • bending: tensile posteriorly, compressive anteriorly
  • rotation: reorientation of collagenous fibres - tightening in 1D, loosening in other
18
Q

Nucleus pulposus

A

Structure

  • gelatinous (75-90% water)
  • PG (aggrecan), collagen II + matrix proteins
  • collagen fibres random + loose - isotropic material properties

Function

  • imbibes water (increase in 200% volume in saline)
  • internal pressure - fixed charge of PGs

Mechanical behaviour

  • pressure in all directions
  • lateral pressure restricted by tension in annulus fibres
  • superior pressure against end plates (increase stiffness)
19
Q

Cartilaginous endplates

A

Structure

  • hyaline cartilage 0.6cm thick
  • similar to articular cartilage (70-80% water, PG)

Function

  • nutrients diffuse through endplate to nucleus
  • permeability of endplates control waste product accumulation
  • transport through annulus periphery

Mechanical behaviour

  • deforms under compression
  • fluid pressurisation maintains uniform stress distribution at boundary
20
Q

Disc degeneration - changes in composition and behaviour

A

Changes in composition:

  • increase fibre content
  • decreased hydration of nucleus + annulus (decrease PG)
  • endplate thickness irregular - ossification (less cushioning)
  • annulus - alterations in collagen I and II distribution

Behaviour:

  • pressure + flexibility affected
  • nucleus - shear mod increases 8 fold
  • annulus - increase compressive + shear mod, decrease permeability
21
Q

Maximum safe value of disc compressive force (NIOSH)

A

3.4kN

22
Q

Intervertebral disc injuries

A

Annular injury:

  • rings: softened, overstretched, torn
  • normal viscoelasticity exceeded
  • cannot stabilise / resist pressure –> buckling

Nucleus pulposus:

  • extrusion - breaches annulus fibrosus
  • prolapse - fluid escapes through fissures - chronic back pain
23
Q

Slipped disc

A
  • prolapsed / herniated disc

- annulus fibrosus ruptures and nucleus protrudes from disc –> impinging spinal cord nerves

24
Q

Sciatica

A
  • irritation / compression of sciatic nerve

- slipped disc = most common cause

25
Q

Last resort surgical treatments for back pain

A

Discectomy:
- ruptured part of nucleus pulposus removed

Spinal fusion:
- intervertebral joint fused using implants

Orthopaedic Biomechanics (8 decks)

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