Lecture 3 - Spine Flashcards
Lumbar spine
- L1-L5
- carries weight of upper body (larger, broader)
- peripheral nerves (legs, pelvis)
Cervical spine
- C1-C7
- more flexible
- supports head
- wide ROM
- peripheral nerves (arms, shoulder, chest and diaphragm)
Sacral and coccygeal region
Sacrum:
- triangular
- base of spine
- connects spine to pelvis
- nerves to pelvic organs
Coccyx:
- few small tailbones
Thoracic spine
- T1-T12
- mid-back / dorsal
- ribs attach to vertebrae
- immobile
- peripheral nerves (intercostal)
Spinal functional unit
- 2 vertebrae + intervertebral disc
- two joints between vertebrae:
- intervertebral disc (symphysis)
- 2 gliding facet joints - 6 degrees of freedom
Spinal curvature over the ages
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
Lordosis
Lordosis:
- excessive inward lordotic curvature
Kyphosis:
Kyphosis:
- outward (convex) curvature
Scoliosis:
Scoliosis:
- sideways curvature - ‘S’ or ‘C’ shaped
Facet joints
- channel and limit the range of motion
- assist in load bearing (30% compressive load)
Upright position - biomechanical effects
- spinal compression (body weight + weight held by arms and hands)
- COG anterior to spinal column –> constant forward bending moment on spine
Forces acting on the spine
- body weight (curved spine - compressive and shear components)
- tension (spinal ligaments + surrounding muscles)
- intra-abdominal pressure
- external loads
Why should you lift with your legs and not your back?
What effect does spinal rotation have on the IVDs?
- 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
Torque
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
Backrests
Support spinal loads by supporting a portion of the weight of the trunk
- reduce the moment arm of the trunk
Intervertebral disc
- 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
Annulus fibrosus
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
Nucleus pulposus
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)
Cartilaginous endplates
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
Disc degeneration - changes in composition and behaviour
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
Maximum safe value of disc compressive force (NIOSH)
3.4kN
Intervertebral disc injuries
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
Slipped disc
- prolapsed / herniated disc
- annulus fibrosus ruptures and nucleus protrudes from disc –> impinging spinal cord nerves
Sciatica
- irritation / compression of sciatic nerve
- slipped disc = most common cause
