Biomechanics of the spine

Question 1

What are the 3 main functions of the spine?

Answer 1

• Allow movement
• Carry loads
• Protect neural structures

Question 2

What is the key role of the spine?

Answer 2

• Stability
• “the ability of the spine under physiological loads to limit patterns of displacement so as not to damage or irrate the spinal cord or nerve roots and in addition to prevent incapacitating deformity or pain due to structural changes”

Question 3

How is stability achieved?

Answer 3

• Punjabi 1992
• passive musculoskeletal subsystem
• active musculoskeletal subsystem
• neural subsystem
• all 3 work together to provide overal stability

Question 4

What does the passive musculoskeletal subsystem consist of?

Answer 4

• vertebra
• facet joints
• intervertebral dics
• spinal ligaments
• joint capsule
• flexion resisted by posterior structures
  
  • ligaments
  • facet joints
  • capsule
  • post part of intervertebral discs
• extension limited by
  
  • anterior longitudinal lig,
  • ant part of annulus fibrosis
  • facet joints
• rotation limited by
  
  • facet joints
  • intevertebral discs

Question 5

How many vertebra are there?

Answer 5

• 33
• 7 cervical
• 12 thoracic
• 5 lumbar
• 5 sacral
• 4 coccygeal

Question 6

What does each vertebra consist of ?

Answer 6

• Anterior body
• Posterior neural arch
  
  • 2 pedicles
  • 2 laminae
  • 2 transverse processes
  • single spinous process

Question 7

How does the vertebral bodies change as you move caudally?

Answer 7

• Width and depth increase in size

Question 8

What is the internal architecture of the cancellous vetebral body ?

Answer 8

• Horzontal & vertical trabeculae
  
  • horizontal prevent the side walls from collapsing when subjected to large compression forces
  • ( loss of horizontal trabeculae in Osteoporosis -> decreased vertebral body stiffness
  • Vertical trabeculae aid the transmission of the force from superior surface to the inferior surface of the body ( compression forces occur at end plate)

Question 9

What shape is the spine in the sagittal plane?

Answer 9

• S shaped
• Cervical and lumbar lordosis
• thoracic kyphosis
• adv to
  
  • ensure the bdy massis distrubuted away from the central axis of motion so maintaining balance is easier
  • curvatures provide shock absorbing qualities as they change with each strike during walking
  • coupling of the lumbar spine with pelvic rotation allow rotation of the lumbar spine to occur whenever there is lateral bend of the spine
  • this couping is also dependent on the facet joint orientation & lordosis of the lumbar spine

Question 10

What is the role of the spinous processes in the biomechanics of the spine?

Answer 10

• provides insertion for interspinous ligaments and paraspinal muscles
• it provides a long moment arm for the muscles and ligaments so reduce the force required to resist flexion deformities

Question 11

What is coupling? How is this illustrated in the spine?

Answer 11

• when movements in one plane causes an associated movement in another plane
• occurs in cervical and lumbar spine
• cervical spine, lateral bending is assoc with rotation of the spinous processes towards the convexity of the curve
• in lumbar spine lateral bending is assoc with rotation of the spinous processes towards the concavity of the curve

Question 12

Where is the axis of rotation in the spine?

Answer 12

• Posterior part of the vertebral body

Question 13

What is the cervical facet orientation?

Answer 13

• 45 degrees to sagittal plane
• parallel to coronal plane
• => AP translation resisted but
• less resistance to flexion/extension/ lateral bend and rotation

Question 14

What is the thoracic facet joint orientation?

Answer 14

• 60 degrees to sagittal plane
• 20 degrees to coronal plane

Question 15

What is the lumbar facet joint orientation?

Answer 15

• oreintated in sagital plane proximally but become coronally orientated towards the lumbosacral junction
• tilted in coronal plane by 45 degrees
• => allow flexion and translation

Question 16

What happens to the facet joints as you move caudally?

Answer 16

• Progressively** tilt up** (45-60-90 degrees)
• and in (neutral -20 45 degrees)

Question 17

What is the role of the facet joints?

Answer 17

• support some of the compressive load to the spine
• torsional stability- contribute 40% of torsional load resistance in the lumbar spine

Question 18

What 2 factors determine the degree of support provided for the spinal column by the spinal ligaments?

Answer 18

• Intrinsic strength of the ligament
• length of the lever arm ( moment) thru the lig acts, lies posterior part of vertbral body so a weak lig might still be effective as it is attached further away from the axis of rotaton

Question 19

What is biomechanical function of the intervertebral discs?

Answer 19

• Distribute compressive forces evenly from one vertebral body to the next
• shock absorption

Question 20

What are the intervertebral discs composed of?

Answer 20

• Anulus fibrosis- peripheral ring of fibrous tissue
  
  • concentric lamella of collagen
  • alternate layers of fobres positioned at 90 degrees to each other= some resistance to forces applied in any direction & hoop stresses generated by physiological loading of the nucleus
• central nucleus pulposus
  
  • ​consists of hyrophilic proteoglycans
  • h20 content reduces to 70% in old age
  • high water content enables the nucleus to deform like fluid when compressive force applied, spread hydraulically to equalise the forces adn keeping disc anulus under balanced tension during axial loading
  • with reduction in h20 ability to beahve hydraulically reduces, less able to deform, with time -> fissure formation and disc degeneration

Question 21

Name the muscles invovled in the active musculoskeletal subsystem?

Answer 21

• Extensor spinal muscles
• intercostal muscles
• abdominal wall
• lower turnk muscles

Question 22

describe the layers of the extensor spinal muscles involved?

Answer 22

• superifical layer
  
  • erector spinae
  • from lateral to medial =
    
    • iliocostalis
    • longissimus
    • spinalis
    • splenius in the neck
• Intermediate layer
  
  • **transversospinalis- **between transverse process and spine
    
    • thoracic spine rotators
    • multifidus
    • semispinalis
• Deep layer
  
  • interspinalis and intertransversalis

Question 23

Can you draw the displacement vs force graph for the spine?

Answer 23

• inital zone of high flexibilty= neutral zone
• zone of increased resistance= elastic zone
• in neutral zone studies suggest
  
  • ​passive subsystem applies very little resistane to movement. Thought to act as force transducer supplying feedback to neural subsystem.
  • most resistance from active and neural subsystems (skeleton itself is unstable without muscles)
  • thought muscle activation reduces the size of neutral important in pathological condiitons when increased in disc degeneration/spinal injuries/burst fx
• in elastic zone the passive subsystem resists motion

Question 24

When does instability of the spine occur?

Answer 24

• When the neutral zone is increased (e.g disc degeneation / injury) beyond the physiological range of movements and so the stabilising subsystems do not reach the the siffening phase of the force-displacement curve within the physioloigcal range of movement
• where the elastic zone is increased in pathological processes, muscular activity, with coordination of the active and neural systems will attempt to decrease the neutral zone. neural and active subsystems are then compensating for a fault in the passive subsystem