biomechanics of the spine

Functional unit

· disc-vertebra-disc unit is motion segment

Anterior

· consists of vertebral bodies and intervertebral disc

· designed to bear compressive loeads

Intervertebral disc

· two functions

Load

· hydrostatic function

· acts as cushion

· stores energy and distributes loads

· during load

· nucleus pulposus undergoes compressive stress

· annulus fibrosus undergoes tensile stress

· nucles pulposus has intrinsic pressure at rest

· called prestress

· exerted by longitudinal ligaments and ligamentum flavum

Movement

· prevents excessive movement

Posterior

· consists of posterior elements

Facet joints

· two functions

Movement

· primary function is to guide movements

· orientation of facet joints dictates direction and degree of movement

Cervical

· upper 2 vertebrae specialised

· C3 to C7 typical

· facets orientated

· parallel to coronal plane

· angled 45^o upwards from transverse plane

· means pure rotation impossible

· abduction associated with rotation

· with abduction, neural arch slides downwards and thus backwards on concave side and vice versa

· thus concomitant rotation produced

Thoracic

· facets orientated

· at 20^o outwards to coronal plane

· angled 60^o upwards from transverse plane

· lie on circumference of circle

· centre of circle in vertebral body

· means pure rotation possible

· abduction associated with rotation

Lumbar

· facets orientated

· at 45^o inwards to coronal plane

· angled 90^o upwards from transverse plane

· lie vertically

· means no rotation possible

Load

· some load-bearing function

· maximal during hyperextension

· up to 30%

Kinematics

segmental motion

Flexion-extension

Cervical

· total of 150^o

· 15^o at O-C1

· 10^o at C1-C2

· 65^o at C2-C7

Thoracic

· average of 8^o per level

· progressively increases

· 4^o at top

· 12^o at bottom

Lumbar

· average of 16^o per level

· progressively increases

· 12^o at top

· 20^o at bottom

Lateral flexion

Cervical

· total of 100^o (50^o to each side)

· 5^o at O-C1

· none at C1-2

· 45^o at C2-C7

Thoracic

· average of 6^o per level

Lumbar

· average of 6^o per level

Rotation

Cervical

· total of 180^o (90^o to each side)

· none at O-C1

· 45^o at C1-C2

· 45^o at C3-C7

Thoracic

· average of 9^o per level

Lumbar

· average of 2^o per level

· 5^o possible at lumbosacral junction

functional motion

Flexion-extension

Cervical

· C1 can move independently

· below this, coupled with transverse translation

· flexion results in forward shift

Thoracic

· thoracic spine contributes little

· orientation of facets

· restriction of ribs

Lumbar

· 1st 60^o occurs in lumbar spine

· esp. lower segments

· then tilting of pelvis occurs

· reverse sequence occurs during extension

· pelvis

· ribs

Lateral flexion

Cervical

· C1 can move independently

· below this, coupled with rotation

· lateral flexion results in rotation to same side

Thoracic

· can predominate in thoracic spine

· allowed by facet orientation

· limited by rib cage

Lumbar

· can also predominate in lumbar spine

· similar degree permitted

Rotation

Cervical

· coupled with lateral flexion

· rotation results in lateral flexion to same side

Thoracic

· significant amount occurs

· associated with lateral flexion

· vertebral bodies rotate towards concavity

Lumbar

· only occurs at lumbosacral junction

Kinetics

cervical

· loads relatively low during

· upright standing

· siting

· increased moderately during

· rotation

· lateral flexion

· more significant with extremes of flexion-extension

lumbar

· loads produced by

· body weight

· muscle activity

· ligamentous prestress

· external loads

Standing

· line of gravity passes

· through body of T1

· anterior to body of L4

· means there is constant forward-bending moment

· requires counterbalance by erector spinae force

· load on spine is twice body weight

Sitting

· increased force

· line of gravity shifted further forwards

· creates longer lever arm for force exerted by weight of upper body

· forces are 2 times that standing with relaxed unsupported sitting

· decreased by

· erect cf. relaxed sitting

· supported cf. unsupported sitting

· backward inclination cf. forward inclination

Reclining

· load minimal

· because body weight eliminated

· even further decreased by traction with knees semiflexed

Lifting

· significantly increases load

· related to

· position of object wrt. centre of motion of spine

· size, shape, weight and density of object

· degree of flexion or extension of spine

· load reduced by

· holding object close to body (reduces lever arm)

· keeping body erect (reduces lever arm)

· bending at knees (keeps object closer to trunk)

· contracting abdominal muscles (supports spine)

· with compressive testing to failure

· fracture point of end-plate reached before disc damage occured

· disc damage may occur from recurrent tensile stresses