Spine Biomechanics

stability of the spine

the ability of the vertebral column to maintain its integrity under normal loading and physiologic movement that preserves function and protects associated neurologic structures

instability of the spine

the loss os stability leading to a loss of spinal stiffness which results in abnormal and increased movement in the motion segments

the spinal motion segment is also known as

the functional spinal unit

instability of the spine is associated with

abnormal coupling patterns or quantity of motion which tends to result in dysfunctional movement in more than one direction

antelisthesis

upper vertebrae moves forward

with flexion and extension the AOR is through

the center of the vertebral body below

coupled motion of the spine

lateral bending and acial rotation

what is the AOR for lateral bending

between facet joints

what is the AOR for rotation

the center of the vertebral body in the T spine and posterior SP in the L spine

facet opening is

the increase of intervertebral space

facet closing is the

decrease in intervertebral foramen space

opening of the facets occurs with

flexion, opposite SB, same side rotation

closing of the facets occurs with

extension, same side bend, opposite side rotation

what occurs in the neutral zone of the spine

there is relatively little resistance to motion, high spinal flexibility, low energy expenditure, and people can move very well

what happens in the elastic zone of the spine

there is significant increase in soft tissue stiffness, which results in an overall increase in spinal stiffness and an increase in resistance

punjabis bowl

it is easier to move in the neutral part of the bowl and harder to move as you approach the edges

in a person without LBP and a normal bowl

the pain free zone is outside the neutral zone

in a person with LBP and an abnormal bowl

the pain free zone is smaller than the neutral zone which creates instability

regional interdependence

you need a lot of systems to work together to allow proper mobility and stability

the 3 column model of the spine

portions anterior, middle, and posterior spine which helps to organize stability and loac

the anterior column is related to

injury to the vertebral body

the middle column is related to

spinal cord and neural content

the posterior column is associated with

instability of the vertebrae

trabecular integrity involves

vertical components to help with axial load, horizontal components to prevent bowing, and curved oblique components to withstand shear

loading with spinal flexion

compression anteriorly and traction posteriorly but traction is reduced by paraspinals

under compression what fails first

bone before intervertebral disc

with an exponential effect of stiffness what happens

horizontal lamellae break down, vertical columns are lengthened, and vertical columns thin

with a decrease in density that is not distributed homogeneously what happens

the anterior half of the vertebral body is at higher risk

what is the first phase of bone mineral density break down

there is increased space between lamellae and the horizontal lamella break down first. this reduces the amount of force

what is phase two of bone mineral density break down

the vertical portion gets thiner which decreases the strength of the system as a whole

a bone loss of 25% leads to

a reduction of resistance of 50%

what is the stress rider effect of the lumbar spine

a point of the vertebrae where stress is significantly concentrated making it more susceptible to fracture due to repetitive loading which occurs at the area or above it

with bone cement or fusion there is a risk of injury

at the level above or below due to an increase in stress. These segments typically become hypermobile to compensate

facet joints function

guide motion, take up to 33% load, resist shear forces, and maintain spacial symmetry

spatial symmetry is due to

intervertebral disc health, ligamentous support, size and shape, and posture

IVD health is interrelated to

facets

facet joints are very sensitive to

asymmetry and overloading

shear force is

a horizontal force that is generally a pain generator

intervertebral disc function

uniquely mitigates forces via acting as a shock absorber for compression and supporting movement in NZ and EZ for tension

annular layers of the disc help with

fiber orientation to maintain strength

annulus fibrosis is stiffer

anteriorly and in the outer layers

nucleus function

exerts force on the annulus and endplates with compression and mitigates to areas of lower pressure and puts the annulus under tension with traction

what happens to the IVD during forward bending

compression to the anterior IVD and traction to the posterior IVD

what is the weakest portion of the IVD

posterolateral

ligaments function

mainly in distraction to keep things close, provide stability closer to COR

the moment arm distance from IAR for ligaments leads to

greater resistance to load

ligamentum flavum

has a higher elastin component which is good for higher compliance and acts to preload the IVD posteriorly

lumbar lordosis enable

humans to have an upright posture, normal gait, and allow humans to use the hands as tools

cervical and lumbar COG

passes through the vertebral bodies, takes very low loads to maintain, have a higher risk of a burst fracture

thoracic COG

passes anterior to the vertebral body, are biased into flexion, have a higher risk of compression fracture