Biomechanics of the Lumbar Spine

6. Which of the following statements about the NIOSH Action Limit (AL) for L5/S1 compression force is true? (a)The Action Limit is 6376 N (b) L5/S1 compression forces above the AL are associated with a significant risk of spinal injury (c) Only 25% of men and 1% of women can lift loads above the AL (d)None of the above

(d) none of the above

What are some risk factors fro reporting LBP?

-Cumulative lumbar moment -Typical hand force -Peak L4/5 shear force -Peak trunk velocity

Why is it recommended to avoid lifting following prolonged trunk flexion?

Allows time for nucleus to resume neutral position and ligaments to regain their resting length following creep response

Which recommendation about listing technique cannot be justified by biomechanical evidence?

Always use a leg rather than back lift

What type of shear is the motion segment designed to resist?

Anterior shear

The interspinous ligament exerts what type of shear when the lumbar spine is flexed?

Anterior shear load on L4 relative to L5

What are reasons for why it is recommended that the lumbar lordosis be maintained during lifting?

Equalises strain on nucleus pulpous, posterior ligaments are not strained/cannot be injured, facet joints can bear some load, spine is stronger, helps avoid large anterior shear forces

What resists shear loads?

Facet joints and the erector spinae

What is a true statement about spinal flexion

Facet joints are unloaded

What type of moment is the spine constantly experiencing due to COM placement?

Flexion moment due to the weight being slightly anterior to the lumbar spine

What are possible mechanisms for disc prolapse?

Flexion, axial compression, or extension

What is the instantaneous axis of rotation?

For every motion there is a point about which the body can be considered to rotate

Why should you avoid lifting straight after getting up from bed?

Forward bending stress on disc is high in the morning as all the fluid has returned to the disc

Explain how sagittal plane pelvic rotation during standing influences the curvature of the lumbar spine

Forward pelvic rotation > increase lumbar lordosis Backward pelvic rotation > decrease lumbar lordosis

What type of pelvic tilt causes more lumbar lordosis?

Forward tilt

What is the Nucleus Pulposus?

Gel-like substance, 90% water and is the inner part of the IV disc

What is the Annulus Fibrosus?

Outer ring of fibrous cartilage that is made up of concentric bands of collagenous tissue.

What is spondylolysis?

Pars fracture

What resists flexion moments in the lumbar spine?

Posterior ligaments and erector spinae

What type of shear force do the ES fibres produce?

Posterior shear force

Which type of lift is more likely to result in loss of balance?

Squat lift

Which lift would exert the highest anterior shear load on the lumbar spine?

Stoop lift

What type of joint is the zygapophyseal joints?

Synovial

Explain how contraction fo the TA muscle can increase intra-abdominal pressure without increasing the compression load experienced by the lumbar spine

TA has a horizontal muscle fibre alignment, so does not generate loading along the compression axis when activated

What does positive joint shear mean?

Tendency of L4 to move anterior to L5

What is the posterior portion of the motion segment?

vertebral arches, facet joint, transverse and spinous processes, and posterior ligaments

What is the semi-squat?

• Between stoop and squat • Common freely chosen technique • Uses mid ROM for all joints

What are some advantages and disadvantages of a squat lift?

• Increase energy cost • Increase fatigue • Increase RPE • Decrease compliance • Decreased compliant loads for objects from b/w feet • Decrease shear forces • Decreased balance • Maintains lumbar lordosis

What are the advantages and disadvantages of stoop lift?

• Loss of lumbar lordosis • Load is closer • Difficult to maintain lumbar lordosis --> muscles are aligned with the compression axis - BAD

Backwards tilt of the pelvis is produced by what muscles?

is produced by the abdominal muscles

What happens to injury rate when exposed to action limits?

moderately increased injury risk

What occurs to load placement with degeneration of the discs?

shift in loading towards the posterior when it should be mainly anterior

What is reaction shear?

Due to upper body mass and hand loads

What are the three parts of longissimus?

Thoracic, cervical and capitis

What are the guidelines for safe lifting?

1. minimise bending moments, 2. maintain normal lordosis 3. avoid lifting after prolonged flexion 4. avoid lifting shorty after rising from bed 5. Pre-stress the trunk 6. avoid twisting and lateral bending

What is the anterior portion of the motion segment?

2 adjacent IV bodies, IV disc and longitudinal ligament

What does the motion segment consist of?

2 adjacent vertebrae and intervening soft tissue

How much force can the ligaments of the spine resist?

20N of compression

How many vertebrae are there?

33

What is the L5/S1 Action limit?

3433N

What is the moment arm for erector spinae?

5cm-6cm

What is the L5/S1 Maximum Permissible limit?

6376N

What are the three parts of spinalis?

Thoracic, cervical and capitis

Where does a pars fracture occur?

Between the upper and lower articular facets - this is the weakest part of the neural arch

What is spondylolisthesis?

Bilateral separation - displacement of posterior vertebral part

What is vertebroplasty?

Cement augmentation of damaged (osteoporotic) vertebrae

What are disadvantages of fusion?

Changes regional segmental motion and increases stress on the adjacent levels

Why is it recommended that twisting the trunk during lifting be avoided?

Co-contraction of trunk muscles is substantially increased during twisting, which increases the compressive loading on the lumbar spine

What type of load does erector spinae produce on the spine?

Compression load as is squeezes the IV discs and vertebrae as it contracts

Explain how the intervertebral disc is able to convert an applied compressive stress to tensile stress in the annulus fibrosus

Compression on nucleus pulpous causes bulging which strains the annulus fibrosus

What are symptoms of vertebral compression fractures?

Deformity (loss of height and pronounced kyphosis), acute and chronic pain, respiratory and intestinal compromised and psychosocial challenges

What effect does bent legs have?

Inactivates iliopsoas (prevents it from increasing lumbar load) and promotes rectus abdomens (backward pelvic tilt - decreases lumbar lordosis)

What happens to the ISL when the spine is flexed?

It produces a large anterior shear load

Where is disc degeneration most likely to occur?

L4-5 and L5-S1

The erector spina muscle contributes to which force during lifting?

L5/S1 compression force

L5/1 compression force is not influenced by

L5/S1 shear force

What are some characteristics of degenerative disc disease?

Loss in hydrostatic capabilities and decrease in disc height and protrusion of the nucleus into the vertebral body.

What are two problems associated with inter body fusion of adjacent lumbar vertebrae?

Loss of ROM, increase stress on adjacent joints/increase adjacent segment disease

What are the three parts of iliocostalis?

Lumbar, thoracic and cervical

What should individuals which moderate and high risk of injury do?

Moderate risk should load share and try to swap for some other tasks and high risk should let a machine take over the task

What are two advantages of a stoop-lift compared to squat lifting technique?

More stable/better balance, reduced fatigue, reduced perceived exertion, reduced energy cost, may be able to reduce moment arm of load relative to lumbar spine

What is joint shear?

Reaction shear + muscle/ligament shear

What are 2 ways in which the bending moment not he spine can be decreased during a lifting task?

Reduce moment arm of load or decrease weight of load

What are the three main roles of the ligaments?

Restrain motion, protect the spinal cord & proprioception

what happens to injury rate when exposed to maximum permissible limit?

Significant increase in injury risk

Explain why loss of lumbar lordosis during lifting is associated with increase in shear forces acting on the lumbar spine?

The erector spina muscle group can no longer produce posterior shear force and the interspinous ligament generates increased anterior shear force when the spine goes from lordotic to flexed

How does the annulus fibrosis work to distribute force?

The fibres in adjacent layers are oriented at different angles and hence resist both tension and compression. A compression load on the disc results in tension in the AF due to the bulging of the NP and placing tension of the AF fibres

What are some advantages of fusion?

eliminates segmental instability and plan and restores correct spinal shape

What is imbibition?

exponential change in thickness for constant load due to the constant compressive force that results in a net flow of fluid out of the disc during the day

What are the altered biomechanics of degenerative disc disease?

facet joints are overloaded, IV foramen is constricted and osteophytes

What is explicitly not included in the motion segment?

The muscles

Why is intra-abdominal pressure important for the spine?

Theory is that when internal pressure increases the trunk becomes stiff and therefore where the muscle meets the spine braces the spine

What happens to the ligaments in a lordotic position?

They are slack

The main muscle primarily responsible for generating intra-abdominal pressure is?

Transverse abdominus

How should you pull an object towards you?

You should pull towards your lumbar spine to reduce or eliminate the flexion moment

What are some problems with fusion of vertebrae?

adjacent disc stress, non-union and iatrogenic effect