Spine Biomechanics Revision

Question 1

In the annulus fibrosus, what is the difference between the inner and outer annulus?

Answer 1

Outer annulus (ligamentous portion) has greatest density of type 1 collagen
- it resists excessive bending or twisting (torsion) of vertebrae
Inner fibres (capsular portion) along with the endplate encases the nucleus pulposus

Question 2

What is the Nucleus Pulposus?

Answer 2

The inner gel like centre of the intervertebral disc
Contains loose fibres suspended in a mucoprotein gel
High proteoglycan content:
- affinity and retention of water
- hydrodynamic properties
➡️ deforms in any direction, to equal the forces applied on it
➡️ acts as a shock absorber
- loose collagen network
➡️ some limit to deformation
- disruption of proteoglycan content = loss of disc height
- gravitational effects:
➡️ 2cm taller in the morning
➡️ astronauts 5cm taller in space I

Question 3

What is the effect of ageing on the intervertebral disc?

Answer 3

• nucleus pulposus begins to dehydrate and concentrations of proteoglycans in the matrix decreases therefore limiting the ability of the disc to absorb shock
• annulus fibrosus becomes weaker with age and has an increased risk of tearing
• the cartilage end plates begin thinning, fissures begin to form, and there is sclerosis of the subchondral bone
• as the fissures are formed, the nucleus pulposus can seep out and put pressure on nerves

Question 4

What do the spinal ligaments do, what are they made of and where are they situated?

Answer 4

They check reigns to movement
They are bundles of collagen fibres arranged along lines of most stress
The majority of ligamentous structures sit posterior to the vertebral body

Question 5

Why are the facet joints important in load bearing?

Answer 5

As disc height decreases, facet load increases by 47%

Question 6

What is the difference in discs between cervical discs and lumbar discs?

Answer 6

There are no discs above C2
In adulthood cervical discs are:
- Crescent shaped
- thick anteriorly, tapering laterally with only a few fibres posteriorly
- no annulus at the posterolateral margins
- no recognisable nucleus

Question 7

What is the difference between the upper thoracic region and the lower thoracic region?

Answer 7

All movements are coupled with rib movement
Upper thoracic: functionally linked to cervical spine
- facets orientated in the coronal plane
Lower thoracic: functionally linked to lumbar spine
- facets more oriented towards sagittal plane

Question 8

Why are the lumbar vertebrae large in body?

Answer 8

It is a high load region and they have resilient discs

Question 9

What is the Neutral zone in the spine?

Answer 9

The range of motion where the muscle system has most influence of movement and load control

Question 10

What is the elastic zone in the spine?

Answer 10

The range beyond the neutral zone where stability and load attenuation is dependent upon bony and ligamentous restraint

Question 11

What are the local muscles attaching directly to the spine and what are their function?

Answer 11

Lumbar multifidus has a segmental innervation pattern
- maintains lumbar lordosis within the neutral zone

Hip flexors = Psoas Major, Iliacus, rectus femoris
Spine extensors = erector spinae (made up of, iliocostalis, longissimus, spinalis), multifidus
Lateral Flexors = quadratus lumborum, intertransversari lateralis and medialis

Control segmental stability

Question 12

What are the large torque producing muscles in the trunk?

Answer 12

Rectus abdominis
External Oblique
Arms, Chest, Legs

Question 13

How do we control loading in the spine?

Answer 13

Pre-tension ligaments
Intra-abdominal pressure
Gravity
Muscle contraction

Question 14

What happens when we incorrectly load the spine?

Answer 14

There is loss of control and deviation into the elastic zone ➡️ overloading of passive structures ➡️ progressive micro-trauma and remodelling

Overload in the neutral zone➡️ loss of structural integrity ➡️ trauma (with massive levels of force)

Question 15

What are the effects of overloading (compression/shear/torsion)?

Answer 15

• sustained loading in elastic zone may lead to creep
  ➡️ mechanical creep is defined as elongation of tissue beyond its intrinsic extensibility resulting from a constant load over time
• loss of motor control may lead to overload
  ➡️ this leads to tissue remodelling being triggered ➡️ osteophyte formation ➡️ facet joint Hypertrophy ➡️ morphological changes in the end plate ➡️ annular tearing

Question 16

What is the annulus fibrosus?

Answer 16

Outer fibrous ring of the intervertebral disc
15-25 concentric lamellae
Large collagen bundles arranged in alternate directions obliquely
- resists multi directional force
- resists any potential cleavage plane through which nucleus may escape