Biomechanics of the Vertebral Body

Question 1

What is the primary function of the vertebral body?

Answer 1

To bear weight.

Their structure and internal architecture reflects the loads they can stably sustain (Bogduk, 2005).

Question 2

What stress does flexion of the vertebral column result in?

Answer 2

Large compressive stress in the posterior part of the vertebral body, but leaves the centre of the vertebral body largely unstressed.

Question 3

What does axial compression of the vertebral body cause?

Answer 3

Leads to large stresses all over the vertebral body.

This allows the load to be divided.

The vertebrae is designed to transmit applied force.

Question 4

What does anterior shear force on the vertebral body cause?

Answer 4

Large anterior compressive stresses at the pedicle bases.

Smit et al. (1997)

Question 5

Describe the vertebral body of a cervical vertebra

Answer 5

• Whilst the cervical vertebrae carry the weight of the head and resist different compressive forces, the bodies of the cervical vertebrae are smaller than those of other vertebrae.
• Typically small and kidney-shaped.
• Superior surface is raised laterally and flat posteriorly.

Question 6

Which cervical vertebrae are exceptions in terms of vertebral body typical structure?

Answer 6

• Atlas (C1) - has no body. Defined by anterior and posterior arches, allowing free rotation of C1 on C2.
• Axis (C2) - has the typical features of the cervical vertebrae despite the additional odontoid process and irregular body.
  
  • It is the strongest of the cervical vertebrae.

Question 7

Describe the body of a typical thoracic vertebrae

Answer 7

• When viewed superiorly, bodies can be described as heart-shaped.
• Typically larger than cervical.

Question 8

Describe the body of a typical lumbar vertebra and why does this structure suit function?

Answer 8

• Much stronger and fatter than cervical and thoracic.
• Kidney-shaped body.
• Superior and inferior surfaces of the body are parallel.
• These flat surfaces are dedicated to withstanding lonitudinal loading and the flatness gives stability to the intervertebral joints.
• Bogduk, 2005

Question 9

Describe the sacrum

Answer 9

• Five fused vertebrae with no defined body.
• Bogduk (2005) describes rectangular areas, resembling bodies, embedded in the sacrum.
• Triangular shape with apex inferiorly.

Question 10

Describe the coccyx

Answer 10

• Four fused vertebrae with no defined body.
• Articulates with the apex of the sacrum.

Question 11

Which type of bone comprises the internal architecture of vertebral bodies?

Answer 11

Trabecular bone

Question 12

Describe the internal structure of a vertebral body

Answer 12

• Not solid bone but not hollow.
• Body can be thought of as a box containing vertical struts, supported by transverse/oblique struts.
• These struts are trabeculae - lightweight and strong.

Question 13

What is found in the spaces between the trabeculae inside the vertebral bodies?

Answer 13

• Blood
• Bone marrow - sometimes the site of haematopoiesis.
• Blood vessels for supply and drainage

Question 14

What are vertebral endplates?

Draw a rough diagram of these within an IV disc

Answer 14

• Component of the intervertebral disc.
• They are very thin.
• There is a superior and inferior enplate.

Question 15

Describe the structure of the vertebral endplate

Answer 15

• Hyaline cartilage + osseous component
• They are very thin (~1mm) and are thinnest in the centre

Question 16

What happens to the vertebral endplate in later life?

Answer 16

• Blood vessels are obliterated
• End plate calcifies / shows signs of ossification
• Less nutrients can reach the IV disc by osmosis or diffusion

Question 17

What are the functions of the vertebral endplates?

Answer 17

• They prevent the nucleus pulposus from bulging into the vertebral body.
• They absorb forces / pressure from the mechanical loading of the vertebral column - SHOCK ABSORBER.
• Have a role in fluid / nutrient exchange between the vertebral bodies and the rest of the IV disc.

Question 18

Grant et al. (2001)

Answer 18

• The centre of the endplate is the weakest
• Areas in the posterior endplate are the strongest
  
  • Lumbar - around the pedicles
  • Sacral - posterior aspect, unspecified
• Inferior endplates are stronger than superior endplates
• Limitations:
  
  • ​Study only looked at lumbar and sacral vertebrae
  • Mean specimen age 75 years old

Question 19

Describe the structure of trabecular bone

Answer 19

• Highly porous
• Anisotropic
• Organised to optimise the transfer of loads

Question 20

What causes the specific pattern of the trabeculae in the trabecular bone of the vertebral bodies?

Answer 20

The stresses which they are subjected to

Question 21

What are the three zones of the trabeculae of the vertebral bodies?

Answer 21

1. Central zone
2. Zones on either side of the central zone
3. Anterior superior and anterior inferior zone

Question 22

Describe the paracentral zones of the vertebral body trabeculae

Answer 22

• Zones to either side of the central zone
• Regularly spaced trabeculae
• Longitudinal and transverse trabeculae
• Directly beneath the endplate region

Question 23

Describe the anterior superior and anterior inferior zones of the vertebral body trabeculae

Answer 23

• Oblique trabecular systems do not reach the anterior margin of the vertebral body.
• So, there is a decreased amount of trabecular bone
  
  • Creates areas of weakness - reason for compression fractures.
• Compression fractures caused by hyperflexion, especially in osteoporosis.

Question 24

Describe the central zone of the vertebral body trabeculae

Answer 24

• Large cylinders
• Vertical arrangement
• Cylinder walls comprised of lamellar bone (thin but solid plates) with circular trabeculae around the basivertebral veins.

Question 25

Which trabecular systems does each vertebral body contain?

Answer 25

* One principal vertical system * Various horizontal and secondary oblique systems

Question 26

Describe the principal vertical trabecular system in the vertebral bodies

Answer 26

* Found throughout the bodies of the whole vertebral column with the exception of the IV discs. * Sustains body weight. * Sustains all shocks to the vertebral column in a perpendicular direction. * Resists atrophy more than any other system.

Question 27

Describe the secondary oblique trabecular systems in the vertebral bodies

Answer 27

* 4 tracts which oblique accessory systems run in: * Left superior * Left inferior * Right superior * Right inferior * Spirally wound oblique systems resist torsion * Together with the vertical system they resist bending and shear

Question 28

Describe the horizontal and accessory trabecular systems in the vertebral bodies

Answer 28

* Principally tension resistant. * Minor accessory systems of the transverse and spinous processes are also tension resistant. * They all resist muscular pull.

Question 29

Describe the origin and termination of each superior oblique trabecular system

Answer 29

Runs from the superior articular process on one side downwards towards through the pedicle to the lower surface of the vertebral body on the opposite side.

Question 30

Describe the origin and termination of each inferior oblique trabecular system

Answer 30

Runs from the inferior articular process of one side, upwards via the pedicle, the the upper surface of the vertebral body on the opposite side.

Question 31

What is the only trabecular system found in the anterior margin of the vertebral body and why?

Answer 31

* Vertical compression trabeculae only * Because the oblique systems do not reach the anterior margins of the vertebral bodies

Question 32

Describe the origin and termination of the horizontal accessory trabecular systems of the vertebral bodies

Answer 32

Begin in each transverse process and pass into the body of the vertebra where they intersect in the midline.

Question 33

Ebraheim et al. (2000)

Answer 33

* Investigated the trabecular architecture in the sacrum * They found that: * the sacrum does bear weight (superiorly at least) * trabeculae are mostly concentrated in S1 and S2 * Trabeculae coincide with load transmission through the sacrum - the systems extend from weight bearing areas (body, facets, alae) towards the auricular surface.

Question 34

How is the strength of trabecular bone defined?

Answer 34

The maximum stress it can withstand before failing. Oftadeh et al. (2015)

Question 35

Smit et al. (1976) on bone functional adaptation

Answer 35

* **Wolff's law** (known as bone functional adaptation) - structural adaptations of bones are due to their mechanical usage. * Bone's ability to adapt to mechanical loads results in bones with architecture which suits its individual needs. * Principal stresses on the vertebral body are not always compressive; they may be tensile: * Tensile stresses under lateral shear, flexion and lateral bending.

Question 36

Describe the typical architecture of a vertebral body

Answer 36

Vertically orientated trabeculae and horizontal connections near the endplate with denser areas of bone near the pedicle bases.

Question 37

Describe the trabecular organisation in the pedicles

Answer 37

* On an X-ray, the pedicles appear as a bone cylinder, from which trabeculae run in a fan-shaped form toward the superior and and inferior endplate. * The density of trabecular bone here is much higher than elsewhere in the vertebral body.

Question 38

Which features of the lumbar vertebral bodies suggest that walking ma be the activity which determines their architecture?

Answer 38

* Architecture of the trabecular bone * Vertical orientation of the facet joints

Question 39

The trabecular bone architecture of lumbar vertebral bodies indicate that they are loaded by what? ## Footnote **Smit et al. (1976)**

Answer 39

* Axial compression * Compressive forces within the facet joints during anterior and lateral shear, as well as axial torsion.

Question 40

Which everyday activities lead to axial compression and anterior shear?

Answer 40

* Axial compression - present during any action due to gravitation and muscular forces. * Anterior shear - flexion and weight lifting.

Question 41

Smit et al. (1976) summary

Answer 41

* Study based on the assumption that bone material is arranged to optimally bear the applied loads. * Results show that architecture of vertebral trabecular bone is actually related to mechanical function. *

Question 42

Compare and contrast cortical and trabecular bone

Answer 42

* Both types are comprised of hydroxyapatite, collagen and water * however, trabecular bone has lower calcium content, tissue density (1.874 g/mm³) and ash fraction (33.9%) compared to cortical bone. * Trabecular bone has higher water content (27%) compared to cortical bone (23%). * Trabecular bone is more active in remodelling and consequently less mineralised.

Question 43

Describe the cortical thickness of vertebral bodies

Answer 43

* Varies throughout the vertebral column * Ventral shell is thicker than dorsal shell * Cervical and lumbar bodies have higher cortical thickness than thoracic bodies (biphasic curvatures)

Question 44

Ritzel et al. (1997) on cortical thickness

Answer 44

* Suggest that disruption of the posterior cortex by veins and difference in loading are possible explanations for the thinner cortical bone posteriorly. * They also suggest that this could be caused by the differences in weight bearing on the cortices due to the s-shaped spine.