Biomechanics Of The Vertebral Body Flashcards
What regions of the vertebral body are stressed and unstressed in vertebral Flexion?
Large compressive stresses in posterior parts
Centre is pretty unstressed
What stresses are present during vertebral axial loading?
Large stresses all over vertebral body:
- Load is divided
- Allows vertebra to transmit applied force
What is a shear force?
A force acting parallel to a material so as to tend to deform it
What is the shear angle?
Angle between deformed body and original position
What stresses result from anterior shear forces on the vertebrae?
Large anterior compressive stresses at pedicel bases
What is the reference for basic loads and the resultant vertebral stresses?
Smit et al., 1997
What is the typical shape of the cervical vertebral body?
Small
Kidney shaped
How does the superior surface of the cervical vertebral bodies appear? (Reference?)
Raised laterally
Flat posteriorly
(Palastanga and Soames, 2012)
What is the typical shape of a thoracic vertebral body when viewed superiorly? (Reference?)
Heart shaped
Larger than cervical vertebrae
(Palastanga and Soames, 2012)
Why are the superior and inferior surfaces of the lumbar vertebral bodies parallel? (Reference?)
The flat surfaces:
- Help withstand longitudinal loading
- Give stability to the intervertebral joints
(Bogduk, 2005)
How does Bogduk (2005) describe the vertebral bodies of the sacrum?
Rectangular areas, resembling bodies, embedded in sacrum
Where does the apex of the sacrum lie? (Reference?)
Inferiorly
Strandring et al., 2008
What is the internal structure of the vertebral body? (Reference?)
Not completely solid
A ‘box’ filled with transverse and oblique ‘struts’
- Struts = Trabeculae
What influences the structure of the trabeculae within vertebral bodies?
Basivertebral vein
What fills the spaces between trabeculae within vertebral bodies?
Bone marrow Blood vessels (arterial and venous)
How do trabeculae affect the structure of the vertebral bodies?
Make it lightweight but strong
What is the reference for the trabecular structure of vertebral bodies?
Bogduk, 2005
What is the structure of a vertebral endplate? (References?)
Hyaline cartilage: - Immediately adjacent to IV disc - Strandring et al., 2008 Osseous component: - Surrounding hyaline component - Moore, 2000 and 2006
Why does the endplate calcify as we age?
Blood vessels obliterate
What is the sequelae for the IV disc due to endplate calcification? (Reference?)
Less nutrients to IV disc via diffusion/osmosis
Palastanga and Soames, 2012
What is the thickness of a vertebral endplate and where are they thinnest?
1mm
Thinner in centre
What are the functions of vertebral endplates?
Prevent nucleus pulposus bulging into vertebral body
Absorb forces from mechanical loading of spine
Play a role in fluid/nutrient exchange between:
- Vertebral bodies
- Rest of IV disc
Grant et al., (2001) did a study on endplate strength. What were some of the findings?
Centre is weakest
Areas in posterior endplate are strongest:
- Lumbar around pedicels
- Sacral (unspecified)
Inferior endplate stronger than superior endplate of each IV disc
What are the limitations of the Grant et al., (2001) study on endplate strength?
Focused on lumbar and sacral vertebrae only (L3-S1)
Mean specimen age around 75 years
What are the implications of endplate strengths?
Surgical implications:
- Fixations
- Implants
The trabeculae bone of vertebral bodies is highly porous and anisotropic. What does this mean?
It’s physical property has different values when measured, via micro-CT on cross-sections, in different directions
Where else is trabecular bone found? (Reference?)
Epiphyses of long bones
Oftadeh et al., 2015
What do the trabecular systems of vertebral bodies reflect? (Reference?)
The stresses they are subject to
Palastanga and Soames, 2012
How is the central zone of trabecular bone described?
Large cylinders (walls of lamellar bone) Vertical in arrangement (around basivertebral veins)
How are the trabecular zones lying laterally to the central zone described?
Consist of longitudinal and transverse trabeculae
Spaced regularly:
- Directly beneath endplate
Why are the superoanterior and inferoanterior zones of trabecular bone the reasons for compression vertebral fractures?
Less trabecular bone present = Areas of weakness
Due to oblique systems not reaching the anterior margin of the vertebral body
Where is the principal vertical trabecular system found?
In all vertebral bodies
What are the functions of the principal vertical trabecular system?
Sustains body weight
Sustains all shocks perpendicular to vertebral column
What is the course of the superior oblique trabecular systems?
Run from superior articulate process through pedicle to the lower surface of the vertebral body on the contralateral side
What is the course of the inferior oblique trabecular systems?
Run from inferior articulate process through pedicle to upper surface of the vertebral body on the contralateral side
How many oblique systems are there per vertebral body?
2 superior (one on each side) 2 inferior
What are the functions of the oblique trabecular systems?
Resist torsion
Resist bending and shear (with vertical system)
What trabecular system at the oblique systems continuous with?
Continuous posteriorly with trabeculae in spinous process
What is the course of the horizontal trabecular systems?
Begin in each transverse process
Pass into vertebral body and interest in midline
What is the function of the horizontal trabecular system?
Resist tension along with accessory systems of transverse and spinous processes:
- Resist muscle pull
What is the reference for the trabecular systems of vertebral bodies?
Palastanga and Soames, 2012
What did Ebraheim et al., (2000) discover when they investigated the trabecular architecture in the sacrum?
Sacrum does not bear weight
Trabeculae mostly concentrated in S1 and S2
What did Bogduk, (2005) find in regards to Sacral trabeculae?
Sacrum and L5 bear similar load
Implies sacrum is large because of its pelvic function and not due to weight bearing
What trabecular systems exist in the sacrum?
Vertical
Transverse
What is the new name for Wolff’s law?
Bone functional adaptation
How is trabecular bone strength defined? (Reference?)
The maximum stress a structure can withstand before failing
Oftadeh et al., 2015
What is Wolff’s law?
Structural adaptations of bones due to mechanical usage:
- Result in vertebral body architecture optimally suiting site individual needs
What stresses contribute to bone functional adaptation? (Reference?)
Compressive Tensile: - Lateral shear - Flexion - Lateral Flexion (Smit et al., 1997)
What does a lateral x-ray through a pedicle of a lumbar vertebra show?
Pedicle are a bony cylinder:
- Trabeculae run in a fan-shape to each endplate
- Density of trabecular bone higher here than elsewhere
What does the trabecular architecture and vertical orientation of facet joints suggest?
Walking determines architecture of lumbar vertebrae
What did Smit et al., (1997) find regarding the loading of lumbar vertebrae and the relationship of this to trabecular architecture?
They are loaded by:
- Axial compression (gravity and muscular forces)
- Compression within facet joints during:
1. Anterior and lateral shear
2. Axial torsion (walking)
Why do vertebrae need loaded regularly?
To prevent bone resorption (bending may not be sufficient)
What does Smit et al., (1997) suggest is the order of events that results in the trabecular architecture of lumbar vertebrae?
- Walking
- Rotation of body segments
- Axial torsion
- Transformed to horizontal shear
- Facet joint loading
- Trabecular architecture
What are the components of both trabecular and cortical bone?
Hydroxyapatite
Collagen
Water
What component of bone is reduced in trabecular bone and what effects does this have on density and ash fraction?
Hydroxyapatite
Reduced density (1.874g/mm3)
Reduced ash fraction (33.9%)
What component of bone is increased in trabecular bone and what effect does this have?
Water
Results in lower mineralisation of trabecular and new bone
How does the surface area to volume ratio of cortical bone compare to trabecular bone?
Lower in cortical bone
What are the turnover rates for cortical and trabecular bones?
Cortical = 3% Trabecular = 26%
What are the functional units of cortical and trabecular bones?
Cortical = Osteon (Haversian systems) Trabecular = Trabeculae
Which part of the vertebral cortical bone is thicker? Why is the other part thinner?
Ventral shell
Dorsal shell thinner perhaps due to posterior cortex being disrupted by veins and different loading
Where is cortical thickness greatest in the vertebral column and why? (Reference?)
Greatest in cervical and lumbar regions
Due to Spinal curvatures
What is the reference for cortical thickness?
Ritzel et al., 1997
