Bones Flashcards
What does morphological mean?
Relating to the form or structure of things
What does anisotropic mean?
Having a physical property which has a different value when measured in different directions
What two types of bone structure are found in long bones?
Cortical (compact) bone
Cancellous/trabecular (spongy) bone
What are the two phases or bone remodelling?
Resorption (break down of old bone)
Ossification (formation of new bone)
What are the 3 types of cell related to bone creation and remodelling?
Osteoblasts
Osteoclasts
Osteocytes
Osteoblasts:
- what is their function?
- where are they found (in relation to the bone)?
Ossification (formation) of new bone)
Found near the surface of bones
How do osteoblasts create new bone?
They:
- make osteoid (mainly collagen)
- secrete alkaline phosphatase (creates sites for calcium and phosphate deposition)
- allows crystals of bone mineral to grow
- osteoid becomes mineralised and forms bone
-
What is osteoid?
Osteoid is an unmineralized organic tissue (mainly comprised of collagen) that eventually undergoes calcification and is deposited as lamellae or layers in the bone matrix
Where are osteoclasts found?
How do they break down bone?
On the surface of bones
They secrete acid phosphatase which un-fixes calcium in mineralised bone, breaking it down.
What are osteocytes?
Where are they found?
What is their function?
Osteoblasts which are no longer on the bone surface
Found in lacunae between lamellae
Maintain oxygen and mineral levels in bone
What are lacunae in bones?
Gaps within the lamellae (rings) of bone tissue in which osteocytes are to be found
What are the 2 mechanisms for creating bone in foetal development and what type of bone do they create?
- Endochronal ossification (long bones)
- Intramembraneous ossification (flat bones)
What are the stages of endochronal ossification?
- Development of cartilage model (rough shape of bone being formed)
- Osteoid calcifies, blood vessels grow into cavities within matrix
- Osteoblasts use calcified matrix as support structure to lay down more osteoid and form trabecullae
- Osteoclasts break down spongy bone to create medullary cavity
How does bone strength over time (from initial formation)?
Initially bone material deposited with collagen fibres in random directions (called woven bone)
Overtime, converted into lamellar bone (stronger due to aligned fibres)
Osteoid initially calcifies to produce cancellous bone
Where cortical bone required, more deposition of osteoid increeases bone density
What is woven bone?
What is the reason for its name?
Bone material initially deposited with collagen fibres in random directions
Collagen fibres woven together randomly
Where do secondary ossification centres appear in long bones?
At either end of bone
What is found between the primary and secondary ossification centres in long bones?
What is this made of?
What happens to this as the bone grows?
What eventually happens to this as bone growth ceases?
Epiphyseal plate
Cartilage
Continues to form new cartilage and replaced by bone (the bone increases in length)
Mid 20’s : the plate is replaced by bone and no further growth
What are the reasons for remodelling of bone?
- Reshape skeleton during growth
- Maintain calcium levels in body
- Repair microfractures caused by stress
What are the 5 stages of bone remodelling?
- Quiesence - resting state of bone surface (lining cells - inactive osteoblasts on bone surface)
- Activation - osteoclasts on bone surface in small resorption pits (Howships lacunae)
- Resorption - osteoclasts remove old bone by removing the the mineral matrix and break down collagen fibres
- Reversal - resorption process ends. Osteoclasts replaced by osteoblasts
- Formation - osteoblasts lay down new bone:
i. create a matrix of collagen (osteoid)
ii. osteoblasts mineralise the matrix to form new bone - Quiescence
How does the process of remodelling cortical bone compare to remodelling trabecular bone?
Same process
Different geometry to form concentric lamellae in osteons
Why is bone considered a responsive material?
Continuous resorption and formation of bone
Body responds to stress levels in different areas to ensure right amount of healthy bone is maintained and reshaped
What factors cause a variation in bone properties?
- Age
- Gender
- Location in body
- Temperature
- Mineral content
- Amount of water present
- Disease e.g. osteoperosis
What category of material does bone fall within?
Fibre composite (collagen fibres and inorganic matrix)
What are the 5 types of loading (of bone)?
- Tension
- compression
- Torsion
- Bending
- Shear
Explain stress
How is it calculated?
What are its units?
Load acting on material in body
Force/cross sectional area
Pascal or N/m2
Explain strain
How is it calculated
What are its units?
Deformation experienced by material in a body
Change in dimension/Original dimension
No units
What are 3 prefixes for increasing orders of numbers?
Kilo (k) - 1,000
Mega (M) - 1,000,000
Giga (G) - 1,000,000,000
Define yield stress/strain
The level of stress/strain up to which strain is proportional to stress
What is the name for the region for which strain is proportional to stress?
Elastic region
Define ultimate stress/strain
Fracture point
The level of stress/strain at which fracture of the material occurs
What measure is often used to define the strength of a material?
Ultimate stress
What is the measure of stiffness of a material?
How is it calculated?
What are its units?
Young Modulus
Change in stress/Change in strain (within the elastic region)
Pa or N/m2
How does osteoporosis affect the Youngs Modulus of bone?
What causes this?
Higher Young’s Modulus
More brittle
Lower bone mineral and therefore collagen content (which would otherwise allow bone to flex and bend)
What does the Young’s Modulus of trabecular bone vary with?
Bone density squared
What is the dominant form of linear elastic deformation in both high and low density trabecular bone?
Bending
What two factors account for 70-95% of the variance in elastic constant of trabecular bone?
Density
Architecture
What is the strength of trabecular bone proportionate to?
Density to the power of 3/2
What does certain evidence suggest the mechanism of compressive failure by trabecular bone result from?
Elastic buckling rather than brittle crushing
How does osteoporosis affect trabeculae in relation to modulus and strength?
10% reduction in trabeculae
leads to
70% reduction in modulus and strength
What are the 2 mechanisms whereby osteoporosis affects modulus and strength of bone.
How do these two mechanisms compare in terms of impact?
Trabecular thinning as well as resorption
Resorption more severely reduces modulus and strength than uniform thinning
What are the 3 methods of measuring bone density?
- Dual energy x-ray absorpitometry (DEXA)
- Ultrasound
- Quantitative computed tomography (QCT)
What is the most common method of measuring bone density?
DEXA
How is ultrasound used to measure bone density?
How does ultrasound compare to DEXA?
Usually performed at calcaneus (not poss. to measure at hip or spine).
Mainly measures bone mass rather than quality
Measurement at hip can predict hip fracture nearly as well ass DEXA
Using in combination with DEXA does not improve prediction of fractures
What are the pros and cons of QCT compared to other methods of measuring bone density?
- Must be done following strict protocols in lab. (In-community setting reproducibility of results is poor)
- Greater radiation exposure
- Worse precision (partic. in older individuals)
- Can be used to study changes in cortical vs trabecular bone
What are the 2 measure of bone quality?
T-score
- number of standard deviations for bone mineral density (e.g. as measured by DEXA) below average for young adult at peak bone density (differs depending on group of adults used as reference)
Z-score
- as above but with ref to average for a person of same age
What is the WHO categorisation of bone density
What population is this based on?
Normal bone: T-score > -1
Osteopenia: T-score between -2.5 and -1
Osteoporosis: T-score < -2.5
Based on bone density in white women
