Fracture through the Ages Flashcards
Define bone strength.
The ability of bone to resist fracture
What factors contribute to bone strength?
Density
Structure
What method has been used for diagnosing osteoporosis and what are the limitations of this method?
DEXA scans
This gives a reading of bone mineral density (BMD) but it doesn’t tell you anything about the bone structure
Describe the effects of oestrogen on osteoclasts.
Oestrogen stimulates apoptosis in osteoclasts
What are the two main divisions of bone composition?
Cell (10% of volume)
Matrix (90%)
What are the two subsets of bone matrix and what falls undereach?
Organic – collagen, non-collagenous proteins, mucopolysaccharides
Inorganic – hydroxyapatite crystals (calcium and phosphorus)
What are the four types of bone cells?
Osteoprogenitor cells
Osteocytes
Osteoblasts
Osteoclasts
What is the role of osteoprogenitor cells?
These differentiate into the other types of bone cell
What is the role of osteocytes?
They are involved in bone homeostasis
Stimulate the activity of osteoblasts and clasts depending on the bone load
How often does bone normally turnover?
120 days
Describe normal bone turnover.
The osteoclast will dissolve away the bone
Preosteoblasts will move in and differentiate into osteoblasts
Osteoblasts lay down matrix
Matrix gets ossifed by hydroxyapetite
In a healthy person, the osteoblasts will lay down more bone than the osteoclasts dissolved (so you don’t get any bone loss)
How is bone turnover different in an elderly person?
There is less apoptosis of osteoclasts and the resorption pits are very big and don’t get filled in by osteoblasts so you get loss of bone
How do bisphosphonates work?
Bisphosphonates are absorbed into the bone during remodelling.
When osteoclats dissolve the bone they release the bisphosphonates
This is absorbed by osteoclasts
They damage their cytoskeleton so that the osteoblasts lose their RUFFLED BORDER, and without this they can’t function
What is a major problem with bisphosphonate use?
Atypical fatigue fractures
What causes the major problem with bisphosphonates?
They reduce bone remodelling (which replaces old and damaged bone) so you get premature ageing of the bone
Furthermore, microcracks form in the bones due to day-to-day use and if these microcracks are not filled in by bone remodelling they will eventually join up and cause stress fractures
What is the half-life of alendronate?
Around 10 years
What new drug has come onto the market that has a similar action to bisphosphonates but with a shorter half-life?
Denusomab (half-life = 6 months)
An antibody to RANK-L
Describe the action of RANKL.
RANKL binds to RANK receptors on macrophages and promotes their maturation to osteoclasts
In a healthy person, what protein is responsible for regulating the bone remodelling process and how does it do this?
Osteoprotegrin
It prevents the binding to RANKL to the RANK receptor (this is what denusomab also does)
State Wolff’s Law.
Bone remodels according to the stresses applied to it
At what age is peak bone mass reached?
30-40 years
State some other factors that contribute to bone mass
Genetics
Nutrition
Vitamin D
Exercise
What are the five stages of fracture healing?
Haematoma Inflammation Soft Callous Hard Callous Remodelling
Which type of collagen is deposited in the soft callous?
Type 2 collagen
What prevents mineralisation in the soft callous?
Proteoglycans
What happens in stage 3 of fracture healing?
The soft callous is invaded by blood vessels
Chondroblasts break down the calcified callous
It is replaced by osteoid (type I collagen) produced by osteoblasts
Osteoid calcifies to form woven bone
What happens in stage 4 of fracture healing?
Woven bone remodels to lamellar bone
It is shaped relative to the load (Wolff’s law)
Medullar canal reforms
Name four types of fracture and how they’re caused
Spiral - twisting
Oblique - direct compression
Butterfly fragment - bending
Transverse - pulling
What type of fracture can occur in the bones of children due to their plasticity?
Greenstick fractures
One cortex could break but the other cortex could bend but stay intact
What are osteocytes?
Osteoblasts that become stuck in the bone when it is being remodelled
Where are osteocytes found?
In the lacuna of bone
These are connected by Canaliculi
How do osteoclasts sense bone load?
Have projections into fluid filled canaliculi
Load is applied to the bone:
If the bone structure is too dense for the load then there will be low pressure in the fluid and the cytes signal for osteoclasts to thin the bone
If the bone strucutre is too weak for the load then the pressure in the liquid will be higher and the cytes will signal to blasts to strengthen the bone
What are the results from a DEXA scan?
Compares your BMD to that of the average healthy adult
If more than 2.5 SD from mean - osteopeosis
Why might denusomab be favoured over bisphosphonates?
Bisphosphonates last for over a decade in the bone
denusomab has a half life of 6 months
