Bone tutorial Wk 3 Flashcards
Compare the mechanical properties of cortical and cancellous bone by distinguishing between their material and structural properties.
Cortical bone has higher modulus, greater ultimate stress (c 150 – 250 MPa), but small ultimate strain (<2%). Cancellous bone has highly variable values, dependent on how much bone material is in the section, its orientation, and how it is interconnected.
The differences in the mechanical properties / behaviour is a result of the difference in the structural characteristics as the material is the same in cortical and cancellous bone.
Explain the functions of the major bone cells in modelling and remodelling of bone. (and the difference between the two processes.)
Osteocytes – as signal transducers, ‘sensors’ that monitor the matrix
Osteoblasts (OB) – that produce osteoid, the non-mineralised organic component of bone matrix
Osteoclasts (OC) – that secrete acid phosphatase and other enzymes to eat away bone – both the mineral and organic phase (which is mostly collagen)
In modelling, the OBs and OCs may both be active, or just one type, and their respective activities are not directly linked to each other. Modelling can occur at any bone surface.
Remodelling entails the linked (at one site and through time) activity of OCs and OBs, such as the formation of a Haversian system (secondary osteon), or surface remodelling of trabecular bone.
What are osteocytes?
signal transducers, ‘sensors’ that monitor the matrix
What are osteoblasts (OB)?
that produce osteoid, the non-mineralised organic component of bone matrix
What are osteoclasts (OC)?
secrete acid phosphatase and other enzymes to eat away bone – both the mineral and organic phase (which is mostly collagen)
Explain how and why the type of fracture varies with the rate of application of force. Give examples.
High strain rates generate more brittle fractures of bone (often with high energy release and bones shattering). Low strain cause more ductile behaviours, perhaps just cracks and not total failure.
Transverse- bending (over fairly brittle bone, has ductile bone will most probably cause greenstick)
Linear- Mike is not sure
Oblique (non displaced)
Oblique (displaced)
Spiral- torsion loading of bone (non-uniform of stress, stress at perimeter is higher than the middle; surface fails first)
Greenstick- tensile surface that fails; half broken, possible- fracture surface starts at tensile and then half way runs longitudinally
Comminuted- bone fragments, brittle, high energy
Describe the effect of repetitive loading on the mechanical properties of bone
POSSIBLE EXAM QUESTION
Studies in the rat show that bone formation in response to mechanical loading is greatest when the total volume of loading is broken up into smaller bouts with rest periods in between - that is, to get a bone growth effect from exercise - you only need short periods of intense activity - not long endurance bouts of loading.
Mechanical loading is a potent osteogenic stimulus, but desensitisation occurs rapidly and periods of rest are needed to resensitise the cells…the principle of intermittent loading being of greater value than continuous loading.
Exercise in separate bouts for greatest effect.
If bone loading is excessively repetitive then fatigue may occur, wherein microdamage to the matrix accrues through time, possibly to a point where catastrophic failure occurs.
Cortical bone has ______ modulus, ______ ultimate stress, but _____ ultimate strain.
higher; greater; small
Cancellous bone has _______values, dependent on how much bone _____ is in the section, its _____, and how it is interconnected.
highly variable; material; orientation
High strain rates generate more_______fractures of bone (often with high energy release and bones shattering).
brittle
Low strain cause more ______ behaviours, perhaps just cracks and not total failure.
ductile
