Fractures, fracture healing + non-surgical fixation Flashcards
to what type of force is a long bone generally weakest
shear
in tension and compression, what are stiffness and load required to cause failure of a bone proportional to?
cross sectional area
- larger the area, stronger and stiffer the bone
what 2 things affect mechanical behaviour of bone in bending loading and what quantity takes these into account?
cross sectional area
distribution of bone tissue around neutral axis
second moment of area takes both of these into account
to resist bending is it best to have bone close to or at a distance from the neutral axis?
at a distance
what factors affect bone strength and stiffness in torsional loading? what is the quantity that takes these into account?
same as for bending
- cross sec area
- distribution of bone around neutral axis
polar moment of inertia
are the tibia and fibula more prone to fracture proximally or distally?
tibia - more prone distally
fibula - more prone proximally
(they have diff geometry to one another)
in bending, which side of a bone (convex/concave) is in compression and which is in tension? which will fail first in adults and children?
bending:
convex side in tension
concave side in compression
convex will fail first in adults and concave will fail first in children
what is the likely fracture pattern from bending loading on bone
transverse
fracture pattern of compressive force
oblique
under what circumstances does a butterfly fracture pattern occur
compressive force coupled with bending force
- the bending force causes a transverse crack on the side in tension
- the compressive force causes an oblique fracture
what type of load results in a spiral fracture
pure torsion
where in the bone will the fracture occur in pure axial compression
close to or within the metaphases because the cancellous bone is weaker
is bone stronger if loaded slowly or fast
stronger at faster loading rate
what takes longer to heal, long bone fractures or cancellous bone fractures?
long bone
how long do long bones take to heal roughly
6 - 12 weeks
when would union be referred to as atrophic or fibrous
bony union not taking place due to no blood supply being re-established
what is laid down instead of bone cells at the fracture site if there is excessive movement
cartilage
what is ‘elephants foot’ appearance
a lot of movement at the fracture site causing a false joint (or pseudo arthritis) to form between rapidly proliferating cartilage cells at either end
fracture healing
- weeks 0-2
macrophages mop up the haematoma and dead cells
fracture healing
- weeks 2-6
new capillaries grow into the fracture haematoma bringing fibroblasts and osteoblasts
surviving periosteum begins to regenerate and grow
fracture healing
- weeks 6-12
new bone tissue laid down and eventually the 2 ends reunite as a provisional callus
under the right circumstances what will the provisional callus continue to form up until about 12 months
woven bone which gradually remodels to form a cortex
how does the provisional callus appear on X-ray
dense area
what do fibroblasts form
fibrin (scar tissue)
what is another name for secondary bone healing
natural bone healing
what happens in primary bone healing
new Haversian systems grow directly across the fracture gap
what is the problem with primary bone healing
although faster at healing the fracture, it does not quickly recover its original strength
what is Wolffs law
the ability of bone to remodel itself depending upon the mechanical demands placed upon it
what makes up for the fact that the callus has lower strength and rigidity than bone
the second moment of area is increased
what maintains the rigidity of the callus (back up with an equation)
R = E I although E (yougs modulus) is lower for the callus, I (second moment of area) is higher
along what axis does loading encourage bone union
along its long axis
Hydroxyapatite is known to be piezoelectric, what does this mean?
it develops an electric charge when loaded
what hormone is thought to be produced at fracture sites
substance P
how is pain relief achieved in fracture mx
strong opioids usually justified - added benefit of reduced anxiety and fear
splint reduces muscle spasm
how much blood is roughly lost from a femoral fracture and a pelvic fracture
femoral - about 1 litre
pelvic - up to 3 litres
what are the 2 primary objectives of all kinds of fixation device
reduction of displacement minimise deformation (/movement)
what types of force does a splint allow bone to resist / not resist
they help resist bending forces
little use in resisting torsional and compression forces
what material is plaster of paris
calcium sulphate
why does care need to be taken with plaster of paris
heat is produced from the calcium sulphate hemihydrate (the more of this compound in the bandage, the more heat)
what type of solvent are commercial plaster bandages made with
an organic solvent such as ether, which contains no water
what is added to help the calcium sulphate ‘hold on’ to the bandage
starch
why is starch referred to as an accelerator in plaster cast setting
it speeds up the chemical reaction and therefore influences the speed of setting
how can plaster setting be slowed down
- reducing the amount of starch added
- adding a retarder
- altering the temp of the water used to wet the bandages
examples of retarders
alum and borax
whats the difference between long and short crystals in plaster of paris
long - occur naturally as alabaster - give a hard quality
short - give the cast a softer feel
plaster functions in 2 ways - describe them
- forms a rigid exoskeleton which support the soft tissues, which in turn supports the broken bone
- gentle 3 point fixation achieved by moulding the cast against the fracture - giving a so called periosteal hinge
how do standard casts control rotation
by incorporating the whole of the broken bone and limb segment in the cast including the joints
what is the disadvantage of prolonged casting and how can it be overcome
prolongs rehab due to making joints stiff and muscles waste
overcome with careful moulding and the application of hinges to the cast - FUNCTIONAL BRACING -
how does Sarmiento’s below the knee cast achieve rotary control
moulding around upper third of tibia
extensions which encapture the femoral condyles in knee flexion
principle disadvantage of skeletal traction
bone infection risk
advantages of skeletal traction
can be used to apply large loads
load can be precisely relative to the long axis of the bone (important in dynamic traction)
why is a Thomas splint only used for a week or two
the immobility prevents joint movement, does not induce axial movement at the fracture site, and leads to muscle disuse
What age group is a Thomas splint usually used for
children because their fractures usually heal quickly and they dont cope well with complicated traction
what is the purpose of balanced traction
principally used to offset pressure effects caused by splints
e.g. thomas splint with small load applied to the splint as a whole to offset the pressure at the groin
traction is the safest method of treating fractures, why then is it not commonly used?
requires prolonged hospitalisation
clinical complications include bed sores, chest and urinary unfections, muscle and bone atrophy
