Load Support Flashcards
What is a major role of orthopaedic implants?
structural support
In a plated region, applied load is transferred from the bone to the plate - what kind of region is this?
load transfer region
In a plated region, some part of the load is taken by the bone and some by the plate - what kind of region is this?
load sharing
In a plated region - load is transferred from the plate to the bone - what kind of region is this?
load transfer region
Why does the cement not take much load from the implant stem?
the stiffness is so much lower than the implant and bone
what is an interface stress?
load generated as load is transferred from one material to another at an interface
What is an interface?
area where load is transferred between to materials
if a top material is much softer than the bottom one, what happens?
load is concentrated centrally i.e. soft office chair.
if a top material is stiffer than the bottom one, what happens?
load is distributed more evenly over surfaces
what kind of loading occurs between stems and surrounding bone?
shear
How does shear stress vary over the structure?
At the region of load sharing there is no shear stress - greatest at points of load transfer
What can happen as a result of high shear stresses between stems and bone?
loosening and failure
What can cause stress concentrations?
non-uniformity of contact non-uniformity of mechanical properties variations in surface contours sharp corners notches holes
What kind of load transfer is most likely to cause bone loss?
where there is load transfer over 2 or more areas
What does the amount of load transferred depend on?
stiffness of the materials
What two factors influence structural stiffness?
material stiffness of the material
geometric of the material
What is material stiffness?
basic property of the material
What is geometric stiffness?
shape fo the cross section of the structural component
What is the formula for shear modulus (G)?
shear stress/shear strain
What is stiffness?
force required to produce a unit deflection
What is the formula for axial stiffness?
S= EA/L
What is the approximate ratio of the material stiffness of steel to cortical bone?
10x
What geometrical factors affect the racial stiffness of a structural component?
cross-sectional area
length
What is the formula for rigidity?
R=EA
What is the formula for bending rigidity?
Rb = EI
I - second moment of area
What is the formula for I of a rectangle?
bd3/12
What is the formula for I of a hollow circle?
pi/64(D4-d4)
What is the equation for torsional rigidity?
Rt = GJ
What is G?
shear modulus
What is J?
Polar moment of inertia
In a load sharing region the load taken by the bone and the stem is the ratio of what?
their rigidities
What is the equation for load sharing?
Lbone/Ltotal = Rbone/Rtotal = Rbone/Rbone + Rstem
