Fracture Fixation and Rigidity Flashcards
What are the 3 aims of fracture fixation?
produce optimum rigidity for fracture healing
maintain fixation until fracture has united
avoid excessive stress shielding
Over what percentage of strain does no bone healing occur?
10%
What is the optimum strain for bone healing?
2-10%
What happens if a fixation plate is too thick or too thin?
too thick - non union/delayed union
too thin - breakage, bending
What factors influence the mechanical properties of a fixation device?
material of device
dimensions of the device
distribution of material within the device
What is the youngs modulus of cortical bone and how does it compare to titanium and steel?
20GPa
Ti - 100GPa
Steel - 200GPa
What dimension of a device impact its strength?
length
width (proportional)
thickness (resistance to deformation proportional to the cube)
Why is a construct with the resisting material further away from the centroidal axis more effective at resisting deformation?
compression and tension forces increase with distance from the neutral axis
What quantity describes distribution of material around the neutral axis?
second moment of area
What is the formula for second moment of area of a rectangular cross section?
1/12 bh^3
What is the formula for second moment of area of a cylindrical cross section?
I = (1/4)pi x r^4
In torsion, shear forces increase with distance from the central axis - what quantity describes this?
polar moment of inertia
What two quantities is resistance to torsion dependent on?
shear modulus
polar moment of inertia(J)
What is the relationship between hollow tubes and solid rods in terms of stiffness?
for any external diameter, the solid rod is stiffer than a hollow tube made from the same material
What material is particularly vulnerable to notching?
titanium
