Fracture Repair Flashcards
Define martensitic stainless steel
Highly magnetic, high corrosion resistance, high hardness.
12-18% chromium
Used for cutting tools (drills, taps) and screwdrivers, wrenches
Common types: 400 series
Define precipitation hardenable stainless steel
High magnetic attraction, moderate corrosion resistance, high hardness
E.g. Type 630 - 17% chromium, 4% nickel
Don’t have a high carbon content so edge retention and wear resistance are inferior to martensitic.
Define austenitic stainless steel
Low magnetic attraction, high corrosion resistance, moderate hardness
16-18% chromium, 8-10% nickel.
L = low carbon
Implant quality stainless steel is austenitic
What are the properties of aluminium?
Low strength, highly ductile, no magnetic, lightweight.
Aluminium alloys have more strength and less ductility.
Uses: depth gauges, IM nail insertion instruments, screw racks, graphic case models, hollow ex-fix rings.
What is anodising and why is it done?
An electrolytically passivation process that increases the thickness of the aluminium oxide surface coating on aluminium alloys.
Increases corrosion resistance and surface hardness. Also allows colour coding.
What is galling?
Adhesive wear that occurs when two metals rub together at high points on their mating surfaces.
What is the composition of implant quality 316L stainless steel?
What properties does each compound confer?
Wrought 18% chromium-14% nickel-2.5% molybdenum implant alloy.
Cr - corrosion resistance
Ni - microstructural stability
Mb - improved resistance to pitting and crevice corrosion
Low carbon = improved resistance to intergranular corrosion
Completely nonmagnetic
Austenitic
What is galvanic corrosion?
An accelerated form of corrosion that occurs when stainless steel and titanium implants are mixed
What is cold working?
Metal working that consists of permanently deforming the material at room temp to increase the strength
What is electropolishing?
Surface treatment used for stainless steel implants. An electric current is applied to an implant immersed in a specially formulated chemical solution under specified conditions of time and voltage.
—> removes microscopic amount of metal, decreases the surface toughness of the implant, provides a low coefficient of friction, improves corrosion resistance and creates a chemically passivated surface.
How can the surface of stainless steel implants be treated?
Chemical passivation
Electropolishing
Shot peening
What is chemical passivation?
Immersion of a stainless steel implant in 20-45 volume percent nitric acid.
Removes surface contaminants and restores maximum corrosion resistance.
What is shot peening?
Surface tx for stainless steel implants. The implant surface is subjected to high-velocity impaction by metallic or ceramic particles under well-defined conditions.
—> roughened surface with increased residual compressive stress for enhanced fatigue life
Is there an advantage of titanium or stainless steel wrt implant infection?
No
BUT tissue adjacent to titanium becomes well vascularised with less tendency towards capsule formation
What is the main disadvantage of titanium implants?
Removal is difficult due to soft tissue and bone adhesion to the implant
Difference btw titanium and stainless steel
Titanium has a lower density, lower modulus of elasticity, provides less MR artefact and has superior pitting and crevice corrosion resistance.
What is the modulus of elasticity?
Stress per unit strain in the elastic region.
Constant physical property.
High modulus of elasticity = material will transfer less stress from the implant to the bone.
Titanium is lower than stainless steel
What is stress shielding?
When a material has a high modulus of elasticity it transfer less stress from the implant to the bone —> bone is not adequately stress to consolidate properly during bone healing.
What are the main differences between commercially pure titanium and titanium alloy?
Alloys have higher tensile strength capability, similar modulus or elasticity and equivalent density.
What is notch sensitivity?
The relative effect that local irregularities or stress raisers have on mechanical properties.
Stainless steel = titanium alloy > unalloyed titanium
What is fatigue?
A phenomenon leading to fracture under repeated or fluctuating stresses
What is the endurance limit?
The maximum stress below which a material can endure an infinite number of stress cycles
What is runout?
The maximum cyclic stress that does not create fatigue fracture at a predetermined number of cycles (usually 10^6 or greater)
What is a S-N curve?
S = applies cyclic stress N = number of cycles required to fracture the test specimen or achieve runout
