Fracture Repair

Question 1

Define martensitic stainless steel

Answer 1

Highly magnetic, high corrosion resistance, high hardness.

12-18% chromium

Used for cutting tools (drills, taps) and screwdrivers, wrenches

Common types: 400 series

Question 2

Define precipitation hardenable stainless steel

Answer 2

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.

Question 3

Define austenitic stainless steel

Answer 3

Low magnetic attraction, high corrosion resistance, moderate hardness

16-18% chromium, 8-10% nickel.
L = low carbon

Implant quality stainless steel is austenitic

Question 4

What are the properties of aluminium?

Answer 4

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.

Question 5

What is anodising and why is it done?

Answer 5

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.

Question 6

What is galling?

Answer 6

Adhesive wear that occurs when two metals rub together at high points on their mating surfaces.

Question 7

What is the composition of implant quality 316L stainless steel?

What properties does each compound confer?

Answer 7

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

Question 8

What is galvanic corrosion?

Answer 8

An accelerated form of corrosion that occurs when stainless steel and titanium implants are mixed

Question 9

What is cold working?

Answer 9

Metal working that consists of permanently deforming the material at room temp to increase the strength

Question 10

What is electropolishing?

Answer 10

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.

Question 11

How can the surface of stainless steel implants be treated?

Answer 11

Chemical passivation
Electropolishing
Shot peening

Question 12

What is chemical passivation?

Answer 12

Immersion of a stainless steel implant in 20-45 volume percent nitric acid.

Removes surface contaminants and restores maximum corrosion resistance.

Question 13

What is shot peening?

Answer 13

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

Question 14

Is there an advantage of titanium or stainless steel wrt implant infection?

Answer 14

No

BUT tissue adjacent to titanium becomes well vascularised with less tendency towards capsule formation

Question 15

What is the main disadvantage of titanium implants?

Answer 15

Removal is difficult due to soft tissue and bone adhesion to the implant

Question 16

Difference btw titanium and stainless steel

Answer 16

Titanium has a lower density, lower modulus of elasticity, provides less MR artefact and has superior pitting and crevice corrosion resistance.

Question 17

What is the modulus of elasticity?

Answer 17

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

Question 18

What is stress shielding?

Answer 18

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.

Question 19

What are the main differences between commercially pure titanium and titanium alloy?

Answer 19

Alloys have higher tensile strength capability, similar modulus or elasticity and equivalent density.

Question 20

What is notch sensitivity?

Answer 20

The relative effect that local irregularities or stress raisers have on mechanical properties.

Stainless steel = titanium alloy > unalloyed titanium

Question 21

What is fatigue?

Answer 21

A phenomenon leading to fracture under repeated or fluctuating stresses

Question 22

What is the endurance limit?

Answer 22

The maximum stress below which a material can endure an infinite number of stress cycles

Question 23

What is runout?

Answer 23

The maximum cyclic stress that does not create fatigue fracture at a predetermined number of cycles (usually 10^6 or greater)

Question 24

What is a S-N curve?

Answer 24

S = applies cyclic stress
N = number of cycles required to fracture the test specimen or achieve runout

Question 25

What is the primary method for identifying implant failure modes?

Answer 25

Fracture surface examination with the scanning electron microscope

Question 26

What is fretting corrosion?

Answer 26

An accelerated form of corrosion that can occur when the protective passive film is mechanically disrupted in a corrosive environment.

E.g. the relative motion of the underside of a bone screw head with the contact surface of a bone plate