Properties of metals used in orthopedics implants depend upon their formation, treatment, and corrosion status. Most commonly used alloys in orthopedics are Stainless steel, titanium, and cobalt-chromium.
In Mechanics,
The normal force is perpendicular to
- Tension
- Compression
Shear force is parallel to the surface of the plane
Axial Force is alone the axis (along the central axis cause linear displacement)
Moment
Effect of force acting on the lever arm ( Force x Distance)
Torque
Turning, twisting, and rotational effects of force
Types of loading
- Axial Loading
- Tension: Traction or pulling apart
- Compression: Pressing together
- Bending: Effect of the force applied perpendicular to the axis of the beam
- Torsion: Twisting
- Direct Shear
- Contact loading
Fracture Pattern + Load
The body under load reacts in 2 ways:
- Stress
- The intensity of internal force generates
- That resist deformation
- Force/area
- Unit = Pascal
- Strain
- Technical term to explain deformation
- The relative measurement of deformation
- Change in length/ Original length
Both are of 3 types: Compressive, Tensile, and Shear
Some Important definitions
Strength: Ability of a material to resist an applied force without rupture
Elasticity: Ability of a material to recover its original shape after deformation
Plasticity: To be formed in a new shape without fracture and retain the shape after load removal
Ductility: Ability to be stretched without fracture ( Imp factor to contour wire, plates in OT)
Toughness: Ability of a material to withstand suddenly applied force without fracture
Brittleness: Opposite of toughness, no evidence of plastic prior to fracture
Helix: While turning around a straight line moves in one direction parallel to the line
Hooke’s Law: When a material is loaded in an elastic zone, Strain is proportional to strain
Derived from axially loaded on objects
Elastic Zone: The zone where a material will return to its original shape for a given amount of force
Yield Point: Transition between elastic and plastic deformation
Yield Strength: Amount of stress necessary to produce a specific amount of permanent deformation
Plastic zone: Material will not return to its original shape for a given amount of stress
Breaking Point: Object fails and break
Young Modulus of elasticity
Material property
The measure of stiffness (ability to resist deformation in the elastic zone)
Calculating by measuring the slope of the Stress/ Strain curve
Higher the modulus of elasticity higher the stiffness
Relative values of Young Modulus of Elasticity
Near the material’s Young modulus of elasticity ( cortical bone and titanium) stress transfer is uniform so the load is equally distributed to the bone and implants but in higher differences load is distributed in the higher curve so the bone will get resorbed due to low load distribution in bone (Osteopenia) and failure
Metal bio-compatibility is directly related to its corrosion resistance
Grain structure is affected by the method of fabrication i.e. the finer grain is stronger and more ductile
Properties of metals are associated with Surface treatment, nitriding, and mechanism of formation
Surface treatment
Polishing removes scratches (acts as a local stress riser)
Passivation process=> Produce protective oxide layer by immersing in a strong nitric acid solution
It dissolves the iron particles left by machine operation and leaves a thin, transparent but dense oxide film on the surface of the alloy (Imp. enhances corrosion resistance to implants)
Stainless steel=> Forms Chromium oxide layer
Titanium => dioxide layer
These are damaged by cold working, scratching, and mechanical trauma but are self-reparative in various degrees in presence of oxygen which is called RE-PASSIVATION
Nitriding
Allowing the surface to react with ammonia and potassium cyanate=> This process hardens the surface of the titanium implant
Corrosion and its varieties
Body in the electrolytic environment:- Gradual degradation of a metal by electrochemical attack
Initiation of corrosion depends upon
- pH
- Oxygen tension
In pH 7.4 = Protective oxide layer is stable but in an acidic environment like infection, it damages the oxide layer and produces corrosion
Stress Corrosion
Occurs in the area of the high-stress gradient
Maybe the presence of crack, encouraged by corrosion
The crack appears in the tensile site
Galvanic Corrosion
Electron flow from more (-VE) to more (+VE) material when emerged in a liquid
Due to two different metals
Crevice Corrosion
The narrow gap between implants (Screws and plates)
Common in places where O2 tension is low
Increase concentration of H+ Cl–
Intergranular Corrosion
If impurities aggregates between grains of relatively pure alloys
Localized galvanic corrosion may exist between crystals and alloys in grain boundaries
Fretting Corrosion
A very small oscillation movement, vibration, or slip between components of devices
Abrasive damage permitting initiating of reaction
Pitting Corrosion
A localized reaction similar to service corrosion
Starting as a defect in the passive surface layer
Corrosion develops
GF In PSC
Ion release
Implanted metal release ion into tissue
Which decreases with time
Not a major clinical factor (So, mostly doesn’t require removal)
In Some patient-> Sensitive to Chronic/ Nickel of SS Implant=> Required removal
Composition of metals
Properties of metals depend upon the composition of the metals
| Stainless Steel (316L Alloy) | Cobalt | Titanium |
|---|---|---|
| Fe: 62.5% | Cobalt:55-65% | Titanium:55-60% (60) |
| Cr: 17.6% | Cr:19-21% | Cr:27-30% (30) |
| Ni: 14.5% | Ni:9-11% | Ni: 2.5% |
| Mo: 2.5% | Tugsten:14-16% | Al: 5.5-6.5% (6) |
| C: < 0.03% | C:0.05-0.15% | Vanadium: 3.5-4.5% (4) |
| Fe, Co, C, Mg, Si, etc |
TITANIUM VS STAINLESS STEEL
SS can be produced with high elastic modulus and ductility, which decrease stress shielding and cause osteoporosis
SS is cheaper
Titanium is more corrosion resistant and free of toxic ions
Titanium is less allergic (Minimal Nickel)
The mechanical properties of titanium are closer to the bone than SS
Radio friendly is titanium
Second Operation is often unnecessary and prolonged absence of work is avoided
Titanium is less prone to fatigue failure
See also: NanoTechnology in orthopedics
See also: Robotic Surgery in Orthopedics
