2.Metallic biomaterials Flashcards
physical properties of metals
- heat and eletrical conductivity
- ductile and malleable
- high density
- high melting point
- excellent mechanical properties
chemical properties
high reactivity
resist to corrosion (metal alloys used in the body e.g., stainless steel, titanium and Cr-Co)
high wear resistance
forces experienced by metallic biomaterials during application
tension
compression
shearing
torsion
bending
[mechanical properties of metals and alloys] yield strength
the stress at which plastic
strain occurs (“elastic to plastic transition”)
[mechanical properties of metals and alloys] Breaking strength
point where the material
breaks
ductility [mechanical properties of metals and alloys]
the value of plastic strain required to break the material
Resilience [mechanical properties of metals and alloys]
elastic energy that can be stored in a unit volume of stressed material
Toughness [mechanical properties of metals and alloys]
energy required to deform a unit volume of material to its breaking point
[mechanical properties of metals and alloys] Ultimate tensile strength
maximum nominal stress versus nominal strain plot
Aseptic loosening
failure of joint prostheses (~10–20 years postsurgery) often associated to osteolysis (bone resorption) and inflammatory cellular response within the joint.
What are the key requirements that metals should exhibit for such applications?
- Biocompatibility, nontoxicity, not allergenicity;
- Corrosion resistance;
- Adequate mechanical properties;
- Wear resistance.
Lanzutti and co-workers analyzed the failure of a metallic hip prosthesis in a
patient. In your opinion, what are the most common causes of implant
failure?
The most probably cause(s) of the failure can be attributed to:
a) Poor integration and bonding with the adjacent tissue.
b) Stress shielding effect.
c) Corrosion mediated failure.
d) Manufacturing and quality control issues
Why metals corrode in the presence of biological fluids?
- Metal atoms react spontaneously with oxygen, hydrogen protons and ionic salts
over timeàmetal oxides; - ~96% of the body weight consists of oxygen, carbon, hydrogen, and nitrogen
(building blocks of water and proteins);
Release of metal ions from the materials into the surrounding tissue, which
can concentrate locally or diffuse systemically (biocompatibility)
Impact of corrosion for:
* Patient
Localized pain
Inflammation
Accumulation of metallic ions in the body
Impact of corrosion for:
Implant
Fracture and failure
Loss of function
Why most of the metals are used in combination with other metals or
nonmetal elements?
Increase the strength
Confer higher corrosion resistance
Improve specific properties
Alloys: mixture of two or more metals or nonmetal elements
Alloys
Alloys: mixture of two or more metals or nonmetal elements
Polarization resistance
resistance of the specimen to oxidation during the application of an external
potential.
How to minimize corrosion of metallic biomaterials?
- Addition of metal elements with high stability – high resistance to
corrosion, such as Zr, Ti, Nb, Ta, Pt, Ag, and Au; - Surface finishing
- Avoid to use dissimilar metals in the same implant
Major application of stainless steel
surgical instrumentation
Screws, rods, and plates for bone fixation and in spinal fusion devices
CoCrMo alloys caracteristics
- Higher resistance to fatigue or fracture than stainless steel or titanium;
- Good resistance to corrosion
- Higher Mo content to compensate the reduction on Cr and maintain the
corrosion resistance - High resistance
- Cr=high resistance to corrosion;
Co=high mechanical properties
Titanium and its alloys
*Light weight, excellent corrosion resistance, and enhanced biocompatibility
* Excellent mechanical properties
* Density of Ti«stainless steel<cobalt chromium alloy
* good resistance to corrosion
* low wear resistance
* high chemical reactivity at high temperatures in the presence of oxygen
Vandium can be
carcinogenic
Aluminium can cause
nerological side effects
and
genetoxicity
Which problems/consideration should we take into account when considering porous metallic biomaterials?
- decreased moduli
- bone fixation via bone ingrowth
- resistance to corrosion and fatigue
- entrapped powder
Titanium alloys limitations
- Relatively poor wear resistance in an articulating situation, compared to cobalt
alloys: - Due to their high reactivity in the presence of oxygen during the hightemperature
processing an inert atmosphere or vacuum is required; - High cost – high reactivity and poor machinability;
Titanium alloys mojor applications
- Dental implants
- total joint replacements
- Good performance at interfacing with the biological system
- Bone ingrowth into porous titanium surfaces, known as biological fixation, is a
primary means by which orthopedic implants affix to bone directly
Shape memory alloys
Titanium–nickel alloys (Nitinol)
Relatively stable cyclic performance (stable memory), good workability, and good resistance to corrosion and fatigue
Shape-memory alloys deformation
Plastically deformed at a low temperature (martensitic phase)
Return back to their original predeformed
shape when exposed to a high temperature (austenite phase)
Definition of biodegradable metals
“Corrode gradually in vivo, with an appropriate host response elicited by released
corrosion products, which can pass through or be metabolized or assimilated by cells and/or
tissue, and then dissolve completely upon fulfilling the mission to assist with tissue healing with no implant residues.”
Absorbable
biodegradation products are metabolized or assimilated by cells/tissue
Biodegradable
The material/device itseld undergoes biodegradation process
Which base metals can be used for biodegradable implants?
Ferro
Zinco
Magnesio
Characteristics of using magnesium as a base material for biodegradable implants
low density
young modulus similar to the bone
fast degradation => mechanical integraty (????)
corrosion produces H2(g)
Wolff’s Law
living bones will remodel in adaptation to
the external loads they experience
How the level of internal strain experienced within the bone affects biological processes?
Net mineral loss
Mineral homeostasis
Net mineral gain
Damage formation.
stress-shielding
Stress shielding is the reduction in bone density (osteopenia) as a result of removal of typical stress from the bone by an implant (for instance, the femoral component of a hip prosthesis). This is because by Wolff’s law, bone in a healthy person or animal remodels in response to the loads it is placed under.
Pros of Iron (Fe) based metallic biomaterials
- good corrosion resistance and fatigue resistance in short term applications
- low cost
- easy to be machined
Cons of Iron (Fe) based metallic biomaterials
- corrosion in long term applications
- high modulus => The higher the modulus, the stiffer the material (+ rígido)
*stress shielding effect
Applications of Iron (Fe) based metallic biomaterials
instruments
temporary devices
permanent implants (stem of hip prostetics)
Pros of Cobalt (Co) based metallic biomaterials
- long term corrosion resistance
- best fatigue and wear resistance
- biocompatibility
Cons of Cobalt (Co) based metallic biomaterials
- difficult to machine = expensive to process
- high modulus
- stress-shielding effect
- Co allergy
Application of Cobalt (Co) based metallic biomaterials
Permanent joint implants
Pros of Titanium (Ti) based metallic biomaterials
- light
- greatest corrosion resistance
- best biocompatibility
- free of metal-related allergy
- low young’s mudulus (indicates a material that undergoes large (elastic) deformation under a relatively low load. Such materials stretch easily. )
Cons of Titanium (Ti) based metallic biomaterials
- lower shear strength
- low wear resistance
- still have. though to a lesser degree, stress-shielding effect
Applications of Titanium (Ti) based metallic biomaterials
- permanent implant
- stem of hip prostheses
- dental screws
- temporary device
How is it possible to reduce the stress-shielding effect?
- Biomaterial selection
- Implant design
- Topology optimization
