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
