- See:
-
Biomechanics Menu
-
Cobalt Based Alloys
-
Metal Fatigue
-
Stainless steel
-
Titanium
-
Ultimate Tensile Strength
-
Vitallium
-
Yield Strength:
- Flexibility of Metals:
- a brittle material breaks before any plastic deformation;
- a ductile material has a plastic behavior before it breaks;
- Battery Effect:
- even in a single metal a battery effect can be produced;
- if strip of iron is immersed in a salt solution, the portion nearest surface, where the oxygen tension is the greatest, becomes cathode;
- anode is a zone at a deeper level;
- where cathode is large & anode is small, corrosion is greatest;
- if cathode is plate & anode is screw, severe corrosion takes place;
- clinically, mixing of
plates &
screws made of more inert metals such as
vitallium &
titanium does not give rise to significant corosion;
-
combinations of metals:
- dissimilar metal alloys are used in combination in total joint implants;
-
titanium based alloys &
cobalt based alloys can be combined with themselves and with each other;
-
stainless steel alloys also can be combined w/ each other, but not w/ either
titanium or
cobalt;
- references:
-
Coexistence of dissimilar metals after conversion of intertrochanteric osteotomy to total hip arthroplasty. 18 patients followed for 5-20 years after conversion.
-
Should the galvanic combination of titanium and stainless steel surgical implants be avoided?
- Corrosion:
- corrosion is release of ions and compounds as result of chemical action;
- in contrast wear is loss of solid fragments from surfaces due to mechanical action;
-
fretting corrosion:
- a process in which abrasive wear is accompanied by corrosion;
- protective oxide layer on metal is removed by abrasion process;
- because new passivation layer that forms after abrasion is neither as durable nor as chemically inert as original layer, metal is more susceptible to corrosion;
- this form of corrosion often occurs between screw heads &
plates;
-
stainless steel & cobalt-chromium alloys are susceptible to fretting corrosion;
- Grain Size:
- grain size or cystal size of metal is broadly indicative of its quality;
- in general larger grain, less the tensile strength of metal;
- conversely, smaller or finer the grain, greater toughness or strength;
- heating metal to approx its melting point increases grain size;
- forging decreases the grain size;
- deforming
stainless steel stem increases the grain size primarily on outer segment;
- Oxidation:
- chloride ions interfere w/ oxidation & formation of passivation layer in
stainless steel implants;
- practice of steam sterilization of implants w/ saline in environment gives rise to surface corrosion in both instruments & implants and should be prohibited;
- rough usage of implants and scraatches will break the oxide film on surface of an implant and be the nidus where corrosion, especially stress corrosion, may start;
- implants should never be thrown around in basins or shaken together in basket, nor immersed in saline;
- oxide layer inhibits metal egress and thus inhibits corrosion;
- this layer serves to protect the metal by insulating it from electrolyte solution;
- chromium oxide passivation layer forms on
stainless steel & cobalt-chromium alloy;
-
titanium oxide layer forms on titanium and titanium alloys;
- Toughness:
- ability of a metal to absorb energy by bending without breakage (the strain energy in the metal at the point of
ultimate stress;
- it is the area under the stress
strain curve;
- energy a structure absorbs as it deformed by applied force is equal to the work done by that force;
Detection of Orthopaedic Implants in Vivo by Enhanced-Sensitivity, Walk-Through Metal Detectors.
