- Discussion:
- Biomechanics Menu
- Metal Fatigue
- Ultimate Tensile Strength
- Yield Strength:
- Types of Metals:
- Cobalt Based Alloys
- Titanium
- Vitallium
- Stainless steel
- 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?
- 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;
- 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;
- 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
- Flexibility of Metals:
- a brittle material breaks before any plastic deformation;
- a ductile material has a plastic behavior before it breaks;
- 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:
- metal is heated and hammered or squeezed into shape
- decreases the grain size;
- causes an orientation of the grain flow making the metal strong
- deforming stainless steel stem increases the grain size primarily on outer segment;
Detection of Orthopaedic Implants in Vivo by Enhanced-Sensitivity, Walk-Through Metal Detectors.
Original Text by Clifford R. Wheeless, III, MD.
Last updated by Clifford R. Wheeless, III, MD on Sunday, August 4, 2013 10:04 am