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Characteristics of Metals and Implants

- Discussion:
       - Biomechanics Menu
Metal Fatigue
Ultimate Tensile Strength
Yield Strength
       - Types of Metals: 
Cobalt Based Alloys 
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

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