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Metallic Implant Materials

Metallic Implant Materials. 40% of annual 3.6 million orthopaedic operations $6 billion market 5 out of 100 Americans carry a piece of metal in them!. Dental Implants. Prepared by: Dental Materials Department Yenepoya Dental College, Yenepoya University, Mangalore.

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Metallic Implant Materials

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  1. Metallic Implant Materials • 40% of annual 3.6 million orthopaedic operations • $6 billion market • 5 out of 100 Americans carry a piece of metal in them!

  2. Dental Implants Prepared by: Dental Materials Department Yenepoya Dental College, Yenepoya University, Mangalore.

  3. Metallic Implant Materials • Reduction of fracture, internal fixation • Replacement of hip, knee and shoulder • Oral and maxillofacial surgery

  4. Types of Metallic Implants • Stainless steel • Cobalt Based Alloys • Titanium Alloys Composition? Properties? Manufacturing?

  5. Stainless Steels • The most common stainless steel: 316L • Fe 60-65 wt% • Cr 17-19 wt % • Ni 12-14 wt%

  6. Stainless Steels • Carbon content reduced to 0.03 wt%: • better resistance to in vivo corrosion. • Why reduce carbon? • reduce carbide (Cr23C6) formation at grain boundary • carbide impairs formation of surface oxide

  7. Corrosion at the Grain Boundary High risk of corrosion when Cr drops down to less then 9% at the boundary C Cr Cr Cr23C6 Cr C C

  8. Stainless Steels • Why add chromium? • corrosion resistance by formation of surface oxide • Why add nickel? • improve strength by increasing face centered cubic phase (austenite)

  9. Stainless Steels • Good stainless steel: • Austenitic (face centered cubic) • No ferrite (body centered cubic) • No carbide • No sulfide inclusions • Grain size less then 100m • Uniform grain size

  10. Stainless Steels • Improved mechanical properties by cold working (a.k.a. strain hardening) • How? excessive number of dislocations are induced prior to in-vivo use, newer dislocations will be harder to induce

  11. Stainless Steels • How to cold work: load plastically. • Pros: increased yield strength, ultimate strength and fatigue strength • Cons: reduced ductility

  12. Cobalt Based Alloys • Common types for surgical applications: • ASTM F75 • ASTM F799 • ASTM F790 • ASTM F 562

  13. Cobalt Alloys: ASTM F75 • Co-Cr-Mo • Surface oxide; thus corrosion resistant • Wax models from molds of implants • Wax model coated with ceramic and wax melted away • Alloy melted at 1400 C and cast into ceramic molds.

  14. Cobalt Alloys: ASTM F75 • Three caveats: • Carbide formation  corrosion. Solution: anneal at 1225 C for one hour. • Large grain size  reduced mechanical strength (WHY?) • Casting defects  stress concentration, propensity to fatigue failure

  15. The enemy within: Casting defect Polished-etched view of a cast ASTM F75 femoral hip stem. Note dendrites and large grains In vivo fracture initiated from a an inclusion formed during the casting process

  16. Cobalt Alloys: ASTM F799 • Modified form of F75: hot forged after casting • Mechanical deformation induces a shear induced transformation of FCC structure to HCP. • Fatigue, yield and ultimate properties are twice of F75.

  17. Cobalt Alloys: ASTM F90 • W and Ni are added to improve machinability and fabrication • Mechanical properties similar to F75 • Mechanical properties double F75 if cold worked

  18. Titanium Based Alloys • Lighter • Good mechanical properties • Good corrosion resistance due to TiO2 solid oxide layer

  19. Titanium Based Alloys • Ti-6% wt Al-4% wt V (ASTM F136) is widely used • Contains impurities such as N, O, Fe, H, C • Impurities increase strength and reduce ductility

  20. Titanium Alloys: ASTM F136 • HCP structure transforms to BCC for temperatures greater than 882 C. • Addition of Al stabilizes HCP phase by increasing transformation temperature • V has the inverse effect

  21. Yield Strength of Metals

  22. Dental Metals • Amalgam: • Solid alloy • silver, tin, copper, zinc and mercury • deformable mixture packed in cavity • cures over time • 25% of total strength in 1 hour • full strength in a day • Gold: • Durable, stable, corrosion resistant as fillings

  23. Dental Metals: Nitinol • NIckel-TItanium-Naval Ordinance Lab • Shape memory alloy (SMA): ability to return to a predetermined shape when heated

  24. Dental Metals: Nitinol Orthodontic applications

  25. Corrosion • Corrosion is the degradation of metals to oxide, hydroxide or other compounds through chemical reactions. • Human body is an aggressive environment: • water • dissolved oxygen • proteins • chloride • hydroxide • pH (after surgery pH around 5.3-5.6) • flow rate

  26. Corrosion: Basic Reactions • Ionization: formation of metallic cations under acidic or reducing (i.e. oxygen poor) conditions M  M++e-

  27. Corrosion: Basic Reactions • Oxidation: reaction of metal with oxygen M + O2 MO2

  28. Corrosion: Basic Reactions • Hydroxylation: reaction of water under alkaline (i.e. basic) or oxidizing conditions • yields a hydroxide or hydrated oxide 2M + O2 (aq) + 2H2O  2M(OH)2

  29. Corrosion: Basic Reactions • Reaction: combination with other cations and anions MO22- + HCl  MOCl- + OH-

  30. Corrosion • MECHANISM: • corroded state is preferred since lowest energy state • metal atoms ionize, go into solution and combine with oxygen • metal flakes off • Corrosion of most metallic biomaterials occur by “gaseous reduction”

  31. +ve electrode --ve electrode Corrosion: Gaseous Reduction Oxygen deficient location is the ANODE. Anode oxidizes i.e. rusts! Electron requirement for right cell induces ferrous iron formation on the left cell (see the e- flow) Reaction in which electrons are gained Oxidation: Reaction in which electrons are lost

  32. RUST Corrosion: Terminology • Formation of rust: Water, oxygen and metal essential • Oxidation of iron to ferrous (iron +2) ion Fe  Fe+2 + 2 e- • Oxidation of ferrous ions to ferric (iron +3) ion Fe+2 Fe+3 + e- • Reduction of oxygen using electrons generated by oxidation O2 (g) + 2H2O + 4e- 4OH- Overall Reaction 1: 4Fe+2+O2+8H2O  2Fe2O3. H2O (s) + 8H+ Overall Reaction 2: 4Fe+2 + O2 + 2H2O  4Fe+3+ 4OH- 4Fe+3 + 12OH-  4Fe(OH)3

  33. Corrosion: Terminology How it happens on the surface?

  34. Corrosion:Closer look at the “gaseous reduction”: Crevice Corrosion • Pitting Corrosion: special case of crevice corrosion where corrosion is induced by handling damage such as scratches. Surgeons should be careful about this type of corrosion.

  35. Corrosion Memorabilia! • Electron flow direction is FAT CAT (from anode to cathode) • Oxidation at the anode, reduction at the cathode (OAR-CAT !) • ANO (anode attracts negative ions and results in oxidation) • CPR (cathode attracts positive ions and results in reduction)

  36. Galvanic Corrosion • Occurs when there are two dissimilar metals • Much more rapid than corrosion by gaseous reduction • Avoid implantation of dissimilar metals

  37. Conventional current Electrons Fe2+ Fe2+ O2 Galvanic Corrosion Fe Cu O2 taken from Bob Cottis Corrosion and Protection Centre, UMIST

  38. 1.6 B oxygen 0.8 A EM (Volts) 0 hydrogen -0.8 -1.6 0 4 8 12 pH Pourbaix Diagram (poor-bay) • classification of all possible reactions between a metallic element and water for combinations of pH and electrical potential difference • above B oxygen is released • below A hydrogen is released • between A and B water is stable

  39. 1.6 B 0.8 A EM (Volts) 0 -0.8 -1.6 0 4 8 12 pH Passivation Corrosion Immunity • Corrosion: concentration of the metal is greater than 10-6 [M] • Metal and the passive coating is attacked resulting in corrosion. Pourbaix Diagram • Passivation: formation of “oxides and hydroxides” • Concentration of the metal is less than [10-6] M • Reaction products cling to the interface between the metal and the solution, reducing further reactions. • Thus, the metal is covered with passive coating. • Immunity: Dominant reaction is “ionization”. • Concentration of the metal is less than [10-6] M • At this concentration the reaction is immune from corrosion

  40. 1.6 B 0.8 A EM (Volts) 0 -0.8 -1.6 0 4 8 12 pH Pourbaix Diagram Chromium in water Chromium in water + chloride • Note smaller passivation region. Greater chance of corrosion. • Most physiological solutions in the corrosion region

  41. Against Corrosion • Use appropriate metals • Avoid implantation of dissimilar metals • Minimize pits and crevices • Avoid transfer of metal from tools to the implant during surgery • A metal that does not corrode in one part of the body may corrode somewhere else

  42. Next: Ceramics • Please revisit the previous lecture on ionic bond, coordination number, and electronegativity • Bring along handout #2

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