1 / 42

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.

maried
Download Presentation

Metallic Implant Materials

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related