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Biomaterials metallic & nonmetallic implants

Biomaterials metallic & nonmetallic implants

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Biomaterials metallic & nonmetallic implants

  1. 1. Metallic & Nonmetallic Implants In Orthopaedics Dr Debasis Mukherjee Dept of Orthopaedics IPGMER
  2. 2. HISTORY • More than 2000 years ago, Romans, Chinese, and Aztec’s used gold in dentistry. • Earliest evidence of fracture treatment with a metallic wire was way back in 1770. • In 18th century, came ‘Antisepsis’ and ‘Anaesthesia’, along with development in metallurgy and plastic and X-RAY. • 1960 Charnley uses PMMA, ultrahigh-molecular-weight polyethylend, and stainless steal for total hip replacement. • Discovery of newer metals and newer alloys.
  3. 3. WHY TO KNOW ??? To make an informed selection of the Surgical implant
  4. 4. BIOMATERIALS • These are natural or synthetic substances , capable of being tolerated permanently or temporarily by the human body.
  5. 5. BIOMATERIALS • Biomaterials used in orthopedics are – 1. Metals and metal alloys 2. Ceramic and ceramometallic materials 3. Tissue adhesives 4. Bone replacement materials 5. Carbon materials and composites
  6. 6. Orthopedic Implant • An orthopedic implant is a medical device manufactured to replace a missing joint or bone or to support a damaged bone. • Implants can be Metallic or Non-metallic.
  7. 7. ISSUES • Biocompatibility • Strength parameters(tensile,comressive and torsional strength;stiffness,fatigue resistance,contourability) • Resistance to degradation and erosion • Ease of integration when appropriate • Minimal adverse effect on immaging
  8. 8. Orthopedic Terms Osteoconductive – The property of a material that allows for the possible integration of new bone with the host bone. Osteoinductive – Characteristic in materials that promote new bone growth. Bioresorbable – The ability of a material to be entirely adsorbed by the body.
  9. 9. Orthopedic Terms • Fatigue • Endurance limit • Cyclic Failure- High and Low • Elongation • Corrosion and Passivation
  10. 10. Basic Biomechanics • Material Properties – Elastic-Plastic – Yield point – Brittle-Ductile – Toughness – Hardness • Independent of Shape! • Structural Properties – Bending Stiffness – Torsional Stiffness – Axial Stiffness • Depends on Shape and Material!
  11. 11. Basic Biomechanics Force, Displacement & Stiffness Force Displacement Slope = Stiffness = Force/Displacement
  12. 12. Basic Biomechanics Stress = Force/Area Strain Change Height (L) / Original Height(L0) Force Area L
  13. 13. Basic Biomechanics Stress-Strain & Elastic Modulus Stress = Force/Area Strain = Change in Length/Original Length (L/ L0) Slope = Elastic Modulus = Stress/Strain
  14. 14. Basic Biomechanics Common Materials in Orthopaedics • Elastic Modulus (GPa) • Stainless Steel 200 • Titanium 100 • Cortical Bone 7-21 • Bone Cement 2.5-3.5 • Cancellous Bone 0.7-4.9 • UHMW-PE 1.4-4.2 Stress Strain
  15. 15. Basic Biomechanics • Elastic Deformation • Plastic Deformation • Energy Energy Absorbed Force Displacement PlasticElastic
  16. 16. Basic Biomechanics • Stiffness-Flexibility • Yield Point • Failure Point • Brittle-Ductile • Toughness-Weakness Force Displacement PlasticElastic Failure Yield Stiffness
  17. 17. Stiff Ductile Tough StrongStiff Brittle Strong Ductile Weak Brittle Weak Strain Stress
  18. 18. Flexible Ductile Tough Strong Flexible Brittle Strong Flexible Ductile Weak Flexible Brittle Weak Strain Stress
  19. 19. Basic Biomechanics • Load to Failure – Continuous application of force until the material breaks (failure point at the ultimate load). – Common mode of failure of bone and reported in the implant literature. • Fatigue Failure – Cyclical sub-threshold loading may result in failure due to fatigue. – Common mode of failure of orthopaedic implants and fracture fixation constructs.
  20. 20. Bone Properties • Density – 2.3g/cm3 • Tensile Strength – 3-20MPa • Compressive Strength – 15,000 psi • Shear Strength – 4,000 psi • Young’s Modulus – 10-40 MPa
  21. 21. Metals For Implants • Must be corrosion resistant • Mechanical properties must be appropriate for the desired application • Areas subjected to cyclic loading must have good fatigue properties -- implant materials cannot heal themselves
  22. 22. Metals For Implants • Must be corrosion resistant • Mechanical properties must be appropriate for the desired application • Areas subjected to cyclic loading must have good fatigue properties -- implant materials cannot heal themselves
  23. 23. Metals Used in Implants • Three main categories of metals for orthopedic implants – stainless steels – cobalt-chromium alloys – titanium alloys – Metallic Foam
  24. 24. Stainless Steel • Generally about 12% chromium,13 to 15.5% nickel (316L, Fe-Cr-Ni-Mo) • High elastic modulus, rigid-results in Stress Shielding. • Low resistance to stress corrosion cracking, pitting and crevice corrosion, better for temporary use • Corrosion accelerates fatigue crack growth rate in saline (and in vivo) • Intergranular corrosion at chromium poor grain boundaries -- leads to cracking and failure • Wear fragments - found in adjacent giant cells • Cheap
  25. 25. Cobalt – Based Alloys • Co-Cr-Mo – Used for many years in dental implants; more recently used in artificial joints – good corrosion resistance • Co-Cr-Ni-Mo – Finer grains – Typically used for stems of highly loaded implants, such as hip and knee arthroplasty • Very high fatigue strengths, high elastic modulus – High degree of corrosion resistance in salt water when under stress – Poor frictional properties with itself or any other material
  26. 26. Titanium and Titanium Alloys • Minimal attenuation problem on MRI • High strength to weight ratio – Density of 4.5 g/cm3 compared to 7.9 g/cm3 for 316 SS • Modulus of elasticity for alloys is about 110 GPa – Not as strong as stainless steel or cobalt based alloys, but has a higher “specific strength” or strength per density – Low modulus of elasticity
  27. 27. Titanium Alloys • Co-Ni-Cr-Mo-Ti, Ti6A4V • Poor shear strength which makes it undesirable for bone screws or plates • Tends to seize when in sliding contact with itself or other metals • Poor surface wear properties - may be improved with surface treatments such as nitriding and oxidizing
  28. 28. Best Performance • Titanium has the best biocompatibility of the three. – Metal of choice where tissue or direct bone contact required (endosseous dental implants or porous un-cemented orthopedic implants) – Corrosion resistance due to formation of a solid oxide layer on surface (TiO2) -- leads to passivation of the material
  29. 29. Metallic Foam • Types of metallic foams – Solid metal foam is a generalized term for a material starting from a liquid-metal foam that was restricted morphology with closed, round cells. – Cellular metals:A metallic body in which a gaseous void is introduced. – Porous metal: Special type of cellular metal with certain types of voids, usually round in shape and isolated from each other. – Metal Sponges: A morphology of cellular metals with interconnected voids.
  30. 30. Magnesium Foam Open cellular structure permits ingrowths of new-bone tissue and transport of the body fluids – Strength & Modulus can be adjusted through porosity to match natural bone properties
  31. 31. Why Magnesium? • Bioresorbable • Biocompatible – Osteoconductive – Osteoinductive • Properties of bone can be easily attained using varying processing techniques
  32. 32. Tantalum • A newer material, tantalum, is a trabecular metal composed of a carbon substrate with elemental tantalum deposited on the surface. Forms a biological scaffold for new bone formation. • Modulus of elasticity closer to that of bone than stainless steel or the cobalt-based alloys. • Not been used in the manufacture of implants until more recently. Because of its remarkable resistance to corrosion, tantalum seems well suited to a biological ingrowth • Because of its remarkable resistance to corrosion, tantalum seems well suited to a biological ingrowth setting, but long-term studies are needed to confirm its usefulness.
  33. 33. Comparisons Material Density Youngs Modulus Tensile Strength Estimated Cost Ranking Bone 2.3 10 – 40 3 – 20 Na Stainless Steel 7.9 196 290 1 Co Alloys 8.9 211 345 4 Ti Alloys 4.5 105 200 3 Mg Foam 2.33 10.476 2.843 2
  34. 34. Composites • Manufactured in several ways – Mechanical bonding between materials (matrix and filler) – Chemical bonding – Physical (true mechanical) bonding • Young’s modulus = 200 GPa • Benefits – Extreme variability in properties is possible • Problems – Matrix cracking – Debonding of fiber from matrix • Examples: concrete, fiberglass, laminates, bone
  35. 35. Ceramics • Materials resulting from ionic bonding of – A metallic ion and – A nonmetallic ion (usually oxygen) • Benefits – Very hard, strong, and good wear characteristics – High compressive strength – Ease of fabrication • Examples – Silicates , Metal Oxides - Al2O3, MgO – Carbides - diamond, graphite, pyrolized carbons – Ionic salts - NaCl, CsCl, ZnS
  36. 36. Ceramics (cont’d) • Uses – Surface Replacement – Joint Replacement • Problems – Very brittle & Low tensile strength • Undergo static fatigue – Very biocompatible – Difficult to process • High melting point • Expensive
  37. 37. Polyethylene • Ultra high molecular weight (UHMWPE) • High density – Molecular weight 2-6 million • Benefits – Superior wear characteristics – Low friction – Fibers included • Improve wear properties • Reduce creep • Used – Total joint arthoplasty
  38. 38. Bone Cement • Used to fill gaps between bone and implant • Example: total hip replacement – If implant is not exactly the right size, gaps are filled regardless of bone quality
  39. 39. Bone Cement • Polymethylmethacrylate • Mixed from powder polymer and liquid monomer – In vacuum • Reduce porosity • Increase strength – Catalyst (benzoyl peroxide) may be used • Benefits – Stable interface between metal and bone http://www.totaljoints.info/bone_cement.htm
  40. 40. Bone Cement (cont’d) • Problems – Inherently weak • Stronger in compression than tension • Weakest in shear – Exothermic reaction • May lead to bone necrosis – By handling improperly or less than optimally • Weaker – Extra care should be taken to • Keep debris out of the cement mantle (e.g., blood, fat) • Make uniform cement mantle of several mm • Minimize voids in the cement : mixing technique • Pressurize
  41. 41. Biodegradable materials • These are Poly-Glycolic Acid (PGA), PDS, polylevolactic acid (PLLA), and racemic poly(D, L)-lactic acid (PDLLA) • Recently SR-PGA and SR-PLLA • High glass transition temperature • Can be covalently linked with HRP, IL-2, BMP-2 • Advantages are gradual load transfer to the healing tissue, reduced need for hardware removal, and radiolucency • Disadvantages like aseptic inflammation and sinus tract formation,severe synovitis are more prominent with PGA than PLA
  42. 42. Biodegradable materials • Uses- Reattachment of ligaments,tendons,meniscal tears and other soft tissue structures. Stabilization of fractures,osteotomies,bone grafts and fusions.
  43. 43. Mechanical Properties of IM • As Implant materials have to function as bones, the mechanical properties of interest are – Elastic modulus – Ultimate tensile strength • They are listed in order of increasing modulus or strength (in next 2 slides)
  44. 44. Elastic Modulus in increasing order of strength 1. Cancellous bone 2. Polyethylene 3. PMMA (bone cement) 4. Cortical bone 5. Titanium alloy 6. Stainless steel 7. Cobalt-chromium alloy
  45. 45. Ultimate Tensile Strength in increasing order of strength 1. Cancellous bone 2. Polyethylene 3. PMMA (bone cement) 4. Cortical bone 5. Stainless steel 6. Titanium alloy 7. Cobalt-chromium alloy
  46. 46. THANK YOU

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