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Biomaterials Used in Orthopedic Implants

Biomaterials Used in Orthopedic Implants. Magnesium Foam As a Bioresorbable Implant Presented By Joe Barker. Orthopedics. Bone properties, Osteoconductivity, & Biocompatibility. Bone Properties. Density – 2.3g/cm 3 Tensile Strength – 3-20MPa Compressive Strength – 15,000 psi

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Biomaterials Used in Orthopedic Implants

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  1. Biomaterials Used in Orthopedic Implants Magnesium Foam As a Bioresorbable Implant Presented By Joe Barker

  2. Orthopedics Bone properties, Osteoconductivity, & Biocompatibility

  3. 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

  4. 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. Trochanter The second segment of the leg, after the coxa and before the femur

  5. Screw Types OBLIQUE SCREWS • In subtrochanteric and high femoral fractures oblique screws may be required to be inserted up the femoral neck • Screws are 4.5mmX150mm

  6. Screw Types CANNULATED SCREW • Screw Sizes • 6.5mm X 102mm • 4.5 X 12.5mm

  7. Screw Types CANNULATED SCREW Continued... A bulbous ended nail with cannulated 12.5 mm screws is shown here successfully stabilizing a subtrochanteric non-union of the femur following a failed Gamma nail

  8. Screw Types TRANSVERSE SCREWS In most subtrochanteric and upper femoral fractures it is much easier to insert transverse screws in the upper femur, than use oblique screws up the neck of the femur.

  9. Screw Types Transverse Screws Continued....

  10. Orthopedic Materials: Metals

  11. 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

  12. Devices Were Metals Are Used • Orthopedic devices • Plates and screws, Pins and Wires, rods (temporary) • Total joints (permanent) • Clips and staples

  13. Metals Used in Implants • Three main categories of metals for orthopedic implants • stainless steels • cobalt-chromium alloys • titanium alloys • Material looked at in this project: • Magnesium Foam

  14. Stainless Steel • Generally about 12% chromium (316L, Fe-Cr-Ni-Mo) • High elastic modulus, rigid • 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

  15. 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 • 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 • Molybdenum is added to produce finer grains

  16. Titanium and Titanium Alloys • 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 - does not match bone causing stress shielding

  17. 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

  18. 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

  19. Orthopedic Materials Metallic Foams

  20. 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.

  21. Magnesium Foam The type of Magnesium foam used in this study would be classified as a porous metal. Why 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

  22. Requirements for Porous Implant • Pore Morphology (Spherical) • Pore Size (200m - 500m) • Porosity • High Purity (99.9%) • Biocompatibility

  23. Why Magnesium? • Bioresorbable • Biocompatible • Osteoconductive • Osteoinductive • Properties of bone can be easily attained using varying processing techniques

  24. Processing the Mg by Powder Metallurgy Techniques Powder • Mg powder • 99.9% purity • particle size 180m • Binder: Ammonium Bicarbonate • Spherical Shape • 99.0% purity • Size between 200m – 500m

  25. Processing the Mg by Powder Metallurgy Techniques Processing Steps • Blend powders until a homogenous mixture is attained. • Uniaxially press at 100MPa into green compacts • Heat treat at 200ºC for 5hrs, for binder burnout • Sinter at 500ºC for 2hrs

  26. Results From Processing • Optical Micrograph of Porous Mg: • Small isolated micropores distributed in the wall of the interpenetrated macropores. • The micropores are on the order of microns, while the macropores are in the range of 200m – 500m

  27. Results of Processing SEM Micrograph of Mg: • Micropores result from the volume shrinkage during sintering and are to small for bone growth • Macropores are made on the appropriate size level to promote the ingrowths of new-bone tissues and transport of body fluid

  28. Determination of Mechanical Properties The Stress – Strain Curve shows a large plateau region From this you can see that the plateau stress of the Mg foam is roughly 2.33 MPa Using Gibson – Ashby model the following properties can be attained: • pl/ys=C(/s)3/2; C=0.3 • E/Es=A(/s)2; A=1

  29. Properties Attained from Processing With a porosity of  50% • Density = 0.87g/cm3 • pl = 2.33MPa • ys = 2.843 MPa • E = 10.476GPa

  30. Adsorption and Toxicity • Adsorption Rates for Mg • The bone will adsorb around 40% of the Mg in the screw per year. • From this the lifetime of the screw would be between 5 7 years before no traces are left. • Toxicity • Recommended dosage of Mg per day is 350mg • 60% of Mg in the body is found in bones • In recent studies, a diet rich in Mg resulted in increases in bone density in postmenopausal women • Relatively low toxicity issues, but in vivo testing would clarify.

  31. Cost of Materials Price of Mg powder for particle size  200m: • $56.24 for 1Kg Price of Binder powder within set size limits: • $40.66 for 1Kg Cost for smallest screw size: 4.5mm X 12.5mm • $0.07 Cost for largest screw size: 6.5mm X 150mm • $0.50

  32. Processing Costs Processing features: • Simple Uniaxially pressed operation • No need for mass production • No continuous processing (more costly) • No need for multiple large industrial scale facilities • Less workers and utility costs

  33. Comparisons

  34. Conclusions • Mg Foam has mechanical properties better suited for bone substitute than the other commercial products • You have the advantage of Mg being bioresorbable and osteoconductive. • Promotes new bone growth • Is completely replaced by new bone, making the bone stronger • You don’t have to walk around with a foreign body inside your leg for the duration of your life, or you don’t have to have a second surgery to remove the implant

  35. Conclusion Continued.... • Processing cost is comparative to products all ready in use • Toxicity issues are small since the amounts of Mg is low, but medical workups would be advised before implantation • Mg Foam is a viable option for use in a screw implant.

  36. Questions Any Questions about the project?

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