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Polymers in Orthopaedics October 16, 2002

Polymers in Orthopaedics October 16, 2002. Steven M. Kurtz, Ph.D. 1,2,3,4,5 1 Exponent, Inc. 2 Drexel University 3 Thomas Jefferson University 4 Temple University 5 Princeton University. Basic Definitions . Basic Definitions . Thermal Transitions . Glass Transition Temperature (T g )

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Polymers in Orthopaedics October 16, 2002

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  1. Polymers inOrthopaedicsOctober 16, 2002 Steven M. Kurtz, Ph.D.1,2,3,4,51Exponent, Inc.2Drexel University3Thomas Jefferson University 4Temple University 5Princeton University

  2. Basic Definitions

  3. Basic Definitions

  4. Thermal Transitions • Glass Transition Temperature (Tg) • PE: Tg = -130 to -80°C • PMMA: Tg = 120°C • Bone Cements: Tg = 60-90°C • Peak Melting Temperature (Tm) • PE: Tm = 135°C

  5. Melt Transition in UHMWPE

  6. Elastic Properties (Room Temperature) • Polyethylene, E = 1 GPa • Bone Cement, E = 3 GPa • Cortical Bone, E = 15 GPa • Ti Alloy, E = 110 GPa • CoCr Alloy, E = 200 GPa

  7. Rate Dependence

  8. Temperature Dependence of UHMWPE

  9. Rate Dependence of UHMWPE

  10. Range of Relevant Structural Length Scales • Chemical Composition • Linear • Branched • Copolymer • Macromolecular Structure • Supermolecular Morphology

  11. Chemical Structure • Poly methyl methacrylate • Polyethylene CH3 -(-CH2–C-)n- COO-CH3 -(-CH2–CH2-)n-

  12. Crystallinity in Polymers -(-CH2–CH2-)n-

  13. Crystalline Morphology

  14. Microstructure: Lamellae and Amorphous amorphous regions crystalline lamellae

  15. Superstructure of UHMWPE:Resin Flakes Virgin Resin Flake

  16. Basic Polymer Concepts Review • Polymers are large molecules • Morphology reflects structure • Amorphous • Semi-Crystalline • Crosslinked • Properties are structure, temperature, and rate dependent

  17. Bone Cements • Amorphous polymer • “Brittle” at room temperature • Molecular weight of 200-800,000 CH3 -(-CH2–C-)n- C = O O CH3

  18. Time to Mix!!

  19. Bone Cement Curing In Vitro

  20. Bone Cement Curing In Vivo

  21. BoneCementConstituents

  22. Composite Beam Analysis Bone Stem M M Bone Cement

  23. Composite Beam Analysis Tensile Stress M M Compressive Stress

  24. Relevant Material Properties for Bone Cements • Elastic Modulus • Compressive Yield/Ultimate Stress • Tensile Yield/Ultimate Stress • Fracture Toughness • Fatigue Resistance • Initiation • Propagation

  25. Factors Influencing Material Properties for Bone Cements • Formulation • Mixing Technique • Temperature, Strain Rate • Hydration • Additives (e.g., Antibiotics) • Radiation Sterilization • In Vivo Exposure (?)

  26. UHMWPE • Semi-crystalline linear homopolymer • “Ductile” at room temperature • Molecular weight of 2-6 million • www.uhmwpe.org

  27. Morphology • Composite Material • Crystalline Lamellae • Amorphous Matrix • Crystalline Architecture • 10-50 nm thickness • 10-50 mm long • 50 nm spacing

  28. Two-Phase “Sandwich” Model Areas of Mechanical Attachment “Weld Areas” Lamellar Crystal Tie Molecules E. H. Andrews (1972) Bhateja and Andrews (1985)

  29. Two-Phase “Sandwich” Model • Elastic Modulus • Proportional to Xc • Yield Stress • Proportional to (0.1 + Xc)2 s E. H. Andrews (1972) Bhateja and Andrews (1985) s

  30. Mechanisms of Plastic Deformation in UHMWPE • Deformations in Amorphous Layers • Interlamellar Shear • Interlamellar Separation • Lamella-stack Rotation • Internal Cavitation

  31. Mechanisms of Plastic Deformation in UHMWPE • Crystalline Plasticity • Chain Slip • Transverse Slip • Dislocation Generation • Twinning • Martensitic Transformations

  32. Uniaxial Tension Test Kurtz et al., Biomaterials, 1998

  33. Plasticity Induced Damage Layer Edidin et al., J. Arthroplasty, 1999

  34. Effect of Processing on Morphology • UHMWPE • 10-50 nm thick lamellae • Enhanced UHMWPE • 200-500 nm thick lamellae

  35. Morphology & Mechanical Behavior Processing Mechanical Environment Chemical Environment Morphology Mechanical Behavior

  36. Chain scission ‘degradation’ Crosslinking ‘improvement’ Two Major ReactionsDuring Radiation Sterilization

  37. Small Punch Testing

  38. Simulated Gait: Highly Crosslinked* vs. Conventional UHMWPE Flexion-Extension Abduction Adduction Internal-External Rotation Conventional 2.5g in N2 Head Sizes: ____ 32 mm ____ 28 mm ____ 22 mm Highly Crosslinked

  39. —Greenwald et al., 2002 AAOS

  40. RetrievalAnalysis

  41. What is Retrieval Analysis? • Study of implants retrieved from the human body • NIH Technology Assessment Conference (January 10-12, 2000) • Improving Medical Implant Performance Through Retrieval Information • Regional Research Centers

  42. NIH Initiative • Technology Assessment (01/00) • NIH/AAOS Conference (10/00) • www.aaos.org • Search for “implant wear” • Program Announcement (11/01)

  43. Drexel Implant Research Center ROTHMAN, CWRU Hip/Knee/Spine Retrievals Patient Data, Radiographs Implant Lot Data DREXEL IRC Implant Repository Retrieval Databases Hip/Knee/Spine CORPORATE PARTNERS Exponent, SHO

  44. Why are Implants Revised? • Patient Factors • Weight, Activity Level • Surgical Factors • Implantation Angle • Implant Factors • Design, Thickness

  45. Why Perform Retrieval Analysis? • Design Validation • Material Validation • Training/Education • Due Diligence

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