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Polymers in Orthopaedics: An In-depth Exploration of Material Properties and Structural Dynamics

This article discusses the fundamental definitions, thermal transitions, elastic properties, and morphological characteristics of polymers in orthopaedics. It covers topics such as rate and temperature dependence, chemical structure, crystallinity, and the behavior of specific polymers like polyethylene and poly methyl methacrylate. Additionally, it delves into the properties and influences on bone cements, focusing on relevant material properties and factors affecting them. The text analyzes the morphology, mechanical behavior, and processing effects on ultra-high molecular weight polyethylene (UHMWPE), including mechanisms of plastic deformation, processing-induced damage, and radiation sterilization reactions. It also explores advanced testing methods, such as small punch testing and simulated gait analysis, comparing conventional and highly crosslinked UHMWPE. The significance of retrieval analysis in improving medical implant performance via research initiatives and retrieval databases is highlighted. Overall, this comprehensive review provides valuable insights into the intricate world of polymers in orthopaedic applications.

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Polymers in Orthopaedics: An In-depth Exploration of Material Properties and Structural Dynamics

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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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