Biomaterials

1
Biomaterials

• The objective of these lectures is to review the
  fundamental requirements for biomaterials used in
  biomechanical engineering applications.

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Material Attributes for Medical Applications

• Biocompatibilty
• Non-carinogenic, non-pyrogenic, non-toxic,
  non-allergenic, blood compatible,
  non-inflammatory
• Sterilizability
• Not destroyed or severely altered by sterilizing
  techniques such as autoclaving, dry heat,
  radiation, ethylene oxide
• Physical Characteristics
• Strength, toughness, elasticity,
  corrosion-resistance, wear-resistance, long-term
  stability
• Manufacturability
• Machinable, moldable, extrudable

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Biocompatibility

• Early Definition
• Lack of interaction between material and
  tissue
• Implies inert, non-toxic, non-carcinogenic,
  non-allergenic, non-inflammatory, non-degradable
• Thus, material has zero influence

4
Biocompatibility

• Contemporary Definition
• Ability of a material to perform with an
  appropriate host response, in a specific
  application
• Refers to a collection of processes and
  interdependent mechanisms of interaction between
  material and tissue
• Ability of material to perform and not just
  reside in the body
• Appropriate host response must be acceptable
  given the desired function
• Specific application must be defined

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Biocompatibility

• Specific application must also consider the time
  scale over which the host is exposed to the
  material

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Host Response

• Types of Reactions
• Normal wound healing response
• Protein adsorption ? Acute Inflammation ?
  Resolution
• Persistent Inflammation
• Acute ?? Chronic
• Effect of relatively reactive tissue environment
  on material (i.e. corrosion, degradation
  products)
• Possibility of remote or systemic effects
  (transient or chronic) if reaction products are
  transported away from implant site

7
Host Response

• Types of Reactions
• Infection (early or late onset)
• Osteolysis
• Neoplasia (cancer)

8
Biocompatibility Testing

• Considerations
• Type of device, principle tissue(s) in contact,
  period of implantation
• Tests for Chronically Implanted Devices
• In Vitro cytotoxicity, carcinogenicity,
  mutagenicity
• In Vivo pyrogenicity, systemic/acute toxicity
• Chronic Animal Implantation Studies
• (3 species for 6, 12 and 24 months)
• Human Clinical Trials

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Implantable Materials

• Metals
• Polymers
• Ceramics
• Composites

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Biomaterials Metals
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Biomaterials Metals
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Biomaterials Polymers
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Biomaterials Polymers
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Biomaterials Ceramics
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Biomaterials Ceramics
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Biomaterial Properties
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Corrosion

• Galvanic

Crevice
Stress-Corrosion Cracking
Fretting
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Galvanic Corrosion

• Electrochemical circuit between two dissimilar
  metals
• Anodic material is more basic and oxidizes
  (corrodes)
• Cathodic material is more noble and is protected

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Implant Fixation Methods

• No such thing as absolute rigidity since both the
  implant and the underlying bone are deformable.
• Some deformation will occur at the bone/implant
  interface and is only acceptable if
• Magnitude does not progressively increase
• Does not give rise to pain
• Does give rise to unacceptable quantities of
  debris
• Biological restrictions
• Cortical and cancellous bone are significantly
  weaker in tension and compression
• Fibrous tissue layer that is laid down at the
  bone/implant interface during initial healing
  phase is also weak in tension and shear
• Therefore, try to avoid tension and shear when
  condidering fixation method

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Implant Fixation Methods

• Interference Fits
• Can provide good fixation
• Bone remodeling can remove interference on which
  fixation depends and can lead to loosening

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Implant Fixation Methods

• Screws
• Do not ensure tightness regardless of how many
  screws are present and can result in loosening
• Crevice corrosion is a common problem under screw
  heads (observed in fracture fixation plates) and
  can lead to loosening
• Locally high contact stresses at bone/screw
  interface
• Only suitable for temporary fixation (e.g.
  fracture fixation)

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Implant Fixation Methods

• Bone Cement
• Gap filling agent
• Polymethylmethacrylate (PMMA) which is
  polymerized in situ
• Distributes load over largest possible area (low
  contact stresses)
• Provides mechanically interlocking between
  implant and cancellous bone
• Problems monomers are toxic, polymerization
  process is exothermic (gt50C) and cement is
  generally brittle

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Implant Fixation Methods

• Bone Ingrowth
• Porous coats, grooves and/or meshes
• Good for long-term fixation
• Relative motion must be restricted to ensure bone
  ingrowth
• Pore size has a distinct effect on the amount of
  ingrowth
• Common approach is to create a layer of partially
  sintered beads on the surface of the implant

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Wear

• In any system if there is contact and relative
  motion between two materials, then wear will
  occur.
• The extent of wear is the key issue in
  biomaterials
• Biological response to wear debris
• Degradation of implant ? premature failure
• Wear is still the major unsolved problem of joint
  replacements
• Early failures (lt 7 years for TKRs)
• Requires revision surgery (typically less
  effective than primary surgery)

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Wear

• Factors to consider
• Material Selection
• Select more wear resistant materials (e.g. Co-Cr
  gtgt Ti)
• Develop surface modifications (e.g. TiN)
• Materials Combinations
• Same (metal-on-metal)
• Mixed (metal-on-plastic)
• Contact Mechanics
• Loads (magnitude, static, dynamic)
• Mechanical properties of materials
• Geometry of contacting bodies (e.g. congruency)

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Wear

• Factors to consider
• Lubrication
• Lubricant properties
• Mechanism of lubrication (e.g. elastohydrodynamic)
  
• Surface Finish
• 2nd body wear, 3rd body wear
• Kinematics of Articulation
• Velocity, rolling/sliding
• Biological Response
• Bulk versus particulate debris

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Material Combinations (THRs)
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Mechanisms of Wear

• Flat surfaces, even those polished to a mirror
  finish, are not truly flat on an atomic scale.
  They are rough, with sharp, rough or rugged
  outgrowth peaks, termed asperities.
• Under compression, the asperities deform, leading
  to increased contact area (lower stresses) with
  higher coefficients of friction (µs, µd).
• Depending on how the asperities interact under
  relative motion, different wear mechanisms can
  occur.

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Mechanisms of Wear

• Fatigue
• Primarily related to one material (UHMWPE)
• Cyclic subsurface tension and compression
• Adhesive
• Related to two materials (metal UHMWPE)
• Surface energy between materials in contact
• Abrasive
• Related to three materials (metal, UHMWPE and
  debris)
• Hard, rough material removes soft material
• Combinations of above can occur

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Wear Testing

• Screening tests are typically used to reproduce
  the mechanisms of wear observed in retrieved
  implants in a controlled environment.
• Stimulators
• Pros actual implants used
• Cons difficult to model actual biomechanics
• Rotating Pin-on-Flat
• Pros simpler model than simulator
• Cons does not actually model kinematics/dynamics
  
• Reciprocating Pin-on-Flat
• Pros sliding motion modeled well (good for
  THRs)
• Cons does not actually model knee
  kinematics/dynamics

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Consequences of Wear

• Excessive wear can lead to premature failure of
  the component however, there can also be a
  biological response to the generated wear debris,
  such as inflammation and/or osteolysis.
• Osteolysis refers to the active resorption of
  bone tissue as part of a biological reaction to
  wear particles generated from artificial joint
  replacements. This process ultimately results in
  implant loosening and eventually requiring
  revision surgery.
• The magnitude of the osteolytic response is
  dependent on the nature of the wear particles
  generated
• chemical composition
• size (smaller particles have greater effect)
• shape (shaper particles have greater effect)

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Osteolysis
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Sterilization Methods

• Autoclave (Steam)
• High temperature process (121 134C)
• Commonly for repeat sterilization (e.g.
  instruments)
• Cheap
• Ethylene Oxide (EO)
• Low temperature process (for heat sensitive
  materials, e.g. UHMWPE)
• Residual gas can linger
• Environmental impact and occupational hazard
• Gamma Radiation
• Very effective
• Can cause polymer oxidation and crosslinking