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Introduction
  • Biomaterials encompasses all synthetic and natural materials used during orthopaedic procedures
  • Basic definitions
    • load
      • a force that acts on a body
    • stress
      • definition
        • intensity of an internal force
      • calculation
        • force / area
      • units
        • Pascal's (Pa) or N/m2
    • strain
      • definition
        • relative measure of the deformation of an object
      • calculation
        • change in length / original length
      • units
        • none
  • Mechanical property definitions
    • elastic deformation
      • reversible changes in shape to a material due to a load
      • material returns to original shape when load is removed
    • plastic deformation
      • irreversible changes in shape to a material due to a load
      • material DOES NOT return to original shape when load is removed
    • toughness
      • definition
        • amount of energy per volume a material can absorb before failure (fracture)
      • calculation
        • area under the stress/strain curve
      • units
        • joules per meter cubed, J/m3
    • creep
      • increased load deformation with time under constant load
    • load relaxation
      • decrease in applied stress under conditions of constant strain
    • hysteresis (energy dissipation)
      • characteristic of viseoelastic materials where the loading curve does not follow the unloading curve
      • the difference between the two curves is the energy that is dissipated
    • finite element analysis
      • breaking up a complex shape into triangular or quadrilateral forms and balancing the forces and moments of each form to match it with its neighbor
Material Strength: Stress vs Strain Curve
  • Derived from axially loading an object and plotting the stress verses strain curve
  • Elastic zone
    • the zone where a material will return to its original shape for a given amount of stress
    • "toe region"
      • applies to a ligaments stress/strain curve
      • represents straightening of the crimped ligament fibrils
  • Yield point
    • the transition point between elastic and plastic deformation 
  • Yield strength
    • the amount of stress necessary to produce a specific amount of permanent deformation
  • Plastic zone
    • the zone where a material will not return to its orginal shape for a given amount of stress
  • Breaking point
    • the object fails and breaks
  • Ultimate (Tensile) strength
    • defined as the load to failure
  • Hooke's law
    • when a material is loaded in the elastic zone, the stress is proportional to the strain
  • Young's modulus of elasticity 
    • measure of the stiffness (ability to resist deformation) of a material in the elastic zone
    • calculated by measuring the slope of the stress/strain curve in the elastic zone
    • a higher modulus of elasticity indicates a stiffer material
Young's Modulus of Metals and Biologics
  • Relative values of Young's modulus of elasticity (numbers correspond to numbers on illustration to right)  
    1. Ceramic (Al2O3)
    2. Alloy (Co-Cr-Mo)
    3. Stainless steel
    4. Titanium
    5. Cortical bone
    6. Matrix polymers
    7. PMMA
    8. Polyethylene
    9. Cancellous bone
    10. Tendon / ligament
    11. Cartilage

Material Descriptions
  • Brittle material 
    • a material that exhibits linear stress stain relationship up until the point of failure
    • undergoes elastic deformation only, and little to no plastic deformation
    • examples
      • PMMA
      • ceramics
  • Ductile Material 
    • undergoes large amount of plastic deformation before failure
    • example
      • metal
  • Viscoelastic material
    • a material that exhibits a stress-strain relationship that is dependent on duration of applied load and the rate by which the load is applied (strain rate) q
      • a function of the internal friction of a material
      • examples
        • ligaments
        • bone  
  • Isotropic materials
    • possess the same mechanical properties in all directions
      • example
        • golf ball
  • Anisotropic materials
    • possess different mechanical properties depending on the direction of the applied load
    • examples
      • ligaments
      • bone
Metal Characteristics
  • Fatigue failure
    • failure at a point below the ultimate tensile strength secondary to repetitive loading   
      • depends on magnitude of stress and number of cycles
  • Endurance limit
    • defined as the maximal stress under which an object is immune to fatigue failure regardless of the number of cycles
  • Creep  
    • phenomenon of progressive deformation of metal in response to a constant force over an extended period of time
  • Corrosion
    • refers to the chemical dissolving of metal. Types include
      • galvanic corrosion
        • dissimilar metals leads to electrochemical destruction q
        • mixing metals 316L stainless steel and cobalt chromium (Co-Cr) has highest risk of galvanic corrosion
        • can be reduced by using similar metal
      • crevice corrosion
        • occurs in fatigue cracks due to differences in oxygen tension
        • 316L stainless steel most prone to crevice corrosion 
      • fretting corrosion
        • description
          • a mode of destruction at the contact site from the relative micromotion of two materials or two components
        • clinical significance
          • common at the head-neck junction in hip arthroplasty
          • most common cause of mid-stem failure in modular revision type stems 
            • arthroplasty involving modular implants are at risk for fretting corrosion and failure between the components of the final implant 
            • increased risk with the increased number of interfaces between the various components
Specific Metals
  • Titanium
    • uses
      • fracture plates
      • screws
      • intramedullary nails
      • some femoral stems
    • advantages
      • very biocompatable
      • forms adherent oxide coating through self passivation
        • corrosion resistant
      • low modulus of elasticity makes it more similar to biologic materials as cortical bone
    • disadvantages
      • poor resistance to wear (notch sensitivity) (do not use as a femoral head prosthesis)
      • generates more metal debris than cobalt chrome
  • Stainless Steel (316L)
    • components
      • primarily iron-carbon alloy with lesser elements of
        • chromium
        • molybdenum
        • manganese 
        • nickel
    • advantages
      • very stiff
      • fracture resistant
    • disadvantages
      • susceptible to corrosion
      • stress shielding of bone due to superior stiffness
  • Cobalt alloy
    • components
      • cobalt
      • chromium
      • molybdenum
    • advantages
      • very strong
      • better resistance to corrosion than stainless steel
Specific Non-Metals
  • Ultra-high-molecular-weight polyethylene
    • advantages
      • tough
      • ductile
      • resilient
      • resistant to wear
    • disadvantages
      • susceptible to abrasion
        • wear usually caused by third body inclusions
      • thermoplastic (may be altered by extreme temperatures)
      • weaker than bone in tension
    • other
      • gamma irradiation
        • increases polymer chain cross-linking which improves wear characteristics
        • decreases fatigue and fracture resistance
  • Polymethylmethacrylate (PMMA, bone cement)
    • functions
      • used for fixation and load distribution in conjunction with orthopaedic implants
      • functions by interlocking with bone
      • may be used to fill tumor defects and minimize local recurrence
    • properties
      • 2 component material
        • powder
          • polymer
          • benzoyl peroxide (initiator)
          • barium sulfate (radio-opacifier)
          • coloring agent (green chlorophyll or blue cobalt)
        • liquid
          • monomer
          • DMPT (N,N-Dimethyl para-toluidine, accelerator)
          • hydroquinone (stabilizer)
    • advantages
      • reaches ultimate strength at 24 hours
      • strongest in compression
      • Young's modulus between cortical and cancellous bone
    • disadvantages
      • poor tensile and shear strength
      • insertion can lead to dangerous drop in blood pressure
      • failure often caused by microfracture and fragmentation
  • Silicones
    • polymers that are often used for replacement in non-weight bearing joints
    • disadvantages
      • poor strength and wear capability responsible for frequent synovitis
  • Ceramics
    • advantages
      • best wear characteristics with PE
      • high compressive strength
    • disadvantages
      • typically brittle, low fracture toughness 
      • high Young's modulus
      • low tensile strength
      • poor crack resistance characteristics
Bone
  • Bone composition
    • composed of collagen and hydroxyapatite
    • collagen
      • low Young's modulus
      • good tensile strength
      • poor compressive strength
    • hydroxyapatite
      • stiff and brittle
      • good compressive strength
  • Mechanical properties
    • advantages
      • strongest in compression
      • a dynamic structure
        • remodels geometry to increase inner and outer cortex to alter the moment of inertia and minimize bending stresses
    • disadvantages
      • weakest in shear
  • Failure (fracture)
    • tension
      • usually leads to transverse fracture secondary to muscle pull
    • compression
      • due to axial loading
      • leading to a crush type fracture
      • bone is strongest in resisting compression
    • bending
      • leads to butterfly fragment
    • torsion
      • leads to spiral fracture
      • the longer the bone the greater the stresses on the outer cortex under torsion
Ligaments & Tendons
  • Characteristics
    • viscoelastic with nonlinear elasticity
    • displays hysteresis (see definition above)
  • Advantages
    • strong in tension (can withstand 5-10% as opposed to 1-4% in bone)
  • Disadvantages
    • demonstrate creep and stress relaxation
 

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