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Updated: Sep 2 2024

Material Properties

Images
https://upload.orthobullets.com/topic/9062/images/biomaterials-key image.jpg
https://upload.orthobullets.com/topic/9062/images/elastic deformation.jpg
https://upload.orthobullets.com/topic/9062/images/creep.jpg
https://upload.orthobullets.com/topic/9062/images/Basic Stress Strain Curve_moved.jpg
  • Definitions
    • 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 
      • 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)
        • a function of the internal friction of a material
        • examples
          • ligaments
          • articular cartilage
    • 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
          • 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
          • titanium least 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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