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Material Properties

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Topic updated on 09/28/14 4:57pm
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 the load and the rate by which the load is applied 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
        • occurs at contact sites between two materials that are subject to micromotion
        • common at the head-neck junction in hip arthroplasty
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 
    • 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 orthopeadic 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)
        • liquid
          • monomer
          • DMPT (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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Qbank (20 Questions)

TAG
(OBQ13.134) Which of the following is a potential cause of fretting corrosion? Topic Review Topic

1. The micromotion at the femoral head-neck junction in a modular total hip replacement
2. A stainless-steel cerclage wire is in contact with a titanium-alloy femoral stem
3. Friction between polyethylene liner and femoral head leading to osteolysis
4. The formation of pits within a stainless-steel plate and the subsequent release of metal ions
5. The formation of an adherent oxide coating on titanium implants

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TAG
(OBQ13.189) Which of the following statements is true regarding polymethylmethacrylate (PMMA)? Topic Review Topic

1. Barium sulfate initiates the polymerization of monomethacrylate
2. It is a ductile material
3. The Young's modulus is between cortical and cancellous bone
4. It resists shear better than compressive forces
5. The polymerization of PMMA is endothermic

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TAG
(OBQ12.76) Compared to cold-forged cobalt chrome, titanium alloys have which property? Topic Review Topic

1. Increased fatigue strength
2. Increased yield strength
3. Increased endurance limit
4. Decreased ductility
5. Decreased tensile strength

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TAG
(OBQ11.86) Which of the following definitions best describes the phenomenon of load relaxation? Topic Review Topic

1. Constant loading causing material to continue to deform over time
2. Stress at failure (the ultimate stress) divided by the strain at failure (the ultimate strain)
3. Decreased peak loads over time with the same amount of elongation
4. Stress is proportional to strain up to a limit
5. Strain divided by the time that the load is applied

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TAG
(OBQ11.104) Which of the following best describes plastic deformation? Topic Review Topic

1. Change in length of a material under loading that returns to its original length once the load is removed
2. Progressive deformation of a material in response to a constant force over an extended period
3. The ability of a material to resist deformation
4. Change in length of a material under loading that does not return to the original length once the load is removed
5. The relative measure of the deformation of an object due to a load

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TAG
(OBQ11.111) When discussing metal implants and devices, which of the following best describes fatigue? Topic Review Topic

1. Load at which a material fractures
2. Progressive deformation due to a constant force over an extended period
3. Change in the stress-strain relationship dependent on the rate of loading
4. Failure at a submaximal tensile strength level after numerous loading cycles
5. Change in mechanical properties as a result of the direction of a load

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TAG
(OBQ11.121) Which of the following statements defines creep, as it relates to material properties? Topic Review Topic

1. Progressive deformation response to constant force over an extended period of time
2. A solid material's ability to deform under tensile stress
3. The ability of a materials mechanical properties to vary according to the direction of load
4. The rupture of a material under repeated cyclic stresses, at a point below the normal static breaking strength
5. The ability of a material to absorb energy and plastically deform without fracturing

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TAG
(OBQ10.58) The elements chromium, molybdenum, and cobalt are basic components of which of the following implant materials? Topic Review Topic

1. Aluminum oxide
2. Cobalt alloy
3. Stainless steel
4. PMMA
5. Tantalum

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TAG
(OBQ09.47) What description below best describes galvanic corrosion? Topic Review Topic

1. corrosion resulting from an electrochemical potential created between two metals in conductive medium
2. corrosion resulting from contact sites between materials under load
3. corrosion resulting from oxygen tension differences
4. corrosion from localized pits on metal surfaces
5. corrosion from allergic reaction

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TAG
(OBQ09.115) Which of the following defines the stress at which a material begins to undergo plastic deformation? Topic Review Topic

1. Toughness
2. Ultimate strength
3. Yield strength
4. Fatigue strength
5. Endurance limit

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TAG
(OBQ08.155) Bone is biomechanically weakest to resistance of which of the following forces? Topic Review Topic

1. Tension
2. Compression
3. Inertia
4. Centripetal
5. Shear

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TAG
(OBQ08.191) Which of the following most accurately describes stainless steel? Topic Review Topic

1. Composed of iron-carbon alloy, modulus of elasticity less stiff than bone
2. Composed of cobalt-chrome-molybdenum alloy, modulus of elasticity more stiff than bone
3. Composed of iron-carbon alloy, modulus of elasticity more stiff than titanium
4. Composed of cobalt-chrome-molybedenum alloy, modulus of elasticity less stiff than titanium
5. Composed of iron-carbon alloy, modulus of elasticity is more stiff than bone, cobalt-chrome, and aluminum-oxide (ceramic)

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TAG
(OBQ08.243) Which of the following best describes the process of galvanic corrosion? Topic Review Topic

1. Degradation from exposure to a harsh environment
2. Differences in oxygen tension within and outside of a crevice
3. Micromotion between material when under a load
4. Free radical oxidation
5. Electrochemical potential created between two metals in physical contact when immersed in a conductive medium

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TAG
(OBQ08.259) A typical load-elongation curve of a ligament is shown in Figure A. What region of the curve represents elastic deformation occurring after the crimped ligament fibrils have been straightened? Topic Review Topic
FIGURES: A          

1. A
2. B
3. C
4. D
5. E

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TAG
(OBQ06.211) Low toughness is a disadvantage of which of the following bearing surfaces used in total hip arthroplasty? Topic Review Topic

1. Cobalt chromium
2. Titanium
3. Ceramic
4. Polyethylene
5. Stainless steel

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TAG
(OBQ05.59) Ligaments are viscoelastic, meaning that their tensile strength is affected by: Topic Review Topic

1. Torsion and tension only
2. Orientation of applied strain
3. Rate of applied load
4. Compression only
5. Tension only

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TAG
(OBQ05.183) Which of the following materials has a Young's modulus of elasticity that is most similar to cortical bone Topic Review Topic

1. Titanium
2. Zirconia
3. Stainless steel
4. Ceramic (Al2O3)
5. Alloy (Co-Cr-Mo)

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TAG
(OBQ04.181) Which of the following materials is most susceptible to galvanic corrosion? Topic Review Topic

1. Titanium
2. Zirconia
3. Polyethylene
4. Cobalt-chromium
5. Alumina

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TAG
(OBQ04.202) An 18-year-old female soccer player sustains a non-contact deceleration injury while making a sharp pivot to strike the ball. She hears a loud pop in her knee, is unable to bear weight initially following the injury, and develops an immediate knee effusion. The structure most likely injured in this athlete exhibits all of the following biomechanical properties EXCEPT: Topic Review Topic

1. Viscoelasticity
2. Creep
3. Isotropism
4. Stress relaxation
5. Nonlinear elasticity

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TAG
(OBQ04.238) When analysing complex geometric form and material property distributions, the structure of interest may be divided up into numerous connected subregions or elements within which approximate functions are used to represent the unknown quantity. What is the name for this technique? Topic Review Topic

1. Breakdown synthesis
2. Finite element method
3. Algebraic conclusion
4. Differential equations
5. Isogeometric analysis

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