Biomaterials

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Biomaterials
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Fundamentals of Material Science
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Elastic DeformationStress

• Internal reaction to externally applied force
  (equal to the applied force in magnitude but
  opposite direction)
• Distributed over the cross-sectional area of the
  specimen
• Stress (?) Force / Area (N/m2) (Pa)

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Elastic DeformationStress

• Resolved into three types
• Tension
• Compression
• Shear

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Bending Stress in a cylinder
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Elastic DeformationStrain

• Stress produces deformation. The measurement of
  deformation, normalized by the original length is
  called strain
• Strain (?) Change in length/Original length
• (Deformed length original length)/ original
  length
• (DL-OL)/OL
• Strain is dimensionless

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Elastic DeformationStress Strain Curve

• Plot of load versus deformation
• Stress on ordinate (y axis) strain on abscissa
  (x axis)
• Indicate the material properties of specimen
  tested

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Stress-Strain Curve
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Elastic DeformationModulus of Elasticity

• At low levels of stress there is a linear
  relationship between applied stress and the
  resultant deformation
• This proportionality is called modulus of
  elasticity or Youngs modulus
• Modulus of Elasticity (E) Stress/Strain
• It is a measure of the stiffness of the material

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Elastic Deformation

• Elastic limit
• Maximum stress that a material can withstand
  without permanent deformation or failure
• Ultimate Strength Strain
• The point at which the material fails

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Stress-Strain curves of Idealized Elastic
Materials
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Elastic DeformationArea under the curve

• The area under the stress-strain curve is the
  work required to produce the deformation (work
  expressed in Joules, a unit of energy)
• Energy put into deforming a purely elastic
  material to a point before failure can be
  recovered by removing the stress (Resilience)

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Work Energy in Elastic Material
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Plastic deformation

• Residual deformation remaining after all the
  initial stresses have been removed
• Yield strength of material is defined as the
  stress at which the material exhibits a specified
  deviation from linear proportionality between
  stress strain (yield point)
• Plastic deformation represents change in the
  molecular structure of the material and often
  indicates change in its material properties

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Stress-Strain curve of an idealized
Elastic-Plastic Material
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Ductility

• Brittle material fracture or fail before they
  undergo any permanent deformation. They have a
  straight stress-strain curve.
• Examples Ceramic, bone
• Ductile material reach a yield point and then
  undergo deformation before failure.
• Examples Aluminum, ligament, capsule

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Toughness

• The area under both the elastic plastic portion
  of the stress-strain curve is the total energy
  required to stress the material to a point of
  failure. This is the measure of the materials
  toughness.

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Toughness
Stiff Brittle
Flexible ductile
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Stress-Strain curves of idealized materials with
various combinations of material properties
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Fatigue

• Repeated loading and unloading (cyclical loading)
  of a material will cause it to fail, even if the
  loads are below the ultimate stress
• The fatigue life of a material is recorded on a
  curve of stress (s) versus number of cycles, or
  S-N curve
• Endurance limit of a material is a level of
  stress tolerated for extended period of time

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S-N curve
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Fatigue failure

• Two stages
• Crack Initiation
• Occurs at the site of structural or geometric
  weakness (surface stretch, sharp changes in cross
  section)
• Crack Propagation

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Hardness

• Surface property
• Ability to resist plastic deformation at the
  material surface
• It is measured by pressing an indenter into a
  surface (Rockwell hardness scale)
• This is an important consideration for
  articulating surface in an artificial joint
  (ceramic on ceramic)

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Material behavior based on the direction of the
applied load

• Isotropic
• Material is called isotropic if its mechanical
  properties is identical in all the coordinates to
  an applied load
• Example Tennis ball
• Anisotropic
• Material that have different properties in
  different direction of the applied load
• All biological tissues (bone, cartilage,
  ligaments) are anisotropic

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Stress-Strain curves demonstrating anisotropic
behavior of cortical bone
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Viscoelasticity

• Mechanical properties of the material depends on
  the rate of loading or rate of strain
• At low strain rates the material flows like a
  viscous liquid. At high strain rates, the same
  material can behave as a elastic solid (Silly
  Putty)
• Exhibits properties of creep, stress relaxation
  hysteresis
• Most biological tissues are viscoelastic

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Stress-Strain curves to illustrate viscoelastic
behavior
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Creep

• Creep occurs when a viscoelastic material is
  subjected to the action of a constant load
• The material undergoes a rapid initial
  deformation followed by a slow (time-dependent),
  progressively increasing deformation know as
  creep, until an equilibrium is reached

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Stress relaxation

• SR occurs when a viscoelastic material is
  subjected to a constant deformation
• Typically there is a high initial stress followed
  by a slow (time-dependent) progressively
  decreasing stress required to maintain the
  deformation

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Hysteresis

• The elastic recovery of a viscoelastic material
  stressed below its yield point does not always
  coincide with the deformation curve
• The area between the two curves represents the
  energy that is dissipated (heat)
• This loss of strain energy is called hysteresis

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Hysteresis
Energy dissipated
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Fluids Viscosity

• Viscosity is the resistance of a fluid to flow.
  It characterizes the internal resistance of a
  fluid to shear deformation
• When viscosity of a fluid is independent of the
  shear rate it is said to exhibit Newtonian
  behavior
• Example Water, plasma

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Fluids Viscosity

• Shear thinning fluids exhibit nonnewtonian
  behavior where the viscosity decreases with
  increasing shear rates (the faster they are
  loaded the easier they flow)
• Example Synovial fluid, whole blood
• There is an inverse relationship between
  viscosity shear rates

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Viscosity versus Strain Rate
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Structural Properties of MaterialBending

• Neutral Axis
• When a beam is subjected to bending loads one
  side of the beam undergoes tensile load and the
  other side compressive. There are zero stresses
  in the center of the beam this is called as the
  neutral axis

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Structural Properties of MaterialBending
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Structural Properties of MaterialBending

• The magnitude of the tensile or compressive
  stress varies linearly with distance from the
  neutral axis
• The resistance to bending (areal moment of
  inertia) is created by the shape of the beam. It
  is independent of the material factors
• In a solid rod it is directly related to the 4th
  power of the radius

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¼ ? r4
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Structural Properties of MaterialTorsion

• Torsion applies shear load whose magnitude
  increases with the distance from the center of
  rotation

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Structural Properties of MaterialTorsion

• Mass further from the center of rotation imposes
  a greater restriction to the twisting than does
  equal amount of mass more centrally located
• This is reflected in polar moment of inertia
• Physiologic effect of mass location is seen in
  the diaphyseal expansion of bone with age

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1/2 ? r4
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Axial Load Sharing

• Two materials placed adjacent to one another in
  structural application (fracture fixation by
  plate) will carry part of the applied load
• The stress distribution will depend on the
  material properties, cross sectional areas of two
  material the nature of bond between them
• The less stiff material will be stress shielded

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Tribological PropertiesFriction

• When two material in contact are in relative
  motion, the resistance to the movement is called
  the frictional force
• The frictional force is directly proportional to
  the load across the interface
• µ (coefficient of friction) Ff / R (load)

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Friction
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Tribological PropertiesFriction
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Wear

• The essential feature of wearing process is loss
  of material from one of the bearing surfaces
• Several types of wear mechanisms
• Adhesive wear
• Abrasive wear
• Transfer wear
• Fatigue wear
• Third body wear
• Corrosive wear

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Adhesive Wear

• When two bodies slide against each other, small
  fragments of each surface adhere to the other
  surface. When subsequent movement occurs, the
  material breaks, not at the interface but through
  one of the material

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Abrasive wear

• When a rough material slides on a relatively soft
  surface, it can plow through the softer material
  and produce needles or curls of loose debris

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Fatigue wear

• Local strains gradients in the softer material
  may cause sufficient subsurface stress
  concentration to produce fatigue failure after
  repetitive or cyclical loading

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Third Body Wear

• Trapping of wear debris within the moving
  interfaces or the introduction of foreign
  particles, such as bone or PMMA produce local
  stress concentrations results in abrasion of one
  or both moving surfaces

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Corrosive wear

• Loss of substance as a result of chemical attack
• Corrosion accelerated by motion (fretting)
• Example Plates screws, Morse taper

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Wear of Total Joint Component

• Charnley reported wear rate of 0.15 mm per year
  for the acetabular cup
• Acetabular cup wear is primarily adhesive
• In total knees local contact stresses and
  subsurface fatigue wear becomes the dominant
  mechanism

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Corrosion

• Corrosion is gradual degradation of materials by
  electrochemical attack
• Corrosion weakens the implanted material, changes
  the surface of the material and releases metal
  ions into the body fluids
• Passivation is the process by which a metal is
  surface coated by the oxide of the metal leading
  to a decrease in the corrosion rate.

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Galvanic corrosion

• When two different metals are placed in contact
  in an electrolytic environment, one material
  gives up electrons to the other
• Metals in an implant system should not be mixed
• In case of Co-Cr titanium alloy, passivation
  (TiO2) prevents formation of galvanic couple

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Crevice corrosion

• Intense localized corrosion within crevices on
  metal surface
• Isolated areas of restricted fluid conduction
  leading to accelerated accumulation of positive
  ions with influx of Cl- to maintain
  electroneutrality leading to corrosion
• Example Beneath screw heads

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Pitting corrosion

• Extremely localized corrosion similar to crevice
  corrosion, starting at the defect in the passive
  surface layer
• Chromium, nickel molybdenum are added to
  stainless steel to increase resistance to pitting
  corrosion

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Fretting corrosion

• Corrosion occurring at contact areas between
  materials under load subjected to vibration and
  slip

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Metals In Orthopedics
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Material Processing its effect on structure

• Casting is pouring of molten metal into a mold
  to produce upon cooling specific shape. Voids and
  impurities are major problems with casting
  resulting area of structural weakness
• Forging is a process by which a block of metal is
  heated and pressed into a die by the application
  of a single force

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Material Processing its effect on structure

• Hot isostatic pressing (HIPing) involves the
  consolidation under high temperature and pressure
  of metal powder into fine grained material

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Stainless Steel

• Graded as 316 or 316L are used for clinical
  application
• 316L is an iron based alloy with chromium,
  nickel, molybdenum lt 0.03 carbon
• Alloying with chromium generates a protective
  layer of oxide that resists corrosion
• The elastic modulus is in the range of 200 GPa
  approximately 12 times that of cortical bone

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Cobalt-Chromium Alloys

• Cast Co-Cr alloys are weak and are not sufficient
  for high stress / high cyclical loading
• Forging HIPing can improve the yield strength
  fatigue life producing material with fewer
  defects

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Titanium

• Titanium is highly biocompatible
• Commercially pure titanium is not used in
  manufacturing implants, however it is the
  material of choice for fabrication of porous
  surface in cementless implants
• In clinical practice the titanium- aluminum
  vanadium alloy is used in implants (high strength
  to weight ratio)

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Titanium-AluminumVanadium alloy

• Elastic modulus of titanium (100 GPa) is half
  that of stainless steel Co-Cr.
• It is still six times that of cortical bone (17
  GPa)
• Notch sensitivity External stress riser shortens
  the fatigue life
• Because of notch sensitivity it is not a suitable
  material for porous coating
• High wear rate with UHMWPE not suitable for
  articular surface

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PolymersPolyethylene

• Plastic formed by polymerization of ethylene
  CH2CH2
• The dominant variable determining the functional
  properties of the molecular weight. Ultrahigh
  MWPE is used for acetabular cups tibia trays
• Radiation sterilization produces free radicals
  that can either combine to form cross links
  between the chains or oxidize starting a cascade
  of environmental degradation reactions

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PolymersPolyethylene

• Keeping radiation dosage low performing the
  sterilization in vacuum or nitrogen atmosphere
  can minimize the damage
• Subsurface fatigue has proved to be a major cause
  of polyethylene delamination in TKA

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PolymersPolymethylmethacrylate (PMMA)

• Polymerization Curing
• Powder contains 88 by weight polymer, 10 W/
  radio-opaque material (barium) 2 W/
  initiator (benzoyl peroxide)
• The liquid is the solution of the monomer (97
  W/), activator (demethyl-p-toludine, 2.5 W/)
  and a small amount of stabilizer
• The powder is sterilized by gamma radiation the
  liquid by ultrafiltration

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Polymethylmethacrylate

• Polymerization Curing
• The polymerization reaction is self-catalyzing
  exothermic
• The entire setting process is shortened by
  increased ambient temperature
• Increasing the relative amount of monomer
  increases the heat given off during
  polymerization, prolongs the setting time
  increases the amount of free monomer in the
  tissue (toxic)

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Curing curve of PMMA
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Polymethylmethacrylate

• Polymerization Curing
• Size of the cement mass determines the peak
  exothermic temperature
• If PMMA is heated too quickly or allowed to be
  come too hot, there is increased porosity with a
  resultant decrease in the mechanical strength
  (converse is true for chilling)
• Reduction in porosity also improves the physical
  properties of the cement. This is achieved by
  centrifugation or vacuum mixing

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Polymethylmethacrylate

• Mechanical Properties
• The endurance limit of PMMA in fatigue has been
  found to be higher in compression tests than in
  tension
• Early insertion of the acrylic cement while the
  viscosity is low, prevents laminations that
  significantly weakens the polymerized cement mass
• Pressurization of cement within the bone can
  enhance both the ultimate tensile compressive
  strength by 30

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Polymethylmethacrylate

• Mechanical Properties
• Presence of blood tissue inclusions, stress
  risers reduced thickness contribute to weak
  cement
• When polymerized at 37ºC, PMMA achieve 90 of its
  strength in 4 hours ultimate strength in 24
  hours
• PMMA is not a glue it is a grout. The bone-cement
  interface strengths are directly related to the
  surface area of fixation the degree of
  penetration (up to maximum of 5 mm)

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Polymethylmethacrylate

• Mechanical Properties
• If 1 gm of powder antibiotic is thoroughly mixed
  with powder component of bone cement, the final
  cement has 4 decrease in the ultimate
  compressive strength

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Ceramics

• Alumina Aluminum oxide (Al2O3)
• Zerconia Zirconium oxide (ZrO2)
• Excellent abrasion resistance with low surface
  roughness (low coefficient of friction)
• Used for articulating components in hip
  arthroplasties
• Brittle with low tensile strength very
  sensitive to microstructural flaws

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Casting Material

• Plaster
• Heating gypsum salt to decrease the naturally
  occurring water
• The setting reaction produces crystalline calcium
  sulfate dihydrate
• The reaction is exothermic
• A soft solid cast is produced within 4 5
  minutes with 35 to 50 of its ultimate strength

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Casting Material

• Fiberglass
• Polyurethane resin
• Water for polymerization
• Radiolucent, decreased weight, increased
  endurance water resistant

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Biodegradable materials

• Biodegradable refers to any material that breaks
  down when placed in a biologic environment
• Bioresorbable specifies a material that is not
  only broken down but also removed from the site
• Examples Polyglycolic acid (PGA) polylactic
  acid (PLA)
• Small incidence of inflammatory reaction in soft
  tissue applications. Intra-osseous reaction is
  uniformly benign

79
When a long bone is subjected to pure torsional
loads as shown in the Figure, the greatest
stresses (shear) are located

• at the neutral axis.
• within the cortical wall.
• on the ends near the grips.
• on the periosteal surface.
• on the endosteal surface.

80
Virtually all biological materials are
viscoelastic, which means that their mechanical
behavior is dependent on what factor?

• Load applied
• Cross-sectional area
• Rate of loading
• Mode of loading
• Direction of loading

81
The coefficient of friction between the
cobalt-chromium metal and the ultra-high
molecular weight polyethylene in the total
artificial hip joint is in the general range of

• 2.0 to 3.0.
• 0.5 to 1.0.
• 0.2 to 0.4.
• 0.05 to 0.15.
• 0.002 to 0.04.

82
A brittle material such as a ceramic femoral head
prosthesis undergoes what type(s) of deformation
when loaded to failure?

• Elastic and plastic
• Elastic
• Plastic
• Viscoelastic
• Viscoelastic and plastic

83
What is the minimum pore size (in ?m) of the
porous surface of a metallic implant that allows
cellular and extracellular elements of bone
growth?

• 25
• 100
• 750
• 1000
• 1500

84
Titanium, an extremely reactive metal, is one of
the most biocompatible implant materials because

• nothing in the biological environment reacts with
  titanium.
• physiologic conditions inhibit titanium
  reactions.
• proteins coat the titanium and insulate it from
  the body.
• titanium spontaneously forms a stable oxide
  coating.
• titanium alloys are less reactive than pure metal.

85
Which of the following materials is most likely
to undergo pitting and crevice corrosion in vivo?

• Alumina
• Zirconia
• CoCr alloy
• Ti6A14V
• 316L stainless steel

86
What term describes a material that has the same
mechanical properties in all directions?

• Elastic
• Homogenous
• Isotropic
• Orthogonal
• Uniform

87
Which of the following factors is most likely to
cause increased wear damage to an ultra high
molecular weight polyethylene (UHMWPE)
articulating surface?

• Ethylene oxide sterilization
• Third body inclusions
• Cold flow deformation
• Gamma radiation sterilization
• Ion implantation of the UHMWPE surface

88
Of the stress-strain curves shown in the Figure,
which material has the greatest ultimate tensile
strength?

• A
• B
• C
• D
• E

89
Of the stress-strain curves shown in the Figure,
which material has the greatest ductility?

• A
• B
• C
• D
• E

90
One year after operative treatment of a hip
fracture, the radiograph in the Figure was
obtained. Failure of the device is most likely a
result of what factor(s)?

• Friction between the nail and barrel and the rate
  of loading
• Corrosion of the screws
• Magnitude of loading and number of loading cycles
• Low bone density and nail/barrel friction
• Rate of loading and number of load cycles

91
One year after operative treatment of a hip
fracture, the radiograph in the Figure was
obtained. Failure of the device is most likely a
result of what factor(s)?

• Friction between the nail and barrel and the rate
  of loading
• Corrosion of the screws
• Magnitude of loading and number of loading cycles
• Low bone density and nail/barrel friction
• Rate of loading and number of load cycles

92
What is the primary disadvantage of using
hydroxyapatite as bone graft substitute?

• Local tissue toxicity
• Slow biodegredation rate
• Incites a foreign body reaction
• Poor binding with bone
• Induction of immunologic response

93
A solid rectangular rod that measures 10 mm on
each side and 40 cm in length is subjected to
mechanical testing in compression. The
experimental data are shown in the table below.
What is the calculated elastic modulus of the
material?

• 200 Pa
• 200 MPa
• 200 GPA
• 1,000 Pa
• 10,000 Pa

94
Friction is defined as the resistance to sliding
motion of two objects in contact. For one body
to slide across another body in contact, it must
overcome a frictional force that is dependent on
the coefficient of friction and the

• area of contact.
• moment of inertia.
• normal load applied to the body.
• stiffness of the body.
• curvature of the surface.

95
What is the most likely mode of failure seen in a
fracture fixation device about which a nonunion
of the bone has developed?

• A single tensile overload
• A single compressive overload
• A single torsional overload
• Creep loading
• Fatigue loading

96
When a long bone is subjected to a bending
moment, the greatest tensile stresses are located

• within the cortex.
• at the neutral axis.
• at a periosteal surface.
• at an endosteal surface.
• along the bending axis.

97
What is the correct order (ranking) lowest to
highest) for the tensile modulus of elasticity of
the following materials?

• Trabecular bone, PMMA, cortical bone, titanium
  alloy, stainless steel
• Trabecular bone, cortical bone, PMMA, titanium
  alloy, stainless steel
• Trabecular bone, cortical bone, PMMA, stainless
  steel, titanium alloy
• Trabecular bone, PMMA, cortical bone, stainless
  steel, titanium alloy
• PMMA, trabecular bone, cortical bone, titanium
  alloy, stainless steel

98
Two intramedullary rods of the same length and
made from the same material differ in that one
has a 10 increase in its solid circular
cross-sectional area. What is the approximate
percent increase in the bending rigidity between
these two rods?

• 5
• 10
• 20
• 30
• 50

99
Experimentally, the degree of viscoelastic
behavior under a specific set of conditions can
be determined by measuring the stress-strain
curve at different

• specimen geometries.
• strain rates.
• strain levels.
• stress levels.
• torque levels.

100
Which of the following is considered the most
important factor that influences the torsional
strength of a hollow stainless steel
intramedullary rod?

• Wall thickness
• Shear modulus
• Cross-sectional diameter
• Yield stress
• Curvature of the rod

101
Most natural biologic materials are anisotropic,
meaning that their stress-strain curve exhibits

• different moduli for compressive and tensile
  tests.
• a high degree of nonlinearity.
• a high sensitivity to the size of the test
  specimen.
• dependence on the rate of loading.
• dependence on the direction of load application.

102
What is the most likely mechanism of wear for the
acetabular implant shown in the Figure ?

• Adhesive
• Fatigue
• Third body
• Abrasive
• Corrosive

103
Which of the following variables most influences
the volumetric wear of polyethylene occurring on
secondary surfaces (backside wear) in modular
total hip and total knee components?

• Total contact area
• Roughness of the metal surface
• Composition of the metal surface
• Magnitude of the load
• Relative motion

104
Which of the following factors is most commonly
associated with late aseptic loosening of
cemented acetabular components?

• Increased frictional torque
• Recurrent neck-socket impingement
• Fatigue failure of cement
• Poor initial component fixation
• Polyethylene wear

105
Contact stresses in the bearing surfaces of
polyethylene tibial components in total knee
replacement prostheses can be minimized to reduce
wear by designing

• thinner polyethylene inserts
• all-polyethylene components without a tray.
• components with greater flexibility.
• lesser sagittal plane conformity.
• greater frontal plane (medial-lateral) conformity.

106
Ceramic materials, such as zirconia and alumina,
are typically inert and act as electrical and
thermal insulators. The atomic basis for this
behavior is that the bonds holding ceramic
materials together are mostly

• ionic bonds that localize the binding electrons
• ionic bonds that are highly directional.
• covalent bonds that tie up outer shell electrons.
• van der Waals bonds that require close nuclear
  proximity.
• metallic bonds that produce close-packed
  structures.

107
Which of the following processes will most
greatly increase the wear damage to an ultrahigh
molecular weight polyethylene articulating
surface?

• Ethanol sterilization
• Third body inclusion
• Cold flow deformation
• Gamma radiation sterilization
• Ion implantation on the mating metallic surface

108
The clinical use of bioresorbable polymeric
materials (eg, poly(lactic acid), poly(glycolic
acid)) in fracture devices is currently limited
most by which of the following factors?

• Rate of degradation
• Release of debris
• Biocompatibility
• Mechanical strength
• Availability

109
Cross-linking of the polymer chains of ultrahigh
molecular weight polyethylene has been shown to
improve wear resistance in in vitro hip simulator
studies. Other physical and mechanical
properties are also altered by cross-linking.
Which of the following properties is considered
the major concern in using cross-linked ultrahigh
molecular weight polyethylene in total joint
replacements?

• Elastic modulus
• Percent of crystallinity
• Fracture resistance
• Yield stress
• Molecular weight

110
Which of the following statements best describes
why titanium, an extremely reactive metal, is
considered on of the most biocompatible implant
materials?

• The body has no mechanism to degrade titanium.
• Macrophages surround and isolate the implant.
• Titanium has a very slow corrosion rate.
• Titanium is implanted as an alloy with aluminum
  and vanadium
• Titanium forms titanium dioxide on its surface.

111
At higher rates of loading, bone absorbs more
energy prior to failure because

• the modulus of elasticity decreases.
• bone is anisotropic
• bone is viscoelastic.
• bone deforms plastically.
• bone is stronger in compression than in tension.

112
Which of the following design parameters has the
most favorable impact on polyethylene wear rate
following total knee replacement?

• Grit-blast surface finish on the tibial tray
• Increased medial-lateral articular surface
  conformity
• Heat-pressed polyethylene surface finish
• Titanium counterface-bearing surface
• Carbon-fiber inclusion within the polyethylene

113
Which of the following material combinations has
the lowest coefficient of friction?

• Steel/steel
• High-density polyethylene/steel
• High-density polyethylene/cobalt chrome
• High-density polyethylene/titanium
• Hyaline cartilage/hyaline cartilage

114
In a fatigue test, the maximum stress under which
the material will not fail, regardless of how
many loading cycles are applied, is defined as

• endurance limit.
• failure stress
• critical stress.
• yield stress.
• elastic limit.

115
A biologic or an artificial material in which the
direction of loading does not influence its
mechanical properties can be defined as

• isotropic
• anisotropic.
• homogeneous.
• nonhomogeneous.
• orthotropic.

116
The change over time in strain of a material
under a constant load is defined as

• creep.
• relaxation.
• energy dissipation.
• plastic deformation.
• elastic deformation.

117
The bending stiffness of a slotted stainless
steel intramedullary nail will be increased most
by

• changing to a titanium nail.
• changing to a nonslotted nail.
• changing the cross-sectional shape of the nail.
• increasing the diameter of the nail by 3 mm.
• increasing the diameter of the interlocking
  screws.

118
What is the primary mechanism of wear of
polyethylene acetabular components?

• Crevice corrosion
• Oscillatory fretting
• Oxidative degradation
• Adhesion and abrasion
• Fatigue and delamination

119
What is the most important surface geometry
design parameter associated with decreased
contact stress and wear reduction in total knee
prostheses?

• Unrestrained roll-back
• Unrestrained rotational conformity
• Medial-lateral conformity
• Anteroposterior conformity in flexion
• Anteroposterior conformity in extension

120
Gamma ray irradiation for sterilization of
ultra-high molecular weight polyethylene in an
oxygen environment can have what effect on the
material?

• Increase stiffness
• Increase fracture toughness
• Increase fatigue strength
• Decrease mechanical strength
• Decrease wear rate

121
The white oxidation bands observed in
polyethylene components are associated with which
of the following sterilization techniques?

• Autoclaving
• Electron beam irradiation in nitrogen
• Gamma radiation in air
• Ethylene oxide sterilization
• Gas plasma sterilization

122
Which of the following properties is most
commonly associated with titanium alloy implants
when compared with cobalt-chromium alloys?

• Lower elastic modulus
• Lower corrosive resistance
• Better wear characteristics
• Lower notch sensitivity
• Greater hardness

123
Which of the following is considered a potential
advantage of using ceramic materials in total hip
arthroplasty?

• High surface roughness
• High wear resistance
• Brittle nature
• Low tensile strength
• Low cost