Updated: 10/23/2021

# Material Properties

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• Definitions
• Biomaterials encompasses all synthetic and natural materials used during orthopaedic procedures
• Basic definitions
• 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
• decrease in applied stress under conditions of constant strain
• hysteresis (energy dissipation)
• 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
• 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
• 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
• 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
• very stiff
• fracture resistant
• susceptible to corrosion
• stress shielding of bone due to superior stiffness
• Cobalt alloy
• components
• cobalt
• chromium
• molybdenum
• very strong
• better resistance to corrosion than stainless steel
• Specific Non-Metals
• Ultra-high-molecular-weight polyethylene
• tough
• ductile
• resilient
• resistant to wear
• susceptible to abrasion
• wear usually caused by third body inclusions
• thermoplastic (may be altered by extreme temperatures)
• weaker than bone in tension
• other
• 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)
• reaches ultimate strength at 24 hours
• strongest in compression
• Young's modulus between cortical and cancellous bone
• 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
• poor strength and wear capability responsible for frequent synovitis
• Ceramics
• best wear characteristics with PE
• high compressive strength
• 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
• strongest in compression
• a dynamic structure
• remodels geometry to increase inner and outer cortex to alter the moment of inertia and minimize bending stresses
• weakest in shear
• Failure (fracture)
• tension
• usually leads to transverse fracture secondary to muscle pull
• compression
• 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)
• strong in tension (can withstand 5-10% as opposed to 1-4% in bone)
• demonstrate creep and stress relaxation
Flashcards (26)
Cards
1 of 26
Questions (45)

(OBQ20.123) Figures A and B demonstrate two different fracture patterns. What is the percent change in strain at fracture site Y relative to fracture site X when the fractures are unloaded?

QID: 215534
FIGURES:

50% increase

8%

(67/813)

80% decrease

46%

(377/813)

100% increase

9%

(74/813)

20% decrease

20%

(160/813)

No change

15%

(126/813)

L 4 E

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(SBQ18BS.11) Which of the following types of corrosion is defined by the formation of pits, grooves, and oxide debris due to the relative micromotion between two affixed materials placed under a load?

QID: 211222

Crevice

10%

(174/1823)

Fretting

62%

(1128/1823)

Galvanic

7%

(133/1823)

Pitting

17%

(314/1823)

Stress

3%

(56/1823)

L 3 A

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(OBQ18.152) Based on the stress-strain curve in Figure A, which of the following materials has the highest modulus of elasticity?

QID: 213048
FIGURES:

Ceramic

67%

(1180/1766)

PMMA

2%

(40/1766)

Titanium

3%

(59/1766)

Cancellous bone

1%

(14/1766)

Cartilage

26%

(453/1766)

N/A A

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(OBQ17.156) A 60-year-old patient presents to clinic complaining of left hip squeaking and stiffness following a ceramic-on-ceramic total hip replacement five years earlier. Initially, he was extremely pleased with the operation, but these symptoms of squeaking and stiffness have developed fairly abruptly over the past 6 months. On physical exam, he has a well-healed incision, a 1 centimeter leg length discrepancy (left shorter than right), decreased hip flexion, and decreased internal rotation. A radiograph is shown in Figure A. C-reactive protein is 0.2 mg/dL (nl <0.9 mg/dL) and erythrocyte sedimentation rate is 5 mm/hr (nl 0-15 mm/hr). What would you recommend for this patient?

QID: 210243
FIGURES:

Observation and physical therapy

7%

(147/2114)

Left hip corticosteroid injection

0%

(9/2114)

Left hip aspiration

7%

(155/2114)

Thorough debridement and two-stage revision

2%

(34/2114)

Thorough debridement and one-stage head and liner revision

83%

(1751/2114)

N/A A

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

QID: 4824

Barium sulfate initiates the polymerization of monomethacrylate

11%

(446/4130)

It is a ductile material

6%

(243/4130)

The Young's modulus is between cortical and cancellous bone

77%

(3161/4130)

It resists shear better than compressive forces

3%

(127/4130)

The polymerization of PMMA is endothermic

3%

(121/4130)

L 2 B

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

QID: 4769

The micromotion at the femoral head-neck junction in a modular total hip replacement

74%

(3230/4355)

A stainless-steel cerclage wire is in contact with a titanium-alloy femoral stem

11%

(471/4355)

Friction between polyethylene liner and femoral head leading to osteolysis

3%

(141/4355)

The formation of pits within a stainless-steel plate and the subsequent release of metal ions

9%

(411/4355)

The formation of an adherent oxide coating on titanium implants

2%

(71/4355)

L 3 B

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(SBQ12SP.45) Which of the following lists these materials in order of increasing modulus of elasticity?:

QID: 3743

Cortical bone; Titanium; Cobalt-chrome; Stainless steel; Ceramic

16%

(722/4630)

Titanium; Cortical bone; Ceramic; Cobalt-chrome; Stainless steel

5%

(233/4630)

Cortical bone; Titanium; Stainless steel; Cobalt-chrome; Ceramic

70%

(3218/4630)

Stainless steel; Titanium; Cortical bone; Ceramic; Cobalt Chrome

4%

(193/4630)

Cortical bone; Stainless steel; Titanium; Cobalt-chrome; Ceramic

5%

(244/4630)

L 3 B

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

QID: 4436

Increased fatigue strength

15%

(677/4508)

Increased yield strength

11%

(514/4508)

Increased endurance limit

8%

(378/4508)

Decreased ductility

8%

(339/4508)

Decreased tensile strength

57%

(2573/4508)

L 3 C

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

QID: 3544

Progressive deformation response to constant force over an extended period of time

87%

(2985/3422)

A solid material's ability to deform under tensile stress

3%

(88/3422)

The ability of a material's mechanical properties to vary according to the direction of load

2%

(61/3422)

The rupture of a material under repeated cyclic stresses, at a point below the normal static breaking strength

3%

(86/3422)

The ability of a material to absorb energy and plastically deform without fracturing

5%

(176/3422)

L 1 B

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

QID: 3509

Constant loading causing material to continue to deform over time

21%

(408/1907)

Stress at failure (the ultimate stress) divided by the strain at failure (the ultimate strain)

2%

(33/1907)

Decreased peak loads over time with the same amount of elongation

69%

(1309/1907)

Stress is proportional to strain up to a limit

3%

(60/1907)

Strain divided by the time that the load is applied

5%

(87/1907)

L 3 B

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

QID: 3527

Change in length of a material under loading that returns to its original length once the load is removed

10%

(281/2743)

Progressive deformation of a material in response to a constant force over an extended period

6%

(165/2743)

The ability of a material to resist deformation

0%

(10/2743)

Change in length of a material under loading that does not return to the original length once the load is removed

82%

(2246/2743)

The relative measure of the deformation of an object due to a load

1%

(28/2743)

L 1 B

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

QID: 3534

Load at which a material fractures

5%

(123/2550)

Progressive deformation due to a constant force over an extended period

10%

(254/2550)

Change in the stress-strain relationship dependent on the rate of loading

4%

(106/2550)

Failure at a submaximal tensile strength level after numerous loading cycles

79%

(2004/2550)

Change in mechanical properties as a result of the direction of a load

2%

(51/2550)

L 1 B

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

QID: 3146

Aluminum oxide

0%

(12/2482)

Cobalt alloy

85%

(2101/2482)

Stainless steel

13%

(317/2482)

PMMA

1%

(17/2482)

Tantalum

1%

(32/2482)

L 2 C

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

QID: 2860

Corrosion resulting from an electrochemical potential created between two metals in conductive medium

94%

(1914/2042)

Corrosion resulting from contact sites between materials under load

2%

(50/2042)

Corrosion resulting from oxygen tension differences

2%

(41/2042)

Corrosion from localized pits on metal surfaces

1%

(28/2042)

Corrosion from allergic reaction

0%

(4/2042)

L 1 C

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

QID: 2928

Toughness

2%

(27/1459)

Ultimate strength

2%

(24/1459)

Yield strength

85%

(1234/1459)

Fatigue strength

11%

(154/1459)

Endurance limit

1%

(13/1459)

L 2 C

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

QID: 577

Composed of iron-carbon alloy, modulus of elasticity less stiff than bone

3%

(30/890)

Composed of cobalt-chrome-molybdenum alloy, modulus of elasticity more stiff than bone

15%

(137/890)

Composed of iron-carbon alloy, modulus of elasticity more stiff than titanium

66%

(588/890)

Composed of cobalt-chrome-molybedenum alloy, modulus of elasticity less stiff than titanium

5%

(44/890)

Composed of iron-carbon alloy, modulus of elasticity is more stiff than bone, cobalt-chrome, and aluminum-oxide (ceramic)

9%

(83/890)

L 2 C

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(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?

QID: 645
FIGURES:

A

9%

(201/2193)

B

75%

(1644/2193)

C

6%

(134/2193)

D

4%

(98/2193)

E

5%

(107/2193)

L 1 C

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

QID: 629

Degradation from exposure to a harsh environment

2%

(15/994)

Differences in oxygen tension within and outside of a crevice

2%

(17/994)

Micromotion between material when under a load

4%

(44/994)

4%

(38/994)

Electrochemical potential created between two metals in physical contact when immersed in a conductive medium

88%

(876/994)

L 1 C

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

QID: 541

Tension

29%

(337/1165)

Compression

2%

(20/1165)

Inertia

1%

(13/1165)

Centripetal

6%

(66/1165)

Shear

62%

(724/1165)

L 3 D

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(SAE07HK.42) What property of titanium alloys accounts for their high corrosion resistance in vivo?

QID: 6002

Self-passivation

57%

(217/379)

Ductility

9%

(33/379)

Hardness

11%

(40/379)

Modulus of elasticity

9%

(33/379)

Conductivity

15%

(56/379)

N/A E

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

QID: 222

Cobalt chromium

1%

(45/3439)

Titanium

23%

(800/3439)

Ceramic

48%

(1664/3439)

Polyethylene

24%

(838/3439)

Stainless steel

2%

(80/3439)

L 4 C

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

QID: 945

Torsion and tension only

5%

(47/923)

Orientation of applied strain

18%

(170/923)

Rate of applied load

73%

(675/923)

Compression only

0%

(4/923)

Tension only

2%

(22/923)

L 1 D

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

QID: 1069

Titanium

88%

(1286/1458)

Zirconia

1%

(9/1458)

Stainless steel

4%

(55/1458)

Ceramic (Al2O3)

4%

(56/1458)

Alloy (Co-Cr-Mo)

3%

(50/1458)

L 1 C

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(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:

QID: 1307

Viscoelasticity

9%

(88/968)

Creep

23%

(226/968)

Isotropism

50%

(481/968)

Stress relaxation

8%

(79/968)

Nonlinear elasticity

9%

(87/968)

L 1 C

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(OBQ04.63) The bending rigidity of the implant shown in Figure A is proportional to what power of the measured radius of the implant?

QID: 1168
FIGURES:

2

9%

(153/1701)

3

20%

(339/1701)

4

69%

(1181/1701)

5

1%

(14/1701)

6

1%

(10/1701)

L 2 D

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

QID: 1286

Titanium

18%

(153/851)

Zirconia

4%

(33/851)

Polyethylene

3%

(25/851)

Cobalt-chromium

58%

(495/851)

Alumina

16%

(139/851)

L 3 D

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Evidence (28)
VIDEOS & PODCASTS (3)