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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
        • occurs at contact sites between two materials that are subject to micromotion
        • common at the head-neck junction in hip arthroplasty
        • most common cause of mid-stem failure in modular revision type stems 
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)
        • 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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Questions (21)

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

QID:4769
1

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

71%

(1650/2315)

2

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

13%

(296/2315)

3

Friction between polyethylene liner and femoral head leading to osteolysis

3%

(72/2315)

4

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

10%

(231/2315)

5

The formation of an adherent oxide coating on titanium implants

2%

(47/2315)

Select Answer to see Preferred Response

PREFERRED RESPONSE 1

Micromotion at the femoral head-neck junction can lead to fretting corrosion, one of the most common causes of failure of a modular implant.

Modular components give surgeons excellent intraoperative flexibility, but are susceptible to various types of corrosion. While titanium and cobalt-chrome contain a protective surface oxide layer, continued micromotion at the modular junction may disrupt the protective layer leading to fretting corrosion, defined as micromotion at contact sites under load. This may eventually lead to a painful synovitis that necessitates a revision procedure.

Srinivasan et al. review modularity in total hip arthroplasty. Amongst other things, they discuss the modularity of the femoral head/neck junction, describing the morse taper interlocking system that provides both axial and rotational stability.

Illustration A shows an example of corrosion at the head/neck junction of a total hip arthroplasty.

Incorrect Answers:
Answer 2: This is an example of galvanic corrosion, as two dissimilar metals are in contact with each other.
Answer 3: This is an example of adhesive wear.
Answer 4: This is an example of pitting corrosion, or crevice corrosion.
Answer 5: This process is called self-passivization, enabling titanium to become corrosion resistant.

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

QID:4824
1

Barium sulfate initiates the polymerization of monomethacrylate

12%

(243/2034)

2

It is a ductile material

5%

(103/2034)

3

The Young's modulus is between cortical and cancellous bone

77%

(1571/2034)

4

It resists shear better than compressive forces

3%

(52/2034)

5

The polymerization of PMMA is endothermic

2%

(49/2034)

Select Answer to see Preferred Response

PREFERRED RESPONSE 3

Young's modulus measures the ability of a material to resist deformation. PMMA has a Young's modulus between cortical and cancellous bone.

PMMA, or bone cement, is frequently used in joint replacement surgery, tumor surgery, and spine surgery. Bone cements are provided as two-component materials, a powder and a liquid. The powder usually contains a polymer, benzoyl peroxide (initiator), and barium sulfate (radio-opacifier). The liquid usually contains a monomer, DMPT (accelerator), and hydroquinone (stabilizer). The two components are mixed and a free radical polymerization occurs when the initiator is mixed with the accelerator.

Webb and Spencer review the current uses and limitations of polymethylmethacrylate in orthopaedic surgery. Amongst other things, they describe PMMA as a brittle, notch-sensitive material with a modulus of elasticity 10x lower than cortical bone and 100x lower than metal stems used as femoral components.

Illustration A shows the components of bone cement.

Incorrect Answers:
Answer 1: Benzoyl peroxide is the initiator when the liquid monomer (monomethacrylate) is added to polymer powder (polymethylmethacrylate).
Answer 2: PMMA is considered brittle, meaning that it exhibits linear stress stain relationship up until the point of failure.
Answer 4: PMMA resists compression quite well, but handles shear forces poorly.
Answer 5: The polymerization of PMMA is exothermic, meaning it gives off heat.

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

QID:4436
1

Increased fatigue strength

13%

(422/3149)

2

Increased yield strength

11%

(332/3149)

3

Increased endurance limit

8%

(266/3149)

4

Decreased ductility

7%

(232/3149)

5

Decreased tensile strength

60%

(1881/3149)

Select Answer to see Preferred Response

PREFERRED RESPONSE 5

Titanium implants have decreased tensile (ultimate) strength when compared to cobalt chrome.

Ultimate strength, or tensile strength, is the maximum stress a material can withstand before undergoing breakage or failure. The ranking of ultimate strength, from highest to lowest is: 1) cobalt chrome, 2)titanium, 3)stainless steel, and 4) cortical bone.

Young's modulus of elasticity is defined as the measure of stiffness of a material in the elastic zone. A higher Young's modulus indicates a stiffer material. While titanium is highly biocompatible with a low modulus of elasticity (Young's modulus), it has poor wear characteristics making it non-suitable for femoral heads in total hip arthroplasty.

Long et al. present a review on titanium implants with a focus on bio-mechanical properties. Their study supports previous data which showed high rates of ultra-high molecular weight polyethylene wear due to accelerated breakdown when in contact with a titanium surface.

Incorrect Answers:
Answer 1: Fatigue strength, or the maximum cyclic load (10 million cycles) that a standard sized metal can absorb before fracture, is lower in titanium compared to cobalt chrome.
Answer 2: Yield strength, or the maximal stress a material can take before permanent deformation, is decreased in titanium compared to cobalt chrome.
Answer 3: Endurance limit is another way of saying fatigue strength, which is discussed in incorrect answer 1.
Answer 4: Ductility, or the measure of how much strain a material can take before rupturing, is higher for titanium than cobalt chrome


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

QID:3509
1

Constant loading causing material to continue to deform over time

21%

(202/949)

2

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

1%

(10/949)

3

Decreased peak loads over time with the same amount of elongation

71%

(675/949)

4

Stress is proportional to strain up to a limit

3%

(24/949)

5

Strain divided by the time that the load is applied

4%

(35/949)

Select Answer to see Preferred Response

PREFERRED RESPONSE 3

Load relaxation is characterized by decreased peak loads over time with the same amount of elongation.

Screen performed a study on tenocytes and tendon fascicles. It was found that viscoelasticity and relaxation behavior within isolated tendon fascicles is dominated by fiber sliding mechanisms and proteoglycans have a role in the mechanisms of strain transfer within the tendon.

Incorrect Answers:
Answer 1: Creep is defined as the constant loading causing material to continue to deform over time
Answer 2: Young's modulus is defined as the stress divided by the strain. It is important to note that this only applies during the linear portion of the stress/strain curve (during elastic behavior).
Answer 4: Hooke's law states that stress is proportional to strain up to a limit
Answer 5: Strain rate is defined as the strain divided by the time that the load is applied


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

QID:3527
1

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

10%

(168/1760)

2

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

4%

(69/1760)

3

The ability of a material to resist deformation

0%

(6/1760)

4

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

85%

(1497/1760)

5

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

1%

(15/1760)

Select Answer to see Preferred Response

PREFERRED RESPONSE 4

Plastic deformation is defined as an irreversible change in length after removing the load during the plastic range on a stress-strain curve.

The stress-strain curve is found in Illustration A. Objects in the elastic zone of the curve will return to their normal shape when the load is removed. This is termed elastic deformation. Objects in the plastic zone will not return to their normal shape when the load is removed. This is termed plastic deformation. The yield point marks the transition between the elastic and plastic zones.

Incorrect Answers:
Answer 1: This is the definition of elastic deformation.
Answer 2: This is the definition of creep.
Answer 3: This is Young's Modulus.
Answer 5: This is the definition of strain.

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

QID:3534
1

Load at which a material fractures

5%

(57/1180)

2

Progressive deformation due to a constant force over an extended period

7%

(80/1180)

3

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

3%

(36/1180)

4

Failure at a submaximal tensile strength level after numerous loading cycles

83%

(981/1180)

5

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

2%

(22/1180)

Select Answer to see Preferred Response

PREFERRED RESPONSE 4

Fatigue is a characteristic of metal defined as failure below the ultimate tensile strength after numerous loading cycles.

Bong et al reviewed the biomechanics of lower extremity intramedullary nailing. They detailed the intrinsic (material properties, cross-sectional shape, anterior bow, diameter) properties and extrinsic (reaming, comminution and locking screws) properties on nail biomechanics.

Hou et al investigated the effects of design and microstructure of tibial screws on nail biomechanics. They tested the mechanical strength of a both-ends-threaded screw and an unthreaded bolt and compared them to five commercially available screws in 3-point bending. As the main cause of failure was mechanical overloading, they concluded that screw thread removal could increase the fatigue life of interlocking devices.

Incorrect answers:
Answer 1: Load at which a material fractures is ultimate strength
Answer 2: Progressive deformation due to a constant force over an extended period is creep
Answer 3: Change in the stress-strain relationship dependent on the rate of loading is viscoelasticity
Answer 5: Change in mechanical properties as a result of the direction of a load describes an anisotropic property


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Question COMMENTS (1)

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

QID:3544
1

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

89%

(1349/1519)

2

A solid material's ability to deform under tensile stress

3%

(42/1519)

3

The ability of a materials mechanical properties to vary according to the direction of load

1%

(22/1519)

4

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

2%

(28/1519)

5

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

5%

(69/1519)

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PREFERRED RESPONSE 1

Creep is the tendency of a solid material to move slowly or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material. Creep is an undesirable property of orthopaedic bio-materials because they release frictional forces necessary to maintain rigid internal fixation. In total hip arthroplasty polyethylene liners, creep is the plastic deformation of the acetabular liner that occurs due to loading without the production of wear debris or particles.

Incorrect Answers:
2-This is the definition of ductility
3-This is the definition of anisotropy. Bone is anisotropic.
4-This is the definition of fatigue failure
5-This is the definition of toughness.


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

QID:3146
1

Aluminum oxide

0%

(5/1780)

2

Cobalt alloy

86%

(1535/1780)

3

Stainless steel

12%

(215/1780)

4

PMMA

0%

(7/1780)

5

Tantalum

1%

(17/1780)

Select Answer to see Preferred Response

PREFERRED RESPONSE 2

Cobalt alloys are extremely strong and are well-suited to applications requiring longevity. Strength of the implant is improved by the addition of molybdenum. Corrosion resistance is addressed by the addition of chromium, which also increases the hardness of the implant.

Incorrect Answers:
Answer 1: Aluminum oxide (Al2-03) is a ceramic used in bearing surface applications.
Answer 3: Stainless steel is an iron-carbon alloy, which also has silicon, manganese, molybdenum, and chromium in lesser amounts. It is much more susceptible to both galvanic and crevice corrosion than cobalt alloys.
Answer 4: PMMA is a cement made of poly-methyl-methacrylate.
Answer 5: Tantalum is very resistant to corrosion, and is often used in implants where bony ingrowth is desired.


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

QID:2860
1

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

97%

(1262/1307)

2

Corrosion resulting from contact sites between materials under load

1%

(19/1307)

3

Corrosion resulting from oxygen tension differences

1%

(13/1307)

4

Corrosion from localized pits on metal surfaces

1%

(10/1307)

5

Corrosion from allergic reaction

0%

(3/1307)

Select Answer to see Preferred Response

PREFERRED RESPONSE 1

There are many modes of corrosion in orthopaedic implants and galvanic corrosion is a type of corrosion which results from an electrochemical potential created between two metals in a conductive medium.

Galvanic corrosion is often seen at the interface of metals (e.g plates and screws) when different metals are used. The conductive medium is usually serum or interstitial fluid.

Incorrect Answers:
Answer 2 describes fretting corrosion and answer 3 describes crevice corrosion. Answer 4 describes pitting corrosion and answer 5 does not make sense.


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Question COMMENTS (3)

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

QID:2928
1

Toughness

2%

(15/768)

2

Ultimate strength

1%

(10/768)

3

Yield strength

88%

(674/768)

4

Fatigue strength

8%

(59/768)

5

Endurance limit

1%

(9/768)

Select Answer to see Preferred Response

PREFERRED RESPONSE 3

A material undergoing plastic deformation will not return to its original form once the stress is removed. This occurs once a material has been subject to stress past the yield strength, also called the yield point. Prior to the yield point, elastic deformation occurs, and the material will return to its original form once the stress is removed. Illustration A demonstrates these definitions in the classic stress-strain curve. The amount of energy a material can absorb before failure is defined as toughness. Ultimate strength is the highest point on the stress-strain curve. It represents the maximum stress a material can absorb while being stretched before "necking", when the cross-sectional area of the material begins to contract. Fatigue strength refers to cyclic testing of a material. Also called fatigue limit or endurance limit, it is the amount of cyclic stress that can be applied to a material before failure. Endurance limit has also been used to define the maximum level of stress that can be applied to a material cyclically and never cause failure.

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

QID:541
1

Tension

26%

(102/388)

2

Compression

2%

(6/388)

3

Inertia

1%

(2/388)

4

Centripetal

6%

(23/388)

5

Shear

65%

(252/388)

Select Answer to see Preferred Response

PREFERRED RESPONSE 5

Bone is weakest in shear and strongest in compression. When a force creates a tensile stress in a particular region of a loaded bone, failure will occur in that region first. A transverse fracture occurs in a long bone that is subjected to pure bending. The convex portion of the bone is under tension and fails first, the fracture then propagates transversely. A butterfly fragment results from a combination of bending (transverse) and compression (oblique/shear) as the ends of the failing bone are driven together. The production of a butterfly fragment likely depends on the rate and magnitude of the applied load.


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

QID:577
1

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

3%

(9/265)

2

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

13%

(34/265)

3

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

69%

(183/265)

4

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

4%

(10/265)

5

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

9%

(24/265)

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PREFERRED RESPONSE 3

Stainless steel is primarily an iron-carbon alloy with other elements including molybdenum, chromium, and manganese. Illustration A demonstrates Young's modulus of elasticity for multiple orthopaedic biomaterials. Stainless steel is stiffer than bone and titanium but less stiff than ceramics and cobalt-chrome. Titanium most closely emulates the modulus of elasticy of bone. Friedman, et al reviews the basic sciences of orthopaedic biomaterials.

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

QID:629
1

Degradation from exposure to a harsh environment

2%

(5/293)

2

Differences in oxygen tension within and outside of a crevice

2%

(5/293)

3

Micromotion between material when under a load

2%

(7/293)

4

Free radical oxidation

4%

(11/293)

5

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

90%

(264/293)

Select Answer to see Preferred Response

PREFERRED RESPONSE 5

Galvanic corrosion occurs when two dissimilar metals are in contact in an electrolyte solution. In this situation, there is an electrical potential difference resulting in a flow of electrons from the more active to the more noble metal. This results in a corrosive attack on the active metal (anode). It is most commonly seen at the screw-plate interface when used to treat fractures.

With regards to orthopaedic implants, it should be noted that stainless steel is highly susceptible to galvanic corrosion, and that the highest risk of galvanic corrosion is the combination of cobalt chromium and 316L stainless steel.

Hol et al. performed a study to determine the fretting corrosion between screws and plates made of dissimilar metals, namely titanium and stainless steel. They did not find an increase in fretting corrosion when combining the two different metals.

Incorrect Answers:
Answer 1: This type of corrosion, degradation corrosion, most commonly occurs with orthopaedic biomaterials such as polymers.
Answer 2: This refers to crevice corrosion. It most commonly occurs between the countersunk region of holes in plates and cementless acetabular components.
Answer 3: Fretting is physical movement (micro motion) of two plates against each other leading to mechanical wear and material transfer at the surface.
Answer 4: Free radical oxidation, or oxidative corrosion, is a chemical reaction involving a change in the oxidation state of polyethylene or metal.


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Question COMMENTS (2)

(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? Review Topic

QID:645
FIGURES:
1

A

8%

(113/1385)

2

B

77%

(1071/1385)

3

C

6%

(87/1385)

4

D

4%

(52/1385)

5

E

4%

(58/1385)

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PREFERRED RESPONSE 2

Region B represents elastic deformation (can return to original length if the load is removed) of the ligament where the collagen fibril backbone itself is stretched.

Incorrect Answers:
Region A represents elastic deformation during the "toe-region" of the load-elongation curve, where the load causes the the crimped collagen fibers in the ligament to stretch out.
Region C is the ultimate load.
Region D is the total energy absorbed
Region E is the ultimate elongation.


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

QID:222
1

Cobalt chromium

1%

(29/2076)

2

Titanium

25%

(527/2076)

3

Ceramic

46%

(955/2076)

4

Polyethylene

25%

(512/2076)

5

Stainless steel

2%

(48/2076)

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PREFERRED RESPONSE 3

Low toughness is a disadvantage of ceramic bearings in total hip arthroplasty.

Ceramic is a non-metal that demonstrates excellent wear characteristics when used with polyethylene in total hip arthroplasty. Although it has a high Young's modulus, it has a low fracture toughness. Subsequently, ceramic is poorly resistant to crack formation. In contrast, UHMWPE has a high fracture toughness because of the presence of very long hydrocarbon chains.

Santavirta et al. review alternative bearing materials to improve wear in total hip arthroplasty. Alumina ceramics are noted to be biostable and bioinert. The best wear properties are noted with ceramic-on-ceramic bearings. For current ceramic constructs, fracture risk is less than 1 per 1000.

Lang et al. review the use of ceramics in total hip replacement. The authors note that ceramic has high compressive strength and high wettability. Low fracture toughness and linear elastic behavior increase the risk of breakage of ceramic components under stress. Processing improvements, enhanced head-neck interfaces and liner modifications have lead to a decrease in the rate of ceramic fracture.

Illustration A shows a compromised ceramic head as a manifestation of the low fracture toughness of the material.

Incorrect Answers:
Answers 1, 2, 4, 5: Low fracture toughness is a characteristic of ceramic that risks component compromise during placement.

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

QID:945
1

Torsion and tension only

4%

(11/270)

2

Orientation of applied strain

17%

(45/270)

3

Rate of applied load

76%

(206/270)

4

Compression only

1%

(2/270)

5

Tension only

1%

(4/270)

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PREFERRED RESPONSE 3

Ligaments are viscoelastic material which means their stress-strain curve patterns are time/rate dependent (as a result of the internal friction).

The inital portion of the stress-strain curve, called the toe region, exhibits a high deformation/low force characteristic due to the uncrimping of collagen fibers and the elasticity of elastin. Next is the linear region where slippage within and then between collagen fibrils occurs. In this stage, ligaments gets stiffer (increased tensile strength) at higher strain rates.

Illustration A shows the different regions of the stress-strain curve.

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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 Review Topic

QID:1069
1

Titanium

91%

(722/792)

2

Zirconia

0%

(3/792)

3

Stainless steel

3%

(23/792)

4

Ceramic (Al2O3)

3%

(20/792)

5

Alloy (Co-Cr-Mo)

3%

(24/792)

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PREFERRED RESPONSE 1

Of the materials listed Titanium has an elastic moduli closest to cortical bone. Titanium is extra-ordinarily light, strong, highly ductile, and corrosion resistant. Titanium is however very notch sensitive and has poor wear resistance.

Young Modulus of Elasticity is defined as the stiffness (ability to maintain shape under external loading) of a material. On the stress vs. strain curve it is defined as the slope of the line in the elastic zone (see Illustration A). Young’s modulus is constant and different for each material. The relevant moduli (unit GPa) are approximated below:

1) UHMWPE = 0.8-1.5.
2) Cancellous Bone = 2
3) PMMA = 3.1
4) Cortical Bone = 18
5) Titanium = 115
6) Tantalum = 186
7) Stainless Steel = 240
8) Cobalt-Chromium Alloy = 240
9) Zirconia (Ceramic) = 248
10) Alumina = 340

Illustration A shows a stress vs. strain curve. Young Modulus of Elasticity is defined is defined as the slope of the line in the elastic zone

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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? Review Topic

QID:1168
FIGURES:
1

2

10%

(48/479)

2

3

17%

(83/479)

3

4

71%

(340/479)

4

5

1%

(4/479)

5

6

1%

(3/479)

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PREFERRED RESPONSE 3

The bending rigidity of a solid cylindrical pin is related to the fourth power of the pin’s radius.

A hollow, cannulated intramedullary nail has a bending rigidity related to the 3rd power. The rigidity of a fracture plate is proportional to the plate thickness to the third power. Thus, doubling the fracture plate thickness increases its bending stiffness 8 times.

The bending rigidity of an external fixator pin is proportional to the fourth power of the pin diameter. The bending stiffness of each pin is proportional to the third power of the bone-rod distance. However, the most important factor external fixator stability is for the the fracture ends to come into contact with each other.




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

QID:1286
1

Titanium

17%

(42/250)

2

Zirconia

2%

(4/250)

3

Polyethylene

4%

(9/250)

4

Cobalt-chromium

60%

(150/250)

5

Alumina

18%

(45/250)

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PREFERRED RESPONSE 4

Of the materials listed, cobalt-chromium is the only material that is most susceptible to galvanic corrosion. Galvanic corrosion is defined as intense localized electrochemical attack between two metal components exposed to corrosive environments.

Incorrect Answers:
Answer 1: Titanium has less galvanic corrosion than cobalt-chromium alloys because it chemically protects itself by a reaction called self-passivation, which is the formation of a protective surface oxide.
Answer 2 & 5: Zirconia and alumina are both ceramics, and are immune to metallic galvanic corrosion.
Answer 3: Polyethylene is a plastic polymer which is also immune to metallic galvanic corrosion.


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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: Review Topic

QID:1307
1

Viscoelasticity

10%

(29/297)

2

Creep

19%

(55/297)

3

Isotropism

54%

(161/297)

4

Stress relaxation

9%

(26/297)

5

Nonlinear elasticity

8%

(25/297)

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PREFERRED RESPONSE 3

The clinical presentation is consistent for an ACL tear. The ACL has the biomechanical properties of viscoelasticity, creep, stress relaxation, and nonlinear elasticity. It does NOT demonstrate isotropism. Isotropic materials such as metals exhibit the same mechanical properties in all directions.

All ligaments and tendons are anisotropic and exhibit different mechanical properties depending on the direction of the applied load. Ligaments are viscoelastic indicating they exhibit a time-dependent mechanical behavior. Thus, the relationship between stress and strain is not constant but depends on the time of displacement or load. One characteristic of viscoelasticity is creep, whereby there is an increasing deformation under constant load. Viscoelastic materials also exhibit stress relaxation whereby stress will be reduced or will relax under a constant deformation. Ligaments also demonstrate nonlinear elasticity (see illustration paragraph below).

Screen et al. studied viscoelasticity within isolated tendon fascicles. Their results provide further evidence of the complex anisotropic and viscoelastic nature of tendons. They conclude proteoglycans play an important functional role in controlling the viscoelastic behaviour and the mechanisms of strain transfer within tendon.

Illustration A demonstrates, nonlinear elasticity, which is another characteristic of ligaments. The toe region (labeled A in Illustration A) represents "un-crimping" of the crimp in the collagen fibrils. Since it is easier to stretch out the crimp of the collagen fibrils, this part of the stress strain curve (the "toe region") shows a relatively low stiffness. As the collagen fibrils become uncrimped, the collagen fibril backbone itself is being stretched (labeled B in Illustration A), which gives rise to a stiffer material. As individual fibrils within the ligament or tendon begin to fail damage accumulates, stiffness is reduced and the ligament/tendons begins to fail (labeled C in Illustration A).

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(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? Review Topic

QID:1343
1

Breakdown synthesis

14%

(50/357)

2

Finite element method

58%

(208/357)

3

Algebraic conclusion

2%

(8/357)

4

Differential equations

6%

(21/357)

5

Isogeometric analysis

19%

(68/357)

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PREFERRED RESPONSE 2

To solve a problem with complex geometric form and material property distributions, the finite element approach is used to break the problem up into smaller “finite elements” with simple geometric form. Usually triangular or quadrilateral elements are used. A computer program is written to balance the forces and moments acting on each element, and match these forces and moments with those of its neighboring elements. For large structures with a large number of elements, the computer must solve thousands of algebraic equations to make sure all the forces are balanced in the interior of the body and at the surface where the forces are applied. In orthopedics, stress analysis of the cement fixation of implants to bone is frequently carried out using finite element analysis.


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