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Which of the following is a potential cause of fretting corrosion?
The micromotion at the femoral head-neck junction in a modular total hip replacement
A stainless-steel cerclage wire is in contact with a titanium-alloy femoral stem
Friction between polyethylene liner and femoral head leading to osteolysis
The formation of pits within a stainless-steel plate and the subsequent release of metal ions
The formation of an adherent oxide coating on titanium implants
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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.
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.
Srinivasan A, Jung E, Levine BR.
J Am Acad Orthop Surg. 2012 Apr;20(4):214-22. PMID: 22474091 (Link to Abstract)
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Average 4.0 of 6 Ratings
Which of the following statements is true regarding polymethylmethacrylate (PMMA)?
Barium sulfate initiates the polymerization of monomethacrylate
It is a ductile material
The Young's modulus is between cortical and cancellous bone
It resists shear better than compressive forces
The polymerization of PMMA is endothermic
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.
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.
Webb JC, Spencer RF
J Bone Joint Surg Br. 2007 Jul;89(7):851-7. PMID: 17673574 (Link to Abstract)
Average 4.0 of 7 Ratings
Compared to cold-forged cobalt chrome, titanium alloys have which property?
Increased fatigue strength
Increased yield strength
Increased endurance limit
Decreased tensile strength
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.
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
Long M, Rack HJ.
Biomaterials. 1998 Sep;19(18):1621-39. PMID: 9839998 (Link to Abstract)
Average 4.0 of 12 Ratings
Which of the following definitions best describes the phenomenon of load relaxation?
Constant loading causing material to continue to deform over time
Stress at failure (the ultimate stress) divided by the strain at failure (the ultimate strain)
Decreased peak loads over time with the same amount of elongation
Stress is proportional to strain up to a limit
Strain divided by the time that the load is applied
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.
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
J Mech Behav Biomed Mater. 2008 Jan;1(1):51-8. PMID: 19627771 (Link to Abstract)
Average 3.0 of 24 Ratings
Which of the following best describes plastic deformation?
Change in length of a material under loading that returns to its original length once the load is removed
Progressive deformation of a material in response to a constant force over an extended period
The ability of a material to resist deformation
Change in length of a material under loading that does not return to the original length once the load is removed
The relative measure of the deformation of an object due to a load
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.
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.
Average 4.0 of 21 Ratings
When discussing metal implants and devices, which of the following best describes fatigue?
Load at which a material fractures
Progressive deformation due to a constant force over an extended period
Change in the stress-strain relationship dependent on the rate of loading
Failure at a submaximal tensile strength level after numerous loading cycles
Change in mechanical properties as a result of the direction of a load
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.
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
Bong MR, Kummer FJ, Koval KJ, Egol KA
J Am Acad Orthop Surg. 2007 Feb;15(2):97-106. PMID: 17277256 (Link to Abstract)
Hou SM, Wang JL, Lin J.
J Orthop Trauma. 2002 Nov-Dec;16(10):701-8. PMID: 12439193 (Link to Abstract)
Average 4.0 of 16 Ratings
Which of the following statements defines creep, as it relates to material properties?
Progressive deformation response to constant force over an extended period of time
A solid material's ability to deform under tensile stress
The ability of a materials mechanical properties to vary according to the direction of load
The rupture of a material under repeated cyclic stresses, at a point below the normal static breaking strength
The ability of a material to absorb energy and plastically deform without fracturing
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.
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.
The elements chromium, molybdenum, and cobalt are basic components of which of the following implant materials?
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.
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.
Average 3.0 of 18 Ratings
What description below best describes galvanic corrosion?
Corrosion resulting from an electrochemical potential created between two metals in conductive medium
Corrosion resulting from contact sites between materials under load
Corrosion resulting from oxygen tension differences
Corrosion from localized pits on metal surfaces
Corrosion from allergic reaction
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.
Answer 2 describes fretting corrosion and answer 3 describes crevice corrosion. Answer 4 describes pitting corrosion and answer 5 does not make sense.
Average 3.0 of 20 Ratings
Which of the following defines the stress at which a material begins to undergo plastic deformation?
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.
Average 3.0 of 17 Ratings
Bone is biomechanically weakest to resistance of which of the following forces?
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.
Average 2.0 of 37 Ratings
Which of the following most accurately describes stainless steel?
Composed of iron-carbon alloy, modulus of elasticity less stiff than bone
Composed of cobalt-chrome-molybdenum alloy, modulus of elasticity more stiff than bone
Composed of iron-carbon alloy, modulus of elasticity more stiff than titanium
Composed of cobalt-chrome-molybedenum alloy, modulus of elasticity less stiff than titanium
Composed of iron-carbon alloy, modulus of elasticity is more stiff than bone, cobalt-chrome, and aluminum-oxide (ceramic)
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.
Friedman RJ, Black J, Galante JO, Jacobs JJ, Skinner HB.
Instr Course Lect. 1994;43:233-55. PMID: 9097153 (Link to Abstract)
Average 3.0 of 22 Ratings
Which of the following best describes the process of galvanic corrosion?
Degradation from exposure to a harsh environment
Differences in oxygen tension within and outside of a crevice
Micromotion between material when under a load
Free radical oxidation
Electrochemical potential created between two metals in physical contact when immersed in a conductive medium
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.
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.
Holl PJ, Mollster A, Gjerdet NR
Injury. 2008 Feb;39(2):161-9. PMID: 18054018 (Link to Abstract)
Average 4.0 of 29 Ratings
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?
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.
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.
Low toughness is a disadvantage of which of the following bearing surfaces used in total hip arthroplasty?
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.
Answers 1, 2, 4, 5: Low fracture toughness is a characteristic of ceramic that risks component compromise during placement.
Santavirta S, Bahler M, Harris WH, Konttinen YT, Lappalainen R, Muratoglu O, Rieker C, Salzer M
Acta Orthop Scand. 2003 Aug;74(4):380-8. PMID: 14521286 (Link to Abstract)
Lang JE, Whiddon DR, Smith EL, Salyapongse AK.
J Surg Orthop Adv. 2008 Spring;17(1):51-7. PMID: 18284905 (Link to Abstract)
Average 3.0 of 15 Ratings
Ligaments are viscoelastic, meaning that their tensile strength is affected by:
Torsion and tension only
Orientation of applied strain
Rate of applied load
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.
Average 3.0 of 11 Ratings
Which of the following materials has a Young's modulus of elasticity that is most similar to cortical bone
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
Average 3.0 of 13 Ratings
The bending rigidity of the implant shown in Figure A is proportional to what power of the measured radius of the implant?
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.
Average 3.0 of 31 Ratings
Which of the following materials is most susceptible to galvanic corrosion?
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.
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.
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:
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).
Average 4.0 of 23 Ratings
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?
Finite element method
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.
Average 1.0 of 186 Ratings