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Biomechanics & Biomaterials MCQs

Practice Set 63 of 88

This practice set contains high-yield board review questions covering key concepts in Biomechanics & Biomaterials. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 1241

Topic: Biomechanics & Biomaterials

Which of the following orthopedic implant materials has a modulus of elasticity that most closely resembles that of intact cortical bone?

. Cobalt-chromium alloy
. Stainless steel
. Titanium alloy
. Tantalum
. Polymethylmethacrylate (PMMA)

Correct Answer & Explanation

. Cobalt-chromium alloy

Explanation

Titanium alloys have a modulus of elasticity closer to that of cortical bone compared to stainless steel or cobalt-chromium, reducing stress shielding. However, it is still stiffer than native cortical bone.

Question 1242

Topic: Biomechanics & Biomaterials
In ultra-high-molecular-weight polyethylene (UHMWPE) manufacturing, what is the primary purpose of adding Vitamin E (alpha-tocopherol) to the highly cross-linked polyethylene?
. To increase the elasticity modulus
. To reduce abrasive wear by hardening the surface
. To quench free radicals and prevent oxidation
. To improve the radiopacity of the implant
. To stimulate local bone ingrowth into the polymer

Correct Answer & Explanation

. To quench free radicals and prevent oxidation

Explanation

Vitamin E is an antioxidant added to highly cross-linked UHMWPE to scavenge and quench residual free radicals generated during the irradiation process. This prevents in vivo oxidation, thereby preserving the material's mechanical properties.

Question 1243

Topic: Biomechanics & Biomaterials

Galvanic corrosion in orthopedic implants occurs when two dissimilar metals are placed in contact within a conductive fluid. Which of the following combinations is most susceptible to significant galvanic corrosion?

. Titanium alloy and commercially pure titanium
. Cobalt-chromium and titanium alloy
. Stainless steel and titanium alloy
. Cobalt-chromium and oxidized zirconium
. Tantalum and titanium alloy

Correct Answer & Explanation

. Titanium alloy and commercially pure titanium

Explanation

Mixing stainless steel and titanium implants can lead to severe galvanic corrosion due to a significant difference in their electrochemical potentials. Mixing titanium and cobalt-chromium is generally well-tolerated and less susceptible to galvanic corrosion.

Question 1244

Topic: Biomechanics & Biomaterials

A 48-year-old female presents with acute medial knee pain after squatting to pick up a heavy box. She felt a "pop" in the back of her knee. MRI reveals a complete radial tear of the posterior horn of the medial meniscus within 5 mm of its bony attachment.

Biomechanically, what is the consequence of nonoperative management of this specific injury pattern?

. It functions similarly to a structurally intact meniscus due to the preservation of the circumferential fibers.
. It leads to peak contact pressures equivalent to a total meniscectomy due to the loss of hoop stresses.
. It causes an isolated increase in anterior tibial translation.
. It results in varus alignment without altering tibiofemoral contact pressures.
. It predominantly affects the popliteomeniscal fascicles leading to lateral instability.

Correct Answer & Explanation

. It functions similarly to a structurally intact meniscus due to the preservation of the circumferential fibers.

Explanation

A posterior root tear of the medial meniscus disrupts the circumferential fibers, leading to a complete loss of hoop stresses. Biomechanical studies demonstrate that this results in contact pressures and kinematics equivalent to a total meniscectomy, rapidly accelerating the progression of osteoarthritis. Early surgical repair is recommended to restore joint mechanics.

Question 1245

Topic: Biomechanics & Biomaterials
What is the primary mechanism of degradation of conventional ultra-high molecular weight polyethylene (UHMWPE) components in total joint arthroplasty when they are sterilized via gamma irradiation in an oxygen-rich environment?
. Excessive cross-linking
. Galvanic corrosion
. Chain scission and oxidation
. Creep deformation
. Third-body abrasive wear

Correct Answer & Explanation

. Chain scission and oxidation

Explanation

Historically, gamma irradiation of UHMWPE in air (oxygen) led to the formation of free radicals. These free radicals react with oxygen over time, causing chain scission (breaking of the polymer chains) and oxidation. This chemical degradation drastically reduces the mechanical properties and wear resistance of the polyethylene, leading to premature catastrophic failure. Modern UHMWPE is irradiated in inert environments and subsequently melted or annealed to extinguish free radicals.

Question 1246

Topic: Biomechanics & Biomaterials

On a standard stress-strain curve for an orthopedic biomaterial, the total area under the curve up to the point of material failure represents which of the following properties?

. Toughness
. Resilience
. Yield strength
. Young's modulus
. Ultimate tensile strength

Correct Answer & Explanation

. Toughness

Explanation

Toughness is defined as the amount of energy a material can absorb before it fractures, which graphically equates to the total area under the stress-strain curve. Resilience is the area under the elastic portion only (energy absorbed without permanent deformation). Young's modulus is the slope of the elastic region (stiffness), and ultimate tensile strength is the peak stress the material endures.

Question 1247

Topic: Biomechanics & Biomaterials

In orthopedic biomechanics, the phenomenon of 'creep' is best defined as:

. Progressive, time-dependent deformation of a material subjected to a constant load
. A time-dependent decrease in stress within a material held at a constant strain
. The exact point at which a material transitions from elastic to plastic deformation
. The loss of energy in the form of heat during cyclical loading and unloading
. The sudden mechanical failure of a material after repeated loading below its ultimate strength

Correct Answer & Explanation

. Progressive, time-dependent deformation of a material subjected to a constant load

Explanation

Creep is the time-dependent, progressive deformation (strain) of a viscoelastic material under a constant applied stress (load). Conversely, stress relaxation is the progressive decrease in stress over time when a material is held at a constant strain (deformation). Both are fundamental properties of viscoelastic biological tissues like tendons, ligaments, and cartilage.

Question 1248

Topic: Biomechanics & Biomaterials

Galvanic corrosion is an electrochemical process that can lead to orthopedic implant failure. Which of the following conditions is necessary for galvanic corrosion to occur?

. Two dissimilar metals must be in physical contact within an electrolytic solution
. Fretting micro-motion must mechanically strip away the titanium passivation layer
. The implant must be exposed to an environment with exceptionally high oxygen tension
. An acidic microenvironment must form in narrow gaps or crevices around screws
. Metal wear debris must aggregate and form immune complexes with local macrophages

Correct Answer & Explanation

. Two dissimilar metals must be in physical contact within an electrolytic solution

Explanation

Galvanic corrosion occurs when two different (dissimilar) metals, which have different electrochemical potentials (anodic/cathodic indices), are placed in direct physical or electrical contact within a conductive fluid (such as human serum or interstitial fluid). The less noble metal acts as an anode and undergoes accelerated corrosion. Using a stainless steel screw with a titanium plate is a classic example.

Question 1249

Topic: Biomechanics & Biomaterials

A surgeon elects to use a stainless steel screw to secure a commercially pure titanium plate to the diaphysis of a femur. Which type of corrosion is most likely to occur at the screw-plate interface due to the difference in electrochemical potential?

. Crevice corrosion
. Fretting corrosion
. Galvanic corrosion
. Pitting corrosion
. Stress corrosion cracking

Correct Answer & Explanation

. Crevice corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in contact within an electrolytic medium (such as body fluids). The less noble metal acts as an anode and undergoes accelerated corrosion.

Question 1250

Topic: Biomechanics & Biomaterials

A researcher applies a constant strain (fixed length deformation) to a harvested anterior cruciate ligament specimen in a mechanical testing apparatus. Over a period of 60 seconds, the load required to maintain this specific strain gradually decreases. This viscoelastic property is termed:

. Creep
. Stress relaxation
. Hysteresis
. Fatigue failure
. Isotropic degradation

Correct Answer & Explanation

. Creep

Explanation

Stress relaxation is the decrease in stress (force/load) over time when a viscoelastic material is held at a constant strain. Creep, in contrast, is the gradual increase in strain (deformation) over time when subjected to a constant stress (load).

Question 1251

Topic: Biomechanics & Biomaterials

When mixing polymethylmethacrylate (PMMA) bone cement, which of the following statements regarding the addition of prophylactic antibiotics is true?

. Adding antibiotics in a liquid form increases the ultimate compressive strength of the cement.
. Thermal necrosis of surrounding bone is prevented due to a massive decrease in the exothermic reaction temperature.
. Adding more than 2 grams of antibiotic powder per 40-gram bag of cement substantially decreases the mechanical strength of the cement.
. Aminoglycosides are contraindicated because they are inactivated by the exothermic polymerization reaction.
. The porosity of the cement is decreased, preventing long-term antibiotic elution.

Correct Answer & Explanation

. Adding antibiotics in a liquid form increases the ultimate compressive strength of the cement.

Explanation

For prophylaxis, usually < 2g of antibiotic per 40g bag of PMMA is used to preserve structural integrity. Using more than 2g (as in a spacer) significantly weakens the mechanical properties of the cement. Antibiotics must be added in powder form; liquid antibiotics severely degrade cement strength. Aminoglycosides and vancomycin are heat-stable and survive the exothermic reaction. Adding powder increases porosity.

Question 1252

Topic: Biomechanics & Biomaterials
Highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is commonly used in total hip arthroplasty to reduce volumetric wear. What is the primary mechanical trade-off associated with increasing the degree of cross-linking in UHMWPE?
. Increased oxidation resistance but decreased stiffness
. Decreased abrasive wear but increased adhesive wear
. Increased wear resistance but decreased fatigue strength and fracture toughness
. Increased fracture toughness but decreased wear resistance
. Decreased elastic modulus but increased yield strength

Correct Answer & Explanation

. Increased wear resistance but decreased fatigue strength and fracture toughness

Explanation

Cross-linking UHMWPE significantly improves its wear resistance, particularly against adhesive and abrasive wear. However, the cross-linking process reduces the material's ductility, ultimate tensile strength, fatigue strength, and fracture toughness, making it more prone to rim cracking in certain applications like thin acetabular liners.

Question 1253

Topic: Biomechanics & Biomaterials

Cortical bone is a viscoelastic material. When loaded at a very high strain rate, such as during a high-speed motor vehicle accident, how do the mechanical properties of the bone change compared to a low strain rate?

. Decreased stiffness and decreased ultimate strength
. Increased stiffness and increased ultimate strength
. No change in stiffness but increased ductility
. Decreased stiffness and increased energy to failure
. Increased ductility and decreased ultimate strength

Correct Answer & Explanation

. Decreased stiffness and decreased ultimate strength

Explanation

Because bone is viscoelastic, its mechanical behavior is highly dependent on the rate of loading. At high strain rates, bone becomes both stiffer (higher modulus of elasticity) and stronger (higher ultimate strength), allowing it to absorb more energy before failure. When it does fail, this high energy release often results in comminution.

Question 1254

Topic: Biomechanics & Biomaterials

Mixing dissimilar metals in orthopedic implants (e.g., using a stainless steel screw in a titanium plate) can potentially lead to galvanic corrosion. In such a mixed-metal construct, which classic electrochemical process occurs?

. The less noble (more anodic) metal undergoes accelerated corrosion
. The more noble (more cathodic) metal undergoes accelerated corrosion
. Both metals corrode equally due to fluid electrolyte mediation
. Titanium acts as the anode and corrodes rapidly
. Passivation prevents any electron transfer regardless of the metal types

Correct Answer & Explanation

. The less noble (more anodic) metal undergoes accelerated corrosion

Explanation

In a galvanic couple formed by two dissimilar metals in an electrolyte (body fluid), the metal with the lower electrode potential (the less noble or more anodic metal, such as stainless steel) will lose electrons and undergo accelerated corrosion. The more noble metal (cathodic, such as titanium) is protected.

Question 1255

Topic: Biomechanics & Biomaterials

When a stainless steel screw is used in conjunction with a titanium plate, galvanic corrosion may occur. Which of the following best describes the electrochemical roles of the two metals in this scenario?

. Stainless steel acts as the anode and corrodes
. Stainless steel acts as the cathode and corrodes
. Titanium acts as the anode and corrodes
. Titanium acts as the cathode and corrodes
. Both metals act as anodes and corrode equally

Correct Answer & Explanation

. Stainless steel acts as the anode and corrodes

Explanation

In galvanic corrosion, the less noble metal acts as the anode and undergoes active corrosion. Stainless steel is less noble than titanium, so it acts as the anode and corrodes when the two are coupled in an electrolytic environment.

Question 1256

Topic: Biomechanics & Biomaterials
Highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is widely used in total joint arthroplasty to reduce wear. What is the primary mechanical trade-off associated with the radiation-induced cross-linking of UHMWPE?
. Increased abrasive wear
. Decreased fatigue strength and fracture toughness
. Increased oxidation potential without remelting
. Decreased ultimate tensile strength but increased ductility
. Increased elastic modulus leading to stress shielding

Correct Answer & Explanation

. Decreased fatigue strength and fracture toughness

Explanation

While high-dose irradiation cross-linking significantly decreases adhesive and abrasive wear of UHMWPE, it reduces mechanical properties such as fatigue strength, fracture toughness, and ductility. Post-irradiation melting is often used to eliminate free radicals and prevent oxidation, though it further reduces crystallinity.

Question 1257

Topic: Biomechanics & Biomaterials

To minimize stress shielding around a femoral stem in total hip arthroplasty, the implant's stiffness should ideally match that of cortical bone. Which of the following lists materials in order of decreasing Young's modulus (from stiffest to most flexible)?

. Cobalt-Chrome, Titanium alloy, Cortical bone
. Titanium alloy, Cobalt-Chrome, Cortical bone
. Cobalt-Chrome, Cortical bone, Titanium alloy
. Cortical bone, Titanium alloy, Cobalt-Chrome
. Titanium alloy, Cortical bone, Cobalt-Chrome

Correct Answer & Explanation

. Cobalt-Chrome, Titanium alloy, Cortical bone

Explanation

Young's modulus is a measure of material stiffness. Cobalt-Chrome is the stiffest (~210 GPa), followed by Titanium alloy (~110 GPa), while human cortical bone is much more flexible (~15-20 GPa). Using titanium instead of cobalt-chrome helps reduce stress shielding.

Question 1258

Topic: Biomechanics & Biomaterials

When a tendon is subjected to tensile loading, the initial portion of the stress-strain curve exhibits a non-linear 'toe region' before becoming linear. What physiological change at the microscopic level corresponds to this toe region?

. Microscopic failure of individual collagen fibers
. Uncrimping of the naturally wavy collagen fibrils
. Plastic deformation of the extracellular matrix
. Slippage of proteoglycan cross-links
. Macroscopic rupture of the epitenon

Correct Answer & Explanation

. Microscopic failure of individual collagen fibers

Explanation

The non-linear 'toe region' of the ligament/tendon stress-strain curve (up to ~2% strain) occurs as the naturally crimped (wavy) collagen fibers straighten out. Once uncrimped, the fibers stretch linearly until they reach the yield point.

Question 1259

Topic: Biomechanics & Biomaterials

During the correction of a pediatric clubfoot using the Ponseti method, the tightened ligaments gradually elongate over time under the constant applied load of the cast. This phenomenon is an example of which viscoelastic property?

. Stress relaxation
. Hysteresis
. Creep
. Fatigue failure
. Isotropic strain

Correct Answer & Explanation

. Stress relaxation

Explanation

Creep is the time-dependent continuous deformation (elongation) of a viscoelastic material under a constant load. In contrast, stress relaxation is the decrease in internal stress over time when the material is held at a constant deformation/length.

Question 1260

Topic: Biomechanics & Biomaterials

When a stainless steel screw is placed through a titanium plate, which type of corrosion is most likely to occur, and which metal will undergo accelerated oxidation?

. Galvanic corrosion; stainless steel
. Galvanic corrosion; titanium
. Fretting corrosion; stainless steel
. Fretting corrosion; titanium
. Pitting corrosion; stainless steel

Correct Answer & Explanation

. Galvanic corrosion; stainless steel

Explanation

Galvanic corrosion occurs when two dissimilar metals are in contact within an electrolytic solution. The less noble metal, which is stainless steel in this scenario, acts as the anode and undergoes accelerated corrosion.

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