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Biomechanics & Biomaterials MCQs

Practice Set 65 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 1281

Topic: Biomechanics & Biomaterials

A surgeon selects a titanium alloy intramedullary implant over a cobalt-chromium implant for fracture fixation in a patient with severe osteopenia, hoping to minimize long-term stress shielding. This clinical decision relies on which mechanical property of titanium being closer to that of cortical bone?

. Yield strength
. Ultimate tensile strength
. Young's modulus
. Fatigue limit
. Ductility

Correct Answer & Explanation

. Yield strength

Explanation

Young's modulus (modulus of elasticity) measures material stiffness. Titanium is significantly less stiff (lower Young's modulus) than cobalt-chromium or stainless steel, bringing it closer to the stiffness of cortical bone and thereby reducing stress shielding.

Question 1282

Topic: Biomechanics & Biomaterials

A patient undergoes rehabilitation following an Achilles tendon repair. During normal physiological loading, the tendon exhibits a non-linear stress-strain response. The initial 'toe region' of this curve is primarily due to which structural phenomenon?

. Breaking of enzymatic collagen cross-links
. Microfailure of individual collagen fibrils
. Uncrimping of the naturally wavy collagen fibers
. Elastic deformation of elastin molecules
. Viscoelastic stress relaxation

Correct Answer & Explanation

. Breaking of enzymatic collagen cross-links

Explanation

The non-linear toe region of a ligament or tendon stress-strain curve represents the uncrimping of relaxed, wavy collagen fibers as they are pulled taut. Once straightened, the tissue enters the linear elastic region.

Question 1283

Topic: Biomechanics & Biomaterials

During revision of a metal-on-polyethylene total hip arthroplasty, copious black debris is noted specifically around the modular head-neck taper junction. This phenomenon is primarily initiated by which type of corrosion mechanism?

. Galvanic corrosion
. Pitting corrosion
. Fretting corrosion
. Intergranular corrosion
. Stress corrosion cracking

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Trunnionosis is primarily initiated by fretting corrosion, where micromotion at the modular head-neck taper mechanically disrupts the protective oxide (passivation) layer of the alloy. This exposed metal then undergoes chemically assisted crevice corrosion.

Question 1284

Topic: Biomechanics & Biomaterials

When a corrective brace is applied to a pediatric patient with a clubfoot, the straps must be tightened periodically because the tension in the material decreases over time despite the foot remaining in the exact same corrected position. What viscoelastic property does this describe?

. Creep
. Stress relaxation
. Hysteresis
. Fatigue failure
. Anisotropy

Correct Answer & Explanation

. Stress relaxation

Explanation

Stress relaxation is a viscoelastic property where the stress (internal force) decreases over time when a material is held at a constant strain (deformation). Creep, conversely, is increasing deformation over time under a constant load.

Question 1285

Topic: Biomechanics & Biomaterials

To minimize stress shielding and subsequent bone resorption around a femoral stem, materials with an elastic modulus close to cortical bone are preferred. Which of the following orthopedic metals most closely approximates the modulus of elasticity of human cortical bone?

. Cobalt-chromium-molybdenum alloy
. 316L Stainless steel
. Titanium-6Aluminum-4Vanadium alloy
. Tantalum
. Zirconium

Correct Answer & Explanation

. Titanium-6Aluminum-4Vanadium alloy

Explanation

Cortical bone has a modulus of elasticity around 15-20 GPa. Titanium alloys (~110 GPa) are substantially closer to bone than cobalt-chromium (~220 GPa) or stainless steel (~200 GPa), making them less prone to causing severe stress shielding.

Question 1286

Topic: Biomechanics & Biomaterials

In materials science, the area under the entire stress-strain curve up to the point of failure represents the material's:

. Stiffness
. Yield strength
. Toughness
. Ductility
. Ultimate tensile strength

Correct Answer & Explanation

. Toughness

Explanation

The area under the entire stress-strain curve represents toughness, which is the amount of energy a material can absorb before fracturing. Stiffness is represented by the slope of the elastic region (Young's modulus). Yield strength is the point where plastic deformation begins. Ductility is the amount of plastic deformation before failure.

Question 1287

Topic: Biomechanics & Biomaterials
High-dose gamma irradiation of ultra-high-molecular-weight polyethylene (UHMWPE) followed by remelting is primarily performed to improve which of the following properties?
. Fracture toughness
. Fatigue strength
. Wear resistance
. Oxidation resistance
. Yield strength

Correct Answer & Explanation

. Wear resistance

Explanation

Highly cross-linking UHMWPE significantly increases its wear resistance by forming covalent bonds between polymer chains. However, this process decreases mechanical properties such as fracture toughness, fatigue strength, and ultimate tensile strength. Remelting or annealing is done subsequently to eliminate free radicals and prevent oxidation, though remelting further reduces mechanical properties compared to annealing.

Question 1288

Topic: Biomechanics & Biomaterials

A 45-year-old patient undergoes an Ilizarov frame application with gradual distraction for a tibial defect. Over several hours after tensioning the wires, the tension in the wires gradually decreases without any change in their length. This phenomenon is known as:

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

Correct Answer & Explanation

. Stress relaxation

Explanation

Stress relaxation is a viscoelastic property where the stress (tension) in a material decreases over time when it is held at a constant strain (constant length). Creep is the progressive deformation (increase in strain) over time under a constant load (stress). Hysteresis represents energy lost during the loading and unloading cycles of a viscoelastic material.

Question 1289

Topic: Biomechanics & Biomaterials

During fracture fixation, a surgeon inadvertently uses a stainless steel screw to secure a titanium alloy plate. Several months later, rapid corrosion is noted at the screw-plate interface. Which of the following electrochemical phenomena best explains this specific type of corrosion?

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

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in physical contact within an electrolytic environment (such as physiologic tissue fluid or blood). An electrochemical cell is created, and the less noble metal acts as the anode, undergoing accelerated corrosion while protecting the more noble metal.

Question 1290

Topic: Biomechanics & Biomaterials

During mechanical testing of a native human ligament, the initial non-linear portion of the stress-strain curve exhibits increasing stiffness with elongation. What structural event primarily dictates this initial 'toe region'?

. Microscopic failure of individual collagen fibers
. Plastic deformation of the extracellular matrix
. Irreversible sliding of proteoglycan molecules
. Uncrimping of the resting wavy collagen fibers
. Macroscopic yield of the entire ligament structure

Correct Answer & Explanation

. Uncrimping of the resting wavy collagen fibers

Explanation

The non-linear 'toe region' of the stress-strain curve for ligaments and tendons is caused by the uncrimping (straightening) of the naturally wavy collagen fibrils. As the fibers straighten, the structure becomes stiffer, transitioning into the linear elastic region where further deformation stretches the actual collagen molecules.

Question 1291

Topic: Biomechanics & Biomaterials

Stress shielding around orthopedic implants is heavily influenced by the modulus of elasticity of the implant material relative to the surrounding bone. Among the following common orthopedic structural materials, which has a modulus of elasticity closest to that of mature human cortical bone (~15-20 GPa)?

. Cobalt-chromium alloy
. 316L Stainless steel
. Titanium alloy (Ti-6Al-4V)
. Zirconia ceramic
. Alumina ceramic

Correct Answer & Explanation

. Titanium alloy (Ti-6Al-4V)

Explanation

Titanium alloys (e.g., Ti-6Al-4V) have a modulus of elasticity of approximately 110 GPa. While still stiffer than cortical bone (15-20 GPa), titanium is significantly less stiff and closer to bone than both 316L Stainless Steel (~200 GPa) and Cobalt-chromium alloys (~240 GPa), thus reducing the extent of stress shielding.

Question 1292

Topic: Biomechanics & Biomaterials

Which of the following metallic biomaterials has a Young's modulus most closely matching that of human cortical bone, thereby minimizing the risk of stress shielding in orthopedic implants?

. 316L Stainless steel
. Cobalt-chromium-molybdenum alloy
. Titanium alloy (Ti-6Al-4V)
. Alumina ceramic
. Zirconia ceramic

Correct Answer & Explanation

. 316L Stainless steel

Explanation

The Young's modulus (modulus of elasticity) of cortical bone is approximately 15-20 GPa. Titanium alloys (e.g., Ti-6Al-4V) have a modulus of roughly 110 GPa, which is significantly lower and closer to cortical bone than 316L stainless steel (~200 GPa) or Cobalt-chromium alloys (~210-230 GPa). This lower stiffness reduces stress shielding and subsequent bone resorption.

Question 1293

Topic: Biomechanics & Biomaterials

During revision trauma surgery, a 316L stainless steel screw is inadvertently used to lag a fracture fragment through a Titanium alloy (Ti-6Al-4V) plate. Which electrochemical process explains why this combination is contraindicated?

. Fretting corrosion
. Galvanic corrosion
. Crevice corrosion
. Pitting corrosion
. Intergranular corrosion

Correct Answer & Explanation

. Fretting corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals with differing anodic/cathodic potentials are placed in direct physical contact within an electrolytic solution, such as human extracellular fluid. The less noble metal acts as an anode and undergoes accelerated oxidative corrosion, which can lead to rapid implant failure, adverse tissue reactions, and metallosis.

Question 1294

Topic: Biomechanics & Biomaterials

A patient with a modular metal-on-polyethylene total hip arthroplasty presents with groin pain and a palpable mass. Revision surgery reveals blackened debris and local tissue necrosis at the head-neck junction (trunnion), but no significant wear at the bearing surface.

Which of the following wear mechanisms is primarily responsible for the failure at the modular junction?

. Galvanic corrosion
. Fretting corrosion
. Pitting corrosion
. Adhesive wear
. Abrasive wear

Correct Answer & Explanation

. Galvanic corrosion

Explanation

The scenario describes mechanically assisted crevice corrosion (MACC), commonly referred to as fretting corrosion, occurring at the modular head-neck junction (taperosis/trunnionosis). Fretting involves micromotion between two loaded surfaces that destroys the passivation layer, allowing crevice corrosion to rapidly degrade the metal, producing metallic debris and adverse local tissue reactions (ALTR).

Question 1295

Topic: Biomechanics & Biomaterials

During cyclic compressive loading of articular cartilage, the tissue initially deforms rapidly, followed by a slow, time-dependent deformation until an equilibrium is reached. Which of the following tissue components and physical mechanisms primarily dictate this time-dependent viscoelastic response (creep)?

. Intrinsic stiffness of Type II collagen
. Fluid exudation out of the extracellular matrix
. Superficial zone chondrocyte density
. Covalent cross-linking of Type IX collagen
. Molecular weight of the hyaluronan backbone

Correct Answer & Explanation

. Intrinsic stiffness of Type II collagen

Explanation

The viscoelastic behavior of articular cartilage in compression is primarily governed by fluid flow. The highly negatively charged glycosaminoglycans (GAGs) on aggrecan create high osmotic pressure that draws water in. When compressed, fluid is forced out of the extracellular matrix. The frictional drag of fluid moving through the porous solid matrix causes the time-dependent creep response.

Question 1296

Topic: Biomechanics & Biomaterials
Highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) reduces volumetric wear in total hip arthroplasty. However, the gamma irradiation used to induce cross-linking generates free radicals. What is the primary biomechanical trade-off of "remelting" the UHMWPE above its melting temperature to completely eliminate these free radicals?
. Decreased wear resistance
. Decreased yield strength and fatigue resistance
. Increased rate of in vivo oxidation
. Increased generation of third-body debris
. Loss of radiolucency on plain radiographs

Correct Answer & Explanation

. Decreased yield strength and fatigue resistance

Explanation

Irradiation causes cross-linking but leaves free radicals that can cause long-term oxidation and degradation. Remelting (heating above 150°C) completely eliminates the free radicals but alters the crystalline structure of the polymer, resulting in decreased yield strength, ultimate tensile strength, and fatigue resistance. Annealing (heating below the melting point) preserves mechanical properties better but leaves residual free radicals.

Question 1297

Topic: Biomechanics & Biomaterials

Titanium alloy (Ti-6Al-4V) is frequently chosen over 316L stainless steel for the manufacturing of diaphyseal fracture plates. Compared to 316L stainless steel, titanium alloy is characterized by:

. A higher modulus of elasticity, resulting in increased stress shielding.
. A lower modulus of elasticity, resulting in decreased stress shielding.
. Higher ultimate tensile strength and increased ductility.
. Increased susceptibility to crevice corrosion.
. A higher density and higher modulus of elasticity.

Correct Answer & Explanation

. A lower modulus of elasticity, resulting in decreased stress shielding.

Explanation

Titanium alloy has a modulus of elasticity (Young's modulus) of approximately 110 GPa, which is about half that of 316L stainless steel (~200 GPa). Because it is closer to the modulus of cortical bone (~15-20 GPa), titanium plates distribute load more evenly, thereby decreasing the effect of stress shielding compared to stiffer stainless steel implants.

Question 1298

Topic: Biomechanics & Biomaterials

Polymethylmethacrylate (PMMA) bone cement is widely utilized for implant fixation. Which of the following statements regarding the physical and handling properties of PMMA is true?

. It is strongest in tension and weakest in compression.
. The polymerization reaction is highly endothermic.
. Increasing the ambient operating room temperature will increase the setting time.
. Antibiotics added in low doses (e.g., 1g per 40g) dramatically reduce compressive strength below mechanical failure limits.
. Barium sulfate or zirconium dioxide is added to the powder component to render the cement radiopaque.

Correct Answer & Explanation

. Barium sulfate or zirconium dioxide is added to the powder component to render the cement radiopaque.

Explanation

Barium sulfate or zirconium dioxide is added to PMMA to provide radiopacity. PMMA is strongest in compression and weak in tension/shear. The polymerization is an exothermic reaction. Increasing ambient temperature or humidity decreases the setting time (cures faster). Low doses of antibiotics do not clinically compromise compressive strength.

Question 1299

Topic: Biomechanics & Biomaterials

On a stress-strain curve representing a typical orthopaedic implant material, the total area under the elastic portion of the curve represents which mechanical property?

. Ultimate tensile strength
. Yield strength
. Modulus of resilience
. Toughness
. Ductility

Correct Answer & Explanation

. Ultimate tensile strength

Explanation

The area under the elastic (linear) portion of the stress-strain curve represents the modulus of resilience, which is the energy a material can absorb without undergoing permanent deformation. Toughness is the total area under the entire curve until failure.

Question 1300

Topic: Biomechanics & Biomaterials

The primary mechanism of wear in a properly functioning metal-on-highly cross-linked polyethylene total hip arthroplasty is best described as:

. Abrasive wear
. Adhesive wear
. Third-body wear
. Fretting wear
. Galvanic corrosion

Correct Answer & Explanation

. Abrasive wear

Explanation

Adhesive wear is the predominant mode of wear in a well-functioning metal-on-polyethylene articulation. It occurs when microscopic asperities on the two bearing surfaces bond together and then tear away, generating polyethylene debris.

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