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Biomechanics & Biomaterials MCQs

Practice Set 7 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 121

Topic: Biomechanics & Biomaterials
When designing a femoral stem for total hip arthroplasty, preventing fatigue failure due to cyclic bending moments is crucial. To achieve this, the stem's cross-sectional geometry should be optimized to:
. Minimize its overall length to reduce stress.
. Maximize its Area Moment of Inertia, especially in high-stress regions.
. Concentrate the stem material along its neutral axis.
. Use a material with a very low Young's Modulus.
. Increase the surface area for bone ingrowth.

Correct Answer & Explanation

. Maximize its Area Moment of Inertia, especially in high-stress regions.

Explanation

Fatigue failure in a femoral stem is caused by repeated cyclic stresses. Bending stress is inversely proportional to the Area Moment of Inertia. Therefore, maximizing the Area Moment of Inertia, particularly in regions prone to high bending moments, is the most effective geometric strategy to reduce stress and increase fatigue life.

Question 122

Topic: Biomechanics & Biomaterials

Bending rigidity of a solid cylindrical intramedullary nail is mathematically proportional to the nail's radius raised to which of the following powers?

. First power
. Second power
. Third power
. Fourth power
. Fifth power

Correct Answer & Explanation

. Fourth power

Explanation

The bending rigidity and polar moment of inertia of a solid cylinder are proportional to the radius to the fourth power (r^4). Therefore, even a small increase in the nail diameter dramatically increases its resistance to bending and torsion.

Question 123

Topic: Biomechanics & Biomaterials

In the mechanical testing of orthopedic implants, the area under the stress-strain curve represents which of the following material properties?

. Elastic modulus
. Yield strength
. Ultimate tensile strength
. Toughness
. Ductility

Correct Answer & Explanation

. Toughness

Explanation

The area under the stress-strain curve represents the toughness of the material, defined as the total energy absorbed prior to failure. The slope of the elastic region, in contrast, represents the elastic modulus (stiffness).

Question 124

Topic: Biomechanics & Biomaterials

Long bones subjected to high-energy, rapid loading rates fail at a higher load and absorb more energy than when subjected to slow loading rates. This phenomenon is a direct result of which biomechanical property of bone?

. Anisotropy
. Ductility
. Viscoelasticity
. Creep
. Fatigue

Correct Answer & Explanation

. Viscoelasticity

Explanation

Viscoelasticity indicates that the mechanical properties of bone are rate-dependent. At higher loading rates, bone is stiffer, absorbs more energy, and fails at higher loads, which typically results in high-energy comminuted fracture patterns.

Question 125

Topic: Biomechanics & Biomaterials

Which of the following orthopedic implant materials has a modulus of elasticity most closely resembling that of human cortical bone, thereby theoretically minimizing stress shielding?

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

Correct Answer & Explanation

. Titanium alloy

Explanation

Titanium alloy has a lower modulus of elasticity compared to stainless steel and cobalt-chromium, making it much closer to the elasticity of cortical bone. This reduces the stiffness mismatch and minimizes stress shielding at the fracture site.

Question 126

Topic: Biomechanics & Biomaterials

Cortical bone exhibits different mechanical properties depending on the direction of the applied load, being strongest in longitudinal compression and weakest in transverse tension. This biomechanical property is known as:

. Viscoelasticity
. Creep
. Isotropism
. Anisotropy
. Plasticity

Correct Answer & Explanation

. Anisotropy

Explanation

Anisotropy is the property of a material exhibiting different mechanical properties when loaded in different directions. Cortical bone is highly anisotropic, being strongest parallel to the osteons (longitudinal compression) and weakest in transverse tension or shear.

Question 127

Topic: Biomechanics & Biomaterials

A skier sustains a tibial plateau fracture during a high-speed collision. Biomechanically, because cortical bone is viscoelastic, it responds to this high rate of loading by becoming:

. Less stiff and fracturing at a lower energy threshold
. More stiff and absorbing more energy before failure
. More elastic and failing primarily in tension
. Less brittle and failing with a pure transverse pattern
. Unaffected in stiffness but demonstrating increased ductility

Correct Answer & Explanation

. More stiff and absorbing more energy before failure

Explanation

Due to viscoelasticity, cortical bone becomes both stronger and stiffer when subjected to higher rates of loading. Consequently, it absorbs significantly more energy prior to failure, which is explosively released, often causing severe soft tissue injury.

Question 128

Topic: Biomechanics & Biomaterials

When selecting an intramedullary nail, the surgeon must consider its cross-sectional geometry. The nail's resistance to bending forces is proportional to its:

. Polar moment of inertia
. Area moment of inertia
. Working length
. Thread pitch
. Yield strain

Correct Answer & Explanation

. Area moment of inertia

Explanation

A nail's resistance to bending is governed by its area moment of inertia. Conversely, its resistance to torsion is dictated by its polar moment of inertia; both are proportional to the fourth power of the radius (r^4) for a solid cylinder.

Question 129

Topic: Biomechanics & Biomaterials

The torsional rigidity of a solid cylindrical intramedullary nail is proportional to its radius raised to which power?

. Square (r^2)
. Cube (r^3)
. Fourth power (r^4)
. Fifth power (r^5)
. Directly proportional to the radius (r)

Correct Answer & Explanation

. Fourth power (r^4)

Explanation

The polar moment of inertia, which determines a solid cylinder's resistance to torsion, is proportional to the radius raised to the fourth power (r^4). A small increase in nail diameter therefore yields an exponential increase in torsional rigidity.

Question 130

Topic: Biomechanics & Biomaterials

A surgeon inadvertently mixes a titanium alloy fracture plate with 316L stainless steel screws. This dissimilar metal combination risks galvanic corrosion. Which material acts as the anode, and what is its fate?

. Titanium acts as the anode and undergoes accelerated corrosion
. Stainless steel acts as the anode and undergoes accelerated corrosion
. Titanium acts as the cathode and undergoes accelerated corrosion
. Stainless steel acts as the cathode and undergoes accelerated corrosion
. Both metals corrode equally due to passivation layer breakdown

Correct Answer & Explanation

. Stainless steel acts as the anode and undergoes accelerated corrosion

Explanation

In a galvanic couple between stainless steel and titanium, titanium is the more noble (cathodic) metal. Stainless steel acts as the anode and undergoes accelerated galvanic corrosion, which can lead to implant failure and adverse local tissue reactions.

Question 131

Topic: Biomechanics & Biomaterials

Cortical bone exhibits viscoelastic behavior, meaning its mechanical properties depend on the loading rate. How does cortical bone adapt biomechanically when subjected to a high-velocity impact (fast loading rate) compared to a low-velocity force?

. It becomes less stiff and absorbs less energy before failure
. It becomes stiffer and absorbs more energy before failure
. Its stiffness remains constant, but the yield point occurs at a lower strain
. It exhibits plastic deformation earlier in the stress-strain curve
. It converts primarily into a perfectly elastic material

Correct Answer & Explanation

. It becomes stiffer and absorbs more energy before failure

Explanation

Due to its viscoelastic nature, cortical bone becomes both stiffer and stronger at higher loading rates. It absorbs significantly more energy prior to failure in high-velocity trauma, which is why high-energy fractures typically result in greater comminution and soft tissue damage upon energy release.

Question 132

Topic: Biomechanics & Biomaterials

A fracture fixation plate manufactured from titanium alloy (Ti-6Al-4V) is compared to an identical plate made of 316L stainless steel. Which of the following correctly describes the titanium construct's biomechanical profile relative to the stainless steel construct?

. Higher modulus of elasticity and higher fatigue strength.
. Lower modulus of elasticity and higher yield strength.
. Lower modulus of elasticity resulting in lower stiffness.
. Higher stiffness and greater ductility.
. Lower yield strength and higher stiffness.

Correct Answer & Explanation

. Lower modulus of elasticity resulting in lower stiffness.

Explanation

Titanium alloy has a modulus of elasticity (approx. 110 GPa) roughly half that of stainless steel (approx. 200 GPa). This gives titanium implants lower stiffness, resulting in less stress shielding and improved load sharing with the healing bone.

Question 133

Topic: Biomechanics & Biomaterials

A 65-year-old female presents with an open, highly comminuted tibia shaft fracture and severe osteopenia. A circular external fixator (Ilizarov frame) is applied. What specific modification to the external fixator pins will most significantly increase their pull-out strength in this osteoporotic bone?

. Using fully threaded cortical screws instead of half-pins.
. Decreasing the pin diameter to preserve bone stock.
. Placing the pins perfectly parallel to the joint line.
. Applying pre-tension to all half-pins.
. Using hydroxyapatite-coated half-pins.

Correct Answer & Explanation

. Using hydroxyapatite-coated half-pins.

Explanation

In osteoporotic bone, external fixation pin loosening is a major complication. Using hydroxyapatite-coated half-pins enhances osteointegration at the pin-bone interface, significantly increasing pull-out strength and reducing the incidence of pin tract infections.

Question 134

Topic: Biomechanics & Biomaterials

During the initial 'run-in' phase of a metal-on-metal bearing articulation, wear debris is generated at a higher rate. What is the primary mode of wear occurring during this specific phase?

. Third-body wear from acrylic cement
. Boundary lubrication leading to adhesive and abrasive wear
. Galvanic corrosion at the head-neck taper
. Fatigue wear of the subchondral bone
. Fretting wear at the acetabular rim

Correct Answer & Explanation

. Boundary lubrication leading to adhesive and abrasive wear

Explanation

The 'run-in' phase relies on boundary lubrication where asperities on the metal surfaces interact directly. This early adhesive and abrasive wear eventually smooths the surfaces to allow fluid-film lubrication.

Question 135

Topic: Biomechanics & Biomaterials

A 70-year-old female falls directly onto her femur, sustaining a subtrochanteric fracture. Considering the primary load-bearing function of the femoral diaphysis, cortical bone is optimally designed to resist which type of force?

. Torsion
. Shear
. Tension
. Compression
. Bending

Correct Answer & Explanation

. Compression

Explanation

Correct Answer: DCortical bone, with its dense, anisotropic structure, is strongest in compression. It has high stiffness and can withstand significant compressive loads before failure. It is weaker in tension, and even weaker in shear and torsion. Bending creates both tension and compression, so while it resists bending, its primary strength lies in resisting direct compression.

Question 136

Topic: Biomechanics & Biomaterials

When evaluating the mechanical properties of an orthopedic implant material, the 'yield point' on a stress-strain curve represents what phenomenon?

. The point of ultimate material failure or breakage
. The transition from elastic to plastic deformation
. The region of maximum stiffness
. The point where the material undergoes minimal strain under maximum stress
. The limit of viscoelastic creep

Correct Answer & Explanation

. The transition from elastic to plastic deformation

Explanation

The yield point on a stress-strain curve marks the end of the elastic region and the beginning of the plastic region. Beyond this point, the material will no longer return to its original shape when the load is removed, representing permanent deformation.

Question 137

Topic: Biomechanics & Biomaterials

What process is most commonly used to eliminate free radicals in highly cross-linked polyethylene components used in total joint arthroplasty?

. Gamma irradiation in air
. Ethylene oxide sterilization
. Melting or annealing the polyethylene
. Addition of polymethylmethacrylate
. Cold forging

Correct Answer & Explanation

. Melting or annealing the polyethylene

Explanation

After cross-linking via irradiation, free radicals are eliminated by either melting or annealing the polyethylene. This critical step prevents long-term oxidation and subsequent abrasive wear in vivo.

Question 138

Topic: Biomechanics & Biomaterials

What is the typical water content of a healthy adult meniscus?

. Approximately 30-40% of wet weight.
. Approximately 50-60% of wet weight.
. Approximately 70-80% of wet weight.
. Approximately 90-95% of wet weight.
. Less than 20% of wet weight.

Correct Answer & Explanation

. Approximately 70-80% of wet weight.

Explanation

Correct Answer: CThe meniscus is rich in water, which accounts for approximately 70-80% of its wet weight in healthy adults. This high water content, along with the proteoglycan matrix, contributes to its viscoelastic properties and ability to distribute loads and absorb shock. With age and degeneration, water content can decrease, affecting its biomechanical function.

Question 139

Topic: Biomechanics & Biomaterials

During a two-stage revision for a periprosthetic joint infection of the knee, the surgeon mixes 4 grams of Vancomycin and 4 grams of Tobramycin into a 40-gram bag of polymethylmethacrylate (PMMA) bone cement to create an articulating spacer. Which of the following describes the most significant consequence of this high-dose antibiotic loading on the PMMA?

. It increases the ultimate compressive strength of the cement
. It significantly decreases the structural compressive strength of the cement
. It accelerates the exothermic polymerization time
. It completely prevents the elution of Tobramycin
. It has negligible effects on mechanical properties compared to standard-dose PMMA

Correct Answer & Explanation

. It significantly decreases the structural compressive strength of the cement

Explanation

High-dose antibiotic loading (typically >2g per 40g bag of PMMA) creates significant stress risers within the cement mantle, substantially decreasing its compressive strength and overall structural integrity.

Question 140

Topic: Biomechanics & Biomaterials

Regarding the biomechanics of bone, cortical bone is optimally designed to resist which type of force?

. Torsion
. Shear
. Tension
. Compression
. Bending

Correct Answer & Explanation

. Compression

Explanation

Correct Answer: DCortical bone, with its dense, anisotropic structure, is strongest in compression. It has high stiffness and can withstand significant compressive loads before failure. It is weaker in tension, and even weaker in shear and torsion. Bending creates both tension and compression, so while it resists bending, its primary strength lies in resisting direct compression.

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