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Biomechanics & Biomaterials MCQs

Practice Set 79 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 1561

Topic: Biomechanics & Biomaterials

Which of the following is the primary mechanism of wear in a well-functioning, non-impinging metal-on-polyethylene total joint arthroplasty?

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

Correct Answer & Explanation

. Adhesive wear

Explanation

Adhesive wear is the primary mode of wear in a normally functioning metal-on-polyethylene articulation. It occurs when microscopic asperities on the articulating surfaces temporarily weld together and are subsequently sheared off during motion.

Question 1562

Topic: Biomechanics & Biomaterials

During tensile testing of a normal tendon, the initial 'toe region' of the stress-strain curve represents which of the following physiological events?

. Microscopic failure of collagen fibers
. Macroscopic failure of the tendon
. Uncrimping of the collagen fibers
. Plastic deformation of the tendon
. Yield point of the tendon

Correct Answer & Explanation

. Uncrimping of the collagen fibers

Explanation

The toe region of the stress-strain curve (up to about 2% strain) represents the straightening or 'uncrimping' of the normally crimped collagen fibers. Continued stress leads to the linear (elastic) region, followed by microscopic failure (yield point) and then macroscopic failure.

Question 1563

Topic: Biomechanics & Biomaterials

When a stainless steel screw is placed through a titanium plate in a physiological environment, which of the following phenomena is most likely to occur?

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

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals (like stainless steel and titanium) are placed in contact within an electrolytic solution (like bodily fluids). The less noble metal (typically stainless steel) undergoes accelerated anodic dissolution.

Question 1564

Topic: Biomechanics & Biomaterials

A 65-year-old man presents with an acutely swollen, red, and painful knee. Aspiration yields cloudy yellow fluid with a WBC count of 45,000 cells/mm3. Polarized light microscopy reveals rhomboid-shaped crystals that are weakly positively birefringent. What is the composition of these crystals?

. Monosodium urate
. Calcium pyrophosphate dihydrate
. Basic calcium phosphate
. Cholesterol
. Hydroxyapatite

Correct Answer & Explanation

. Calcium pyrophosphate dihydrate

Explanation

Rhomboid-shaped, weakly positively birefringent crystals are characteristic of pseudogout, which is caused by the deposition of calcium pyrophosphate dihydrate (CPPD) crystals in the joint. Monosodium urate crystals (gout) are needle-shaped and strongly negatively birefringent.

Question 1565

Topic: Biomechanics & Biomaterials

In total joint arthroplasty, the phenomenon of 'stress shielding' occurs when an implant carries a disproportionate share of the mechanical load, leading to resorption of the surrounding bone. Comparing common orthopedic implant materials, which of the following characteristics makes Titanium alloys less prone to causing stress shielding than Cobalt-Chromium alloys?

. Higher ultimate tensile strength
. Higher resistance to galvanic corrosion
. Modulus of elasticity closer to that of cortical bone
. Higher density and radiopacity
. Greater resistance to abrasive wear

Correct Answer & Explanation

. Modulus of elasticity closer to that of cortical bone

Explanation

Titanium alloys have a lower modulus of elasticity compared to Cobalt-Chromium (CoCr) or stainless steel, making Titanium more flexible and mechanically closer to the stiffness of cortical bone. This similarity allows for better load sharing with the host bone, thereby reducing stress shielding and subsequent osteopenia.

Question 1566

Topic: Biomechanics & Biomaterials
To improve the longevity of ultra-high-molecular-weight polyethylene (UHMWPE) bearings in total joint arthroplasty, cross-linking is performed followed by either remelting, annealing, or Vitamin E doping. What is the primary biomechanical advantage of incorporating Vitamin E (alpha-tocopherol) compared to the remelting process?
. It prevents oxidation without compromising fatigue strength
. It increases the coefficient of friction to improve stability
. It eliminates the need for initial gamma irradiation
. It decreases the ultimate tensile strength to increase elasticity
. It enhances the osteoinductive properties of wear debris

Correct Answer & Explanation

. It prevents oxidation without compromising fatigue strength

Explanation

Remelting eliminates free radicals created during irradiation but alters the crystalline structure, which reduces fatigue strength and crack propagation resistance. Vitamin E scavenges free radicals without the need for thermal remelting, thereby preventing oxidation while preserving the polymer's fatigue strength and mechanical properties.

Question 1567

Topic: Biomechanics & Biomaterials

A surgeon is revising a femur fracture nonunion originally fixed with a 316L stainless steel plate. The surgeon plans to add a lag screw across the nonunion site using a titanium screw. Which of the following types of corrosion is most likely to occur if these two distinct metals are in direct contact in vivo?

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

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two electrochemically dissimilar metals (such as stainless steel and titanium) are placed in direct physical contact within a conductive fluid medium (like body fluids). The less noble metal becomes the anode and corrodes at an accelerated rate. Mixing metals in orthopedic implants should generally be avoided to prevent this complication.

Question 1568

Topic: Biomechanics & Biomaterials
Increasing the radiation dose during the cross-linking process of ultra-high molecular weight polyethylene (UHMWPE) for total joint arthroplasty has which of the following mechanical effects?
. Increases oxidative degradation
. Decreases adhesive wear but increases abrasive wear
. Decreases volumetric wear but decreases fatigue strength
. Increases ultimate tensile strength
. Decreases the risk of impingement

Correct Answer & Explanation

. Decreases volumetric wear but decreases fatigue strength

Explanation

Highly cross-linked polyethylene is manufactured by exposing UHMWPE to radiation. While this significantly increases wear resistance (decreasing volumetric wear and lowering the risk of osteolysis), it concomitantly alters the mechanical properties, leading to a reduction in ultimate tensile strength, yield strength, and fatigue strength. Remelting is subsequently used to reduce free radicals and prevent oxidative degradation.

Question 1569

Topic: Biomechanics & Biomaterials

In the biomechanics of articular cartilage, which lubrication mechanism is characterized by the elastic deformation of the articular surfaces under load, thereby increasing the surface area and trapping a pressurized fluid film between them?

. Boundary lubrication
. Elastohydrodynamic lubrication
. Weeping lubrication
. Hydrodynamic lubrication
. Boosted lubrication

Correct Answer & Explanation

. Elastohydrodynamic lubrication

Explanation

Elastohydrodynamic lubrication occurs when the load is high enough to cause elastic deformation of the compliant articular cartilage. This deformation increases the contact surface area and alters the geometry, trapping a fluid film (synovial fluid) that separates the surfaces, thereby protecting them during rapid motion under high loads.

Question 1570

Topic: Biomechanics & Biomaterials

A 45-year-old patient undergoes an Achilles tendon repair. During rehabilitation, the physical therapist applies a constant load to the tendon over time, resulting in continued progressive deformation of the tendon. Which biomechanical property is described?

. Creep
. Stress relaxation
. Hysteresis
. Isotropy
. Anisotropy

Correct Answer & Explanation

. Creep

Explanation

Creep is defined as the progressive deformation of a material under a constant load over time. Viscoelastic materials, such as tendons and ligaments, exhibit this property. Stress relaxation, conversely, is the decrease in stress over time when a material is held at a constant strain (deformation).

Question 1571

Topic: Biomechanics & Biomaterials

In the context of orthopedic implants, which of the following material properties describes the ability to absorb energy and plastically deform prior to failure?

. Stiffness
. Elasticity
. Toughness
. Fatigue limit
. Ductility

Correct Answer & Explanation

. Toughness

Explanation

Toughness is represented by the total area under the stress-strain curve and indicates the amount of energy a material can absorb before failure. It incorporates both the strength and ductility of the material.

Question 1572

Topic: Biomechanics & Biomaterials

A surgeon revises a femur fracture initially treated with a stainless steel plate and screws. Due to stripped screw holes, the surgeon replaces several screws with titanium screws, leaving the stainless steel plate in place. What biomechanical complication is most likely to occur?

. Fretting corrosion
. Galvanic corrosion
. Crevice corrosion
. Stress shielding
. Pitting corrosion

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., stainless steel and titanium) are placed in contact within a conductive medium (body fluid). The electrochemical potential difference causes the less noble metal to corrode rapidly. Mixing distinct metal types in implants should be avoided.

Question 1573

Topic: Biomechanics & Biomaterials

A 65-year-old male presents with acute swelling and severe pain in his right knee. Aspiration yields cloudy synovial fluid with a WBC count of 45,000 cells/mm3. Polarized light microscopy reveals rhomboid-shaped crystals with weak positive birefringence. What is the composition of these crystals?

. Monosodium urate
. Calcium pyrophosphate dihydrate
. Basic calcium phosphate
. Hydroxyapatite
. Cholesterol

Correct Answer & Explanation

. Calcium pyrophosphate dihydrate

Explanation

The clinical and microscopic findings are diagnostic of pseudogout (CPPD). The crystals are made of Calcium Pyrophosphate Dihydrate, which are classically rhomboid-shaped and exhibit weak positive birefringence under polarized light. Monosodium urate (gout) crystals are needle-shaped and strongly negatively birefringent.

Question 1574

Topic: Biomechanics & Biomaterials

When analyzing the biomechanical properties of an orthopedic implant material using a standard stress-strain curve, the 'yield point' specifically represents:

. The point at which the material fractures completely.
. The transition from reversible elastic deformation to irreversible plastic deformation.
. The maximum stress a material can withstand before necking.
. The area under the curve representing overall material toughness.
. The slope of the linear elastic region representing stiffness.

Correct Answer & Explanation

. The transition from reversible elastic deformation to irreversible plastic deformation.

Explanation

The yield point on a stress-strain curve defines the exact point at which a material transitions from elastic deformation (where it will return to its original shape when the load is removed) to plastic deformation (irreversible, permanent structural change). The ultimate tensile strength is the maximum stress on the curve. The slope of the elastic region is Young's modulus (stiffness). The total area under the curve represents toughness.

Question 1575

Topic: Biomechanics & Biomaterials

Regarding the inherent mechanical properties of orthopedic implant materials and bone, which of the following materials possesses the lowest Young's modulus (thereby exhibiting the greatest relative elasticity/least stiffness)?

. Cobalt-chromium-molybdenum alloy
. 316L Stainless steel
. Titanium alloy (Ti-6Al-4V)
. Cortical bone
. Cancellous bone

Correct Answer & Explanation

. Cancellous bone

Explanation

Young's modulus is a measure of the stiffness of a solid material. A lower Young's modulus indicates less stiffness (greater elasticity). Cancellous bone has a significantly lower Young's modulus (approx. 0.1 - 2 GPa) compared to cortical bone (15 - 20 GPa). Among the common metals, Titanium alloy has the lowest modulus (~110 GPa), followed by Stainless steel (~200 GPa) and Cobalt-chromium (~210-250 GPa). Therefore, cancellous bone is the least stiff of the options provided.

Question 1576

Topic: Biomechanics & Biomaterials
Highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) is widely used in total joint arthroplasty to minimize wear debris. However, which critical mechanical property is significantly decreased as a direct result of the high-dose irradiation process used to cross-link the polyethylene?
. Abrasive wear resistance
. Oxidation resistance (if remelted appropriately)
. Ultimate tensile strength and fatigue crack propagation resistance
. Young's modulus
. Compressive yield strength

Correct Answer & Explanation

. Ultimate tensile strength and fatigue crack propagation resistance

Explanation

Highly cross-linked polyethylene (HXLPE) is manufactured by subjecting UHMWPE to high-dose gamma or electron-beam irradiation, which severs molecular chains and induces dense cross-linking. While this process vastly improves adhesive and abrasive wear resistance, the trade-off is a measurable decrease in mechanical properties, specifically ultimate tensile strength, fatigue crack propagation resistance, and ductility. For this reason, HXLPE is used with caution in high-stress, non-conforming joints or in very thin acetabular liners where fatigue failure could occur.

Question 1577

Topic: Biomechanics & Biomaterials

During a revision open reduction internal fixation of a distal femur fracture, a surgeon applies a titanium locking plate and secures it using stainless steel screws. What type of corrosion is most likely to occur at the interface between the plate and the screws?

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

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., titanium and stainless steel) are placed in direct physical contact within a conductive fluid environment (like serum or interstitial fluid). The metal with the lower electrochemical potential (anodic) will corrode preferentially to protect the more noble metal (cathodic).

Question 1578

Topic: Biomechanics & Biomaterials

Which type of corrosion is most likely to occur in the restricted, oxygen-depleted space between the head and neck taper of a modular total hip arthroplasty implant?

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

Correct Answer & Explanation

. Crevice corrosion

Explanation

Crevice corrosion occurs in restricted spaces (crevices) such as modular tapers or under screw heads, where oxygen depletion leads to a localized acidic environment that breaks down the implant's passivation layer.

Question 1579

Topic: Biomechanics & Biomaterials

On a stress-strain curve representing the mechanical properties of a bone plate, what does the area under the curve up to the yield point represent?

. Toughness
. Ductility
. Resilience
. Ultimate tensile strength
. Fatigue strength

Correct Answer & Explanation

. Resilience

Explanation

Resilience is the capacity of a material to absorb energy when deformed elastically and then recover completely upon unloading. It corresponds to the area under the elastic portion of the stress-strain curve (up to the yield point).

Question 1580

Topic: Biomechanics & Biomaterials
The incorporation of Vitamin E (alpha-tocopherol) into highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) components in total joint arthroplasty primarily serves to:
. Increase the melting temperature of the polymer
. Promote biological fixation to the bone
. Scavenge free radicals and prevent long-term oxidation
. Increase the elastic modulus of the bearing surface
. Prevent galvanic corrosion at modular interfaces

Correct Answer & Explanation

. Scavenge free radicals and prevent long-term oxidation

Explanation

Irradiation used to cross-link UHMWPE produces free radicals, which can lead to oxidation and embrittlement over time. Vitamin E is an antioxidant added to the polyethylene to scavenge these free radicals, preventing oxidation without the mechanical property compromises associated with post-irradiation remelting.

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