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Biomechanics & Biomaterials MCQs

Practice Set 74 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 1461

Topic: Biomechanics & Biomaterials

Polymethylmethacrylate (PMMA) bone cement is frequently used in joint arthroplasty. Which of the following statements regarding the mechanical properties and functional characteristics of PMMA is true?

. It is strongest in tension and weakest in compression.
. The addition of barium sulfate increases its ultimate tensile strength.
. The polymerization process is a strongly endothermic reaction.
. It acts as an adhesive that chemically bonds directly to the host bone.
. It functions via mechanical interlock and is strongest in compression.

Correct Answer & Explanation

. It functions via mechanical interlock and is strongest in compression.

Explanation

PMMA acts as a grout, relying on mechanical interlocking with cancellous bone rather than chemical adhesion. It is significantly stronger in compression than in tension or shear, and its polymerization is highly exothermic.

Question 1462

Topic: Biomechanics & Biomaterials

Mixing stainless steel and titanium implants in the same surgical site increases the risk of which specific type of material degradation?

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

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in contact within a conductive fluid environment, such as the human body. This leads to accelerated corrosion of the less noble metal.

Question 1463

Topic: Biomechanics & Biomaterials
In total joint arthroplasty, the process of highly cross-linking ultra-high molecular weight polyethylene (UHMWPE) achieves which of the following mechanical changes?
. Decreased wear resistance
. Increased fatigue strength
. Decreased wear rate but decreased fatigue strength
. Increased ductility and toughness
. Increased yield strength

Correct Answer & Explanation

. Decreased wear rate but decreased fatigue strength

Explanation

Highly cross-linking UHMWPE significantly improves its wear resistance, thereby decreasing the wear rate. However, this process alters the mechanical properties by decreasing its fatigue strength, yield strength, and toughness.

Question 1464

Topic: Biomechanics & Biomaterials
Which of the following modifications to ultra-high-molecular-weight polyethylene (UHMWPE) most effectively reduces adhesive and abrasive wear when used in total joint arthroplasty?
. Gamma irradiation in an oxygen-rich environment
. Ethylene oxide gas sterilization
. Highly cross-linking the polyethylene with subsequent thermal treatment
. Decreasing the molecular weight of the polymer
. Adding a pure cobalt-chromium backing to the liner

Correct Answer & Explanation

. Highly cross-linking the polyethylene with subsequent thermal treatment

Explanation

Highly cross-linking UHMWPE, typically achieved through gamma or electron beam irradiation followed by thermal treatment (remelting or annealing to eliminate free radicals), significantly improves its wear resistance. This process drastically decreases both adhesive and abrasive wear. However, highly cross-linking the material can slightly reduce its mechanical properties, such as fatigue strength and ductility.

Question 1465

Topic: Biomechanics & Biomaterials

On a standard stress-strain curve for a typical ductile metal used in orthopedic implants, which of the following best defines the yield point?

. The point of ultimate mechanical failure and fracture
. The transition point from elastic deformation to plastic deformation
. The maximum stress the material can withstand before necking begins
. The exact point where the modulus of elasticity reaches zero
. The region representing the material's toughness

Correct Answer & Explanation

. The transition point from elastic deformation to plastic deformation

Explanation

The yield point on a stress-strain curve marks the limit of elastic behavior and the beginning of plastic behavior. Prior to the yield point, deformation is completely reversible (elastic phase); once the load is removed, the material returns to its original shape. Beyond the yield point, the material undergoes permanent (plastic) deformation.

Question 1466

Topic: Biomechanics & Biomaterials

An orthopedic surgeon is revising a failed internal fixation and considers placing a titanium screw through a 316L stainless steel plate. Which of the following electrochemical principles explains why this specific mixing of metals should generally be avoided?

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

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals (such as stainless steel and titanium) are placed in direct physical contact within a conductive electrolyte environment (such as bodily fluids). The difference in their anodic index creates a small electrical current, leading to accelerated corrosion of the less noble metal (in this case, the stainless steel).

Question 1467

Topic: Biomechanics & Biomaterials
In total hip arthroplasty, highly cross-linked polyethylene (HXLPE) has significantly reduced the rate of osteolysis compared to conventional ultra-high molecular weight polyethylene (UHMWPE). What is the primary mechanism by which vitamin E is utilized during the manufacturing of modern HXLPE?
. To increase the melting temperature of the polymer
. To prevent adhesive wear by increasing crystallinity
. To quench free radicals and prevent in vivo oxidation
. To promote protein binding and improve fluid film lubrication
. To substitute for gamma irradiation in the cross-linking process

Correct Answer & Explanation

. To quench free radicals and prevent in vivo oxidation

Explanation

Highly cross-linked polyethylene is manufactured using radiation to create cross-links that improve wear resistance, but this process generates free radicals. If left untreated, these free radicals react with oxygen, leading to oxidation and subsequent degradation of the polymer. Vitamin E (alpha-tocopherol) is an antioxidant added to HXLPE to quench these free radicals, thereby preventing oxidation without the need for post-irradiation remelting, which can decrease mechanical strength.

Question 1468

Topic: Biomechanics & Biomaterials

In the application of orthopedic implants, mixing a titanium screw with a stainless steel plate is generally avoided to prevent galvanic corrosion. In this specific bimetallic coupling, which of the following accurately describes the electrochemical roles of the two metals?

. Titanium acts as the anode and corrodes, while stainless steel acts as the cathode.
. Stainless steel acts as the anode and corrodes, while titanium acts as the cathode.
. Both metals corrode equally due to uniform electron exchange.
. Titanium undergoes passivation, accelerating its own corrosion.
. Stainless steel undergoes fretting corrosion, protecting the titanium.

Correct Answer & Explanation

. Stainless steel acts as the anode and corrodes, while titanium acts as the cathode.

Explanation

Galvanic corrosion occurs when two dissimilar metals are in contact within an electrolytic solution (like body fluid). The more noble (less reactive) metal acts as the cathode, while the less noble (more reactive) metal acts as the anode and undergoes accelerated corrosion. Titanium is more noble than stainless steel; therefore, in a titanium-stainless steel coupling, the stainless steel acts as the anode and preferentially corrodes.

Question 1469

Topic: Biomechanics & Biomaterials

A surgeon is considering mixing different metallic implants in a single surgical site. To prevent severe galvanic corrosion, which of the following combinations of metals should be strictly avoided due to their significant differences in anodic/cathodic potential?

. Cobalt-chrome and titanium
. Titanium and oxidized zirconium
. Cobalt-chrome and stainless steel
. Tantalum and titanium
. Titanium and ceramic

Correct Answer & Explanation

. Cobalt-chrome and stainless steel

Explanation

Mixing stainless steel and cobalt-chromium in the same electrolyte environment can result in severe galvanic corrosion, particularly degrading the stainless steel. Titanium and cobalt-chromium can occasionally be used together with less risk, but stainless steel mixed with other alloys should be avoided.

Question 1470

Topic: Biomechanics & Biomaterials

When a tensile force is applied to a normal ligament, the initial "toe region" observed on its stress-strain curve represents which of the following biomechanical phenomena?

. Macroscopic failure of collagen fibers
. Microscopic failure of collagen fibers
. Uncrimping of resting collagen fibrils
. Plastic deformation of the ligament
. Elastic deformation of intermolecular cross-links

Correct Answer & Explanation

. Uncrimping of resting collagen fibrils

Explanation

The toe region of a ligament or tendon stress-strain curve occurs at low strains. It corresponds to the physical uncrimping (straightening) of the naturally wavy collagen fibrils before they begin to bear significant tensile load.

Question 1471

Topic: Biomechanics & Biomaterials

To minimize the complication of stress shielding following cementless total hip arthroplasty, engineers seek biomaterials with mechanical properties similar to host bone. Which of the following implant materials possesses a Young's modulus of elasticity most closely matching that of human cortical bone?

. Stainless steel
. Cobalt-chromium alloy
. Titanium alloy
. PMMA bone cement
. Alumina ceramic

Correct Answer & Explanation

. Titanium alloy

Explanation

Titanium alloy has a Young's modulus (stiffness) closer to that of cortical bone compared to cobalt-chromium or stainless steel. This relative flexibility helps reduce stress shielding around the implant.

Question 1472

Topic: Biomechanics & Biomaterials

Polymethylmethacrylate (PMMA) bone cement is frequently used for implant fixation in joint arthroplasty. Which of the following statements regarding the physical and mechanical properties of PMMA is most accurate?

. It acts as an adhesive that bonds chemically to the host bone.
. It demonstrates its highest mechanical strength in tension.
. It undergoes a highly endothermic reaction during polymerization.
. It acts as a grout, relying purely on mechanical interlock for fixation.
. Its compressive strength significantly increases with the deliberate inclusion of blood or fat.

Correct Answer & Explanation

. It acts as a grout, relying purely on mechanical interlock for fixation.

Explanation

PMMA bone cement acts strictly as a grout, filling voids to create a mechanical interlock between the implant and bone; it has no chemical adhesive properties. It polymerizes via a strongly exothermic reaction and is much stronger in compression than in tension.

Question 1473

Topic: Biomechanics & Biomaterials

Which of the following combinations of orthopedic implants poses the highest risk for severe galvanic corrosion if placed in direct physical contact within the body?

. Titanium and Titanium alloy
. Stainless steel and Cobalt-Chromium
. Titanium alloy and Cobalt-Chromium
. Stainless steel and Titanium
. Two highly cross-linked polyethylene components

Correct Answer & Explanation

. Stainless steel and Titanium

Explanation

Galvanic corrosion occurs when two dissimilar metals are in contact within an electrolytic environment. Stainless steel and titanium have a large difference in their anodic index, leading to significant galvanic corrosion and should not be mixed.

Question 1474

Topic: Biomechanics & Biomaterials

When evaluating the biomechanical properties of a titanium orthopedic implant, the point on the stress-strain curve where the material transitions from elastic deformation to plastic deformation is known as the:

. Ultimate tensile strength
. Yield point
. Young's modulus
. Failure point
. Proportional limit

Correct Answer & Explanation

. Yield point

Explanation

The yield point marks the transition from the elastic region (where deformation is reversible) to the plastic region (where deformation is permanent). Young's modulus represents the stiffness in the elastic region, represented by the slope of the elastic line.

Question 1475

Topic: Biomechanics & Biomaterials

A surgeon plans to use a stainless steel plate with titanium screws for fracture fixation. Which of the following best describes the likely outcome of mixing these two metals in vivo?

. Enhanced osteointegration due to micro-currents
. Galvanic corrosion primarily affecting the titanium screws
. Galvanic corrosion primarily affecting the stainless steel plate
. Crevice corrosion strictly isolated to the screw heads
. No significant corrosive reaction will occur

Correct Answer & Explanation

. Galvanic corrosion primarily affecting the stainless steel plate

Explanation

When dissimilar metals are in contact in an electrolytic fluid (body fluid), galvanic corrosion occurs. The less noble metal (anodic) corrodes more rapidly. Stainless steel is less noble than titanium; thus, the stainless steel plate will undergo accelerated corrosion.

Question 1476

Topic: Biomechanics & Biomaterials

In total joint arthroplasty, the type of wear generated when two articulating surfaces come into direct contact and micro-welds form and break off, generating particulate debris, is classified as:

. Abrasive wear
. Adhesive wear
. Third-body wear
. Corrosive wear
. Fretting wear

Correct Answer & Explanation

. Adhesive wear

Explanation

Adhesive wear occurs when two surfaces slide against each other under pressure, causing localized bonding (micro-welds) that are sheared off during motion. Abrasive wear occurs when a harder surface cuts into a softer one.

Question 1477

Topic: Biomechanics & Biomaterials

Which of the following orthopedic implant materials has a Young's modulus most closely matching that of cortical bone?

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

Correct Answer & Explanation

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

Explanation

Cortical bone has a Young's modulus of approximately 15-20 GPa. Titanium alloys (~110 GPa) are closer to cortical bone than stainless steel (~200 GPa) or Co-Cr (~210-240 GPa), which helps to minimize stress shielding. PMMA is significantly less stiff (~2-3 GPa).

Question 1478

Topic: Biomechanics & Biomaterials

A 45-year-old sustains a femur fracture and undergoes Ilizarov external fixation. During the lengthening process, the tension in the transfixing wire gradually decreases over time while the length remains strictly constant. This biomechanical phenomenon is known as:

. Creep
. Stress relaxation
. Fatigue failure
. Hysteresis
. Load sharing

Correct Answer & Explanation

. Stress relaxation

Explanation

Stress relaxation is the decrease in stress (tension) over time when a viscoelastic material is held at a constant strain (length). Creep, conversely, is the progressive deformation of a material over time under a constant load.

Question 1479

Topic: Biomechanics & Biomaterials

Polymethylmethacrylate (PMMA) is frequently utilized in arthroplasty to secure metallic components to the host bone. By what primary mechanism does PMMA stabilize the orthopedic prosthesis?

. Chemical bonding to hydroxyapatite
. Osteoinduction of the surrounding bone
. Mechanical micro-interlocking with cancellous bone
. Covalent bonding to the metallic implant
. Biological integration with osteoblasts

Correct Answer & Explanation

. Mechanical micro-interlocking with cancellous bone

Explanation

PMMA bone cement acts strictly as a grout, not an adhesive. It stabilizes the implant by mechanical micro-interlocking with the trabeculae of the host cancellous bone and the roughened surface of the prosthesis.

Question 1480

Topic: Biomechanics & Biomaterials
Highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is commonly used in total joint arthroplasty to reduce wear. Which of the following is the primary mechanical disadvantage of increasing the cross-linking of UHMWPE?
. Increased adhesive wear
. Decreased fatigue resistance and fracture toughness
. Increased coefficient of friction
. Decreased oxidation resistance
. Increased susceptibility to third-body wear

Correct Answer & Explanation

. Decreased fatigue resistance and fracture toughness

Explanation

While high cross-linking decreases the volumetric wear of UHMWPE, it significantly reduces its mechanical properties, such as fatigue resistance, ductility, and fracture toughness. This increases the risk of mechanical failure or rim fracture.

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