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Biomechanics & Biomaterials MCQs

Practice Set 69 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 1361

Topic: Biomechanics & Biomaterials
The historical practice of sterilizing ultra-high-molecular-weight polyethylene (UHMWPE) components using gamma irradiation in an air (oxygenated) environment resulted in premature implant failure primarily due to:
. Incomplete sterilization of bacterial spores
. Immediate reduction in the molecular weight of the polymer during packaging
. Oxidation and chain scission occurring over time during shelf storage and in vivo
. Excessive cross-linking causing the material to become too brittle
. Leaching of toxic free radicals into the surrounding periprosthetic host tissues

Correct Answer & Explanation

. Oxidation and chain scission occurring over time during shelf storage and in vivo

Explanation

Gamma irradiation of UHMWPE in air generates free radicals that readily react with oxygen. Over time (both during shelf storage and in vivo), this causes subsurface oxidation, leading to chain scission, decreased molecular weight, embrittlement, and drastically accelerated wear. Modern UHMWPE is sterilized in inert gas or vacuum and thermally treated.

Question 1362

Topic: Biomechanics & Biomaterials

On a standardized stress-strain curve for a given orthopaedic material, the total area under the entire curve up to the point of structural fracture represents the material's:

. Stiffness
. Resilience
. Toughness
. Yield strength
. Ultimate tensile strength

Correct Answer & Explanation

. Stiffness

Explanation

Toughness is defined as the amount of energy a material can absorb before it eventually fractures, which is represented geometrically by the total area under the entire stress-strain curve. Resilience is the energy absorbed prior to yielding (the area under the elastic portion only). Stiffness is the slope of the elastic region.

Question 1363

Topic: Biomechanics & Biomaterials

Titanium alloys, such as Ti-6Al-4V, are frequently utilized for cementless femoral stems in total hip arthroplasty. Compared to cobalt-chromium alloys, titanium alloys are biomechanically preferred in this application primarily because they possess:

. A higher modulus of elasticity, increasing stress transfer to the surrounding cortical bone
. A lower modulus of elasticity, reducing proximal stress shielding
. Greater resistance to abrasive and third-body wear at articular interfaces
. Higher ultimate tensile strength for modular neck applications
. Absolute inertness to mechanically assisted crevice corrosion

Correct Answer & Explanation

. A higher modulus of elasticity, increasing stress transfer to the surrounding cortical bone

Explanation

Titanium alloys have a significantly lower modulus of elasticity (closer to that of cortical bone) compared to cobalt-chromium or stainless steel. This lowers the stiffness of the implant, which reduces stress shielding of the proximal femur, helping to preserve host bone density. Titanium, however, has poor wear characteristics.

Question 1364

Topic: Biomechanics & Biomaterials
Highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is manufactured to decrease wear rates in total joint arthroplasty. What is the primary disadvantage of highly cross-linking UHMWPE?
. Increased oxidative degradation
. Decreased ultimate tensile strength
. Increased volumetric wear
. Decreased modulus of elasticity
. Increased osteolysis risk

Correct Answer & Explanation

. Decreased ultimate tensile strength

Explanation

Highly cross-linking UHMWPE significantly reduces abrasive wear but simultaneously decreases its mechanical properties, including ultimate tensile strength, yield strength, and fatigue resistance. This makes it more susceptible to catastrophic failure in high-stress applications like tibial posts.

Question 1365

Topic: Biomechanics & Biomaterials

A dynamic splint applies a constant deformation to a contracted joint over time. Which viscoelastic property explains the gradual decrease in force required to maintain this constant deformation?

. Creep
. Stress relaxation
. Hysteresis
. Fatigue failure
. Isotropy

Correct Answer & Explanation

. Creep

Explanation

Stress relaxation is the phenomenon where a viscoelastic material experiences a decrease in stress (force) over time when subjected to a constant strain (deformation). Creep, conversely, is increasing deformation under a constant load.

Question 1366

Topic: Biomechanics & Biomaterials

The addition of barium sulfate or zirconium dioxide to polymethylmethacrylate (PMMA) bone cement serves which primary purpose?

. Increases the fatigue strength of the cement
. Accelerates the polymerization rate
. Decreases the exothermic reaction temperature
. Provides radiopacity
. Enhances antibiotic elution

Correct Answer & Explanation

. Increases the fatigue strength of the cement

Explanation

Barium sulfate and zirconium dioxide are added to PMMA bone cement exclusively to make the cement radiopaque, allowing for radiographic visualization. However, they act as impurities and can decrease the cement's overall mechanical strength.

Question 1367

Topic: Biomechanics & Biomaterials

In total joint arthroplasty, wear debris generated by particles of bone cement caught between the femoral head and polyethylene liner is best classified as:

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

Correct Answer & Explanation

. Adhesive wear

Explanation

Third-body wear occurs when hard, loose particles (such as bone fragments, PMMA cement, or metal debris) become trapped between the two articulating bearing surfaces. This significantly accelerates the wear of the softer polyethylene component.

Question 1368

Topic: Biomechanics & Biomaterials

Which metal alloy used in orthopedic implants possesses a modulus of elasticity most closely matching that of cortical bone?

. Cobalt-chromium-molybdenum
. 316L Stainless steel
. Titanium alloy (Ti-6Al-4V)
. Tantalum
. Commercially pure titanium

Correct Answer & Explanation

. Cobalt-chromium-molybdenum

Explanation

Titanium alloys (like Ti-6Al-4V) have a modulus of elasticity approximately half that of cobalt-chromium or stainless steel, making them the closest match to cortical bone among standard solid metals. This helps reduce stress shielding.

Question 1369

Topic: Biomechanics & Biomaterials

In a standard stress-strain curve for an orthopedic biomaterial, the total area under the curve prior to the point of failure represents the material's:

. Stiffness
. Toughness
. Ductility
. Yield strength
. Viscoelasticity

Correct Answer & Explanation

. Stiffness

Explanation

Toughness is defined as the amount of energy a material can absorb before it fractures, represented by the total area under the stress-strain curve. Stiffness is represented by the slope of the linear elastic region.

Question 1370

Topic: Biomechanics & Biomaterials
Highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is manufactured primarily to reduce which of the following?
. Adhesive wear
. Abrasive wear
. Fatigue failure
. Oxidation degradation
. Fretting wear

Correct Answer & Explanation

. Adhesive wear

Explanation

Highly cross-linked UHMWPE significantly reduces adhesive and abrasive wear in total joint arthroplasty. However, the cross-linking process can reduce the material's overall fatigue strength and fracture toughness.

Question 1371

Topic: Biomechanics & Biomaterials

Which of the following orthopedic implant materials has a modulus of elasticity most similar to that of cortical bone, thereby minimizing stress shielding?

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

Correct Answer & Explanation

. Stainless steel

Explanation

Titanium alloys have a lower modulus of elasticity compared to stainless steel and cobalt-chromium, making them closer to the modulus of cortical bone. This reduces the risk of stress shielding and subsequent periprosthetic bone loss.

Question 1372

Topic: Biomechanics & Biomaterials

A surgeon places a stainless steel screw into a titanium plate to fix a fracture. This combination places the construct at highest risk for which type of corrosion?

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

Correct Answer & Explanation

. Fretting corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in physical contact within an electrolytic medium, such as human body fluid. The less noble metal (anode) undergoes accelerated corrosion.

Question 1373

Topic: Biomechanics & Biomaterials

A 42-year-old male is involved in a high-speed motor vehicle collision and sustains an isolated extra-articular fracture of the scapular neck. Which of the following radiographic parameters is the most widely accepted absolute indication for operative internal fixation?

. Glenopolar angle of 45 degrees
. Medial/lateral translation of 5 mm
. Glenopolar angle of less than 22 degrees
. Angulation of 15 degrees in the sagittal plane
. Glenoid articular step-off of 1 mm

Correct Answer & Explanation

. Glenopolar angle of 45 degrees

Explanation

The normal glenopolar angle (GPA) is between 30 to 45 degrees. A severely decreased GPA (< 22 degrees) indicates significant rotational malalignment of the glenoid fragment relative to the scapular body, which alters rotator cuff biomechanics and glenohumeral kinematics, leading to poor clinical outcomes. Therefore, a GPA < 22 degrees is a primary indication for open reduction and internal fixation of a scapular neck fracture. Other relative indications include medial translation > 20 mm, or angular displacement > 40 degrees.

Question 1374

Topic: Biomechanics & Biomaterials

Manufacturers have introduced Vitamin E-blended highly cross-linked polyethylene for total joint arthroplasty components. The primary biomechanical purpose of incorporating Vitamin E is to prevent which of the following processes?

. Adhesive wear
. Abrasive wear
. Oxidative degradation
. Cold flow (creep)
. Third-body wear

Correct Answer & Explanation

. Adhesive wear

Explanation

Vitamin E acts as a potent antioxidant. It neutralizes free radicals introduced during the irradiation cross-linking process, thereby preventing long-term oxidative degradation of the polyethylene component.

Question 1375

Topic: Biomechanics & Biomaterials

A 45-year-old active female sustains a medial meniscus posterior root tear. From a biomechanical perspective, how does this injury alter the knee joint kinematics and contact pressures?

. Normalizes contact pressure across the medial compartment
. Decreases peak contact pressure by 50%
. Increases the inherent hoop stresses within the remaining meniscus
. It is biomechanically equivalent to a total meniscectomy
. Shifts the mechanical axis to a valgus alignment

Correct Answer & Explanation

. Normalizes contact pressure across the medial compartment

Explanation

A medial meniscus posterior root tear severely compromises the meniscal hoop tension. Biomechanically, the loss of hoop stress containment results in peak contact pressures equivalent to a complete meniscectomy, predisposing the joint to rapid arthrosis.

Question 1376

Topic: Biomechanics & Biomaterials

Orthopedic implants and connective tissues exhibit viscoelastic properties. Which of the following statements provides the most accurate biomechanical definition of 'stress relaxation'?

. A decrease in stress within a material over time when it is subjected to a constant strain
. An increase in strain within a material over time when it is subjected to a constant stress
. The energy lost as heat during the loading and unloading cycles of a material
. The point at which a material transitions from elastic to plastic deformation
. The gradual increase in microcracks leading to failure under repetitive cyclic loading

Correct Answer & Explanation

. A decrease in stress within a material over time when it is subjected to a constant strain

Explanation

Viscoelastic materials exhibit time-dependent responses to loading. 'Stress relaxation' is defined as the progressive decrease in internal stress over time when the material is held at a constant strain (deformation). An example is the decreasing tension in a tightened ligament graft over time. Option 1 defines 'creep', which is the progressive increase in strain (deformation) over time when subjected to a constant stress (load). Option 2 defines 'hysteresis'. Option 3 describes the yield point. Option 4 describes fatigue failure.

Question 1377

Topic: Biomechanics & Biomaterials
In the manufacturing of modern highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) for total joint arthroplasty, what is the primary biomechanical and chemical purpose of adding vitamin E (alpha-tocopherol) to the polymer?
. To increase the melting temperature of the polymer allowing for easier machining
. To improve the initial yield strength of the polyethylene under heavy loads
. To scavenge free radicals generated during irradiation and prevent in vivo oxidation
. To facilitate more rapid bony ingrowth into the porous metal backing
. To increase the overall degree of cross-linking without needing gamma irradiation

Correct Answer & Explanation

. To scavenge free radicals generated during irradiation and prevent in vivo oxidation

Explanation

Irradiation is used to heavily cross-link UHMWPE to reduce wear, but it leaves behind free radicals. Over time, these free radicals can react with oxygen in vivo, leading to oxidative degradation, embrittlement, and failure. Vitamin E (alpha-tocopherol) is a potent antioxidant added to scavenge these free radicals, neutralizing them without needing secondary thermal treatments (like remelting) which can reduce mechanical strength.

Question 1378

Topic: Biomechanics & Biomaterials

In orthopedic biomechanics, stress shielding around an implant is largely determined by the mismatch in stiffness between the implant material and the host bone. Which of the following values best approximates the modulus of elasticity (Young's modulus) of normal human cortical bone?

. 0.5 - 1 GPa
. 15 - 20 GPa
. 100 - 110 GPa
. 200 - 220 GPa
. 500 - 550 GPa

Correct Answer & Explanation

. 0.5 - 1 GPa

Explanation

The modulus of elasticity (stiffness) of cortical bone is approximately 15-20 GPa. For comparison, cancellous bone is much more flexible (0.1-2 GPa). Titanium alloys used in orthopedics have a modulus around 110 GPa, and Stainless Steel/Cobalt-Chrome alloys are around 200-220 GPa. The large difference between metal implants and cortical bone is the primary driver of stress shielding.

Question 1379

Topic: Biomechanics & Biomaterials

When selecting metallic alloys for orthopedic implants, engineers often consider the modulus of elasticity to minimize stress shielding of the surrounding bone. Which of the following implant materials has a modulus of elasticity that most closely approximates that of cortical bone?

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

Correct Answer & Explanation

. Stainless steel

Explanation

Titanium and its alloys have a modulus of elasticity that is much closer to that of cortical bone compared to stiffer metals like cobalt-chromium or stainless steel. This property helps ensure better load sharing and reduces the risk of long-term stress shielding and bone resorption.

Question 1380

Topic: Biomechanics & Biomaterials

The introduction of highly cross-linked polyethylene (HXLPE) has significantly reduced wear rates and osteolysis in total hip arthroplasty. However, the electron beam or gamma irradiation used to create the cross-linking alters the mechanical properties of the plastic. Which of the following mechanical changes is an adverse consequence of increased irradiation and cross-linking?

. Increased ultimate tensile strength
. Increased resistance to fatigue crack propagation
. Decreased yield strength, ductility, and fracture toughness
. Increased generation of free radicals leading to accelerated in vivo oxidation
. Decreased modulus of elasticity resulting in component deformation

Correct Answer & Explanation

. Increased ultimate tensile strength

Explanation

While cross-linking polyethylene dramatically improves adhesive and abrasive wear resistance, it compromises several structural mechanical properties. Highly cross-linked polyethylene has decreased yield strength, decreased ultimate tensile strength, decreased ductility, and decreased fracture toughness compared to conventional polyethylene. Free radicals are generated during irradiation, but modern processing involves melting or annealing to quench these radicals, preventing oxidation.

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