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 941
Topic: Biomechanics & Biomaterials
When a continuous, constant load is applied to a ligament over time, the ligament will gradually elongate. This viscoelastic property is best described as:
Correct Answer & Explanation
. Creep
Explanation
Creep is the time-dependent increase in strain (deformation or elongation) of a viscoelastic material when subjected to a constant stress (load). Stress relaxation is the time-dependent decrease in stress when the material is held at a constant strain. Hysteresis refers to the energy lost (usually as heat) during the loading and unloading cycle of a viscoelastic material.
Question 942
Topic: Biomechanics & Biomaterials
The compressive stiffness of normal articular cartilage is primarily dependent on which of the following structural interactions?
Correct Answer & Explanation
. Repulsive forces between negatively charged glycosaminoglycan chains and restricted water flow
Explanation
The compressive stiffness of articular cartilage is mainly due to the swelling pressure exerted by the highly negatively charged glycosaminoglycan (GAG) chains (part of the aggrecan molecule). These fixed negative charges repel each other and attract cations (and water via osmosis). The collagen network restricts the swelling, and the resistance to fluid flow through this matrix provides the viscoelastic compressive strength.
Question 943
Topic: Biomechanics & Biomaterials
When designing an intramedullary nail for a diaphyseal long-bone fracture, minimizing stress shielding while maintaining adequate fatigue strength is a primary biomechanical goal. Which of the following orthopaedic implant materials has a modulus of elasticity closest to that of human cortical bone?
Correct Answer & Explanation
. Titanium-6Aluminum-4Vanadium alloy
Explanation
Human cortical bone has a modulus of elasticity (Young's modulus) of approximately 15-20 GPa. Titanium alloys (e.g., Ti-6Al-4V) have a modulus of about 110 GPa, which is the closest metallic implant material to cortical bone. Stainless steel is approximately 200 GPa, and Cobalt-chromium alloys are around 220-240 GPa. The lower modulus of elasticity of Titanium reduces the degree of stress shielding compared to stiffer metals.
Question 944
Topic: Biomechanics & Biomaterials
What is the primary mechanism of corrosion at the modular taper junction (head-neck interface) of a total hip arthroplasty?
Correct Answer & Explanation
. Mechanically assisted crevice corrosion
Explanation
Mechanically assisted crevice corrosion (MACC), also known as fretting corrosion, is the primary mode of failure at modular head-neck taper junctions in total hip arthroplasty. The cyclic loading causes mechanical disruption of the protective oxide layer (fretting), and the fluid in the confined modular space becomes acidic and depleted of oxygen (crevice), leading to synergistic corrosion.
Question 945
Topic: Biomechanics & Biomaterials
A 30-year-old athlete sustains a suspected Achilles tendon injury. During the physiological loading of the tendon, the initial 'toe region' of the stress-strain curve represents which specific microscopic change?
Correct Answer & Explanation
. Uncrimping of the naturally wavy collagen fibers
Explanation
The stress-strain curve for ligaments and tendons begins with a non-linear 'toe region.' This region corresponds to the straightening out (uncrimping) of the naturally crimped collagen fibers as initial tension is applied. Once uncrimped, the curve becomes linear (the elastic region), where the fibers are stretched until yielding occurs.
Question 946
Topic: Biomechanics & Biomaterials
During the manufacturing of ultra-high molecular weight polyethylene (UHMWPE) for total joint arthroplasty, highly cross-linked polyethylene is typically subjected to gamma irradiation. What is the primary purpose of the subsequent thermal treatment (melting or annealing) of the polyethylene?
Correct Answer & Explanation
. To eliminate or reduce residual free radicals generated during irradiation.
Explanation
Gamma irradiation is used to cross-link UHMWPE, which significantly improves wear resistance. However, irradiation breaks carbon-carbon bonds, generating free radicals. If these free radicals are left in the material, they can react with oxygen in vivo or on the shelf, leading to oxidative degradation, chain scission, embrittlement, and increased wear. Thermal treatments, such as remelting or annealing, are applied after irradiation to increase the mobility of the polymer chains, allowing the free radicals to recombine and thus reducing the risk of oxidation. Remelting actually reduces crystallinity and ultimate tensile strength, making Options A and C incorrect.
Question 947
Topic: Biomechanics & Biomaterials
Corrosion of metallic orthopaedic implants can lead to catastrophic implant failure and adverse local tissue reactions. Which of the following scenarios best describes the mechanism underlying galvanic corrosion?
Correct Answer & Explanation
. Electrochemical dissolution occurring when two dissimilar metals are placed in direct physical contact within an electrolytic medium
Explanation
Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials (e.g., a stainless steel wire around a titanium rod) are placed in direct contact within an electrolytic solution (such as human body fluid). The less noble metal acts as an anode and undergoes accelerated corrosion. Fretting corrosion, in contrast, results from micromotion at modular interfaces (such as a head-neck taper) that repetitively strips the protective oxide (passivation) layer. Corrosion fatigue combines cyclic mechanical loading with a corrosive environment.
Question 948
Topic: Biomechanics & Biomaterials
What is the primary biomechanical tradeoff of heating highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) above its melting point (remelting) during processing to quench residual free radicals?
Correct Answer & Explanation
. Decreased ultimate tensile strength and fatigue resistance
Explanation
Highly cross-linked polyethylene is irradiated to improve wear resistance, but this creates free radicals that can lead to oxidation and degradation in vivo. To eliminate these free radicals, the material is either annealed (heated below the melting point) or remelted (heated above the melting point). Remelting effectively eliminates all free radicals, preventing oxidation, but the structural changes associated with melting significantly decrease the ultimate tensile strength, yield strength, and fatigue resistance of the material. Annealing preserves mechanical strength but leaves residual free radicals.
Question 949
Topic: Biomechanics & Biomaterials
Among the following solid metallic materials commonly used in orthopedic implants, which possesses the lowest modulus of elasticity (Young's modulus), thereby theoretically minimizing the risk of stress shielding when utilized for diaphyseal fixation?
Correct Answer & Explanation
. Titanium alloy (Ti-6Al-4V)
Explanation
The modulus of elasticity (Young's modulus) represents the intrinsic stiffness of a material. Cortical bone has a modulus of approximately 15-20 GPa. Titanium alloys (e.g., Ti-6Al-4V) have a modulus of about 100-110 GPa, which is roughly half the stiffness of stainless steel (approx. 200 GPa) and cobalt-chromium alloys (approx. 210-230 GPa). Although titanium is still significantly stiffer than bone, its relatively lower elastic modulus compared to other common solid surgical metals translates to improved load-sharing and theoretically less severe stress shielding around intramedullary stems and plates.
Question 950
Topic: Biomechanics & Biomaterials
In an orthopedic biomechanics laboratory, a researcher is testing the viscoelastic properties of a human anterior cruciate ligament (ACL) graft. A constant, unchanging tensile load is applied to the graft, and the researcher observes a gradual, time-dependent increase in the deformation (length) of the tissue. Which of the following biomechanical phenomena does this observation describe?
Correct Answer & Explanation
. Creep
Explanation
Creep is the progressive deformation of a viscoelastic material when subjected to a constant load over time. Stress relaxation, another viscoelastic property, is the decrease in stress (force) within a material over time when it is held at a constant deformation or length. Hysteresis represents the energy lost (usually as heat) during the loading and unloading cycles of a material. Anisotropy refers to a material possessing different mechanical properties depending on the direction of the applied load.
Question 951
Topic: Biomechanics & Biomaterials
Viscoelastic materials, such as articular cartilage and tendons, exhibit time-dependent mechanical behaviors. Which of the following best describes the phenomenon of 'stress relaxation'?
Correct Answer & Explanation
. Decreasing stress over time under a constant deformation
Explanation
Viscoelastic materials display time-dependent responses to loading. Stress relaxation is the decrease in internal stress over time when the material is held at a constant deformation (strain). Creep is the gradual increase in deformation (strain) over time under a constant load (stress). Hysteresis is the energy lost (as heat) during the loading and unloading cycle. Strain-rate dependency means the material becomes stiffer when loaded at a faster rate.
Question 952
Topic: Biomechanics & Biomaterials
In total hip arthroplasty, the use of highly cross-linked polyethylene (HXLPE) has significantly reduced wear rates compared to conventional ultra-high molecular weight polyethylene (UHMWPE). Which of the following trade-offs is most commonly associated with increasing the radiation dose to maximize cross-linking in HXLPE?
Correct Answer & Explanation
. Decreased fatigue and fracture resistance
Explanation
Cross-linking of polyethylene by irradiation improves its wear resistance significantly. However, increasing the radiation dose leads to decreased mechanical properties, specifically reducing ultimate tensile strength, elongation to failure, and fracture/fatigue resistance. To prevent in vivo oxidation caused by free radicals generated during irradiation, the polyethylene is usually melted or annealed, or infused with an antioxidant like Vitamin E.
Question 953
Topic: Biomechanics & Biomaterials
Galvanic corrosion is a mode of implant failure that involves the electrochemical destruction of metal. It is most likely to occur in vivo when which of the following two metal alloys are placed in direct contact?
Correct Answer & Explanation
. Stainless steel and titanium alloy
Explanation
Galvanic corrosion occurs when two dissimilar metals are placed in an electrolytic solution (such as bodily fluids). The risk of galvanic corrosion is highest when there is a large difference in the anodic index between the two metals. Stainless steel and titanium have significantly different electrochemical potentials, making their combination highly susceptible to galvanic corrosion. Cobalt-chromium and titanium are frequently used together because their electrochemical potentials are similar.
Question 954
Topic: Biomechanics & Biomaterials
A 65-year-old active man is undergoing total hip arthroplasty. The surgeon opts to use a highly cross-linked polyethylene (HXLPE) liner. Which of the following best describes the mechanical trade-off when comparing HXLPE to conventional ultra-high-molecular-weight polyethylene (UHMWPE)?
Correct Answer & Explanation
. Decreased volumetric wear but reduced fracture toughness and ultimate tensile strength
Explanation
Highly cross-linked polyethylene (HXLPE) is manufactured by exposing conventional UHMWPE to radiation, which breaks polymer chains and allows them to recombine (cross-link). This process significantly decreases volumetric wear, thereby drastically reducing the incidence of particle-induced osteolysis. However, the cross-linking process alters the mechanical properties of the material, resulting in decreased fracture toughness, reduced ultimate tensile strength, and reduced ductility (elongation to failure).
Question 955
Topic: Biomechanics & Biomaterials
Highly cross-linked polyethylene (HXLPE) is widely used in total hip arthroplasty to reduce the incidence of wear and subsequent osteolysis. What is the primary negative biomechanical consequence of increasing the radiation dose used for cross-linking the ultra-high-molecular-weight polyethylene (UHMWPE)?
Correct Answer & Explanation
. Decreased fatigue resistance and fracture toughness
Explanation
While high levels of radiation used to cross-link polyethylene dramatically decrease wear rates, they negatively impact the material's mechanical properties. Specifically, high radiation doses lead to decreased ultimate tensile strength, decreased fatigue resistance, and decreased fracture toughness. This makes the material more susceptible to fracture, which is why highly cross-linked polyethylene use in total knee arthroplasty (which has higher contact stresses and varied kinematics) required careful optimization.
Question 956
Topic: Biomechanics & Biomaterials
A 50-year-old female experiences a sudden "pop" in the posterior aspect of her knee while descending stairs. She is diagnosed with a complete medial meniscus posterior root tear with 3 mm of extrusion on MRI. Biomechanically, what is the direct consequence of leaving this tear untreated compared to the native knee?
Correct Answer & Explanation
. Increased peak contact pressure equivalent to a total medial meniscectomy
Explanation
A complete posterior root tear of the medial meniscus leads to an immediate loss of circumferential hoop stresses, which effectively renders the meniscus nonfunctional (meniscal extrusion). Biomechanically, this is equivalent to a total medial meniscectomy, leading to significantly increased peak contact pressures and accelerated onset of osteoarthritis.
Question 957
Topic: Biomechanics & Biomaterials
A 20-year-old collegiate baseball pitcher reports progressive medial elbow pain and decreased pitching velocity. The moving valgus stress test is positive. MRI shows a high-grade partial tear of the anterior bundle of the ulnar collateral ligament (UCL). During surgical reconstruction, where is the most appropriate anatomical insertion site on the ulna for the graft?
Correct Answer & Explanation
. Sublime tubercle
Explanation
The anterior bundle of the ulnar collateral ligament (UCL) is the primary restraint to valgus stress at the elbow during the throwing motion. It originates on the anteroinferior aspect of the medial epicondyle and inserts on the sublime tubercle of the proximal ulna. Reconstruction grafts must be routed through or anchored at this anatomic footprint to restore proper biomechanics.
Question 958
Topic: Biomechanics & Biomaterials
The creation of highly cross-linked polyethylene (HXLPE) for THA involves irradiating the material. What is the primary purpose of post-irradiation thermal treatment (remelting or annealing) in the manufacturing process of HXLPE?
Correct Answer & Explanation
. To reduce the generation of free radicals
Explanation
Irradiation of polyethylene creates beneficial cross-links but also generates free radicals that can lead to in vivo oxidation and degradation. Post-irradiation thermal treatments extinguish these free radicals and improve long-term oxidative stability.
Question 959
Topic: Biomechanics & Biomaterials
To significantly reduce the wear rate in total hip arthroplasty, highly cross-linked polyethylene (HXLPE) is manufactured by subjecting ultra-high molecular weight polyethylene (UHMWPE) to gamma or electron-beam irradiation. What is the primary mechanical trade-off associated with increasing the irradiation dose?
Correct Answer & Explanation
. Decreased fatigue resistance and ultimate tensile strength
Explanation
While high doses of irradiation increase cross-linking and drastically reduce wear rates, the primary trade-off is a decrease in the material's mechanical properties, including fatigue resistance, ultimate tensile strength, and fracture toughness.
Question 960
Topic: Biomechanics & Biomaterials
Regarding the material properties of highly cross-linked polyethylene (HXLPE) used in total hip arthroplasty, increasing the radiation dose to enhance cross-linking primarily results in a trade-off characterized by which of the following mechanical changes?
Correct Answer & Explanation
. Decreased fatigue crack propagation resistance
Explanation
Highly cross-linked polyethylene (HXLPE) is created by exposing standard ultra-high-molecular-weight polyethylene to ionizing radiation, which forms free radicals that recombine to create cross-links between polymer chains. While this significantly decreases the volumetric wear rate, it comes at the cost of decreasing several mechanical properties, particularly fatigue resistance, ductility, yield strength, and ultimate tensile strength. Therefore, decreased fatigue crack propagation resistance is the primary mechanical trade-off of higher cross-linking doses. Oxidation resistance is typically addressed by subsequent thermal treatments (remelting or annealing) or adding antioxidants like Vitamin E.
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