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Biomechanics & Biomaterials MCQs

Practice Set 55 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 1081

Topic: Biomechanics & Biomaterials

Which biomechanical property best describes the time-dependent deformation of a material under a constant load, exemplified by the continued lengthening of a tendon when subjected to sustained tensile force?

. Elasticity
. Plasticity
. Viscosity
. Creep
. Stress relaxation

Correct Answer & Explanation

. Creep

Explanation

Creep is the time-dependent increase in deformation (strain) of a material under a constant applied load (stress). Tendons and ligaments, being viscoelastic materials, exhibit creep. Elasticity refers to reversible deformation. Plasticity refers to permanent deformation after load removal. Viscosity describes resistance to flow. Stress relaxation is the time-dependent decrease in stress within a material when it is held at a constant strain.

Question 1082

Topic: Biomechanics & Biomaterials

A 65-year-old female presents with acute, intense pain, swelling, and warmth in her left knee. Aspiration yields cloudy synovial fluid. Microscopic analysis of the fluid reveals rhomboid-shaped crystals that exhibit weak positive birefringence under polarized light. What is the primary constituent of these crystals?

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

Correct Answer & Explanation

. Calcium pyrophosphate dihydrate

Explanation

Rhomboid-shaped, weakly positively birefringent crystals are pathognomonic for pseudogout. The crystals are composed of calcium pyrophosphate dihydrate (CPPD).

Question 1083

Topic: Biomechanics & Biomaterials
In total hip arthroplasty, using highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) alters its mechanical properties compared to conventional polyethylene. Which of the following represents the most accurate mechanical trade-off of cross-linking?
. Increases ultimate tensile strength and decreases oxidation resistance
. Decreases volumetric wear but reduces fatigue resistance and fracture toughness
. Decreases adhesive wear but significantly increases abrasive wear
. Increases resistance to third-body wear while increasing creep
. Enhances the elastic modulus but decreases yield strength

Correct Answer & Explanation

. Decreases volumetric wear but reduces fatigue resistance and fracture toughness

Explanation

Highly cross-linked polyethylene significantly decreases the volumetric wear rate (both adhesive and abrasive), reducing osteolysis. However, the cross-linking process and subsequent thermal treatments (remelting/annealing) alter its mechanical properties, resulting in decreased ultimate tensile strength, decreased yield strength, decreased fatigue resistance, and reduced fracture toughness compared to conventional UHMWPE.

Question 1084

Topic: Biomechanics & Biomaterials

An orthopedic surgeon utilizes a stainless steel screw to secure a titanium alloy plate. This mixing of dissimilar metals can lead to galvanic corrosion. In this specific construct, which of the following best describes the electrochemical process?

. Titanium acts as the anode and undergoes preferential corrosion
. Stainless steel acts as the anode and undergoes preferential corrosion
. Both metals undergo simultaneous anodic degradation due to crevice corrosion
. Titanium acts as the cathode and dissolves into the surrounding tissue
. Stainless steel acts as the cathode and becomes protected from fretting

Correct Answer & Explanation

. Stainless steel acts as the anode and undergoes preferential corrosion

Explanation

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

Question 1085

Topic: Biomechanics & Biomaterials

A ligament undergoes a constant load over a prolonged period. It is observed that the ligament continues to deform slowly over time despite the load remaining unchanged. Which of the following biomechanical properties best describes this phenomenon?

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

Correct Answer & Explanation

. Creep

Explanation

Creep is defined as the progressive deformation of a viscoelastic material under a constant load over time. Stress relaxation, conversely, is the decrease in stress over time when the material is held at a constant deformation. Hysteresis represents the energy lost (usually as heat) during the loading and unloading cycles of a viscoelastic material.

Question 1086

Topic: Biomechanics & Biomaterials

A patient with a stainless steel dynamic compression plate for a femur fracture undergoes revision surgery where a titanium screw is inadvertently used to secure a loose segment of the plate. Over time, localized corrosion is noted at the titanium-stainless steel interface. Which of the following statements regarding galvanic corrosion in this scenario is true?

. Titanium acts as the anode and corrodes
. Stainless steel acts as the anode and corrodes
. Titanium acts as the cathode and corrodes
. Stainless steel acts as the cathode and corrodes
. Galvanic corrosion cannot occur between titanium and stainless steel in vivo

Correct Answer & Explanation

. Stainless steel acts as the anode and corrodes

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in an electrolytic solution (like bodily fluids). The less noble metal (anode) undergoes accelerated corrosion, while the more noble metal (cathode) is protected. Stainless steel is less noble than titanium; therefore, the stainless steel acts as the anode and corrodes.

Question 1087

Topic: Biomechanics & Biomaterials
To reduce wear rates in total joint arthroplasty, ultra-high molecular weight polyethylene (UHMWPE) is often highly cross-linked via irradiation. However, irradiation creates free radicals that can lead to oxidation and embrittlement. Which of the following is the most definitive traditional method used to eliminate these free radicals post-irradiation?
. Cold sterilization using ethylene oxide
. Melting the polyethylene above its crystalline melting point
. Immersing the polyethylene in a saline bath
. Doping the polyethylene with cobalt-chromium particles
. Irradiating the polyethylene in an oxygen-rich environment

Correct Answer & Explanation

. Melting the polyethylene above its crystalline melting point

Explanation

Highly cross-linked polyethylene is produced by gamma or electron beam irradiation, which leaves free radicals. Heating the polymer above its crystalline melting point (remelting, ~150°C) provides enough molecular mobility for the free radicals to recombine, effectively eliminating them and reducing future oxidation (though it slightly reduces mechanical strength). Alternatively, annealing or doping with Vitamin E can be used.

Question 1088

Topic: Biomechanics & Biomaterials

In orthopedic implants, placing a stainless steel plate in contact with a titanium screw in the presence of body fluids primarily leads to which type of corrosion?

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

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two electrochemically dissimilar metals are placed in contact within a conductive medium (such as body fluids). The less noble metal becomes the anode and corrodes, while the more noble metal acts as the cathode. While fretting and crevice corrosion are common in modular components of the same material (e.g., head-neck tapers), mixing stainless steel and titanium specifically drives galvanic corrosion.

Question 1089

Topic: Biomechanics & Biomaterials

A pediatric orthopedic surgeon applies a Ponseti cast to an infant with a clubfoot. Over the next week, the constant deformation of the contracted medial soft tissues leads to a gradual decrease in the applied tension within those tissues. This biomechanical phenomenon is known as:

. Creep
. Stress relaxation
. Hysteresis
. Fatigue failure
. Isotropic behavior

Correct Answer & Explanation

. Stress relaxation

Explanation

Stress relaxation is a viscoelastic property where the stress (internal tension) within a material decreases over time when it is held at a constant strain (deformation). In serial casting, holding the soft tissues in a stretched position leads to stress relaxation. Creep, conversely, is the progressive deformation of a material when subjected to a constant load (stress).

Question 1090

Topic: Biomechanics & Biomaterials

When comparing the biomechanics of a locking compression plate (LCP) construct to a conventional non-locking plate construct, which of the following best describes the fundamental mechanism by which a locking plate provides stability?

. Locking plates rely on high friction between the plate and the underlying bone surface.
. Locking plates require perfect anatomical contouring to maintain fracture reduction.
. Locking plates function as a single-beam construct dependent on the secure screw-plate interface.
. Conventional plates provide greater preservation of the periosteal blood supply.
. Conventional plates inherently provide relative stability rather than absolute stability.

Correct Answer & Explanation

. Locking plates function as a single-beam construct dependent on the secure screw-plate interface.

Explanation

Locking plates achieve stability by threading the screw head directly into the plate, creating a fixed-angle, single-beam construct. This does not rely on compressing the plate against the bone, thus it preserves periosteal blood supply and does not require perfect contouring. Conventional plates rely on friction generated by compressing the plate against the bone.

Question 1091

Topic: Biomechanics & Biomaterials

Among the commonly used synthetic bone graft substitutes, which material possesses the highest compressive strength but exhibits the slowest rate of in vivo resorption?

. Calcium sulfate
. Tricalcium phosphate
. Hydroxyapatite
. Demineralized bone matrix
. Cancellous allograft

Correct Answer & Explanation

. Hydroxyapatite

Explanation

Hydroxyapatite is a crystalline calcium phosphate ceramic that has excellent compressive strength but is resorbed extremely slowly in vivo, often taking years or persisting indefinitely. In contrast, calcium sulfate resorbs very rapidly (weeks to months), and tricalcium phosphate has an intermediate resorption rate.

Question 1092

Topic: Biomechanics & Biomaterials

When cortical bone is subjected to mechanical loading at higher strain rates (e.g., a high-speed motor vehicle collision compared to a low-speed fall), which of the following describes its biomechanical response?

. It becomes less stiff and fails at a lower load
. It becomes more stiff and fails at a higher load
. Its stiffness remains constant but it undergoes greater plastic deformation
. It becomes more brittle and fails at a lower load
. Its energy-absorbing capacity decreases significantly

Correct Answer & Explanation

. It becomes more stiff and fails at a higher load

Explanation

Bone is a viscoelastic material, meaning its biomechanical properties depend on the rate of loading (strain rate). At higher strain rates, bone becomes stiffer, absorbs more energy before failure, and sustains higher loads before fracturing. The increased stored energy at failure is what leads to highly comminuted fractures in high-energy trauma.

Question 1093

Topic: Biomechanics & Biomaterials

A patient undergoes revision total hip arthroplasty. The surgeon uses a stainless steel cerclage wire around the proximal femur in contact with a titanium femoral stem. What specific type of corrosion is most likely to occur at the interface of these two metals?

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

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two electrochemically dissimilar metals (e.g., stainless steel and titanium) are placed in direct physical contact within an electrolytic solution (such as bodily fluids). The less noble metal acts as an anode and corrodes.

Question 1094

Topic: Biomechanics & Biomaterials

The observation that bone architecture remodels in response to the mechanical stresses placed upon it is mathematically and mechanistically explained by which of the following principles?

. Heuter-Volkmann principle
. Wolff's Law
. Hooke's Law
. Young's modulus
. Poisson's ratio

Correct Answer & Explanation

. Wolff's Law

Explanation

Wolff's Law states that bone grows and remodels in response to the forces that are placed upon it ('form follows function'). The Heuter-Volkmann principle applies to physeal growth, stating that compressive forces inhibit growth while tensile forces stimulate it.

Question 1095

Topic: Biomechanics & Biomaterials

Which of the following clinical scenarios is most likely to result in galvanic corrosion of an orthopedic implant?

. Fretting at the head-neck junction of a modular titanium hip prosthesis
. Using a stainless steel screw to secure a titanium alloy plate
. Scratching the passivation layer of a cobalt-chrome femoral stem
. High-cycle fatigue loading of a stainless steel intramedullary nail
. Contact between ultra-high-molecular-weight polyethylene and a roughened metallic surface

Correct Answer & Explanation

. Using a stainless steel screw to secure a titanium alloy plate

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., stainless steel and titanium) are placed in contact within an electrolytic environment, such as the human body. This leads to an electrochemical potential difference and accelerated corrosion of the less noble metal.

Question 1096

Topic: Biomechanics & Biomaterials

On a standard load-elongation (stress-strain) curve for a healthy ligament, what structural phenomenon is primarily responsible for the initial 'toe region'?

. Microscopic failure of individual collagen fibers
. Macroscopic plastic deformation
. Uncrimping of type I collagen fibrils
. Breaking of enzymatic cross-links between proteoglycans
. Elastic deformation of the bone-ligament interface

Correct Answer & Explanation

. Uncrimping of type I collagen fibrils

Explanation

The non-linear 'toe region' of the stress-strain curve represents the straightening out, or 'uncrimping,' of the wavy collagen fibrils in tendons and ligaments. Once uncrimped, the structure enters the linear elastic region.

Question 1097

Topic: Biomechanics & Biomaterials

According to the mechanical principles of intramedullary nailing, how is the torsional rigidity of a solid cylindrical titanium rod affected if its radius is doubled?

. It increases by a factor of 2
. It increases by a factor of 4
. It increases by a factor of 8
. It increases by a factor of 16
. It remains unchanged

Correct Answer & Explanation

. It increases by a factor of 16

Explanation

The torsional rigidity of a solid cylinder is proportional to the polar moment of inertia, which is directly proportional to the fourth power of the radius (r^4). Therefore, doubling the radius increases the torsional rigidity by a factor of 16.

Question 1098

Topic: Biomechanics & Biomaterials
Which of the following processing methods for ultra-high-molecular-weight polyethylene (UHMWPE) in total joint arthroplasty provides the greatest reduction in long-term oxidative degradation while maintaining wear resistance?
. Gamma irradiation in air followed by terminal sterilization
. Gamma irradiation in an inert gas environment followed by remelting
. Ethylene oxide sterilization without irradiation
. Gamma irradiation followed by cold sterilization
. Cross-linking with electron beam irradiation without thermal treatment

Correct Answer & Explanation

. Gamma irradiation in an inert gas environment followed by remelting

Explanation

Irradiation of UHMWPE creates cross-links that improve wear resistance but also creates free radicals that lead to oxidative degradation and embrittlement. Processing the polyethylene in an inert (oxygen-free) environment and subsequently thermally treating it (remelting or annealing) extinguishes the free radicals, significantly reducing long-term oxidative degradation while maintaining the wear properties of highly cross-linked polyethylene.

Question 1099

Topic: Biomechanics & Biomaterials
Highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is widely used in modern total joint arthroplasty to reduce wear. While cross-linking successfully decreases adhesive and abrasive wear, it significantly diminishes which of the following mechanical properties?
. Hardness
. Fatigue resistance
. Oxidation resistance
. Elastic modulus
. Melting temperature

Correct Answer & Explanation

. Fatigue resistance

Explanation

The irradiation process used to highly cross-link UHMWPE improves wear resistance but decreases its fatigue resistance, ductility, ultimate tensile strength, and yield strength. To mitigate free radicals generated during this process and prevent oxidation, manufacturers utilize techniques such as remelting, annealing, or adding an antioxidant like Vitamin E.

Question 1100

Topic: Biomechanics & Biomaterials

An intramedullary nail utilized for a tibial shaft nonunion presents with a broken distal locking screw, as conceptualized in

. Microscopic analysis of the retrieved screw shows beach-mark lines on the fracture surface. What is the primary mechanism of failure for this implant?

. Ductile failure
. Brittle failure
. Galvanic corrosion
. Fatigue failure
. Creep

Correct Answer & Explanation

. Fatigue failure

Explanation

Orthopedic implants in the setting of delayed union or nonunion typically fail via fatigue failure. Fatigue is the structural failure of a material subjected to repeated cyclic loading at stress levels well below the ultimate tensile strength. Macroscopically, fatigue failure does not present with severe plastic deformation (unlike ductile failure) and is classically identified by 'beach marks' or striations on the failure surface under electron microscopy.

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