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Biomechanics & Biomaterials MCQs

Practice Set 14 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 261

Topic: Biomechanics & Biomaterials

The wear resistance of ultra-high molecular weight polyethylene can be improved by exposing the polymer to high-energy radiation (eg, gamma or electron beam), followed by a thermal treatment. What is one detrimental side effect of this process?

. Decreased resistance to oxidative degradation
. Decreased toughness
. Increased density of nonconsolidated particles
. Increased elastic modulus
. High levels of residual free radicals

Correct Answer & Explanation

. Decreased toughness

Explanation

Highly cross-linked polyethylene has gained widespread acceptance for joint arthroplasty components because of reported experimental and early clinical accounts of significant reductions in wear. Cross-linking is increased by imparting additional energy into the polymer (above that conventionally used for sterilization). The thermal treatments after cross-linking stabilize the material against oxidative degradation by quenching free radicals and also reduce the elastic modulus. One disadvantage of the increased cross-linking is a reduction in toughness that makes the polyethylene more susceptible to crack initiation and propagation. The reduced toughness raises concerns for gross component fracture and fracture at stress concentrations that can arise with the locking mechanisms used to secure polyethylene inserts into metallic backings. Nonconsolidated polyethylene particles have been associated with increased subsurface density secondary to oxidative degradation in conventional polyethylene implants. The quenching of free radicals by thermal treatment in highly cross-linked polyethylene should prevent this problem. Collier JP, Currier BH, Kennedy FE, et al: Comparison of cross-linked polyethylene materials for orthopaedic applications. Clin Orthop 2003;414:289-304. Buckwalter JA, Einhorn TA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System, ed 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2000, pp 203-208.

Question 262

Topic: Biomechanics & Biomaterials

Plots of stress versus strain for four orthopaedic biomaterials are shown in Figure 3. Referring to the figure, what is the correct identification of the curves?

Basic Science 2006 Practice Questions: Set 3 (Solved) - Figure 1

. A = cortical bone; B = bone cement; C = cobalt alloy; D = titanium alloy
. A = cortical bone; B = bone cement; C = cobalt alloy; D = stainless steel
. A = bone cement; B = cortical bone; C = titanium alloy; D = stainless steel
. A = bone cement; B = cortical bone; C = cobalt alloy; D = titanium alloy
. A = polyethylene; B = cortical bone; C = stainless steel; D = titanium alloy

Correct Answer & Explanation

. A = bone cement; B = cortical bone; C = cobalt alloy; D = titanium alloy

Explanation

Stress-strain plots allow easy comparison of a number of important mechanical properties, including elastic modulus (the slope of the initial straight line portion of the curve) and yield stress (the stress at the break in the curves for bone, steel, and titanium alloy). Important considerations here are much lower modulus and ultimate stress of bone and cement compared to the two metallic alloys, the fact that titanium is lower modulus but higher strength than stainless steel, and the identification of cement as the only brittle material among the four. Burstein AH, Wright TM: Fundamentals of Orthopaedic Biomechanics. Baltimore, MD, Williams and Wilkins, 1994, pp 97-129.

Question 263

Topic: Biomechanics & Biomaterials

The load versus deformation curve of the functional spinal unit (FSU) is made up of the neutral zone, the elastic zone, and the plastic zone. What is the plastic zone of the curve believed to represent?

. Lining up of collagen fibers within the ligaments
. Stretching of elastin within the disk and ligaments
. Transition between flexion and extension
. Reversible elongation of the soft tissues
. Traumatic range of motion, resulting in damage to the soft tissues

Correct Answer & Explanation

. Traumatic range of motion, resulting in damage to the soft tissues

Explanation

Plastic deformation of viscoelastic tissues represents deformation of the soft tissues to the point of failure. The lining up of collagen fibers would be in the "toe region" of the curve, which, in the case of the FSU, would be mainly in the neutral zone. Elastin is a minor contributor to the composition of the ligaments and would be protected by the stiffer collagen fibers. The transition between flexion and extension occurs in the neutral zone, and reversible elongation occurs in the elastic zone. Fardon DF, Garfin SR, Abitbol J, et al (eds): Orthopaedic Knowledge Update: Spine 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, pp 15-23.

Question 264

Topic: Biomechanics & Biomaterials

A patient with a cobalt-chromium alloy (Co-Cr) femoral stem has a periprosthetic fracture that is to be fixed with a cable-plate device. The surgeon should make sure that the plate, screws, and cable, respectively, are made of

. Co-Cr, stainless steel, stainless steel.
. stainless steel, stainless steel, Co-Cr.
. stainless steel, Co-Cr, Co-Cr.
. titanium alloy, titanium alloy, titanium alloy.
. titanium alloy, stainless steel, Co-Cr.

Correct Answer & Explanation

. stainless steel, Co-Cr, Co-Cr.

Explanation

Contact between metals in a biologic environment leads to galvanic corrosion. Reduction potentials of Co-Cr and stainless steel produce the worst combination of metals in commonly used implants. Because the fixation implants are not intended to contact the existing implant, it is not as great a consideration as the plate and the screws and cables that will directly contact each other. Miller MD (ed): Review of Orthopaedics, ed 3. Philadelphia, PA, WB Saunders, 2000, pp 119-144.

Question 265

Topic: Biomechanics & Biomaterials

Figure 19 shows the current radiograph of a 48-year-old man who reports hip pain and marked difficulty walking after undergoing revision of a failed total hip replacement 2 years ago. What is the mechanism of failure?

Hip Board Review 2001: High-Yield MCQs (Set 2) - Figure 19

. Fatigue
. Crevice corrosion
. Galvanic corrosion
. Loosening
. Wear

Correct Answer & Explanation

. Fatigue

Explanation

Fatigue from repetitive loading of the stem with the distal aspect well-fixed resulted in stem failure. If the stem had loosened, it would not have broken. Crevice corrosion occurs at a taper interface; galvanic corrosion occurs at the junction of two metals of differing electrochemical potentials, not along a uniform portion of the implant. Simon SR (ed): Orthopaedic Basic Science. Rosemont, IL, American Academy of Orthopaedic Surgeons, 1994, pp 449-486.

Question 266

Topic: Biomechanics & Biomaterials

When polyethylene is exposed to radiation and subsequently heated, certain chemical changes occur in the material. Which of the following statements best describes these changes?

. The process converts an otherwise interpenetrating networking structure of polymer chains into a linear, high molecular weight polyethylene macromolecule.
. The process increases the ductility of the material.
. The process leads to fewer particles that are larger in size than the untreated material.
. The process improves (lowers) the wear rate but may increase the risk of fracture.
. The process decreases the wear rate of the material, compared to untreated polyethylene, when tested against a rough counterface.

Correct Answer & Explanation

. The process improves (lowers) the wear rate but may increase the risk of fracture.

Explanation

Exposure of polyethylene to radiation and then heating it to quench the free radicals leads to a cross-linked material. It converts a high molecular weight polyethylene macromolecule to an interpenetrating network structure of polymer chains. The ductility of the material is decreased, hence the greater risk of fracture. While the wear rate (measured as fewer and smaller particles) against a smooth counterface is markedly reduced, cross-linked polyethylene has shown a larger increase in wear rate when a rougher counterface is used compared to noncross-linked material. Due to reduced mechanical strength, highly cross-linked polyethylene is less resistant to abrasive wear.

Question 267

Topic: Biomechanics & Biomaterials

Why is tendon considered an anisotropic material?

. Young's modulus is greater than that of bone.
. Young's modulus is greater than that of ligament.
. Mechanical properties change with preconditioning.
. Intrinsic mechanical properties vary depending on the direction of loading.
. Intrinsic mechanical properties vary depending on the rate of loading.

Correct Answer & Explanation

. Intrinsic mechanical properties vary depending on the direction of loading.

Explanation

Anisotropic materials have mechanical properties that vary based on the direction of loading. The relative values of Young's modulus for tendon, ligament, and bone are not relevant to isotropy. The mechanical properties of tendon do change with preconditioning, but this change is related to viscoelasticity. The intrinsic mechanical properties of tendon do vary with the rate of loading, but this variance is related to viscoelasticity. Buckwalter JA, Einhorn TA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System, ed 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2000, pp 134-180.

Question 268

Topic: Biomechanics & Biomaterials

A 45-year-old man has severe pain in both feet after his boots become wet while hunting. Examination 3 hours after the onset of symptoms reveals that his feet are cold to touch and the skin appears blanched. Management should consist of

. slow rewarming in cool 77 degrees F (25 degrees C) water.
. rapid rewarming in a footbath at 104.0 degrees F to 107.6 degrees F (40 degrees C to 42 degrees C).
. rewarming in 98.6 degrees F (37 degrees C) water.
. heated blankets at 100.4 degrees F (38 degrees C).
. a heating pad at 104.0 degrees F (40 degrees C).

Correct Answer & Explanation

. rapid rewarming in a footbath at 104.0 degrees F to 107.6 degrees F (40 degrees C to 42 degrees C).

Explanation

The patient has frostbite involving both feet. Rapid rewarming in a protected environment is the initial treatment. A footbath with water at 104.0 degrees F to 107.6 degrees F (40 degrees C to 42 degrees C) is ideal. This facilitates a uniform rewarming of the involved tissue. The other choices are less than ideal. Appliances such as heating pads provide uneven heating and may actually burn the skin. Pinzur MS: Frostbite: Prevention and treatment. Biomechanics 1997;4:14-21.

Question 269

Topic: Biomechanics & Biomaterials

Clinical evidence suggests that grafts for replacing a torn anterior cruciate ligament often stretch after surgery. What is the most probable mechanism for this behavior?

. Gross failure at the attachment sites
. Fatigue failure of the ligament tissue
. Creep of the graft material
. Water absorption by the graft material
. Elastic stretch of collagen fibers

Correct Answer & Explanation

. Creep of the graft material

Explanation

The stretching of the graft occurs over time as the graft is loaded. Time-dependent deformation under load is called creep and is common in viscoelastic materials such as ligament tissue. Creep can occur under both static and cyclic load conditions; time-dependent deformation will occur as long as load is applied to the tissue. Similarly, when a graft is initially tensioned to a given deformation at surgery, the load generated in the graft will decrease over time; this behavior is called stress relaxation and also is indicative of a viscoelastic material. Water content may affect the viscoelastic properties by changing the friction between collagen fibers, but studies have shown little difference in water content between grafts and normal ligaments. Fatigue failures may manifest themselves through damage to the ligament tissue, but this would require higher loads than are routinely experienced by grafts. Elastic stretch is recoverable and, therefore, does not contribute to a permanent stretch. Similarly, gross failure at the attachment would not cause a stretch, but rather a catastrophic instantaneous instability. Boorman RS, Thornton GM, Shrive NG, et al: Ligament grafts become more susceptible to creep within days after surgery. Acta Orthop Scand 2002;73:568-574. Buckwalter JA, Einhorn TA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System, ed 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2000, pp 596-609.

Question 270

Topic: Biomechanics & Biomaterials

Which of the following statements best characterizes polymethylmethacrylate (PMMA) when it is used to secure joint components in bone and to distribute the forces evenly across the bone-implant interface?

Hip Board Review 2004: High-Yield MCQs (Set 2) - Figure 2

. PMMA is stronger in tension than compression.
. Porosity reduction increases the fatigue strength of PMMA.
. Hypotension that occasionally results after PMMA is placed in the femoral canal is independent of a patient's intraoperative blood volume.
. Inclusion of antibiotics does not alter the strength of PMMA.
. PMMA bonds chemically to bone and the implant surface.

Correct Answer & Explanation

. Porosity reduction increases the fatigue strength of PMMA.

Explanation

PMMA has no adhesive properties and can be more accurately described as grout than glue. It does not chemically bond to bone or implants; however, mechanical bonding is accomplished with porous or coated components and with cancellous bone. PMMA is approximately three times stronger in compression than in tension. Peak blood levels of monomer are usually seen approximately 3 minutes after the cement is placed. The monomer is cleared by the lungs. Associated hypotension is more closely related to diminished blood volume than to circulating monomer levels. High porosity decreases the tensile and fatigue properties of cement. Manually mixed cement may have porosity as high as 27%. Porosity may be reduced to less than 1% through vacuum mixing or centrifugation of the cement. When adding antibiotics to cement, the compressive and tensile forces are not appreciably decreased, but the overall fatigue strength may be reduced. Canale ST (ed): Campbell's Operative Orthopaedics, ed 9. St Louis, MO, Mosby, 1998, pp 221-224.

Question 271

Topic: Biomechanics & Biomaterials

When analyzing the mechanical properties of an orthopedic plate, the area under the stress-strain curve up to the point of failure represents which of the following material properties?

. Elastic modulus
. Yield strength
. Ultimate tensile strength
. Toughness
. Ductility

Correct Answer & Explanation

. Toughness

Explanation

Toughness is defined as the amount of energy a material can absorb before it fractures, which is graphically represented by the total area under the stress-strain curve. Elastic modulus is the slope of the linear elastic region. Yield strength is the stress at which plastic deformation begins.

Question 272

Topic: Biomechanics & Biomaterials
A biomaterials researcher is studying the wear properties of highly cross-linked polyethylene (HXLPE) used in total hip arthroplasty. Which of the following manufacturing processes is critical for eliminating the free radicals generated during the cross-linking process, thereby reducing the risk of oxidative degradation?
. Gamma irradiation in an inert argon environment
. Remelting the polyethylene at temperatures above its melting point
. Sterilization using ethylene oxide
. Cold drawing of the polyethylene sheet prior to machining
. Addition of polymethylmethacrylate (PMMA) to the resin

Correct Answer & Explanation

. Remelting the polyethylene at temperatures above its melting point

Explanation

The irradiation of ultra-high molecular weight polyethylene (UHMWPE) creates highly cross-linked polyethylene (HXLPE) which has vastly improved wear resistance. However, irradiation leaves behind reactive free radicals that can react with oxygen in vivo, leading to oxidative degradation and embrittlement. Thermal treatments are used to extinguish these free radicals. Remelting (heating above the melting point, ~135-150°C) eliminates all free radicals but slightly reduces mechanical properties. Annealing (heating below the melting point) preserves mechanical properties better but leaves residual free radicals.

Question 273

Topic: Biomechanics & Biomaterials
A total hip arthroplasty is planned for a 55-year-old active patient. The surgeon selects highly cross-linked polyethylene (HXLPE) to reduce wear. Which of the following represents a known mechanical tradeoff associated with the irradiation and remelting/annealing process used to create HXLPE?
. Increased elastic modulus
. Increased ultimate tensile strength
. Decreased fatigue strength
. Decreased oxidation resistance
. Increased coefficient of friction

Correct Answer & Explanation

. Decreased fatigue strength

Explanation

While the irradiation process creates cross-links that significantly improve wear resistance, and the subsequent remelting or annealing processes eliminate free radicals to improve oxidation resistance, this comes at a mechanical cost. Highly cross-linked polyethylene has decreased fatigue strength, ultimate tensile strength, and fracture toughness compared to conventional UHMWPE.

Question 274

Topic: Biomechanics & Biomaterials
How does highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) differ from conventional polyethylene in total joint arthroplasty?
. Increased wear resistance and decreased fracture toughness
. Increased wear resistance and increased yield strength
. Decreased wear resistance and increased ultimate tensile strength
. Decreased wear resistance and decreased fracture toughness
. Increased wear resistance with no change in mechanical properties

Correct Answer & Explanation

. Increased wear resistance and decreased fracture toughness

Explanation

Highly cross-linked polyethylene is manufactured by exposing the material to radiation, which forms covalent bonds between the polymer chains. This process significantly decreases adhesive and abrasive wear. However, the cross-linking process and subsequent thermal treatment (melting or annealing) to extinguish free radicals also alter its mechanical properties, resulting in decreased yield strength, ultimate tensile strength, and fracture toughness.

Question 275

Topic: Biomechanics & Biomaterials

A ligament is stretched to a fixed length during a biomechanical study, and the force required to maintain that specific length gradually decreases over time. What intrinsic viscoelastic property of the ligament does this phenomenon describe?

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

Correct Answer & Explanation

. Stress relaxation

Explanation

Stress relaxation is the decrease in stress (force) over time when a viscoelastic material is held at a constant strain (length). In contrast, creep is the increase in strain (deformation) over time when the material is held at a constant stress (load). Hysteresis is the energy lost as heat during the loading and unloading cycle.

Question 276

Topic: Biomechanics & Biomaterials

During anterior cruciate ligament (ACL) reconstruction, an allograft is tensioned prior to final fixation. Over time, the graft experiences a gradual increase in deformation despite being held under a constant applied load. What biomechanical property does this describe?

. Creep
. Stress relaxation
. Hysteresis
. Fatigue
. Anisotropy

Correct Answer & Explanation

. Creep

Explanation

Creep is the viscoelastic property defined as the progressive deformation of a material over time when subjected to a constant load. Stress relaxation is the decrease in stress over time when a material is held at a constant strain/deformation. Hysteresis represents the energy lost (as heat) during the loading and unloading cycles of a viscoelastic material. Anisotropy means a material's physical properties depend on the direction of applied load.

Question 277

Topic: Biomechanics & Biomaterials

In the context of orthopedic implant design, utilizing a material with a Young's modulus (modulus of elasticity) that closely approximates that of human cortical bone primarily reduces the incidence of which of the following phenomena?

. Galvanic corrosion
. Fretting wear
. Stress shielding
. Third-body wear
. Macrophage-induced osteolysis

Correct Answer & Explanation

. Stress shielding

Explanation

Stress shielding occurs when a stiff implant takes the majority of the mechanical load, depriving the surrounding bone of the stress required to maintain its mass (per Wolff's law), leading to bone resorption and potential implant loosening. Using a material with a Young's modulus closer to cortical bone (such as titanium alloy compared to stiffer cobalt-chromium) allows better load sharing with the bone, thereby reducing stress shielding.

Question 278

Topic: Biomechanics & Biomaterials
In modern total hip arthroplasty, the use of highly cross-linked polyethylene (HXLPE) has significantly reduced the incidence of wear-related osteolysis compared to traditional ultra-high-molecular-weight polyethylene (UHMWPE). Which of the following manufacturing processes is primarily responsible for generating the cross-links that increase the wear resistance of HXLPE?
. High-dose gamma or electron beam irradiation
. Sterilization with ethylene oxide gas
. Addition of Vitamin E prior to consolidation
. Calcium phosphate surface coating
. Cold working the polyethylene sheet under high pressure

Correct Answer & Explanation

. High-dose gamma or electron beam irradiation

Explanation

Highly cross-linked polyethylene (HXLPE) is created by subjecting standard UHMWPE to high doses of gamma or electron beam irradiation (typically 50-100 kGy). The irradiation cleaves polymer chains, creating free radicals that subsequently bond to form a dense network of cross-links, vastly improving wear resistance. This is typically followed by a thermal treatment (annealing or remelting) to extinguish residual free radicals and prevent long-term oxidation.

Question 279

Topic: Biomechanics & Biomaterials
In total hip arthroplasty, the use of highly cross-linked polyethylene (HXLPE) compared to conventional ultra-high-molecular-weight polyethylene (UHMWPE) is associated with which of the following material property changes?
. Increased yield strength
. Decreased wear resistance
. Decreased fatigue crack propagation resistance
. Increased ductility
. Increased ultimate tensile strength

Correct Answer & Explanation

. Decreased fatigue crack propagation resistance

Explanation

Cross-linking of polyethylene significantly improves its wear resistance, which is highly beneficial for the longevity of THA bearings. However, this process alters the mechanical properties of the material, leading to decreased fatigue crack propagation resistance, reduced ductility, decreased ultimate tensile strength, and decreased yield strength.

Question 280

Topic: Biomechanics & Biomaterials

In the context of orthopedic biomechanics, which of the following accurately defines the 'Young's modulus' of a material?

. The point at which the material permanently deforms
. The area under the stress-strain curve representing toughness
. The slope of the stress-strain curve in the elastic region
. The maximum stress a material can withstand before failure
. The point of total structural failure

Correct Answer & Explanation

. The slope of the stress-strain curve in the elastic region

Explanation

Young's modulus (or elastic modulus) is a measure of the stiffness of a solid material. It is defined mathematically as the slope of the linear, elastic portion of the stress-strain curve. The higher the Young's modulus, the stiffer the material.

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