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Biomechanics & Biomaterials MCQs

Practice Set 61 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 1201

Topic: Biomechanics & Biomaterials

A 25-year-old athlete undergoes an ACL reconstruction with a hamstring autograft. The surgeon applies a constant 20 N tension to the graft for 10 minutes prior to fixation. The graft elongates over this time without an increase in the applied force. This viscoelastic phenomenon is best described as:

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

Correct Answer & Explanation

. Stress relaxation

Explanation

Creep is a viscoelastic property defined as progressive deformation (elongation) of a material over time when subjected to a constant load. In contrast, stress relaxation is the decrease in stress (tension) within a material over time when it is held at a constant length (deformation). Pre-tensioning an ACL graft utilizes both creep and stress relaxation to minimize post-implantation elongation.

Question 1202

Topic: Biomechanics & Biomaterials
Which of the following manufacturing processes is essential in creating highly cross-linked polyethylene (HXLPE) to significantly reduce wear in total hip arthroplasty?
. Gamma irradiation followed by melting or annealing
. Addition of high-dose vitamin C
. Cold drawing of the polymer chains under tension
. Decreasing the molecular weight of the polyethylene base resin
. Introduction of calcium phosphate into the polymer matrix

Correct Answer & Explanation

. Gamma irradiation followed by melting or annealing

Explanation

Highly cross-linked polyethylene is manufactured by exposing ultra-high molecular weight polyethylene (UHMWPE) to gamma or electron beam irradiation. This creates free radicals that bond to form cross-links, drastically increasing wear resistance. To prevent long-term oxidative degradation from residual free radicals, the material is subsequently either melted or annealed. Modern alternatives include doping with Vitamin E.

Question 1203

Topic: Biomechanics & Biomaterials
A 55-year-old active male is undergoing total hip arthroplasty. The surgeon chooses a highly cross-linked polyethylene (HXLPE) liner. Which of the following is an expected trade-off compared to conventional ultra-high-molecular-weight polyethylene (UHMWPE)?
. Increased adhesive wear
. Decreased fatigue resistance
. Increased abrasive wear
. Increased oxidation potential after remelting
. Decreased volumetric wear linearly proportional to cup diameter

Correct Answer & Explanation

. Decreased fatigue resistance

Explanation

Cross-linking of polyethylene significantly reduces adhesive and abrasive wear, decreasing the volumetric wear rate. However, the irradiation process decreases the mechanical properties of the material, specifically reducing its fracture toughness, yield strength, ultimate tensile strength, and fatigue crack propagation resistance.

Question 1204

Topic: Biomechanics & Biomaterials

During a cemented total hip arthroplasty, which of the following techniques is most important to maximize the fatigue strength of the polymethylmethacrylate (PMMA) bone cement?

. Hand-mixing the cement in an open bowl
. Adding aqueous antibiotics to the cement powder
. Vacuum mixing to reduce porosity
. Delaying insertion until the cement is in the doughy phase
. Minimizing pressurization during insertion

Correct Answer & Explanation

. Hand-mixing the cement in an open bowl

Explanation

Vacuum mixing significantly reduces the porosity of PMMA bone cement by eliminating air bubbles trapped during mixing. Reduced porosity decreases stress risers within the cement mantle, thereby maximizing its intrinsic fatigue strength.

Question 1205

Topic: Biomechanics & Biomaterials

A 62-year-old male with a primary metal-on-polyethylene THA presents with a 6-month history of worsening hip pain. Blood tests reveal elevated serum cobalt and chromium levels. Radiographs demonstrate a well-fixed modular titanium stem and acetabular cup. What is the most likely source of the metal ion elevation in this specific clinical scenario?

. Impingement of the femoral neck on the metallic acetabular rim
. Mechanically assisted crevice corrosion at the head-neck junction
. Third-body wear of the polyethylene transferring to the trunnion
. Corrosion at the interface of a modular distal femoral sleeve
. Galvanic corrosion between the titanium stem and cortical bone

Correct Answer & Explanation

. Impingement of the femoral neck on the metallic acetabular rim

Explanation

In a metal-on-polyethylene bearing with elevated metal ions, the most common source of cobalt and chromium is mechanically assisted crevice corrosion (trunnionosis) at the modular head-neck taper junction. This is increasingly recognized with large diameter metal heads on titanium stems.

Question 1206

Topic: Biomechanics & Biomaterials

Early formulations of zirconia ceramic femoral heads in total hip arthroplasty experienced an unacceptably high rate of catastrophic failure in vivo. This mechanical failure was fundamentally attributed to which of the following material properties?

. Galvanic corrosion at the interface between the ceramic head and the titanium trunnion
. Phase transformation from the stable tetragonal phase to the weaker monoclinic phase in an aqueous environment
. High levels of continuous alpha radiation emitting from the zirconia matrix
. Severe impingement leading to focal stripe wear and eventual shattering
. Rapid third-body wear initiated by hydroxyapatite shedding from the acetabular cup

Correct Answer & Explanation

. Galvanic corrosion at the interface between the ceramic head and the titanium trunnion

Explanation

Yttria-stabilized zirconia was introduced for its high fracture toughness. However, in the aqueous, warm environment of the human body, the material undergoes a slow phase transformation from the strong tetragonal phase to the weaker monoclinic phase. This transformation is accompanied by a volume expansion that creates microcracks, ultimately leading to catastrophic shattering of the femoral head.

Question 1207

Topic: Biomechanics & Biomaterials

In the manufacturing of highly cross-linked polyethylene (HXLPE) for total hip arthroplasty, the polymer is often subjected to a thermal treatment of remelting (heating above its melting point of 135°C). What is the primary advantage of remelting compared to annealing (heating below the melting point)?

. It completely eliminates free radicals, maximizing oxidation resistance.
. It increases the ultimate tensile strength of the polyethylene.
. It enhances the fatigue crack propagation resistance.
. It promotes further cross-linking to decrease wear rates.
. It increases the crystallinity of the polyethylene structure.

Correct Answer & Explanation

. It completely eliminates free radicals, maximizing oxidation resistance.

Explanation

Remelting highly cross-linked polyethylene above its melting point allows polymer chains enough mobility to recombine and completely eliminate free radicals, which maximizes long-term oxidation resistance. However, it does cause a slight reduction in mechanical properties (like fatigue strength and crystallinity) compared to annealing. Annealing preserves more mechanical strength but leaves residual free radicals.

Question 1208

Topic: Biomechanics & Biomaterials

Polymethylmethacrylate (PMMA) bone cement is frequently used for component fixation in arthroplasty. What is the primary mechanism by which PMMA secures a prosthesis to the host bone?

. Covalent chemical bonding to the hydroxyapatite of the bone.
. Covalent chemical bonding to the metallic surface of the prosthesis.
. Osteoinductive stimulation of local bone ingrowth.
. Mechanical interlock by acting as a grout within the interstices of cancellous bone.
. Thermal necrosis of the bone bed resulting in a fibrous ankylosis.

Correct Answer & Explanation

. Covalent chemical bonding to the hydroxyapatite of the bone.

Explanation

PMMA bone cement acts strictly as a grout, not an adhesive. It does not chemically bond to bone or the implant. Its primary mechanism of fixation relies on mechanical interdigitation as the liquid cement flows into the trabecular interstices of the prepared cancellous bone bed and subsequently cures and hardens into a solid mantle.

Question 1209

Topic: Biomechanics & Biomaterials

Which of the following orthopedic biomaterials has a modulus of elasticity (Young's modulus) that most closely matches that of normal human cortical bone?

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

Correct Answer & Explanation

. Cobalt-chromium alloy

Explanation

Titanium alloy has a modulus of elasticity (approx. 110 GPa) that is closer to that of cortical bone (approx. 15-20 GPa) compared to stainless steel (200 GPa) or cobalt-chromium (220 GPa). This closer match helps reduce stress shielding around implants.

Question 1210

Topic: Biomechanics & Biomaterials

An orthopedic surgeon is revising a failed internal fixation of a distal femur fracture. The original construct utilized a 316L stainless steel plate secured with Ti-6Al-4V (titanium alloy) screws. Which type of corrosion is most likely to occur at the plate-screw interface in this scenario?

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

Correct Answer & Explanation

. Crevice corrosion

Explanation

Galvanic corrosion occurs when two electrochemically dissimilar metals (such as stainless steel and titanium) are placed in direct physical contact within a conductive fluid medium (e.g., blood or interstitial fluid). The less noble metal (anode) undergoes accelerated corrosion. Mixing stainless steel with titanium or cobalt-chromium alloys is generally contraindicated in orthopedics due to this risk.

Question 1211

Topic: Biomechanics & Biomaterials

In a standard stress-strain curve of a given orthopaedic material, what does the area under the entire curve up to the point of failure represent?

. Stiffness
. Yield strength
. Toughness
. Ductility
. Fatigue limit

Correct Answer & Explanation

. Stiffness

Explanation

The area under the stress-strain curve represents the toughness of the material, which is the total energy absorbed by the material before it fractures. Stiffness is represented by the slope of the elastic region (Young's modulus). Yield strength is the point at which plastic deformation begins. Ductility is a measure of how much plastic deformation a material can undergo before failure.

Question 1212

Topic: Biomechanics & Biomaterials
In total joint arthroplasty, highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is commonly used to reduce wear. Which of the following is a recognized mechanical trade-off associated with increasing the cross-linking of UHMWPE?
. Increased abrasive wear
. Decreased fatigue strength and fracture toughness
. Increased oxidative degradation
. Decreased elastic modulus
. Increased adhesive wear

Correct Answer & Explanation

. Decreased fatigue strength and fracture toughness

Explanation

While high cross-linking of UHMWPE significantly reduces abrasive and adhesive wear rates (thereby minimizing particle-induced osteolysis), the cross-linking process alters the polymer chains, leading to a reduction in certain mechanical properties, particularly fatigue strength, tensile strength, and fracture toughness.

Question 1213

Topic: Biomechanics & Biomaterials

In orthopaedic internal fixation, why is it generally contraindicated to use a stainless steel screw in direct combination with a titanium plate?

. Mismatch in elastic modulus will cause early plate fatigue
. It predisposes the hardware construct to galvanic corrosion
. It significantly increases the risk of a severe local foreign body giant cell reaction
. Titanium induces a local osteolytic reaction when adjacent to steel
. Stainless steel has a lower yield strength than titanium leading to screw breakage

Correct Answer & Explanation

. Mismatch in elastic modulus will cause early plate fatigue

Explanation

Mixing dissimilar metals (such as stainless steel and titanium) in a continuous electrolytic environment (such as human body fluids) creates an electrochemical cell. This leads to galvanic corrosion, where the less noble metal (stainless steel in this pairing) undergoes accelerated degradation. This can compromise implant integrity and release metallic debris.

Question 1214

Topic: Biomechanics & Biomaterials

A 72-year-old patient presents with an acutely painful, swollen knee. Joint aspiration yields synovial fluid with a white blood cell count of 25,000 cells/mcL. Microscopic examination under polarized light reveals rhomboid-shaped crystals with weakly positive birefringence. What is the predominant composition of these crystals?

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

Correct Answer & Explanation

. Monosodium urate

Explanation

The presence of rhomboid-shaped, weakly positively birefringent crystals on polarized light microscopy is the hallmark finding for pseudogout, which is caused by the deposition of Calcium Pyrophosphate Dihydrate (CPPD) crystals. In contrast, monosodium urate crystals (seen in gout) are needle-shaped and exhibit strong negative birefringence.

Question 1215

Topic: Biomechanics & Biomaterials
Which sterilization method for ultra-high-molecular-weight polyethylene (UHMWPE) creates free radicals that most significantly increase the risk of oxidative degradation and accelerated wear in total joint arthroplasty?
. Gamma irradiation in air
. Gamma irradiation in a vacuum
. Ethylene oxide gas
. Gas plasma
. Electron beam irradiation in an inert gas

Correct Answer & Explanation

. Gamma irradiation in air

Explanation

Gamma irradiation in air introduces free radicals that combine with oxygen, leading to chain scission and oxidative degradation over time. This severely compromises the mechanical properties and increases wear rates of the polyethylene.

Question 1216

Topic: Biomechanics & Biomaterials

In the setting of a total hip arthroplasty, assembling a cobalt-chromium femoral head onto a titanium femoral stem can lead to which type of corrosion at the trunnion?

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

Correct Answer & Explanation

. Fretting corrosion only

Explanation

Mixing two dissimilar metals, such as cobalt-chromium and titanium, exposes them to body fluids acting as an electrolyte, creating an electrochemical gradient that leads to galvanic corrosion. Mechanically induced fretting corrosion often occurs concurrently at this modular junction.

Question 1217

Topic: Biomechanics & Biomaterials

What is the primary mechanism by which polymethylmethacrylate (PMMA) bone cement secures a total joint arthroplasty component to the host bone?

. Chemical bonding to the hydroxyapatite of the bone matrix
. Biological osteointegration directly into the cement surface
. Covalent chemical bonding to the metal prosthesis
. Mechanical interlock with the interstices of cancellous bone
. Formation of a dense fibrocartilaginous anchoring layer

Correct Answer & Explanation

. Chemical bonding to the hydroxyapatite of the bone matrix

Explanation

PMMA acts as a grout, not an adhesive glue. It provides initial rigid fixation by penetrating the trabecular interstices of cancellous bone and creating a robust mechanical interlock once it polymerizes and hardens.

Question 1218

Topic: Biomechanics & Biomaterials

To correct a severe humpback deformity in a scaphoid waist nonunion through a volar approach, what specific shape of bone graft is most frequently required to restore normal carpal kinematics?

. Cylindrical cortical strut
. Wedge-shaped corticocancellous graft
. Spherical cancellous bone chips
. Dorsal-spanning vascularized pedicle
. Flexible osteochondral allograft

Correct Answer & Explanation

. Cylindrical cortical strut

Explanation

A wedge-shaped corticocancellous graft (typically from the iliac crest) is placed volarly to pry open the collapsed scaphoid. This corrects the flexion deformity of the distal pole and restores scaphoid length.

Question 1219

Topic: Biomechanics & Biomaterials

A 28-year-old man presents with a 2-year-old scaphoid waist nonunion. CT imaging demonstrates a 'humpback' deformity with a lateral intrascaphoid angle of 65 degrees. Which of the following is the most appropriate surgical strategy to restore carpal kinematics?

. Dorsal approach with a headless compression screw and demineralized bone matrix
. Volar approach with a structural opening wedge cancellous bone graft and rigid internal fixation
. Dorsal approach with a 1,2 ICSRA pedicled vascularized graft
. Proximal row carpectomy
. Volar approach with radial styloidectomy alone

Correct Answer & Explanation

. Dorsal approach with a headless compression screw and demineralized bone matrix

Explanation

A humpback deformity (lateral intrascaphoid angle > 45 degrees) represents fixed flexion of the distal pole. It is best corrected via a volar approach utilizing a structural opening wedge graft (Fisk-Fernandez technique) to restore scaphoid length and alignment.

Question 1220

Topic: Biomechanics & Biomaterials

A patient undergoes revision total hip arthroplasty. The retrieved femoral component demonstrates evidence of galvanic corrosion at the modular head-neck junction. Which of the following material combinations is most susceptible to this specific type of corrosion?

. Cobalt-chromium alloy head on a titanium alloy stem
. Ceramic head on a titanium alloy stem
. Cobalt-chromium head on a cobalt-chromium stem
. Stainless steel head on a stainless steel stem
. Ceramic head on a cobalt-chromium stem

Correct Answer & Explanation

. Cobalt-chromium alloy head on a titanium alloy stem

Explanation

Galvanic corrosion occurs when two dissimilar metals are in contact in an electrolyte solution (like body fluid). A cobalt-chromium head on a titanium stem is a classic combination susceptible to galvanic corrosion due to their different resting electropotentials, leading to mechanically assisted crevice corrosion (trunnionosis). Ceramic heads do not undergo metallic corrosion.

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