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Biomechanics & Biomaterials MCQs

Practice Set 13 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 241

Topic: Biomechanics & Biomaterials

A 55-year-old male is 15 years post-operative from a total hip arthroplasty utilizing a highly cross-linked polyethylene (HXLPE) liner. Which of the following best describes the wear and mechanical characteristics of HXLPE compared to conventional ultra-high-molecular-weight polyethylene?

. Decreased volumetric wear but increased susceptibility to fatigue cracking and fracture
. Increased volumetric wear and increased susceptibility to fatigue cracking
. Decreased volumetric wear and decreased susceptibility to fatigue cracking
. Increased volumetric wear and decreased susceptibility to fatigue cracking
. No significant difference in volumetric wear, but improved fracture toughness

Correct Answer & Explanation

. Decreased volumetric wear but increased susceptibility to fatigue cracking and fracture

Explanation

Highly cross-linked polyethylene (HXLPE) is manufactured using irradiation to create cross-links between polymer chains, followed by a heating process (melting or annealing) to eliminate free radicals. This significantly reduces adhesive and abrasive volumetric wear. However, the cross-linking process alters the mechanical properties, leading to a reduction in ductility, yield strength, and ultimate tensile strength, which increases its susceptibility to fatigue cracking and rim fracture, particularly in thin liners or with malpositioned components.

Question 242

Topic: Biomechanics & Biomaterials

A 65-year-old male presents with new-onset right groin pain 5 years after an uncomplicated metal-on-polyethylene total hip arthroplasty. Inflammatory markers are normal. Aspiration yields cloudy fluid with negative cultures, but significantly elevated cobalt levels compared to chromium. What is the primary mechanism of failure?

. Galvanic corrosion at the head-neck junction
. Abrasive wear of the polyethylene liner
. Type IV delayed hypersensitivity to bone cement
. Third-body wear from retained cement
. Impingement of the femoral neck on the acetabular rim

Correct Answer & Explanation

. Galvanic corrosion at the head-neck junction

Explanation

Mechanically assisted crevice corrosion (trunnionosis) at the modular head-neck junction can occur in metal-on-polyethylene THAs. It classically presents with elevated serum or synovial cobalt levels out of proportion to chromium.

Question 243

Topic: Biomechanics & Biomaterials

An opening wedge high tibial osteotomy (HTO) is planned for a 45-year-old male laborer with isolated medial compartment osteoarthritis and varus malalignment. What is a common unintended consequence on the sagittal plane biomechanics of the knee if the osteotomy gap is opened equally anteriorly and posteriorly?

. Decreased posterior tibial slope
. Increased posterior tibial slope
. Anterior translation of the tibia relative to the femur in extension
. Increased patellofemoral tracking laterally
. Decreased tension on the posterior cruciate ligament

Correct Answer & Explanation

. Decreased posterior tibial slope

Explanation

During a medial opening wedge HTO, opening the osteotomy gap equally anteriorly and posteriorly typically leads to an unintended increase in the posterior tibial slope. This occurs because the proximal tibia is triangular, being narrower anteriorly than posteriorly. To maintain the native sagittal slope, the anterior opening gap must be approximately half the size of the posterior gap. An increased posterior slope can inadvertently increase anterior tibial translation and stress the ACL.

Question 244

Topic: Biomechanics & Biomaterials

Polymethylmethacrylate (PMMA) bone cement is frequently used to deliver local antibiotics in the treatment of orthopedic infections. Which of the following properties is a critical requirement for an antibiotic to be effectively and safely incorporated into PMMA?

. Thermolability
. High protein binding
. Hydrophobicity
. Bacteriostatic mechanism of action
. Thermal stability and water solubility

Correct Answer & Explanation

. Thermal stability and water solubility

Explanation

To be effectively mixed with PMMA, an antibiotic must possess thermal stability (to withstand the exothermic reaction of cement polymerization which can reach high temperatures) and water solubility (to allow it to elute out of the hydrophobic cement matrix into the surrounding aqueous tissue environment). Aminoglycosides (tobramycin) and glycopeptides (vancomycin) fit these criteria well.

Question 245

Topic: Biomechanics & Biomaterials

To optimize gait biomechanics and minimize adjacent joint arthritis, what is the ideal position for a tibiotalar arthrodesis?

. Neutral dorsiflexion, 0 to 5 degrees of valgus, and 5 to 10 degrees of external rotation
. 5 degrees of plantarflexion, neutral coronal alignment, and neutral rotation
. 5 degrees of dorsiflexion, 5 degrees of varus, and neutral rotation
. Neutral dorsiflexion, 5 degrees of varus, and 15 degrees of external rotation
. 5 degrees of plantarflexion, 5 degrees of valgus, and 15 degrees of external rotation

Correct Answer & Explanation

. Neutral dorsiflexion, 0 to 5 degrees of valgus, and 5 to 10 degrees of external rotation

Explanation

The optimal position for ankle arthrodesis is neutral dorsiflexion, slight valgus (0-5 degrees), and slight external rotation (5-10 degrees) matching the contralateral limb. Plantarflexion or varus malalignment significantly impairs gait and accelerates adjacent joint arthritis.

Question 246

Topic: Biomechanics & Biomaterials

Which of the following combinations of implant characteristics confers the highest risk for mechanically assisted crevice corrosion (trunnionosis) at the head-neck junction in total hip arthroplasty?

. Titanium alloy stem, cobalt-chromium head, large head diameter (>36 mm)
. Cobalt-chromium stem, ceramic head, small head diameter (28 mm)
. Titanium alloy stem, ceramic head, short taper design
. Cobalt-chromium stem, cobalt-chromium head, standard offset
. Stainless steel stem, stainless steel head, large head diameter

Correct Answer & Explanation

. Titanium alloy stem, cobalt-chromium head, large head diameter (>36 mm)

Explanation

Trunnionosis (mechanically assisted crevice corrosion) is most heavily associated with mixed-metal modular junctions, particularly a titanium alloy stem paired with a cobalt-chromium head. The risk is significantly amplified by the use of larger femoral heads (>36 mm), which increase the frictional torque and the resultant lever arm forces transmitted to the trunnion, leading to micromotion, fretting, and corrosion.

Question 247

Topic: Biomechanics & Biomaterials

In the context of scapholunate dissociation, a patient is planned for a capsulodesis and ligamentous reconstruction. Which distinct region of the scapholunate interosseous ligament is thickest, strongest, and most critical to reconstruct to restore normal carpal kinematics?

. Volar (palmar) region
. Proximal membranous region
. Central fibrocartilaginous region
. Dorsal region
. Radioscaphocapitate complex

Correct Answer & Explanation

. Dorsal region

Explanation

The dorsal region of the scapholunate interosseous ligament is the thickest and mechanically most important stabilizer of the scapholunate joint. Reconstruction efforts primarily focus on restoring this dorsal continuity.

Question 248

Topic: Biomechanics & Biomaterials

A surgeon chooses to use calcium phosphate cement to fill a metaphyseal void following the reduction of a distal radius fracture. Which of the following correctly describes a biomechanical or physical property of calcium phosphate cement?

. It cures via an endothermic reaction, protecting surrounding tissue
. It provides high tensile and shear strength
. It is rapidly resorbed within 4 to 6 weeks, matching the rate of new bone formation
. It cures via an exothermic reaction and provides high compressive strength
. It is primarily osteoinductive, promoting cell differentiation

Correct Answer & Explanation

. It cures via an exothermic reaction and provides high compressive strength

Explanation

Calcium phosphate cement cures in vivo via an exothermic reaction into a hydroxyapatite-like structure. It is highly osteoconductive and provides excellent compressive strength (equivalent to or greater than cancellous bone), making it ideal for filling metaphyseal voids. However, it lacks tensile/shear strength and resorbs very slowly (often taking years), unlike calcium sulfate which resorbs rapidly.

Question 249

Topic: Biomechanics & Biomaterials

A surgeon incorporates calcium sulfate pellets as a bone graft substitute to fill a benign bone cyst defect in the hand. Three weeks postoperatively, the patient presents with non-purulent, serous drainage from the wound. Inflammatory markers are normal and cultures are negative. What is the most likely cause of this drainage?

. Exothermic necrosis of surrounding soft tissue during setting
. Allergic reaction to the carrier matrix
. Rapid dissolution of calcium sulfate leading to a hyperosmotic serous effusion
. Undiagnosed low-grade mycobacterial infection
. Premature weight-bearing causing micromotion and inflammatory exudate

Correct Answer & Explanation

. Rapid dissolution of calcium sulfate leading to a hyperosmotic serous effusion

Explanation

Calcium sulfate is an osteoconductive bone graft substitute known for its very rapid absorption rate, which often outpaces new bone formation. Its dissolution can create a localized hyperosmotic environment, drawing fluid into the area and resulting in a sterile, serous wound exudate in up to 30% of patients. It does not set with an exothermic reaction (unlike calcium phosphate or PMMA).

Question 250

Topic: Biomechanics & Biomaterials
In total hip arthroplasty, the use of highly cross-linked polyethylene (HXLPE) compared to conventional ultra-high molecular weight polyethylene (UHMWPE) alters the wear profile. Which of the following trade-offs is most characteristic of increasing the radiation dose during HXLPE manufacturing?
. Increased wear rate and increased fatigue strength
. Decreased wear rate and decreased fatigue strength
. Decreased wear rate and increased oxidation potential
. Increased wear rate and increased oxidation potential
. No change in wear rate but decreased fatigue strength

Correct Answer & Explanation

. Decreased wear rate and decreased fatigue strength

Explanation

Highly cross-linking polyethylene significantly decreases the volumetric wear rate, lowering the risk of osteolysis. However, it inversely decreases the material's mechanical properties, such as fatigue strength and fracture toughness.

Question 251

Topic: Biomechanics & Biomaterials

A 55-year-old female experiences a 'pop' in her posterior knee while squatting. MRI demonstrates a medial meniscus posterior root tear with 4 mm of meniscal extrusion. Which of the following best describes the biomechanical consequence of this injury?

. Decreased peak contact pressures in the medial compartment
. Loss of hoop stresses leading to contact pressures equivalent to a total meniscectomy
. Increased varus alignment of the knee by 5 degrees
. Shift of the mechanical axis laterally
. Hypertrophy of the adjacent medial articular cartilage

Correct Answer & Explanation

. Loss of hoop stresses leading to contact pressures equivalent to a total meniscectomy

Explanation

A complete tear of the medial meniscus posterior root disrupts the circumferential hoop stresses of the meniscus. Biomechanically, this results in altered kinematics and elevated peak contact pressures that are nearly equivalent to those seen following a total meniscectomy.

Question 252

Topic: Biomechanics & Biomaterials

The ability of compressed cortical bone to resist greater applied force in the longitudinal plane than in the transverse plane is an illustration of what material property?

. Yield strength
. Elastic modulus
. Viscoelasticity
. Anisotropy

Correct Answer & Explanation

. Elastic modulus

Explanation

Material properties characterize mechanical functional limits of a material independent of the size or shape of that material. Anisotropic materials are those for which properties behave differently dependent on the direction of applied force. Yield strength is the load at which permanent plastic deformation begins to occur. Elastic modulus is the mathematical description of the tendency of a material to be deformed elastically in response to an applied force. The elastic modulus of a material is defined as the slope of its stress-strain curve in the elastic deformation region. Viscoelastic materials such as bone exhibit time-rate-dependent stress-strain behavior as a function of internal friction. The modulus of viscoelastic materials increase as the strain rate increases.

Question 253

Topic: Biomechanics & Biomaterials

A surgeon is preparing antibiotic-loaded bone cement (ALBC) to create a temporary articulating spacer for a two-stage exchange of an infected total knee arthroplasty. To maximize the elution profile of the antibiotics, which of the following preparation techniques is most appropriate?

. Vacuum-mixing the cement to decrease porosity
. Decreasing the ratio of antibiotic powder to cement powder to < 5% by weight
. Hand-mixing the cement and using high-dose antibiotics (>10% by weight)
. Increasing the liquid monomer-to-powder ratio significantly
. Pre-heating the monomer to accelerate the exothermic reaction

Correct Answer & Explanation

. Hand-mixing the cement and using high-dose antibiotics (>10% by weight)

Explanation

To maximize antibiotic elution from a PMMA spacer, the cement should be highly porous. This is best achieved by hand-mixing (non-vacuum mixing) the cement and adding high doses of antibiotics (typically >10% of the cement weight, e.g., 4 to 8 grams of antibiotic per 40-gram bag of PMMA). Vacuum mixing decreases porosity and improves mechanical strength, which is desired for definitive fixation but counterproductive for a temporary spacer where maximum antibiotic elution is the primary goal.

Question 254

Topic: Biomechanics & Biomaterials

During a two-stage exchange for a periprosthetic joint infection, the surgeon prepares an articulating antibiotic-loaded polymethylmethacrylate (PMMA) spacer. To maximize the local elution of antibiotics, which of the following preparation techniques should be employed?

. Mixing the cement under a strong vacuum
. Increasing the ratio of liquid monomer to powder
. Hand-mixing the cement to increase porosity
. Using entirely pre-mixed commercial antibiotic bone cement
. Adding more than 10 grams of antibiotics per 40-gram bag of PMMA

Correct Answer & Explanation

. Hand-mixing the cement to increase porosity

Explanation

Hand-mixing cement increases its porosity, which significantly enhances the surface area and subsequent elution profile of the incorporated antibiotics. Vacuum mixing decreases porosity and therefore decreases antibiotic elution.

Question 255

Topic: Biomechanics & Biomaterials

A 70-year-old diabetic male undergoes an aspiration of a painful TKA. Cultures grow Candida albicans. A two-stage exchange is planned. Which of the following is the most appropriate local antibiotic spacer strategy for this patient?

. Vancomycin and Tobramycin loaded PMMA
. Voriconazole or Amphotericin B loaded PMMA
. Caspofungin loaded PMMA
. Micafungin loaded calcium sulfate beads
. Fluconazole administered intravenously only with standard antibiotic PMMA

Correct Answer & Explanation

. Voriconazole or Amphotericin B loaded PMMA

Explanation

Fungal periprosthetic joint infections require local antifungal delivery to effectively clear the joint space. Voriconazole and Amphotericin B maintain their stability and elute effectively from polymethylmethacrylate (PMMA) cement, unlike echinocandins.

Question 256

Topic: Biomechanics & Biomaterials

When preparing antibiotic-loaded polymethylmethacrylate (PMMA) for a structural articulating spacer in a two-stage revision, what is the maximum recommended weight of antibiotic powder per 40-gram bag of cement to avoid catastrophic mechanical failure?

. 1 gram
. 4 grams
. 8 grams
. 12 grams
. 20 grams

Correct Answer & Explanation

. 4 grams

Explanation

For structural PMMA spacers, adding more than 4 grams (approx 10%) of antibiotics per 40-gram bag significantly compromises the mechanical compressive strength of the cement. Non-structural beads can accommodate higher ratios up to 20% without clinical consequence.

Question 257

Topic: Biomechanics & Biomaterials

When formulating an antibiotic-loaded polymethylmethacrylate (PMMA) spacer for a two-stage exchange arthroplasty, which of the following mixing techniques is recommended to maximize antibiotic elution?

. Vacuum mixing to reduce cement porosity
. Adding liquid antibiotics to the monomer phase prior to mixing
. Hand-mixing the cement in an open bowl at atmospheric pressure
. Increasing the ratio of liquid monomer to polymer powder
. Cooling the monomer to 4 degrees Celsius prior to vacuum mixing

Correct Answer & Explanation

. Hand-mixing the cement in an open bowl at atmospheric pressure

Explanation

Hand-mixing PMMA cement in an open bowl at atmospheric pressure increases the porosity of the cement mantle. This high porosity is undesirable for structural long-term fixation but is necessary to maximize the elution of antibiotics in a temporary spacer.

Question 258

Topic: Biomechanics & Biomaterials

Figure 40 shows the plain radiograph of a 30-year-old woman who has had a long history of standing bilateral anterior knee pain and a sense of patellar instability without frank dislocation. Nonsurgical management consisting of anti-inflammatory drugs and physical therapy has failed to provide relief. Examination reveals full range of motion of both knees, with moderate patellofemoral crepitance. Patellar apprehension and patellar grind tests are positive. The Q-angle measures 20 degrees. Management should now consist of

Sports Medicine Board Review 2001: High-Yield MCQs (Set 4) - Figure 11

. bilateral arthroscopic lateral releases.
. bilateral arthroscopic lateral releases and medial retinacular thermal shrinkage.
. bilateral lateral releases and anteromedialization of the tibial tubercles.
. physical therapy and the use of patella-stabilizing braces.
. physical therapy with taping.

Correct Answer & Explanation

. bilateral lateral releases and anteromedialization of the tibial tubercles.

Explanation

The history, physical examination, and radiographs indicate that the patellofemoral pain is most likely caused by excessive lateral patellar pressure and patellar maltracking. Because the radiographs reveal the lateral tilt of the patella and lateral subluxation, the treatment of choice is bilateral lateral releases with anteromedialization of the tibial tubercles. This procedure corrects not only the excessive lateral patellar pressure, but also the lateral subluxation. The use of patella-stabilizing braces or taping may provide temporary relief, but these implements are not well-tolerated and they will not change the underlying biomechanics of the knee. Simple lateral release is indicated for isolated lateral tilt, but it does not correct the lateral subluxation. The use of thermal capsular shrinkage for the medial retinaculum has not been proven to provide long-term correction of the deformity. Boden BP, Pearsall AW, Garrett We Jr, et al: Patellofemoral instability: Evaluation and management. J Am Acad Orthop Surg 1997;5:47-57.

Question 259

Topic: Biomechanics & Biomaterials

The acceleration of an object under the influence of a force depends directly on the mass of the object. The angular acceleration of an object under the influence of a moment depends directly on the

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

. area moment of inertia.
. weight squared.
. length of the lever arm.
. mass moment of inertia.
. initial velocity.

Correct Answer & Explanation

. mass moment of inertia.

Explanation

Similar to the action of a force, a moment tends to angularly accelerate an object in a manner proportional to a quantity related to the mass of the object. The concept of a more massive object requiring a larger force to cause the same straight line acceleration is straightforward. The concept for changing angular velocity is similar but not identical. The proportional constant between the moment and the resulting angular acceleration is the mass moment of inertia, which depends not only on the mass of the object, but also its distribution. The unit of mass moment of inertia is obtained by multiplying the mass of the object by the square of the distance between an equivalent location of the center of rotation of the object and an equivalent location of the center of mass. Orthopaedic surgeons can change both the amount of mass carried by a limb and the way that the mass is distributed. For example, in applying a cast to the leg, the physician can affect the mass of the cast by the choice of casting material and by the size of the cast. As the mass moment of inertia of the limb increases (eg, by applying the cast farther down on the leg or using a heavier casting material), the patient will need to exert larger moments to angularly accelerate the leg during gait. Andriacchi T, Natarajan RN, Hurwitz DE: Musculoskeletal dynamics, locomotion, and clinical applications, in Mow VC, Hayes WC (eds): Basic Orthopaedic Biomechanics, ed 2. New York, NY, Lippincott-Raven, 1997, pp 43-47.

Question 260

Topic: Biomechanics & Biomaterials

Figure 6 shows an object being held in an outstretched hand. To offset the moment created by the object (ignoring the weight of the forearm), the biceps must generate a force of

Basic Science Board Review 2006: High-Yield MCQs (Set 4) - Figure 1

. 7.5 N.
. 15 N.
. 30 N.
. 75 N.
. 150 N.

Correct Answer & Explanation

. 75 N.

Explanation

Answering this question requires understanding of two important biomechanics concepts. First, because neither the object being held in the hand nor the body is moving and, hence, their accelerations are zero, the problem is one of static equilibrium in which the sum of the moments acting on the body is zero. Second, a moment is the action of a force that causes an object to rotate about any point away from its line of action. The magnitude of the moment is the magnitude of the force multiplied by the perpendicular distance between the line of action and the point (often called the moment arm or lever arm). In this problem, two forces are causing moments about the elbow. The magnitude of the moment caused by the object in the hand is 5 N times 30 cm or 150 N-cm. To maintain equilibrium, the moment caused by the biceps force must also have a magnitude of 150 N-cm. Its moment arm is 2 cm, so the magnitude of the biceps force is 150 N-cm divided by 2 cm, which equals 75 N. In general, functional loads such as the object are always at a mechanical advantage (ie, have a longer moment arm) over the muscle. Therefore, muscles must generate large forces to overcome the moments caused by even small functional loads. An KN, Chao ES, Kaufman KR: Analysis of muscle and joint loads, in Mow VC, Hayes WC (eds): Basic Orthopaedic Biomechanics, ed 2. New York, NY, Lippincott-Raven, 1997, pp 1-14.

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