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1. General Principles & Basic Science MCQs

Practice Set 273 of 789

This practice set contains high-yield board review questions covering key concepts in 1. General Principles & Basic Science. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 5441

Topic: Biomechanics & Biomaterials

Stress shielding in cementless femoral stems leads to proximal bone loss. Which of the following implant materials has a Young's modulus closest to that of cortical bone, thereby theoretically minimizing stress shielding?

. Cobalt-Chromium alloy
. Stainless steel (316L)
. Titanium alloy (Ti-6Al-4V)
. Alumina ceramic
. Zirconia ceramic

Correct Answer & Explanation

. Titanium alloy (Ti-6Al-4V)

Explanation

Titanium alloy has a Young's modulus (approx. 110 GPa) that is lower than Cobalt-Chromium (approx. 210 GPa) and Stainless Steel (approx. 200 GPa), making it closer to that of cortical bone (approx. 15-20 GPa). This reduced stiffness mismatch helps decrease stress shielding.

Question 5442

Topic: 1. General Principles & Basic Science

When evaluating the pullout strength of a cortical screw, which of the following design modifications will yield the greatest increase in pullout strength?

. Increasing the inner (core) diameter
. Decreasing the thread pitch
. Increasing the outer (thread) diameter
. Decreasing the length of engagement
. Changing the screw material from stainless steel to titanium

Correct Answer & Explanation

. Increasing the outer (thread) diameter

Explanation

Screw pullout strength is most heavily influenced by the outer diameter, thread depth, thread pitch, and the length of engagement in the bone. Increasing the outer diameter has the most profound effect on maximizing pullout resistance.

Question 5443

Topic: Biology, Genetics & Bone Healing

A 55-year-old patient receives a recombinant human Bone Morphogenetic Protein (rhBMP-2) graft during a spinal fusion. Which intracellular signaling molecules are directly phosphorylated after BMP-2 binds to its cell surface receptors?

. JAK/STAT
. Smad 1/5/8
. beta-catenin
. cAMP/PKA
. MAP kinase

Correct Answer & Explanation

. Smad 1/5/8

Explanation

BMP-2 binds to serine/threonine kinase receptors, leading to the intracellular phosphorylation of Smad 1, 5, and 8. These receptor-regulated Smads then form a complex with Smad 4 and translocate to the nucleus to induce osteogenic gene transcription.

Question 5444

Topic: Biology, Genetics & Bone Healing

A 70-year-old woman is treated with intermittent low-dose teriparatide. The anabolic effect of this drug on bone is primarily mediated by the stimulation of which of the following?

. Osteoclast apoptosis
. Osteoblast differentiation and lifespan extension
. Inhibition of the Wnt/beta-catenin pathway
. Increased sclerostin expression
. Direct mineralization of osteoid

Correct Answer & Explanation

. Osteoblast differentiation and lifespan extension

Explanation

Teriparatide (recombinant PTH 1-34) given intermittently has an anabolic effect by stimulating osteoblast differentiation, activity, and extending their lifespan by inhibiting osteoblast apoptosis. Continuous PTH, conversely, leads to net bone resorption.

Question 5445

Topic: Biomechanics & Biomaterials

Which of the following forms of corrosion is most likely to occur at the modular junction between a titanium femoral stem and a cobalt-chromium femoral head due to variations in local oxygen tension?

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

Correct Answer & Explanation

. Crevice corrosion

Explanation

Crevice corrosion occurs in restricted spaces (like modular tapers) where oxygen depletion prevents the reformation of the protective oxide layer. While galvanic corrosion can also occur with dissimilar metals, taper corrosion is primarily a mechanically assisted crevice corrosion (fretting-crevice mechanism).

Question 5446

Topic: Biomechanics & Biomaterials

When testing the viscoelastic properties of cortical bone, how does an increased rate of loading (strain rate) affect the biomechanical behavior of the bone?

. Decreases stiffness and decreases ultimate strength
. Decreases stiffness and increases ultimate strength
. Increases stiffness and increases energy absorption to failure
. Increases stiffness but decreases energy absorption to failure
. No change in stiffness but decreases ultimate strength

Correct Answer & Explanation

. Increases stiffness and increases energy absorption to failure

Explanation

Because bone is a viscoelastic material, its mechanical properties change with the rate of loading. At higher strain rates (e.g., trauma), bone becomes both stiffer and stronger, absorbing more energy before failure.

Question 5447

Topic: Biomechanics & Biomaterials

Polymethylmethacrylate (PMMA) bone cement is primarily utilized in arthroplasty for implant fixation. Which of the following best describes its mechanical characteristics?

. It acts as a true adhesive bonding to bone and metal
. It is stronger in tension than in compression
. It undergoes an endothermic reaction during polymerization
. It is strongest in compression and weak in tension and shear
. It expands significantly during the curing phase

Correct Answer & Explanation

. It is strongest in compression and weak in tension and shear

Explanation

PMMA does not act as an adhesive; it provides fixation via mechanical interlock. It is highly resistant to compressive forces but is notably weak in tension and shear, and it undergoes an exothermic reaction, shrinking slightly during polymerization.

Question 5448

Topic: Biology, Genetics & Bone Healing
During the soft callus phase of secondary fracture healing, the initial cartilaginous matrix is predominantly composed of which type of collagen?
. Type I
. Type II
. Type III
. Type IX
. Type X

Correct Answer & Explanation

. Type II

Explanation

The soft callus is a cartilaginous intermediate in endochondral fracture healing and is predominantly composed of Type II collagen. As the soft callus is replaced by hard callus (woven bone), Type I collagen becomes predominant.

Question 5449

Topic: Biology, Genetics & Bone Healing

A surgeon decides to treat a nonunion with distraction osteogenesis. The process of bone formation during distraction osteogenesis occurs primarily through which mechanism?

. Endochondral ossification
. Intramembranous ossification
. Creeping substitution
. Osteoconduction
. Appositional chondrogenesis

Correct Answer & Explanation

. Intramembranous ossification

Explanation

Distraction osteogenesis heals primarily through intramembranous ossification, where bone forms directly from mesenchymal cells without a cartilaginous intermediate. A stable mechanical environment and adequate vascularity are essential for this to occur.

Question 5450

Topic: Biomechanics & Biomaterials

In the stress-strain curve of a normal human ligament, the initial 'toe region' is characterized by low stiffness. What microscopic structural change corresponds to this region?

. Microfailure of collagen bundles
. Straightening out of the natural crimp of collagen fibers
. Plastic deformation of elastin fibers
. Slippage of proteoglycan molecules
. Macroscopic complete failure of the ligament

Correct Answer & Explanation

. Straightening out of the natural crimp of collagen fibers

Explanation

The non-linear 'toe region' of a ligament or tendon stress-strain curve occurs at low strains. It represents the straightening (uncrimping) of the resting, wavy collagen fibers before they begin to stretch elastically.

Question 5451

Topic: Biology, Genetics & Bone Healing

Romosozumab is a bone-building medication recently approved for severe osteoporosis. It exerts its dual effect of increasing bone formation and decreasing bone resorption by binding to and inhibiting:

. Cathepsin K
. RANKL
. Sclerostin
. Parathyroid hormone
. Alkaline phosphatase

Correct Answer & Explanation

. Sclerostin

Explanation

Romosozumab is a monoclonal antibody that binds to and inhibits sclerostin. Sclerostin is an inhibitor of the Wnt/beta-catenin signaling pathway, so its inhibition leads to increased osteoblastic bone formation and secondarily decreased bone resorption.

Question 5452

Topic: Biomechanics & Biomaterials

In designing an intramedullary nail, if the solid cross-sectional radius of the nail is increased by 10%, how is its torsional rigidity affected?

. It increases by 10%
. It increases by 21%
. It increases by 46%
. It increases by 100%
. It remains unchanged

Correct Answer & Explanation

. It increases by 46%

Explanation

The torsional rigidity of a solid cylinder is proportional to the polar moment of inertia, which varies with the radius to the fourth power (r^4). An increase of 10% (1.1) results in a rigidity of 1.1^4 = 1.4641, which is approximately a 46% increase.

Question 5453

Topic: 1. General Principles & Basic Science

Passivation is a manufacturing process used to increase the corrosion resistance of orthopedic metal implants. For stainless steel implants, this process creates a protective surface layer primarily composed of:

. Titanium dioxide
. Chromium oxide
. Iron oxide
. Aluminum oxide
. Cobalt oxide

Correct Answer & Explanation

. Chromium oxide

Explanation

Passivation of stainless steel (and cobalt-chromium alloys) involves treating the metal with nitric acid to produce a highly adherent, protective, and self-healing layer of chromium oxide on the surface. This layer significantly improves corrosion resistance.

Question 5454

Topic: Biology, Genetics & Bone Healing

Vitamin D plays a critical role in calcium homeostasis and bone health. The final step in the formation of the most active form of Vitamin D (1,25-dihydroxyvitamin D3) occurs in the kidney and is catalyzed by which enzyme?

. 25-hydroxylase
. 1-alpha-hydroxylase
. 24-hydroxylase
. Alkaline phosphatase
. Tartrate-resistant acid phosphatase

Correct Answer & Explanation

. 25-hydroxylase

Explanation

Cholecalciferol is hydroxylated in the liver to 25-hydroxyvitamin D3 by 25-hydroxylase. The final conversion to the active 1,25-dihydroxyvitamin D3 occurs in the kidney, catalyzed by 1-alpha-hydroxylase, an enzyme stimulated by PTH.

Question 5455

Topic: Biology, Genetics & Bone Healing

A fracture is rigidly fixed with a compression plate, achieving absolute stability and a gap of less than 0.01 mm. Under these conditions, the fracture will heal via primary bone healing, which is microscopically characterized by:

. An initial cartilaginous soft callus
. Granulation tissue maturation into woven bone
. Cutting cones formed by osteoclasts followed by osteoblasts
. Intramembranous ossification from the periosteum
. Chondrocyte hypertrophy and apoptosis

Correct Answer & Explanation

. Cutting cones formed by osteoclasts followed by osteoblasts

Explanation

Primary (direct) bone healing occurs under conditions of absolute stability and minimal gap strain. It bypasses callus formation and relies on remodeling units (cutting cones), where osteoclasts drill across the fracture line followed closely by bone-depositing osteoblasts.

Question 5456

Topic: Biology, Genetics & Bone Healing

A 65-year-old woman is treated with a monoclonal antibody for osteoporosis that mimics the physiological role of osteoprotegerin (OPG). What is the primary mechanism of action of this medication?

. Binds to RANKL, preventing its interaction with RANK
. Binds directly to osteoclasts, inducing apoptosis
. Stimulates Wnt signaling by inhibiting sclerostin
. Competitively inhibits the ruffled border of osteoclasts
. Increases intestinal calcium absorption

Correct Answer & Explanation

. Binds to RANKL, preventing its interaction with RANK

Explanation

Denosumab is a monoclonal antibody that acts like OPG by binding to RANKL, preventing it from activating RANK on osteoclasts. This inhibits osteoclast survival and function, thereby decreasing bone resorption.

Question 5457

Topic: Biomechanics & Biomaterials
When selecting an intramedullary nail for a tibial shaft fracture, the surgeon considers the material properties. Which of the following orthopedic biomaterials has a Young's modulus closest to that of cortical bone?
. Cobalt-chromium alloy
. Titanium alloy
. Stainless steel
. Alumina ceramic
. Ultra-high-molecular-weight polyethylene (UHMWPE)

Correct Answer & Explanation

. Titanium alloy

Explanation

Titanium has a Young's modulus of approximately 110 GPa, which is closer to cortical bone (15-30 GPa) compared to stainless steel (200 GPa) or cobalt-chromium (220 GPa). This closer match helps reduce stress shielding.

Question 5458

Topic: Biomechanics & Biomaterials
Highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is widely used in total hip arthroplasty to reduce wear. What is the primary trade-off regarding the mechanical properties of UHMWPE when the degree of cross-linking is increased?
. Decreased fatigue resistance and fracture toughness
. Decreased resistance to adhesive wear
. Increased risk of oxidation in vivo
. Increased ultimate tensile strength
. Decreased elastic modulus leading to stress shielding

Correct Answer & Explanation

. Decreased fatigue resistance and fracture toughness

Explanation

Increasing cross-linking in UHMWPE significantly improves its resistance to adhesive and abrasive wear. However, this comes at the expense of decreased fatigue resistance, ductility, and fracture toughness.

Question 5459

Topic: Biomechanics & Biomaterials

A ligament graft used for anterior cruciate ligament (ACL) reconstruction elongates over time when subjected to a constant physiological load. This time-dependent biomechanical property is best described as:

. Stress relaxation
. Creep
. Hysteresis
. Fatigue failure
. Anisotropy

Correct Answer & Explanation

. Creep

Explanation

Creep is the progressive deformation of a viscoelastic 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 deformation.

Question 5460

Topic: Biomechanics & Biomaterials

A surgeon decides to use a solid intramedullary nail instead of a hollow nail of the same outer diameter. According to the polar moment of inertia, how does increasing the inner diameter (hollowing the nail) affect its torsional rigidity?

. It increases proportionally to the inner radius squared
. It decreases proportionally to the inner radius to the fourth power
. It decreases linearly with the inner radius
. It remains unchanged as it depends only on the outer diameter
. It increases proportionally to the outer radius cubed

Correct Answer & Explanation

. It decreases proportionally to the inner radius to the fourth power

Explanation

The polar moment of inertia for a hollow cylinder is proportional to the difference between the outer radius to the fourth power and the inner radius to the fourth power. Therefore, hollowing a solid nail decreases its torsional rigidity proportionally to the inner radius to the fourth power.

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