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1. General Principles & Basic Science MCQs

Practice Set 554 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 11061

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of bone healing, primary bone healing (direct lamellar bone formation via cutting cones without intermediate callus) can only occur if the interfragmentary strain is below what critical threshold?

. Less than 2%
. 2% to 10%
. 10% to 30%
. 30% to 50%
. Greater than 50%

Correct Answer & Explanation

. Less than 2%

Explanation

Perren's strain theory dictates that the tissue capable of forming within a fracture gap is determined by the strain. Primary lamellar bone healing requires absolute stability with an interfragmentary strain of less than 2%. Strains between 2% and 10% permit secondary healing with woven bone/callus formation. Cartilage can tolerate ~10% strain, and granulation tissue can tolerate up to 100% strain.

Question 11062

Topic: Biomechanics & Biomaterials
To reduce volumetric wear and subsequent osteolysis in total joint arthroplasty, manufacturers often utilize highly cross-linked ultra-high molecular weight polyethylene (UHMWPE). What is the primary mechanical trade-off resulting from the irradiation and cross-linking of UHMWPE?
. Decreased resistance to adhesive wear
. Decreased resistance to abrasive wear
. Decreased fatigue crack propagation resistance (decreased toughness)
. Increased oxidation potential in vivo
. Decreased modulus of elasticity

Correct Answer & Explanation

. Decreased fatigue crack propagation resistance (decreased toughness)

Explanation

Cross-linking UHMWPE (usually via gamma or electron beam irradiation) significantly increases its resistance to adhesive and abrasive wear. However, this process alters the polymer's mechanical properties, leading to decreased ductility, decreased ultimate tensile strength, and reduced fatigue crack propagation resistance (toughness). Remelting post-irradiation decreases oxidation potential.

Question 11063

Topic: 1. General Principles & Basic Science

Which of the following statements best describes the structural incorporation and remodeling process of a massive cortical bone autograft?

. Early revascularization leading to an immediate increase in mechanical strength
. Osteoclastic resorption via cutting cones followed by osteoblastic formation, resulting in temporary mechanical weakness
. Direct endochondral ossification without a cartilaginous intermediate
. Rapid creeping substitution that maintains the graft's mechanical integrity throughout the process
. Solely osteoinductive activity without providing any structural support

Correct Answer & Explanation

. Early revascularization leading to an immediate increase in mechanical strength

Explanation

Cortical bone grafts incorporate via creeping substitution, which relies heavily on osteoclasts moving in to resorb the bone via cutting cones, followed by osteoblasts laying down new bone. Because resorption initially outpaces formation, the graft becomes significantly weaker and more porous before it eventually regains its strength over a period of months to years.

Question 11064

Topic: Biology, Genetics & Bone Healing

Nitrogen-containing bisphosphonates (e.g., Alendronate, Zoledronate) are commonly prescribed to treat osteoporosis. They decrease osteoclast activity and induce osteoclast apoptosis primarily through the inhibition of which of the following enzymes?

. Cathepsin K
. Receptor activator of nuclear factor kappa-B ligand (RANKL)
. Farnesyl pyrophosphate (FPP) synthase
. Carbonic anhydrase II
. Osteoprotegerin (OPG)

Correct Answer & Explanation

. Cathepsin K

Explanation

Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate (FPP) synthase, an enzyme in the mevalonate pathway. This prevents the prenylation of small GTPase proteins (like Ras, Rho, Rac) which are essential for osteoclast function and survival, ultimately leading to osteoclast apoptosis. Non-nitrogenous bisphosphonates work by forming toxic ATP analogues.

Question 11065

Topic: 1. General Principles & Basic Science

Review the basic structure of the sarcomere and its regulatory proteins.

During excitation-contraction coupling in skeletal muscle, calcium ions are released from the sarcoplasmic reticulum. To initiate contraction, these calcium ions bind directly to which of the following molecules?

. Actin
. Myosin
. Tropomyosin
. Troponin C
. Troponin I

Correct Answer & Explanation

. Actin

Explanation

In skeletal muscle, the release of calcium from the sarcoplasmic reticulum leads to calcium binding directly to Troponin C. This causes a conformational change in the troponin-tropomyosin complex, exposing the myosin-binding sites on the actin filament and allowing cross-bridge cycling to occur. Troponin I inhibits the interaction, and Troponin T binds tropomyosin.

Question 11066

Topic: Biomechanics & Biomaterials

Minimizing the mismatch in the modulus of elasticity between an orthopaedic implant and the host bone is critical to reduce stress shielding and subsequent bone resorption. Which of the following implant materials has a Young's modulus most closely approximating that of human cortical bone?

. 316L Stainless Steel
. Cobalt-Chromium-Molybdenum alloy
. Titanium alloy (Ti-6Al-4V)
. Polymethylmethacrylate (PMMA)
. Alumina ceramic

Correct Answer & Explanation

. 316L Stainless Steel

Explanation

Human cortical bone has a Young's modulus of approximately 15-20 GPa. Titanium alloys (Ti-6Al-4V) have a modulus of roughly 110 GPa, which is significantly lower and closer to bone than Stainless Steel (~200 GPa) or Cobalt-Chromium alloys (~220-240 GPa). This lower modulus makes titanium more flexible, reducing stress shielding.

Question 11067

Topic: 1. General Principles & Basic Science

Consider the geometric properties of a standard orthopaedic bone screw.

When a surgeon is inserting a cortical screw into dense diaphyseal bone, resistance is met. Which of the following design parameters is the primary determinant of the screw's torsional yield strength (its resistance to breaking during insertion)?

. Outer (major) diameter
. Inner (core) diameter
. Thread pitch
. Thread depth
. Screw length

Correct Answer & Explanation

. Outer (major) diameter

Explanation

A screw's resistance to breaking under torsional loads (torsional yield strength) is proportional to the cube of its inner (core) diameter. In contrast, the screw's pull-out strength is directly proportional to its outer diameter, thread engagement length, and the shear strength of the bone.

Question 11068

Topic: Biology, Genetics & Bone Healing
A 10-year-old boy presents with bleeding gums, petechiae, and evidence of impaired wound healing. Radiographs show a distinct radiolucent line in the metaphysis (Trümmerfeld zone). The underlying biochemical defect is a failure to properly hydroxylate proline and lysine residues. Which step of collagen synthesis is directly impaired?
. Extracellular cleavage of procollagen propeptides
. Intracellular post-translational modification in the rough endoplasmic reticulum
. Intracellular assembly of the triple helix
. Extracellular cross-linking by lysyl oxidase
. Transcription of the COL1A1 gene in the nucleus

Correct Answer & Explanation

. Intracellular post-translational modification in the rough endoplasmic reticulum

Explanation

The clinical presentation is classic for scurvy (Vitamin C deficiency). Vitamin C is an essential cofactor for prolyl hydroxylase and lysyl hydroxylase. These enzymes function intracellularly within the rough endoplasmic reticulum to hydroxylate proline and lysine residues on preprocollagen chains. This hydroxylation is required for stable triple-helix formation.

Question 11069

Topic: Biology, Genetics & Bone Healing

Laboratory evaluation of a 55-year-old female reveals elevated serum calcium, decreased serum phosphate, elevated intact parathyroid hormone (PTH), and elevated alkaline phosphatase. If a bone biopsy were performed, which of the following histological findings would most likely be observed?

. Abundant unmineralized osteoid seams (defective mineralization)
. A mosaic pattern of lamellar bone with prominent, haphazard cement lines
. Increased osteoclastic tunneling, subperiosteal resorption, and peritrabecular fibrosis
. Thinning of cortical bone with completely normal trabecular mineralization and architecture
. Osteoclasts lacking a ruffled border leading to dense, sclerotic bone

Correct Answer & Explanation

. Abundant unmineralized osteoid seams (defective mineralization)

Explanation

The laboratory profile indicates primary hyperparathyroidism. Excess PTH stimulates massive osteoclastic bone resorption. Histologically, this presents as dissecting osteitis (osteoclastic tunneling into trabeculae), subperiosteal bone resorption, and replacement of the marrow space with highly vascular fibrous tissue (osteitis fibrosa cystica/brown tumors). Option A is osteomalacia, B is Paget's, D is osteoporosis, and E is osteopetrosis.

Question 11070

Topic: Biology, Genetics & Bone Healing

The differentiation of multipotent mesenchymal stem cells (MSCs) into various skeletal lineages is governed by specific transcription factors. The commitment of an MSC specifically into the osteoblast lineage requires the expression of which master transcription factor?

. Sox9
. Runx2 (Cbfa1)
. PPAR-gamma
. MyoD
. NF-kappaB

Correct Answer & Explanation

. Sox9

Explanation

Runx2 (also known as Cbfa1) is the master transcription factor required for the differentiation of mesenchymal stem cells into osteoblasts. A mutation in this gene leads to cleidocranial dysplasia. Sox9 drives chondrogenesis, PPAR-gamma drives adipogenesis, and MyoD drives myogenesis.

Question 11071

Topic: 1. General Principles & Basic Science

Proteoglycans represent a critical component of the extracellular matrix of articular cartilage. What is the primary biomechanical function of the most abundant proteoglycan, aggrecan, in this tissue?

. It primarily resists pure tensile and shear loads across the joint
. It provides compressive stiffness by creating an osmotic swelling pressure that is constrained by the collagen network
. It acts as a boundary lubricant on the most superficial surface of the cartilage layer
. It covalently tethers chondrocytes to the extracellular matrix to sense mechanical strain
. It enzymatically regulates the intermolecular cross-linking of type II collagen fibrils

Correct Answer & Explanation

. It primarily resists pure tensile and shear loads across the joint

Explanation

Aggrecan consists of a protein core with numerous negatively charged glycosaminoglycan (GAG) side chains (chondroitin sulfate and keratan sulfate). These negative charges repel each other and attract cations (like Na+) and water, creating a massive osmotic swelling pressure. The type II collagen network restrains this swelling, rendering the cartilage stiff and capable of resisting large compressive loads.

Question 11072

Topic: Biology, Genetics & Bone Healing

During fracture healing and normal bone remodeling, the canonical Wnt signaling pathway plays a crucial role in promoting osteoblast differentiation and bone formation. Activation of this pathway primarily results in the intracellular accumulation and nuclear translocation of which of the following molecules?

. NFATc1
. Smad4
. Beta-catenin
. STAT3
. c-Fos

Correct Answer & Explanation

. NFATc1

Explanation

In the canonical Wnt signaling pathway, Wnt proteins bind to Frizzled receptors and LRP5/6 co-receptors. This inhibits the destruction complex (which includes GSK-3beta), preventing the degradation of beta-catenin. Accumulated beta-catenin then translocates to the nucleus where it binds TCF/LEF transcription factors to upregulate genes that promote osteoblastogenesis and bone formation. Sclerostin (SOST) inhibits this pathway.

Question 11073

Topic: Biomechanics & Biomaterials

A patient presents with pain and a sterile effusion 4 years after a metal-on-polyethylene total hip arthroplasty utilizing a modular titanium neck and cobalt-chromium head.

Retrieval analysis of the head-neck taper junction would most likely reveal which primary mechanism of failure?

. Galvanic corrosion alone
. Fretting corrosion
. Abrasive wear
. Adhesive wear
. Third-body wear

Correct Answer & Explanation

. Galvanic corrosion alone

Explanation

Mechanically assisted crevice corrosion (MACC), often initiated by fretting corrosion, is the primary mode of failure at modular junctions in total hip arthroplasty. Micromotion at the taper junction disrupts the passivating oxide layer (fretting), exposing reactive metal to fluid, which then undergoes crevice and galvanic corrosion.

Question 11074

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) are essential for osteoinduction. Following the binding of BMP-2 to its transmembrane serine/threonine kinase receptor, which of the following intracellular signaling molecules is directly phosphorylated to translocate to the nucleus and modulate gene transcription?

. JAK
. STAT
. Smad 1/5/8
. beta-catenin
. c-AMP

Correct Answer & Explanation

. JAK

Explanation

BMPs signal via cell surface serine/threonine kinase receptors. Upon activation, they phosphorylate receptor-regulated Smads (R-Smads), specifically Smad 1, 5, and 8. These bind to the common-mediator Smad 4, forming a complex that translocates into the nucleus to act as a transcription factor.

Question 11075

Topic: 1. General Principles & Basic Science

During skeletal muscle contraction, calcium ions are released from the sarcoplasmic reticulum. To initiate the power stroke cycle, calcium must bind directly to which of the following proteins?

. Myosin heavy chain
. Tropomyosin
. Troponin I
. Troponin C
. Troponin T

Correct Answer & Explanation

. Myosin heavy chain

Explanation

In skeletal muscle, calcium binds to Troponin C. This causes a conformational change in the troponin complex, shifting tropomyosin away from the myosin-binding sites on the actin filament, allowing cross-bridge cycling to occur. Troponin I is inhibitory, and Troponin T binds to tropomyosin.

Question 11076

Topic: 1. General Principles & Basic Science
A patient sustains a peripheral nerve injury characterized by disruption of the axons, endoneurium, and perineurium, but the epineurium remains intact. According to the Sunderland classification, what grade of injury is this?
. Grade I
. Grade II
. Grade III
. Grade IV
. Grade V

Correct Answer & Explanation

. Grade IV

Explanation

Sunderland Grade IV injury involves disruption of the axon, endoneurium, and perineurium, with an intact epineurium. Grade I is neurapraxia. Grade II is axonotmesis (axon disrupted, endoneurium intact). Grade III involves axon and endoneurium disruption (perineurium intact). Grade V is complete nerve transection.

Question 11077

Topic: Biomechanics & Biomaterials

A freshly harvested tendon is subjected to biomechanical testing. The tendon is stretched to a specific length, and that length is maintained constant over time. The gradual decrease in the measured force required to maintain this constant length is best defined as:

. Creep
. Stress relaxation
. Hysteresis
. Fatigue
. Anisotropy

Correct Answer & Explanation

. Creep

Explanation

Stress relaxation is the viscoelastic property where the stress (force) decreases over time when a material is held at a constant strain (length). Creep is the gradual increase in strain (deformation) when held at a constant stress (load).

Question 11078

Topic: Biology, Genetics & Bone Healing

Analysis of normal synovial fluid reveals it is a dialysate of blood plasma combined with a specific macromolecule that imparts high viscosity. Which cell type is primarily responsible for synthesizing this macromolecule?

. Type A synoviocytes
. Type B synoviocytes
. Chondrocytes
. Osteoblasts
. Mast cells

Correct Answer & Explanation

. Type A synoviocytes

Explanation

Synovial fluid's viscosity is largely due to hyaluronic acid (hyaluronan), which is synthesized by Type B synoviocytes. Type B synoviocytes are fibroblast-like cells. Type A synoviocytes are macrophage-like and are responsible for phagocytosis.

Question 11079

Topic: Biomechanics & Biomaterials

Titanium alloy (Ti-6Al-4V) is widely used in orthopedic implants due to its excellent biocompatibility and a modulus of elasticity closer to bone than cobalt-chromium. However, its primary biomechanical disadvantage, which limits its use in certain articulating applications, is:

. High galvanic corrosion potential
. Poor notch sensitivity and wear resistance
. Low fatigue strength
. High density
. Excessive stiffness

Correct Answer & Explanation

. High galvanic corrosion potential

Explanation

Titanium alloys have poor wear characteristics and are highly notch sensitive, meaning surface scratches significantly reduce their fatigue life. Consequently, they are rarely used for articulating surfaces (e.g., femoral heads) and are typically reserved for stems, cups, and fracture fixation where wear is less of an issue.

Question 11080

Topic: Biology, Genetics & Bone Healing

According to Perren’s strain theory of fracture healing, lamellar bone can only form when the interfragmentary strain is below which of the following thresholds?

. 100%
. 50%
. 10%
. 2%
. 0.1%

Correct Answer & Explanation

. 100%

Explanation

Perren's strain theory postulates that specific tissues can only form under certain strain conditions. Granulation tissue tolerates up to 100% strain, fibrous tissue and cartilage up to 10%, and lamellar bone can only form when strain is < 2%.

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