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1. General Principles & Basic Science MCQs

Practice Set 400 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 7981

Topic: Physiology & Rehabilitation

A thorough understanding of clinical gait analysis is essential for evaluating neuromuscular pathologies. During a normal human gait cycle, the quadriceps muscle group exhibits its peak electromyographic (EMG) activity during which of the following phases?

. Mid-stance
. Terminal stance
. Initial swing
. Terminal swing into early loading response
. Mid-swing

Correct Answer & Explanation

. Terminal swing into early loading response

Explanation

The quadriceps muscle group shows biphasic activity during the normal gait cycle, but it reaches its absolute peak electromyographic (EMG) activity during terminal swing and continues into the early loading response (initial contact to foot flat). During terminal swing, the quadriceps fire concentrically to extend the knee and prepare the limb for heel strike. Immediately following heel strike (loading response), they fire eccentrically to control knee flexion and absorb the shock of impact.

Question 7982

Topic: Biology, Genetics & Bone Healing

A 45-year-old male sustains a closed, isolated midshaft tibia fracture and undergoes fixation with a dynamically locked intramedullary nail. Which of the following mechanical environments or systemic factors would most significantly upregulate the expression of Bone Morphogenetic Protein-2 (BMP-2) and promote the formation of a robust cartilaginous callus?

. Absolute rigid plate fixation with interfragmentary compression
. High-dose systemic nonsteroidal anti-inflammatory drug (NSAID) administration
. Controlled cyclic micromotion at the fracture site
. Systemic hyperbaric oxygen therapy
. Selective depletion of local inflammatory macrophages

Correct Answer & Explanation

. Controlled cyclic micromotion at the fracture site

Explanation

Secondary bone healing relies on endochondral ossification, characterized by the formation of a cartilaginous callus. This process is heavily mechanically driven by relative stability, which allows for controlled cyclic micromotion at the fracture site (e.g., dynamic intramedullary nailing, bridge plating, external fixation). This micromotion physically stimulates the upregulation of osteogenic factors, primarily BMP-2, driving chondrogenesis and callus formation. Absolute rigid fixation leads to primary bone healing without a callus. NSAIDs and macrophage depletion delay or inhibit fracture healing.

Question 7983

Topic: 1. General Principles & Basic Science

The unique biomechanical ability of articular cartilage to resist compressive loads is primarily a function of its extracellular matrix. Which molecule is directly responsible for generating the high intrinsic swelling pressure within the matrix?

. Type II collagen
. Aggrecan
. Decorin
. Type X collagen
. Lubricin

Correct Answer & Explanation

. Aggrecan

Explanation

Aggrecan is the most abundant large aggregating proteoglycan in articular cartilage. It is densely packed with highly negatively charged glycosaminoglycan (GAG) side chains, primarily chondroitin sulfate and keratan sulfate. The dense concentration of negative charges creates strong electrostatic repulsion and attracts cations and water via the Donnan osmotic effect. This creates a massive internal swelling pressure that is restrained by the tensile network of Type II collagen. The combination of this swelling pressure and the restraining collagen network allows cartilage to resist high compressive loads.

Question 7984

Topic: Biology, Genetics & Bone Healing

A 35-year-old man sustains a closed midshaft femur fracture and is treated with a locked intramedullary nail. Callus formation is noted on radiographs at 6 weeks. According to Perren's strain theory, what maximum level of interfragmentary strain allows for the formation of lamellar bone at the fracture site?

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

Correct Answer & Explanation

. 2% to 10%

Explanation

According to Perren's strain theory, the type of tissue that forms in a fracture gap depends on the interfragmentary strain. Lamellar bone is brittle and can only form when the strain is less than 2%. Woven bone can tolerate strains between 2% and 10%. Cartilage can tolerate strains up to approximately 10%, while granulation tissue can form and persist in high-strain environments up to 100%. Intramedullary nailing allows for secondary bone healing via callus formation by keeping strain in the optimal range.

Question 7985

Topic: Biomechanics & Biomaterials

A 65-year-old man undergoes total hip arthroplasty using a highly cross-linked polyethylene (HXLPE) liner. Which of the following manufacturing processes is primarily responsible for reducing the wear rate of HXLPE compared to conventional polyethylene, while simultaneously decreasing its fatigue strength?

. Addition of vitamin E prior to consolidation
. Electron beam or gamma irradiation
. Ethylene oxide gas sterilization
. Remelting below the melting temperature (annealing)
. Cold forging of the polymer resin

Correct Answer & Explanation

. Electron beam or gamma irradiation

Explanation

Highly cross-linked polyethylene (HXLPE) is created by exposing conventional polyethylene to high doses of gamma or electron beam irradiation. This process cleaves carbon-hydrogen bonds, creating free radicals that combine to form cross-links between polymer chains. This drastically improves adhesive and abrasive wear resistance but concurrently reduces mechanical properties such as fatigue strength, ultimate tensile strength, and ductility. Remelting or adding Vitamin E is done to eliminate or neutralize residual free radicals to prevent long-term oxidation, rather than to create the cross-links.

Question 7986

Topic: 1. General Principles & Basic Science

In normal articular cartilage, the extracellular matrix is uniquely designed to resist varying mechanical loads. During the initial application of a compressive load, which of the following components is primarily responsible for generating the swelling pressure that resists tissue compression?

. Type II collagen network
. Hyaluronic acid backbone
. Aggrecan via its negatively charged glycosaminoglycan side chains
. Type X collagen
. Superficial zone chondrocytes

Correct Answer & Explanation

. Aggrecan via its negatively charged glycosaminoglycan side chains

Explanation

Aggrecan is the most abundant proteoglycan in articular cartilage. It possesses glycosaminoglycan (GAG) side chains, specifically chondroitin sulfate and keratan sulfate, which contain highly concentrated negatively charged sulfate and carboxyl groups. This fixed negative charge creates a high Donnan osmotic pressure (swelling pressure) that draws water into the tissue. When compressive loads are applied, the resistance to water extrusion by these fixed negative charges provides the primary resistance to compression. Type II collagen primarily provides tensile strength.

Question 7987

Topic: Biomechanics & Biomaterials

An orthopedic surgeon applies a dynamic splint to correct a knee flexion contracture in a 12-year-old child. The splint is designed to hold the knee at a constant, fixed amount of extension. Over a 30-minute period, the patient notes that the splint feels less tight, and the surgeon measures a decrease in the force required to maintain this exact degree of extension. Which viscoelastic property of ligaments best explains this phenomenon?

. Creep
. Stress relaxation
. Hysteresis
. Strain-rate dependency
. Anisotropy

Correct Answer & Explanation

. Stress relaxation

Explanation

Stress relaxation is a hallmark viscoelastic property defined as the decrease in stress (force) over time when a material is held at a constant strain (deformation or length). Because the dynamic splint holds the knee at a constant degree of extension (constant strain), the decline in the resisting force over time is due to stress relaxation. Creep, by contrast, is the progressive deformation (increase in length) over time when a material is subjected to a constant stress (force). Hysteresis refers to the energy lost as heat during a full cycle of loading and unloading.

Question 7988

Topic: Biology, Genetics & Bone Healing

Which of the following best describes the molecular mechanism of action of nitrogen-containing bisphosphonates (e.g., zoledronic acid) in the treatment of osteoporosis?

. Inhibition of RANKL binding to its receptor
. Inhibition of farnesyl pyrophosphate synthase
. Incorporation into ATP analogues leading to osteoclast apoptosis
. Stimulation of Wnt/beta-catenin signaling
. Inhibition of sclerostin

Correct Answer & Explanation

. Inhibition of farnesyl pyrophosphate synthase

Explanation

Nitrogen-containing bisphosphonates (such as alendronate, risedronate, and zoledronic acid) exert their effect by inhibiting farnesyl pyrophosphate synthase, an essential enzyme in the mevalonate pathway. This prevents the prenylation of small GTP-binding proteins (like Ras, Rho, and Rac), which are crucial for normal osteoclast function, ruffled border formation, and cell survival. Non-nitrogenous bisphosphonates (like etidronate) act differently by being incorporated into non-hydrolyzable ATP analogues, which ultimately induce osteoclast apoptosis. Denosumab inhibits RANKL, and romosozumab inhibits sclerostin.

Question 7989

Topic: Biomechanics & Biomaterials

During the application of a cast to correct a clubfoot deformity using the Ponseti method, the orthopedist applies a constant corrective stretch to the ligaments over time. The observed gradual decrease in the force required to hold the foot in the corrected position is an example of which of the following viscoelastic properties?

. Creep
. Hysteresis
. Stress relaxation
. Anisotropy
. Isotropic behavior

Correct Answer & Explanation

. Stress relaxation

Explanation

Stress relaxation is a viscoelastic property defined as the progressive decrease in stress (or internal force) over time when a material is held at a constant strain (or deformation). In serial casting, holding the foot in a fixed, deformed position allows the soft tissues to undergo stress relaxation. In contrast, creep is the progressive deformation of a material over time under a constant load. Hysteresis refers to the energy lost (usually as heat) during the cyclic loading and unloading of a viscoelastic material.

Question 7990

Topic: Biology, Genetics & Bone Healing

A surgeon decides to use demineralized bone matrix (DBM) as a bone graft substitute during a spinal fusion. Which of the following bone grafting properties is DBM most reliably recognized for possessing?

. Osteoconduction only
. Osteoinduction only
. Osteoconduction and osteogenesis
. Osteoconduction and osteoinduction
. Osteogenesis, osteoinduction, and osteoconduction

Correct Answer & Explanation

. Osteoconduction and osteoinduction

Explanation

Demineralized bone matrix (DBM) is an allograft that has been chemically processed to remove its mineral component. This processing exposes bone morphogenetic proteins (BMPs) and other inherent growth factors, conferring osteoinductive properties to the graft. Additionally, the remaining structural collagenous matrix provides a physical scaffold for new bone ingrowth, conferring osteoconductive properties. DBM does not contain living cells (like osteoblasts or mesenchymal stem cells), and therefore lacks osteogenic properties. Autograft is the only classic graft type providing all three: osteogenesis, osteoinduction, and osteoconduction.

Question 7991

Topic: Biology, Genetics & Bone Healing
Bone morphogenetic proteins (BMPs) are used clinically to promote bone healing. At the cellular level, the canonical signaling pathway for BMPs involves binding to a transmembrane receptor followed by the direct intracellular phosphorylation of which of the following?
. Beta-catenin
. Janus kinases (JAK)
. Smad proteins
. Cyclic AMP (cAMP)
. Tyrosine kinase

Correct Answer & Explanation

. Smad proteins

Explanation

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta (TGF-β) superfamily. They bind to cell-surface serine/threonine kinase receptors. Upon activation, the type II receptor phosphorylates the type I receptor, which in turn phosphorylates intracellular receptor-regulated Smad proteins (typically Smads 1, 5, and 8 for BMPs). These phosphorylated R-Smads bind to a common-mediator Smad (Smad 4) to form a complex that translocates into the nucleus to regulate the transcription of target genes. Wnt signaling utilizes beta-catenin, while JAK-STAT and tyrosine kinases are associated with other cytokine and growth factor pathways.

Question 7992

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of bone healing, primary (osteonal) bone healing without intermediate callus formation requires the interfragmentary strain at the fracture site to be maintained below what specific threshold?

. 2%
. 10%
. 15%
. 25%
. 100%

Correct Answer & Explanation

. 2%

Explanation

Perren's strain theory dictates that a specific tissue can only form in a fracture gap if the interfragmentary strain is below the critical tolerance level of that tissue. Interfragmentary strain is defined as the change in gap length divided by the original gap length. Granulation tissue can tolerate up to 100% strain, whereas cartilage can tolerate up to 10% strain, allowing for secondary bone healing via endochondral ossification (callus). For primary bone healing to occur—where osteonal cutting cones directly cross the fracture gap without callus formation—absolute stability is required, meaning the interfragmentary strain must be maintained at less than 2%.

Question 7993

Topic: 1. General Principles & Basic Science

During normal physiologic loading of a native synovial joint, different lubrication regimes predominate depending on the activity level. When an individual is standing still for a prolonged period, which of the following is the primary mechanism of joint lubrication?

. Elastohydrodynamic lubrication
. Hydrodynamic lubrication
. Squeeze-film lubrication
. Boundary lubrication
. Weeping lubrication

Correct Answer & Explanation

. Boundary lubrication

Explanation

Boundary lubrication predominates under extreme conditions of high loads and low relative speeds, such as prolonged standing. In this regime, the joint surfaces are closest together, and the fluid film alone cannot completely separate them. Lubrication is primarily provided by a protective monomolecular layer of surface-active molecules—predominantly lubricin (PRG4) and surface-active phospholipids—that bind to the articular surface, drastically reducing friction and preventing direct cartilage-to-cartilage adhesion. Fluid-film mechanisms (like elastohydrodynamic, hydrodynamic, and squeeze-film lubrication) require motion and joint loading cycles to generate a pressurized layer of synovial fluid that separates the articulating surfaces during dynamic activities.

Question 7994

Topic: 1. General Principles & Basic Science
To reduce wear rates in total joint arthroplasty, highly cross-linked polyethylene (HXLPE) is irradiated to create cross-links. However, this process produces free radicals. Which of the following is the primary mechanism by which the addition of Vitamin E (alpha-tocopherol) improves the performance of HXLPE?
. Increases the yield strength of the polyethylene
. Eliminates the need for gamma irradiation
. Acts as an antioxidant to quench free radicals and prevent oxidative degradation
. Promotes osteoblast adhesion to the polyethylene surface
. Increases the crystallinity of the polyethylene to reduce third-body wear

Correct Answer & Explanation

. Acts as an antioxidant to quench free radicals and prevent oxidative degradation

Explanation

Gamma irradiation of UHMWPE creates free radicals. If left unquenched, these react with oxygen, leading to oxidative degradation, embrittlement, and increased wear. Vitamin E (alpha-tocopherol) is a potent antioxidant that quenches free radicals without the need for post-irradiation melting (which decreases mechanical strength).

Question 7995

Topic: Biology, Genetics & Bone Healing

Denosumab is frequently used in the treatment of osteoporosis and giant cell tumor of bone. What is the specific mechanism of action of this medication?

. Binds to the ruffled border of osteoclasts, directly causing apoptosis
. Acts as a competitive antagonist of the Wnt signaling pathway
. Binds directly to RANKL, preventing it from activating the RANK receptor on osteoclasts
. Stimulates osteoprotegerin (OPG) production by osteoblasts
. Inhibits the enzyme cathepsin K, preventing degradation of bone collagen

Correct Answer & Explanation

. Binds directly to RANKL, preventing it from activating the RANK receptor on osteoclasts

Explanation

Denosumab is a fully human monoclonal antibody that binds to RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand). By binding to RANKL, it prevents the interaction between RANKL and the RANK receptor on the surface of osteoclast precursors and mature osteoclasts. This leads to the inhibition of osteoclast formation, function, and survival, thereby decreasing bone resorption.

Question 7996

Topic: 1. General Principles & Basic Science
The pullout strength of a cortical bone screw is directly proportional to all of the following parameters EXCEPT:
. The outer (major) diameter of the screw
. The inner (core) diameter of the screw
. The length of thread engagement in the bone
. The shear strength of the host bone
. The thread depth

Correct Answer & Explanation

. The inner (core) diameter of the screw

Explanation

The pullout strength of a screw is directly proportional to the outer (major) diameter of the screw, the length of thread engagement, and the shear strength of the bone material. It is inversely proportional to the inner (core) diameter, assuming the outer diameter remains constant, because a larger inner diameter reduces the thread depth. The inner diameter primarily determines the tensile strength of the screw, not the pullout strength.

Question 7997

Topic: 1. General Principles & Basic Science

Which zone of articular cartilage is characterized by collagen fibers oriented perpendicular to the joint surface and contains the highest concentration of proteoglycans?

. Superficial (tangential) zone
. Middle (transitional) zone
. Deep (radial) zone
. Calcified zone
. Subchondral bone

Correct Answer & Explanation

. Deep (radial) zone

Explanation

Articular cartilage is divided into four structural zones. The deep (radial) zone contains collagen fibers that are oriented perpendicular to the articular surface. This zone provides the greatest resistance to compressive forces and has the highest concentration of proteoglycans and the lowest concentration of water. The superficial zone has collagen parallel to the joint surface and the highest concentration of water.

Question 7998

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of bone healing, which of the following tissue types can tolerate the highest amount of strain before failure?

. Intact cortical bone
. Woven bone
. Lamellar bone
. Fibrocartilage
. Granulation tissue

Correct Answer & Explanation

. Granulation tissue

Explanation

Perren's strain theory dictates that a specific tissue type can only form and survive under a specific mechanical strain environment. Granulation tissue is highly compliant and can tolerate up to 100% strain before failure. Fibrocartilage can tolerate roughly 10-15% strain, woven bone can tolerate about 5% strain, and lamellar (cortical) bone can tolerate only about 2% strain.

Question 7999

Topic: Biomechanics & Biomaterials

A 65-year-old man undergoes revision of a total hip arthroplasty. The surgeon utilizes a titanium alloy femoral stem and a cobalt-chromium femoral head. What type of corrosion is specifically driven by the electrochemical potential difference between these two dissimilar metals?

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

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in contact within an electrolytic solution (such as body fluid). The less noble metal acts as an anode and corrodes, while the more noble metal acts as a cathode. While fretting and crevice corrosion also occur at modular junctions, galvanic corrosion is the specific term for corrosion driven by the coupling of mixed metals.

Question 8000

Topic: Biomechanics & Biomaterials

In the stress-strain curve of a healthy human ligament, the initial non-linear 'toe region' represents which of the following microscopic physiological events?

. Breaking of cross-links between collagen fibrils
. Plastic deformation of the extracellular matrix
. Uncrimping of the relaxed collagen fibers
. Microfailure of individual collagen bundles
. Complete macroscopic failure of the ligament

Correct Answer & Explanation

. Uncrimping of the relaxed collagen fibers

Explanation

The stress-strain curve for ligaments and tendons begins with a non-linear 'toe region.' This phase represents the physiological uncrimping or straightening of the wavy collagen fibers as tension is initially applied. After uncrimping, the curve enters the linear (elastic) region where the fibers themselves are stretched. Microfailure begins later at the yield point, entering the plastic region.

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