Orthopedic Basic Science 2026 MCQs: Board Review Pt 1

Question 1

A 20-year-old woman with a history of subtotal meniscectomy has a painful knee. What associated condition is a contraindication to proceeding with a meniscal allograft?

Explanation

Patients with significant joint malalignment place increased stresses on the allograft, and this malalignment must be corrected to decrease the likelihood of meniscal allograft failure. None of the other options would lead to failure of the allograft.

Question 2

Long bone fracture repair following intramedullary stabilization occurs primarily through which of the following healing mechanisms?

Explanation

The mechanical environment represents a major factor in the type of healing that occurs after a fracture. Intramedullary nail fixation allows for motion at the fracture site, which promotes bone formation both directly (intramembranous ossification) and through a cartilage intermediate (endochondral ossification). Absolute stability, as would be obtained with a compression plate, favors healing through the direct formation of bone without a cartilage intermediate (intramembranous ossification), or primary fracture repair. This type of healing would include the remodeling of the bone ends through the direct contact of bone, often referred to as contact healing or haversian remodeling. Buckwalter JA, Einhorn TA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System, ed 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2000, pp 385-386.

Question 3

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

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 4

Figures 1a through 1c show the radiograph and MRI scans of a 16-year-old patient who has a painful hip. Examination reveals a significant limp, limited abduction and internal rotation, and severe pain with internal rotation and adduction. A biopsy specimen is shown in Figure 1d. What is the deposited pigment observed in this condition?

Explanation

1b 1c 1d Pigmented villonodular synovitis (PVNS) is a synovial proliferative disorder that remains a diagnostic difficulty. The most common clinical features are mechanical pain and limited joint motion. On radiographs, the classic finding is often a large lesion, associated with multiple lucencies. Other findings may include a normal radiographic appearance, loss of joint space, osteonecrosis of the femoral head, or acetabular protrusion. MRI is the imaging modality of choice and will show the characteristic findings of a joint effusion, synovial proliferation, and bulging of the hip. The synovial lining has a low signal on T1- and T2-weighted images, secondary to hemosiderin deposition. Copper deposition occurs in patients with Wilson's disease, which mainly affects the liver. Bhimani MA, Wenz JF, Frassica FJ: Pigmented villonodular synovitis: Keys to early diagnosis. Clin Orthop 2001;386:197-202.

Question 5

Titanium and its alloys are unsuitable candidates for which of the following implant applications?

Explanation

Titanium alloy is highly biocompatible, has higher strength than stainless steel, and is highly resistant to corrosion. It is particularly suited for use in fracture plates, bone screws, and intramedullary nails because of its low modulus of elasticity (low stiffness), which can reduce stress shielding. It is also widely used for porous-ingrowth coatings. However, clinical experience has shown that titanium alloy bearing surfaces such as a femoral ball are highly susceptible to severe metallic wear, particularly in the presence of third-body abrasive particles (PMMA fragments, bone chips, metal debris, etc). McKellop HA, Sarmiento A, Schwinn CP, et al: In vivo wear of titanium-alloy hip prostheses. J Bone Joint Surg Am 1990;72:512-517. Salvati EA, Betts F, Doty SB: Particulate metallic debris in cemented total hip arthroplasty. Clin Orthop 1993;293:160-173.

Question 6

What percent of patients initially diagnosed with classic, high-grade osteosarcoma of the extremity have visible evidence of pulmonary metastasis on CT of the chest?

Explanation

CT studies show that approximately 10% to 20% of patients with high-grade osteosarcoma have pulmonary metastases at diagnosis. Although not visible on current staging studies, it is believed that up to 80% of patients have micrometastatic disease that requires systemic chemotherapy. Because it is not possible to identify those patients who do not have disseminated micrometastatic disease, most patients are treated presumptively with chemotherapy. Kaste SC, Pratt CB, Cain AM, et al: Metastases detected at the time of diagnosis of primary pediatric extremity osteosarcoma at diagnosis: Imaging features. Cancer 1999;86:1602-1608.

Question 7

Staphylococcus aureus develops methicillin resistance through production of which of the following agents?

Explanation

Organisms may develop resistance to antibiotics by production of specific enzymes. S aureus develops methicillin resistance by production of penicillin-binding protein 2a (PBP2a). The strains of S aureus that have acquired the mecA gene for PBP2a are designated as methicillin-resistant S aureus (MRSA). The enzyme manifests resistance to covalent modification by beta-lactam antibiotics at the active-site serine residue in two ways. First, the microscopic rate constant for acylation (k2) is attenuated by three to four orders of magnitude over the corresponding determinations for penicillin-sensitive PBPs. Second, the enzyme shows elevated dissociation constants (Kd) for the noncovalent preacylation complexes with the antibiotics, the formation of which ultimately would lead to enzyme acylation. The two factors working in concert effectively prevent enzyme acylation by the antibiotics in vivo, giving rise to drug resistance. Catalase and oxidase are enzymes produced by some bacteria that confer virulence to the organism but do not make the organisms methicillin resistant. Glycocalix is the pericellular layer produced by bacteria that serves many functions including attachment to surfaces. Beta-lactam describes a class of antibiotics. Fuda C, Suvorov M, Vakulenko SB, et al: The basis for resistance to beta-lactam antibiotics by penicillin-binding protein 2a (PBP2a) of methicillin-resistant staphylococcus aureus. J Biol Chem 2004;279:40802-40806. Lim D, Strynadka NC: Structural basis for the beta lactam resistance of PBP2a from methicillin-resistant Staphylococcus aureus. Nat Struct Biol 2002;9:870-876.

Question 8

A 30-year-old man reports pain and weakness in his right arm. Examination reveals grade 4 strength in wrist flexion and elbow extension, decreased sensation over the middle finger, and decreased triceps reflex. These symptoms are most compatible with impingement on what spinal nerve root?

Explanation

Motor impulses to the triceps, wrist flexion and elbow extension, and sensation to the middle finger are associated most commonly with the C7 root. Hoppenfeld S: Physical Examination of the Spine and Extremities. Upper Saddle River, NJ, Prentice Hall, 1976, p 125.

Question 9

Why is tendon considered an anisotropic material?

Explanation

Anisotropic materials have mechanical properties that vary based on the direction of loading. The relative values of Young's modulus for tendon, ligament, and bone are not relevant to isotropy. The mechanical properties of tendon do change with preconditioning, but this change is related to viscoelasticity. The intrinsic mechanical properties of tendon do vary with the rate of loading, but this variance is related to viscoelasticity. Buckwalter JA, Einhorn TA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System, ed 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2000, pp 134-180.

Question 10

What does Dual Energy X-ray Absorptiometry (DEXA) testing, as a technique, measure?

Explanation

DEXA can provide data on bone mineral content and soft-tissue composition, and requires cross-sectional dimension for accuracy. DEXA provides a quantitative, not qualitative, measurement of bone mineral content and is incapable of differentiating between trabecular and cortical bone. Osteoarthritis falsely elevates the values, especially in the AP spinal analysis. Genant HK, Faulkner KG, Gluer CC: Measurement of bone mineral density: Current status. Am J Med 1991;91:49S-53S. Genant HK, Engelke K, Fuerst T, et al: Review: Noninvasive assessment of bone mineral density and stature: State of the art. J Bone Miner Res 1996;11:707-730.

Question 11

A 20-year-old athlete sustains a 2- x 3-cm grade IV chondral injury to the right knee. After failure of nonsurgical management, which of the following procedures would ensure the highest percentage of hyaline-like cartilage?

Explanation

Autologous chondrocyte implantation was first reported by Brittberg in 1994 and has resulted in predominantly type II collagen (hyaline-like articular cartilage) in the repair tissue. The extracellular matrix in articular cartilage is made up primarily of type II collagen, proteoglycans, and water. Arthroscopic chondroplasty, microfracture, drilling, and abrasion arthroplasty all result eventually in fibrocartilage fill of the defect (predominantly type I collagen). Brittberg M, Lindahl A, Nilsson A, et al: Treatment of deep cartilage defects in the knee with autologous chondrocyte transplantation. N Engl J Med 1994;331:889-895.

Question 12

Which of the following changes to heart rate, blood pressure, and bulbocavernosus reflex are typical of spinal shock?

Explanation

The term 'spinal shock' applies to all phenomena surrounding physiologic or anatomic transection of the spinal cord that results in temporary loss or depression of all or most spinal reflex activity below the level of the injury. Hypotension and bradycardia caused by loss of sympathetic tone is a possible complication, depending on the level of the lesion. The mechanism of injury that causes spinal shock is usually traumatic in origin and occurs immediately, but spinal shock has been described with mechanisms of injury that progress over several hours. Spinal cord reflex arcs immediately above the level of injury also may be depressed severely on the basis of the Schiff-Sherrington phenomenon. The end of the spinal shock phase of spinal cord injury is signaled by the return of elicitable abnormal cutaneospinal or muscle spindle reflex arcs. Autonomic reflex arcs involving relay to secondary ganglionic neurons outside the spinal cord may be affected variably during spinal shock, and their return after spinal shock abates is variable. The returning spinal cord reflex arcs below the level of injury are irrevocably altered and are the substrate on which rehabilitation efforts are based.

Question 13

What is the primary intracellular signaling mediator for bone morphogenetic protein (BMP) activity?

Explanation

BMPs signal through the activation of a transmembrane serine/threonine kinase receptor that leads to the activation of intracellular signaling molecules called SMADs. There are currently eight known SMADs, and the activation of different SMADs within a cell leads to different cellular responses. The other mediators are not believed to be directly involved with BMP signaling. Lieberman J, Daluiski A, Einhorn TA: The role of growth factors in the repair of bone: Biology and clinical applications. J Bone Joint Surg Am 2002;84:1032-1044. Li J, Sandell LJ: Transcriptional regulation of cartilage-specific genes, in Rosier RN, Evans C (eds): Molecular Biology in Orthoapedics, Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, pp 21-24.

Question 14

Which of the following properties primarily provides the excellent corrosion resistance of metallic alloys such as stainless steel and cobalt-chromium-molybdenum?

Explanation

All of the metals and metallic alloys used in orthopaedic surgery obtain their corrosion resistance from an adherent oxide layer. For stainless steel and cobalt alloy, the addition of chromium as an alloying element ensures the formation of a chromium oxide passive layer that forms on the surface and separates the bulk material from the corrosive body environment. Titanium alloy achieves the same result without chromium by forming an adherent passive layer of titanium oxide. Although these layers can indeed be hard, hardness does not in and of itself provide corrosion resistance. Adding nickel to both metallic alloys adds to strength but does not influence corrosion resistance appreciably. Galvanic potential can influence corrosion but does so by differences in potential between two contacting materials; for example, stainless steel and cobalt alloy have substantially different potentials, and if they were in contact within an aqueous environment, corrosion would commence with the stainless steel becoming the sacrificial anode. Metallic carbides are important in strengthening the alloys but have no role in providing corrosion resistance. Williams DF, Williams RL: Degradative effects of the biological environment on metal and ceramics, in Ratner BD, Hoffman AS, Shoen FJ, et al (eds): Biomaterials Science. San Diego, CA, Academic Press, 1996, pp 260-265. Buckwalter JA, Einhorn TA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System, ed 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2000, pp 190-193.

Question 15

Which of the following infectious organisms may be associated with underlying malignancy?

Explanation

Evidence implicates an association, albeit unexplained, between Clostridium septicum infection and malignancy, particularly hematologic or intestinal malignancy. The malignancy is often at an advanced stage, compromising survival of the patients. A bowel portal of entry is postulated for most patients. In the absence of an external source in the patient with clostridial myonecrosis or sepsis, the cecum or distal ileum should be considered a likely site of infection. Increased awareness of this association between Clostridium septicum and malignancy, and aggressive surgical treatment, may result in improvement in the present 50% to 70% mortality rate. Other organisms associated with malignancy include group Clostridium streptococci that are occasionally associated with upper gastrointestinal malignancies. Schaaf RE, Jacobs N, Kelvin FM, et al: Clostridium septicum infection associated with colonic carcinoma and hematologic abnormality. Radiology 1980;137:625-627.

Question 16

Immobilization of human tendons leads to what changes in structure and/or function?

Explanation

Recent in vivo and in vitro experiments demonstrate that immobilization of tendon decreases its tensile strength, stiffness, and total weight. Microscopically, there is a decrease in cellularity, overall collagen organization, and collagen fibril diameter.

Question 17

Human menisci are made up predominantly of what collagen type?

Explanation

Type I collagen accounts for more than 90% of the total collagen content. Other minor collagens present include types II, III, V, and VI. Mow VC, Arnoczky SP, Jackson DW (eds): Knee Meniscus: Basic and Clinical Foundations. New York, NY, Raven Press, 1992, p 41.

Question 18

Type II collagen in nondiseased adult human articular cartilage has a half-life that is generally

Explanation

Type II collagen in articular cartilage is amazingly stable. This is important to know because matrix homeostasis generally is associated with minimal synthesis and degradation of type II collagen. Passive glycation has a consistent rate and occurs over decades. The relative amount of glycation in cartilage with age has been used as a measure of stability. Also, the rate of racemization of aspartic acid from the L to D form occurs spontaneously at a very slow rate. The relative stability of collagen can be estimated by calculating the percentage of D aspartic acid per dry weight of type II collagen. Maroudas A, Palla G, Gilav E: Racemization of aspartic acid in human articular cartilage. Connect Tissue Res 1992;28:161-169.

Question 19

What changes in muscle physiology would be expected in an athlete who begins a rigorous aerobic program for an upcoming marathon?

Explanation

Muscle fibers can be categorized grossly into two types. Type I muscle, also known as slow-twitch muscle, is responsible for aerobic, oxidative muscle metabolism. It has a much lower strength and speed of contraction than fast-twitch type II muscle but is significantly more fatigue resistant. With training for endurance sports, the type I muscle undergoes adaptive changes to the increased stress. Increases in capillary density, oxidative capacity, mitochondrial density, and subsequent fatigue resistance are all observed changes. Hypertrophy of type IIb muscle is seen in strength training. Simon SR (ed): Orthopaedic Basic Science. Rosemont, IL, American Academy of Orthopaedic Surgeons, 1994, pp 89-125.

Question 20

When a structure like a long bone is under a bending load, its maximum stress is most dependent on what factor?

Explanation

The maximum stress in a bone occurs at the periosteal surface (the greatest distance from the center of the bone). The magnitude of the stress is equal to the magnitude of the applied moment (M) multiplied by the distance to the surface (roughly the radius of the bone, r) divided by the area moment of inertia (I), so that stress = Mr/I. Of the possible answers, only area moment of inertia of the cross section contains any of these three items. The stress can also depend on the length of the bone, but it cannot be determined without knowing the location at which the bending load is applied, information that was not given in the problem. The type of structural support may influence local stresses where the support contacts the bone, but it has little effect on the maximum stress in the bone. The cross-sectional area is not as important as the area moment of inertia because the stress is not evenly distributed over the cross-section. Plastic modulus is a material property, not a geometric or structural property, and it does not affect stress. Hayes WC, Bouxsein ML: Biomechanics of cortical and trabecular bone: Implications for assessment of fracture risk, in Mow VC, Hayes WC (eds): Basic Orthopaedic Biomechanics, ed 2. New York, NY, Lippincott-Raven, 1997, pp 76-82.

Question 21

A 24-year-old man who works at a local oyster and clam farm sustained a laceration on his hand at work. Examination reveals a cellulitic index finger without evidence of tenosynovitis. After appropriate irrigation and debridement, what antibiotic is most appropriate?

Explanation

Injuries involving brackish water and shellfish can have devastating consequences caused by Vibrio vulnificus infections. Patients may have a severe invasive infection, with three main clinical features: primary septicemia, wound infection, and gastroenteritis. Antibiotic administration is crucial because mortality rates of up to 50% have been observed with Vibrio septicemia. The current recommendation is to give a third-generation cephalosporin such as ceftazadime. Chiang SR, Chuang YC: Vibrio vulnificus infection: Clinical manifestations, pathogenesis, and antimicrobial therapy. J Microbiol Immunol Infect 2003;36:81-88.

Question 22

A 16-year-old girl has had anterior leg pain and a mass for the past 8 months. Figures 2a and 2b show a radiograph and an H & E histologic specimen. Which of the following disorders is believed to be a precursor of this lesion?

Explanation

2b The radiograph and pathology are consistent with adamantinoma. While the mechanism underlying adamantinoma has not been identified, it is believed to be closely related to osteofibrous dysplasia, which may represent a precursor. The other diagnoses are not known to give rise to adamantinoma.

Question 23

Acetaminophen is an antipyretic medication. It exerts its pharmacologic effects by inhibiting which of the following enzymes?

Explanation

Acetaminophen inhibits prostaglandin E2 production via IL-1 B, without affecting cyclooxygenase-2 enzymatic activity. The therapeutic concentrations of acetaminophen induce an inhibition of IL-1 B-dependent NF-kappa B nuclear translocation. The selectivity of this effect suggests the existence of an acetaminophen-specific activity at the transcriptional level that may be one of the mechanisms through which the drug exerts its pharmacologic effects. Acetaminophen does not affect any of the other enzymes named above.

Question 24

Nutritional rickets is associated with which of the following changes in chemical blood level?

Explanation

Nutritional rickets is associated with decreased dietary intake of Vitamin D, resulting in low levels of Vitamin D that result in decreased intestinal absorption of calcium and low to normal serologic levels of calcium. To boost serum calcium levels, there is a compensatory increase in PTH and bone resorption, leading to increased alkaline phosphatase levels. Brinker MR: Cellular and molecular biology, immunology, and genetics in orthopaedics, in Miller MD (ed): Review of Orthopaedics, ed 3. Philadelphia, PA, WB Saunders, 2001, pp 81-94. Pettifor J: Nutritional and drug-induced rickets and osteomalacia, in Farrus MJ (ed): Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, ed 5. Philadelphia, PA, Lippincott Williams and Wilkins, 2003, pp 399-466.

Question 25

What assay most directly assesses gene expression at the posttranslational level?

Explanation

Gene expression at the posttranslational level refers to proteins, as opposed to DNA or RNA. The only assay above that targets protein expression directly is the Western blot. Standard PCR is amplification of targeted DNA segments, regardless of whether or not they are actively expressed. Real-time PCR, Northern blot, and microarray expression profile analysis all quantify RNA as a means to determine posttranscriptional gene expression. Brinker MR: Cellular and molecular biology, immunology, and genetics in orthopaedics, in Miller MD (ed): Review of Orthopaedics, ed 3. Philadelphia, PA, WB Saunders, 2001, pp 81-94.

Question 26

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

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 27

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?

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 28

Articular cartilage has a specific zonal anatomy that determines its mechanical properties. Which of the following accurately describes the composition of the deep zone compared to the superficial zone?

Explanation

The deep (basal) zone of articular cartilage is characterized by the highest concentration of proteoglycans and the lowest water content. The collagen fibers in this zone (primarily Type II) are arranged perpendicularly to the joint surface and cross the tidemark into the calcified zone, anchoring the cartilage to the subchondral bone. The superficial (tangential) zone, on the other hand, possesses the highest water content, the lowest proteoglycan content, and collagen fibers oriented parallel to the joint surface to effectively resist shear forces.

Question 29

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?

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 30

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?

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 31

Mutations in the gene encoding for Type X collagen are most commonly associated with which of the following skeletal dysplasias?

Explanation

Type X collagen is a short-chain, network-forming collagen exclusively expressed by hypertrophic chondrocytes in the hypertrophic zone of the physis (growth plate) and in calcifying cartilage. Mutations in the COL10A1 gene, which encodes for Type X collagen, cause Schmid metaphyseal chondrodysplasia. Osteogenesis imperfecta is caused by mutations in Type I collagen (COL1A1 or COL1A2). Achondroplasia is due to a gain-of-function mutation in the FGFR3 gene. Cleidocranial dysplasia is caused by an alteration in the RUNX2/CBFA1 transcription factor. Multiple epiphyseal dysplasia is genetically heterogeneous, often involving COMP or Type IX collagen mutations.

Question 32

In total joint arthroplasty, different mechanisms of polyethylene wear dictate implant longevity. Which of the following best describes the primary mechanism of ultra-high-molecular-weight polyethylene (UHMWPE) failure historically seen in total knee arthroplasty compared to total hip arthroplasty?

Explanation

Historically, the primary mode of UHMWPE failure in total knee arthroplasty (TKA) is subsurface delamination and fatigue wear. This occurs due to the relatively non-conforming nature of the tibiofemoral joint, leading to high cyclical, non-uniform contact stresses that exceed the fatigue strength of the polyethylene, particularly if it has undergone oxidative degradation (e.g., gamma-irradiated in air). In contrast, a total hip arthroplasty (THA) is a highly conforming ball-and-socket joint, and its primary wear mechanism is a combination of adhesive and abrasive wear, which generates millions of submicron volumetric wear particles that drive osteolysis.

Question 33

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?

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 34

A 25-year-old athlete undergoes an anterior cruciate ligament (ACL) reconstruction. The native ACL has a direct insertion into the bone of the femur and tibia. Which of the following represents the correct order of histological transition zones at a direct ligamentous insertion (enthesis)?

Explanation

A direct ligament or tendon insertion into bone (enthesis) is designed to gradually transmit forces from a pliable soft tissue to a rigid bone, thereby mitigating stress concentrations that could lead to avulsion. It consists of four distinct, progressively stiffening histological zones. The correct transitional order from soft tissue to bone is: 1) Tendon/Ligament proper, 2) Uncalcified fibrocartilage, 3) Tidemark (a distinct, highly basophilic line separating uncalcified and calcified tissue), and 4) Calcified fibrocartilage, which ultimately interdigitates with the subchondral bone. Indirect insertions do not have this distinct four-zone transition and often insert over a broader area via Sharpey's fibers directly into the periosteum.

Question 35

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?

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 36

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?

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 37

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?

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 38

The pullout strength of a cortical bone screw is directly proportional to all of the following parameters EXCEPT:

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 39

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

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 40

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

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 41

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?

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 42

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

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.

Question 43

Following a peripheral nerve transection (neurotmesis), Wallerian degeneration occurs distal to the injury site. Which cell type is primarily responsible for clearing myelin debris and facilitating the creation of endoneurial tubes to guide regenerating axons in the peripheral nervous system?

Explanation

In the peripheral nervous system, Schwann cells are critical for both myelinating axons and facilitating regeneration after injury. Following nerve transection, Wallerian degeneration occurs distally. Schwann cells, along with recruited macrophages, phagocytose myelin debris. Schwann cells then proliferate and form the Bands of Büngner within the endoneurial tubes, which guide regenerating axons back to their targets. Oligodendrocytes serve a similar myelinating function in the central nervous system but do not aid in robust regeneration.

Question 44

A marathon runner relies heavily on muscle fibers adapted for prolonged endurance. Which of the following characteristics best describes the predominant muscle fiber type utilized during this activity?

Explanation

Marathon running requires prolonged endurance, which relies on Type I (slow-twitch) muscle fibers. These fibers are characterized by a slow contraction speed, high resistance to fatigue, and a strong reliance on aerobic (oxidative) metabolism. To support this aerobic metabolism, they have a high density of mitochondria, a rich capillary supply, and a high concentration of myoglobin, which gives them a distinct red color.

Question 45

A patient is prescribed apixaban for deep vein thrombosis (DVT) prophylaxis following a total knee arthroplasty. What is the specific mechanism of action of this anticoagulant medication?

Explanation

Apixaban and rivaroxaban are direct oral anticoagulants (DOACs) that act as highly selective, direct inhibitors of Factor Xa (the 'xa' in their generic names denotes their target). They bind directly to Factor Xa, inhibiting both free and clot-bound Factor Xa, as well as prothrombinase activity, thereby preventing the conversion of prothrombin to thrombin. Dabigatran is a direct thrombin inhibitor. Aspirin inhibits COX-1. Warfarin is a vitamin K antagonist.

Question 46

A histologic section of healthy adult articular cartilage is analyzed. In which zone are the collagen fibers oriented primarily parallel to the joint surface to resist shear forces?

Explanation

The superficial (tangential) zone of articular cartilage makes up about 10-20% of the thickness and consists of tightly packed collagen fibers (mostly Type II) oriented parallel to the articular surface. This orientation provides the cartilage with high tensile strength to resist shear forces. The deep zone has collagen fibers oriented perpendicular to the joint surface to resist compressive forces.

Question 47

An orthopedic surgeon is considering intramedullary nailing for a tibial shaft fracture. According to the principles of biomechanics, if the diameter of a solid cylindrical nail is increased by a factor of 2, how does its bending stiffness change?

Explanation

The bending stiffness of a solid cylindrical structure is directly proportional to its area moment of inertia. For a solid cylinder, the area moment of inertia is proportional to the radius (or diameter) raised to the fourth power (r^4). Therefore, increasing the diameter by a factor of 2 increases the bending stiffness by 2^4, which equals 16.

Question 48

A 4-year-old boy presents with a history of recurrent fractures following minimal trauma. On examination, he is noted to have blue sclerae and mild hearing loss. Which of the following describes the underlying molecular defect most likely responsible for his condition?

Explanation

The clinical presentation is classic for osteogenesis imperfecta (OI). OI is primarily caused by autosomal dominant mutations in the COL1A1 or COL1A2 genes, which encode the alpha-1 and alpha-2 chains of Type I collagen, leading to defective collagen synthesis. FGFR3 mutations cause achondroplasia. Fibrillin-1 mutations cause Marfan syndrome. CBFA1/RUNX2 mutations cause cleidocranial dysplasia.

Question 49

A researcher is studying the failure mechanisms of orthopedic implants in an animal model. A stainless steel screw is deliberately used to secure a titanium alloy plate. What is the most likely mode of corrosion to occur at the interface of these two implants in vivo?

Explanation

Galvanic corrosion occurs when two electrochemically dissimilar metals are placed in direct contact within an electrolytic medium, such as bodily fluids. The less noble metal becomes the anode and undergoes accelerated corrosion, while the more noble metal becomes the cathode. To prevent this, mixing dissimilar metals like stainless steel and titanium in a single construct is generally avoided.

Question 50

Denosumab is utilized in the management of osteoporosis and giant cell tumor of bone. Which of the following accurately describes its primary cellular mechanism of action?

Explanation

Denosumab is a fully human monoclonal antibody that directly binds to and inhibits Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). By neutralizing RANKL, it prevents RANKL from binding to the RANK receptor on the surface of osteoclasts and their precursors, thereby potently inhibiting osteoclast maturation, function, and survival. OPG is the body's natural decoy receptor for RANKL.

Question 51

During the static stretching phase of a physical therapy protocol for Achilles tendinopathy, a therapist notes that when the tendon is stretched to a specific length and held constant, the internal force required to maintain that length gradually decreases over time. This specific biomechanical property is termed:

Explanation

Stress relaxation is a hallmark viscoelastic property characterized by a decrease in stress (force) over time when a material is held at a constant strain (length or deformation). Creep, conversely, is the progressive increase in strain (deformation) over time when a constant stress (load) is applied. Hysteresis refers to the energy lost as heat during the loading and unloading cycles of a viscoelastic material.

Question 52

Recombinant human Bone Morphogenetic Protein-2 (rhBMP-2) is commonly utilized to enhance posterolateral spinal fusions. BMPs induce osteoblastic differentiation primarily through which of the following intracellular signaling pathways?

Explanation

Bone Morphogenetic Proteins (BMPs) belong to the TGF-beta superfamily. When BMPs bind to their specific serine/threonine kinase membrane receptors, they trigger the intracellular phosphorylation of Smad proteins, specifically Smad 1, 5, and 8 (receptor-regulated Smads). These form a complex with Smad 4 (common-partner Smad) and translocate to the nucleus to regulate target gene transcription for osteoblast differentiation.

Question 53

A 68-year-old male presents with groin pain 15 years after an uncomplicated primary total hip arthroplasty. Radiographs demonstrate significant eccentric wear of the polyethylene liner and substantial periprosthetic acetabular osteolysis. What is the primary cellular mediator responsible for initiating the biological cascade leading to this osteolysis?

Explanation

Aseptic loosening and periprosthetic osteolysis in joint arthroplasty are primarily driven by the generation of ultra-high-molecular-weight polyethylene (UHMWPE) wear debris. The debris particles (specifically those 0.1 to 1.0 micrometers in size) are phagocytosed by macrophages. This triggers the macrophages to release a cascade of pro-inflammatory cytokines (e.g., TNF-alpha, IL-1, IL-6), which subsequently stimulate osteoclastic bone resorption.

Question 54

A patient undergoes open reduction and internal fixation (ORIF) of a transverse radial shaft fracture utilizing a dynamic compression plate to achieve absolute stability. Assuming an anatomic reduction with rigid fixation is obtained, which of the following best describes the expected primary mechanism of fracture healing?

Explanation

Absolute stability achieved through rigid internal fixation (such as compression plating) eliminates interfragmentary motion and bypasses the callus formation seen in secondary healing. Instead, the bone heals via direct (primary) bone healing. This occurs through direct Haversian remodeling, where osteoclasts create 'cutting cones' that cross the fracture site, immediately followed by osteoblasts depositing new lamellar bone.

Question 55

When evaluating an MRI of the knee to confirm a suspected anterior cruciate ligament (ACL) tear, a specific sequence is chosen wherein the repetition time (TR) is 2500 ms and the echo time (TE) is 100 ms. On this sequence, the joint effusion appears uniformly bright (hyperintense). What type of sequence is being utilized?

Explanation

A T2-weighted MRI sequence is defined by a long Repetition Time (TR > 2000 ms) and a long Echo Time (TE > 80 ms). On T2-weighted images, tissues with high water content, such as joint effusions, CSF, or edema, appear hyperintense (bright), making it excellent for evaluating pathology. T1 sequences (short TR, short TE) show fat as bright and fluid as dark.

Question 56

Which of the following statements best describes the biomechanical property of an implant that determines its rigidity and is represented by the slope of the linear portion of the stress-strain curve?

Explanation

Modulus of elasticity (Young's modulus) is the measure of a material's intrinsic stiffness. It is calculated as the slope of the linear (elastic) portion of the stress-strain curve. A higher modulus indicates a stiffer material. Toughness is the energy absorbed up to the point of failure (area under the entire curve). Yield strength is the stress at which plastic deformation begins. Ultimate tensile strength is the maximum stress the material can withstand.

Question 57

In articular cartilage, which zone has the highest concentration of water and collagen, but the lowest concentration of proteoglycans?

Explanation

The superficial (tangential) zone of articular cartilage contains the highest concentration of water (approx. 80%) and collagen (Type II, arranged parallel to the joint surface to resist shear stress), and the lowest concentration of proteoglycans. The deep zone contains the lowest water content and the highest concentration of proteoglycans, with collagen fibers oriented vertically to resist compressive forces.

Question 58

During fracture healing via endochondral ossification, which transcription factor is strictly required for the differentiation of mesenchymal stem cells into chondrocytes?

Explanation

SOX9 is the master transcription factor for chondrogenesis, essential for driving mesenchymal stem cells to differentiate into chondrocytes during endochondral ossification and cartilage formation. RUNX2 (also known as Cbfa1) and Osterix are critical transcription factors for osteoblast differentiation.

Question 59

A child presents with rhizomelic short stature, frontal bossing, and midface hypoplasia. Genetic testing reveals a mutation in the FGFR3 gene. Which of the following describes the molecular consequence of this mutation?

Explanation

Achondroplasia is caused by a gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. FGFR3 normally functions to negatively regulate endochondral bone growth. A gain-of-function mutation leads to its constitutive activation, which prematurely inhibits chondrocyte proliferation in the proliferative zone of the physis, resulting in dwarfism.

Question 60

In total hip arthroplasty, the primary initiator of aseptic loosening and periprosthetic osteolysis is the biological response to wear debris. Which cell type phagocytoses these particles and subsequently secretes cytokines that stimulate osteoclastogenesis?

Explanation

Periprosthetic osteolysis is primarily initiated by macrophages that phagocytose ultra-high-molecular-weight polyethylene (UHMWPE) wear particles. These activated macrophages secrete inflammatory cytokines (such as TNF-alpha, IL-1, IL-6, and PGE2), which increase RANKL expression. This cascade ultimately drives osteoclast activation and subsequent periprosthetic bone resorption.

Question 61

Demineralized bone matrix (DBM) is widely used in orthopedic surgery as an adjunct for spinal fusion and fracture nonunions. Which of the following combinations of properties does DBM possess?

Explanation

Demineralized bone matrix (DBM) provides a collagenous scaffold that serves as an osteoconductive matrix. Because the demineralization process preserves biologically active bone morphogenetic proteins (BMPs) and other growth factors, DBM is also osteoinductive. However, because it contains no living cells (osteoprogenitors or osteoblasts), it lacks osteogenic properties.

Question 62

A patient develops a methicillin-resistant Staphylococcus aureus (MRSA) infection following a posterior spinal fusion. Intravenous vancomycin is initiated. What is the primary mechanism of action of vancomycin?

Explanation

Vancomycin is a glycopeptide antibiotic that treats Gram-positive bacterial infections, including MRSA, by inhibiting cell wall synthesis. It achieves this by binding tightly to the D-alanyl-D-alanine terminus of cell wall precursor units, effectively preventing their incorporation into the peptidoglycan matrix. Quinolones inhibit DNA gyrase, macrolides bind the 50S subunit, and aminoglycosides bind the 30S subunit.

Question 63

A 65-year-old woman with a history of a distal radius fracture undergoes a dual-energy x-ray absorptiometry (DEXA) scan yielding a T-score of -3.0. She is prescribed denosumab. What is the precise mechanism of action of this medication?

Explanation

Denosumab is a fully human monoclonal antibody that targets and binds to the Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). By neutralizing RANKL, it prevents it from binding to the RANK receptor on the surface of osteoclasts and their precursors. This profoundly inhibits osteoclast differentiation, function, and survival, leading to decreased bone resorption.

Question 64

You are evaluating a high-performance marathon runner for a tibial stress fracture. During your review of basic muscle physiology, you recall that Type I (slow-twitch) muscle fibers possess specific characteristics tailored for endurance. Which of the following best describes Type I muscle fibers compared to Type IIb fibers?

Explanation

Type I muscle fibers are 'slow-twitch' oxidative fibers specialized for endurance activities. They have a high myoglobin content (giving them a red appearance), high mitochondrial and capillary densities, and rely heavily on oxidative phosphorylation. They have high fatigue resistance but lower peak force, slower contraction velocity, and smaller motor neuron sizes compared to Type II (fast-twitch) fibers.

Question 65

Mixed-metal constructs, such as using a stainless steel screw with a titanium plate, are generally avoided in orthopedic surgery. Which specific type of corrosion is most likely to occur in this scenario?

Explanation

Galvanic corrosion occurs when two electrochemically dissimilar metals (e.g., stainless steel and titanium) are placed in physical contact within a conductive electrolyte medium, such as human bodily fluid. The less noble metal becomes the anode and undergoes accelerated corrosion, while the more noble metal acts as the cathode. Fretting corrosion, by contrast, is caused by micromotion between two surfaces.

Question 66

Sclerostin is a key regulatory glycoprotein in bone metabolism and the target of the monoclonal antibody romosozumab. What is its primary mechanism of action at the molecular level?

Explanation

Sclerostin is produced primarily by mature osteocytes and acts as a negative regulator of bone formation. It functions by binding to the LRP5/6 co-receptors on the surface of osteoblasts, which competitively blocks the binding of Wnt ligands. This prevents the activation of the canonical Wnt/beta-catenin signaling pathway, leading to the phosphorylation and proteasomal degradation of beta-catenin, ultimately downregulating osteoblastogenesis and bone formation. Romosozumab is a neutralizing antibody against sclerostin that harnesses this pathway to stimulate bone formation.

Question 67

During an anterior cruciate ligament (ACL) reconstruction, a surgeon tensions the tendon graft to a specific length and fixes it. Over the next several minutes, the tension (force) required by the fixation device to maintain that exact length slowly decreases. Which of the following viscoelastic properties best describes this phenomenon?

Explanation

Tendons and ligaments are viscoelastic materials, meaning their mechanical response depends on the rate and duration of loading. Stress relaxation is defined as the decrease in stress (or force) over time when the material is held at a constant strain (constant length). Conversely, creep is the progressive deformation (increase in length) over time when a material is subjected to a constant load (constant stress). Hysteresis refers to the energy lost (usually as heat) during a loading and unloading cycle.

Question 68

Articular cartilage relies on a unique structural organization to withstand both compressive and shear forces in synovial joints. Which zone of articular cartilage is characterized by the highest water content, the lowest proteoglycan concentration, and collagen fibrils oriented parallel to the joint surface?

Explanation

The superficial (tangential) zone of articular cartilage comprises 10-20% of the thickness. It features densely packed Type II collagen fibrils oriented parallel to the articular surface, making it highly adept at resisting shear forces. It also contains the highest concentration of water and the lowest concentration of proteoglycans. In contrast, the deep zone contains collagen fibrils oriented perpendicular to the joint surface, the highest proteoglycan content, and the lowest water content, making it optimal for resisting compressive loads.

Question 69

A surgeon performs an open reduction and internal fixation of a diaphyseal fracture using a titanium alloy (Ti-6Al-4V) plate and secures it with 316L stainless steel screws. The physiologic environment surrounding this mixed-metal construct will most likely predispose it to which of the following phenomena?

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in contact within a conductive electrolytic solution (like bodily fluids). The less noble metal (anode) will corrode more rapidly, while the more noble metal (cathode) remains protected. In a titanium-stainless steel construct, stainless steel acts as the anode and corrodes, while titanium acts as the cathode. Thus, mixing these specific metals is generally avoided in orthopedic surgery.

Question 70

Following a traumatic transection of the radial nerve, the distal nerve segment undergoes Wallerian degeneration. Which of the following cells are primarily responsible for the rapid phagocytosis and clearance of myelin debris in the peripheral nervous system, thereby creating a permissible environment for axonal regeneration?

Explanation

In the peripheral nervous system (PNS), Wallerian degeneration involves the breakdown of the axon and its myelin sheath distal to the injury. Schwann cells dedifferentiate, proliferate, and work alongside recruited blood-borne macrophages to rapidly phagocytose and clear the myelin debris. This clearance is crucial because myelin debris contains inhibitory factors for axon growth. In the central nervous system (CNS), oligodendrocytes and microglia clear debris much more slowly, which contributes to the poor regenerative capacity of the CNS.

Question 71

Biomechanical compatibility between orthopedic implants and native bone is essential to prevent stress shielding. Which of the following lists correctly orders orthopedic materials from the lowest modulus of elasticity (most flexible) to the highest modulus of elasticity (stiffest)?

Explanation

The modulus of elasticity (Young's modulus) represents the stiffness of a material. To minimize stress shielding, implant materials ideally should have a modulus closer to that of bone. Cortical bone has a modulus of approximately 15-20 GPa. Titanium alloy (Ti-6Al-4V) is approximately 110 GPa. Stainless steel (316L) is approximately 200 GPa. Cobalt-chromium alloys are the stiffest, with a modulus around 220-240 GPa. Thus, the correct order from lowest to highest stiffness is Cortical bone < Titanium alloy < Stainless steel < Cobalt-chromium alloy.

Question 72

During the active phase of bone remodeling, an osteoclast attaches to the bone surface and creates an isolated, acidic microenvironment known as Howship's lacuna. While the acidic pH dissolves the inorganic mineral, which of the following enzymes is primarily secreted to degrade the organic Type I collagen matrix?

Explanation

Bone resorption by osteoclasts is a biphasic process. First, the inorganic hydroxyapatite is dissolved by hydrochloric acid, which is produced intracellularly by carbonic anhydrase II and actively pumped into the ruffled border cleft via a vacuolar H+-ATPase. Following demineralization, the organic matrix (predominantly Type I collagen) is degraded by proteolytic enzymes. Cathepsin K, a highly specific lysosomal cysteine protease secreted by the osteoclast, is the primary enzyme responsible for this organic matrix degradation. TRAP is a marker of osteoclast activity but not the primary collagenase.

Question 73

The excitation-contraction coupling of skeletal muscle relies heavily on calcium handling. Upon an action potential traveling down the T-tubule, calcium is released from the sarcoplasmic reticulum. What is the immediate direct effect of this intracellular calcium release?

Explanation

According to the sliding filament theory, an action potential triggers calcium release from the sarcoplasmic reticulum. This calcium binds to troponin C, which undergoes a conformational change. This movement pulls tropomyosin away from the myosin-binding sites on the actin filament. Once these sites are exposed, the energized myosin heads can bind to actin to form cross-bridges, leading to the power stroke and muscle contraction.

Question 74

A 35-year-old male undergoes bridge plating for a comminuted midshaft femur fracture. The surgeon opts to leave four empty screw holes in the plate directly overlying the comminuted fracture segment. How does this increased 'working length' biomechanically alter the construct?

Explanation

Working length in bridge plating is defined as the distance between the two innermost screws on either side of the fracture. Increasing the working length makes the construct less rigid (more flexible), which correspondingly decreases both the bending and torsional stiffness of the plate. This deliberate flexibility allows for controlled interfragmentary micromotion (strain), which stimulates callus formation and secondary bone healing. It does not increase pull-out strength, and high flexibility prevents primary bone healing.

Question 75

A 7-year-old boy sustains a closed Salter-Harris Type II fracture of the distal radius. The initial fracture plane traverses transversely through the physis before exiting vertically through the metaphysis. Biomechanically, this initial transverse failure across the growth plate characteristically occurs through which histological zone?

Explanation

Physeal fractures classically occur through the hypertrophic zone of the growth plate (specifically near the zone of provisional calcification). Biomechanically, this zone is the weakest layer of the physis due to the high volume of swollen, hypertrophic chondrocytes and the relatively sparse extracellular matrix separating them. The lack of robust structural matrix predisposes this specific zone to transverse shearing or tension failure, which is the pathognomonic starting point for Salter-Harris injuries.

Question 76

Articular cartilage is a highly specialized tissue responsible for minimizing friction and distributing loads across joints. Which of the following zones of articular cartilage contains the highest concentration of water and the lowest concentration of proteoglycans?

Explanation

The superficial (tangential) zone of articular cartilage is the thinnest layer but contains the highest water content (approximately 80%) and the lowest concentration of proteoglycans. The collagen fibers (primarily Type II) are aligned parallel to the joint surface to resist shear forces. The deep zone, in contrast, has the lowest water content, highest proteoglycan content, and collagen fibers aligned perpendicular to the joint surface to resist compressive loads.

Question 77

When evaluating the mechanical properties of an orthopedic implant, the stress-strain curve provides essential data.

Which of the following points on the curve represents the maximum stress a material can withstand before it begins to neck and ultimately fail?

Explanation

The Ultimate Tensile Strength (UTS) is the peak point on the stress-strain curve and represents the maximum stress a material can withstand before failure. After this point, the material begins to 'neck' (in ductile materials) and stress decreases until the failure point is reached. The yield point is the point at which elastic deformation transitions to plastic (irreversible) deformation.

Question 78

During the process of secondary fracture healing, various growth factors are expressed to orchestrate cellular differentiation and matrix synthesis. Which of the following growth factors is powerfully osteoinductive, signals predominantly through serine/threonine kinase receptors, and activates intracellular Smad proteins?

Explanation

Bone morphogenetic proteins (BMPs), particularly BMP-2 and BMP-7, are potent osteoinductive growth factors of the TGF-beta superfamily. They bind to serine/threonine kinase receptors on the cell surface, which subsequently phosphorylate and activate intracellular Smad proteins (e.g., Smad 1, 5, 8). These activated Smads form a complex with Smad 4, translocate to the nucleus, and upregulate the transcription of osteogenic genes such as Runx2.

Question 79

A surgeon is struggling with poor screw purchase in osteopenic bone during an open reduction and internal fixation of a distal humerus fracture. To maximize the pullout strength of a cortical screw, which of the following screw design modifications would be most effective?

Explanation

The pullout strength of a bone screw is directly proportional to the outer (thread) diameter, the length of thread engagement, and the shear strength of the bone. It is inversely proportional to the pitch. The formula for pullout strength is F = S * L * π * D, where S is the shear strength of the bone, L is the length of engagement, and D is the outer diameter. Increasing the outer diameter has the most profound effect on increasing pullout strength. Increasing the inner (root or core) diameter improves the bending strength of the screw but decreases the thread depth, which reduces pullout strength.

Question 80

Denosumab is an effective pharmacological agent used in the management of osteoporosis and giant cell tumor of bone. Which of the following describes the exact mechanism of action of this medication?

Explanation

Denosumab is a fully human monoclonal antibody (IgG2) that targets and binds to Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). RANKL is typically expressed by osteoblasts and stromal cells. By binding to RANKL, denosumab prevents the interaction between RANKL and the RANK receptor on the surface of osteoclasts and their precursors. This inhibits osteoclast formation, function, and survival, thereby reducing bone resorption. It effectively mimics the physiological role of osteoprotegerin (OPG).

Question 81

A 12-year-old male sustains a Salter-Harris Type II fracture of the distal femur. The fracture cleavage plane primarily propagates through which specific histologic zone of the physis?

Explanation

Salter-Harris fractures typically occur through the zone of hypertrophy of the physis. This zone is mechanically the weakest layer because the chondrocytes are significantly enlarged (hypertrophic), resulting in a relatively low ratio of extracellular matrix to cell volume. Furthermore, the zone lacks the structural reinforcement of calcification, which begins in the adjacent zone of provisional calcification.

Question 82

In total joint arthroplasty, the wear properties of ultra-high-molecular-weight polyethylene (UHMWPE) are critical to long-term implant survival. Which of the following processing techniques optimizes wear resistance while minimizing the risk of oxidative degradation?

Explanation

Highly cross-linked polyethylene (HXLPE) is manufactured by subjecting UHMWPE to gamma or electron beam irradiation, which creates free radicals that form cross-links between polymer chains. This significantly improves wear resistance. However, residual free radicals can react with oxygen over time, leading to oxidative degradation and embrittlement. To prevent this, the irradiated polyethylene undergoes a thermal treatment—either remelting (heating above the melting point) or annealing (heating just below the melting point)—often in an inert gas environment or vacuum, to quench the residual free radicals.

Question 83

During the clinical examination of a patient with a chronic knee flexion contracture, dynamic splinting is applied to provide a constant, low-load stretch to the posterior soft tissues. Over time, the contracted ligaments and capsule gradually elongate. Which of the following viscoelastic properties does this process demonstrate?

Explanation

Creep is defined as the progressive deformation (elongation) of a viscoelastic material when subjected to a constant, sustained load over time. This principle is applied clinically in dynamic splinting and serial casting. In contrast, stress relaxation refers to the decrease in stress (force) within a material over time when it is held at a constant deformation (length). Hysteresis is the energy lost as heat during the loading and unloading cycle of a viscoelastic material.

Question 84

A 28-year-old male presents with a slow-growing, painful mass near his knee joint. Biopsy reveals a biphasic tumor with both epithelial and spindle cell components. Cytogenetic analysis is most likely to reveal which of the following chromosomal translocations?

Explanation

The clinical presentation and biphasic histology (epithelial and spindle cells) are characteristic of synovial sarcoma. Synovial sarcoma is uniquely associated with the t(X;18)(p11;q11) chromosomal translocation, which results in the SYT-SSX fusion gene. The t(11;22) translocation is characteristic of Ewing sarcoma; t(12;16) is associated with myxoid liposarcoma; t(2;13) is seen in alveolar rhabdomyosarcoma; and t(9;22) in extraskeletal myxoid chondrosarcoma (as well as CML).

Question 85

A 65-year-old female is prescribed fondaparinux for deep vein thrombosis (DVT) prophylaxis following a total knee arthroplasty. What is the specific mechanism of action of this pharmacological agent?

Explanation

Fondaparinux is a synthetic pentasaccharide that exerts its anticoagulant effect by binding specifically to antithrombin III (ATIII). This binding induces a conformational change in ATIII, accelerating its ability to irreversibly inactivate Factor Xa by a factor of 300. Because it is an indirect inhibitor of Factor Xa and does not directly bind the factor itself, it differs from agents like rivaroxaban and apixaban (direct Factor Xa inhibitors). Dabigatran is a direct thrombin inhibitor, aspirin inhibits COX-1, and warfarin inhibits vitamin K epoxide reductase.

Question 86

In normal articular cartilage, which of the following best describes the structural and biochemical properties of the deep zone compared to the superficial zone?

Explanation

The deep zone of articular cartilage has the lowest water content, highest proteoglycan content, and collagen fibers (primarily type II) arranged perpendicularly to the subchondral bone to best resist compressive forces. The superficial zone has the highest water content, lowest proteoglycan content, and collagen fibers arranged parallel to the joint surface to resist shear forces.

Question 87

A surgeon is planning to use a solid intramedullary nail to treat a tibial shaft fracture. If the diameter of the nail is increased by a factor of 2, how does the torsional rigidity of the nail change?

Explanation

Torsional rigidity of a solid cylindrical implant is directly proportional to its polar moment of inertia, which is proportional to the radius (or diameter) to the fourth power (r^4). Therefore, increasing the diameter by a factor of 2 increases the torsional rigidity by a factor of 2^4, which equals 16.

Question 88

Which of the following factors is directly secreted by osteoblasts to inhibit osteoclastogenesis and limit bone resorption?

Explanation

Osteoprotegerin (OPG) is a decoy receptor produced by osteoblasts that binds to RANKL, preventing it from interacting with RANK on osteoclast precursors. This prevents osteoclast differentiation and activation, thereby limiting bone resorption. RANKL and M-CSF promote osteoclastogenesis. Sclerostin is produced by osteocytes to inhibit bone formation via the Wnt pathway, and Cathepsin K is an enzyme secreted by osteoclasts to degrade collagen.

Question 89

During a marathon, an endurance runner predominantly utilizes a specific type of skeletal muscle fiber. Which of the following best characterizes this muscle fiber type?

Explanation

Type I muscle fibers (slow-twitch) are highly utilized during endurance activities like marathons. They are characterized by high oxidative capacity, high mitochondrial density, high myoglobin content (giving them a red color), and strong resistance to fatigue. Type II fibers (fast-twitch) are utilized for short, explosive movements and have higher glycolytic capacity and fatigue more rapidly.

Question 90

A 45-year-old man sustains a closed midshaft femur fracture treated with a reamed, statically locked intramedullary nail. Fracture healing in this construct is predominantly expected to occur via:

Explanation

Intramedullary nailing provides relative mechanical stability rather than absolute stability. This strain environment stimulates secondary bone healing, which primarily occurs via endochondral ossification. This process progresses through stages of inflammation, soft callus (cartilaginous), hard callus, and remodeling. Primary bone healing (direct osteonal remodeling without callus) requires absolute stability and direct cortical contact, typically achieved with rigid compression plating.

Question 91

In total hip arthroplasty, the use of highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) compared to conventional UHMWPE results in which of the following mechanical trade-offs?

Explanation

Highly cross-linked UHMWPE significantly decreases volumetric wear, which is highly beneficial for reducing particle-induced osteolysis and improving the longevity of the joint replacement. However, the cross-linking process (via irradiation) and subsequent thermal treatment (to eliminate free radicals) alter the material's mechanical properties, generally resulting in decreased fatigue strength, ultimate tensile strength, and ductility compared to conventional UHMWPE.

Question 92

According to the Seddon classification of nerve injuries, which of the following best describes an injury characterized by disruption of the axon and myelin sheath, but with preservation of the endoneurium, perineurium, and epineurium?

Explanation

Axonotmesis (which corresponds to Sunderland Grade II) involves disruption of the axon and its myelin sheath, while the surrounding connective tissue frameworks (endoneurium, perineurium, and epineurium) remain intact. This allows for Wallerian degeneration distally but provides an intact anatomical path for axonal regeneration, typically resulting in favorable functional recovery. Neurapraxia is a temporary conduction block, while neurotmesis involves complete nerve transection.

Question 93

A 6-year-old boy presents with multiple recurrent fractures with minimal trauma, blue sclerae, and early-onset hearing loss. A genetic defect involving which of the following is most likely responsible for his condition?

Explanation

The clinical presentation is classic for Osteogenesis Imperfecta (OI), which is most commonly caused by autosomal dominant mutations in the genes (COL1A1 or COL1A2) encoding Type I collagen. Type I collagen is the primary structural collagen in bone, skin, tendons, and the sclera. FGFR3 mutations are associated with achondroplasia. Fibrillin-1 mutations are the cause of Marfan syndrome.

Question 94

During the remodeling phase of normal tendon healing, the biochemical composition of the extracellular matrix shifts to restore tensile strength. Which of the following changes is most characteristic of this phase?

Explanation

Tendon healing occurs in three overlapping phases: inflammatory, proliferative (reparative), and remodeling. During the early proliferative phase, fibroblasts rapidly synthesize Type III collagen, which is disorganized and weaker. In the remodeling phase, cellularity decreases, and Type III collagen is gradually replaced by Type I collagen. These Type I fibers align along the axis of mechanical stress, restoring the tendon's mature architecture and tensile strength.

Question 95

Which of the following best describes the specific mechanism of action of rivaroxaban, an oral anticoagulant frequently used for deep vein thrombosis prophylaxis following total joint arthroplasty?

Explanation

Rivaroxaban is a Direct Oral Anticoagulant (DOAC) that exerts its effect by directly and competitively inhibiting active Factor Xa. This interrupts both the intrinsic and extrinsic pathways of the blood coagulation cascade, preventing thrombin generation and subsequent thrombus formation. Dabigatran directly inhibits thrombin (Factor IIa). Warfarin is a vitamin K antagonist. Low-molecular-weight heparins activate antithrombin III. Clopidogrel targets ADP receptors on platelets.

Question 96

A 45-year-old man undergoes revision internal fixation of a femoral nonunion. The surgeon plans to use a titanium alloy plate for stabilization. Which of the following screw materials, if used in combination with this titanium plate, would be most likely to result in significant galvanic corrosion, and which material would preferentially undergo corrosive degradation?

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in an electrolytic solution (like bodily fluids), causing the less noble (more anodic) metal to corrode. In the galvanic series, titanium and cobalt-chromium are more noble (cathodic) compared to 316L stainless steel (anodic). If stainless steel screws are used with a titanium plate, the stainless steel acts as the anode and will preferentially corrode. Mixing stainless steel with titanium or cobalt-chromium implants is generally contraindicated in orthopedics for this reason.

Question 97

A 72-year-old woman is prescribed romosozumab, a monoclonal antibody, for the treatment of severe postmenopausal osteoporosis. What is the primary molecular mechanism by which this medication exerts its osteoanabolic effect?

Explanation

Romosozumab is a monoclonal antibody that targets and neutralizes sclerostin. Sclerostin is a glycoprotein secreted primarily by osteocytes that acts as a negative regulator of bone formation by binding to the LRP5/6 co-receptors on osteoblasts, thereby inhibiting the canonical Wnt/beta-catenin signaling pathway. By neutralizing sclerostin, romosozumab disinhibits this pathway, leading to a potent osteoanabolic (bone-building) effect.

Question 98

Which of the following best describes the fundamental biochemical changes that occur in the extracellular matrix of early osteoarthritic articular cartilage, as compared to the changes seen in normal aging articular cartilage?

Explanation

The hallmark of early osteoarthritis (OA) is an increase in cartilage water content due to damage to the restraining type II collagen network. Additionally, the cartilage attempts to repair itself by synthesizing new, immature proteoglycans that have a higher concentration of chondroitin sulfate relative to keratan sulfate (increased CS:KS ratio), although total proteoglycan content eventually drops. In contrast, normal aging cartilage is characterized by a decrease in water content and a decrease in the CS:KS ratio (as keratan sulfate concentration increases with age).

Question 99

In the manufacturing of ultra-high-molecular-weight polyethylene (UHMWPE) for total joint arthroplasty, highly cross-linking the polyethylene via gamma irradiation provides which of the following combinations of material property changes when compared to conventional UHMWPE?

Explanation

Highly cross-linked polyethylene (HXLPE) is created by exposing UHMWPE to gamma or electron beam irradiation, which forms covalent bonds between adjacent polymer chains. This cross-linking significantly improves the adhesive and abrasive wear resistance of the material, substantially reducing osteolysis rates. However, this process alters the crystalline structure, resulting in a degradation of mechanical properties, including decreased ductility, decreased fracture toughness, decreased fatigue strength, and decreased yield/ultimate tensile strength.

Question 100

A 25-year-old man sustains a mid-shaft humerus fracture resulting in a complete radial nerve palsy. Three weeks later, an electromyogram (EMG) reveals fibrillation potentials and positive sharp waves in the brachioradialis. Which of the following pathophysiologic changes at the cellular level best explains these electrodiagnostic findings?

Explanation

Fibrillation potentials and positive sharp waves on EMG are classic signs of active muscle denervation. Following Wallerian degeneration of the motor nerve, the muscle undergoes denervation supersensitivity. At the cellular level, the acetylcholine receptors (AChRs), which are normally strictly localized to the neuromuscular junction (motor endplate), are upregulated and widely synthesized/distributed across the entire extrajunctional sarcolemma (muscle cell membrane). This makes the entire muscle fiber hypersensitive to minute amounts of free acetylcholine, leading to spontaneous, independent depolarizations of single muscle fibers, recorded as fibrillation potentials.

AAOS & ABOS Basic Science MCQs (Set 3): Bone Biology, Biomechanics & Anatomy Review

Orthopedic Basic Science 2026 MCQs: Board Review Questions & Answers (Part 1)

Key Takeaway

Orthopedic Basic Science 2026 MCQs: Board Review Questions & Answers (Part 1)

Chapter Index