Orthopedic Basic Science 2026 MCQs: Part 2 Review

Question 1

Fully dense alumina and zirconia materials have been used as bearing materials in hip arthroplasty to provide greater wear resistance than polished metallic surfaces. Although both have shown reduced wear clinically, what concerns continue to exist about the use of zirconia?

Explanation

Zirconia as a bearing surface is "metastable," meaning that, in the microstructure of the material the zirconia molecules are ordered in a tetragonal fashion, but they can easily transform to a monoclinic microstructure that is less wear resistant. Transformation can occur with input of enough energy (eg, thermal energy imparted by steam sterilization or mechanical energy at the bearing surface). Zirconia was introduced as an alternative to alumina because it has a higher toughness, making it less susceptible to gross fracture (ceramics do not undergo fatigue fracture, but rather fail from a process of slow crack growth). Zirconia is highly biocompatible (as are many ceramics) and is essentially immune to corrosive processes that can plague metallic alloys such as stainless steel. Clarke IC, Manaka M, Green DD, et al: Current status of zirconia used in total hip implants. J Bone Joint Surg Am 2003;85:73-84. Haraguchi K, Sugano N, Nishii T, et al: Phase transformation of a zirconia ceramic head after total hip arthroplasty. J Bone Joint Surg Br 2001;83:996-1000.

Question 2

A 36-year-old skier sustains a grade III posterior cruciate ligament (PCL) tear. Where will increased contact pressures develop over time?

Explanation

Complete rupture of the PCL leads to increased contact pressures in the patellofemoral and medial compartments of the knee. However, whether degenerative arthritis will develop and in which compartments still remains controversial.

Question 3

A 46-year-old patient with cervical myelopathy undergoes a multilevel posterior cervical laminectomy from C3 to C7. The risk of postlaminectomy kyphosis is greatest with removal of which of the following structures?

Explanation

Removal of more than 50% of a facet joint can lead to segmental instability and compromises the overall strength of the joint. Removal of the lamina, interspinous ligament, and ligamentum flavum are standard features of a cervical laminectomy. Most surgeons favor fusion with instrumentation of a laminectomized cervical spine. If the anterior part of the spine is already ankylosed from previous surgery or from degenerative conditions, or a posterior fusion with instrumentation is included, then the risk of kyphosis or instability is reduced.

Question 4

A patient with a below-the-knee amputation is being evaluated for a new prosthesis. He wants to improve his ability to walk on uneven surfaces. What modification to the prosthesis can be made to accommodate this request?

Explanation

Changing from a solid keel to a keel with a sagittal split allows an amputee to navigate uneven terrain more easily. Changing the length of the keel affects the responsiveness of the prosthesis but does not address the surface conditions for ambulation. The SACH is not used as frequently anymore, because overload problems to the nonamputated foot have been observed. Koval K (ed): Orthopaedic Knowledge Update 7. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, pp 31-45.

Question 5

What is the relative amount of type II collagen synthesis in disease-free adult articular cartilage compared to developing teenagers?

Explanation

Adult articular cartilage has less than 5% of the synthesis rate of type II collagen than that seen in developing teenagers. Both synthesis and degradation of type II collagen in normal adult articular cartilage is very low compared to children. In osteoarthrosis, both synthesis and degradation are increased, but the collagen does not properly incorporate into the matrix. Lippiello L, Hall D, Mankin HJ: Collagen synthesis in normal and osteoarthritic human cartilage. J Clin Invest 1977;59:593-600.

Question 6

What gene is expressed the earliest during the differentiation of a chondrocyte during endochondral ossification?

Explanation

Transcription factors regulate the activation or repression of cartilage-specific genes. Sox-9, considered a major regulator of chondrogenesis, regulates several cartilage-specific genes during endochondral ossification, including collagen types II, IV, and XI and aggrecan. 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 7

The vascular supply to the medial meniscus comes primarily from what artery?

Explanation

The vascular supply to the medial and lateral menisci originates predominantly from the medial and lateral genicular arteries. The popliteal artery splits into the superior genicular, which splits into medial and lateral branches supplying the patellar cartilage and the posterior cruciate ligament. The middle genicular artery also supplies the anterior curciate ligament, posterior cruciate ligament, and collateral ligaments. The inferior genicular splits into medial and lateral branches and supplies the menisci and other knee ligaments. Despite propagation of incorrect terminology, there is no superior or lateral genicular artery.

Question 8

A patient with a cobalt-chromium alloy (Co-Cr) femoral stem has a periprosthetic fracture that is to be fixed with a cable-plate device. The surgeon should make sure that the plate, screws, and cable, respectively, are made of

Explanation

Contact between metals in a biologic environment leads to galvanic corrosion. Reduction potentials of Co-Cr and stainless steel produce the worst combination of metals in commonly used implants. Because the fixation implants are not intended to contact the existing implant, it is not as great a consideration as the plate and the screws and cables that will directly contact each other. Miller MD (ed): Review of Orthopaedics, ed 3. Philadelphia, PA, WB Saunders, 2000, pp 119-144.

Question 9

A college football player performs bicep curls as part of his weight lifting routine. During the flexion phase of the curl, what term defines the type of muscle contraction?

Explanation

Muscle contractions can be classified by tension, length, and velocity. Isometric contractions involve changing tension in the muscle while the muscle stays at a constant length. An example would be pushing against a wall. Isokinetic contractions occur when the muscle maximally contracts at a constant velocity. Isotonic contractions involve constant tension throughout the range of motion. Eccentric contraction is when the muscle lengthens during contraction. Eccentric contractions have the highest risk of injury. Plyometrics are eccentric contractions at a rapid rate. Simon SR (ed): Orthopaedic Basic Science. Rosemont, IL, American Academy of Orthopaedic Surgeons, 1994, pp 89-125.

Question 10

Which of the following statements most accurately describes the layers of articular cartilage?

Explanation

Normal articular cartilage is composed of three zones that are based on the shape of the chondrocytes and the distribution of the type II collagen. The tangential zone has flattened chondrocytes, condensed collagen fibers, and sparse proteoglycan. The intermediate zone is the thickest layer with round chondrocytes oriented in perpendicular or vertical columns paralleling the collagen fibers. The basal layer is deepest with round chondrocytes. The tidemark is deep to the basal layer and separates the true articular cartilage from the deeper cartilage that is a remnant of the cartilage anlage, which participated in endochondral ossification during longitudinal growth in childhood. The tidemark divides the superficial uncalcified cartilage from the deeper calcified cartilage and also is the division between nutritional sources for the chondrocytes. The tidemark is the zone in which chondrocyte renewal took place in childhood. The tidemark is found only in joints and not in the cap of an enchondroma. It is seen most prominently in the adult, nongrowing joint.

Question 11

What term best describes the process involved when a growth factor produced by an osteoblast stimulates the differentiation of an adjacent undifferentiated mesenchymal cell during fracture repair?

Explanation

Growth factors are proteins secreted by cells that can act on target cells to produce certain biologic actions. These actions can be described as autocrine, paracrine, and endocrine. Autocrine actions are those in which the growth factor influences an adjacent cell of its origin or identical phenotype. Paracrine actions are those in which the protein influences an adjacent cell that is different in its origin or phenotype. Endocrine actions are those in which the factor influences a cell located at a distant anatomic site. 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.

Question 12

Vertebral fractures are common in the thoracolumbar spine. What is the most important factor that determines the strength of the cancellous bone in the vertebral body?

Explanation

Cancellous bone strength and stiffness are determined primarily by the apparent density (the amount of bone per unit volume). Strength varies approximately as the square of the density, and stiffness as the cube of the density; therefore, these are very strong relationships. Cancellous bone strength also depends on the mineral content, the rate of loading (it is viscoelastic), the anatomic level, and the trabecular number (an histomorphometry term), but all to a markedly lesser extent than density. Carter DR, Hayes WC: The compressive behavior of bone as a two-phase porous structure. J Bone Joint Surg Am 1977;59:954-962.

Question 13

What additional percentage of energy expenditure above baseline is required for ambulation after an above-the-knee amputation?

Explanation

Patients with an above-the-knee amputation have a 65% increase in energy expenditure. A patient with a transtibial amputation requires 25% more energy above baseline values; however, bilateral transtibial amputations are associated with a 40% increase in energy expenditure. Otis JC, Lane JM, Kroll MA: Energy cost during gait in osteosarcoma patients after resection and knee replacement and after above-the-knee amputation. J Bone Joint Surg Am 1985;67:606-611.

Question 14

Bacitracin is a topical antibiotic agent that may be added to solutions and used for intraoperative lavage. What is this agent effective against?

Explanation

Bacitracin is a polypeptide obtained from a strain (Tracy strain) of Bacillus subtilis. It is stable and poorly absorbed from the intestinal tract; its only use is for topical application to skin, wounds, or mucous membranes. Concentrations of 500 to 2,000 units per milliliter of solution or gram of ointment are used for topical application. Bacitracin is mainly bactericidal for gram-positive bacteria, including penicillin-resistant staphylococci. In combination with polymixin B or neomycin, bacitracin is useful for suppression of mixed bacterial flora in surface lesions. Bacitracin is toxic for the kidney, causing proteinuria, hematuria, and nitrogen retention; therefore, it has no place in systemic therapy. Bacitracin is said not to induce hypersensivity readily, but reactions to this agent have been described. Rosenstein BD, Wilson FC, Funderburk CH: The use of bacitracin irrigation to prevent infection in postoperative skeletal wounds: An experimental study. J Bone Joint Surg Am 1989;71:427-430.

Question 15

Ceramic bone substitutes have which of the following properties?

Explanation

Ceramics have the following properties: They are resorbed at varying rates, and the chemical composition of the ceramic significantly affects the rate of resorption. For example, tricalcium phosphate (TCP) undergoes biologic resorption 10 to 20 times faster than hydroxyapatite. The partial conversion of TCP to hydroxyapatite once it is in the body significantly reduces the rate of resorption. Some segments of hydroxyapatite can remain in place in the body for 7 to 10 years. In clinical trials, TCP more readily remodels because of its porosity, but it is weaker. The success of converted corals as a bone graft substitute relies on a complex sequence of events of vascular ingrowth, differentiation of osteoprogenitor cells, bone remodeling, and graft resorption occurring together with host bone ingrowth into and on the porous coralline microstructure or voids left behind during resorption. Lane JM, Bostrom MP: Bone grafting and new composite biosynthetic graft materials. Instr Course Lect 1998;47:525-534. Walsh WR, Chapman-Sheath PJ, Cain S, et al: A resorbable porous ceramic composite bone graft substitute in a rabbit metaphyseal defect model. J Orthop Res 2003;21:655-661.

Question 16

Linazolid exerts its antimicrobial action by inhibiting bacterial

Explanation

Linazolid is the first agent of the oxazolidinone group of antibiotics and is very active against methicillin-sensitive Staphylococus aureus, S epidermidis, and vancomycin-resistant enterococci. The drug has no gram-negative activity. Linazolid inhibits protein synthesis by blocking formation of the 70S ribosomal translation complex. This mechanism of action is unique to the oxazolidinones. Rybak MJ, Cappelletty DM, Moldovan T, et al: Comparative in vitro activities and postantibiotic effects of the oxazolidinone compounds eperezolid (PNU-100592) and linezolid (PNU-100766) versus vancomycin against Staphylococcus aureus, coagulase-negative staphylococci, Enterococcus faecalis, and Enterococcus faecium. Antimicrob Agents Chemother 1998;42:721-724.

Question 17

Which of the following changes of calcium metabolism accompany the loss of bone during menopause?

Explanation

There is a negative change of calcium balance with a decrease in intestinal absorption and an increase in urinary calcium loss. The reduction of intestinal absorption is accompanied by reduced circulating concentrations of total, but not free 1,25-dihydroxyvitamin D. However, estrogen may also directly regulate intestinal calcium resorption independent of vitamin D. Tubular resorption of calcium is higher in the presence of estrogen. Studies of the levels of PTH in the presence of estrogen are controversial. Oh KW, Rhee EJ, Lee WY, et al: The relationship between circulating osteoprotegerin levels and bone mineral metabolism in healthy women. Clin Endocrinol (Oxf) 2004;61:244-249.

Question 18

When compared with fresh-frozen bone allograft, freeze-dried bone allograft (FDBA) is characterized by

Explanation

The compaction of FDBA is faster than that of fresh-frozen bone. The maximal stiffness reached by both materials when tested was the same (55 MPa), but the FDBA required fewer impactions to achieve that stiffness. Because it is easier to impact, the FDBA may be mechanically more efficient than the fresh-frozen bone in surgical conditions. The brittleness of irradiated FDBA, caused by loss of the capacity to absorb energy in a plastic way, increases the compactness and stiffness of morcellized grafts. The failure rate of fusion in adolescent idiopathic scoliosis has been shown to be much higher in FDBA than in either iliac crest bone graft or composite autograft with demineralized bone matrix. There is a greater erosive surface response to allograft when compared to autograft or frozen allograft, with a larger number of osteoclast and osteoblast nuclei seen microscopically. Cornu O, Libouton X, Naets B, et al: Freeze-dried irradiated bone brittleness improves compactness in an impaction bone grafting model. Acta Orthop Scand 2004;75:309-314. Price CT, Connolly JF, Carantzas AC, et al: Comparison of bone grafts for posterior spinal fusion in adolescent idiopathic scoliosis. Spine 2003;28:793-798.

Question 19

A study is designed that examines fractures in children with osteogenesis imperfecta after being treated with bisphosphonates compared with a placebo. A difference is found for which the P value is greater than what is considered to be statistically significant. What is the next appropriate statistical analysis?

Explanation

When a study yields a negative result between treatment groups, the next step is to perform a power analysis. The power, by definition, is the probability of rejecting the null hypothesis: in this example the null hypothesis would be that children treated with bisphosphonates would have fewer fractures than the untreated control population. The power analysis helps answer the question as to whether the null hypothesis should be rejected and the finding is real, or whether the sample size was too small or the effect of treatment too subtle to demonstrate a difference between the treatment and control groups. 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, p 7.

Question 20

Which of the following best describes the relative content of the components of articular cartilage in decreasing order?

Explanation

Water is the most abundant component of articular cartilage with a wet weight of 65% to 80%. Of the water, 80% is at the surface and 65% at the deep zone. Collagen accounts for 10% to 20% of the wet weight, with type II collagen accounting for 90% to 95% of the total collagen content. Small amounts of types V, VI, IX, X, and XI collagen are also present. Proteoglycans comprise 10% to 15% of the wet weight of collagen. The remainder of the wet weight is made up of other collagens, noncollagenous proteins, and chondrocytes.

Question 21

Human tendons are made up primarily of what collagen type (~95%)?

Explanation

Tendons are dense, primarily collagenous tissues that attach muscle to bone. Collagen content of the dry weight is slightly greater than that found in ligaments and is predominantly type I. Type III collagen makes up the remaining ~5% of total collagen content. Kasser JR (ed): Orthopaedic Knowledge Update 5. Rosemont, IL, American Academy of Orthopaedic Surgeons, 1996, pp 10-12. Garrett WE, Speer KP, Kirkendall DT (eds): Principles & Practice of Orthopaedic Sports Medicine. Philadelphia, PA, Lippincott Williams & Wilkins, 2000, pp 21-37.

Question 22

The therapeutic effect of etanercept in the treatment of rheumatoid arthritis is primarily mediated through

Explanation

Etanercept is a fusion protein that combines the ligand-binding domain of the TNF-a receptor to the Fc portion of human immunoglobulin G (IgG). Protein serves as a competitive inhibitor of TNF-a signaling. COX2 is the target of NSAIDs, including newer formulations that are more COX2-specific. The remaining responses are not direct targets of etanercept. Weinblatt ME, Kremer JM, Bankhurst AD, et al: A trial of etanercept, a recombinant tumor necrosis factor receptor: Fc fusion protein, in patients with rheumatoid arthritis receiving methotrexate. N Engl J Med 1999;340:253-259.

Question 23

A 21-year-old woman has a nontraumatic rupture of the Achilles tendon. Which of the following commonly prescribed medications has been associated with this condition?

Explanation

Fluoroquinolones have been associated with increased rates of tendinitis, with special predilection for the Achilles tendon. Tenocytes in the Achilles tendon have exhibited degenerative changes when viewed microscopically after fluoroquinolone administration. Recent clinical studies have shown an increased relative risk of Achilles tendon rupture of 3.7. The other listed drugs have no known increase in tendon rupture rates nor tendinitis. van der Linden PD, van de Lei J, Nab HW, et al: Achilles tendinitis associated with fluoroquinolones. Br J Clin Pharmacol 1999;48:433-437. Bernard-Beaubois K, Hecquet C, Hayem G, et al: In vitro study of cytotoxicity of quinolones on rabbit tenocytes. Cell Biol Toxicol 1998;14:283-292.

Question 24

Bioabsorbable polymers are used in a wide range of orthopaedic devices, including anchors, staples, pins, plates, and screws. What is the primary drawback for bioabsorbable implants?

Explanation

A number of bioabsorbable polymers are used in orthopaedic applications, and all have in common reports of foreign body reactions, which occur in more than 50% of patients in some series. In general, the high cost of these polymers is offset by the elimination of a second surgery to remove the implant. Bioabsorbable polymers are low strength in comparison to metallic alloys but of sufficient strength for many orthopaedic applications. The elastic modulus is not as high as many other orthopaedic biomaterials, making them suitable for applications where lower stiffness is an asset. Ambrose CG, Clanton TO: Bioabsorbable implants: Review of clinical experience in orthopedic surgery. Ann Biomed Eng 2004;32:171-177.

Question 25

What ligament is the primary restraint to applied valgus loading of the knee?

Explanation

The superficial portion of the MCL contributes 57% and 78% of medial stability at 5 degrees and 25 degrees of knee flexion, respectively. The deep MCL and posteromedial capsule act as secondary restraints at full knee extension. The anterior cruciate ligament and PCL also provide secondary resistance to valgus loads.

Question 26

A 65-year-old active male undergoes a total hip arthroplasty using a highly cross-linked polyethylene (HXLPE) liner. While cross-linking polyethylene significantly improves its wear characteristics, the process of irradiation and subsequent thermal treatment alters other mechanical properties. Compared to conventional ultra-high-molecular-weight polyethylene (UHMWPE), which of the following is an expected biomechanical trade-off of using HXLPE?

Explanation

Highly cross-linked polyethylene is manufactured by exposing UHMWPE to gamma or electron beam irradiation, creating free radicals that bond to form cross-links. While this significantly decreases adhesive and abrasive wear, it negatively impacts other mechanical properties. The necessary thermal treatments (melting or annealing) used to extinguish residual free radicals decrease the material's crystallinity. This results in decreased ultimate tensile strength, yield strength, and fracture toughness (fatigue crack propagation resistance).

Question 27

A 72-year-old female with severe osteoporosis is treated with denosumab, a monoclonal antibody that targets RANKL. By blocking RANKL, this medication prevents the activation and maturation of osteoclasts. What natively produced physiological molecule performs an identical biological function by acting as a decoy receptor for RANKL?

Explanation

Osteoprotegerin (OPG) is a glycoprotein naturally secreted by osteoblasts that acts as a soluble decoy receptor for RANKL. By binding to RANKL, OPG prevents RANKL from interacting with the RANK receptor on osteoclast precursors, thereby inhibiting osteoclastogenesis and reducing bone resorption. Denosumab mimics this exact mechanism therapeutically. Sclerostin is an inhibitor of the Wnt pathway. M-CSF promotes osteoclast precursor survival. Cathepsin K is an enzyme secreted by mature osteoclasts to degrade bone matrix.

Question 28

A core biopsy of articular cartilage is analyzed histologically and biochemically. The researcher notes an area characterized by the highest concentration of proteoglycans, the lowest concentration of water, and thick type II collagen fibers oriented perpendicular to the joint surface. Which zone of articular cartilage is being observed?

Explanation

Articular cartilage is divided into four zones. The deep (radial) zone contains the highest concentration of proteoglycans and the lowest water content. Its collagen fibers (primarily type II) are large in diameter and oriented perpendicular to the joint surface to resist compressive forces. The superficial zone has the highest water content, parallel collagen fibers, and lowest proteoglycan content. The calcified zone is separated from the deep zone by the tidemark.

Question 29

Stress shielding and subsequent bone resorption around a femoral stem are related to the difference in the modulus of elasticity between the metallic implant and the host bone. Cortical bone has a modulus of elasticity of approximately 15-20 GPa. Which of the following standard orthopedic implant materials has a modulus of elasticity that most closely approximates that of native cortical bone?

Explanation

The modulus of elasticity (Young's modulus) represents a material's stiffness. Cortical bone has a modulus of roughly 15-20 GPa. Titanium alloy is roughly 100-110 GPa, which is the closest of the commonly used bulk structural metals. Stainless steel has a modulus of roughly 200 GPa, and Cobalt-chromium alloy is approximately 220-240 GPa. Ceramics like alumina and zirconia are much stiffer (>300 GPa). Because titanium is closest to cortical bone, it typically generates less stress shielding than stainless steel or cobalt-chromium.

Question 30

A surgeon utilizes recombinant human bone morphogenetic protein-2 (rhBMP-2) on an absorbable collagen sponge to augment an anterior lumbar interbody fusion. Upon BMP-2 binding to its specific cell surface serine/threonine kinase receptors on mesenchymal stem cells, which of the following intracellular signaling pathways is primarily activated to promote osteoblastic differentiation?

Explanation

Bone morphogenetic proteins (BMPs) belong to the TGF-beta superfamily. When a BMP ligand binds to its transmembrane serine/threonine kinase receptor complex, it phosphorylates receptor-regulated SMADs, specifically SMAD 1, 5, and 8. These phosphorylated SMADs then form a complex with the co-SMAD (SMAD 4) and translocate to the nucleus to regulate the transcription of osteogenic genes like Runx2. The Wnt pathway uses beta-catenin.

Question 31

A 28-year-old male presents with a slowly enlarging, deep soft tissue mass in the posterior thigh. Core needle biopsy reveals a biphasic tumor consisting of both epithelial glandular elements and spindle cell components. Molecular cytogenetic analysis is most likely to reveal which of the following chromosomal translocations?

Explanation

The clinical and histological description (biphasic appearance with epithelial and spindle cells) is classic for synovial sarcoma. Synovial sarcoma is characterized molecularly by the t(X;18)(p11;q11) translocation, which results in the SYT-SSX fusion gene. Options 0 [t(11;22)] is classic for Ewing sarcoma. Option 2 [t(12;16)] represents myxoid liposarcoma. Option 3 [t(9;22)] can be seen in extraskeletal myxoid chondrosarcoma. Option 4 [t(2;13)] is found in alveolar rhabdomyosarcoma.

Question 32

A neonate with a clubfoot deformity undergoes weekly manipulations and applications of a Ponseti cast. The cast holds the foot in a constantly deformed, corrected position. Over the course of the week, the tension felt within the contracted medial soft tissues gradually decreases without any further manipulation. Which biomechanical property of viscoelastic materials does this clinical phenomenon best represent?

Explanation

Stress relaxation is a classic viscoelastic property defined as a decrease in stress (tension or internal force) over time when a material is held under a constant deformation (constant strain). This is exactly what happens when a cast is applied holding a joint in a static position. Creep, in contrast, is the progressive deformation of a material over time when subjected to a constant load (stress). Hysteresis is the loss of energy (usually as heat) between the loading and unloading cycles.

Question 33

A patient who previously underwent total hip arthroplasty with a cobalt-chromium femoral head impacted onto a titanium alloy stem presents with groin pain and an expanding pseudotumor. Retrieval analysis of the explanted components demonstrates significant material degradation, pitting, and black debris exclusively at the modular head-neck taper junction. Which of the following is the primary initiating mode of corrosion responsible for this phenomenon?

Explanation

The failure at the modular head-neck taper in total hip arthroplasty (often termed trunnionosis) is primarily initiated by fretting corrosion. Micro-motion at the taper junction physically disrupts the protective passivation (oxide) layer on the metal surfaces (fretting). Once this layer is breached, local changes in the micro-environment (depletion of oxygen, drop in pH) lead to mechanically assisted crevice corrosion (MACC). While galvanic potential exists between differing metals, fretting is recognized as the necessary inciting mechanical event.

Question 34

According to Perren's strain theory, the phenotype of tissue that differentiates within a fracture gap is strictly dependent on the local interfragmentary strain. In order for primary (direct) bone healing with Haversian remodeling to occur without the intermediate formation of a visible fracture callus, the absolute interfragmentary strain must be kept below what critical threshold?

Explanation

Perren's strain theory states that tissues will not form if the local strain exceeds the maximum strain that tissue can withstand before rupturing. Granulation tissue tolerates up to 100% strain, fibrous tissue up to 17%, and fibrocartilage up to 10%. Bone tissue is highly brittle and can only withstand <2% strain. Therefore, primary (direct) bone healing—which bypasses callus formation—requires rigid fixation that maintains interfragmentary strain below 2%.

Question 35

Tranexamic acid (TXA) is routinely administered either intravenously or topically during total joint arthroplasty to reduce perioperative blood loss and the need for transfusions. Which of the following best describes the specific molecular mechanism of action of this pharmacological agent?

Explanation

Tranexamic acid (TXA) is a synthetic analog of the amino acid lysine. It functions as an antifibrinolytic by reversibly binding to the lysine receptor sites on plasminogen. This action competitively inhibits the activation of plasminogen into the active enzyme plasmin by tissue plasminogen activator (tPA). Because plasmin is responsible for the degradation of fibrin clots, TXA effectively stabilizes the fibrin matrix and prevents early clot breakdown.

Question 36

A surgeon is reviewing bearing surfaces for total hip arthroplasty. To reduce oxidation and improve wear resistance without sacrificing fatigue strength, vitamin E (alpha-tocopherol) is increasingly added to highly cross-linked polyethylene (HXLPE). What is the primary mechanism by which Vitamin E achieves this effect?

Explanation

Vitamin E acts as a powerful antioxidant by scavenging free radicals produced during the gamma irradiation process of HXLPE. This prevents long-term oxidation in vivo, eliminating the need for post-irradiation thermal treatments (like remelting), which are known to decrease the mechanical and fatigue strength of the polyethylene.

Question 37

Bone morphogenetic proteins (BMPs) play a crucial role in osteoinduction during fracture healing. Following the binding of a BMP (such as BMP-2) to its cell-surface serine/threonine kinase receptor, which of the following intracellular signaling molecules is directly phosphorylated to translocate to the nucleus and regulate gene transcription?

Explanation

BMPs bind to serine/threonine kinase receptors on the cell surface, leading to the phosphorylation of receptor-regulated Smads (Smad 1, 5, and 8). These phosphorylated Smads form a complex with the common-partner Smad (Smad 4) and translocate to the nucleus to regulate target gene transcription, promoting osteoblast differentiation (e.g., upregulating Runx2). Beta-catenin is involved in the Wnt signaling pathway, while NF-kappaB is downstream of RANKL.

Question 38

Cortical bone exhibits viscoelastic properties, meaning its biomechanical response is dependent on the rate of loading. When cortical bone is subjected to a rapid rate of loading (high strain rate) compared to a slow rate of loading, how do its mechanical properties change?

Explanation

Bone is a viscoelastic material, meaning its mechanical properties are strain-rate dependent. At higher strain rates (e.g., high-speed motor vehicle trauma), cortical bone becomes stiffer (exhibits an increased modulus of elasticity), sustains higher loads before failing (increased ultimate strength), and absorbs more energy before fracture compared to slower loading rates.

Question 39

A basic science researcher is analyzing a full-thickness sample of human articular cartilage. Compared to the superficial tangential zone, the deep zone of normal articular cartilage is characterized by which of the following?

Explanation

The deep zone (radial zone) of articular cartilage has the highest proteoglycan content, the lowest water content, and thick collagen fibers oriented perpendicular to the joint surface to resist high compressive forces. The superficial (tangential) zone has the highest water content, highest collagen content, flattened chondrocytes, and collagen fibers oriented parallel to the surface to resist shear forces.

Question 40

A 4-year-old boy presents with disproportionate short stature, frontal bossing, and rhizomelic shortening of the limbs. Genetic testing confirms a mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. What is the precise molecular effect of this mutation on the physeal growth plate?

Explanation

Achondroplasia is caused by an autosomal dominant, gain-of-function mutation in the FGFR3 gene. In normal physiology, FGFR3 signaling functions to inhibit chondrocyte proliferation and differentiation at the growth plate. The gain-of-function mutation constitutively activates this pathway, leading to excessive inhibition of chondrocyte proliferation in the proliferative zone of the physis, resulting in characteristic dwarfism.

Question 41

A 65-year-old male undergoes a revision total hip arthroplasty due to an adverse local tissue reaction (ALTR). The primary surgery utilized a titanium alloy stem with a cobalt-chromium modular head. Intraoperatively, significant black tissue debris is noted around the head-neck taper junction. Which of the following best describes the primary mechanism of wear and corrosion at this specific interface?

Explanation

Trunnionosis at the modular head-neck taper of a total hip arthroplasty is primarily caused by mechanically assisted crevice corrosion (MACC). Micromotion (fretting) at the modular junction mechanically disrupts the protective passivating oxide layer of the metals. This disruption, combined with an oxygen-depleted environment within the crevice (which prevents the spontaneous repassivation of the oxide layer), leads to accelerated corrosive dissolution of metal ions and subsequent adverse local tissue reaction.

Question 42

Following an acute Achilles tendon rupture treated nonoperatively, the tendon undergoes a predictable sequence of healing phases. During the remodeling phase (beginning approximately 6 weeks post-injury), which of the following cellular and biochemical changes predominantly occurs?

Explanation

Tendon healing occurs in three overlapping phases: inflammatory, proliferative/repair, and remodeling. During the proliferative phase, Type III collagen is synthesized rapidly to form a provisional, disorganized matrix. In the remodeling phase, cellularity and matrix synthesis decrease, and the weaker Type III collagen is gradually replaced by stronger, highly cross-linked Type I collagen, which aligns along the axis of mechanical stress to maximize tensile strength.

Question 43

A 24-year-old female is undergoing physical therapy for patellofemoral pain syndrome. Her therapist prescribes a rehabilitation protocol utilizing both open and closed kinetic chain exercises. To minimize the patellofemoral joint reaction force (PFJRF) and resultant joint stress, in which ranges of motion are open and closed kinetic chain exercises respectively most safely performed?

Explanation

In open kinetic chain (OKC) exercises (e.g., seated leg extension), the PFJRF and stress are highest in early flexion (0-45 degrees) due to decreasing patellofemoral contact area and an increasing lever arm of the lower leg; therefore, OKC is safest performed in deeper flexion (90 to 45 degrees). Conversely, in closed kinetic chain (CKC) exercises (e.g., squats), the PFJRF and stress are highest in deep flexion due to the significantly increased quadriceps force required to resist the body weight lever arm; thus, CKC is safest in early flexion (0-45 degrees).

Question 44

The addition of antibiotics to polymethylmethacrylate (PMMA) bone cement is routinely used for the treatment of periprosthetic joint infection. To optimize the elution profile and mechanical properties of an antibiotic-loaded PMMA spacer, which of the following principles must be adhered to?

Explanation

PMMA polymerization is highly exothermic, reaching core temperatures that can easily degrade heat-sensitive medications. Therefore, antibiotics added to PMMA must be thermally stable (e.g., vancomycin, tobramycin, gentamicin). Liquid antibiotics severely degrade the mechanical properties of PMMA. Doses exceeding 10-15% significantly compromise the structural integrity of the cement. Furthermore, hand mixing (rather than vacuum mixing) is preferred for spacer creation because it increases porosity, which beneficially increases the surface area for antibiotic elution.

Question 45

A 55-year-old female with chronic kidney disease presents with secondary hyperparathyroidism. The persistently elevated levels of parathyroid hormone (PTH) have a profound effect on bone remodeling. Which of the following accurately describes the primary cellular mechanism by which PTH stimulates bone resorption?

Explanation

Osteoclasts do not possess receptors for Parathyroid Hormone (PTH). Instead, PTH binds to specific receptors on osteoblasts, stimulating them to upregulate the expression of Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL) and downregulate the secretion of Osteoprotegerin (OPG), a decoy receptor. The increased RANKL binds to RANK on osteoclast precursors, driving their differentiation and activation into mature, active bone-resorbing osteoclasts.

Question 46

A 65-year-old male undergoes revision total hip arthroplasty due to an adverse local tissue reaction. Intraoperatively, significant black debris and tissue necrosis are noted localized strictly around the modular head-neck taper junction, while the articular bearing surfaces show minimal wear. What is the primary mechanism of corrosion initiating at this specific modular interface?

Explanation

Modular tapers in total joint arthroplasty are primarily susceptible to mechanically assisted crevice corrosion (MACC). This process is initiated by fretting (micromotion) which disrupts the protective passive titanium oxide layer. Once the oxide layer is breached, crevice corrosion ensues in the oxygen-depleted, acidic environment of the taper junction, leading to the release of metal ions and subsequent adverse local tissue reactions (ALTR).

Question 47

A 72-year-old female with severe osteoporosis sustains a distal radius fracture. She is started on daily subcutaneous teriparatide to improve bone mass and potentially assist with fracture healing. Through which primary cellular mechanism does intermittent administration of this medication promote an anabolic effect on bone?

Explanation

Teriparatide is a recombinant human parathyroid hormone (PTH 1-34) analog. While continuous PTH exposure is catabolic, intermittent PTH administration is highly anabolic. It exerts its anabolic effect by increasing osteoblast lifespan (preventing apoptosis), stimulating osteoblast differentiation, and activating the Wnt/beta-catenin signaling pathway. A key part of this mechanism is the downregulation of sclerostin (a Wnt inhibitor encoded by the SOST gene) expressed by osteocytes.

Question 48

A researcher is analyzing an articular cartilage biopsy from a 50-year-old patient with early osteoarthritis. When evaluating the structural and biochemical properties of the normal superficial (tangential) zone of articular cartilage, which of the following characteristic findings should be expected?

Explanation

The superficial (tangential) zone of articular cartilage comprises 10-20% of the articular cartilage thickness. It is characterized by flattened chondrocytes, a high density of collagen fibers (predominantly Type II and IX) oriented parallel to the articular surface to resist shear forces, the highest water content, and the lowest proteoglycan concentration. The deep zone features vertically oriented collagen and highest proteoglycan content.

Question 49

During internal fixation of a severely osteoporotic proximal humerus fracture, a surgeon wishes to optimize the biomechanics of the construct to prevent screw pullout. According to the mathematical principles of screw mechanics, which of the following modifications is most proportional to an increase in screw pullout strength?

Explanation

Screw pullout strength is directly proportional to the major (outer) diameter of the screw, the length of thread engagement, and the shear strength of the host bone. While decreasing thread pitch increases the number of threads engaged, its effect is less proportionally significant than increasing the major diameter. The minor (inner root) diameter determines the tensile and bending strength of the screw itself, not its pullout resistance.

Question 50

A physical therapist designs a rehabilitation protocol for a patient recovering from a hamstring injury, emphasizing exercises that generate the absolute maximum muscle force to stimulate tendon remodeling. According to muscle physiology and the force-velocity relationship, which type of muscle contraction generates the highest maximal tension?

Explanation

The force-velocity curve of muscle physiology demonstrates that eccentric contractions (lengthening under tension) generate significantly more force than isometric and concentric contractions. Within eccentric contractions, higher velocity (fast eccentric) yields higher tension/force up to a plateau, whereas in concentric contractions, slower velocity generates more force. Therefore, fast eccentric contractions are capable of producing the greatest absolute muscle force.

Question 51

A 4-year-old boy presents with rhizomelic short stature, frontal bossing, and midface hypoplasia. Radiographs show narrowing of the interpedicular distances in the lumbar spine. The genetic mutation responsible for this condition primarily disrupts endochondral ossification in which of the following zones of the physis?

Explanation

Achondroplasia is an autosomal dominant skeletal dysplasia caused by a gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. FGFR3 normally acts as a negative regulator of bone growth. The constitutively active mutated receptor inappropriately inhibits chondrocyte proliferation, thereby profoundly affecting the proliferative zone of the physis and resulting in impaired endochondral ossification and rhizomelic dwarfism.

Question 52

A 28-year-old male sustains a midshaft humerus fracture resulting in an acute radial nerve palsy. At 4 weeks, EMG confirms denervation. During the ongoing process of Wallerian degeneration and subsequent peripheral nerve regeneration, which cell type undergoes dedifferentiation, forms the bands of Büngner, and upregulates the production of neurotrophic factors (e.g., NGF) to guide the regenerating axon?

Explanation

Following a peripheral nerve injury, Schwann cells in the distal nerve stump undergo a massive phenotypic modulation. They dedifferentiate, assist in initial myelin clearance, proliferate, and align to form longitudinal conduits known as the bands of Büngner. Crucially, they upregulate the synthesis of neurotrophic factors (such as Nerve Growth Factor, NGF) and cell adhesion molecules that physically and chemically guide the regenerating axonal sprouts.

Question 53

A 68-year-old female is scheduled for a bilateral total knee arthroplasty. To minimize perioperative blood loss, the surgeon administers intravenous tranexamic acid (TXA). What is the specific molecular mechanism of action of tranexamic acid in this setting?

Explanation

Tranexamic acid (TXA) is a synthetic analog of the amino acid lysine. It acts as a powerful antifibrinolytic agent by reversibly binding to the lysine receptor sites on plasminogen. This competitive binding prevents plasminogen from adhering to fibrin. Consequently, it inhibits the activation of plasminogen to plasmin by tissue plasminogen activator (tPA), effectively stabilizing the fibrin clot and preventing premature fibrinolysis.

Question 54

A researcher is performing biomechanical testing on a novel synthetic graft for anterior cruciate ligament (ACL) reconstruction. The graft is clamped in a testing machine, instantly elongated to a fixed length, and held constant over time. The researcher observes that the force required to maintain this exact length progressively decreases. Which viscoelastic property is being demonstrated?

Explanation

Viscoelastic materials exhibit time-dependent mechanical behaviors. 'Stress relaxation' is defined as the steady decrease in stress (or force) over time when the material is deformed and held at a constant strain (or length). Conversely, 'creep' is the progressive deformation (increase in length/strain) over time when a material is subjected to a constant load (or force). 'Hysteresis' refers to the energy lost as heat during the loading and unloading cycle.

Question 55

A 35-year-old female presents with a destructive, eccentric, lytic lesion in the distal femur extending to the subchondral bone. Biopsy confirms a Giant Cell Tumor of Bone (GCTB). Due to the size and joint proximity, neoadjuvant denosumab is prescribed. Denosumab exerts its therapeutic effect in this condition by targeting which molecule, and which cell population primarily expresses this target?

Explanation

Denosumab is a fully human monoclonal antibody that binds to and inhibits Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). In Giant Cell Tumor of Bone (GCTB), the neoplastic cells are actually the spindle-shaped mononuclear stromal cells. These stromal cells pathologically overexpress RANKL, which aggressively recruits and stimulates the formation of the reactive, non-neoplastic multinucleated giant cells (osteoclasts) responsible for the massive bone destruction. Thus, Denosumab targets RANKL secreted by the neoplastic mononuclear stromal cells.

Question 56

A 45-year-old man undergoes a posterolateral lumbar fusion using recombinant human bone morphogenetic protein-2 (rhBMP-2). Upon binding to its specific cell surface receptor, which of the following intracellular signaling molecules is directly phosphorylated to propagate the osteoinductive signal?

Explanation

Bone morphogenetic proteins (BMPs) are members of the TGF-beta superfamily. When a BMP binds to its specific heterodimeric serine/threonine kinase receptor, it causes the intracellular phosphorylation of receptor-regulated SMADs (typically SMAD 1, 5, and 8). These phosphorylated SMADs then form a complex with a Co-SMAD (SMAD 4) and translocate to the nucleus to regulate the transcription of osteogenic genes, such as Runx2. Beta-catenin is involved in the Wnt signaling pathway. The JAK/STAT pathway is typically utilized by cytokines, and NF-kappaB is central to the RANKL pathway in osteoclasts.

Question 57

A 62-year-old woman presents with groin pain three years after a total hip arthroplasty using a cobalt-chromium modular head on a titanium alloy stem. Joint aspiration is negative for infection, but MRI demonstrates a solid periprosthetic pseudotumor. Revision surgery reveals black debris at the head-neck junction. Which of the following mechanisms is primarily responsible for the generation of this specific debris?

Explanation

The scenario describes trunnionosis, which manifests as an adverse local tissue reaction (ALTR) or pseudotumor due to metal debris at the modular head-neck junction. This is primarily caused by mechanically assisted crevice corrosion (MACC), also known as fretting corrosion. Micro-motion at the junction mechanically disrupts the protective passive oxide layer on the metals (fretting), which exposes the underlying reactive metal to the physiologic fluid environment. This accelerates electrochemical dissolution (corrosion). While galvanic corrosion plays a role due to dissimilar metals, the primary driver at a modular THA junction is the mechanical disruption coupled with crevice corrosion.

Question 58

A 6-month-old infant is evaluated for short-limbed dwarfism, frontal bossing, and midface hypoplasia. Genetic testing confirms a mutation in the FGFR3 gene. Which of the following best describes the pathophysiologic consequence of this specific genetic alteration?

Explanation

Achondroplasia is the most common form of short-limbed dwarfism and is caused by an autosomal dominant mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. Normally, FGFR3 functions as a negative regulator of linear bone growth. The mutation (most commonly a G380R substitution) causes a 'gain of function', meaning the receptor is constitutively active even without its ligand. This leads to profound, continuous inhibition of chondrocyte proliferation and hypertrophy in the proliferative zone of the physis, ultimately resulting in stunted endochondral ossification.

Question 59

A researcher is studying the biomechanical properties of human articular cartilage in an osteoarthritic knee model. During normal joint kinematics, which zone of articular cartilage is primarily responsible for resisting shear stresses, and what is the predominant orientation of its collagen fibers?

Explanation

Articular cartilage is divided into distinct structural zones. The superficial (tangential) zone makes up the top 10-20% of articular cartilage and has a high concentration of water and collagen (predominantly Type II). The collagen fibers in this zone are densely packed and oriented strictly parallel to the articular surface. This orientation provides the highest tensile strength and is primarily responsible for resisting the severe shear stresses generated during joint motion. In contrast, the deep zone features collagen fibers oriented perpendicularly to resist compressive loads.

Question 60

A 72-year-old man requires revision of a cemented total knee arthroplasty due to extensive aseptic loosening and osteolysis. Histologic analysis of the periprosthetic tissue demonstrates a dense macrophage infiltrate. Which of the following ultra-high-molecular-weight polyethylene (UHMWPE) particle sizes is most highly reactive and responsible for maximizing macrophage activation and subsequent osteolysis?

Explanation

Polyethylene wear particles are the primary initiators of aseptic loosening in joint arthroplasty. Local macrophages phagocytose these particles and subsequently release pro-inflammatory cytokines (such as TNF-alpha, IL-1, and IL-6) that stimulate osteoclastogenesis via the RANK/RANKL pathway. The most biologically active particles that maximize this macrophage activation are in the submicron range, specifically between 0.1 and 1.0 micrometers. Particles larger than 10 micrometers are generally too large for individual macrophages to ingest and typically elicit a foreign-body giant cell response instead.

Question 61

Tranexamic acid (TXA) is routinely utilized in major orthopedic procedures to reduce perioperative blood loss. Which of the following best describes its mechanism of action at the molecular level?

Explanation

Tranexamic acid (TXA) is a synthetic analog of the amino acid lysine. It acts as a potent antifibrinolytic agent by reversibly and competitively binding to the lysine-binding sites on plasminogen molecules. This action prevents plasminogen from binding to the surface of fibrin, thereby inhibiting its subsequent activation into plasmin. Without active plasmin, the degradation (fibrinolysis) of formed blood clots is halted, preserving the integrity of the clot and reducing surgical bleeding.

Question 62

A multicenter randomized controlled trial is designed to evaluate a novel intramedullary nail versus standard plating for distal tibia fractures. The investigators determine that a sample size of 150 patients is required to achieve 80% power to detect a clinically significant difference in union rates at a significance level of 0.05. Which of the following modifications to the study design would strictly increase the statistical power of this trial?

Explanation

Statistical power (1 - Beta) represents the probability of correctly rejecting the null hypothesis when it is indeed false (i.e., successfully detecting a true difference between groups). Power increases when you increase the sample size, increase the true effect size (the clinical difference between groups), decrease the variance of the data, or increase the significance level (alpha, e.g., moving from 0.05 to 0.10). Decreasing alpha (e.g., to 0.01) makes it harder to reject the null hypothesis, thereby decreasing power. Increasing the sample size gives more precision to the estimates, directly increasing statistical power.

Question 63

A 28-year-old athlete undergoes an Achilles tendon repair. During the remodeling phase of tendon healing, the biomechanical strength of the tendon gradually increases. This process is primarily driven by the replacement of the initial scar tissue matrix with a more organized collagen structure. Which of the following best describes the predominant collagen transition during this phase?

Explanation

Tendon healing proceeds through three overlapping phases: inflammatory, proliferative (repair), and remodeling. During the early proliferative phase, fibroblasts primarily synthesize Type III collagen, which is disorganized, thinner, and mechanically weak. During the remodeling phase (which can last for months to years), this provisional Type III collagen matrix is gradually degraded and replaced by mature Type I collagen. The newly synthesized Type I collagen fibers orient longitudinally parallel to the axis of tension, which dramatically increases the tensile strength of the healed tendon.

Question 64

A resident is inserting a cortical screw for plate fixation of a transverse radius fracture. To mathematically maximize the pull-out strength of the screw in the diaphyseal bone, which of the following alterations to the screw design would be most effective?

Explanation

The pull-out strength of a bone screw is a critical factor in fracture fixation. It is directly proportional to the outer (thread) diameter of the screw, the length of thread engagement (cortical thickness), and the shear strength of the bone. It is inversely proportional to the thread pitch (the distance between adjacent threads). Decreasing the thread pitch means the threads are closer together, resulting in more threads engaging a given length of bone, which increases the total volume of bone caught between the threads and maximizes pull-out strength. Increasing the core diameter while keeping the outer diameter constant actually decreases thread depth, thereby reducing pull-out strength.

Question 65

A 34-year-old man sustains a mid-shaft humerus fracture resulting in a radial nerve palsy. Electromyography (EMG) performed 3 weeks later shows fibrillation potentials in the brachioradialis. Which of the following processes predominantly characterizes the Wallerian degeneration occurring distal to the injury site?

Explanation

Wallerian degeneration is the process of anterograde breakdown of the axon and myelin sheath distal to a nerve injury (axonotmesis or neurotmesis). Following the injury, local macrophages and Schwann cells phagocytose the axonal and myelin debris (neutrophils are not the primary phagocytes here). Critically, Schwann cells then proliferate within the remaining endoneurial tubes and upregulate neurotrophic factors (such as NGF and BDNF) and surface adhesion molecules. These cells align to form the Bands of Büngner, which provide a supportive microenvironment and physically guide the regenerating axonal sprouts from the proximal stump toward the target organ.

Question 66

A 65-year-old male presents with severe groin pain 5 years after undergoing a total hip arthroplasty with a cobalt-chromium (CoCr) head on a titanium (Ti) alloy stem. Joint aspiration is negative for infection, but a metal artifact reduction sequence (MARS) MRI reveals a large cystic pseudotumor around the hip. What is the primary mechanism of corrosion occurring at the head-neck junction?

Explanation

Trunnionosis in total hip arthroplasty with mixed metal components (e.g., CoCr head on Ti stem) is primarily driven by mechanically assisted crevice corrosion (MACC). Micromotion (fretting) at the modular head-neck junction disrupts the passive protective oxide layer of the metals. This allows crevice corrosion to occur in the secluded, low-oxygen space of the Morse taper, leading to the release of metal ions and subsequent adverse local tissue reactions (ALTR).

Question 67

A 70-year-old woman with severe osteoporosis and a recent vertebral compression fracture is started on romosozumab therapy. What is the primary mechanism of action of this medication at the cellular and molecular level?

Explanation

Romosozumab is a monoclonal antibody that targets and binds to sclerostin, a glycoprotein secreted by osteocytes. Sclerostin normally acts as an endogenous inhibitor of the Wnt/beta-catenin signaling pathway, which is crucial for osteoblast differentiation and bone formation. By inhibiting sclerostin, romosozumab enhances Wnt signaling, resulting in a dual effect: markedly increasing bone formation and moderately decreasing bone resorption.

Question 68

An orthopedic surgeon is evaluating different titanium fracture plates for the internal fixation of a comminuted femoral shaft fracture. Bending stiffness is a critical factor in preventing hardware failure. If the surgeon chooses a plate that has double the thickness (height) of a standard plate, how does this specifically alter the area moment of inertia for bending?

Explanation

The bending stiffness of a rectangular plate is directly proportional to its area moment of inertia (I). The formula for the area moment of inertia of a rectangular cross-section is I = (b × h^3) / 12, where 'b' is the base (width) and 'h' is the height (thickness). Doubling the thickness (height) increases the area moment of inertia by 2 cubed (2^3), which equals 8. Therefore, the bending stiffness increases by a factor of 8.

Question 69

A basic science researcher is studying the tribology of healthy articular cartilage during the stance phase of the gait cycle. Under these conditions of high physiological load and extremely low sliding speeds, the fluid film is largely depleted. Which lubrication regime primarily protects the articular surfaces from wear under these specific conditions?

Explanation

Joint lubrication relies on multiple mechanisms depending on loads and sliding speeds. Under high-load and low-speed conditions (such as the stance phase of gait or standing), fluid films break down, and Boundary Lubrication predominates. This is mediated by surface-active molecules, primarily lubricin (PRG4) and hyaluronic acid, which are bound to the superficial zone of the cartilage, preventing direct cartilage-to-cartilage contact and wear.

Question 70

A 55-year-old male develops a chronic periprosthetic joint infection of the knee caused by Staphylococcus epidermidis. The persistence of this organism on the implant surface is primarily facilitated by a highly adherent biofilm. Which of the following is the key component of the extracellular polymeric substance (EPS) that affords structural integrity and antibiotic resistance in S. epidermidis biofilms?

Explanation

In Staphylococcus epidermidis, biofilm formation and structural integrity are heavily dependent on Polysaccharide Intercellular Adhesin (PIA), which is synthesized by enzymes encoded in the icaADBC operon. PIA forms the main volume of the extracellular polymeric substance (EPS) matrix, shielding the bacterial colony from host immune responses and systemic antibiotics, necessitating surgical removal of the hardware.

Question 71

A 24-year-old elite sprinter sustains an acute Achilles tendon rupture and undergoes open surgical repair. During the remodeling phase of tendon healing, which of the following biochemical and structural changes predominantly occurs to maximize the tensile strength of the repair?

Explanation

Tendon healing proceeds through inflammatory, proliferative, and remodeling phases. During the proliferative phase, fibroblasts rapidly produce disorganized Type III collagen, which is mechanically weak. In the remodeling phase (starting around 6 weeks and lasting for months), this Type III collagen is gradually degraded and replaced by stronger, highly organized Type I collagen parallel to the axis of tension. Concurrently, covalent cross-linking between collagen molecules increases, significantly enhancing the tendon's tensile strength.

Question 72

A 4-year-old child presents with short stature, frontal bossing, and rhizomelic shortening of the upper and lower extremities. Radiographs demonstrate narrowing of the interpedicular distances in the lumbar spine. The pathogenesis of this child's condition is primarily driven by which of the following genetic mechanisms?

Explanation

The clinical and radiographic presentation is classic for achondroplasia, the most common form of short-limb dwarfism. It is caused by a gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. FGFR3 normally acts as a negative regulator of bone growth. The mutation causes constitutive activation of the receptor, which severely inhibits chondrocyte proliferation in the proliferative zone of the physis, leading to diminished enchondral ossification.

Question 73

A 30-year-old weightlifter feels a sharp 'pop' in his distal arm during a heavy preacher curl exercise. According to the sliding filament theory and the length-tension relationship of skeletal muscle, at what sarcomere length is maximum active isometric force generated?

Explanation

The length-tension curve of skeletal muscle demonstrates that maximal active tension is generated at an intermediate (resting) sarcomere length (approx. 2.0-2.2 micrometers). At this length, there is optimal overlap between actin and myosin filaments, allowing the maximum number of cross-bridges to form. Extreme stretch eliminates filament overlap, and extreme shortening causes physical interference between filaments and the Z-discs, both of which reduce active tension.

Question 74

A 45-year-old male sustains a midshaft tibial fracture and is treated with a dynamically locked intramedullary nail. The fracture gap is 2 mm. According to Perren's strain theory, what is the maximum interfragmentary strain that will still permit the direct formation of lamellar bone across the fracture gap?

Explanation

Perren's strain theory dictates that a specific tissue will only form in a fracture gap if the tissue can tolerate the strain (deformation) present. Lamellar bone is brittle and can only form under very low strain conditions (less than 2%). Woven bone can tolerate up to 10% strain, cartilage can tolerate between 10% and 30% strain, and granulation tissue can tolerate up to 100% strain. Absolute stability (strain < 2%) is required for primary (lamellar) bone healing.

Question 75

A 28-year-old patient sustains a closed midshaft humerus fracture resulting in an immediate radial nerve palsy. EMGs performed at 4 weeks show fibrillation potentials but no motor unit action potentials. Which of the following describes the correct order of the connective tissue layers in a peripheral nerve, from innermost (surrounding individual axons) to outermost (surrounding the entire nerve)?

Explanation

The connective tissue layers of a peripheral nerve, from deep to superficial, are: the Endoneurium (surrounds individual axons and their Schwann cells), the Perineurium (surrounds groups of axons to form fascicles, providing primary mechanical strength and maintaining the blood-nerve barrier), and the Epineurium (the outermost layer surrounding the entire nerve and the interfascicular spaces).

Question 76

A 65-year-old woman with severe osteoporosis is treated with romosozumab to reduce her risk of fragility fractures. Which of the following best describes the molecular mechanism of action of this medication?

Explanation

Romosozumab is a monoclonal antibody that binds to and inhibits sclerostin. Sclerostin is a glycoprotein produced primarily by osteocytes that naturally inhibits the Wnt/beta-catenin signaling pathway. By blocking sclerostin, romosozumab disinhibits the Wnt pathway, leading to increased osteoblast differentiation and bone formation, while also secondarily decreasing bone resorption.

Question 77

During an anterior cruciate ligament reconstruction, the surgeon tensions the soft tissue graft and secures it at a constant length for several minutes before final fixation. The surgeon notices that the force required to hold the graft at this specific length gradually decreases over time. Which biomechanical property does this phenomenon represent?

Explanation

Stress relaxation is the viscoelastic property defined as a time-dependent decrease in stress (or force) when a material is held at a constant strain (or length). Creep, conversely, is the gradual increase in strain (deformation) when a material is held under a constant stress (load). Hysteresis refers to the energy lost as heat during a loading and unloading cycle.

Question 78

A basic science researcher is studying a full-thickness osteochondral defect model in an animal. Histologic evaluation of normal articular cartilage demonstrates the tidemark. Which of the following best describes the predominant collagen orientation and primary biomechanical function of the zone located immediately superficial to the tidemark?

Explanation

The zone immediately superficial to the tidemark is the deep zone of articular cartilage. In the deep zone, collagen fibers are oriented perpendicular to the joint surface and anchor the cartilage to the underlying calcified zone and subchondral bone. The primary biomechanical role of this zone and its structural arrangement is to resist high compressive loads.

Question 79

A 55-year-old male undergoes a total hip arthroplasty using highly cross-linked polyethylene (HXLPE). During manufacturing, the polyethylene is subjected to gamma irradiation followed by a remelting process. What is the primary purpose of the remelting step?

Explanation

Irradiation is used to create highly cross-linked polyethylene to significantly improve wear resistance. However, irradiation cleaves polymer chains, generating free radicals. If left untreated, these free radicals react with oxygen in vivo, leading to oxidation, embrittlement, and catastrophic failure. Remelting the plastic above its melting temperature extinguishes these free radicals, preventing oxidation, though it slightly decreases the ultimate yield and fatigue strength.

Question 80

A 28-year-old patient sustains a closed midshaft humerus fracture and presents with a complete radial nerve palsy. Electromyography (EMG) performed at 4 weeks shows fibrillation potentials and positive sharp waves in the brachioradialis, but no motor unit action potentials. What pathophysiologic process best accounts for these findings distal to the injury site?

Explanation

The presence of fibrillation potentials and positive sharp waves on EMG 3 to 4 weeks after injury indicates muscle denervation, signifying a loss of axonal continuity (axonotmesis or neurotmesis) rather than a simple conduction block (neuropraxia). Severed axons undergo Wallerian degeneration distal to the site of injury. Macrophages and Schwann cells clear the myelin and axonal debris, preparing the endoneurial tubes for regenerating axons.

Question 81

A surgeon secures a titanium alloy plate with a stainless steel screw to stabilize a complex periarticular fracture. Over the next year, the patient develops localized pain and swelling. Radiographs show lucency around the screw without signs of infection. Which of the following electrochemical processes is most likely responsible for this complication?

Explanation

Galvanic corrosion occurs when two dissimilar metals (such as titanium and stainless steel) are placed in direct physical contact within an electrolytic environment (like body fluids). The metal with the lower electrochemical potential (anodic metal) undergoes accelerated corrosion to protect the more noble (cathodic) metal. Mixing implant metals should generally be avoided to prevent this complication.

Question 82

According to the Blix length-tension curve for skeletal muscle biomechanics, maximum active isometric tension is generated at an optimal sarcomere resting length. This peak in force production is directly attributed to which of the following?

Explanation

The length-tension relationship dictates that maximum active tension in a muscle fiber is produced at the resting length where there is optimal spatial overlap between the actin and myosin filaments. This allows for the maximum number of cross-bridge formations. If the sarcomere is overly shortened, actin filaments overlap and interfere with each other; if excessively stretched, overlap decreases, reducing cross-bridge potential.

Question 83

A 4-year-old child presents with frontal bossing, midface hypoplasia, and rhizomelic shortening of the upper and lower extremities. Genetic testing reveals an activating mutation in the FGFR3 gene. The pathogenesis of this child's condition is primarily localized to which region of the physis?

Explanation

The child has achondroplasia, the most common form of short-limbed dwarfism, caused by an autosomal dominant, gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene. Normally, FGFR3 negatively regulates chondrocyte proliferation. The activating mutation leads to profound inhibition of chondrocyte proliferation, thereby localizing the primary defect to the proliferative zone of the growth plate.

Question 84

During normal human gait, the center of mass undergoes specific kinematic displacements. Evaluation of the vertical ground reaction force (vGRF) curve reveals a 'double-hump' pattern. Which phases of the gait cycle correspond to the highest magnitudes of vertical ground reaction force?

Explanation

During normal walking, the vertical ground reaction force (vGRF) exhibits a characteristic two-peak (double-hump) pattern. The first peak occurs during the loading response (weight acceptance) as the limb absorbs the downward acceleration of the body's mass. The second peak occurs during terminal stance as the plantarflexors forcefully push off, accelerating the body upward and forward. Mid-stance is represented by the trough between these two peaks.

Question 85

To reduce perioperative blood loss in a patient undergoing bilateral total knee arthroplasty, the surgical team administers intravenous tranexamic acid (TXA). What is the specific molecular mechanism of action of this pharmacologic agent?

Explanation

Tranexamic acid (TXA) is a synthetic derivative of the amino acid lysine. It acts as an antifibrinolytic agent by reversibly and competitively binding to the lysine-binding sites on plasminogen. This interaction prevents plasminogen from binding to fibrin and being activated into plasmin, thereby inhibiting fibrinolysis and stabilizing the existing blood clot.

Question 86

A 25-year-old athlete undergoes an anterior cruciate ligament (ACL) reconstruction using a bone-patellar tendon-bone autograft. During graft preparation, the surgeon applies a constant load of 20 Newtons to the graft for 10 minutes on the back table, observing that the graft slowly elongates over time before its length stabilizes. Which of the following viscoelastic properties does this phenomenon describe?

Explanation

Creep is a classic viscoelastic property defined as the time-dependent increase in strain (deformation) of a material when subjected to a constant stress (load). In ligament reconstruction, pre-tensioning the graft relies on the principle of creep to reduce post-implantation elongation. Stress relaxation, conversely, is the time-dependent decrease in stress under a constant strain. Hysteresis refers to the energy lost (usually as heat) during the loading and unloading cycle of a viscoelastic material.

Question 87

A 68-year-old woman with a history of osteoporosis sustains a distal radius fracture. She has been on denosumab for 2 years prior to the injury. At the cellular and molecular level, how does this specific pharmacological agent primarily inhibit bone resorption?

Explanation

Denosumab is a human monoclonal antibody that directly targets and binds to the Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). By binding to RANKL, it prevents RANKL from interacting with the RANK receptor on the surface of osteoclast precursors and mature osteoclasts. This effectively halts osteoclastogenesis, decreases mature osteoclast function, and reduces survival. Bisphosphonates act by inhibiting farnesyl pyrophosphate synthase (nitrogenous) or inducing apoptosis. Osteoprotegerin (OPG) is the body's natural decoy receptor for RANKL; denosumab mimics the function of OPG but does not act on the RANK receptor itself.

Question 88

In the design and selection of femoral stems for total hip arthroplasty, varying metal alloys are utilized based on their distinct mechanical properties. Compared to Cobalt-Chromium (CoCr) alloys, Titanium (Ti-6Al-4V) alloys exhibit which of the following characteristics?

Explanation

Titanium alloys (e.g., Ti-6Al-4V) have a lower modulus of elasticity compared to Cobalt-Chromium (CoCr) and stainless steel alloys. This lower modulus closer approximates the stiffness of cortical bone, which is highly beneficial for reducing stress shielding around femoral implants. However, titanium is highly notch sensitive (scratches act as significant stress risers) and has poor resistance to abrasive wear, making it inappropriate for use as a bearing surface (e.g., femoral heads).

Question 89

Articular cartilage relies on multiple distinct lubrication mechanisms depending on the load applied and the velocity of the joint articulation. During prolonged standing, which represents a high-load and zero-velocity state, which of the following lubrication mechanisms primarily protects the articular surfaces?

Explanation

Boundary lubrication is the primary mechanism protecting articular cartilage under extreme conditions of high loads and low or zero velocities (e.g., prolonged standing). It depends entirely on a monolayer of protective lubricant molecules, primarily lubricin (PRG4) and surface-active phospholipids, physically bound to the articular surface to prevent direct surface-to-surface adhesion. Fluid-film mechanisms (elastohydrodynamic, hydrodynamic, and squeeze-film) require joint motion or dynamic loading to maintain a wedge or film of fluid between the surfaces.

Question 90

Highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) is the standard bearing surface in modern total hip arthroplasty due to its superior wear characteristics. Which of the following mechanical properties is significantly decreased as a direct consequence of increasing the cross-linking of UHMWPE?

Explanation

While highly cross-linking UHMWPE dramatically improves its resistance to adhesive and abrasive wear (thereby reducing osteolysis), it negatively impacts several bulk mechanical properties. The most significant decreases are seen in ductility, ultimate tensile strength, fracture toughness, and fatigue crack propagation resistance. This reduction in fatigue strength makes highly cross-linked polyethylene more susceptible to mechanical failure, such as rim fracture in thin acetabular liners or post breakage in posterior-stabilized knee designs.

Question 91

According to Perren's strain theory regarding the biomechanics of fracture fixation, what is the maximum mechanical strain tolerated by lamellar bone tissue before it will fail to form across a fracture gap?

Explanation

Perren's strain theory dictates that the type of tissue that forms within a fracture gap is strictly determined by the local mechanical strain it can withstand without rupturing. Lamellar bone can only tolerate up to 2% strain. Therefore, absolute stability (strain < 2%) is required for primary bone healing. Woven bone tolerates up to 10% strain, cartilage up to 30%, and granulation tissue up to 100% strain.

Question 92

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?

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 93

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?

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 94

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?

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 95

A 32-year-old patient sustains a complete peripheral nerve transection (Sunderland Grade V) of the ulnar nerve at the wrist, which undergoes primary epineurial repair. During the subsequent regenerative phase following Wallerian degeneration, what is the primary role of the bands of Büngner?

Explanation

Following a nerve transection, the distal stump undergoes Wallerian degeneration where axons and myelin are cleared by macrophages. Subsequently, Schwann cells proliferate rapidly and align longitudinally within the preserved endoneurial basal lamina tubes. These organized columns of Schwann cells are known as the bands of Büngner. They act as the vital physical and chemical pathways, expressing neurotrophic factors and adhesion molecules, to attract and guide the regenerating axonal growth cones toward their distal target organs.

Question 96

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?

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 97

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?

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 98

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?

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 99

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?

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 100

Following an arthroscopic rotator cuff repair, healing of the tendon to the greater tuberosity footprint progresses through several specific histologic zones to minimize stress concentrations at the interface. Which of the following histologic zones directly connects the unmineralized fibrocartilage to the underlying subchondral bone?

Explanation

The direct insertion of a tendon to bone (enthesis), such as the rotator cuff footprint, consists of four distinct transitional zones designed to gradually transmit mechanical forces: 1) Tendon proper (Type I collagen), 2) Unmineralized fibrocartilage (Type II and III collagen), 3) Mineralized fibrocartilage (Type II and X collagen), and 4) Bone (Type I collagen). The mineralized fibrocartilage zone directly connects the unmineralized fibrocartilage to the underlying bone. The tidemark is the basophilic line that visually separates the unmineralized and mineralized fibrocartilage zones.

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

Orthopedic Basic Science 2026 MCQs: Board Review Questions & Answers (Part 2)

Key Takeaway

Orthopedic Basic Science 2026 MCQs: Board Review Questions & Answers (Part 2)

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