AAOS & ABOS Basic Science MCQs: Biomechanics 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

Which of the following cells produces osteoprotegerin (OPG), acting as a decoy receptor to prevent bone resorption?

Explanation

Osteoblasts and osteocytes secrete OPG, which binds to RANKL and prevents it from interacting with RANK on osteoclast precursors. This inhibits osteoclast differentiation and activation, thereby decreasing bone resorption.

Question 27

Galvanic corrosion is most likely to occur in an orthopaedic implant under which of the following conditions?

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in electrical contact within a conductive fluid medium, such as body fluid. The less noble metal serves as the anode and undergoes accelerated corrosion.

Question 28

A 65-year-old woman is prescribed teriparatide for severe osteoporosis. What is the primary mechanism of action of this medication?

Explanation

Teriparatide is a recombinant human parathyroid hormone (PTH 1-34) analog. When administered intermittently, it strongly stimulates osteoblastic bone formation, unlike continuous PTH exposure which promotes bone resorption.

Question 29

Stress shielding around a femoral stem is most strongly influenced by which of the following material properties of the implant?

Explanation

Modulus of elasticity relates to a material's stiffness. Implants with a high modulus of elasticity bear most of the load when placed in bone, causing stress shielding and subsequent resorption of the unloaded surrounding bone.

Question 30

Tranexamic acid (TXA) is widely used to reduce blood loss in total joint arthroplasty. What is its mechanism of action?

Explanation

TXA is a synthetic analog of the amino acid lysine. It reversibly binds to the lysine receptor sites on plasminogen, preventing its activation to plasmin and thus effectively inhibiting fibrinolysis.

Question 31

During secondary bone healing, which type of collagen is predominantly synthesized during the soft callus phase?

Explanation

The soft callus phase of endochondral ossification is characterized by the production of fibrocartilage and hyaline cartilage, which are predominantly composed of Type II collagen. This is later replaced by Type I collagen during the hard callus phase.

Question 32

The progressive deformation of a polyethylene tibial insert over time under a constant static load is an example of which biomechanical principle?

Explanation

Creep is the progressive deformation of a viscoelastic material when subjected to a constant, continuous stress over time. Stress relaxation, conversely, is the decrease in stress over time when a material is held at a constant strain.

Question 33

When adding antibiotics to polymethylmethacrylate (PMMA) bone cement, which of the following characteristics is essential for the antibiotic to be effective?

Explanation

PMMA polymerization is an exothermic reaction that can reach temperatures exceeding 80 degrees Celsius. Therefore, any antibiotic mixed into the cement (such as tobramycin or vancomycin) must be heat stable to maintain its antimicrobial efficacy.

Question 34

Which of the following factors is most directly proportional to the volumetric wear of ultra-high molecular weight polyethylene (UHMWPE) in total hip arthroplasty?

Explanation

Volumetric wear of polyethylene is directly proportional to the sliding distance per cycle, which is a function of the femoral head diameter. Larger heads increase the sliding distance per articulation, thereby increasing overall volumetric wear.

Question 35

A patient presents with hypocalcemia following a total thyroidectomy. How does the body naturally attempt to correct this via parathyroid hormone (PTH) release?

Explanation

PTH responds to hypocalcemia by increasing bone resorption, increasing renal reabsorption of calcium, and increasing renal excretion of phosphate. It also stimulates 1-alpha-hydroxylase in the kidney to boost active vitamin D synthesis.

Question 36

Denosumab is a targeted biological therapy used to treat severe osteoporosis. What is its specific molecular target?

Explanation

Denosumab is a monoclonal antibody that binds directly to RANKL. By neutralizing RANKL, it prevents interaction with the RANK receptor on osteoclasts, severely inhibiting osteoclast formation, function, and survival.

Question 37

The pullout strength of a standard cortical screw is most heavily dependent on which of the following thread parameters?

Explanation

Pullout strength is directly proportional to the outer thread diameter, the length of thread engagement, and the shear strength of the host bone. Increases in the outer thread diameter significantly enhance the screw's resistance to pullout.

Question 38

Compared to mature lamellar bone, woven bone is characterized by which of the following properties?

Explanation

Woven bone is immature, rapidly formed bone with a random, disorganized collagen fiber orientation. It contains a greater number of osteocytes per unit volume, higher water content, and lower mineral content compared to mature lamellar bone.

Question 39

When a long bone is subjected to pure bending forces, where do the maximum tensile and compressive stresses occur?

Explanation

In bending, normal stresses are zero at the neutral axis and increase linearly with distance from it. Therefore, the maximum tensile and compressive stresses are located at the outermost surfaces of the bone.

Question 40

Rivaroxaban is commonly prescribed for deep vein thrombosis prophylaxis following total joint arthroplasty. Which of the following best describes its mechanism of action?

Explanation

Rivaroxaban is an oral anticoagulant that directly inhibits free and clot-bound Factor Xa. This interrupts the intrinsic and extrinsic pathway of the blood coagulation cascade, preventing thrombin generation.

Question 41

Which enzyme is primarily responsible for generating the acidic environment within the sealing zone of the osteoclast ruffled border?

Explanation

Carbonic anhydrase II generates protons from carbon dioxide and water inside the osteoclast. These protons are then actively pumped across the ruffled border via an H+-ATPase pump, creating the acidic environment necessary for dissolving hydroxyapatite.

Question 42

The "toe region" of a typical load-elongation curve for a normal ligament corresponds to which of the following physiological phenomena?

Explanation

The initial non-linear toe region of a ligament's load-elongation curve represents the straightening out, or uncrimping, of the normally wavy collagen fibers. Once uncrimped, the ligament enters the linear elastic region where stiffness increases.

Question 43

Which of the following best describes the mechanism by which non-steroidal anti-inflammatory drugs (NSAIDs) may impair fracture healing?

Explanation

Fracture healing relies on a coordinated inflammatory response. NSAIDs inhibit COX-2, which drastically decreases the production of prostaglandins (like PGE2) that are critical for normal osteoblast differentiation and early endochondral ossification.

Question 44

Sclerostin, a glycoprotein produced primarily by osteocytes, regulates bone formation by directly inhibiting which of the following intracellular signaling pathways?

Explanation

Sclerostin binds to LRP5/6 receptors on osteoblasts, effectively antagonizing the canonical Wnt/beta-catenin signaling pathway. This inhibition halts osteoblast differentiation and fundamentally decreases bone formation.

Question 45

For a tension band construct to be effective in treating a transverse patella fracture, the implant must convert tensile forces at the anterior surface into which of the following forces at the articular surface?

Explanation

The tension band principle relies on placing a tension-absorbing device (like a wire) on the tension side of a fractured bone. During dynamic loading, the construct converts potentially distracting tensile forces into stabilizing compressive forces at the articular surface.

Question 46

Which of the following alterations to a cortical screw design will yield the greatest theoretical increase in its pullout strength?

Explanation

Pullout strength is directly proportional to the outer diameter of the screw, the length of engagement, and the shear strength of the bone. Increasing the outer diameter has the greatest linear effect on improving pullout strength.

Question 47

During normal bone remodeling, RANKL binds to RANK on the surface of osteoclast precursors to stimulate differentiation. Which of the following molecules acts as a decoy receptor to inhibit this specific interaction?

Explanation

Osteoprotegerin (OPG) is secreted by osteoblasts and binds to RANKL, preventing it from binding to RANK. This inhibition effectively downregulates osteoclastogenesis and reduces bone resorption.

Question 48

A patient with a diaphyseal femur fracture is prescribed an analgesic that severely delays fracture healing. The mechanism of this nonunion is traced to the inhibition of essential endochondral ossification pathways. Which class of medication is most likely responsible?

Explanation

NSAIDs inhibit the cyclooxygenase-2 (COX-2) enzyme, which is critical for mesenchymal stem cell differentiation into osteoblasts during endochondral ossification. This can result in delayed union or nonunion of fractures.

Question 49

A viscoelastic orthopedic implant is subjected to a constant, sustained load over several years, resulting in a slow, progressive deformation of the material. This biomechanical phenomenon is best described as:

Explanation

Creep is the progressive deformation of a viscoelastic material when subjected to a constant load over time. In contrast, stress relaxation is a decrease in stress over time when the material is held at a constant deformation.

Question 50

Intermittent low-dose administration of teriparatide promotes an overall anabolic effect on bone. What is the primary cellular mechanism responsible for this bone formation?

Explanation

Intermittent administration of parathyroid hormone (PTH) analogues like teriparatide strongly stimulates bone formation by increasing osteoblast proliferation, differentiation, and lifespan. This anabolic effect overrides the typical resorptive effects seen with continuous high PTH levels.

Question 51

Tranexamic acid (TXA) is widely utilized in major orthopedic procedures to reduce perioperative blood loss. What is the exact pharmacological mechanism of TXA?

Explanation

TXA is a synthetic analog of the amino acid lysine. It reversibly and competitively binds to the lysine-binding sites on plasminogen, preventing its activation to plasmin and thus inhibiting fibrinolysis.

Question 52

Combining a 316L stainless steel screw with a titanium alloy plate in a fracture construct increases the risk of implant failure due to which specific type of corrosion?

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in contact within an electrolytic environment (like body fluid). The less noble metal undergoes accelerated corrosion.

Question 53

Sclerostin is a glycoprotein secreted by osteocytes that serves as a key negative regulator of bone mass. It achieves this primarily by binding to LRP5/6 receptors and inhibiting which intracellular signaling pathway?

Explanation

Sclerostin antagonizes the canonical Wnt/beta-catenin signaling pathway by binding to LRP5/6. Inhibition of this pathway decreases osteoblastogenesis and bone formation.

Question 54

Highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is routinely used in total hip arthroplasty to reduce volumetric wear. What is the primary biomechanical tradeoff resulting from the irradiation and thermal treatment processes used to create it?

Explanation

While high cross-linking drastically improves wear resistance, the requisite irradiation and subsequent remelting/annealing alter the crystalline structure. This leads to reduced fatigue strength, yield strength, and fracture toughness, making the material more susceptible to catastrophic failure under certain stresses.

Question 55

A patient experiences sudden cardiovascular collapse and malignant arrhythmias shortly after a regional nerve block using bupivacaine. Advanced cardiac life support is initiated. Which of the following is the most critical initial targeted antidote for systemic toxicity from this medication?

Explanation

Local anesthetic systemic toxicity (LAST), particularly severe with bupivacaine, is primarily treated with an intravenous lipid emulsion 20% bolus and infusion. The lipid emulsion acts as a 'lipid sink' to draw the highly lipophilic local anesthetic out of the cardiac tissue.

Question 56

Absolute stability of a transverse radius fracture is achieved using a rigid compression plate, leading to primary bone healing. This specific healing process bypasses callus formation and relies entirely on which of the following cellular mechanisms?

Explanation

Primary bone healing occurs under conditions of absolute stability (strain < 2%). It proceeds via direct Haversian remodeling, where osteoclasts create cutting cones across the fracture site, immediately followed by osteoblasts laying down new lamellar bone.

Question 57

The direct insertion of a ligament into bone is structurally designed to transition mechanical stress and prevent failure at the interface. Which of the following accurately represents the histological progression of this transition from ligament to bone?

Explanation

Direct ligamentous insertions undergo a gradual, four-zone histological transition to minimize stress risers. The sequence strictly follows: tendon/ligament, unmineralized fibrocartilage, mineralized fibrocartilage, and finally bone.

Question 58

When incorporating an antibiotic into polymethylmethacrylate (PMMA) bone cement for an infected arthroplasty spacer, which specific property of the antimicrobial agent is most vital to ensure it remains active after the cement cures?

Explanation

The curing of PMMA is a highly exothermic reaction that can reach temperatures exceeding 80 degrees Celsius. Therefore, any antibiotic mixed into the cement (such as vancomycin or tobramycin) must be thermostable to retain its bactericidal efficacy.

Question 59

When applying a locked plate to a highly comminuted diaphyseal fracture, what is its primary biomechanical advantage over a standard, non-locked dynamic compression plate?

Explanation

Locked plating systems utilize threaded screw heads that lock directly into the plate, creating a rigid fixed-angle construct. This relies on the angular stability of the screw-plate interface rather than friction between the plate and the underlying bone, preserving periosteal blood supply.

Question 60

Demineralized bone matrix (DBM) is commonly used as a bone graft extender. While lacking structural integrity, DBM is highly valued biologically because the demineralization process exposes specific growth factors, providing which primary property?

Explanation

The acid extraction process used to create DBM removes the mineral phase of bone, exposing bone morphogenetic proteins (BMPs) trapped within the matrix. This exposure provides potent osteoinductive signals to recruit and differentiate host mesenchymal stem cells.

Question 61

Romosozumab is a monoclonal antibody recently approved for the treatment of postmenopausal women with severe osteoporosis at high risk of fracture. What is the specific molecular target of this medication?

Explanation

Romosozumab works by binding to and inhibiting sclerostin, thereby disinhibiting the Wnt/beta-catenin signaling pathway. This results in a dual effect: rapidly increasing bone formation while simultaneously decreasing bone resorption.

Question 62

In normal articular cartilage, the remarkable ability to resist massive compressive loads is primarily governed by which of the following biophysical mechanisms?

Explanation

Articular cartilage consists largely of water, Type II collagen, and proteoglycans (aggrecan). The negatively charged glycosaminoglycan chains on aggrecan create a large Donnan osmotic pressure, drawing water in and creating swelling pressure that powerfully resists compressive loads.

Question 63

A healthy individual consumes a highly calcium-rich diet, triggering a transient elevation in serum calcium. In normal homeostasis, which hormone is secreted by the thyroid parafollicular cells to directly counteract this rise by inhibiting osteoclast activity?

Explanation

Calcitonin is released by the parafollicular C-cells of the thyroid gland in response to hypercalcemia. It acts directly on osteoclasts via specific receptors to rapidly inhibit bone resorption, lowering serum calcium levels.

Question 64

Stress shielding around a femoral stem is primarily caused by a mismatch in stiffness between the implant and the surrounding bone. Which of the following sequences represents the correct order of Young's modulus of elasticity from greatest to least?

Explanation

Young's modulus is a measure of material stiffness. Cobalt-chromium is the stiffest (~220 GPa), followed by stainless steel (~200 GPa), titanium alloy (~110 GPa), and cortical bone (~15-20 GPa). Using materials closer to bone's modulus minimizes stress shielding.

Question 65

Fondaparinux is an anticoagulant frequently prescribed for venous thromboembolism prophylaxis following major orthopedic surgery. What is the exact mechanism of action of this synthetic pentasaccharide?

Explanation

Fondaparinux binds exclusively to antithrombin III, inducing a conformational change that dramatically accelerates its natural inhibition of Factor Xa. Unlike apixaban or rivaroxaban, which are direct inhibitors, fondaparinux requires antithrombin III to exert its anticoagulant effect.

Question 66

Osteoprotegerin (OPG) regulates bone mass and prevents excessive bone resorption. It exerts this effect by acting as a decoy receptor for which of the following molecules?

Explanation

OPG is produced by osteoblasts and binds to RANKL, preventing it from binding to the RANK receptor on osteoclasts. This competitive inhibition prevents osteoclast differentiation and activation, thereby reducing bone resorption.

Question 67

A patellar tendon graft is used for ACL reconstruction and is tensioned on the back table. Over time, while held at a constant tension, the graft gradually elongates. Which biomechanical property does this describe?

Explanation

Creep is the progressive deformation (elongation) of a viscoelastic material when subjected to a constant load or tension over time. Stress relaxation, conversely, is the decrease in stress over time when the material is held at a constant length.

Question 68

Tranexamic acid (TXA) is widely used in orthopedic surgery to reduce perioperative blood loss. What is the precise mechanism of action of TXA?

Explanation

TXA is a synthetic lysine analog that competitively binds to the lysine-binding sites on plasminogen. This prevents plasminogen from converting to plasmin, thereby inhibiting the degradation of fibrin clots.

Question 69

When applying a bridging plate to a comminuted diaphyseal femur fracture, the surgeon decides to increase the working length of the plate. What biomechanical effect does this have on the construct?

Explanation

The working length of a plate is the distance between the two closest screws on either side of the fracture. Increasing the working length decreases the bending stiffness of the construct, which allows for more interfragmentary motion and promotes secondary bone healing.

Question 70

Recombinant human Bone Morphogenetic Protein-2 (rhBMP-2) is utilized in spinal fusions to induce osteoinduction. The intracellular signaling pathway activated by BMP-2 binding to its serine/threonine kinase receptor is primarily mediated by which of the following?

Explanation

BMPs bind to cell surface receptors that phosphorylate receptor-regulated Smads, specifically Smad 1, 5, and 8. These complex with Smad 4, translocate to the nucleus, and upregulate the transcription of osteogenic genes like Runx2.

Question 71

A surgeon is evaluating the pullout strength of various cortical screws for rigid internal fixation. Which of the following geometric modifications to a screw will yield the greatest increase in pullout strength?

Explanation

Pullout strength is directly proportional to the outer (major) diameter of the screw, the length of engagement in the bone, and the shear strength of the bone material. Increasing the major diameter most significantly increases pullout resistance.

Question 72

Articular cartilage relies on its distinct zones to resist mechanical forces. Which zone contains collagen fibers oriented parallel to the articular surface, providing the highest resistance to shear stress?

Explanation

The superficial zone of articular cartilage is composed of densely packed collagen fibers oriented parallel to the joint surface. This specific orientation provides the tissue with its ability to resist high shear forces during joint motion.

Question 73

Nitrogen-containing bisphosphonates, such as alendronate, are a first-line treatment for osteoporosis. They cause osteoclast apoptosis by inhibiting which specific enzyme in the mevalonate pathway?

Explanation

Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate (FPP) synthase in the mevalonate pathway. This prevents the prenylation of small GTPase proteins (like Ras and Rho), leading to osteoclast dysfunction and apoptosis.

Question 74

Highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) was developed to improve the longevity of total hip arthroplasty (THA). The primary advantage of cross-linking is the reduction of which type of wear?

Explanation

Cross-linking UHMWPE significantly increases its resistance to adhesive and abrasive wear, which are the main modes of wear generating osteolytic particles in THA. However, highly cross-linked polyethylene has reduced mechanical properties, making it more susceptible to fatigue wear.

Question 75

A patient is prescribed rivaroxaban for DVT prophylaxis following a total knee arthroplasty. What is the mechanism of action of this pharmacological agent?

Explanation

Rivaroxaban and apixaban are direct oral anticoagulants that work by specifically and reversibly binding to and inhibiting free and clot-bound Factor Xa. Unlike heparin, they do not require antithrombin III to exert their effect.

Question 76

During embryonic skeletal development and fracture repair, multipotent mesenchymal stem cells must commit to the osteoblastic lineage. Which of the following transcription factors is the master regulator for osteoblast differentiation?

Explanation

Runx2 (also known as Cbfa1) is the essential transcription factor required for mesenchymal stem cells to differentiate into osteoblasts. A deficiency or mutation in Runx2 leads to cleidocranial dysplasia and absent bone formation.

Question 77

In a posterior-stabilized total knee arthroplasty (TKA), the cam and post mechanism is primarily designed to replicate the function of the resected posterior cruciate ligament (PCL). What specific kinematic function does this substitution promote?

Explanation

The PCL normally functions to roll the femur posteriorly on the tibia during deep flexion, maximizing the range of motion and improving the quadriceps moment arm. In a posterior-stabilized TKA, the interaction of the femoral cam and tibial post forces this femoral rollback.

Question 78

According to Perren's strain theory of fracture healing, the tissue that forms at a fracture site is dictated by the amount of interfragmentary strain. If the local mechanical strain is between 2% and 10%, which type of tissue will predominantly form?

Explanation

Perren's strain theory states that tissues tolerate specific levels of strain before rupturing. Granulation tissue tolerates up to 100% strain, fibrocartilage tolerates 2-10% strain, and bone forms only when the strain is less than 2%.

Question 79

Romosozumab is an anabolic pharmacological agent used to treat severe osteoporosis. It exerts its effect by binding to and inhibiting a specific glycoprotein, thereby activating the Wnt signaling pathway. What is the target of romosozumab?

Explanation

Romosozumab is a monoclonal antibody that binds to and inhibits sclerostin, a glycoprotein produced by osteocytes that normally downregulates bone formation. Inhibiting sclerostin releases the block on the Wnt/beta-catenin pathway, promoting robust osteoblast activity.

Question 80

Mixing stainless steel and titanium implants within the same surgical construct can lead to galvanic corrosion. In this scenario, what happens to the stainless steel component?

Explanation

In a galvanic couple between stainless steel and titanium, titanium is more noble (cathodic) and stainless steel is less noble (anodic). The anodic stainless steel will preferentially lose electrons and undergo accelerated corrosion.

Question 81

Tendon healing occurs in three distinct phases: inflammatory, proliferative, and remodeling. During the remodeling phase, what is the most significant change in the collagen composition of the extracellular matrix?

Explanation

During the initial proliferative phase of tendon healing, fibroblasts rapidly synthesize Type III collagen, which forms disorganized scar tissue. In the remodeling phase, this is gradually replaced by stronger, longitudinally aligned Type I collagen.

Question 82

A researcher is studying the mechanical properties of different orthopedic materials compared to human cortical bone. Which of the following implant materials possesses a Young's modulus (modulus of elasticity) that is closest to that of cortical bone?

Explanation

Cortical bone has a Young's modulus of roughly 15-20 GPa. Titanium alloys have a modulus of around 100-110 GPa, which is much closer to cortical bone than stainless steel (approx. 200 GPa) or cobalt-chromium (approx. 240 GPa), reducing the risk of stress shielding.

Question 83

During a massive rotator cuff repair, a patient receives an interscalene block and subsequently develops seizures followed by cardiac arrest. The anesthesiologist diagnoses severe bupivacaine toxicity. What is the most appropriate specific treatment to reverse the cardiotoxicity?

Explanation

Bupivacaine is highly lipophilic and can cause catastrophic, refractory cardiac arrhythmias if injected intravascularly. Intravenous 20% lipid emulsion therapy is the standard of care to create a "lipid sink" that rapidly extracts the local anesthetic from the myocardial tissues.

Question 84

The meniscus relies on its unique structural composition to distribute axial loads in the knee. The primary function of the circumferentially oriented Type I collagen fibers in the menisci is to?

Explanation

Axial loading of the knee tends to extrude the meniscus peripherally. The robust circumferential Type I collagen fibers resist this outward expansion by converting compressive forces into tensile "hoop stresses," maintaining joint congruity.

Question 85

Nonsteroidal anti-inflammatory drugs (NSAIDs) are often avoided in the acute phase of fracture healing due to concerns about nonunion. What is the specific cellular mechanism by which NSAID use impairs fracture healing?

Explanation

NSAIDs inhibit cyclooxygenase enzymes, specifically COX-2, which is required for the synthesis of prostaglandins (like PGE2) at the fracture site. PGE2 is critical for initiating the inflammatory cascade, angiogenesis, and subsequent enchondral ossification.

Question 86

A 65-year-old woman with severe osteoporosis is started on romosozumab, a monoclonal antibody that binds to sclerostin. What is the primary downstream effect of this medication on bone metabolism?

Explanation

Sclerostin normally inhibits the Wnt signaling pathway by binding to LRP5/6 receptors. Blocking sclerostin allows Wnt signaling to proceed, leading to beta-catenin accumulation, which promotes osteoblast differentiation and bone formation.

Question 87

An orthopedic surgeon is designing a new cortical screw to be used in diaphyseal bone. To maximize the pullout strength of the screw, which of the following geometric modifications should be made?

Explanation

Pullout strength is directly proportional to the outer diameter and length of thread engagement, and inversely proportional to the thread pitch. Decreasing the thread pitch increases the number of threads engaged in the cortical bone, thereby increasing pullout strength.

Question 88

A 55-year-old man undergoes a total knee arthroplasty. To minimize perioperative blood loss, intravenous tranexamic acid (TXA) is administered. What is the primary mechanism of action of this agent?

Explanation

TXA is a synthetic analog of the amino acid lysine that reversibly binds to the lysine receptor sites on plasminogen. This competitively inhibits the activation of plasminogen to plasmin, thereby preventing the degradation of fibrin clots.

Question 89

During an anterior cruciate ligament reconstruction, the surgeon pulls the soft-tissue graft under a constant load for several minutes before final fixation. The graft gradually lengthens during this period. Which viscoelastic property does this demonstrate?

Explanation

Creep is the progressive deformation (lengthening) of a viscoelastic material when it is subjected to a constant load over time. Conversely, stress relaxation is the decrease in stress over time when a material is held at a constant length.

Question 90

Which of the following enzymes is primarily responsible for generating the protons necessary for the acidic environment within the Howship lacuna during osteoclast-mediated bone resorption?

Explanation

Carbonic anhydrase II catalyzes the conversion of carbon dioxide and water into protons and bicarbonate. The protons are then pumped across the osteoclast ruffled border by V-ATPase to create the acidic environment required to dissolve hydroxyapatite.

Question 91

A patient presents with a symptomatic nonunion of a femoral shaft fracture initially treated with a stainless steel plate. The surgeon plans to revise the fixation using a titanium plate and the existing stainless steel screws. Why is this construct contraindicated?

Explanation

Galvanic corrosion occurs when two dissimilar metals are in contact within a conductive physiological fluid, creating an electrochemical gradient. The less noble metal acts as an anode and undergoes accelerated corrosion, making the mixing of stainless steel and titanium implants contraindicated.

Question 92

When preparing antibiotic-loaded polymethylmethacrylate (PMMA) bone cement for a two-stage revision of an infected total hip arthroplasty, which of the following antibiotics is best suited for incorporation due to its thermal stability and elution profile?

Explanation

Antibiotics mixed into PMMA must be heat-stable to withstand the extreme exothermic polymerization reaction of the cement and must be available in powder form. Aminoglycosides (like tobramycin and gentamicin) and vancomycin meet these criteria and are most commonly used.

Question 93

The introduction of highly cross-linked polyethylene (HXLPE) in total hip arthroplasty has significantly reduced volumetric wear. However, the process of cross-linking using irradiation alters its mechanical properties. Which property is significantly decreased in HXLPE compared to conventional ultra-high molecular weight polyethylene?

Explanation

While irradiation and cross-linking dramatically improve wear resistance, they significantly decrease fatigue strength, ultimate tensile strength, and fracture toughness. This makes HXLPE theoretically more susceptible to catastrophic failure under high cyclic stresses.

Question 94

A 32-year-old man undergoes open reduction and internal fixation of a transverse radial shaft fracture with a compression plate. Radiographs at 8 weeks show no visible callus, but the fracture line is disappearing. What is the primary mechanism of bone healing occurring in this scenario?

Explanation

Rigid internal fixation with absolute stability prevents interfragmentary micro-motion, leading to primary bone healing via Haversian remodeling (cutting cones). This process occurs directly across the fracture gap without the formation of an intermediate cartilaginous callus.

Question 95

A 45-year-old patient with a tibial shaft fracture asks for a potent non-steroidal anti-inflammatory drug (NSAID) for pain control. The surgeon advises against prolonged use due to the risk of nonunion. NSAIDs impair fracture healing primarily by inhibiting which of the following processes?

Explanation

NSAIDs inhibit COX-2, an enzyme critical for the early inflammatory phase of fracture healing. COX-2 produces prostaglandins (such as PGE2) that are essential for triggering the differentiation of mesenchymal stem cells into osteoblasts.

Question 96

In a standard stress-strain curve for a metallic orthopedic implant, what does the slope of the curve in the linear elastic region represent?

Explanation

Young's modulus (the elastic modulus) measures a material's intrinsic stiffness and is defined as the slope of the stress-strain curve in the linear elastic region. Toughness is represented by the total area under the curve, and yield strength is the point where plastic deformation begins.

Question 97

In response to hypocalcemia, parathyroid hormone (PTH) is secreted into the bloodstream. Which of the following best describes the direct effect of PTH on the kidneys to restore serum calcium levels?

Explanation

PTH directly increases serum calcium by stimulating calcium reabsorption in the distal convoluted tubule of the kidney. It also decreases phosphate reabsorption in the proximal tubule and upregulates 1-alpha-hydroxylase to increase active vitamin D production.

Question 98

A 72-year-old woman with a history of multiple osteoporotic vertebral compression fractures is started on teriparatide. Which of the following best describes the mechanism of action of this medication?

Explanation

Teriparatide is a recombinant human parathyroid hormone (PTH 1-34) analog. Intermittent, once-daily administration paradoxically stimulates osteoblastic bone formation to a greater extent than osteoclastic resorption, leading to a net increase in bone mass.

Question 99

Ceramic-on-ceramic bearings in total hip arthroplasty offer excellent wear characteristics but are associated with squeaking and catastrophic failure. Which of the following mechanical characteristics best describes ceramics compared to cobalt-chromium alloys?

Explanation

Ceramics (such as alumina and zirconia) possess extreme hardness and high compressive strength, contributing to their low wear rates. However, they are highly brittle materials with very low fracture toughness, making them vulnerable to catastrophic shattering under impact or tensile loads.

Question 100

A surgeon uses demineralized bone matrix (DBM) to augment a posterolateral lumbar fusion. Which of the following properties is characteristic of DBM but absent in synthetic calcium phosphate ceramics?

Explanation

Demineralized bone matrix (DBM) retains bone morphogenetic proteins (BMPs) that confer osteoinductive properties, stimulating undifferentiated cells to become osteoblasts. Synthetic calcium phosphate ceramics only provide a physical scaffold, which is termed osteoconduction.

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

AAOS & ABOS Basic Science MCQs (Set 2): Biomechanics, Bone Physiology & Pharma | Board Review

Key Takeaway

AAOS & ABOS Basic Science MCQs (Set 2): Biomechanics, Bone Physiology & Pharma | Board Review

Chapter Index