ABOS Board Review: Bone Healing & Biomechanics MCQs

Question 1

An orthopaedic surgeon frequently uses hip and knee prostheses from a specific manufacturer. The surgeon becomes acquainted with the manufacturer's representative who provides the support for these prostheses in the hospital. They develop a personal relationship outside of work through a common interest in sailing. Together they become interested in buying a sailboat. The manufacture's representative suggests a partnership in a boat costing $200,000. The manufacture's representative would purchase a 90% interest and the surgeon a 10% interest in the boat. There would be no restrictions on use of the boat by the surgeon. What should the orthopaedic surgeon do?

Explanation

Rejecting this proposal is the only appropriate course of action. Accepting it would, in essence, be receiving a huge gift from industry in the form of a sailboat. Physicians frequently assert that they are not influenced by gifts and relationships with industry representatives, but evidence is to the contrary. Such an arrangement constitutes a tremendous incentive to use the manufacturer's products. The fact that the boat partnership seems completely outside of the orthopaedic business relationship does not excuse it. Conflicts of interest should always be resolved and in the best interest of patient care, and in this case the best course clearly is to avoid the conflict of interest totally. An equal interest in the boat does not eliminate the conflict of interest. AAOS Standard of Professionalism -Orthopaedist -Industry Conflict of Interest (Adopted 4/18/07), Mandatory Standard numbers 6-8. www3.aaos.org/member/profcomp/SOPConflictsIndustry.pdf Opinions on Ethics and Professionalism: The Orthopaedic Surgeon's Relationship with Industry (Document 1204), in Guide to the Ethical Practice of Orthopaedic Surgery, ed 7. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2007, pp 36-40. www.aaos.org/about/papers/ethics/1204eth.asp AdvaMed Code of Ethics on Interactions with Health Care Professionals, Advanced Medical Technology Association, Washington, DC. www.AdvaMed.org

Question 2

When comparing the biomechanical properties of cortical bone to cancellous bone on a stress-strain curve, which of the following statements is true?

Explanation

Cortical bone is stiffer and fails at lower strain levels (around 2%), while cancellous bone is less stiff but can undergo significantly more strain (up to 50%) before failure. Therefore, cancellous bone absorbs more energy and fails at a higher strain.

Question 3

According to Perren's strain theory, what is the maximum tissue strain allowable for primary (direct) bone healing to occur without the formation of a visible callus?

Explanation

Primary bone healing requires absolute stability with an interfragmentary strain of less than 2%. Strains between 2% and 10% result in secondary bone healing characterized by callus formation.

Question 4

An orthopedic surgeon is applying a unilateral external fixator for a comminuted tibia fracture. Which of the following modifications will most significantly increase the bending stiffness of the construct?

Explanation

Increasing the pin diameter has the greatest effect on bending stiffness, as the stiffness is proportional to the radius of the pin to the fourth power (r^4). While decreasing the bone-to-bar distance and using stiffer materials also help, pin diameter is the most mathematically powerful variable.

Question 5

In cortical screw fixation, which of the following variables has the greatest direct influence on the pullout strength of the screw?

Explanation

Screw pullout strength is primarily determined by the outer thread diameter, the length of engagement in the bone, and the shear strength of the bone. The outer diameter dictates the maximum surface area for bone purchase.

Question 6

Bone Morphogenetic Proteins (BMPs) stimulate osteoblast differentiation and bone formation primarily by signaling through which of the following intracellular molecular pathways?

Explanation

BMPs bind to serine/threonine kinase receptors on the cell surface, which subsequently phosphorylate and activate the intracellular Smad 1/5/8 pathway. This leads to the transcription of osteogenic genes like Runx2.

Question 7

When using a locking compression plate (LCP) in a bridging technique for a comminuted diaphyseal fracture, omitting screws in the holes immediately adjacent to the fracture site achieves which of the following?

Explanation

Omitting screws near the fracture site increases the working length of the plate, which decreases the overall construct stiffness. This allows for controlled interfragmentary motion (relative stability), promoting secondary bone healing via callus formation.

Question 8

In total hip arthroplasty, osteolysis is most strongly driven by a macrophage-mediated inflammatory response to ultra-high-molecular-weight polyethylene (UHMWPE) wear particles. What particle size range is most biologically active in stimulating this response?

Explanation

UHMWPE particles in the submicron range, specifically 0.1 to 1.0 micrometers, are most easily phagocytosed by macrophages. This phagocytosis triggers the release of pro-inflammatory cytokines like TNF-alpha and IL-1, leading to severe osteolysis.

Question 9

A surgeon removes a stainless steel plate that was previously placed adjacent to a titanium intramedullary nail. Significant corrosion is noted where the two metals were in contact. Which of the following principles best explains this phenomenon?

Explanation

Galvanic corrosion occurs when two dissimilar metals are in contact within a conductive fluid environment. The less noble metal (stainless steel) acts as the anode and preferentially corrodes, while the more noble metal (titanium) acts as the cathode.

Question 10

A new orthopedic implant is designed to minimize stress shielding in the adjacent host bone. To achieve this, the biomaterial should possess which of the following mechanical properties compared to traditional cobalt-chromium alloys?

Explanation

Stress shielding occurs when a highly rigid implant unloads the adjacent bone, leading to osteopenia. Using a material with a lower modulus of elasticity (closer to that of cortical bone, like titanium rather than cobalt-chromium) allows more physiological load to be transferred to the bone.

Question 11

During the secondary fracture healing process, the soft callus is gradually replaced by a hard callus. This transition is characterized by a shift in the predominant collagen type from:

Explanation

The initial soft callus consists primarily of fibrocartilage, which is rich in Type II collagen. As endochondral ossification progresses, this cartilage is resorbed and replaced by woven bone (hard callus), which is predominantly composed of Type I collagen.

Question 12

On a load-deformation curve for a native human ligament, the initial "toe region" is characterized by low stiffness and high deformation with minimal applied force. This region corresponds physiologically to:

Explanation

The toe region of a ligament's load-deformation curve represents the physiological uncrimping or straightening of the relaxed, wavy collagen fibers. Once these fibers are straightened, the tissue enters the linear elastic region where its stiffness significantly increases.

Question 13

An orthopaedic surgeon uses an unreamed, solid intramedullary nail to treat a diaphyseal femur fracture. The torsional rigidity of this solid nail is proportional to its radius raised to which power?

Explanation

The torsional rigidity of a solid cylinder, such as an intramedullary nail, is proportional to its polar moment of inertia, which scales with the radius to the fourth power (r^4). Therefore, small increases in the nail's diameter result in exponential increases in its resistance to torsional forces.

Question 14

A pediatric patient treated with a dynamic orthosis experiences a progressive change in the shape of their limb over several weeks under a constant applied load. This biomechanical phenomenon is best described as:

Explanation

Creep is a viscoelastic property where a biological material undergoes progressive, time-dependent deformation when subjected to a constant load. Stress relaxation, conversely, is the gradual decrease in stress over time when a material is held at a constant deformation.

Question 15

Which of the following bone graft substitutes possesses osteoconductive and osteoinductive properties, but entirely lacks osteogenic potential?

Explanation

Demineralized bone matrix (DBM) contains natural growth factors like BMPs (providing osteoinduction) and a collagenous scaffold (providing osteoconduction). However, because it is entirely acellular, it lacks osteogenic capability, which requires living osteoprogenitor cells.

Question 16

Denosumab is used to treat osteoporosis and giant cell tumors of bone by inhibiting osteoclast maturation and activity. Which naturally occurring molecule does Denosumab mimic in its mechanism of action?

Explanation

Denosumab is a targeted monoclonal antibody that binds to RANKL, preventing it from interacting with the RANK receptor on osteoclast precursors. This exactly mimics the action of Osteoprotegerin (OPG), an endogenous decoy receptor that regulates and inhibits osteoclastogenesis.

Question 17

Cortical bone exhibits different mechanical properties depending on the direction of the applied load. For example, it is significantly stronger in longitudinal compression than in transverse tension. This fundamental material property is termed:

Explanation

Anisotropy refers to the characteristic of a material having highly variable mechanical properties when loaded in different directions. Bone is strongly anisotropic, being much more robust when loaded parallel to the longitudinal axis of its osteons.

Question 18

What strain environment, as described by Perren's strain theory, is required to dictate secondary bone healing via endochondral ossification?

Explanation

According to Perren's strain theory, secondary bone healing (callus formation) occurs in a strain environment between 2% and 10%. Strains less than 2% allow primary healing, while strains greater than 10% result in granulation tissue and nonunion.

Question 19

A surgeon decides to upsize a solid tibial intramedullary nail from 10 mm to 12 mm. What is the approximate proportional increase in the bending stiffness of the implant?

Explanation

The bending stiffness of a solid cylinder is proportional to the radius to the fourth power based on the area moment of inertia. Upsizing the nail from 10 mm to 12 mm increases the stiffness by (12/10)^4, which equals approximately 2.07.

Question 20

A patient develops a painful mass over a plated clavicle fracture 2 years post-operatively. Surgery reveals black debris where a stainless steel screw was placed through a titanium plate. Which electrochemical process is primarily responsible?

Explanation

Galvanic corrosion occurs when two dissimilar metals are in contact within an electrolytic medium like body fluid. The less noble metal (stainless steel) acts as the anode and corrodes, while the more noble titanium acts as the cathode.

Question 21

During the remodeling phase of fracture healing, which enzyme is primarily responsible for creating the acidic microenvironment at the osteoclast ruffled border to dissolve bone mineral?

Explanation

Carbonic anhydrase II catalyzes the conversion of CO2 and water into protons and bicarbonate. The protons are pumped across the ruffled border via vacuolar H+-ATPases to dissolve hydroxyapatite, while Cathepsin K degrades the organic collagen matrix.

Question 22

When constructing an external fixator for a highly comminuted tibial shaft fracture, which of the following modifications will most effectively increase the axial stiffness of the construct?

Explanation

Moving the pins closer to the fracture site decreases the working length of the bone-pin-rod construct, significantly increasing axial stiffness. Increasing pin diameter or moving the rod closer to the bone also increases stiffness.

Question 23

Which of the following molecules acts as a direct extracellular antagonist to the canonical Wnt signaling pathway, thereby inhibiting osteoblastogenesis and bone formation?

Explanation

Sclerostin, produced primarily by osteocytes, binds to the LRP5/6 coreceptors on osteoblasts, directly inhibiting the canonical Wnt/beta-catenin pathway. This halts osteoblast differentiation and subsequent bone formation.

Question 24

On a stress-strain curve, the total area under the curve before the point of material failure represents which mechanical property?

Explanation

Toughness is defined as the amount of energy a material can absorb before catastrophic failure, represented by the total area under the stress-strain curve. The slope of the linear elastic region represents stiffness or Young's modulus.

Question 25

A patient with a tibia fracture is taking high-dose NSAIDs. This medication impairs fracture healing primarily by inhibiting the synthesis of which downstream mediator vital for endochondral ossification?

Explanation

NSAIDs inhibit cyclooxygenase (COX) enzymes, leading to a profound decrease in prostaglandins, specifically PGE2. PGE2 is essential for osteoblast differentiation and the successful progression of endochondral ossification.

Question 26

In a locked plating construct used for a comminuted metaphyseal fracture, how is the construct's working length defined?

Explanation

The working length of a plate is the distance between the two innermost screws on either side of the fracture. Increasing the working length in a comminuted fracture distributes stress over a longer segment, reducing the risk of fatigue failure.

Question 27

A bone graft incorporates through a slow process where osteoclasts resorb the donor bone and osteoblasts lay down new viable bone. This process, known as creeping substitution, is most characteristic of which graft type?

Explanation

Structural cortical allografts heal via creeping substitution, an extremely slow process where host cutting cones progressively resorb and replace the necrotic donor bone. This often leaves the graft mechanically weak during the intermediate remodeling phase.

Question 28

The pullout strength of a cortical screw is most directly proportional to which of the following variables?

Explanation

Screw pullout strength is proportional to the volume of bone engaged by the threads. This is calculated using a formula dependent on the length of engagement and the difference between the outer diameter squared and the inner (core) diameter squared.

Question 29

During the initial inflammatory phase of fracture healing, which growth factor, released immediately by degranulating platelets, is most responsible for the initial chemotaxis of mesenchymal stem cells?

Explanation

PDGF is released immediately from alpha granules of platelets within the fracture hematoma. It serves as a potent chemoattractant and mitogen for mesenchymal stem cells, macrophages, and fibroblasts to initiate the healing cascade.

Question 30

Fatigue failure of an orthopaedic implant is most likely to occur under which of the following conditions?

Explanation

Fatigue failure occurs when an implant is subjected to repetitive (cyclic) loading at stress levels well below its yield strength. In the presence of a nonunion, the implant absorbs continuous cyclic loads, eventually leading to hardware fracture.

Question 31

Bone Morphogenetic Proteins (BMPs) induce osteoblastic differentiation from mesenchymal stem cells. Which intracellular signaling molecules are directly phosphorylated by BMP receptors to translocate to the nucleus?

Explanation

Binding of BMPs to their serine/threonine kinase receptors leads to the direct phosphorylation of Smad 1, 5, and 8. These complex with Smad 4, translocate to the nucleus, and activate transcription factors such as Runx2.

Question 32

A surgeon secures a cerclage wire around a femoral shaft. Over several months, the wire remains intact but becomes loose due to continuous deformation of the bone under constant strain. What viscoelastic property is this?

Explanation

Stress relaxation is a viscoelastic property where the internal stress within a material decreases over time when it is held at a constant strain or deformation. Creep is the opposite, characterized by increasing deformation under a constant load.

Question 33

Articular cartilage exhibits rate-dependent stiffness, acting much stiffer under rapid impact loading than under slow loading. This biomechanical property is primarily due to:

Explanation

Articular cartilage is a biphasic material. Under rapid loading, the interstitial fluid cannot quickly escape the porous extracellular matrix, resulting in high internal pressure and increased stiffness due to fluid frictional drag.

Question 34

During distraction osteogenesis (Ilizarov technique), bone forms directly under tensional stress without a cartilaginous intermediate. This process is best characterized as:

Explanation

Distraction osteogenesis primarily relies on intramembranous ossification. Mesenchymal stem cells differentiate directly into osteoblasts under the influence of mechanical tension, entirely skipping the cartilaginous phase seen in endochondral healing.

Question 35

When using a dynamic compression plate (DCP) for a transverse radius fracture, eccentric screw placement within the oval plate holes creates which mechanical effect at the fracture site?

Explanation

As the eccentrically placed spherical screw head engages the sloped contour of the DCP hole, it glides down the incline. This translates the bone relative to the plate, generating axial compression across the transverse fracture site.

Question 36

Denosumab is utilized in the treatment of osteoporosis and giant cell tumor of bone. Its mechanism of action most closely mimics which naturally occurring endogenous molecule?

Explanation

Denosumab directly binds to and inhibits RANKL, preventing it from activating RANK on osteoclasts. This mechanism perfectly mimics Osteoprotegerin (OPG), the body's natural decoy receptor for RANKL.

Question 37

According to Perren's strain theory, what is the maximum interfragmentary strain that allows for primary bone healing to occur?

Explanation

Primary bone healing via cutting cones requires absolute stability with an interfragmentary strain of less than 2 percent. Secondary bone healing with callus formation can tolerate up to 10 percent strain.

Question 38

A 45-year-old male undergoes a revision total hip arthroplasty with a fully porous-coated diaphyseal fitting stem. Postoperatively, he experiences severe thigh pain attributed to stress shielding. Which of the following material properties of the stem is most responsible for this phenomenon?

Explanation

Stress shielding occurs when a rigid implant unloads the surrounding bone, leading to osteopenia. Materials with a high modulus of elasticity, such as cobalt-chromium, take on more physiologic loads and cause greater proximal bone resorption.

Question 39

When applying a locking compression plate (LCP) for a comminuted diaphyseal fracture, an orthopaedic surgeon intentionally leaves three screw holes empty directly over the fracture site. What is the primary biomechanical effect of increasing this working length?

Explanation

Increasing the working length of a bridging plate decreases the overall bending stiffness of the construct. This allows for controlled micromotion, which stimulates secondary bone healing through callus formation.

Question 40

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

Explanation

Screw pullout strength is directly proportional to the outer thread diameter, length of engagement, and thread depth. Increasing the outer diameter provides the most significant increase in pullout strength.

Question 41

A surgeon incorrectly mixes a stainless steel screw with a titanium plate during internal fixation of a femur fracture. Which type of corrosion is most likely to occur, and which metal will degrade?

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in a conductive fluid like body tissue. The less noble metal, which is stainless steel in this scenario, acts as the anode and will undergo accelerated degradation.

Question 42

In the biological cascade of particle-induced osteolysis following total joint arthroplasty, which of the following cell types primarily phagocytoses ultra-high-molecular-weight polyethylene (UHMWPE) wear particles?

Explanation

Macrophages are the primary cells that phagocytose UHMWPE wear debris sized 0.1 to 1.0 micrometers. Upon activation, they release pro-inflammatory cytokines like TNF-alpha and IL-1, leading to osteoclast activation and massive osteolysis.

Question 43

During a biomechanical study of the ACL, a constant tensile load is applied to the ligament over an extended period. The investigator notes that the ligament continues to slowly elongate despite the load remaining unchanged. What biomechanical property does this represent?

Explanation

Creep is defined as the time-dependent deformation of a viscoelastic material under a constant load. Stress relaxation, in contrast, is the decrease in internal stress over time when the material is held at a constant deformation.

Question 44

An external fixator is applied to stabilize an open tibia fracture. Which of the following modifications will most significantly increase the bending stiffness of the fixator construct?

Explanation

The bending stiffness of a pin is proportional to the radius to the fourth power. Therefore, increasing the pin diameter provides an exponential and most significant increase in the overall stiffness of the external fixator construct.

Question 45

Demineralized bone matrix (DBM) is commonly used as a bone graft substitute in spinal fusions. Which of the following best describes its biological properties?

Explanation

DBM retains growth factors like BMPs making it osteoinductive, and its residual collagen matrix provides an osteoconductive scaffold. Because the sterilization process removes live cells, it lacks osteogenic potential.

Question 46

The compressive stiffness of articular cartilage is primarily provided by which of its structural components?

Explanation

Proteoglycans, particularly aggrecan, are highly negatively charged and attract water, providing articular cartilage with its compressive stiffness through Donnan osmotic pressure. Type II collagen network provides the tensile strength to resist swelling.

Question 47

During the natural progression of secondary fracture healing, the soft callus phase is characterized by a peak in the synthesis of which of the following?

Explanation

The soft callus phase is heavily cartilaginous, leading to a peak in Type II collagen production by chondrocytes. As the soft callus matures into hard woven bone, Type II collagen is subsequently replaced by Type I collagen.

Question 48

Bone morphogenetic protein-2 (BMP-2) utilizes a specific intracellular signaling pathway to induce osteoblast differentiation. Which of the following molecules acts as the primary intracellular mediator for this pathway?

Explanation

BMP-2 binds to cell surface serine/threonine kinase receptors, which phosphorylate and activate the intracellular SMAD 1/5/8 complex. This complex translocates to the nucleus to upregulate osteogenic transcription factors like RUNX2.

Question 49

An intramedullary nail is inserted for a transverse midshaft femur fracture. Using a solid nail instead of a slotted (split) nail of the exact same diameter most significantly improves which biomechanical property?

Explanation

The torsional rigidity of an intramedullary nail is dramatically reduced if the nail has a longitudinal slot. A solid, closed-section nail has significantly higher torsional rigidity compared to a slotted nail of identical material and size.

Question 50

A massive structural cortical allograft is used for reconstruction after an osteosarcoma resection. Which of the following best describes the healing process of this allograft compared to a cancellous autograft?

Explanation

Cortical allografts heal slowly via creeping substitution, a process where osteoclastic resorption precedes osteoblastic bone formation. This mechanism leaves the graft transiently weaker and structurally compromised, leading to a high risk of late fracture.

Question 51

On a standard stress-strain curve for a ductile biomaterial like stainless steel, what does the total area under the curve up to the point of failure represent?

Explanation

The total area under the stress-strain curve up to the exact point of fracture represents the material's toughness, which is the total energy it can absorb before failing. The area under just the linear elastic portion represents the resilience.

Question 52

Following a tightly reamed intramedullary nailing of a diaphyseal tibia fracture, the local diaphyseal blood supply undergoes a significant physiologic alteration. Which of the following best describes the resulting flow?

Explanation

Normal diaphyseal blood flow in adult long bones is centrifugal, moving from the medullary cavity to the periosteum. After reaming destroys the endosteal supply, the flow temporarily reverses to a centripetal direction to maintain cortical bone viability.

Question 53

A patient with severe osteoporosis is treated with romosozumab. This medication primarily increases bone formation by targeting sclerostin. Which cellular pathway is directly upregulated as a result of this inhibition?

Explanation

Sclerostin is an osteocyte-derived glycoprotein that inhibits the Wnt/beta-catenin signaling pathway, suppressing osteoblast activity. Romosozumab binds and neutralizes sclerostin, unblocking the Wnt pathway to strongly stimulate bone formation.

Question 54

During the remodeling phase of tendon healing, the biomechanical strength of the tendon significantly increases. Which molecular change is most responsible for this specific increase in tensile strength?

Explanation

The remodeling phase is characterized by decreased cellularity and the organization of Type I collagen fibers along the axis of mechanical stress. The formation of covalent cross-links between these Type I collagen fibrils is the primary driver of increased tensile strength.

Question 55

Alumina ceramics are frequently used as bearing surfaces in total hip arthroplasty. Compared to cobalt-chromium, which of the following is a key biomechanical advantage of ceramic bearings?

Explanation

Ceramics like alumina have extreme hardness and are highly scratch-resistant compared to metal alloys, resulting in exceptional wear characteristics and low friction. However, they are highly brittle and possess low fracture toughness, increasing the risk of catastrophic shattering.

Question 56

An orthopaedic implant manufacturer is designing a new cementless hip stem. They wish to use a metal with the lowest modulus of elasticity that still provides adequate fatigue strength to minimize stress shielding. Which of the following metals should they select?

Explanation

Titanium alloys have a modulus of elasticity roughly half that of stainless steel and one-third that of cobalt-chromium. This lower stiffness more closely mimics cortical bone, reducing stress shielding while maintaining excellent fatigue strength.

Question 57

When evaluating the biomechanics of an intramedullary nail for a diaphyseal femur fracture, increasing the diameter of a solid titanium nail from 10 mm to 11 mm will increase its torsional rigidity by approximately what percentage?

Explanation

The torsional rigidity of a solid cylinder is proportional to the radius to the fourth power (r^4). Increasing the diameter from 10 mm to 11 mm increases the rigidity by a factor of (11/10)^4, which is approximately 1.46, representing a 46% increase.

Question 58

In total joint arthroplasty, stress shielding of the proximal femur is primarily determined by the mismatch in which of the following material properties between the metallic implant and the host bone?

Explanation

Stress shielding occurs due to a mismatch in the modulus of elasticity (Young's modulus) between the stiffer metallic implant and the less stiff surrounding bone. This lack of physiologic loading leads to adaptive bone resorption according to Wolff's Law.

Question 59

Sclerostin, a glycoprotein produced by mature osteocytes, regulates bone mass primarily by inhibiting which of the following molecular pathways?

Explanation

Sclerostin binds to LRP5/6 receptors on osteoblasts, effectively inhibiting the canonical Wnt/beta-catenin signaling pathway. This inhibition leads to decreased osteoblastogenesis and a subsequent reduction in bone formation.

Question 60

According to Perren's strain theory, what is the maximum interfragmentary strain that permits the formation of lamellar bone via primary (direct) bone healing without the formation of a visible callus?

Explanation

Primary bone healing requires absolute stability with an interfragmentary strain of less than 2%, allowing osteonal cutting cones to cross the fracture gap. Strains between 2% and 10% permit secondary bone healing with endochondral callus formation.

Question 61

Which of the following biomechanical principles best distinguishes a locking plate construct from a conventional non-locking compression plate?

Explanation

Locking plates function as fixed-angle constructs where the screw heads lock into the plate, acting as a single beam. They do not rely on plate-to-bone friction, thereby preserving periosteal blood supply and providing superior hold in osteoporotic bone.

Question 62

Which of the following intracellular signaling molecules is directly phosphorylated upon the binding of Bone Morphogenetic Protein-2 (BMP-2) to its cell surface serine/threonine kinase receptor?

Explanation

BMP-2 binds to specific cell surface receptors, which then directly phosphorylate Smad 1, 5, and 8. These phosphorylated molecules complex with Smad 4 to translocate to the nucleus and regulate osteogenic gene transcription.

Question 63

When treating a highly comminuted diaphyseal femur fracture with bridge plating, increasing the working length of the plate has which of the following biomechanical effects?

Explanation

The working length is the distance between the two innermost screws spanning the fracture gap. Increasing this length decreases the overall bending stiffness of the construct, allowing micromotion that promotes robust secondary bone healing (callus formation).

Question 64

An ultra-high-molecular-weight polyethylene (UHMWPE) acetabular liner undergoes progressive, time-dependent deformation under the constant compressive load of a patient's body weight. This specific biomechanical phenomenon is known as:

Explanation

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

Question 65

In modern total hip arthroplasty, highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) is widely used. Compared to conventional UHMWPE, what is the primary biomechanical trade-off of the cross-linking process?

Explanation

Highly cross-linked UHMWPE significantly improves wear resistance, dramatically reducing the generation of wear debris and subsequent osteolysis. However, the radiation cross-linking process inherently decreases the material's fatigue strength, yield strength, and overall toughness.

Question 66

An orthopaedic surgeon revises a nonunion of a femur fracture by placing a titanium locking plate adjacent to a previously retained stainless steel intramedullary nail. Which of the following modes of implant failure is most likely accelerated by this specific combination of metals?

Explanation

Galvanic corrosion occurs when two dissimilar metals (such as titanium and stainless steel) are placed in electrical contact within an electrolytic environment like body fluids. This electrochemical process leads to accelerated corrosion of the less noble metal (stainless steel).

Question 67

Which of the following alterations in orthopaedic screw design most significantly increases its pullout strength when inserted into cancellous bone?

Explanation

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. Among the design variables, increasing the major diameter provides the most profound enhancement in pullout resistance.

Question 68

Demineralized bone matrix (DBM) is commonly utilized as a bone graft substitute in spinal fusions. Which of the following combinations of properties best describes its specific biological activity?

Explanation

Demineralized bone matrix (DBM) provides a collagen scaffold (osteoconductive) and retains bone morphogenetic proteins (BMPs) that stimulate local cells to form new bone (osteoinductive). Because it is sterilized and processed, it lacks viable donor cells and is therefore not osteogenic.

Question 69

During the resorption phase of bone remodeling, osteoclasts must firmly adhere to the bone surface. Which specific cell-surface integrin plays a critical role in forming the sealing zone necessary for effective bone resorption?

Explanation

Osteoclasts primarily utilize the Alpha-v Beta-3 integrin to bind to bone matrix proteins containing the RGD (arginine-glycine-aspartic acid) sequence, such as osteopontin. This vital interaction allows the formation of the sealing zone and the isolated acidic microenvironment required for resorption.

Question 70

Which specific alloying element is primarily responsible for the inherent corrosion resistance of 316L stainless steel utilized in orthopaedic implants?

Explanation

Chromium (comprising approximately 17-19% of 316L stainless steel) reacts with oxygen to form a stable, self-repairing passive oxide layer on the implant's surface. This passive layer is the primary mechanism defending against corrosion in physiologic environments.

Question 71

An orthopaedic surgeon elects to use a fracture plate that is exactly twice as thick as the originally planned plate, while maintaining the same width. According to the area moment of inertia for a rectangular cross-section, how much will the bending stiffness of the new construct increase?

Explanation

The bending stiffness of a rectangular plate is proportional to the area moment of inertia, which is calculated as (width x thickness^3) / 12. Doubling the thickness increases the bending stiffness by a factor of 2^3, resulting in an 8-fold increase.

Question 72

During the secondary healing of a fracture, intramembranous ossification occurs predominantly in which of the following specific microenvironments?

Explanation

Intramembranous ossification (direct bone formation without a cartilage intermediate) occurs at the periphery of the fracture site (the hard callus). This area features low interfragmentary strain and relatively high oxygen tension driven by the surrounding periosteal blood supply.

Question 73

According to Perren's strain theory, what range of interfragmentary strain is required to promote secondary bone healing via callus formation?

Explanation

Secondary bone healing (endochondral ossification with callus formation) is stimulated by moderate strain levels between 2% and 10%. Strain below 2% leads to primary bone healing, while strain above 10% typically results in a fibrous nonunion.

Question 74

When evaluating the biomechanical properties of an intramedullary nail, its torsional rigidity is proportional to the radius to what power?

Explanation

The torsional rigidity of a solid cylinder is governed by its polar moment of inertia, which is proportional to the radius to the fourth power (r^4). Therefore, small increases in nail diameter exponentially increase its resistance to torsional deformation.

Question 75

On a standard stress-strain curve for an orthopaedic implant material, the slope of the linear (elastic) portion represents which of the following biomechanical properties?

Explanation

The slope of the linear portion of a stress-strain curve defines the Modulus of Elasticity (Young's modulus), which indicates a material's stiffness. Toughness is the total area under the curve, and yield strength is the point at which plastic deformation begins.

Question 76

According to the principles of screw biomechanics, which of the following geometric modifications most significantly increases the pullout strength of a cortical bone screw?

Explanation

Screw pullout strength is primarily determined by the outer diameter, thread length of engagement, and bone shear strength. Increasing the outer diameter directly and linearly increases the volume of bone captured between threads, maximizing pullout strength.

Question 77

Which of the following bone graft options possesses all three essential properties for bone healing: osteoconduction, osteoinduction, and osteogenesis?

Explanation

Autograft (such as iliac crest) is the gold standard because it provides an osteoconductive scaffold, osteoinductive growth factors, and osteogenic live cells. Allografts and synthetics typically lack live osteoprogenitor cells (osteogenesis).

Question 78

During biomechanical testing of a viscoelastic tissue like articular cartilage, a constant load is applied, resulting in an immediate deformation that slowly continues to increase over time. This phenomenon is termed:

Explanation

Creep is the progressive deformation of a viscoelastic material over time under a constant applied load. Stress relaxation, conversely, is the decrease in stress over time when the material is held at a constant strain (deformation).

Question 79

Bone morphogenetic proteins (BMPs), specifically BMP-2 and BMP-7, initiate their intracellular signaling cascade primarily through which molecular pathway?

Explanation

BMPs bind to cell-surface serine/threonine kinase receptors, leading to the phosphorylation and activation of receptor-regulated SMADs (SMAD 1, 5, and 8). These form a complex with SMAD 4, which translocates to the nucleus to regulate osteogenic gene transcription.

Question 80

When applying a uniplanar external fixator to a tibial shaft fracture, which biomechanical adjustment will most dramatically increase the overall stiffness of the construct?

Explanation

External fixator stiffness is most sensitive to pin diameter because bending stiffness is proportional to the radius to the fourth power (r^4). Decreasing the bar-to-bone distance or spreading pins out also increases stability, but altering pin diameter has an exponential effect.

Question 81

Polymethylmethacrylate (PMMA) acts as a grout rather than an adhesive in total joint arthroplasty. Biomechanically, PMMA exhibits its greatest structural strength against which type of applied load?

Explanation

PMMA is highly resistant to compressive forces, making it an excellent grout for transferring physiological loads from an implant to the bone bed. It is notably weak under tensile and shear forces, which can lead to cement mantle failure.

Question 82

When examining the stress-strain curve of a healthy human ligament, the initial non-linear 'toe region' is anatomically explained by which of the following?

Explanation

The toe region reflects the stretching out, or 'uncrimping,' of the naturally wavy collagen fibers with initial load application. Once straightened, the ligament enters the linear elastic phase where the collagen backbone resists further strain.

Question 83

In a patient undergoing revision total hip arthroplasty for massive retroacetabular osteolysis, what is the primary mechanism of ultra-high-molecular-weight polyethylene (UHMWPE) wear that generated the inciting submicron particles?

Explanation

Adhesive wear occurs when microscopic asperities on the articulating surfaces temporarily bond and then tear away. This mechanism is responsible for producing the millions of submicron polyethylene particles that are phagocytized by macrophages, driving osteolysis.

Question 84

Unlike conventional compression plates, a locked plating construct provides fracture stabilization primarily through which biomechanical principle?

Explanation

Locked plates function as an 'internal external fixator' relying on the threaded fixed-angle interaction between the screw head and the plate hole. They do not require friction between the plate and bone, thus preserving periosteal blood supply.

Question 85

Nonsteroidal anti-inflammatory drugs (NSAIDs) are thought to negatively impact fracture healing. This delay is largely attributed to the inhibition of COX-2, which primarily impairs which stage of bone repair?

Explanation

COX-2 is upregulated in the early phases of fracture healing, producing prostaglandins essential for mesenchymal stem cell differentiation into osteoblasts. NSAID-mediated COX-2 inhibition disrupts this pathway, severely impairing endochondral ossification.

Question 86

A surgeon is performing bridge plating for a comminuted femoral shaft fracture. By intentionally leaving empty screw holes over the fracture zone to increase the 'working length' of the plate, the surgeon biomechanically achieves:

Explanation

Increasing the working length (distance between the innermost screws) decreases the construct's stiffness, distributing deformation over a longer segment. This decreases the overall strain percentage at the fracture gap, facilitating secondary bone healing.

Question 87

Trunnionosis, or failure at the modular head-neck taper junction of a total hip arthroplasty, is best described as mechanically assisted:

Explanation

Trunnionosis is primarily a form of mechanically assisted crevice corrosion. Micromotion (fretting) at the taper junction damages the protective oxide layer, accelerating local crevice corrosion in the fluid-filled, low-oxygen environment.

Question 88

The mechanical behavior of cortical bone varies depending on whether a load is applied longitudinally, transversely, or in shear. This characteristic is defined as:

Explanation

Anisotropy refers to a material having different mechanical properties depending on the direction of the applied load. Cortical bone is highly anisotropic, being much stronger in longitudinal compression than in transverse tension or shear.

Question 89

Achieving absolute stability through anatomic reduction and interfragmentary compression with a lag screw and neutralization plate specifically bypasses callus formation to promote which biological process?

Explanation

Absolute stability reduces interfragmentary strain to less than 2%, preventing the formation of soft callus. This mechanical environment promotes primary (osteonal) bone healing, characterized by osteoclastic cutting cones crossing the fracture directly.

Question 90

Low-intensity pulsed ultrasound (LIPUS) is utilized to accelerate fracture healing. The primary cellular mechanism by which mechanical acoustic waves are transduced into a biologic response involves the upregulation of:

Explanation

LIPUS imparts mechanical micromotion that is sensed by cell-surface integrins on osteoblasts and chondrocytes. This mechanotransduction cascade upregulates intracellular calcium, COX-2, and ultimately osteogenic gene expression to accelerate healing.

Question 91

A surgeon inadvertently utilizes a stainless steel plate with a titanium screw. The direct contact between these dissimilar metals in an electrolytic physiological fluid primarily predisposes the construct to:

Explanation

Galvanic corrosion occurs when two metals with differing electrochemical potentials are in direct contact within an electrolytic medium (like blood or tissue fluid). The less noble metal becomes the anode and preferentially corrodes.

Orthopedic Board Review: Set 861 - 100 High-Yield MCQs

Orthopedic Bone Healing & Biomechanics MCQs for ABOS Board Review

Key Takeaway

Orthopedic Bone Healing & Biomechanics MCQs for ABOS Board Review

Chapter Index