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1. General Principles & Basic Science MCQs

Practice Set 81 of 789

This practice set contains high-yield board review questions covering key concepts in 1. General Principles & Basic Science. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 1601

Topic: 1. General Principles & Basic Science

An 80-year-old female with severe osteoporosis presents with a low-energy transverse subtrochanteric femoral fracture. Compared to a healthy young adult's femur, the osteoporotic bone's reduced resistance to bending, leading to this fracture, is primarily due to a decrease in which biomechanical parameter?

. Bone mineral density alone
. Young's Modulus of cortical bone
. Bone length
. Area Moment of Inertia of the bone cross-section
. Periosteal bone formation rate

Correct Answer & Explanation

. Area Moment of Inertia of the bone cross-section

Explanation

Correct Answer: DOsteoporosis leads to significant thinning of the cortical bone and loss of trabecular architecture. Biomechanically, this translates to a substantial decrease in the Area Moment of Inertia (MOI) of the bone's cross-section. The MOI is a geometric property that directly quantifies resistance to bending and torsion. While reduced bone mineral density (BMD) is a hallmark of osteoporosis, its biomechanical consequence in terms of bending resistance is primarily mediated through the reduction in MOI, as less material is distributed effectively away from the neutral axis. Young's Modulus of the cortical bone material itself may not change as dramatically as its geometric distribution, nor do bone length or periosteal bone formation primarily explain the acute reduction in bending resistance in mature osteoporotic bone.

Question 1602

Topic: Biomechanics & Biomaterials
A biomechanical engineer is evaluating two designs for a new generation of intramedullary nails for tibial fractures. Nail A is a solid rod with a 10mm diameter. Nail B is a cannulated rod with an outer diameter of 12mm and an inner diameter of 8mm. Assuming identical material properties, which nail provides superior resistance to bending and torsion?
. Nail A, due to greater solid mass
. Nail B, due to its larger outer diameter and material distribution
. Both nails offer equal resistance if their cross-sectional areas are identical
. Nail A, if its material's Young's modulus is higher
. Nail B, only if it is made of a stiffer material

Correct Answer & Explanation

. Nail B, due to its larger outer diameter and material distribution

Explanation

Nail B will provide superior resistance to bending and torsion. The Area Moment of Inertia (MOI) is much greater for a cannulated structure where material is distributed further from the neutral axis, even if its cross-sectional area is less than or equal to a solid rod. Nail B has a significantly larger MOI, meaning its material is distributed more effectively further from the center, which dramatically increases its resistance to bending and torsion compared to Nail A.

Question 1603

Topic: Biomechanics & Biomaterials

When analyzing the biomechanics of a pedicle screw construct in the spine, how does increasing the diameter of a pedicle screw influence the overall stiffness of the construct against bending forces?

. Linearly increases stiffness
. Decreases stiffness by increasing flexibility
. Increases stiffness exponentially due to its effect on the Area Moment of Inertia
. Only affects the pullout strength, not bending stiffness
. Has no significant impact on bending stiffness

Correct Answer & Explanation

. Increases stiffness exponentially due to its effect on the Area Moment of Inertia

Explanation

Correct Answer: CIncreasing the diameter of a pedicle screw significantly increases the stiffness of the construct against bending forces. This is due to its exponential effect on the Area Moment of Inertia (MOI) of the screw itself. For a circular cross-section, MOI is proportional to the diameter to the fourth power (I ~ d^4). Therefore, even a small increase in diameter leads to a substantial increase in the screw's individual bending resistance, which is a critical component of the overall construct's bending and torsional rigidity. While a larger diameter also increases pullout strength, its impact on bending stiffness via MOI is exponential.

Question 1604

Topic: 1. General Principles & Basic Science

During fracture healing, a bridging callus forms around the fracture site. The progressive increase in the mechanical stability of the healing construct is directly proportional to the increase in which geometric property of the callus?

. Its overall length
. Its degree of vascularization
. Its stiffness (Young's Modulus)
. Its Area Moment of Inertia
. Its cellularity

Correct Answer & Explanation

. Its Area Moment of Inertia

Explanation

Correct Answer: DAs a bridging callus forms and matures, its primary contribution to the increased mechanical stability of the healing fracture is the progressive increase in its Area Moment of Inertia. The callus effectively increases the overall diameter of the bone at the fracture site, distributing the bone tissue (initially woven bone, later lamellar) further from the neutral axis. This geometric change dramatically enhances the construct's resistance to bending and torsional forces. While the stiffness (material property, Young's Modulus) of the callus also increases as it matures, the geometric effect of MOI is paramount for overall structural integrity and resistance to deformation.

Question 1605

Topic: Biomechanics & Biomaterials

A 60-year-old patient with a comminuted mid-shaft humeral fracture is treated with an intramedullary nail. The surgeon notes that the nail chosen has a significantly higher modulus of elasticity than cortical bone. What is the most likely biomechanical consequence of this material mismatch on fracture healing?

. Increased interfragmentary strain, promoting hypertrophic non-union.
. Reduced risk of implant fatigue failure due to increased stiffness.
. Significant stress shielding of the fracture site, potentially leading to delayed union or atrophic non-union.
. Enhanced load sharing between the nail and the bone, accelerating healing.
. Improved rotational stability of the construct due to material properties.

Correct Answer & Explanation

. Significant stress shielding of the fracture site, potentially leading to delayed union or atrophic non-union.

Explanation

Correct Answer: CThe modulus of elasticity (Young's modulus) is a measure of a material's stiffness. If an intramedullary nail has a significantly higher modulus of elasticity than cortical bone, it means the nail is much stiffer than the bone. This stiffness mismatch leads to significant stress shielding. Stress shielding occurs when the rigid implant carries a disproportionate amount of the physiological load, thereby shielding the adjacent bone from normal mechanical stress. According to Wolff's Law, bone requires mechanical stress to remodel and heal effectively. Insufficient stress due to stress shielding can inhibit callus formation and maturation, potentially leading to delayed union or an atrophic non-union (where there is little or no callus formation).Option A is incorrectbecause a very stiff implant would reduce, not increase, interfragmentary strain. Hypertrophic non-union is typically associated with excessive motion, not excessive stiffness.Option B is incorrectbecause while a stiffer implant might seem more robust, excessive stiffness can lead to stress shielding, which weakens the bone and can ultimately contribute to implant failure or refracture after removal if the bone has not healed adequately.Option D is incorrectbecause a high stiffness mismatch reduces load sharing; the nail takes too much load, preventing the bone from experiencing physiological stress.Option E is incorrectbecause while overall stiffness contributes to stability, the primary mechanism for rotational stability comes from the interlocking screws, not solely the material's modulus of elasticity.

Question 1606

Topic: 1. General Principles & Basic Science

To optimize the pullout strength of a cortical bone screw in osteoporotic bone without changing the length of engagement, which of the following geometric modifications to the screw design is most effective?

. Increasing the core diameter
. Increasing the pitch
. Decreasing the thread depth
. Increasing the outer thread diameter
. Decreasing the pitch

Correct Answer & Explanation

. Increasing the outer thread diameter

Explanation

Screw pullout strength is directly proportional to the outer thread diameter and the length of engagement in the bone. Increasing the outer diameter maximizes the volume of bone caught between the threads, thereby increasing resistance to pullout.

Question 1607

Topic: Biomechanics & Biomaterials

An orthopedic surgeon is choosing between a titanium and a stainless steel plate of identical dimensions for fracture fixation. Which statement correctly describes a biomechanical advantage of the titanium implant?

. Titanium has a lower modulus of elasticity, closer to that of cortical bone, reducing stress shielding.
. Titanium has a significantly higher yield strength than stainless steel.
. Stainless steel has a lower modulus of elasticity, making it less brittle.
. Stainless steel undergoes less galvanic corrosion when mixed with other metals.
. Titanium has greater ductility, allowing for more bending cycles before failure.

Correct Answer & Explanation

. Titanium has a lower modulus of elasticity, closer to that of cortical bone, reducing stress shielding.

Explanation

Titanium alloys have a modulus of elasticity approximately half that of stainless steel, making them closer to the modulus of cortical bone. This allows more physiological load transfer to the bone, thereby reducing stress shielding and the risk of osteopenia.

Question 1608

Topic: Biomechanics & Biomaterials

A pedestrian struck by a high-speed vehicle presents with a severely comminuted femur fracture, whereas a patient suffering a low-energy fall presents with a simple transverse fracture. Which biomechanical property of bone explains this difference in fracture patterns?

. Bone behaves as a purely elastic material under high physiological loads.
. Bone becomes significantly more ductile at high strain rates.
. Bone loses its biphasic properties when subjected to direct trauma.
. High strain rates progressively decrease the modulus of elasticity of cortical bone.
. Bone is viscoelastic, meaning it absorbs more energy and becomes stiffer at high strain rates.

Correct Answer & Explanation

. Bone is viscoelastic, meaning it absorbs more energy and becomes stiffer at high strain rates.

Explanation

Bone exhibits viscoelastic behavior, making its mechanical properties dependent on the rate of loading. At higher strain rates, bone becomes stiffer, stronger, and absorbs more energy before failure, leading to explosive comminution when the energy is eventually released.

Question 1609

Topic: Biomechanics & Biomaterials

A 45-year-old patient requires hardware removal due to localized pain over a healed fracture site. Intraoperatively, the surgeon discovers significant corrosion where a stainless steel screw was inadvertently placed through a titanium plate. What is the mechanism of this material failure?

. Fretting corrosion
. Galvanic corrosion
. Crevice corrosion
. Pitting corrosion
. Stress corrosion cracking

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals are placed in physical contact within an electrolyte solution (such as body fluids). The electrochemical potential difference drives an ion exchange, leading to accelerated degradation of the less noble metal.

Question 1610

Topic: Biomechanics & Biomaterials

During fracture fixation, a lag screw is tightly secured across two bone fragments. Over several weeks, the compressive force across the fracture line gradually decreases, even though the screw does not change in length. Which biomechanical property of bone explains this phenomenon?

. Stress relaxation
. Creep
. Fatigue
. Hysteresis
. Anisotropy

Correct Answer & Explanation

. Stress relaxation

Explanation

Stress relaxation is the decrease in stress (compressive force) over time when a viscoelastic material (bone) is held at a constant strain (deformation from the tightened screw). Creep, by contrast, is progressive deformation under a constant load.

Question 1611

Topic: Biomechanics & Biomaterials

Cortical bone has a higher ultimate strength in compression than in tension, and is weakest in shear. This dependence of mechanical properties on the direction of applied loading is known as:

. Viscoelasticity
. Anisotropy
. Isotropy
. Plasticity
. Ductility

Correct Answer & Explanation

. Anisotropy

Explanation

Anisotropy refers to a material exhibiting different mechanical properties depending on the direction of the applied load. Because of the longitudinal orientation of osteons, bone is highly anisotropic, handling axial compression far better than transverse shear.

Question 1612

Topic: Biomechanics & Biomaterials

A biomedical engineer is testing a new orthopedic alloy in a laboratory. On the resulting stress-strain curve, the specific point where the material transitions from elastic (recoverable) deformation to plastic (permanent) deformation is called the:

. Ultimate tensile strength
. Failure point
. Modulus of elasticity
. Proportional limit (yield point)
. Toughness

Correct Answer & Explanation

. Proportional limit (yield point)

Explanation

The proportional limit, or yield point, marks the end of the elastic region on a stress-strain curve. Loads applied beyond this point will cause permanent, non-recoverable plastic deformation of the implant.

Question 1613

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory, what level of interfragmentary strain at the fracture site is required to allow for primary (osteonal) bone healing?

. Greater than 15%
. Less than 2%
. Between 2% and 10%
. Between 10% and 15%
. Primary healing is independent of the strain level.

Correct Answer & Explanation

. Less than 2%

Explanation

Perren's strain theory states that primary bone healing without callus formation occurs only under conditions of absolute stability, which requires an interfragmentary strain of less than 2%. Strains between 2% and 10% result in secondary healing via callus formation.

Question 1614

Topic: 1. General Principles & Basic Science
A surgeon is evaluating two conventional non-locking plates made of the same stainless steel alloy. Plate B has the exact same width as Plate A, but is twice as thick. How much stiffer in bending is Plate B compared to Plate A?
. 2 times
. 4 times
. 8 times
. 16 times
. 32 times

Correct Answer & Explanation

. 8 times

Explanation

The bending stiffness of a rectangular plate is proportional to its area moment of inertia, calculated by the formula (width × thickness^3) / 12. Therefore, doubling the thickness increases the bending stiffness by a factor of eight (2^3).

Question 1615

Topic: Biomechanics & Biomaterials

Cortical bone exhibits viscoelastic properties, meaning its biomechanical behavior changes depending on the rate of loading. During a high-speed motor vehicle collision, a femur is loaded at a very high strain rate. Compared to a low-energy fall, how does the bone behave?

. It becomes less stiff and absorbs less energy before failure.
. It becomes stiffer and absorbs more energy before fracturing.
. Its modulus of elasticity decreases significantly.
. It undergoes plastic deformation exclusively without elastic deformation.
. It exhibits increased ductility and bends further before breaking.

Correct Answer & Explanation

. It becomes stiffer and absorbs more energy before fracturing.

Explanation

Because bone is viscoelastic, it becomes stiffer and stronger at higher strain rates, allowing it to absorb more energy before failure. When it eventually fails, the energy release results in highly comminuted fractures.

Question 1616

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory, fracture healing is dictated by the amount of mechanical strain at the fracture gap. For primary (direct) bone healing to occur without the formation of a provisional callus, the interfragmentary strain must be kept below what threshold?

. 2%
. 10%
. 15%
. 30%
. 50%

Correct Answer & Explanation

. 2%

Explanation

Primary bone healing via Haversian remodeling requires absolute stability with interfragmentary strain less than 2%. Strains between 2% and 10% promote secondary healing via callus, while strains above 10% typically lead to nonunion.

Question 1617

Topic: Biomechanics & Biomaterials

An orthopaedic implant engineer is comparing the fatigue properties of different orthopaedic biomaterials. Which of the following materials has a Young's Modulus of Elasticity closest to that of human cortical bone, thereby theoretically reducing stress shielding?

. Cobalt-Chromium alloy
. 316L Stainless Steel
. Titanium alloy (Ti-6Al-4V)
. Polymethylmethacrylate (PMMA)
. Zirconia ceramic

Correct Answer & Explanation

. Titanium alloy (Ti-6Al-4V)

Explanation

Titanium alloy has a Young's Modulus (approx. 110 GPa) that is closer to cortical bone (15-20 GPa) than stainless steel (200 GPa) or Cobalt-Chromium (240 GPa). This closer matching reduces stress shielding but makes the implant more susceptible to notch sensitivity.

Question 1618

Topic: Biomechanics & Biomaterials

During a revision total hip arthroplasty, the surgeon notices significant corrosion at the modular head-neck junction. The femoral stem is made of a titanium alloy, while the modular head is cobalt-chromium. Which type of corrosion is most directly caused by the pairing of these dissimilar metals?

. Crevice corrosion
. Pitting corrosion
. Fretting corrosion
. Galvanic corrosion
. Stress corrosion cracking

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in contact within a conductive fluid (like body fluids). It causes accelerated corrosion of the less noble metal.

Question 1619

Topic: Biomechanics & Biomaterials

A mechanical test is performed on an anterior cruciate ligament (ACL) graft. The stress-strain curve initially demonstrates a non-linear 'toe region' before becoming linear. What microstructural event is responsible for this toe region?

. Plastic deformation of the collagen fibers
. Microscopic failure of cross-links
. Uncrimping of the resting collagen fibrils
. Water extrusion from the proteoglycan matrix
. Yield point propagation

Correct Answer & Explanation

. Uncrimping of the resting collagen fibrils

Explanation

The non-linear 'toe region' at the beginning of a ligament's stress-strain curve corresponds to the uncrimping (straightening) of the naturally wavy collagen fibrils as initial tension is applied.

Question 1620

Topic: Biomechanics & Biomaterials

Nine months following open reduction and internal fixation of a diaphyseal femur fracture, a patient returns with sudden thigh pain. Radiographs reveal a broken plate and persistent nonunion. The plate failure occurred despite no single load exceeding the material's ultimate tensile strength. This failure mechanism is termed:

. Creep
. Stress relaxation
. Ductile failure
. Fatigue failure
. Galvanic failure

Correct Answer & Explanation

. Fatigue failure

Explanation

Fatigue failure occurs when a material breaks under repeated, cyclical loading at stress levels well below its ultimate tensile strength. It is commonly seen in orthopaedic implants when delayed union or nonunion subjects the hardware to prolonged cyclical stress.

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