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1. General Principles & Basic Science MCQs

Practice Set 483 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 9641

Topic: Infection, Pharmacology & VTE

A 72-year-old male presents with sudden onset excruciating pain in his right great toe, which is exquisitely tender, swollen, and erythematous. He has a history of hypertension and is on a thiazide diuretic. Serum uric acid level is 9.5 mg/dL. Radiographs show soft tissue swelling but no erosions. He is diagnosed with acute gout. Which of the following is the most appropriate acute management strategy?

. Initiate allopurinol immediately to lower uric acid levels.
. Administer colchicine or NSAIDs, and consider a short course of oral corticosteroids if contraindicated.
. Surgical incision and drainage of the MTP joint to relieve pressure.
. Place the patient on a low-purine diet and advise rest.
. Refer for orthopedic consultation for possible joint aspiration and crystal analysis.

Correct Answer & Explanation

. Administer colchicine or NSAIDs, and consider a short course of oral corticosteroids if contraindicated.

Explanation

This is a classic presentation of acute gout. The goal of acute management is to reduce inflammation and pain. The first-line agents for acute gout are NSAIDs (e.g., indomethacin), colchicine, or oral corticosteroids. Colchicine is most effective if started within 36 hours of symptom onset. Oral corticosteroids are a good option if NSAIDs are contraindicated (e.g., renal insufficiency, peptic ulcer disease) or ineffective. Allopurinol (Option A) is a uric acid-lowering therapy (ULT) used for chronic gout management and prophylaxis, not for acute attacks. Initiating allopurinol during an acute attack can paradoxically worsen or prolong the attack by mobilizing uric acid crystals. Surgical drainage (Option C) is not indicated for acute gout unless septic arthritis is suspected. Low-purine diet and rest (Option D) are supportive measures but insufficient for acute, severe pain. Joint aspiration (Option E) is crucial for diagnosis if uncertain, but assuming the diagnosis is clear (as stated), the priority is symptom control.

Question 9642

Topic: Biology, Genetics & Bone Healing

A 10-year-old boy presents with progressive bilateral genu valgum. Physical examination reveals an intermalleolar distance of 10 cm with the knees touching. Radiographs show widening of the physes, metaphyseal flaring, and irregular physeal margins, particularly at the distal femurs and proximal tibias. Laboratory tests reveal low serum phosphate, elevated alkaline phosphatase, and normal serum calcium and PTH. What is the most likely diagnosis?

. Renal osteodystrophy.
. Vitamin D-dependent rickets Type I.
. Blount's disease.
. X-linked hypophosphatemia.
. Scurvy.

Correct Answer & Explanation

. X-linked hypophosphatemia.

Explanation

The clinical presentation (progressive genu valgum, widening physes, metaphyseal flaring) and laboratory findings (low serum phosphate, elevated alkaline phosphatase, normal serum calcium and PTH) are classic for X-linked hypophosphatemia (XLH), also known as Vitamin D-resistant rickets. XLH is the most common hereditary form of rickets, characterized by impaired renal phosphate reabsorption and decreased 1-alpha hydroxylation of Vitamin D, leading to chronic hypophosphatemia. Renal osteodystrophy (Option A) would typically involve abnormalities in calcium, phosphate, and PTH, often in the context of chronic kidney disease. Vitamin D-dependent rickets Type I (Option B) is characterized by low 1,25-dihydroxyvitamin D and typically presents with hypocalcemia. Blount's disease (Option C) is a growth disturbance of the medial proximal tibia, causing genu varum, not valgum, and is not primarily a metabolic bone disease. Scurvy (Option E) is Vitamin C deficiency and has different radiographic and biochemical features.

Question 9643

Topic: Biomechanics & Biomaterials

Considering the material properties of intramedullary nails, why might a titanium alloy nail be preferred over a stainless steel nail in certain clinical scenarios, from a biomechanical perspective?

. Higher ultimate tensile strength.
. Greater stiffness, leading to more rigid fixation.
. Lower modulus of elasticity, promoting load sharing.
. Superior fatigue life under cyclic loading.
. Increased resistance to bacterial adhesion.

Correct Answer & Explanation

. Lower modulus of elasticity, promoting load sharing.

Explanation

Titanium alloys have a lower modulus of elasticity compared to stainless steel. This property makes them biomechanically more compatible with bone, as their stiffness is closer to that of cortical bone. A lower modulus leads to less stress shielding, allowing more physiological stress to be transmitted to the healing bone, which can promote better callus formation and reduce the risk of non-union or refracture after implant removal. While titanium has good fatigue properties, and stainless steel might have slightly higher ultimate tensile strength in some grades, the primary biomechanical advantage often cited for titanium in IM nailing is its lower elastic modulus and consequent improved load sharing.

Question 9644

Topic: Biomechanics & Biomaterials

What is the primary biomechanical difference between a solid and a cannulated intramedullary nail of the same external diameter?

. Solid nails are easier to insert over a guidewire.
. Cannulated nails have greater torsional stiffness.
. Solid nails generally have greater bending and torsional stiffness.
. Cannulated nails are less prone to stress shielding.
. Solid nails are associated with a higher risk of infection.

Correct Answer & Explanation

. Solid nails generally have greater bending and torsional stiffness.

Explanation

For a given external diameter, a solid nail will always have a greater moment of inertia and polar moment of inertia than a cannulated nail. This means a solid nail will be inherently stiffer in both bending and torsion compared to a cannulated nail of the same outer dimension, as material is removed from the center of the cannulated nail. The choice often balances this biomechanical difference against the surgical advantage of guidewire insertion for cannulated nails.

Question 9645

Topic: Biomechanics & Biomaterials

Which biomechanical property is most enhanced by using a reaming technique during IM nailing, leading to the use of a larger nail?

. Torsional resistance.
. Axial stiffness.
. Bending stiffness.
. Load sharing.
. All of the above.

Correct Answer & Explanation

. All of the above.

Explanation

Reaming allows for the insertion of a larger diameter nail. A larger diameter nail significantly increases the moment of inertia (resistance to bending) and the polar moment of inertia (resistance to torsion). Consequently, this improves the nail's bending stiffness, torsional stiffness, and axial stiffness (as cross-sectional area increases). Furthermore, a larger nail fills the medullary canal more completely, enhancing bone-nail contact and thus improving load sharing between the implant and the bone. Therefore, all listed biomechanical properties are enhanced.

Question 9646

Topic: Biology, Genetics & Bone Healing

What is the biomechanical reason that IM nailing is generally preferred over plating for most diaphyseal long bone fractures?

. IM nails provide absolute stability, promoting direct healing.
. IM nails are less prone to infection due to their internal placement.
. IM nails offer load-sharing, reducing stress shielding compared to plates.
. Plates require larger incisions and more soft tissue stripping.
. IM nails prevent rotational motion completely.

Correct Answer & Explanation

. IM nails offer load-sharing, reducing stress shielding compared to plates.

Explanation

The primary biomechanical advantage of IM nailing over plating for diaphyseal fractures is its load-sharing capability. By being centrally located, the nail shares axial and bending loads with the bone, allowing the bone to be physiologically stressed. This reduces stress shielding, promotes robust secondary callus formation, and often leads to faster and more reliable healing compared to plates which typically function as load-bearing devices and are more prone to stress shielding. While soft tissue stripping is a surgical advantage, load-sharing is a direct biomechanical benefit.

Question 9647

Topic: Biology, Genetics & Bone Healing

In a scenario of a non-union after IM nailing, which change to the locking screw configuration is most likely to promote healing if the non-union is hypertrophic?

. Adding more locking screws to increase stability.
. Converting from static to dynamic locking.
. Replacing all locking screws with larger diameter screws.
. Removing the nail and performing plate fixation.
. Changing the nail material to titanium.

Correct Answer & Explanation

. Converting from static to dynamic locking.

Explanation

A hypertrophic non-union implies adequate biological potential but too much motion. In this scenario, converting from static to dynamic locking (by removing one set of screws) allows for controlled axial micromotion and increased load transfer across the fracture site. This controlled compression and appropriate interfragmentary strain can stimulate callus maturation and bridging, leading to consolidation. Adding more screws would increase stiffness, which is usually not the problem with hypertrophic non-unions.

Question 9648

Topic: Biomechanics & Biomaterials

What biomechanical factor is most responsible for the superior fatigue life of an IM nail compared to a plate for similar diaphyseal fractures?

. The material properties of the nail (e.g., titanium vs. stainless steel).
. The intramedullary location of the nail closer to the neutral axis of the bone.
. The larger cross-sectional area of a typical IM nail compared to a plate.
. The ability of the nail to be reamed, creating a larger canal.
. The reduced soft tissue stripping associated with IM nailing.

Correct Answer & Explanation

. The intramedullary location of the nail closer to the neutral axis of the bone.

Explanation

The intramedullary location of the nail, placing it closer to the neutral mechanical axis of the bone, is a key reason for its superior fatigue life. This central position minimizes the bending stresses experienced by the nail because it is subjected to lower bending moments and compressive/tensile stresses compared to an eccentrically placed plate. Plates experience higher peak stresses on their surfaces, making them more prone to fatigue failure. Load sharing also reduces the overall stress on the nail.

Question 9649

Topic: Biology, Genetics & Bone Healing

What biomechanical concept explains why a small amount of callus formation is desirable around an IM nail, as opposed to direct bone healing?

. Callus formation indicates primary bone healing, which is faster.
. Callus increases the load-bearing capacity of the nail.
. Callus is formed in response to controlled micromotion (relative stability) and contributes to the progressive stiffening and healing of the fracture.
. Direct bone healing leads to stress shielding.
. Callus prevents infection.

Correct Answer & Explanation

. Callus is formed in response to controlled micromotion (relative stability) and contributes to the progressive stiffening and healing of the fracture.

Explanation

Intramedullary nailing provides relative stability, allowing for controlled micromotion at the fracture site. This micromotion, within a specific range of interfragmentary strain, is a potent stimulus for secondary bone healing, which involves callus formation. The progressive maturation and mineralization of this callus lead to the gradual stiffening and eventual consolidation of the fracture. Direct bone healing, requiring absolute stability, typically results in minimal or no visible callus.

Question 9650

Topic: Biomechanics & Biomaterials

Which type of material is typically associated with lower Young's Modulus, making it more 'bone-friendly' in terms of stress shielding for IM nails?

. Cobalt-chromium alloys.
. Stainless steel.
. Titanium alloys.
. Nitinol.
. Zirconium.

Correct Answer & Explanation

. Titanium alloys.

Explanation

Titanium alloys (e.g., Ti-6Al-4V) generally have a lower Young's Modulus (approximately 110 GPa) compared to stainless steel (approximately 200 GPa) or cobalt-chromium alloys (approximately 230 GPa). A lower Young's Modulus means the implant is less stiff and closer to the elastic modulus of cortical bone (17-20 GPa), thus reducing the magnitude of stress shielding. Less stress shielding means the bone carries more physiological load, which is thought to be beneficial for bone remodeling and strength. While titanium still causes some stress shielding, it's less pronounced than with stiffer materials.

Question 9651

Topic: Biomechanics & Biomaterials

What is the primary biomechanical difference between a cannulated and a solid intramedullary nail of the same outer diameter and material?

. Cannulated nails are stiffer in bending.
. Solid nails are less resistant to fatigue failure.
. Cannulated nails have a higher area moment of inertia.
. Solid nails generally have greater bending and torsional stiffness.
. Cannulated nails allow for primary bone healing.

Correct Answer & Explanation

. Solid nails generally have greater bending and torsional stiffness.

Explanation

A solid intramedullary nail, having a greater cross-sectional area of material, possesses a higher area moment of inertia compared to a cannulated nail of the same outer diameter. This translates directly into greater bending and torsional stiffness for the solid nail. The higher stiffness makes solid nails more resistant to deformation and potentially more resistant to fatigue failure. Cannulation, while allowing for guidewire insertion, inherently reduces the material in the cross-section, thus lowering its stiffness. Neither type promotes primary bone healing over the other, as both aim for relative stability.

Question 9652

Topic: 1. General Principles & Basic Science

What is the biomechanical significance of 'nail-bone interface friction' in an unreamed IM nailing construct?

. It is the primary mechanism for rotational stability.
. It determines the overall length of the construct.
. It is negligible and has no biomechanical impact.
. It contributes to bending and torsional stability, especially in unreamed nails where canal fill is less.
. It increases the risk of implant infection.

Correct Answer & Explanation

. It contributes to bending and torsional stability, especially in unreamed nails where canal fill is less.

Explanation

While not as significant as the interlocked construct or tight reamed fit, nail-bone interface friction can contribute to bending and torsional stability, particularly in unreamed nails where there is less canal fill and inherently lower stiffness. The friction helps to resist motion between the nail and the endosteal surface. However, it is not the primary mechanism for rotational stability (that's interlocking screws) and is generally less robust than reamed cortical contact or interlocking screws for overall stability. It's not negligible, but its contribution is modest compared to other factors.

Question 9653

Topic: Biomechanics & Biomaterials

Biomechanically, how does a smaller diameter IM nail (e.g., for pediatric patients or smaller adults) generally compare to a larger diameter nail?

. It has a higher area moment of inertia.
. It is more rigid in bending and torsion.
. It is more susceptible to plastic deformation and fatigue failure.
. It provides greater resistance to screw pullout.
. It causes more stress shielding of the bone.

Correct Answer & Explanation

. It is more susceptible to plastic deformation and fatigue failure.

Explanation

The bending and torsional stiffness of a nail are highly dependent on its diameter (specifically, the fourth power of the radius for a solid cylinder). Therefore, a smaller diameter nail will have a significantly lower area moment of inertia, making it less rigid in bending and torsion. This reduced stiffness means it is more susceptible to elastic and plastic deformation under load, and thus has a higher risk of fatigue failure, especially if fracture healing is delayed. It also provides less resistance to screw pullout as the nail is less robust and provides less support for the screws.

Question 9654

Topic: Biomechanics & Biomaterials

Biomechanical studies on nail materials indicate which property is directly related to the flexibility of the implant and its potential for stress shielding?

. Tensile strength.
. Yield strength.
. Young's Modulus.
. Hardness.
. Fatigue limit.

Correct Answer & Explanation

. Young's Modulus.

Explanation

Young's Modulus (or modulus of elasticity) is a measure of a material's stiffness or resistance to elastic deformation under stress. A higher Young's Modulus indicates a stiffer material. Biomechanically, an implant with a Young's Modulus significantly higher than bone (like stainless steel compared to titanium) will bear a disproportionate amount of the load, leading to stress shielding of the adjacent bone. A lower Young's Modulus (e.g., titanium) brings the implant's stiffness closer to that of bone, reducing stress shielding. Tensile strength, yield strength, hardness, and fatigue limit are important material properties but Young's Modulus directly reflects stiffness and thus flexibility and stress shielding potential.

Question 9655

Topic: Surgical Anatomy & Approaches

In proximal tibia fractures, antegrade IM nailing has specific biomechanical challenges. Which of the following is most accurately described as a biomechanical risk for this approach?

. High risk of superior gluteal artery injury.
. Difficulty achieving sufficient purchase in the proximal metaphysis, leading to varus/valgus malalignment.
. Increased risk of radial nerve palsy.
. High incidence of implant migration into the knee joint.
. Inability to achieve rotational stability.

Correct Answer & Explanation

. Difficulty achieving sufficient purchase in the proximal metaphysis, leading to varus/valgus malalignment.

Explanation

Antegrade nailing of proximal tibia fractures is challenging. The wide proximal medullary canal, combined with often comminuted metaphyseal bone and the natural valgus angulation of the proximal tibia, makes it difficult to achieve stable fixation of the proximal fragment. This can lead to loss of reduction, particularly in the coronal plane, resulting in varus or valgus malalignment. Modern nails for this indication often feature multi-planar proximal locking options, blade configurations, or expanded proximal diameters to address this biomechanical challenge. Gluteal artery and radial nerve injuries are not risks for tibial nailing. Implant migration into the knee is a risk if the nail is too long, but not the primary challenge of proximal fragment stability.

Question 9656

Topic: 1. General Principles & Basic Science

When considering the biomechanics of retrograde IM nailing for distal femoral fractures, what is a primary concern regarding knee joint mechanics?

. Increased risk of patellofemoral pain and arthritis from the nail entry portal.
. Reduced knee flexion due to proximal nail protrusion.
. Damage to the extensor mechanism during insertion.
. Increased risk of deep vein thrombosis.
. Altered gait patterns from a shorter working length.

Correct Answer & Explanation

. Increased risk of patellofemoral pain and arthritis from the nail entry portal.

Explanation

Retrograde intramedullary nailing for distal femoral fractures involves an entry portal through the intercondylar notch, often requiring a patellar tendon split. Biomechanically, this can disrupt the load-bearing surfaces of the patellofemoral joint and lead to scarring or irritation from the nail's proximal end, increasing the risk of patellofemoral pain and potentially contributing to post-traumatic arthritis. While damage to the extensor mechanism can occur, patellofemoral pain is a well-recognized specific biomechanical consequence related to the entry point and implant presence. Reduced knee flexion can occur, but the pain aspect is more directly related to the biomechanics of the joint itself. DVT is a general surgical risk, not a specific biomechanical concern for knee mechanics.

Question 9657

Topic: Biology, Genetics & Bone Healing

The concept of 'biological fixation' with IM nails is rooted in which biomechanical principle?

. Rigid stabilization to promote primary cortical healing.
. Minimizing soft tissue disruption to preserve vascularity and promote secondary healing.
. Maximizing the implant-bone interface for immediate load transfer.
. Using porous-coated nails to encourage osteointegration.
. Achieving absolute stability to prevent micromotion.

Correct Answer & Explanation

. Minimizing soft tissue disruption to preserve vascularity and promote secondary healing.

Explanation

Biological fixation, particularly relevant to IM nailing, emphasizes minimizing iatrogenic soft tissue disruption during surgery. This approach aims to preserve the existing periosteal and endosteal blood supply, which is critical for fracture healing. By maintaining a healthy biological environment around the fracture, the IM nail, acting as a load-sharing device providing relative stability, encourages robust secondary bone healing through callus formation. It is less about rigid stabilization or osteointegration (though porous nails exist), and more about supporting the body's natural healing processes.

Question 9658

Topic: 1. General Principles & Basic Science

What biomechanical property is most relevant when selecting an IM nail diameter for an osteoporotic bone?

. The material's corrosion resistance.
. The nail's surface finish.
. The largest possible diameter to maximize canal fill and stiffness.
. The smallest possible diameter to minimize bone removal.
. The nail's ability to be sterilized.

Correct Answer & Explanation

. The largest possible diameter to maximize canal fill and stiffness.

Explanation

In osteoporotic bone, the intrinsic strength of the bone itself is diminished, making the implant responsible for bearing a greater proportion of the load. Biomechanically, it is critical to maximize the stiffness and strength of the IM nail construct. This is best achieved by using the largest possible nail diameter that can safely fit the reamed medullary canal, as this maximizes the area moment of inertia and thus the bending and torsional stiffness, providing the most robust support to the weakened bone. Smaller diameters would lead to an even more unstable construct and higher risk of implant failure or loss of reduction.

Question 9659

Topic: Biomechanics & Biomaterials

What biomechanical property of titanium nails contributes to their perceived advantage in fracture healing compared to stainless steel nails?

. Higher density.
. Increased hardness.
. Lower Young's Modulus, reducing stress shielding.
. Superior fatigue strength.
. Greater coefficient of friction with bone.

Correct Answer & Explanation

. Lower Young's Modulus, reducing stress shielding.

Explanation

Titanium alloys have a Young's Modulus (approximately 110 GPa) that is closer to that of cortical bone (17-20 GPa) than stainless steel (approximately 200 GPa). This lower stiffness means that titanium nails induce less stress shielding of the bone. Biomechanically, allowing the bone to carry more physiological load promotes its natural remodeling and healing processes. While titanium also has excellent corrosion resistance and biocompatibility, its lower Young's Modulus is a key biomechanical advantage cited in promoting fracture healing by optimizing load transfer with the bone. Superior fatigue strength is often debated and depends on specific alloy and design.

Question 9660

Topic: Surgical Anatomy & Approaches

A 25-year-old male undergoes antegrade intramedullary nailing for a mid-shaft femoral fracture. Two weeks post-operatively, he develops a foot drop and diminished sensation over the dorsum of the foot. Which nerve is most likely injured?

. A. Sciatic nerve.
. B. Femoral nerve.
. C. Common peroneal nerve.
. D. Tibial nerve.
. E. Obturator nerve.

Correct Answer & Explanation

. C. Common peroneal nerve.

Explanation

A foot drop (weakness in dorsiflexion and eversion of the foot) and sensory deficit over the dorsum of the foot are classic signs of common peroneal nerve injury. While sciatic nerve injury (A) can also cause foot drop, the common peroneal nerve (a branch of the sciatic nerve) is more susceptible to injury around the knee, often due to positioning on the fracture table or direct compression, or traction during manipulation. Femoral (B), tibial (D), and obturator (E) nerve injuries present with different motor and sensory deficits.

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