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1. General Principles & Basic Science MCQs

Practice Set 480 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 9581

Topic: Biomechanics & Biomaterials

A surgeon is considering the use of a conventional 316L stainless steel plate versus a titanium plate for internal fixation. Which of the following is a distinct advantage of a titanium plate over a stainless steel plate in the context of long-term implantation?

. Higher yield strength, allowing for smaller implant profiles.
. Significantly greater fatigue resistance.
. Reduced modulus of elasticity, potentially mitigating stress shielding.
. Lower cost and easier manufacturing process.
. Superior radio-opacity, making imaging easier.

Correct Answer & Explanation

. Reduced modulus of elasticity, potentially mitigating stress shielding.

Explanation

Titanium (e.g., Ti-6Al-4V) has a lower modulus of elasticity compared to stainless steel. This property makes titanium more 'bone-friendly' as it more closely matches the elastic modulus of cortical bone. This congruence can help reduce the magnitude of stress shielding, a phenomenon where the implant bears too much load, leading to disuse osteopenia in the underlying bone. Stainless steel generally has higher yield strength and fatigue resistance in traditional bulk forms, though modern titanium alloys are very strong. Titanium is more expensive and less radio-opaque than stainless steel.

Question 9582

Topic: Biomechanics & Biomaterials

When discussing plate contours, what is the primary purpose of 'pre-bending' a conventional plate before application to a transverse or short oblique diaphyseal fracture?

. To ensure uniform screw purchase along the plate length.
. To facilitate primary bone healing by promoting absolute stability.
. To prevent gapping on the opposite cortex when achieving compression.
. To reduce the modulus of elasticity of the plate, thereby decreasing stress shielding.
. To provide dynamic compression across the fracture site.

Correct Answer & Explanation

. To prevent gapping on the opposite cortex when achieving compression.

Explanation

Pre-bending a conventional plate is crucial for transverse or short oblique fractures. When the plate is applied and screws are tightened, the plate attempts to straighten, which drives the fracture fragments together, creating compression on the far cortex and preventing gapping on the opposite side (trans-cortex). This enhances interfragmentary compression and primary bone healing. Without pre-bending, compression of the near cortex can lead to distraction of the far cortex. It does not primarily affect screw purchase uniformity, modulus of elasticity, or dynamic compression, which is achieved through eccentric drilling.

Question 9583

Topic: 1. General Principles & Basic Science

What is a significant disadvantage of using a conventional Dynamic Compression Plate (DCP) for internal fixation in a patient with osteopenic bone?

. It provides excessive stress shielding, leading to cortical hypertrophy.
. The screw pull-out strength is significantly reduced due to compromised bone-screw interface friction.
. It promotes undesirable relative stability, hindering primary bone healing.
. The plate contour cannot be adequately matched to the bone surface, leading to gaps.
. It typically requires a larger incision compared to modern locking plates.

Correct Answer & Explanation

. The screw pull-out strength is significantly reduced due to compromised bone-screw interface friction.

Explanation

Conventional DCPs rely heavily on plate-bone friction and interfragmentary compression. In osteopenic bone, the bone quality around the screw threads is poor, leading to significantly reduced screw pull-out strength. The screws may strip out, or the plate may not achieve adequate purchase or compression, leading to early construct failure. While stress shielding can occur, the primary concern in osteopenia with DCPs is the poor bone-screw interface and loss of fixation. Locking plates are generally preferred in osteopenic bone due to their fixed-angle stability independent of bone quality or compression.

Question 9584

Topic: Biomechanics & Biomaterials

In the context of plate fixation, what is the primary concern when considering 'stress shielding'?

. The potential for the implant to corrode over time due to bodily fluids.
. The concentration of stress at the screw-bone interface leading to screw loosening.
. The bone adjacent to the implant experiencing reduced physiological loading, leading to disuse osteopenia.
. The stress on the plate itself, causing fatigue failure of the implant.
. The uneven distribution of compressive forces across the fracture site.

Correct Answer & Explanation

. The bone adjacent to the implant experiencing reduced physiological loading, leading to disuse osteopenia.

Explanation

Stress shielding occurs when a rigid implant (plate) carries a disproportionate amount of the physiological load, thereby 'shielding' the underlying bone from mechanical stress. According to Wolff's Law, bone adapts to the loads placed upon it. If the bone is shielded from stress, it can lead to disuse osteopenia, weakening of the bone, and potentially refracture after implant removal. This is a significant long-term concern with highly rigid plate constructs, particularly locking plates.

Question 9585

Topic: Biomechanics & Biomaterials
Which of the following plate materials exhibits the lowest modulus of elasticity, making it theoretically most compatible with bone from a stress-shielding perspective?
. 316L Stainless Steel
. Ti-6Al-4V Titanium Alloy
. Cobalt-Chromium Alloy
. Magnesium Alloy
. UHMWPE

Correct Answer & Explanation

. Magnesium Alloy

Explanation

Magnesium alloys have a modulus of elasticity very close to that of cortical bone, making them attractive as biodegradable implants with minimal stress shielding. While titanium alloys (Ti-6Al-4V) have a lower modulus than stainless steel, magnesium is even lower and closer to bone. UHMWPE (Ultra-High Molecular Weight Polyethylene) is a polymer, not typically used for load-bearing plates due to its lower strength and viscoelastic properties. Cobalt-chromium is very stiff, similar to stainless steel.

Question 9586

Topic: 1. General Principles & Basic Science

What is the primary concern regarding heat generation during the drilling of bone for screw insertion in plate fixation?

. It can cause premature polymerization of bone cement.
. It leads to immediate bone necrosis and osteolysis, compromising screw purchase.
. It increases the risk of implant corrosion due to thermal expansion.
. It can burn the surgeon's gloves, posing an infection risk.
. It causes thermal damage to surrounding soft tissues and nerves.

Correct Answer & Explanation

. It leads to immediate bone necrosis and osteolysis, compromising screw purchase.

Explanation

Excessive heat generated during drilling can cause thermal necrosis of osteocytes, leading to an avascular zone around the screw hole. This compromises the bone's ability to heal and can lead to early screw loosening and failure of fixation. Therefore, proper drilling technique with sharp drills, appropriate speed, and irrigation (e.g., saline) is crucial to dissipate heat. While soft tissue can be damaged, the primary concern directly related to the bone-screw interface is osteonecrosis.

Question 9587

Topic: Biomechanics & Biomaterials

Which of the following is considered a primary risk factor for implant failure due to fatigue in a plate construct?

. Using a titanium plate instead of stainless steel.
. Placing too many screws in the main bone fragments.
. Achieving absolute stability with no micromotion at the fracture site.
. Insufficient reduction of the fracture, leading to persistent gapping or motion.
. Early weight-bearing against medical advice.

Correct Answer & Explanation

. Insufficient reduction of the fracture, leading to persistent gapping or motion.

Explanation

Fatigue failure of a plate occurs when the implant is subjected to repetitive stresses over time that are below its yield strength but eventually lead to crack propagation and failure. Insufficient reduction of the fracture, leading to persistent gapping or motion at the fracture site, means the plate is continually subjected to high bending stresses, especially if it's solely bridging a defect or has a long working length with too much unsupported motion. This repetitive high-stress cycling is the classic scenario for fatigue failure. While early weight-bearing can contribute, persistent fracture gap/motion directly loads the plate in a way that leads to fatigue. Material (titanium vs. steel) impacts it but is not the primary risk compared to loading conditions.

Question 9588

Topic: 1. General Principles & Basic Science

What is a major advantage of a biodegradable plate for specific orthopedic applications?

. Superior strength and stiffness compared to metallic implants.
. Elimination of the need for a second surgery for hardware removal.
. Enhanced visualization on plain radiographs post-implantation.
. Reduced risk of infection due to antimicrobial properties.
. Accelerated bone healing due to bio-active surface coatings.

Correct Answer & Explanation

. Elimination of the need for a second surgery for hardware removal.

Explanation

The primary advantage of biodegradable (bioabsorbable) plates is the elimination of the need for a second surgery to remove the implant once healing is complete. This reduces patient morbidity, surgical risks, and costs. While some modern biodegradable materials have good mechanical properties, they generally do not match the strength and stiffness of metallic implants. Their degradation products can sometimes cause inflammatory reactions, and they don't inherently possess antimicrobial properties or necessarily accelerate healing more than metallic plates.

Question 9589

Topic: Biomechanics & Biomaterials

In the context of bone plate materials, what is a specific risk associated with the use of 316L stainless steel implants that is less common with titanium implants?

. Higher rate of infection.
. Increased likelihood of non-union.
. Greater potential for corrosion and metal ion release.
. Reduced fatigue strength over time.
. Higher elastic modulus leading to more stress shielding.

Correct Answer & Explanation

. Greater potential for corrosion and metal ion release.

Explanation

While both metals can corrode to some extent, 316L stainless steel has a greater potential for corrosion and release of metal ions (e.g., nickel, chromium, molybdenum) compared to titanium alloys, particularly in the highly corrosive physiological environment. This can lead to local tissue reactions, hypersensitivity, or systemic effects. Titanium is generally considered more biocompatible and resistant to corrosion. While stainless steel does have a higher elastic modulus and thus more stress shielding, the question asks for aspecific riskthat isless commonwith titanium, and corrosion/ion release fits this perfectly.

Question 9590

Topic: Biomechanics & Biomaterials

Which complication is uniquely associated with the use of a plate for internal fixation, particularly in load-bearing bones, compared to an intramedullary nail?

. Infection at the surgical site.
. Damage to surrounding soft tissues and nerves during implantation.
. Non-union of the fracture.
. Increased risk of refracture after implant removal due to stress shielding.
. Fatigue failure of the implant.

Correct Answer & Explanation

. Increased risk of refracture after implant removal due to stress shielding.

Explanation

A significant and somewhat unique complication associated with plate fixation, particularly rigid plates in load-bearing bones, is the increased risk of refracture after plate removal. This is primarily due to stress shielding, where the underlying bone has become osteopenic and weakened because the plate has carried most of the load during healing. Upon plate removal, the weakened bone is suddenly exposed to full physiological loads, making it susceptible to refracture. While other complications (infection, non-union, nerve damage, fatigue failure) can occur with both nails and plates, refracture due to stress shielding is a more prominent concern with plates.

Question 9591

Topic: Biology, Genetics & Bone Healing

When managing a segmental femur fracture using a bridging locking plate, what is the ideal 'plate-bone distance' (PBD) to optimize biological healing?

. Direct plate-bone contact to ensure maximum stability.
. A gap of 0.5-1.0 mm to preserve periosteal blood flow.
. A gap of 2.0-3.0 mm to promote callus formation.
. The plate should be positioned subcutaneously, away from the bone.
. PBD is irrelevant as long as the screws are locking.

Correct Answer & Explanation

. A gap of 0.5-1.0 mm to preserve periosteal blood flow.

Explanation

For bridging osteosynthesis with locking plates, maintaining a small, consistent plate-bone distance (PBD) of approximately 0.5-1.0 mm is often advocated. This small gap allows for better preservation of the periosteal blood supply beneath the plate and provides space for callus formation, which is desirable for secondary bone healing in comminuted or segmental fractures. Direct plate-bone contact, while providing maximum compression with conventional plates, can compromise periosteal blood flow. Larger gaps could lead to instability or hardware prominence.

Question 9592

Topic: 1. General Principles & Basic Science

What is the primary function of the flutes on the tip of a self-tapping screw?

. To allow for cannulation of the screw for K-wire guidance.
. To increase the surface area for bone ingrowth.
. To create a cutting edge that forms its own threads in the bone.
. To facilitate the removal of bone debris during insertion.
. To reduce friction during screw advancement.

Correct Answer & Explanation

. To create a cutting edge that forms its own threads in the bone.

Explanation

Self-tapping screws have a fluted tip, which acts like a tap, cutting threads into the bone as the screw is advanced. This eliminates the need for a separate tapping step, streamlining the surgical procedure. The flutes also help in clearing bone debris, preventing bone compaction at the screw-bone interface, which could otherwise impede proper thread engagement.

Question 9593

Topic: 1. General Principles & Basic Science

Which factor has the LEAST impact on the pullout strength of a bone screw?

. Screw thread pitch.
. Screw core diameter.
. Bone mineral density.
. Screw material (e.g., stainless steel vs. titanium).
. Screw outer (major) diameter.

Correct Answer & Explanation

. Screw material (e.g., stainless steel vs. titanium).

Explanation

All factors listed influence pullout strength. Screw thread pitch, core diameter, and major diameter directly determine the amount of bone engaged by the threads and the screw's resistance to stripping. Bone mineral density directly relates to the quality of the bone in which the screw is inserted. However, thematerialof the screw (stainless steel vs. titanium) primarily affects its fatigue strength, corrosion resistance, and biocompatibility, but has a relatively minor direct impact on the initial pullout strengthof the bone-screw interfacewhen compared to the geometric factors and bone quality itself, assuming sufficient strength of the screw material. The interface quality is dictated more by geometry and bone properties.

Question 9594

Topic: Biomechanics & Biomaterials

What is the main advantage of titanium over stainless steel for orthopedic screws?

. Higher modulus of elasticity, leading to stiffer constructs.
. Superior corrosion resistance and biocompatibility.
. Lower cost and easier manufacturing.
. Greater radiopacity, improving imaging.
. Increased fatigue strength in load-bearing applications.

Correct Answer & Explanation

. Superior corrosion resistance and biocompatibility.

Explanation

Titanium (and its alloys) offers superior corrosion resistance and excellent biocompatibility compared to stainless steel. This reduces the risk of adverse tissue reactions or allergic responses. While stainless steel is stiffer (higher modulus), titanium's lower modulus of elasticity is often considered advantageous as it is closer to that of bone, potentially reducing stress shielding. Titanium also has good fatigue strength but is generally more expensive and technically more challenging to manufacture than stainless steel. Radiopacity for both is adequate, but titanium can cause less artifact on MRI.

Question 9595

Topic: 1. General Principles & Basic Science

What surgical technique is crucial to prevent screw stripping when inserting a cortical screw into dense bone?

. Using a self-tapping screw.
. Overtapping the pilot hole to create wider threads.
. Ensuring the pilot hole matches the core diameter of the screw, and pre-tapping if necessary.
. Inserting the screw at maximum speed to minimize torque.
. Using a larger diameter screw to engage more bone.

Correct Answer & Explanation

. Ensuring the pilot hole matches the core diameter of the screw, and pre-tapping if necessary.

Explanation

Screw stripping occurs when the threads cut into the bone are damaged or fail to engage properly, often due to excessive torque during insertion or an improperly sized pilot hole. To prevent this in dense cortical bone, it is crucial to use a pilot drill bit that accurately matches the core diameter of the screw. Additionally, pre-tapping the hole with a tap of the correct size before inserting the screw significantly reduces the torque required for insertion and ensures precise thread formation, thereby preventing stripping.

Question 9596

Topic: 1. General Principles & Basic Science

What is the mechanical advantage of a screw having a larger major (outer) diameter compared to a smaller one, assuming all other factors are equal?

. Increased ease of insertion due to reduced bone-screw contact.
. Decreased resistance to pullout forces.
. Greater potential for interfragmentary compression.
. Increased surface area for bone-screw contact, leading to greater pullout strength.
. Reduced risk of thermal necrosis during insertion.

Correct Answer & Explanation

. Increased surface area for bone-screw contact, leading to greater pullout strength.

Explanation

A larger major (outer) diameter screw, with appropriate thread engagement, will have a greater surface area of contact between its threads and the bone. This increased bone-screw interface directly translates to greater resistance to pullout forces and potentially greater torsional strength, provided the bone quality is adequate to support the larger diameter. Ease of insertion, compression, and thermal necrosis are affected by other factors like thread design, pilot hole size, and insertion speed, not solely the major diameter.

Question 9597

Topic: 1. General Principles & Basic Science

What is the primary advantage of a 'self-drilling' screw over a 'self-tapping' screw?

. It eliminates the need for a separate pilot hole drilling step.
. It provides superior interfragmentary compression.
. It has a higher fatigue strength.
. It is always cannulated for guidewire insertion.
. It is easier to remove if revision surgery is needed.

Correct Answer & Explanation

. It eliminates the need for a separate pilot hole drilling step.

Explanation

Self-drilling screws combine the drilling and tapping steps into a single instrument. They have a drill bit tip that creates the pilot hole, followed by flutes that cut the threads as the screw advances. This eliminates the need for a separate drilling step, saving time and reducing the number of instruments used in surgery. Self-tapping screws still require a pilot hole to be drilled first, but then cut their own threads.

Question 9598

Topic: 1. General Principles & Basic Science

What is the significance of the 'pitch' of a screw thread in relation to its mechanical properties?

. It determines the screw's bending stiffness.
. It refers to the number of rotations required to fully insert the screw.
. It is the distance between adjacent threads, influencing pullout strength and insertion torque.
. It defines the angle at which the screw must be inserted.
. It indicates the material composition of the screw.

Correct Answer & Explanation

. It is the distance between adjacent threads, influencing pullout strength and insertion torque.

Explanation

The pitch of a screw thread is the axial distance advanced by one complete turn of the screw, or more simply, the distance between two adjacent threads. A finer pitch (more threads per unit length) provides greater purchase in dense bone but requires more turns to insert. A coarser pitch (fewer threads per unit length) offers less purchase in dense bone but greater purchase in cancellous bone and faster insertion. It directly influences both the pullout strength (more threads engaged per length equals more strength) and the insertion torque required.

Question 9599

Topic: 1. General Principles & Basic Science

Which type of screw design minimizes the risk of vascular or neurological injury when used near delicate structures, particularly in unicortical fixation?

. Long, fully threaded bicortical cortical screws.
. Partially threaded cancellous screws.
. Self-drilling, self-tapping screws.
. Locking screws in a unicortical manner with a locking plate.
. Headless compression screws.

Correct Answer & Explanation

. Locking screws in a unicortical manner with a locking plate.

Explanation

Locking screws, particularly when used unicortically with a locking plate, minimize the risk of injury to structures on the far side of the bone. Since locking plates derive their stability from the fixed-angle screw-plate construct rather than bicortical purchase or screw compression, unicortical screw placement often provides sufficient stability, thus avoiding penetration of the far cortex and protecting adjacent neurovascular structures. Other screw types often require bicortical purchase for optimal stability, increasing the risk.

Question 9600

Topic: 1. General Principles & Basic Science

What is the primary risk associated with placing a screw too close to a previous drill hole or screw track?

. Increased interfragmentary compression.
. Enhanced rotational stability.
. Compromised bone quality and reduced pullout strength.
. Elevated risk of infection.
. Faster bone healing due to localized stress.

Correct Answer & Explanation

. Compromised bone quality and reduced pullout strength.

Explanation

Placing a new screw too close to an existing drill hole or screw track significantly compromises the bone quality and integrity in that area. The previous hole effectively creates a stress riser and reduces the amount of intact bone available for the new screw's threads to engage. This leads to diminished purchase, reduced pullout strength, and an increased risk of stripping the new screw or creating a bone defect that can contribute to non-union or refracture.

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