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1. General Principles & Basic Science MCQs

Practice Set 125 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 2481

Topic: 1. General Principles & Basic Science

In distraction osteogenesis, a corticotomy is often preferred over a standard osteotomy. What is the primary biological rationale for performing a low-energy corticotomy?

. To completely preserve the periosteal and endosteal blood supply
. To ensure immediate rigid stability without any fixation
. To completely eliminate the risk of hardware failure
. To purposefully induce a delayed union for gradual healing
. To prevent intraoperative hemorrhage from the medullary canal

Correct Answer & Explanation

. To completely preserve the periosteal and endosteal blood supply

Explanation

A low-energy corticotomy selectively cuts the cortex while attempting to preserve the endosteal and periosteal blood supply. This robust vascularity is critical for generating quality regenerate bone during distraction.

Question 2482

Topic: Biology, Genetics & Bone Healing

When constructing an Ilizarov circular frame, tensioned smooth wires are routinely utilized. What is the primary mechanical effect of appropriately tensioning these wires?

. Increases axial micromotion at the osteotomy site
. Increases the overall stiffness and stability of the construct
. Decreases the risk of superficial pin tract infection
. Increases rotational instability to stimulate callus
. Decreases the rate of bone regenerate formation

Correct Answer & Explanation

. Increases the overall stiffness and stability of the construct

Explanation

Tensioning the smooth wires in a circular external fixator significantly increases the stiffness and stability of the frame. This stability is essential for maintaining alignment and providing the optimal mechanical environment for osteogenesis.

Question 2483

Topic: Biology, Genetics & Bone Healing

Following a tibial corticotomy for limb lengthening, the surgeon mandates a latency period before beginning the distraction phase. What is the primary physiological purpose of this delay?

. To allow for soft tissue swelling to completely subside
. To permit the formation of a provisional callus and local revascularization
. To ensure the external fixator pins completely osteointegrate
. To prevent common peroneal nerve stretch palsy
. To allow the patient's psychomotor adjustment to the heavy frame

Correct Answer & Explanation

. To permit the formation of a provisional callus and local revascularization

Explanation

The latency period (typically 5 to 7 days) allows for the initial inflammatory phase of bone healing, local revascularization, and the formation of a soft provisional callus. This step is essential for robust and successful distraction osteogenesis.

Question 2484

Topic: 1. General Principles & Basic Science

In the intact femur, the anatomical axis and the mechanical axis are not parallel. What is the normal average angle between the anatomical and mechanical axes of the femur?

. 2 degrees
. 7 degrees
. 12 degrees
. 15 degrees
. 20 degrees

Correct Answer & Explanation

. 7 degrees

Explanation

The anatomical axis of the femur typically diverges from the mechanical axis by an average of 7 degrees (normal range 5 to 9 degrees). This divergence must be accounted for when using intramedullary guides or planning corrections.

Question 2485

Topic: 1. General Principles & Basic Science

When calculating the CORA for an angular deformity, a transverse bisector line is drawn. If an osteotomy is performed exactly on this bisector line but away from the deformity's apex, and the hinge is also placed on the bisector line, what will occur during correction?

. Translation will be completely avoided.
. The bone ends will purely lengthen without rotation.
. Collinear alignment is achieved but with translation of the bone ends.
. The joint orientation angles will paradoxically worsen.
. The correction will fail due to immediate hinge binding.

Correct Answer & Explanation

. Collinear alignment is achieved but with translation of the bone ends.

Explanation

If the osteotomy and hinge are on the transverse bisector line of the CORA but not at the apex, angular correction will result in collinear alignment of the mechanical axes. However, this comes at the expense of translation of the bone ends at the osteotomy site.

Question 2486

Topic: 1. General Principles & Basic Science

What is the normal accepted anatomic range for the mechanical lateral distal femoral angle (mLDFA) and the medial proximal tibial angle (MPTA) in the coronal plane?

. mLDFA 80°-84°, MPTA 80°-84°
. mLDFA 85°-90°, MPTA 85°-90°
. mLDFA 90°-95°, MPTA 85°-90°
. mLDFA 95°-100°, MPTA 90°-95°
. mLDFA 80°-85°, MPTA 90°-95°

Correct Answer & Explanation

. mLDFA 85°-90°, MPTA 85°-90°

Explanation

The normal mLDFA is approximately 87° (range 85°-90°), and the normal MPTA is approximately 87° (range 85°-90°). These parameters are critical for evaluating coronal plane deformities around the knee.

Question 2487

Topic: 1. General Principles & Basic Science

According to Paley's Rule 1 of deformity correction, what geometric outcome is expected when the osteotomy and the axis of correction of angulation (ACA) both pass precisely through the center of rotation of angulation (CORA)?

. Angulation corrects with deliberate translation at the osteotomy site.
. A secondary translation deformity is created.
. Pure translation occurs without angular correction.
. Angulation corrects perfectly with collinear mechanical axes and no translation.
. The mechanical axes become parallel but not collinear.

Correct Answer & Explanation

. Angulation corrects perfectly with collinear mechanical axes and no translation.

Explanation

Paley's Rule 1 states that if the osteotomy and the ACA both pass through the CORA, the angulation corrects completely without any translation at the osteotomy site, resulting in collinear mechanical axes.

Question 2488

Topic: 1. General Principles & Basic Science

A surgeon plans a corrective osteotomy for a tibial deformity. Due to a poor soft tissue envelope, the osteotomy is made 4 cm distal to the center of rotation of angulation (CORA). However, the hinge (ACA) is placed exactly at the CORA. Which of Paley's rules does this follow, and what is the expected result?

. Rule 1; collinear realignment without translation.
. Rule 2; collinear mechanical axes with translation of the bone ends at the osteotomy site.
. Rule 3; creation of a secondary translation deformity (zig-zag deformity).
. Rule 2; pure translation correction without any angular change.
. Rule 3; collinear mechanical axes but with unintended limb shortening.

Correct Answer & Explanation

. Rule 2; collinear mechanical axes with translation of the bone ends at the osteotomy site.

Explanation

Paley's Rule 2 states that if the ACA is at the CORA but the osteotomy is at a different level, the mechanical axes will fully realign (collinear), but there will be translation of the bone segments at the osteotomy site.

Question 2489

Topic: 1. General Principles & Basic Science

If the axis of correction of angulation (ACA) and the osteotomy are placed at the same level, but neither passes through the CORA, what is the resulting geometric consequence?

. The mechanical axes become perfectly collinear.
. Pure translation occurs without altering the anatomical axis.
. A new secondary translation deformity is created, leaving the mechanical axes parallel but not collinear.
. The osteotomy site acts as a perfect opening wedge without translation.
. The limb undergoes spontaneous lengthening without angular change.

Correct Answer & Explanation

. A new secondary translation deformity is created, leaving the mechanical axes parallel but not collinear.

Explanation

Paley's Rule 3 dictates that if the ACA is placed away from the CORA (regardless of osteotomy location), angular correction will result in a secondary translation deformity, leaving the proximal and distal mechanical axes parallel rather than collinear.

Question 2490

Topic: 1. General Principles & Basic Science

A 45-year-old patient presents with a severe varus knee and a mechanical axis deviation (MAD) of 40 mm medial to the knee center. Radiographic analysis reveals an mLDFA of 88° and an MPTA of 87°. Which of the following is the most likely cause of the patient's deformity?

. Femoral diaphyseal bowing.
. Tibial metaphyseal varus.
. Abnormal joint line convergence angle (JLCA).
. Rotational malalignment of the tibia.
. Posterior slope abnormalities.

Correct Answer & Explanation

. Abnormal joint line convergence angle (JLCA).

Explanation

Because the osseous parameters (mLDFA and MPTA) are within normal limits (85°-90°), the varus deformity is intra-articular or ligamentous in nature, which is reflected by an abnormal Joint Line Convergence Angle (JLCA).

Question 2491

Topic: Biomechanics & Biomaterials

A patient with a complex multiplanar deformity is treated with a hexapod circular external fixator (e.g., Taylor Spatial Frame). What is the primary biomechanical advantage of this system over a traditional Ilizarov frame?

. It completely prevents pin tract infections due to hydroxyapatite coating.
. It corrects angulation strictly through Rule 1 without the need for an osteotomy.
. It utilizes a 'virtual hinge' to allow simultaneous correction of 6 degrees of freedom.
. It requires a substantially shorter latency and consolidation period.
. It relies exclusively on translation, avoiding all angular correction.

Correct Answer & Explanation

. It utilizes a 'virtual hinge' to allow simultaneous correction of 6 degrees of freedom.

Explanation

Hexapod frames use a 'virtual hinge' computed by software, allowing simultaneous correction of all 6 degrees of freedom (angulation and translation in three planes) without requiring physical hinge repositioning during treatment.

Question 2492

Topic: 1. General Principles & Basic Science

During deformity planning for a bowed femur, the surgeon draws the proximal mechanical axis line and the distal mechanical axis line. These lines do not intersect at a single point; instead, they intersect the mid-diaphyseal axis at two separate locations. What does this indicate?

. The patient has a pure translational deformity.
. The bone has a multi-apical deformity requiring at least two CORAs.
. The joint line convergence angle is highly abnormal.
. The deformity is exclusively in the sagittal plane.
. An error was made, as every bone can only have one CORA.

Correct Answer & Explanation

. The bone has a multi-apical deformity requiring at least two CORAs.

Explanation

When the proximal and distal mechanical axes do not intersect at a single apex but rather require an intervening axis line to connect them, it signifies a multi-apical (complex) deformity with more than one CORA.

Question 2493

Topic: 1. General Principles & Basic Science

To perform a pure medial opening wedge high tibial osteotomy (HTO) for a varus knee without creating translation, where must the mechanical hinge (ACA) be positioned?

. Exactly at the CORA on the concave (medial) cortex.
. Exactly at the CORA on the convex (lateral) cortex.
. In the center of the medullary canal at the level of the osteotomy.
. Proximal to the CORA along the mechanical axis.
. Distal to the CORA along the anatomic axis.

Correct Answer & Explanation

. Exactly at the CORA on the convex (lateral) cortex.

Explanation

To achieve a pure opening wedge correction (lengthening of the concave side), the ACA must be placed exactly at the CORA on the convex (lateral) cortex. Placing it on the concave cortex would result in a closing wedge correction.

Question 2494

Topic: 1. General Principles & Basic Science

When evaluating an angular deformity in the tibial diaphysis, a surgeon notes that the CORA derived from the mechanical axes and the CORA derived from the anatomical axes are at the exact same location. What is the anatomical reason for this?

. The mechanical and anatomic axes of the tibia are superimposed and parallel.
. The tibia possesses a naturally occurring valgus bow of 7 degrees.
. The tibial joint line is perfectly perpendicular to both axes in all patients.
. The center of rotation is naturally extra-osseous in all long bones.
. The measurement was taken in the sagittal plane rather than the coronal plane.

Correct Answer & Explanation

. The mechanical and anatomic axes of the tibia are superimposed and parallel.

Explanation

In the tibia, the mechanical and anatomical axes are normally parallel and superimposed (unlike the femur, where they diverge by ~7 degrees). Thus, in a diaphyseal deformity, both methods yield the same CORA.

Question 2495

Topic: 1. General Principles & Basic Science

A 28-year-old male undergoing tibial lengthening returns at week 5 with an inability to distract the frame further. Radiographs reveal premature consolidation with a continuous bony bridge across the regenerate gap. Which of the following most likely caused this complication?

. A distraction rate of 1.5 mm/day.
. A latency period of 3 days.
. A distraction rhythm of 0.25 mm four times daily.
. A latency period of 14 days and a rate of 0.5 mm/day.
. Rigid wire fixation preventing micro-motion.

Correct Answer & Explanation

. A latency period of 14 days and a rate of 0.5 mm/day.

Explanation

Premature consolidation is a known complication of distraction osteogenesis and is typically caused by a prolonged latency period (>10 days) or an excessively slow distraction rate (<1 mm/day).

Question 2496

Topic: 1. General Principles & Basic Science

In a patient with a pure translation deformity of the femoral diaphysis and strictly parallel mechanical axes, how is the CORA geometrically defined?

. It is located exactly at the site of maximal translation.
. It is located at the nearest joint line.
. The CORA is considered to be at infinity.
. It is located at the mid-point of the mechanical axis deviation.
. Pure translation deformities possess multiple finite CORAs.

Correct Answer & Explanation

. The CORA is considered to be at infinity.

Explanation

By geometric definition, a pure translation deformity involves parallel proximal and distal axes. Because parallel lines never intersect, the CORA for a pure translation deformity is mathematically located at infinity.

Question 2497

Topic: Biology, Genetics & Bone Healing

During the insertion of a half-pin for an external fixator, the surgeon observes significant bone debris and smoke due to excessive heat generation. What is the most direct consequence of thermal necrosis at the pin-bone interface?

. Increased pin-bone interface strength.
. Enhanced local osteogenesis during the latency period.
. Early pin loosening and potential ring sequestrum formation.
. Prevention of pin tract infections due to thermal sterilization.
. Faster consolidation of the regenerate bone.

Correct Answer & Explanation

. Early pin loosening and potential ring sequestrum formation.

Explanation

Thermal necrosis during pin insertion kills the adjacent osteocytes, leading to bone resorption at the interface. This manifests clinically as early pin loosening, pin tract infections, and characteristic 'ring sequestrum' formation on radiographs.

Question 2498

Topic: 1. General Principles & Basic Science

A 32-year-old male presents with a mid-diaphyseal tibial varus deformity. The center of rotation of angulation (CORA) is determined to be at the apex of the deformity. The surgeon plans an osteotomy. Due to poor skin quality at the apex, the osteotomy is performed 5 cm distal to the CORA, but the mechanical hinge of the external fixator is placed exactly at the CORA. According to Paley's principles, what is the expected geometric outcome?

. Pure angulation of the bone ends with parallel translation of the mechanical axes.
. Angulation and translation at the osteotomy site, with perfect realignment of the mechanical axes.
. Pure angulation at the osteotomy site, with perfect realignment of the mechanical axes.
. Pure translation of the bone ends, with failure to realign the mechanical axes.
. Distraction without angulation at the osteotomy site.

Correct Answer & Explanation

. Angulation and translation at the osteotomy site, with perfect realignment of the mechanical axes.

Explanation

According to Paley's Osteotomy Rule 2, if the osteotomy is performed at a different level than the CORA but the mechanical hinge is placed at the CORA, the bone ends will angulate and translate at the osteotomy site, resulting in collinear realignment of the mechanical axes.

Question 2499

Topic: 1. General Principles & Basic Science

According to Paley's principles of deformity correction, if the osteotomy is performed away from the CORA but the mechanical hinge is placed exactly at the CORA, what is the expected geometric outcome?

. Angulation without translation
. Angulation with translation resulting in collinear mechanical axes
. Angulation with translation resulting in parallel mechanical axes
. Translation only without angulation
. Neither angulation nor translation

Correct Answer & Explanation

. Angulation with translation resulting in collinear mechanical axes

Explanation

Paley's Rule 2 states that if the hinge is at the CORA but the osteotomy is at a different level, the mechanical axes will become collinear through a combination of angulation and translation at the osteotomy site.

Question 2500

Topic: 1. General Principles & Basic Science

In planning a tibial deformity correction, a surgeon places both the osteotomy and the mechanical hinge at a site distant from the true Center of Rotation of Angulation (CORA). What is the expected outcome of this correction?

. Collinear mechanical axes with no translation
. Collinear mechanical axes with translation
. Parallel mechanical axes with translation
. Pure rotational deformity
. Pure length discrepancy

Correct Answer & Explanation

. Parallel mechanical axes with translation

Explanation

Paley's Rule 3 states that if the osteotomy and the hinge are both placed away from the CORA, the correction will result in parallel (but not collinear) mechanical axes, introducing a new translational deformity.

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