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1. General Principles & Basic Science MCQs

Practice Set 121 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 2401

Topic: Physiology & Rehabilitation

In an Ilizarov hip reconstruction (double-level pelvic support osteotomy) for a chronically dislocated hip, what is the primary biomechanical objective of the proximal valgus osteotomy?

. To equalize leg length discrepancy by translating the femoral shaft distally.
. To shift the mechanical axis medially to unload the lateral compartment of the knee.
. To create a bony fulcrum against the pelvis, restoring a stable fulcrum for the abductor mechanism and eliminating Trendelenburg gait.
. To realign the knee joint line parallel to the ground.
. To increase the anatomic lever arm of the gluteus maximus.

Correct Answer & Explanation

. To create a bony fulcrum against the pelvis, restoring a stable fulcrum for the abductor mechanism and eliminating Trendelenburg gait.

Explanation

The proximal valgus osteotomy abuts the ischium during weight-bearing. This creates a new fulcrum that stabilizes the pelvis, effectively substituting for the absent or dislocated femoral head and eliminating the Trendelenburg gait.

Question 2402

Topic: 1. General Principles & Basic Science

What is the primary function of the distal femoral osteotomy in Paley's double-level pelvic support osteotomy?

. To tension the iliotibial band and augment abductor power.
. To correct the mechanical axis deviation and joint line obliquity created by the proximal osteotomy, while providing a site for limb lengthening.
. To correct the fixed hip flexion contracture common in chronic dislocations.
. To create a secondary fulcrum against the inferior pubic ramus.
. To improve patellofemoral tracking by externalizing the tibial tubercle.

Correct Answer & Explanation

. To correct the mechanical axis deviation and joint line obliquity created by the proximal osteotomy, while providing a site for limb lengthening.

Explanation

The proximal valgus osteotomy creates a severe mechanical axis deviation (valgus) and an oblique knee joint line. The distal varus osteotomy realigns the mechanical axis, makes the knee joint horizontal, and serves as the lengthening site.

Question 2403

Topic: Surgical Anatomy & Approaches

To achieve optimal biomechanical abutment in a pelvic support osteotomy, at what precise anatomical level should the proximal femoral osteotomy be performed?

. At the level of the lesser trochanter with the hip in maximum abduction.
. At the level of the ischial tuberosity with the hip in maximum adduction.
. At the distal diaphyseal junction to protect the sciatic nerve.
. At the exact center of rotation of the anatomical neck.
. At the greater trochanteric apex.

Correct Answer & Explanation

. At the level of the ischial tuberosity with the hip in maximum adduction.

Explanation

Paley dictates that the proximal osteotomy must be performed exactly adjacent to the ischial tuberosity while the hip is held in maximum adduction. This ensures the apex of the osteotomy creates a perfect fulcrum against the ischium.

Question 2404

Topic: 1. General Principles & Basic Science



According to Paley's Rule 1 of deformity correction, what occurs when both the osteotomy and the correction hinge are placed exactly at the Center of Rotation of Angulation (CORA)?

. Angular correction is achieved, and the mechanical axes become collinear without translation.
. Angular correction is achieved, but the mechanical axes remain parallel with translation.
. Pure translational correction occurs without angular change.
. The mechanical axis is intentionally shifted medially to unload a joint compartment.
. A secondary z-deformity is created that requires distal compensation.

Correct Answer & Explanation

. Angular correction is achieved, and the mechanical axes become collinear without translation.

Explanation

Rule 1 states that if the osteotomy and the hinge are both located at the CORA, the mechanical or anatomical axes will become completely collinear without any translation at the osteotomy site.

Question 2405

Topic: 1. General Principles & Basic Science

According to Paley's Rule 2 of deformity correction, if the correction hinge is at the CORA but the osteotomy is performed at a different level, what is the geometric consequence?

. The axes realign collinearly with no translation at the osteotomy site.
. Angular correction is achieved, the axes become collinear, but intentional translation occurs at the osteotomy site.
. Angular correction occurs, but the axes become parallel rather than collinear.
. The limb segment is shortened proportionately to the distance from the CORA.
. A permanent angular deformity remains.

Correct Answer & Explanation

. Angular correction is achieved, the axes become collinear, but intentional translation occurs at the osteotomy site.

Explanation

Under Rule 2, placing the hinge at the CORA ensures the axes become collinear. However, because the osteotomy is away from the CORA, the bone ends must translate relative to each other at the osteotomy site to achieve this collinearity.

Question 2406

Topic: 1. General Principles & Basic Science

Which of the following describes Paley's Rule 3 of deformity correction?

. Osteotomy and hinge at CORA; collinear axes without translation.
. Osteotomy away from CORA, hinge at CORA; collinear axes with translation.
. Osteotomy and hinge are both away from the CORA; the proximal and distal axes become parallel but are translated, creating a z-deformity.
. Hinge placed perpendicular to the joint line regardless of CORA location.
. Correction of rotation occurs independently of angulation.

Correct Answer & Explanation

. Osteotomy and hinge are both away from the CORA; the proximal and distal axes become parallel but are translated, creating a z-deformity.

Explanation

Rule 3 involves placing both the osteotomy and the hinge away from the CORA. This results in the mechanical axes becoming parallel rather than collinear, effectively producing a translational or "z-deformity".

Question 2407

Topic: Physiology & Rehabilitation

A 19-year-old female undergoes a double-level pelvic support osteotomy for a chronically dislocated hip. To achieve optimal mechanical support and prevent proximal femur migration, the proximal valgus osteotomy should be performed at which anatomical level?

. Subtrochanteric level, 5 cm below the lesser trochanter.
. At the level of the ischial tuberosity with the hip in maximum adduction.
. At the intertrochanteric line.
. At the mid-diaphysis.
. At the level of the greater trochanter.

Correct Answer & Explanation

. At the level of the ischial tuberosity with the hip in maximum adduction.

Explanation

In a pelvic support osteotomy, the proximal valgus osteotomy must be at the level of the ischial tuberosity during maximum adduction. This allows the proximal femur to rest against the pelvis, restoring the fulcrum and eliminating the Trendelenburg gait.

Question 2408

Topic: 1. General Principles & Basic Science

When planning the proximal osteotomy in a pelvic support osteotomy (PSO), the degree of valgus angulation required is calculated based on which of the following clinical measurements?

. Maximum passive hip abduction plus 10 degrees.
. Maximum passive hip flexion minus 15 degrees.
. Maximum passive hip adduction plus 15 degrees.
. The degree of internal rotation contracture.
. The anatomical mechanical axis of the contralateral femur.

Correct Answer & Explanation

. Maximum passive hip adduction plus 15 degrees.

Explanation

The required valgus at the proximal osteotomy is determined by measuring the maximum passive adduction of the hip and adding 15 degrees of overcorrection. This ensures adequate abutment of the femur against the pelvis to stabilize the hemipelvis during single-leg stance.

Question 2409

Topic: Physiology & Rehabilitation

A patient undergoes a pelvic support osteotomy for a chronically dislocated, painful hip. Postoperatively, the patient continues to exhibit a severe Trendelenburg gait and complains of persistent pelvic drop. Which of the following technical errors most likely occurred?

. The distal osteotomy was lengthened too rapidly.
. Insufficient valgus angulation was achieved at the proximal osteotomy.
. The proximal osteotomy was placed exactly at the level of the ischial tuberosity.
. The distal osteotomy was overcorrected into varus.
. Failure to release the iliopsoas tendon.

Correct Answer & Explanation

. Insufficient valgus angulation was achieved at the proximal osteotomy.

Explanation

If insufficient valgus is achieved at the proximal osteotomy site, the femur will fail to adequately support the pelvis (ischium). This lack of a stable fulcrum leads to persistent pelvic drop and a continuing Trendelenburg gait.

Question 2410

Topic: 1. General Principles & Basic Science



When applying Paley's principles of deformity correction, the intersection of the proximal and distal mechanical axis lines defines the Center of Rotation of Angulation (CORA). If the osteotomy and the hinge of the external fixator are placed exactly at the CORA, what is the geometric result of the correction?

. Pure translation without angulation.
. Pure angulation without translation.
. Simultaneous angulation and translation.
. Pure lengthening without angulation.
. Shortening and rotation.

Correct Answer & Explanation

. Pure angulation without translation.

Explanation

According to Paley's Osteotomy Rule 1, when the osteotomy and the mechanical hinge are both placed at the Center of Rotation of Angulation (CORA), the correction results in pure angulation without any translational displacement. This optimally restores the mechanical axis.

Question 2411

Topic: 1. General Principles & Basic Science

A resident is reviewing the Moseley straight-line graph for LLD prediction. Referring to the provided graph, what mathematical manipulation was employed by Dr. Colin Moseley to transform the curvilinear longitudinal growth data into the straight 45-degree "LONG LEG" line, and why is the x-axis non-linear?

. The y-axis (leg length) was logarithmically scaled to straighten the curve.
. The x-axis (skeletal age) was logarithmically scaled to account for growth spurts.
. Datum points were shifted along the x-axis, and the distance between age scales was altered comparably.
. The graph plots percentage of growth inhibition, which naturally forms a straight line.
. The graph uses a different set of growth data that is inherently linear.

Correct Answer & Explanation

. Datum points were shifted along the x-axis, and the distance between age scales was altered comparably.

Explanation

Correct Answer: CThe case explains Moseley's ingenious insight: 'Moseley's major insight was to mathematically convert this curvilinear growth into a straight line at a 45° slope. He achieved this by shifting the datum points along the x-axis and altering the distance between the age scale on the x-axis by a comparable amount. This ingenious mathematical manipulation is precisely why the age scale is nonlinear on the Moseley straight-line graph.' This direct quote confirms the correct answer.Options A and B incorrectly describe logarithmic scaling. Option D misrepresents the data plotted. Option E is incorrect as Moseley used the same Anderson et al. data, but transformed it.

Question 2412

Topic: 1. General Principles & Basic Science

A 10-year-old boy with a Type 1 LLD is predicted to have a 2.5 cm discrepancy at skeletal maturity. His skeletal age is 10.5 years. Based on this predicted discrepancy and the principles of LLD management outlined in the case, which of the following treatment strategies would be most appropriate to achieve limb length equality at maturity?

. Immediate limb lengthening via distraction osteogenesis of the shorter limb.
. Observation with a shoe lift until skeletal maturity, then re-evaluate.
. Contralateral epiphysiodesis of the longer limb.
. Bilateral femoral and tibial lengthening to maximize height.
. Amputation and prosthetic fitting for the shorter limb.

Correct Answer & Explanation

. Contralateral epiphysiodesis of the longer limb.

Explanation

Correct Answer: CThe case emphasizes that prediction 'dictates the magnitude of the problem we must solve and informs the critical choice between relatively simple, minimally invasive procedures like epiphysiodesis and complex, multi-stage reconstructions like distraction osteogenesis.' A predicted LLD of 2.5 cm at skeletal maturity is typically within the range managed by epiphysiodesis of the contralateral (longer) limb, especially in a 10-year-old with significant remaining growth. This procedure slows the growth of the longer limb to allow the shorter limb to catch up, resulting in equality at maturity.Option A (distraction osteogenesis) is generally reserved for larger discrepancies (typically >5 cm) or when growth arrest has occurred. Option B (observation with shoe lift) is usually for very small discrepancies (e.g., <2 cm) or non-progressive LLDs. Option D (bilateral lengthening) is an extreme measure for severe short stature, not a 2.5 cm LLD. Option E (amputation) is reserved for severe, non-salvageable deformities.

Question 2413

Topic: 1. General Principles & Basic Science

A 15-year-old patient presents with a history of inadequately treated neonatal septic arthritis of the right hip, resulting in complete destruction of the proximal femoral epiphysis. Clinically, the patient exhibits a severe Trendelenburg gait and significant leg length discrepancy. Radiographs confirm superior and lateral migration of the proximal femur. The surgeon explains that the primary biomechanical failure leading to the Trendelenburg gait in this specific pathology is:

. A. Primary weakness of the gluteus maximus muscle due to disuse atrophy.
. B. Shortening of the abductor lever arm due to inferior migration of the greater trochanter.
. C. Loss of the stable femoral head fulcrum, leading to abductor muscle slackness and mechanical disadvantage.
. D. Excessive valgus deformity of the proximal femur, causing abductor over-tensioning.
. E. Fixed adduction contracture of the hip, preventing proper pelvic leveling.

Correct Answer & Explanation

. C. Loss of the stable femoral head fulcrum, leading to abductor muscle slackness and mechanical disadvantage.

Explanation

Correct Answer: CThe case explicitly states that when the femoral head and neck are destroyed by septic arthritis, the critical fulcrum vanishes entirely. Without the constraint of the acetabulum, the proximal femur migrates superiorly and laterally, causing the abductor muscles to lose their resting tension, rendering them slack, mechanically disadvantaged, and functionally incompetent. This directly leads to the inability to stabilize the pelvis during single-leg stance, resulting in a positive Trendelenburg sign and gait.Option A is incorrectbecause while disuse atrophy may occur secondarily, the primary mechanical failure is the loss of the fulcrum and subsequent abductor incompetence, not isolated gluteus maximus weakness (which is primarily an extensor, not a primary abductor for pelvic stability).Option B is incorrectbecause the proximal femur migrates superiorly, not inferiorly, and the greater trochanter also migrates superiorly, which would shorten the abductor lever arm but the primary issue is the loss of the fulcrum and slackness, not just lever arm shortening in isolation.Option D is incorrectbecause the pathology typically leads to a relative varus position of the proximal femur relative to the pelvis due to superior migration, not excessive valgus. Even if valgus were present, it would not cause abductor over-tensioning in the context of a lost fulcrum.Option E is incorrectbecause while hip contractures can occur, the primary cause of the Trendelenburg gait in this specific pathology is the mechanical instability from the lost fulcrum and abductor incompetence, not a fixed adduction contracture.

Question 2414

Topic: Infection, Pharmacology & VTE

A 12-year-old patient with sequelae of neonatal septic arthritis of the hip presents with the radiographic findings shown below. The surgeon is planning a double-level pelvic support osteotomy. Based on the provided image and case description, which of the following is the most accurate description of the biomechanical consequence illustrated on the right side of the diagram?

. A. The mechanical axis passes centrally through the knee, leading to balanced compartment loading.
. B. The superior migration of the hip center causes the mechanical axis to shift laterally, inducing a valgus moment at the knee.
. C. The lateral shift of the hip's center of rotation causes the mechanical axis to deviate medially, inducing a varus moment at the knee.
. D. The anatomical axis of the femur is perfectly aligned with the mechanical axis, indicating no deformity.
. E. The joint line congruency angle (JLCA) is significantly increased, indicating severe patellofemoral instability.

Correct Answer & Explanation

. C. The lateral shift of the hip's center of rotation causes the mechanical axis to deviate medially, inducing a varus moment at the knee.

Explanation

Correct Answer: CThe case explicitly states and the image illustrates that in the sequelae of septic arthritis, the 'functional hip joint' is the unstable, superiorly and laterally migrated point of contact between the proximal femur and the ilium. This lateral shift of the hip's center of rotation causes the mechanical axis to deviate profoundly. The load-bearing line now falls far medial to the center of the knee, a condition known as Mechanical Axis Deviation (MAD), which induces a massive varus moment at the knee. This chronic varus force leads to lateral knee instability and medial compartment overload.Option A is incorrectas this describes a normal limb, not the pathological state shown on the right.Option B is incorrectbecause the mechanical axis shifts medially, not laterally, and induces a varus moment, not a valgus moment, at the knee.Option D is incorrectas the image clearly shows a significant deviation of the mechanical axis, indicating a severe deformity.Option E is incorrectbecause while knee pathology will develop, the primary biomechanical consequence illustrated by the mechanical axis deviation is the varus moment and subsequent medial compartment overload and lateral instability, not specifically an increased JLCA or patellofemoral instability as the direct consequence of the MAD shown.

Question 2415

Topic: Infection, Pharmacology & VTE

A 20-year-old patient is undergoing preoperative planning for an Ilizarov double-level pelvic support osteotomy for sequelae of septic arthritis. The surgeon has identified the desired point of contact between the proximal femur and the lateral wall of the pelvis (ischium) as the new functional hip center. According to Paley's principles, what is the significance of this point in planning the proximal osteotomy?

. A. It defines the Mechanical Proximal Tibial Angle (MPTA) for the distal osteotomy.
. B. It serves as the Center of Rotation of Angulation (CORA) for the proximal osteotomy.
. C. It dictates the Joint Line Congruency Angle (JLCA) for the knee joint.
. D. It is the primary reference for calculating the leg length discrepancy (LLD) at skeletal maturity.
. E. It determines the optimal site for the distal varus-lengthening osteotomy.

Correct Answer & Explanation

. B. It serves as the Center of Rotation of Angulation (CORA) for the proximal osteotomy.

Explanation

Correct Answer: BThe case states, 'The Center of Rotation of Angulation (CORA) is the geometric heart of any deformity correction... However, in the case of an absent femoral head, there is no proximal anatomical landmark to define the proximal axis. The surgeon must therefore define a new, functional hip center. This is achieved by determining the desired point of contact between the proximal femur and the lateral wall of the pelvis—specifically, the ischium. The CORA for the pelvic support osteotomy is then planned at this exact level.'Option A is incorrectbecause MPTA relates to the proximal tibia, not the hip CORA.Option C is incorrectbecause JLCA relates to the knee joint line congruency, not the hip CORA.Option D is incorrectbecause while LLD is a component, the CORA is specifically for angular correction planning, not LLD prediction.Option E is incorrectbecause the CORA for the proximal osteotomy is distinct from the planning of the distal osteotomy site, although they are related in the overall correction strategy.

Question 2416

Topic: Infection, Pharmacology & VTE

A patient with a long-standing hip flexion contracture due to septic arthritis sequelae is undergoing preoperative planning for a double-level Ilizarov reconstruction. The surgeon notes significant compensatory lumbar lordosis. Which component of the proximal osteotomy is specifically designed to address this sagittal plane deformity and its associated symptoms?

. A. The valgus component, to provide pelvic support.
. B. The varus component, to correct mechanical axis deviation.
. C. The extension component, to correct the hip flexion contracture.
. D. The distraction component, to lengthen the limb.
. E. The rotational component, to address femoral anteversion.

Correct Answer & Explanation

. C. The extension component, to correct the hip flexion contracture.

Explanation

Correct Answer: CThe case explicitly addresses sagittal plane analysis: 'Patients almost always present with a significant hip flexion contracture... Therefore, the proximal osteotomy must be a multiplanar valgus-extension osteotomy. The extension component is calculated from a careful clinical examination (Thomas test) and sagittal radiographs. Correcting the flexion contracture allows the patient to stand fully upright, eliminating the severe compensatory lumbar lordosis that causes chronic back pain in these patients.'Option A is incorrectbecause the valgus component primarily addresses pelvic instability and abductor tensioning in the coronal plane.Option B is incorrectbecause a varus component is part of thedistalosteotomy, not the proximal, and its role is to correct lateral MAD, not hip flexion contracture.Option D is incorrectbecause distraction is part of thedistalosteotomy for limb lengthening, not the proximal osteotomy's acute correction of flexion contracture.Option E is incorrectbecause while rotational deformities can exist, the extension component specifically addresses the flexion contracture and lumbar lordosis.

Question 2417

Topic: Infection, Pharmacology & VTE

A 14-year-old patient, as seen in the clinical image, presents with the sequelae of neonatal septic arthritis of the hip. The surgeon is performing the proximal valgus-extension osteotomy as part of a double-level Ilizarov reconstruction. Which of the following statements accurately describes a critical aspect of the operative workflow for this specific osteotomy?

. A. The osteotomy is performed distally at the level of the knee joint to allow for acute correction of the valgus deformity.
. B. The osteotomy is performed percutaneously using a high-energy oscillating saw to ensure rapid bone division.
. C. The osteotomy site must be precisely at the level of the ischial tuberosity when the leg is pulled down to its maximal length under traction.
. D. The correction of the valgus and extension components is performed gradually over several weeks using distraction osteogenesis.
. E. The primary goal of this osteotomy is to correct the leg length discrepancy via acute shortening.

Correct Answer & Explanation

. C. The osteotomy site must be precisely at the level of the ischial tuberosity when the leg is pulled down to its maximal length under traction.

Explanation

Correct Answer: CThe case states, 'The proximal osteotomy is performed first and is typically corrected acutely on the operating table. Level of Osteotomy: The osteotomy site is paramount. It must be performed precisely at the level of the ischial tuberosity when the leg is pulled down to its maximal length under traction. This ensures the apex of the angulation will sit perfectly against its intended bony buttress.'Option A is incorrectbecause the proximal osteotomy is performed proximally, at the level of the ischial tuberosity, not distally at the knee.Option B is incorrectbecause the case specifies a 'percutaneous, low-energy osteotomy is performed using a multiple drill-hole and osteotome technique. This preserves the periosteum and the vital endosteal blood supply necessary for rapid bone healing,' directly contradicting the use of a high-energy oscillating saw.Option D is incorrectbecause the proximal osteotomy is typically correctedacutelyon the operating table, not gradually over weeks. Gradual correction is for the distal lengthening osteotomy.Option E is incorrectbecause the primary goal of the proximal osteotomy is pelvic support and correction of hip instability/flexion contracture, not LLD correction via acute shortening (which is generally avoided in lengthening procedures).

Question 2418

Topic: Infection, Pharmacology & VTE

Why do neonates and infants younger than 18 months have a significantly higher risk of concurrent metaphyseal osteomyelitis and septic arthritis of the hip compared to older children?

. They have an immature blood-brain barrier
. They possess transphyseal blood vessels connecting the metaphysis to the epiphysis
. Their acetabular cartilage is highly vascularized
. They have a higher prevalence of Kingella kingae infections
. Their innate immune system lacks functioning macrophages

Correct Answer & Explanation

. They possess transphyseal blood vessels connecting the metaphysis to the epiphysis

Explanation

In infants under 18 months, transphyseal vessels cross the cartilaginous physis, allowing infection to easily spread from the metaphysis directly into the epiphysis and the joint space.

Question 2419

Topic: 1. General Principles & Basic Science

During deformity correction planning for a diaphyseal angular deformity, the surgeon chooses an osteotomy site that is separated from the Center of Rotation of Angulation (CORA), but the hinges of the fixation device are placed exactly on the CORA. What is the expected geometric outcome during correction?

. Pure angulation without translation
. Angulation combined with expected translation of the bone ends
. Pure translation without angulation
. Joint line convergence angle widening
. Immediate premature consolidation

Correct Answer & Explanation

. Angulation combined with expected translation of the bone ends

Explanation

According to the principles of deformity correction (Paley's Rules), if the hinge is placed at the CORA but the osteotomy is at a different level, correction will result in both angulation and translation at the osteotomy site.

Question 2420

Topic: Infection, Pharmacology & VTE

A 6-year-old boy in an endemic region presents with a massive knee effusion and a refusal to bear weight. He has a low-grade fever, ESR of 30, and CRP of 1.8 mg/dL. Synovial fluid aspiration reveals a WBC count of 55,000 cells/mm3 with 85% neutrophils. Which diagnosis must be strongly considered and differentiated from bacterial septic arthritis?

. Transient synovitis
. Lyme arthritis
. Juvenile Idiopathic Arthritis
. Pigmented Villonodular Synovitis
. Hemophilic arthropathy

Correct Answer & Explanation

. Lyme arthritis

Explanation

Lyme arthritis often mimics septic arthritis with high synovial fluid WBC counts (often 50,000-60,000), but patients typically exhibit less severe systemic toxicity. Lyme serology is crucial in endemic areas.

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