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1. General Principles & Basic Science MCQs

Practice Set 772 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 15421

Topic: 1. General Principles & Basic Science

According to Paley's rules of deformity correction, if a corrective osteotomy is performed at a diaphyseal level different from the Center of Rotation of Angulation (CORA), but the axis of correction (hinge) is placed exactly at the CORA, what is the resulting biomechanical effect on the bone segments?

. The mechanical axes realign completely without any translation at the osteotomy site.
. The mechanical axes realign collinearly, but translational displacement occurs at the osteotomy site.
. The mechanical axes remain parallel, creating a pure translation deformity.
. An obligatory rotational deformity is created.
. An uncorrectable length discrepancy occurs without angular correction.

Correct Answer & Explanation

. The mechanical axes realign collinearly, but translational displacement occurs at the osteotomy site.

Explanation

Paley's Rule 2 dictates that placing the hinge at the CORA but making the osteotomy at a different level ensures the proximal and distal mechanical axes realign collinearly. However, this geometry forces the bone ends to translate relative to each other at the osteotomy site.

Question 15422

Topic: Physiology & Rehabilitation

A patient with severe unilateral hip osteoarthritis exhibits a compensated Trendelenburg gait, characterized by excessive lateral leaning of the trunk over the affected hip during the stance phase. What is the primary biomechanical advantage of this gait modification?

. It increases the abductor moment arm.
. It decreases the body weight moment arm.
. It increases the total joint reaction force.
. It shifts the anatomical center of rotation superiorly.
. It recruits the gluteus maximus to assist in abduction.

Correct Answer & Explanation

. It decreases the body weight moment arm.

Explanation

Leaning the trunk laterally shifts the body's center of gravity closer to the center of the affected femoral head. This significantly decreases the body weight moment arm, thereby reducing the force required by the weakened hip abductors and lowering the joint reaction force.

Question 15423

Topic: 1. General Principles & Basic Science

When programming a hexapod external fixator (e.g., Taylor Spatial Frame) for complex deformity correction, which of the following defines the specific point in three-dimensional space around which the entire mathematical correction is referenced?

. The master tab
. The corresponding strut length
. The mounting parameter
. The origin
. The reference ring

Correct Answer & Explanation

. The origin

Explanation

In hexapod software, the "origin" is the mathematical point in space (usually designated at the CORA or a specific point on the bone) around which all angular and translational corrections are calculated.

Question 15424

Topic: 1. General Principles & Basic Science

A patient presents with a multi-apical bowing deformity of the tibial diaphysis. The surgeon plans a single corrective osteotomy located exactly halfway between the two Centers of Rotation of Angulation (CORAs), with the hinge placed at the osteotomy site. What will be the alignment outcome based on Paley's rules?

. Collinear realignment of the mechanical axes
. A pure translational deformity with parallel mechanical axes
. Complete correction of both the proximal and distal apices
. An obligatory rotational malalignment
. Correction of one apex while doubling the angulation of the other

Correct Answer & Explanation

. A pure translational deformity with parallel mechanical axes

Explanation

According to Paley's Rule 3, if both the osteotomy and the correction hinge are placed at a level away from the CORA, the proximal and distal mechanical axes will become parallel but not collinear, resulting in a pure translational deformity.

Question 15425

Topic: 1. General Principles & Basic Science

A medial opening-wedge high tibial osteotomy (HTO) is performed for a patient with medial compartment osteoarthritis. Because of the triangular cross-section of the proximal tibia, how must the opening gap be contoured to maintain the patient's native posterior tibial slope?

. The anterior gap must be smaller than the posterior gap.
. The anterior gap must be larger than the posterior gap.
. The gap must be perfectly symmetric anteriorly and posteriorly.
. The lateral hinge must be translated anteriorly.
. The osteotomy must be extended intra-articularly.

Correct Answer & Explanation

. The anterior gap must be smaller than the posterior gap.

Explanation

Due to the natural posterior slope and the triangular shape of the proximal tibia, an opening-wedge HTO must have a smaller anterior gap relative to the posterior gap. A symmetric opening will inadvertently increase the posterior tibial slope.

Question 15426

Topic: 1. General Principles & Basic Science

A 14-year-old is undergoing 5 cm of femoral lengthening using a monolateral external fixator. During the 5th week of distraction, the patient develops a 20-degree knee flexion contracture and struggles with physical therapy. Radiographs show excellent regenerate bone formation. What is the most appropriate initial management?

. Perform an immediate open quadriceps lengthening.
. Increase the distraction rate to stretch the soft tissues faster.
. Decrease the distraction rate and intensify physical therapy and splinting.
. Perform an immediate femoral shortening osteotomy through the regenerate.
. Remove the external fixator and place a cast.

Correct Answer & Explanation

. Decrease the distraction rate and intensify physical therapy and splinting.

Explanation

Soft tissue resistance, particularly knee flexion contractures from the hamstrings and IT band, is a common complication during femoral lengthening. The standard initial management is to slow the rate of distraction to allow soft tissue adaptation while intensifying physical therapy and dynamic splinting.

Question 15427

Topic: 1. General Principles & Basic Science

In deformity planning, the center of rotation of angulation (CORA) is determined. According to Paley's osteotomy rules, what occurs if the osteotomy is performed at a level different from the CORA, but the mechanical hinge is placed exactly at the CORA?

. Pure angulation with complete realignment of the mechanical axis without translation of the bone ends.
. Pure translation without angulation.
. Angulation combined with translation of the bone ends, successfully aligning the mechanical axis.
. A new iatrogenic deformity is created because the axes will not align.
. The joint orientation lines will remain un-parallel.

Correct Answer & Explanation

. Angulation combined with translation of the bone ends, successfully aligning the mechanical axis.

Explanation

Paley's Rule 2 states that if the hinge is at the CORA but the osteotomy is at a different level, the mechanical axis will fully realign. This occurs through a combination of angulation and translation of the bone ends at the osteotomy site.

Question 15428

Topic: 1. General Principles & Basic Science

According to the rules of deformity correction

, if an osteotomy is performed at a level different from the center of rotation of angulation (CORA), but the mechanical hinge is placed exactly at the CORA, what is the expected geometric result?

. Pure angular correction without translation
. Pure translation without angular correction
. Angular correction with intended translation of the bone ends
. Angular correction with unintended translation and limb shortening
. Neither angular correction nor translation

Correct Answer & Explanation

. Angular correction with intended translation of the bone ends

Explanation

Osteotomy rule 2 states that if the osteotomy is off the CORA but the hinge axis is on the CORA, the bone ends will angulate and translate relative to each other. This translation is functionally required to perfectly realign the mechanical axis.

Question 15429

Topic: Physiology & Rehabilitation

A patient exhibits a compensated Trendelenburg lurch to the right side during the stance phase of the right leg. Biomechanically, what is the primary purpose of this compensatory trunk shift?

. To increase the lever arm of the right hip abductors
. To decrease the lever arm of body weight acting on the right hip
. To shift the center of gravity medially over the left hip
. To increase the joint reaction force across the right hip for stability
. To substitute for a deficient ipsilateral gluteus maximus

Correct Answer & Explanation

. To decrease the lever arm of body weight acting on the right hip

Explanation

A compensated Trendelenburg gait involves leaning the trunk over the affected hip during stance. This shifts the body's center of gravity laterally, directly over the hip joint, which drastically reduces the body weight moment arm and minimizes the force required by the weak abductors.

Question 15430

Topic: Physiology & Rehabilitation

A patient with severe hip dysplasia exhibits an uncompensated Trendelenburg lurch (Duchenne gait), characterized by shifting the torso over the affected hip during the stance phase. What is the biomechanical effect of this compensatory gait?

. Decreases the body weight lever arm
. Increases the body weight lever arm
. Increases the abductor lever arm
. Decreases the abductor lever arm
. Increases the joint reaction force

Correct Answer & Explanation

. Decreases the body weight lever arm

Explanation

Shifting the torso over the affected hip moves the center of gravity closer to the center of rotation of the femoral head. This decreases the body weight lever arm, effectively reducing the required abductor force and minimizing the overall joint reaction force.

Question 15431

Topic: 1. General Principles & Basic Science

According to Paley's principles of deformity correction, if an osteotomy is performed at a level different from the center of rotation of angulation (CORA) but the hinge is placed at the CORA, what is the geometric result after angular correction?

. Pure angulation without translation
. Angulation with unintended creation of a secondary deformity
. Pure translation without angulation
. Collinear correction of axes with an expected translation at the osteotomy site
. Non-collinear parallel mechanical axes

Correct Answer & Explanation

. Collinear correction of axes with an expected translation at the osteotomy site

Explanation

Paley's Rule 2 states that if the osteotomy is outside the CORA but the hinge remains at the CORA, the mechanical axes will align (collinear correction). However, predictable translation will inevitably occur at the local osteotomy site.

Question 15432

Topic: 1. General Principles & Basic Science

In a structurally normal lower extremity, how does the mechanical axis of the femur (a line connecting the center of the femoral head to the center of the knee) typically relate to the anatomic axis of the femur?

. They are collinear
. The mechanical axis is 3 degrees valgus to the anatomic axis
. The mechanical axis is 5-7 degrees varus to the anatomic axis
. The mechanical axis is 5-7 degrees valgus to the anatomic axis
. The mechanical axis is 9 degrees varus to the anatomic axis

Correct Answer & Explanation

. The mechanical axis is 5-7 degrees varus to the anatomic axis

Explanation

The anatomic axis of the femur normally lies in 5 to 7 degrees of valgus relative to the mechanical axis. Therefore, the mechanical axis is oriented 5 to 7 degrees varus relative to the anatomic axis.

Question 15433

Topic: 1. General Principles & Basic Science

During a 6 cm femoral lengthening using a monolateral external fixator in a 14-year-old boy, he develops an extension contracture of the knee. What is the most common anatomic structure responsible for this specific complication?

. Hamstrings
. Rectus femoris
. Iliotibial band
. Gastrocnemius
. Patellar tendon

Correct Answer & Explanation

. Rectus femoris

Explanation

The rectus femoris muscle crosses both the hip and the knee joint. During significant femoral lengthening, its tension increases dramatically, often restricting knee flexion and resulting in an extension contracture.

Question 15434

Topic: 1. General Principles & Basic Science

To perform an opening wedge osteotomy without creating secondary translation of the mechanical axis, where should the mechanical hinge be placed relative to the CORA?

. On the concave cortex at the level of the CORA
. On the convex cortex at the level of the CORA
. In the center of the medullary canal at the CORA
. On the convex cortex away from the CORA
. On the concave cortex away from the CORA

Correct Answer & Explanation

. On the concave cortex at the level of the CORA

Explanation

According to Paley's Rule 1, placing the hinge at the CORA ensures no translation occurs. Placing the hinge specifically on the concave cortex at this level will produce an opening wedge osteotomy upon angular correction.

Question 15435

Topic: 1. General Principles & Basic Science

When evaluating a sagittal plane deformity of the tibia, a procurvatum deformity is identified. What is the characteristic radiographic description of this deformity?

. Posterior apex angulation
. Anterior apex angulation
. Medial apex angulation
. Lateral apex angulation
. Internal rotation of the distal segment

Correct Answer & Explanation

. Anterior apex angulation

Explanation

Procurvatum is a sagittal plane deformity characterized by an anteriorly directed apex. Conversely, recurvatum refers to a posteriorly directed apex.

Question 15436

Topic: 1. General Principles & Basic Science

A patient with severe varus gonarthrosis has a joint line convergence angle (JLCA) of 8 degrees measured on a standing AP radiograph. What does this abnormally high JLCA most likely indicate?

. Diaphyseal bowing of the tibia
. Fixed extra-articular bony valgus deformity of the femur
. Intra-articular deformity or ligamentous laxity
. Rotational malalignment of the femur
. A normal physiologic finding in varus knees

Correct Answer & Explanation

. Intra-articular deformity or ligamentous laxity

Explanation

The normal JLCA is 0 to 2 degrees. An abnormally high JLCA indicates joint space asymmetry, which typically arises from intra-articular cartilage loss, subchondral bone wear, or collateral ligament laxity.

Question 15437

Topic: Biology, Genetics & Bone Healing

A 6-year-old child with a history of lower extremity bowing and dental abscesses is diagnosed with X-linked hypophosphatemic (XLH) rickets. A targeted therapy is initiated. What is the mechanism of action of the most appropriate targeted medical treatment?

. Inhibits tissue-nonspecific alkaline phosphatase
. Activates the calcium-sensing receptor
. Binds and neutralizes fibroblast growth factor 23 (FGF23)
. Stimulates renal 1-alpha-hydroxylase
. Blocks the RANKL-RANK interaction

Correct Answer & Explanation

. Binds and neutralizes fibroblast growth factor 23 (FGF23)

Explanation

Burosumab is a monoclonal antibody that binds and inhibits FGF23, effectively preventing renal phosphate wasting in XLH rickets. It is the definitive medical treatment for patients with a PHEX mutation.

Question 15438

Topic: Biology, Genetics & Bone Healing

A 65-year-old male presents with increasing thigh pain. Radiographs reveal cortical thickening, coarsened trabeculae, and a radiolucent V-shaped lesion in the femoral diaphysis. What is the primary cellular defect in this disease?

. Defective osteoblast mineralization
. Defective osteoclast recruitment and overactivity
. Mutation in type 1 collagen
. Deficiency of tissue nonspecific alkaline phosphatase
. Impaired renal hydroxylation of 25-hydroxyvitamin D

Correct Answer & Explanation

. Defective osteoclast recruitment and overactivity

Explanation

Paget's disease is driven by abnormal, highly active osteoclasts (often with paramyxovirus-like inclusion bodies), leading to a secondary robust but disorganized osteoblastic response. The classic 'blade of grass' or V-shaped lytic lesion represents the advancing osteolytic front.

Question 15439

Topic: Biology, Genetics & Bone Healing

A 5-year-old boy presents with profound genu varum and short stature. Laboratory tests show low serum phosphate, normal serum calcium, and elevated FGF-23 levels. What is the standard medical treatment?

. Intravenous bisphosphonates
. High-dose Vitamin D alone
. Oral phosphate and calcitriol
. Recombinant human growth hormone
. Cinacalcet

Correct Answer & Explanation

. Oral phosphate and calcitriol

Explanation

X-linked hypophosphatemic rickets is caused by a PHEX mutation leading to elevated FGF-23, which causes renal phosphate wasting and decreased calcitriol synthesis. Treatment requires both oral phosphate supplementation and calcitriol (1,25-dihydroxyvitamin D) to heal rickets and prevent secondary hyperparathyroidism.

Question 15440

Topic: Biology, Genetics & Bone Healing

A 3-year-old child presents with a femoral fracture after a minor fall. Radiographs show a "bone-within-bone" appearance and generalized sclerosis. Laboratory findings reveal mild anemia and thrombocytopenia. What is the most likely genetic defect?

. COL1A1 mutation
. FGFR3 mutation
. TCIRG1 mutation
. RUNX2 mutation
. COMP mutation

Correct Answer & Explanation

. TCIRG1 mutation

Explanation

Malignant infantile osteopetrosis is commonly caused by mutations in TCIRG1, leading to an inability of osteoclasts to acidify Howship's lacunae. This results in failed bone resorption, densely sclerotic but brittle bones, and obliteration of the medullary cavity causing pancytopenia.

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