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Total Hip Arthroplasty (THA) MCQs

Practice Set 36 of 36

This practice set contains high-yield board review questions covering key concepts in Total Hip Arthroplasty (THA). Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 701

Topic: Total Hip Arthroplasty (THA)

A patient presents with symptomatic bilateral coxa valga. How does this specific proximal femoral deformity alter hip biomechanics compared to a normal neck-shaft angle?

. Increases the abductor lever arm and decreases joint reaction force
. Decreases the abductor lever arm and increases joint reaction force
. Increases the body weight lever arm and decreases joint reaction force
. Decreases both the abductor lever arm and joint reaction force

Correct Answer & Explanation

. Decreases the abductor lever arm and increases joint reaction force

Explanation

Coxa valga increases the neck-shaft angle, which decreases the femoral offset and shortens the abductor lever arm. This mechanical disadvantage requires increased abductor muscle force, thereby increasing the overall hip joint reaction force.

Question 702

Topic: Total Hip Arthroplasty (THA)

A 10-year-old girl (skeletal age 10) presents with a predicted leg length discrepancy of 3.2 cm at maturity. Assuming growth follows the Menelaus rule, at what approximate skeletal age should a distal femoral epiphysiodesis be performed to equalize her limb lengths?

. Immediately at skeletal age 10
. At skeletal age 11
. At skeletal age 12
. At skeletal age 13
. At skeletal age 14

Correct Answer & Explanation

. At skeletal age 11

Explanation

Using the Menelaus rule, the distal femur grows approximately 10 mm (1 cm) per year, and girls cease growth at skeletal age 14. To correct a 3.2 cm discrepancy, approximately 3 years of growth arrest are needed, indicating the procedure should be performed at skeletal age 11.

Question 703

Topic: Total Hip Arthroplasty (THA)

The Paley Multiplier method is frequently used to predict leg length discrepancy at skeletal maturity for patients with congenital femoral deficiency. What is the fundamental assumption of this predictive model?

. The leg length discrepancy increases by a fixed absolute number of millimeters each year.
. The inhibition of growth (ratio of the short limb to the normal limb) remains constant throughout development.
. Bone age must always be used instead of chronological age for accurate prediction.
. It is only applicable for predictions after the age of 5 years.
. The contralateral normal leg grows at an exponentially decreasing rate.

Correct Answer & Explanation

. The inhibition of growth (ratio of the short limb to the normal limb) remains constant throughout development.

Explanation

The Multiplier method for congenital limb deficiencies assumes a constant percentage of growth inhibition. This means the affected limb grows at a consistently slower proportional rate compared to the normal limb throughout childhood.

Question 704

Topic: Total Hip Arthroplasty (THA)

A non-ambulatory child with severe cerebral palsy develops bilateral spastic coxa valga. How does the pathophysiology of coxa valga alter the normal biomechanics of the hip joint?

. It increases femoral offset and increases abductor mechanical advantage.
. It decreases femoral offset and decreases abductor mechanical advantage.
. It increases femoral offset and decreases the joint reaction force.
. It decreases femoral offset and increases abductor mechanical advantage.
. It creates an excessive retroversion of the femoral neck without altering offset.

Correct Answer & Explanation

. It decreases femoral offset and decreases abductor mechanical advantage.

Explanation

Coxa valga is characterized by an increased neck-shaft angle, which anatomically decreases the horizontal femoral offset. This shortens the abductor moment arm, reducing the mechanical advantage of the abductors and increasing the joint reaction force.

Question 705

Topic: Total Hip Arthroplasty (THA)

In a single-leg stance static free-body diagram of the hip, if the patient's effective body weight (W) is 600 N, the body weight moment arm is 10 cm, and the abductor moment arm is 5 cm, what is the approximate magnitude of the total hip joint reaction force (JRF)? (Assume all forces act in parallel).

. 600 N
. 1200 N
. 1800 N
. 2400 N
. 3000 N

Correct Answer & Explanation

. 1800 N

Explanation

The required abductor force (F_abd) is calculated by balancing moments: F_abd = (600 N * 10 cm) / 5 cm = 1200 N. The total joint reaction force is the sum of the effective body weight and the abductor force (JRF = 600 N + 1200 N = 1800 N).

Question 706

Topic: Total Hip Arthroplasty (THA)

The Paley multiplier method is highly accurate for predicting leg length discrepancy at skeletal maturity. What is the fundamental physiological assumption underlying the use of this method in congenital limb deficiencies?

. Growth inhibition increases exponentially with advancing age
. Growth inhibition remains a constant proportion of normal growth
. The leg length discrepancy remains a constant absolute value throughout childhood
. Growth velocity always peaks earlier in the congenitally affected limb
. The discrepancy spontaneously corrects during the pubertal growth spurt

Correct Answer & Explanation

. Growth inhibition remains a constant proportion of normal growth

Explanation

The multiplier method assumes that the relative growth inhibition of the affected bone remains constant over time. Because the ratio of the discrepancy to the total length is constant, simple multiplication can predict the discrepancy at skeletal maturity.

Question 707

Topic: Total Hip Arthroplasty (THA)

A 12-year-old girl is calculated to have a projected leg length discrepancy of 1.5 cm at skeletal maturity. She is currently asymptomatic but her parents are highly concerned. What is the most appropriate management recommendation?

. Distal femoral epiphysiodesis
. Proximal tibial epiphysiodesis
. Distraction osteogenesis for femoral lengthening
. No operative intervention
. Proximal femoral shortening osteotomy

Correct Answer & Explanation

. No operative intervention

Explanation

Leg length discrepancies of less than 2.0 cm at skeletal maturity rarely cause functional impairment and are generally well tolerated. Non-operative management, such as observation or a simple shoe lift if symptomatic, is indicated.

Question 708

Topic: Total Hip Arthroplasty (THA)

A distal femoral osteotomy is planned for a 12-degree valgus deformity. If a lateral opening wedge technique is executed, what associated structural change is inherent to this procedure?

. Significant shortening of the femur
. Lengthening of the femur
. Medial translation of the mechanical axis without angulation correction
. Increase in the anatomical femorotibial angle
. Decreased resting tension on the iliotibial band

Correct Answer & Explanation

. Lengthening of the femur

Explanation

Opening wedge osteotomies inherently add length to the bone segment due to the addition of space (and often a bone graft) at the osteotomy site. This can be beneficial if the valgus limb is also short, but may cause an unwanted leg length discrepancy if the limbs were equal preoperatively.

Question 709

Topic: Total Hip Arthroplasty (THA)

A patient uses a cane in the contralateral hand to relieve pain from unilateral hip osteoarthritis. By what biomechanical mechanism does this intervention primarily decrease the hip joint reaction force on the affected side?

. Increasing the moment arm of the hip abductor musculature
. Creating a counter-moment that reduces the required force of the affected hip abductors
. Directly decreasing the body weight moment arm by shifting the center of gravity laterally
. Increasing the total compressive force through the pelvis
. Shifting the center of mass towards the affected hip

Correct Answer & Explanation

. Creating a counter-moment that reduces the required force of the affected hip abductors

Explanation

Using a cane in the contralateral hand creates an upward ground reaction force with a long moment arm to the affected hip. This generates a counter-moment that significantly reduces the force required by the affected hip abductors, thereby exponentially decreasing the total hip joint reaction force.

Question 710

Topic: Total Hip Arthroplasty (THA)

A 10-year-old girl is evaluated for a leg length discrepancy (LLD). According to standard growth charts and multiplier methods, at approximately what age do normal girls achieve 50% of their mature lower extremity length?

. 1 year
. 2 years
. 3 years
. 4 years
. 5 years

Correct Answer & Explanation

. 3 years

Explanation

Girls typically reach 50% of their final mature lower extremity length at approximately 3 years of age, whereas boys reach this milestone at approximately 4 years of age.

Question 711

Topic: Total Hip Arthroplasty (THA)

A surgeon performs a total hip arthroplasty and intentionally decreases the femoral offset compared to the patient's native anatomy. What is the most significant biomechanical consequence of this change?

. Decreased joint reactive force across the hip
. Increased abductor muscle force required for pelvic stability
. Increased tension on the sciatic nerve
. Increased abductor moment arm length
. Increased bending moment on the femoral stem

Correct Answer & Explanation

. Increased abductor muscle force required for pelvic stability

Explanation

Decreasing femoral offset shortens the abductor moment arm. To maintain pelvic equilibrium against body weight, the abductor muscles must generate significantly more force, which concurrently increases the overall joint reactive force.

Question 712

Topic: Total Hip Arthroplasty (THA)

According to Paley's rules of deformity correction, if an osteotomy is performed at a level outside the Center of Rotation of Angulation (CORA) and the hinge is placed at the osteotomy site, what is the geometric result of the correction?

. Collinear realignment with expected translation at the osteotomy
. Correction of angulation but creation of an iatrogenic translational deformity
. Translation of the mechanical axis without any change in angular deformity
. Perfect restoration of the anatomical axis with no translation
. Creation of an iatrogenic leg length discrepancy without changing the axis

Correct Answer & Explanation

. Correction of angulation but creation of an iatrogenic translational deformity

Explanation

Paley's Rule 3 dictates that when both the osteotomy and the hinge are located at a level distinct from the CORA, correcting the angular deformity will inadvertently cause the mechanical axis to translate, creating a secondary translational deformity.

Question 713

Topic: Total Hip Arthroplasty (THA)

In a patient undergoing total hip arthroplasty, the surgeon increases the femoral offset by 10 mm compared to the native anatomy. Which of the following biomechanical effects is most likely to occur as a direct result of this modification?

. Decreased strain on the abductor musculature and decreased joint reaction force
. Decreased strain on the abductor musculature and increased joint reaction force
. Increased joint reaction force and decreased torque on the femoral stem
. Decreased bending moment on the femoral component and increased abductor tension
. Increased bony impingement during hip abduction

Correct Answer & Explanation

. Decreased strain on the abductor musculature and increased joint reaction force

Explanation

Increasing femoral offset lengthens the abductor moment arm, which reduces the required abductor muscle force to maintain pelvic stability. Consequently, this leads to a decrease in the overall hip joint reaction force.

Question 714

Topic: Total Hip Arthroplasty (THA)

According to the principles of deformity correction (Osteotomy Rule 2), if the osteotomy is made at a level different from the CORA, but the axis of rotation is placed perfectly at the CORA, what is the geometric result?

. The bone corrects with pure angulation.
. The bone corrects with angulation and translation, restoring the mechanical axis.
. The mechanical axis is intentionally translated away from the joint center.
. The correction results in an inevitable leg length discrepancy.

Correct Answer & Explanation

. The bone corrects with angulation and translation, restoring the mechanical axis.

Explanation

Under Rule 2, placing the hinge at the CORA but making the osteotomy elsewhere allows for simultaneous angulation and translation. This restores the colinearity of the proximal and distal mechanical axes.

Question 715

Topic: Total Hip Arthroplasty (THA)

During templating for a total hip arthroplasty, the surgeon plans to use a high-offset femoral stem to increase global femoral offset without altering leg length. What is the expected effect of this choice on hip biomechanics and stability?

. Decreases abductor tension and decreases impingement risk
. Increases abductor tension and decreases impingement risk
. Increases abductor tension and increases impingement risk
. Decreases abductor tension and increases impingement risk
. Has no effect on abductor tension but decreases impingement risk

Correct Answer & Explanation

. Increases abductor tension and decreases impingement risk

Explanation

Increasing femoral offset shifts the femur laterally, which restores or increases the abductor lever arm (increasing resting abductor tension and mechanical advantage). It also distances the proximal femur from the pelvis, reducing the risk of bony impingement and improving stability.

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