Orthopedic Biomechanics Practice Questions

Q: Which of the following muscles is primarily responsible for generating propulsive force during the push-off phase of normal gait?

Gastrocnemius. **Explanation:** The **push-off phase** (late stance) of the gait cycle requires a powerful plantarflexion force to propel the body forward and upward. 1. **Why Gastrocnemius is correct:** The **Gastrocnemius** and Soleus (together forming the Triceps Surae) are the primary plantarflexors of the ankle. During the "terminal stance" and "pre-swing" phases, the Gastrocnemius undergoes a powerful concentric contraction. This provides the necessary **propulsive force** to lift the heel off the ground and accelerate the center of mass forward. 2. **Why the other options are incorrect:** * **Popliteus:** Known as the "Key to the knee," its primary role is to unlock the knee by laterally rotating the femur on the fixed tibia to initiate flexion. It does not contribute to propulsion. * **Tibialis Anterior:** This is the primary **dorsiflexor** of the foot. It is most active during the "swing phase" (for foot clearance) and at "heel strike" (to control the lowering of the foot via eccentric contraction). * **Iliopsoas:** This is a powerful hip flexor. While it helps initiate the swing phase by pulling the thigh forward, it is not the primary generator of the distal propulsive force seen in push-off. **High-Yield Clinical Pearls for NEET-PG:** * **Gait Cycle:** Stance phase constitutes 60% and Swing phase 40% of the cycle. * **Trendelenburg Gait:** Caused by weakness of the Gluteus Medius (hip abductor). * **Foot Drop:** Result of Tibialis Anterior paralysis (Common Peroneal Nerve injury), leading to a "High Steppage Gait." * **Calf Muscle Rupture:** Often referred to as "Tennis Leg," involving the medial head of the Gastrocnemius.

Q: What is the primary dynamic stabilizer of the patella against the lateral pull of the vastus lateralis?

Vastus medialis obliquus. ### Explanation The patella is naturally predisposed to lateral displacement due to the **Q-angle**, which creates a lateral vector force during quadriceps contraction. To counteract this, the body employs both static and dynamic stabilizers. **Why Vastus Medialis Obliquus (VMO) is correct:** The **Vastus Medialis Obliquus (VMO)** is the most important **dynamic stabilizer** of the patella. Unlike the Vastus Medialis Longus (VML), which contributes to knee extension, the fibers of the VMO are oriented at an angle of approximately 50–55 degrees. This horizontal orientation allows it to pull the patella medially during the final degrees of extension, directly opposing the lateral pull of the vastus lateralis. **Analysis of Incorrect Options:** * **Vastus Medialis Longus (VML):** While part of the same muscle group, its fibers are longitudinal. Its primary function is knee extension, not medial stabilization. * **Medial Patellofemoral Ligament (MPFL):** This is the primary **static stabilizer** (contributing ~60% of restraint against lateral displacement). The question specifically asks for a *dynamic* (muscular) stabilizer. * **Trochlear Depth:** This is a **geometric/osseous stabilizer**. A shallow trochlea (trochlear dysplasia) predisposes to instability, but it is not a dynamic force. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Static Stabilizer:** MPFL (most commonly torn in acute patellar dislocations). * **Primary Dynamic Stabilizer:** VMO. * **"Screw Home" Mechanism:** Occurs in the last 20° of extension; VMO deficiency often leads to patellofemoral pain syndrome (PFPS) in this range. * **Patellar Tracking:** Influenced by the Q-angle; an increased Q-angle (common in females) increases the risk of lateral subluxation.

Q: The following gait is seen due to weakness of:

Gluteus medius. ***Gluteus medius*** - Weakness of the **gluteus medius** leads to a **Trendelenburg gait**, where the pelvis drops on the unsupported side during the swing phase of gait. - The image suggests pelvic tilting, which is characteristic of the body attempting to compensate for the inability of the gluteus medius to stabilize the pelvis. *Gluteus maximus* - Weakness of the gluteus maximus causes difficulty in **hip extension**, resulting in a **lurching gait** where the trunk is thrown backward at heel strike. - This is commonly known as a **gluteus maximus lurch**, which is not depicted in an obvious manner here. *Psoas major* - Weakness of the psoas major would primarily affect **hip flexion**, making it difficult to lift the leg off the ground (e.g., during the swing phase). - This would result in compensatory movements such as circumduction or hiking the hip, rather than the characteristic pelvic drop. *Tibialis anterior* - Weakness of the tibialis anterior causes **foot drop**, leading to a **steppage gait** where the knee is lifted high to avoid dragging the foot. - The image does not show a foot drop or high stepping, thus ruling out tibialis anterior weakness.

Q: The following gait is due to weakness of:

Gluteus medius. **Gluteus medius** - The image depicts a **Trendelenburg gait**, where the pelvis drops on the unsupported side (**swing phase**) due to weakness of the **contralateral gluteus medius** (on the stance leg). - The gluteus medius is crucial for **stabilizing the pelvis** during single-leg stance by abducting the hip and preventing the opposite hip from dropping. *Gluteus maximus* - Weakness of the gluteus maximus primarily causes difficulty with hip extension, leading to a **lurching backward gait** to maintain an upright posture. - It does not directly cause the classic pelvic drop observed in the Trendelenburg gait. *Psoas major* - The psoas major is a primary hip flexor; weakness would cause difficulty lifting the leg forward, resulting in a **waddling gait** or **inability to flex the hip**. - It is not directly responsible for stabilizing the pelvis laterally during locomotion. *Tibialis anterior* - Weakness of the tibialis anterior causes **foot drop**, leading to a **steppage gait** where the leg is lifted high to clear the foot off the ground. - This muscle is involved in dorsiflexion of the ankle, not in hip stability or pelvic drop.

Q: Following a road traffic accident, a patient develops type IIIa compound tibial fracture. Arrange the following external fixators in decreasing order of their stability (highest to lowest) 1. Ilizarov fixator 2. Uniplanar with a single rod 3. Uniplanar with double rod 4. Biplanar frame/Ring with a cylindrical rod

1>4>3>2. ***1>4>3>2*** - **Ilizarov fixator** utilizes multiple wires **under tension** and rings, providing the most **biologically stable** and rigid fixation due to its distributed force across the bone. - **Biplanar frames/Rings with a cylindrical rod** offer high stability by providing pin fixation in **two different planes**, significantly resisting bending and torsional forces. - **Uniplanar with double rod** provides better stability than a single rod by increasing the **moment of inertia** and reducing deflection under axial and bending loads. - **Uniplanar with a single rod** is the least stable due to its limited resistance to **torsional** and **bending forces** as pin placement is restricted to a single plane.

Q: Trendelenburg sign is positive due to the involvement of:

Gluteus medius. ***Gluteus medius*** - The **Trendelenburg sign** indicates weakness or paralysis of the hip abductor muscles, primarily the **gluteus medius** and **gluteus minimus**. - When standing on one leg, these muscles contract on the supported side to keep the pelvis level; if they are weak, the unsupported side of the pelvis drops. *Gluteus maximus* - This muscle is the primary **extensor of the hip** and is crucial for activities like climbing stairs or standing up from a seated position. - Its weakness would primarily affect hip extension, not the ability to keep the pelvis level during single-leg stance. *Psoas major* - The **psoas major** is a powerful **hip flexor** and contributes to lumbar spine stability. - Weakness of this muscle would impair hip flexion, making it difficult to lift the leg forward, but it is not directly involved in stabilizing the pelvis in the frontal plane during standing. *Adductor magnus* - The **adductor magnus** is an important **hip adductor** and also functions as an extensor in certain positions. - Its primary role is to bring the leg towards the midline, and its weakness would not cause the characteristic pelvic drop seen in a positive Trendelenburg sign.

Q: Wolff's law is:-

Osteogenesis is directly proportional to stress and strain.. ***Osteogenesis is directly proportional to stress and strain.*** - **Wolff's Law** states that **bone adapts to the loads** under which it is placed. This means bone will remodel and strengthen in response to increased mechanical stress and strain. - Increased weight-bearing exercise or physical activity leads to **increased bone density** and strength, while lack of stress (e.g., bed rest, immobility) results in bone resorption and weakening. *Epiphyseal centre which appears first unites last with diaphysis* - This statement describes **Ritter's law**, which pertains to the sequence of epiphyseal fusion rather than bone's response to mechanical stress. - Ritter's law is a concept in anatomy related to the order of **epiphyseal plate ossification** and closure. *None of above.* - This is incorrect because one of the provided options accurately defines Wolff's Law. - The third option precisely articulates the principle behind **bone remodeling** in response to mechanical forces. *Epiphyseal centre which appears first unites first with diaphysis.* - This statement is generally not a recognized law in bone development and is inconsistent with the principles of epiphyseal fusion, often contradicting Ritter's law. - The timing of epiphyseal fusion is complex and influenced by various factors, but not simply an "appears first, unites first" rule.

Q: During an autopsy, a pathologist finds a transverse fracture of the femur. Which type of force is most likely responsible for this fracture?

Bending force. ***Bending force*** - A **transverse fracture** results from a force applied perpendicular to the long axis of the bone, causing it to bend and fracture at the point of maximum stress. - This typically happens when the bone is **bent beyond its elastic limit**, leading to a clean break across its width. *Tensile force* - **Tensile force** involves pulling on the bone, which tends to cause **avulsion fractures** or **spiral fractures** if combined with twisting. - It would result in the bone being stretched apart, not a clean transverse break from bending. *Shear force* - **Shear force** occurs when forces are applied parallel to the surface of the bone but in opposite directions, causing one part to slide over another. - This typically leads to **oblique fractures** or **dislocations**, not a transverse fracture. *Compressive force* - **Compressive force** involves pushing the bone together, which can result in **crush fractures** or **impacted fractures**. - This force would typically shorten the bone or create multiple fragments, unlike a distinct transverse break.

Q: Increased Q angle predisposes to

Lateral patellar subluxation. ***Lateral patellar subluxation*** - An increased **Q angle** signifies a greater lateral pull on the patella due to the alignment of the quadriceps muscle and the patellar ligament. - This increased lateral force disproportionately stresses the **medial patellofemoral ligament** (MPFL) and can lead to the patella moving out of its trochlear groove laterally. *Medial patellar subluxation* - This is a rare condition, typically associated with **iatrogenic causes** (e.g., overcorrection during surgery) or congenital anomalies, not an increased Q angle. - An increased Q angle would exert a **lateral pull** on the patella, making medial subluxation less likely. *Superior patellar subluxation* - This usually results from patella alta (high-riding patella) or a **quadriceps tendon rupture**, which allows the patella to migrate proximally. - An increased Q angle directly influences the **medial-lateral stability** of the patella, not its superior-inferior position. *Inferior patellar subluxation* - This is often due to patella baja (low-riding patella) or conditions like **infrapatellar contracture syndrome**, where the patella is pulled distally. - The Q angle is a measure of the angle between the quadriceps femoris muscle and the patellar tendon, primarily affecting **transverse plane stability**.

Question 1

Which posture is associated with the greatest lumbar intradiscal pressure?

Accepted Answer

Sitting with trunk flexed

Answer

Sitting with trunk erect

Answer

Standing with trunk flexed

Answer

Standing with trunk erect

Question 2

Which of the following muscles is primarily responsible for generating propulsive force during the push-off phase of normal gait?

Accepted Answer

Gastrocnemius

Answer

Popliteus

Answer

Tibialis anterior

Answer

Iliopsoas

Question 3

What is the primary dynamic stabilizer of the patella against the lateral pull of the vastus lateralis?

Accepted Answer

Vastus medialis obliquus

Answer

Vastus medialis longus

Answer

Medial patellofemoral ligament

Answer

Trochlear depth

Question 4

The following gait is seen due to weakness of:

Accepted Answer

Gluteus medius

Answer

Gluteus maximus

Answer

Psoas major

Answer

Tibialis anterior

Question 5

The following gait is due to weakness of:

Accepted Answer

Gluteus medius

Answer

Gluteus maximus

Answer

Psoas major

Answer

Tibialis anterior

Question 6

Following a road traffic accident, a patient develops type IIIa compound tibial fracture. Arrange the following external fixators in decreasing order of their stability (highest to lowest)

1. Ilizarov fixator
2. Uniplanar with a single rod
3. Uniplanar with double rod
4. Biplanar frame/Ring with a cylindrical rod

Accepted Answer

1>4>3>2

Answer

2>4>3>1

Answer

1>2>3>4

Answer

2>3>1>4

Question 7

Trendelenburg sign is positive due to the involvement of:

Accepted Answer

Gluteus medius

Answer

Gluteus maximus

Answer

Psoas major

Answer

Adductor magnus

Question 8

Wolff's law is:-

Accepted Answer

Osteogenesis is directly proportional to stress and strain.

Answer

Epiphyseal centre which appears first unites last with diaphysis

Answer

None of above.

Answer

Epiphyseal centre which appears first unites first with diaphysis.

Question 9

During an autopsy, a pathologist finds a transverse fracture of the femur. Which type of force is most likely responsible for this fracture?

Accepted Answer

Bending force

Answer

Tensile force

Answer

Shear force

Answer

Compressive force

Question 10

Increased Q angle predisposes to

Accepted Answer

Lateral patellar subluxation

Answer

Medial patellar subluxation

Answer

Superior patellar subluxation

Answer

Inferior patellar subluxation

Orthopedic Biomechanics — MCQs

Orthopedic Biomechanics — MCQs

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