Orthopedic Biomechanics Practice Questions

Q: When Class III elastics are used, what movement will the maxillary first molars exhibit?

Move mesially and extrude. **Explanation:** In orthodontic biomechanics, the direction of force determines the displacement of teeth. **Class III elastics** are stretched from the **mandibular anterior region** (usually the canines) to the **maxillary posterior region** (usually the first molars). **1. Why Option B is correct:** The force vector of a Class III elastic on the maxillary molar acts in a **downward and forward** direction. * **Mesial Movement:** The horizontal component of the force pulls the maxillary molar forward (mesially). * **Extrusion:** Because the elastic is attached to the lower arch (which is inferior to the maxilla), the vertical component of the force pulls the molar downward, leading to extrusion. **2. Why the other options are incorrect:** * **Option A & C:** Distal movement is characteristic of **Class II elastics**, where the force pulls the maxillary teeth backward. Intrusion would require a superiorly directed force (like a high-pull headgear), which elastics do not provide to the maxillary molars. * **Option D:** This ignores the vertical vector. In clinical practice, elastics rarely exert a purely horizontal force; the "line of action" always creates a vertical component that results in either extrusion or intrusion. **Clinical Pearls for NEET-PG:** * **Class II Elastics:** Cause **distalization and extrusion** of maxillary incisors/molars and **mesialization and extrusion** of mandibular molars. * **Side Effects:** A common side effect of Class III elastics is the steepening of the occlusal plane and a potential increase in the lower anterior facial height due to molar extrusion. * **Center of Resistance:** If the force does not pass through the center of resistance, rotation (tipping) will occur alongside translation.

Q: A high crural index is typically observed in which of the following groups?

Jumping athletes. **Explanation:** The **Crural Index** is a biomechanical ratio used to describe the proportions of the lower limb. It is calculated as: **Crural Index = (Length of Tibia / Length of Femur) × 100** **1. Why Jumping Athletes is Correct:** A high crural index indicates a **longer tibia relative to the femur**. From a biomechanical standpoint, a longer distal segment (tibia) increases the "lever arm" of the lower limb. In jumping athletes (such as high jumpers or basketball players), this anatomical advantage allows for a faster rate of limb extension and greater velocity at the foot during takeoff. This "long-lever" system is more efficient for explosive power and vertical displacement. **2. Analysis of Incorrect Options:** * **Gymnasts:** Typically have a lower crural index and shorter stature. This provides a lower center of gravity and a smaller moment of inertia, which is advantageous for rotational stability and balance. * **Weight lifters:** Benefit from shorter limbs (lower crural index) because shorter levers reduce the torque required to lift heavy loads, providing a mechanical advantage for strength over speed. * **Long-distance runners:** While they often have lean limbs, they do not necessarily require the extreme distal elongation seen in explosive jumpers; their biomechanics favor metabolic efficiency over maximum vertical power. **3. Clinical Pearls for NEET-PG:** * **Evolutionary Note:** High crural indices are often seen in populations adapted to hot climates (to increase surface area for heat dissipation) and in cursorial (running/jumping) animals. * **Brachial Index:** A similar ratio for the upper limb (Radius length / Humerus length × 100). * **High-Yield Fact:** In orthopedics, limb length ratios are crucial for gait analysis and prosthetic design. A higher crural index generally correlates with a higher center of mass, which is beneficial for high-velocity movements.

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: In the mandibular symphysis region, which additional forces are present that are not usually seen in the body, ramus, or condylar regions?

Torsion. **Explanation:** The mandibular symphysis is the midline junction of the two halves of the mandible. Unlike the body or ramus, which primarily experience linear bending forces, the symphysis is subjected to unique biomechanical stresses due to the "U-shaped" geometry of the mandible. **Why Torsion is Correct:** When the mouth opens or during unilateral loading (chewing on one side), the lateral pterygoid muscles pull the condyles medially. This causes the lower borders of the mandibular rami to flare outward while the superior borders move inward. This twisting motion around the long axis of the mandibular body meets at the midline, resulting in **torsional (twisting) forces** at the symphysis. This is why symphyseal fractures often require two points of fixation (e.g., two miniplates) to counteract these rotational forces. **Analysis of Incorrect Options:** * **A. Tension:** Tension occurs at the superior border (alveolar margin) of the mandible during function. It is common to all regions of the mandible. * **B. Compression:** Compression occurs at the inferior border of the mandible during loading. Like tension, it is a standard force found throughout the mandibular body. * **C. Shearing:** While shearing forces can occur during trauma, they are not the characteristic functional biomechanical force unique to the symphysis compared to other regions. **High-Yield NEET-PG Pearls:** * **Champy’s Lines of Osteosynthesis:** These are the ideal lines for plate placement to counteract tension. In the symphysis, two plates are needed to neutralize **torsion**. * **Tension vs. Compression:** In the mandible, the superior border is the tension zone, and the inferior border is the compression zone. * **Stress Distribution:** The symphysis is the thickest part of the mandible (mentum), yet it is a common site for fractures due to its prominent position and the concentration of torsional stress.

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: Defect in Gluteus Maximus lead to all of the following, EXCEPT:

Positive Trendelenburg's sign. ***Positive Trendelenburg's sign*** - A positive Trendelenburg's sign indicates weakness of the **gluteus medius** and **minimus** muscles, which are primarily responsible for abducting the hip and stabilizing the pelvis during gait. - A defect in the **gluteus maximus** does not directly cause this sign, as its main function is hip extension. *Extension defect* - The **gluteus maximus** is the primary extensor of the hip joint. - A defect in this muscle would indeed lead to difficulty or weakness in performing **hip extension**. *Difficulty in straightening from the bending position* - Straightening from a bending position requires powerful **hip extension**, which is a primary function of the **gluteus maximus**. - Without a functional gluteus maximus, this movement would be significantly impaired. *Difficulty rising from the sitting position* - Rising from a sitting position involves strong **hip extension** and **knee extension**, with the gluteus maximus being crucial for the hip component. - A defect would make this everyday activity much more challenging.

Question 1

When Class III elastics are used, what movement will the maxillary first molars exhibit?

Accepted Answer

Move mesially and extrude

Answer

Move distally and intrude

Answer

Move mesially and intrude

Answer

Move only mesially; there will be no vertical movement

Question 2

A high crural index is typically observed in which of the following groups?

Accepted Answer

Jumping athletes

Answer

Gymnasts

Answer

Weight lifters

Answer

Long-distance runners

Question 3

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 4

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 5

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 6

What is the mechanical advantage obtained from the wheel and axle principle of an elevator?

Accepted Answer

4.6

Answer

2.5

Answer

3

Answer

6

Question 7

In the mandibular symphysis region, which additional forces are present that are not usually seen in the body, ramus, or condylar regions?

Accepted Answer

Torsion

Answer

Tension

Answer

Compression

Answer

Shearing

Question 8

The following gait is seen due to weakness of:

Accepted Answer

Gluteus medius

Answer

Gluteus maximus

Answer

Psoas major

Answer

Tibialis anterior

Question 9

The following gait is due to weakness of:

Accepted Answer

Gluteus medius

Answer

Gluteus maximus

Answer

Psoas major

Answer

Tibialis anterior

Question 10

Defect in Gluteus Maximus lead to all of the following, EXCEPT:

Accepted Answer

Positive Trendelenburg's sign

Answer

Extension defect

Answer

Difficulty in straightening from the bending position

Answer

Difficulty rising from the sitting position

Orthopedic Biomechanics — MCQs

Orthopedic Biomechanics — MCQs

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