Biomechanics of Spine — MCQs
During a routine physical examination a 65-year-old male patient is tested for ease and flexibility of the movements of his lumbar region. Which of the following movements is most characteristic of the intervertebral joints in the lumbar region?
A patient has a herniated intervertebral disc impinging on the right C5 nerve roots. Which of the following movements would most likely be affected?
After a 26-year-old man's car was sideswiped by a large truck, he is brought to the emergency department with multiple fractures of the transverse processes of the cervical and upper thoracic vertebrae. Which of the following muscles might be affected?
Which of the following joints are commonly affected in osteoarthritis? I. First metatarsophalangeal joint II. Proximal interphalangeal joint III. Ankle joint IV. 5th and 6th cervical vertebrae joint Select the correct answer using the code given below :
False about fracture of vertebrae
When Class III elastics are used, what movement will the maxillary first molars exhibit?
A high crural index is typically observed in which of the following groups?
Which posture is associated with the greatest lumbar intradiscal pressure?
Which of the following muscles is primarily responsible for generating propulsive force during the push-off phase of normal gait?
What is the primary dynamic stabilizer of the patella against the lateral pull of the vastus lateralis?
Biomechanics of Spine Indian Medical PG Practice Questions and MCQs
Question 1: During a routine physical examination a 65-year-old male patient is tested for ease and flexibility of the movements of his lumbar region. Which of the following movements is most characteristic of the intervertebral joints in the lumbar region?
- A. Rotation
- B. Circumduction
- C. Lateral flexion (Correct Answer)
- D. Extension
Explanation: ***Lateral flexion*** - **Lateral flexion** (side bending) is the most characteristic movement of the lumbar spine among the given options. - This movement is primarily facilitated by the **frontal plane orientation of the facet joints** in the lumbar region. - The lumbar spine allows approximately **20-30 degrees of lateral flexion** to each side. - The structure of the intervertebral discs and facet joint orientation in the lumbar region particularly favor this movement. *Rotation* - **Rotation** is the LEAST characteristic movement of the lumbar spine. - The lumbar facet joints are oriented in the **sagittal plane**, which significantly **restricts rotational movement**. - The lumbar spine allows only about **5 degrees of rotation**, much less than the thoracic or cervical regions. - Most trunk rotation actually occurs at the thoracic spine, not the lumbar region. *Circumduction* - **Circumduction** is a combined, sequential movement (flexion → lateral flexion → extension → lateral flexion) typically seen in ball-and-socket joints. - While the spine can perform these individual movements, circumduction is not considered a characteristic or distinct movement of intervertebral joints. - This term is more applicable to joints like the shoulder and hip. *Extension* - **Extension** (backward bending) is certainly possible and important in the lumbar spine. - However, it is not the MOST characteristic movement—the lumbar spine allows approximately **20-25 degrees of extension**. [1] - Flexion (forward bending, 40-60 degrees) and lateral flexion are more prominent movements in the lumbar region during functional activities.
Question 2: A patient has a herniated intervertebral disc impinging on the right C5 nerve roots. Which of the following movements would most likely be affected?
- A. Extension of the fingers
- B. Extension of the shoulder
- C. Flexion of the elbow (Correct Answer)
- D. Flexion of the wrist
Explanation: ***Flexion of the elbow*** - The **C5 nerve root** is a primary contributor to the innervation of the **biceps brachii** and **brachialis** muscles, which are the prime movers for elbow flexion. - The C5 myotome specifically includes elbow flexion as one of its key motor functions. - Impingement of the C5 nerve root would therefore most directly impact the strength and function of **elbow flexion**, leading to weakness in this movement. *Extension of the fingers* - Finger extension is primarily mediated by the **C7 and C8 nerve roots** via the posterior interosseous nerve (branch of the radial nerve). - C5 does not significantly contribute to finger extension. *Extension of the shoulder* - Shoulder extension involves muscles primarily innervated by the **C6, C7, and C8 nerve roots** (e.g., latissimus dorsi via thoracodorsal nerve, teres major). - While C5 contributes to some shoulder movements (particularly **shoulder abduction** via the deltoid), it is not primarily responsible for shoulder extension. *Flexion of the wrist* - Wrist flexion is primarily served by muscles innervated by the **C6, C7, and C8 nerve roots** (e.g., flexor carpi radialis - C6/C7, flexor carpi ulnaris - C7/C8). - The C5 nerve root has minimal to no role in wrist flexion.
Question 3: After a 26-year-old man's car was sideswiped by a large truck, he is brought to the emergency department with multiple fractures of the transverse processes of the cervical and upper thoracic vertebrae. Which of the following muscles might be affected?
- A. Serratus Posterior Superior
- B. Rhomboid major
- C. Trapezius
- D. Levator scapulae (Correct Answer)
Explanation: ***Levator scapulae*** - The **levator scapulae** muscle originates from the posterior tubercles of the transverse processes of cervical vertebrae C1-C4. - Fractures to these **transverse processes** could directly impact the attachment and function of the levator scapulae. *Serratus Posterior Superior* - The **serratus posterior superior** originates from the nuchal ligament and spinous processes of C7-T3, inserting onto ribs 2-5 - Its origin is primarily from the **spinous processes**, not the transverse processes, of the cervical and upper thoracic vertebrae. *Rhomboid major* - The **rhomboid major** muscle originates from the spinous processes of T2-T5, inserting into the medial border of the scapula. - Its origins are from the **spinous processes** of the upper thoracic vertebrae, not the transverse processes. *Trapezius* - The **trapezius** is a large muscle with a broad origin from the external occipital protuberance, nuchal ligament, and spinous processes of C7-T12. - While it covers a large area, its attachments are primarily to the **occiput** and **spinous processes**, not the transverse processes of the cervical and upper thoracic vertebrae.
Question 4: Which of the following joints are commonly affected in osteoarthritis? I. First metatarsophalangeal joint II. Proximal interphalangeal joint III. Ankle joint IV. 5th and 6th cervical vertebrae joint Select the correct answer using the code given below :
- A. I, II, III and IV
- B. I and II only
- C. III and IV only
- D. I, II and IV only (Correct Answer)
Explanation: ***I, II and IV only*** - **Osteoarthritis** commonly affects joints that bear significant weight or are subject to repetitive stress, such as the **first metatarsophalangeal joint**, **proximal interphalangeal joints**, and the **cervical spine**. - Degenerative changes in these joints, including cartilage loss and **osteophyte formation**, are characteristic findings in osteoarthritis. *I, II, III and IV* - While the first metatarsophalangeal joint, proximal interphalangeal joints, and cervical vertebrae are commonly affected, the **ankle joint** is typically spared in primary osteoarthritis. - Ankle involvement in osteoarthritis is usually secondary to **trauma** or inflammatory arthritis rather than primary degenerative change. *III and IV only* - This option misses the common involvement of the **first metatarsophalangeal joint** and **proximal interphalangeal joints**, which are frequently affected in osteoarthritis. - The ankle joint is less commonly involved in primary osteoarthritis compared to other load-bearing joints like the **knee** and **hip**. *I and II only* - This option incorrectly omits the **cervical vertebrae**, which are a very common site for osteoarthritis, often leading to neck pain and **radiculopathy**. - While the metatarsophalangeal and proximal interphalangeal joints are correct, the exclusion of the cervical spine makes this option incomplete.
Question 5: False about fracture of vertebrae
- A. Fracture dislocation is common in flexion rotation injury
- B. Chance fracture occurs due to flexion distraction injury
- C. Wedge compression causes flexion injury
- D. Anterior longitudinal ligament runs along the posterior surface of vertebral bodies (Correct Answer)
Explanation: ***Anterior longitudinal ligament runs along the posterior surface of vertebral bodies*** - The **anterior longitudinal ligament (ALL)** runs along the **anterior aspect** of the vertebral bodies, preventing hyperextension. - The **posterior longitudinal ligament (PLL)** runs along the posterior surface of the vertebral bodies, within the vertebral canal. *Fracture dislocation is common in flexion rotation injury* - **Flexion-rotation injuries** are highly unstable and frequently lead to **fracture-dislocations** of the vertebral column. - The combined forces cause significant disruption of both bony and ligamentous structures, increasing the likelihood of displacement. *Chance fracture occurs due to flexion distraction injury* - A **Chance fracture** (or seatbelt fracture) is caused by a **flexion-distraction injury**, typically seen in individuals wearing lap belts during deceleration. - This mechanism results in a horizontal splitting of the vertebral body and posterior elements. *Wedge compression causes flexion injury* - A **wedge compression fracture** is the most common type of vertebral fracture and results from a **flexion injury** (hyperflexion). - The anterior portion of the vertebral body collapses, creating a wedge shape, while the posterior column remains intact.
Question 6: When Class III elastics are used, what movement will the maxillary first molars exhibit?
- A. Move distally and intrude
- B. Move mesially and extrude (Correct Answer)
- C. Move mesially and intrude
- D. Move only mesially; there will be no vertical movement
Explanation: **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.
Question 7: A high crural index is typically observed in which of the following groups?
- A. Jumping athletes (Correct Answer)
- B. Gymnasts
- C. Weight lifters
- D. Long-distance runners
Explanation: **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.
Question 8: Which posture is associated with the greatest lumbar intradiscal pressure?
- A. Sitting with trunk flexed (Correct Answer)
- B. Sitting with trunk erect
- C. Standing with trunk flexed
- D. Standing with trunk erect
Explanation: This question is based on the classic biomechanical studies by **Nachemson**, which measured intradiscal pressure at the L3-L4 level in various positions. ### **Explanation** The intradiscal pressure is determined by the combination of **superincumbent body weight** and **muscle activity** required to maintain balance. 1. **Sitting vs. Standing:** When sitting, the pelvis tilts posteriorly, and the normal lumbar lordosis is flattened. This increases the lever arm of the upper body weight, requiring greater back muscle contraction to maintain the posture, which significantly increases the load on the discs compared to standing. 2. **Flexion vs. Extension:** Flexion (leaning forward) shifts the center of gravity further forward. This creates a large **flexion moment**, forcing the posterior spinal muscles and ligaments to exert a massive counter-traction force to prevent the trunk from falling. This "pincer effect" compresses the disc severely. Therefore, **Sitting with trunk flexed (Option A)** combines the high baseline pressure of sitting with the added mechanical disadvantage of flexion, resulting in the highest intradiscal pressure (approx. 185-200% of standing pressure). ### **Analysis of Other Options** * **B. Sitting with trunk erect:** While higher than standing, the vertical alignment reduces the flexion moment compared to leaning forward. * **C. Standing with trunk flexed:** Pressure is high (approx. 150%), but the lower limbs and pelvis help absorb some load that is otherwise transmitted directly to the spine when sitting. * **D. Standing with trunk erect:** This is used as the baseline (100%). The weight is distributed through the vertebral bodies and facets. ### **High-Yield Clinical Pearls for NEET-PG** * **Lowest Pressure:** **Supine (lying flat)** has the lowest intradiscal pressure (approx. 25%). * **Highest Overall Pressure:** Sitting or standing while **flexed and lifting a weight** (e.g., lifting a bucket) produces the absolute maximum pressure. * **Coughing/Straining:** These maneuvers significantly increase intradiscal pressure due to the Valsalva effect. * **Clinical Application:** Patients with acute disc prolapse are advised to avoid sitting and forward bending to minimize the risk of further herniation.
Question 9: Which of the following muscles is primarily responsible for generating propulsive force during the push-off phase of normal gait?
- A. Popliteus
- B. Gastrocnemius (Correct Answer)
- C. Tibialis anterior
- D. Iliopsoas
Explanation: **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.
Question 10: What is the primary dynamic stabilizer of the patella against the lateral pull of the vastus lateralis?
- A. Vastus medialis longus
- B. Vastus medialis obliquus (Correct Answer)
- C. Medial patellofemoral ligament
- D. Trochlear depth
Explanation: ### 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.
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