Biomechanics of Knee — MCQs
Which activity will be difficult to perform for a patient with an anterior cruciate deficient knee joint?
In acute quadriceps tendon rupture, which radiographic finding is most specific?
The kinetic energy of the body is least in one of the following phases of the walking cycle
Deformity is most commonly seen in primary osteoarthritis of the knee joint -
Patellar tendon-bearing P.O.P. cast is indicated in the following fracture:
Locking of the knee involves which of the following?
Which of the following tests is used to test anterior instability of shoulder?
A young marathon runner is participating in a marathon competition. After running for a short distance, he develops progressive, activity-related pain at the anteromedial aspect of the tibia which was mild to start with, but increased on further running. X-ray was normal. The doctor ordered a bone scan. What is the likely diagnosis?
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?
Biomechanics of Knee Indian Medical PG Practice Questions and MCQs
Question 1: Which activity will be difficult to perform for a patient with an anterior cruciate deficient knee joint?
- A. Getting up from a sitting position
- B. Walk downhill (Correct Answer)
- C. Walk uphill
- D. Sitting cross-legged
Explanation: ***Walk downhill*** - An **anterior cruciate ligament (ACL) deficient knee** experiences anterior tibial translation, especially when the muscles can't compensate, leading to instability. - Walking downhill places higher **anterior shear forces** on the knee joint and often involves knee extension or hyperextension, which dramatically increases the risk of the tibia translating anteriorly relative to the femur. *Getting up from a sitting position* - This activity primarily involves **quadriceps muscle contraction** and a concentric movement of the knee, which stabilizes the joint. - It does not typically place significant **anterior shear stress** on the ACL, even in a deficient knee. *Walk uphill* - Walking uphill often involves knee flexion and places the knee in a more protected position against **anterior tibial translation**. - The quadriceps and hamstrings work synergistically to **stabilize the joint** during this motion, reducing stress on the ACL. *Sitting cross-legged* - This position primarily involves **hip and knee flexion and external rotation**, but it is generally a static and non-weight-bearing position. - It does not impose significant **dynamic loads** or shear forces that would cause instability in an ACL-deficient knee.
Question 2: In acute quadriceps tendon rupture, which radiographic finding is most specific?
- A. Joint Space Narrowing
- B. Osteophytes
- C. Patella Baja
- D. Patella Alta (Correct Answer)
Explanation: ***Patella Alta*** - **Patella alta**, or a high-riding patella, is a classic radiographic sign of acute quadriceps tendon rupture because the **patella** is no longer tethered inferiorly by the intact quadriceps tendon. - The unopposed pull of the patellar tendon elevates the patella, making this a highly **specific finding**. *Joint Space Narrowing* - **Joint space narrowing** is indicative of cartilage loss and is commonly seen in degenerative conditions like **osteoarthritis**. - It does not directly reflect soft tissue injury such as a quadriceps tendon rupture. *Osteophytes* - **Osteophytes**, or bone spurs, are a sign of **degenerative joint disease** and chronic stress on joints, often seen in osteoarthritis. - They are not specific to acute traumatic injuries like tendon ruptures. *Patella Baja* - **Patella baja**, or a low-riding patella, is typically associated with **patellar tendon rupture**, not quadriceps tendon rupture. - In a patellar tendon rupture, the patella *drops* due to the loss of its inferior anchor, which is the opposite of a quadriceps rupture.
Question 3: The kinetic energy of the body is least in one of the following phases of the walking cycle
- A. Double support
- B. Mid-stance (Correct Answer)
- C. Toe-off
- D. Heel strike
Explanation: ***Mid-stance*** - During **mid-stance**, the body's center of gravity is at its **highest point**, and the vertical velocity is near zero as the body transitions from upward to downward motion, contributing to **reduced kinetic energy**. - At this phase, forward velocity is relatively constant but the body is at the apex of its vertical trajectory, representing a point of **minimal total kinetic energy** in the sagittal plane. - The body transitions from deceleration to acceleration, with the limb providing stable support as weight passes over the stance foot. *Double support* - In **double support**, both feet are on the ground during the weight transfer phase, and the body's center of gravity is at a lower position compared to mid-stance. - While some energy is dissipated during weight transfer, this phase involves active muscular work and forward momentum maintenance, with kinetic energy being variable. - This represents a transition phase between single support periods, with complex energy exchanges occurring. *Toe-off* - At **toe-off**, the propulsive phase of gait, the body is generating forward momentum with peak forward velocity, meaning there is **significant kinetic energy** as the foot pushes off the ground. - The body's center of gravity is moving upwards and forwards, indicating a higher kinetic energy state. - Ankle plantarflexors are actively propelling the body forward, maximizing kinetic energy output. *Heel strike* - **Heel strike** is a moment of initial contact where the body's forward velocity is still considerable, possessing **significant kinetic energy**. - The limb is preparing to absorb impact forces while the body's center of mass continues moving forward, representing high kinetic energy just before the deceleration phase. - This marks the beginning of the stance phase with substantial horizontal velocity maintained from the swing phase.
Question 4: Deformity is most commonly seen in primary osteoarthritis of the knee joint -
- A. Genu varus (Correct Answer)
- B. Genu valgum
- C. Genu recurvatum
- D. Flexion contracture
Explanation: ***Genu varus*** - **Genu varus** (bow-legged deformity) is the most common angular deformity seen in **primary osteoarthritis of the knee**, particularly due to greater wear in the medial compartment. - This deformity places increased stress on the medial compartment, exacerbating the progression of osteoarthritis in that region. *Genu valgum* - **Genu valgum** (knock-knee deformity) is less common in primary knee osteoarthritis compared to genu varus. - It typically results from greater involvement of the **lateral compartment** of the knee joint. *Genu recurvatum* - **Genu recurvatum** is characterized by hyperextension of the knee joint. - This deformity is often associated with ligamentous laxity or neuromuscular conditions, rather than being the primary or most common deformity in knee osteoarthritis. *Flexion contracture* - A **flexion contracture** refers to the inability to fully extend the knee, causing the knee to be perpetually bent. - While common in advanced knee osteoarthritis due to pain, muscle spasm, and joint space narrowing, it is a contracture, not an angular deformity like genu varus or valgus.
Question 5: Patellar tendon-bearing P.O.P. cast is indicated in the following fracture:
- A. Fracture of the tibia (Correct Answer)
- B. Fracture of the patella
- C. Fracture of the femur
- D. Fracture of the medial malleolus
Explanation: ***Fracture of the tibia*** - A **patellar tendon-bearing (PTB) cast** is specifically designed to bypass the knee joint and transfer weight from the patellar tendon to the cast, offloading the tibia. - This design is particularly useful for **stable, distal tibia fractures** where partial weight-bearing is desired to promote healing. *Fracture of the patella* - A PTB cast would place direct pressure on the **patella**, which is contraindicated in a patellar fracture. - Patellar fractures often require a **cylinder cast** or surgical fixation to immobilize the knee. *Fracture of the femur* - Femoral fractures are typically **more proximal** and require **traction**, **internal fixation**, or a **spica cast** for stabilization. - A PTB cast would not provide adequate immobilization or weight-bearing relief for a femoral fracture due to its design. *Fracture of the medial malleolus* - Medial malleolus fractures involve the **ankle joint**, which is distal to the area covered by a PTB cast. - These fractures typically require a **short leg cast** or surgical repair, focusing on ankle stabilization.
Question 6: Locking of the knee involves which of the following?
- A. Internal rotation of the tibia with the foot on the ground
- B. Contraction of the popliteus muscle
- C. Internal rotation of the femur with the foot on the ground (Correct Answer)
- D. External rotation of femur with the foot off the ground
Explanation: ***Internal rotation of the femur with the foot on the ground*** - When the foot is on the ground (closed kinematic chain), the **femur rotates internally on the tibia** during the end stages of knee extension. This creates a more stable, "locked" position of the knee. - This **terminal rotation of the femur** increases the contact area and tension in the cruciate ligaments, enhancing joint stability for weight-bearing. *Internal rotation of the tibia with the foot on the ground* - This describes the action of the **popliteus muscle** when "unlocking" the knee from full extension, not the locking mechanism itself. - With the foot on the ground, the tibia is fixed, and internal rotation would typically be a movement for unlocking, not locking. *Contraction of the popliteus muscle* - The **popliteus muscle** is primarily responsible for **unlocking the knee** from full extension, by causing internal rotation of the tibia (or external rotation of the femur). - Its contraction would lead to initial flexion of the knee, releasing the locked position, not establishing it. *External rotation of femur with the foot off the ground* - With the foot off the ground (open kinematic chain), **external rotation of the tibia** occurs during the final degrees of extension to lock the knee, not external rotation of the femur. - The locking mechanism requires specific relative rotation between femur and tibia; external rotation of the femur alone would not achieve the screw-home mechanism necessary for knee locking.
Question 7: Which of the following tests is used to test anterior instability of shoulder?
- A. Push-pull test
- B. Apprehension Test (crank test) (Correct Answer)
- C. Posterior drawer test
- D. Jerk test
Explanation: ***Apprehension Test (crank test)*** - The **apprehension test** assesses for anterior shoulder instability by passively abducting and externally rotating the arm, which is the position of potential anterior dislocation. - A positive test is indicated by the patient's **apprehension** or fear of dislocation, often accompanied by muscle guarding, as the head of the humerus is forced anteriorly. *Push-pull test* - The push-pull test is used to assess for **posterior shoulder stability**, specifically for **posterior labral tears** or instability. - It involves applying axial compression while simultaneously pulling the humerus posteriorly, looking for pain or a clunk. *Posterior drawer test* - The posterior drawer test is primarily used to evaluate **posterior glenohumeral instability**. - It involves stabilizing the scapula and applying a posterior force to the humerus while the arm is flexed, abducted, and internally rotated. *Jerk test* - The jerk test is used to identify **posterior-inferior glenohumeral instability** or a **posterior labral tear**, particularly a reverse Bankart lesion. - It involves axially loading the arm while moving it from an abducted and externally rotated position to an adducted and internally rotated position, looking for a sudden "jerk" or clunk.
Question 8: A young marathon runner is participating in a marathon competition. After running for a short distance, he develops progressive, activity-related pain at the anteromedial aspect of the tibia which was mild to start with, but increased on further running. X-ray was normal. The doctor ordered a bone scan. What is the likely diagnosis?
- A. Shin splint (Correct Answer)
- B. Nutcracker fracture
- C. Lisfranc fracture
- D. Jones fracture
Explanation: ***Medial Tibial Stress Syndrome (Shin Splints)*** - **Anteromedial tibial pain** in a runner that starts mild and **worsens with activity** is the classic presentation of medial tibial stress syndrome. - **Normal X-ray** with **bone scan** ordered indicates suspected **periosteal inflammation** and microtears in the tibial cortex, which are hallmarks of this overuse injury. *Nutcracker fracture* - This is a **cuboid bone fracture** in the foot caused by high-energy compression, not tibial pathology. - Pain would be in the **midfoot**, not the anteromedial tibia, and would be **visible on X-ray**. *Lisfranc fracture* - Involves **tarsometatarsal joint disruption** in the midfoot from trauma or twisting injuries. - Pain occurs in the **midfoot region**, not the tibia, and fracture-dislocation would be **evident on X-ray**. *Jones fracture* - This is a **fifth metatarsal base fracture** causing **lateral foot pain**, not tibial pain. - Would present with **point tenderness** over the lateral foot and be **clearly visible on X-ray**. The combination of **exercise-induced anteromedial tibial pain**, **normal radiographs**, and the need for **bone scan** confirmation makes medial tibial stress syndrome the most likely diagnosis in this marathon runner.
Question 9: 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 10: 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.
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