Biomechanics of Arthroplasty — MCQs
A patient with metastatic breast cancer presents with pathological fracture of femur. What is the best fixation method?
Which of the following is the commonest material used to make an orthopedic implant?
Healing of bone is affected by:
Nickel-titanium alloy has increased flexibility over stainless steel. How does the modulus of elasticity for nickel-titanium alloy compare to that of stainless steel?
Which of the following fractures of the neck of femur are associated with maximal compromise in blood supply ?
All of the following factors affect osseointegration EXCEPT:
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?
Biomechanics of Arthroplasty Indian Medical PG Practice Questions and MCQs
Question 1: A patient with metastatic breast cancer presents with pathological fracture of femur. What is the best fixation method?
- A. Long Intramedullary Nail (Correct Answer)
- B. External Fixator
- C. Dynamic Hip Screw
- D. Plate and Screws
Explanation: ***Long Intramedullary Nail*** - Provides **strong internal fixation** that can bear weight immediately, crucial for patients with a limited life expectancy due to metastatic disease. - Stabilizes the entire bone, preventing further **pathological fractures** in the diaphysis and allowing earlier mobilization and pain relief. *External Fixator* - Primarily used for **temporary stabilization** in severe open fractures or polytrauma, and not for definitive fixation of pathological fractures. - High risk of **pin tract infections** and patient discomfort, making it unsuitable for long-term management in cancer patients. *Dynamic Hip Screw* - Primarily used for **intertrochanteric hip fractures**, which are typically proximal femur fractures. - Less effective for **diaphyseal fractures** or for stabilizing bone weakened by metastatic disease along its entire length. *Plate and Screws* - While effective for some fractures, plates may not provide sufficient **load-bearing capacity** for extensively lytic or weakened bone in metastatic disease without extensive bone grafting. - Risk of **stress shielding** and subsequent re-fracture proximal or distal to the plate, especially when the intramedullary canal is compromised by tumor.
Question 2: Which of the following is the commonest material used to make an orthopedic implant?
- A. Methyl-methacrylate
- B. Polyethylene (UHMWPE)
- C. Titanium (Correct Answer)
- D. Stainless steel
Explanation: ***Titanium*** - **Titanium** and its alloys (e.g., Ti-6Al-4V) are widely favored for orthopedic implants due to their **excellent biocompatibility**, high strength-to-weight ratio, and corrosion resistance. - Its **osseointegrative properties** allow bone to grow directly onto the implant surface, providing stable fixation without an intervening fibrous layer. *Methyl-methacrylate* - **Methyl-methacrylate** is primarily used as a **bone cement** (PMMA) to fix implants to bone, rather than as the primary material for the implant itself. - It provides immediate mechanical stability but does not integrate with bone. *Polyethylene (UHMWPE)* - **Ultra-high molecular weight polyethylene (UHMWPE)** is commonly used as a bearing surface in joint replacements (e.g., acetabular liner in hip replacements) for its **low friction** and good wear resistance. - It is not typically used for the structural components of the implant that bear the primary load. *Stainless steel* - **Stainless steel** (e.g., 316L) was historically a common implant material, particularly for temporary fixation devices like plates and screws. - While it has good strength and corrosion resistance, it generally has a **lower biocompatibility** and more elastic modulus mismatch with bone compared to titanium, making it less preferred for permanent, load-bearing implants.
Question 3: Healing of bone is affected by:
- A. Hypoxia
- B. Micromovement
- C. Muscle interposition
- D. All of the options (Correct Answer)
Explanation: ***All of the options*** - **Hypoxia**, **micromovement**, and **muscle interposition** are all factors known to impede or negatively affect the normal healing process of a bone fracture. - The successful healing of a bone fracture relies on a series of biological events that can be disrupted by these adverse conditions, leading to delayed union or non-union. *Hypoxia* - **Hypoxia**, or insufficient oxygen supply, impairs the metabolic activity of cells essential for bone healing, such as osteoblasts and chondrocytes. - It interferes with **angiogenesis**, the formation of new blood vessels, which is critical for delivering nutrients and oxygen to the healing bone. *Micromovement* - Excessive **micromovement** at the fracture site prevents the formation of a stable callus and can stimulate the development of fibrous tissue or cartilage instead of bone. - While some motion is beneficial, uncontrolled or excessive micromotion can lead to a **non-union** or pseudarthrosis, as it constantly disrupts the delicate tissue bridges attempting to form. *Muscle interposition* - **Muscle interposition** refers to muscle tissue becoming trapped between the bone fragments, physically separating them and preventing direct bone-to-bone contact. - This physical barrier inhibits the formation of the **fracture hematoma** and subsequent callus, thus mechanically hindering the healing process.
Question 4: Nickel-titanium alloy has increased flexibility over stainless steel. How does the modulus of elasticity for nickel-titanium alloy compare to that of stainless steel?
- A. One-fourth to one-fifth that of stainless steel (Correct Answer)
- B. Similar to stainless steel
- C. 2 to 3 times that of stainless steel
- D. Half that of stainless steel
Explanation: ***One-fourth to one-fifth that of stainless steel*** - **Nickel-titanium (NiTi) alloys** are known for their exceptional **superelasticity** and **shape memory properties**, which are directly related to their low modulus of elasticity. - This significantly lower modulus allows NiTi wires to undergo large elastic deformations without permanent deformation, providing increased flexibility and lighter, more continuous forces in orthodontics. *Similar to stainless steel* - This statement is incorrect because NiTi alloys were developed precisely to overcome the limitations of stainless steel, particularly its high stiffness. - If their moduli were similar, NiTi would not offer the clinical advantages of increased flexibility and lower force application. *2 to 3 times that of stainless steel* - This is incorrect as a higher modulus of elasticity would mean increased stiffness and reduced flexibility, which is contrary to the known properties and clinical applications of NiTi alloys. - Materials with higher moduli require greater force to deform and would be less suitable for applications requiring gentle, continuous forces like initial orthodontic tooth movement. *Half that of stainless steel* - While NiTi has a lower modulus than stainless steel, "half" is not an accurate approximation of the difference; the actual reduction is significantly greater, typically in the range of one-fourth to one-fifth. - This difference is crucial for explaining the unique clinical benefits of NiTi, such as its ability to be bent significantly without permanent deformation.
Question 5: Which of the following fractures of the neck of femur are associated with maximal compromise in blood supply ?
- A. Basicervical fracture
- B. Trans cervical fracture
- C. Sub Capital fractures (Correct Answer)
- D. Intertrochanteric fractures
Explanation: ***Sub Capital fractures*** - These fractures occur at the anatomical **neck of the femur**, very close to the femoral head. - Due to their location, they disrupt the main blood supply to the femoral head, primarily from the **retinacular arteries**, leading to a high risk of **avascular necrosis**. *Trans cervical fracture* - This fracture occurs through the **midneck of the femur**, which is still within the intracapsular region. - While it has a significant risk of **ischemia**, the compromise is generally less severe than in subcapital fractures. *Intertrochanteric fractures* - These are **extracapsular fractures** occurring between the greater and lesser trochanters. - They tend to have an **excellent blood supply** and thus a low risk of avascular necrosis, but are associated with more significant blood loss and malunion issues. *Basicervical fracture* - This is an **intracapsular fracture** that occurs at the base of the femoral neck, near the junction with the trochanters. - Although intracapsular, its position is slightly more proximal than subcapital fractures, potentially leaving more of the **retinacular vessels** intact, resulting in a somewhat lower risk of avascular necrosis compared to subcapital fractures.
Question 6: All of the following factors affect osseointegration EXCEPT:
- A. Biocompatibility of implant material.
- B. Implant design.
- C. Patient's blood type (Correct Answer)
- D. Status of the host bed.
Explanation: ***Patient's blood type*** - A patient's **blood type** (e.g., A, B, AB, O) is determined by antigens present on red blood cells and plays no direct role in the biological processes of bone healing or the integration of a dental implant with bone. - While systemic factors can influence osseointegration, blood type itself does not affect the cellular and molecular mechanisms required for direct bone-to-implant contact. *Biocompatibility of implant material* - The **biocompatibility** of the implant material (e.g., **titanium**) is crucial for osseointegration, as it must not elicit adverse reactions and must permit host bone growth on its surface. - Materials that are cytotoxic or inflammatory will prevent bone apposition and lead to fibrous encapsulation rather than direct bone contact. *Implant design* - **Implant design**, including features like **surface roughness**, thread pitch, and macro-geometry, significantly influences the initial stability and long-term success of osseointegration. - A greater surface area and appropriate surface treatments can enhance bone cell attachment and differentiation, promoting faster and stronger bone integration. *Status of the host bed* - The **status of the host bone bed** refers to its quality and quantity (e.g., bone density, vascularity), which are critical for the biological processes of osseointegration. - Adequate bone volume and good bone quality provide a stable foundation and sufficient blood supply for bone regeneration around the implant.
Question 7: 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 8: 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 9: 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 10: 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.
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