Principles of Biomechanics — MCQs
When occlusal forces are increased, the cancellous bony trabeculae adapt by?
Which of the following is the commonest material used to make an orthopedic implant?
Lateral movement is produced by anterior translation of one condyle producing rotation about the
Healing of bone is affected by:
The mechanoreceptors in joints and ligaments are:
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?
When forces are applied on the lateral surface of the mandibular angle region, compression is generated on:
All of the following are indications for open reduction and internal fixation (ORIF) of fractures EXCEPT:
Locking compression plating is commonly indicated in which of the following fracture types?
When Class III elastics are used, what movement will the maxillary first molars exhibit?
Principles of Biomechanics Indian Medical PG Practice Questions and MCQs
Question 1: When occlusal forces are increased, the cancellous bony trabeculae adapt by?
- A. Increase in number and thickness (Correct Answer)
- B. Decrease in number and thickness
- C. Decrease in number and increase in thickness
- D. Remains the same
Explanation: ***Increase in number and thickness*** - According to **Wolff's Law**, bone adapts to the stresses placed upon it; increased occlusal forces lead to increased bone density and strength. - This adaptive response involves an **increase in both the number and thickness of trabeculae** to better distribute and withstand the heightened forces. *Decrease in number and thickness* - This scenario would occur if occlusal forces were *decreased* or absent, leading to bone resorption and weakening, not strengthening. - Reduced number and thickness would result in bone that is more **susceptible to fracture** under normal or increased loads. *Decrease in number and increase in thickness* - While an increase in thickness contributes to strength, a decrease in the number of trabeculae would result in fewer structural units to distribute stress, potentially weakening the overall bone structure. - This pattern is not characteristic of bone adaptation to *increased* occlusal forces, which demands a more comprehensive strengthening response. *Remains the same* - Bone is a dynamic tissue that constantly undergoes remodeling in response to mechanical stimuli. - If bone structure remained unchanged despite increased occlusal forces, it would be unable to adequately withstand the stress, potentially leading to **bone damage or resorption**.
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: Lateral movement is produced by anterior translation of one condyle producing rotation about the
- A. Center in the opposite neck
- B. Center in the opposite ramus
- C. Center in the opposite condyle (Correct Answer)
- D. Center in the opposite angle
Explanation: ***Center in the opposite condyle*** - **Lateral excursion** of the mandible involves the **working side condyle** rotating around a vertical axis, while the **non-working side condyle** translates anteriorly and medially (Bennett movement). - This anterior translation of the non-working condyle causes the entire mandible to pivot, with the center of rotation for the **lateral movement** being located roughly within the **condyle** on the **working (rotating)** side of the jaw. *Center in the opposite neck* - While the neck of the condyle is anatomically close to the condyle head, the **functional center of rotation** for lateral movement is typically described as being within the condyle itself, specifically its rotating component. - Positioning the center of rotation in the neck would imply a different biomechanical axis for the movement, which is not accurately reflected in standard mandibular kinematics. *Center in the opposite ramus* - The **ramus** is a broad part of the mandible, much larger than the condyle, and locating the center of rotation here would imply a much wider arc of movement, which is not consistent with the precise articulation of the **temporomandibular joint**. - The primary movements of the mandible during lateral excursion are centered on the condyle and its articular surfaces, not the entire ramus. *Center in the opposite angle* - The **angle of the mandible** is a distant anatomical landmark from the temporomandibular joint and is primarily involved in muscle attachments, not as a point of rotation for **lateral condylar movement**. - Placing the center of rotation at the angle would be biomechanically inaccurate for describing mandibular kinematics during lateral excursion.
Question 4: 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 5: The mechanoreceptors in joints and ligaments are:
- A. Adapt differentially for different stresses
- B. Slow adapting (Correct Answer)
- C. Fast adapting
- D. Non adapting
Explanation: ***Slow adapting*** - **Mechanoreceptors** in joints and ligaments, such as **Ruffini endings** and **Golgi-type endings**, are primarily **slowly adapting**. - This characteristic allows them to provide continuous information about **joint position** and **pressure** over extended periods. *Adapt differentially for different stresses* - While different mechanoreceptors respond to different types of stimuli (e.g., pressure, stretch), this option describes varying responses rather than the fundamental **adaptation rate**. - The primary characteristic being asked for is how their firing rate changes over time in response to a constant stimulus. *Fast adapting* - **Fast-adapting mechanoreceptors**, like **Pacinian corpuscles** and **Meissner's corpuscles**, respond strongly at the onset and offset of a stimulus. - They are more involved in sensing **vibration** and **changes in pressure** rather than sustained joint position. *Non adapting* - All biological sensory receptors exhibit some degree of **adaptation** to a constant stimulus, meaning their firing rate changes over time. - A truly **non-adapting** receptor would fire at a constant rate indefinitely for a given stimulus, which is not characteristic of mechanoreceptors.
Question 6: 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 7: When forces are applied on the lateral surface of the mandibular angle region, compression is generated on:
- A. Medial surface
- B. Superior surface
- C. Lateral surface (Correct Answer)
- D. Inferior surface
Explanation: ***Lateral surface*** - When a force is applied to the **lateral surface** of the mandibular angle, this is the **point of direct impact and compression**. - According to **biomechanical principles**, compression occurs at the site where external force is applied to bone [1]. - In bending mechanics, the side receiving the load experiences **compressive stress**, while the opposite side experiences tensile stress [1]. - This principle is fundamental in understanding **mandibular fracture patterns** and surgical plating techniques. *Medial surface* - The medial surface, being **opposite to the point of force application**, experiences **tensile (tension) forces**, not compression [1]. - In beam bending theory, when one side is compressed, the opposite side is under tension [1]. - This is why fracture lines in the mandible often propagate from the tension side (medial) when lateral forces are applied. *Inferior surface* - The inferior border of the mandible is classically described as the **tension side during mastication and functional loading**, not lateral impact forces. - When lateral forces are applied to the angle, the inferior surface experiences complex stress patterns but is not the primary site of compression. - The inferior border-superior border axis is different from the lateral-medial force axis described in this question. *Superior surface* - The superior (alveolar) border typically experiences **compression during mastication**, but this relates to occlusal forces, not lateral impact. - For lateral forces applied to the mandibular angle, the superior surface does not experience primary compression. - This surface is more relevant for bite forces and dental occlusion mechanics.
Question 8: All of the following are indications for open reduction and internal fixation (ORIF) of fractures EXCEPT:
- A. Multiple trauma
- B. Stable closed fracture (Correct Answer)
- C. Compound fracture
- D. Intra-articular fracture
Explanation: ***Stable closed fracture*** - A **stable closed fracture** typically does not require surgical intervention with ORIF as it can usually be managed non-surgically with casting or bracing. - The goal of ORIF is to achieve **anatomic reduction and rigid fixation**, which is not necessary for stable fractures that maintain alignment. *Multiple trauma* - In patients with **multiple trauma**, early stabilization of long bone fractures using ORIF can help reduce pain, prevent further injury, and facilitate patient mobilization. - This approach aims to reduce the risk of complications such as **ARDS (acute respiratory distress syndrome)** and fat embolism for critically ill patients. *Compound fracture* - **Compound (open) fractures** involve a break in the skin, exposing the bone to the external environment, and are a classic indication for surgical management. - ORIF in these cases helps to achieve **stabilization** after debridement, crucial for preventing infection and promoting bone healing. *Intra-articular fracture* - **Intra-articular fractures** involve the joint surface, and accurate anatomical reduction is critical to prevent post-traumatic arthritis and preserve joint function. - ORIF provides the precise reduction and stable fixation needed to restore the **joint congruity**.
Question 9: Locking compression plating is commonly indicated in which of the following fracture types?
- A. Periaicular fractures
- B. Transverse or oblique fractures of long bones (Correct Answer)
- C. Interochanteric fractures
- D. Fracture of long bones
Explanation: ***Transverse or oblique fractures of long bones*** - **Locking compression plates (LCPs)** are particularly well-suited for **transverse or oblique fractures of long bones** because they provide angular stability, preventing screw pullout even in compromised bone. - Their design allows for a **fixed-angle construct**, which helps maintain alignment and promotes biological healing by minimizing periosteal stripping. *Periaicular fractures* - While LCPs can be used in some **periarticular fractures**, their primary indication is not specifically these fractures, and their benefit is often related to the bone quality of the metaphysis rather than the articulation itself. - These fractures often require careful contouring of plates to conform to the complex anatomy, and sometimes require different fixation strategies. *Interochanteric fractures* - **Intertrochanteric fractures** of the femur are typically treated with intramedullary nails (e.g., trochanteric entry nails) or dynamic hip screws, which are better suited for load-sharing in this weight-bearing region. - Plates, especially LCPs, are generally not the first-line treatment for these fractures due to the high biomechanical forces and risk of cutout. *Fracture of long bones* - This option is too general; while LCPs are used for some **long bone fractures**, it is not indicated for all types. Many long bone fractures are better treated with intramedullary nailing or traditional non-locked plating. - The specific fracture pattern (e.g., comminuted, transverse, oblique) and location within the long bone determine the most appropriate fixation method.
Question 10: 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.
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