Clinically Relevant Aspects of Dental Materials Science in Orthodontics - Multiple choice questions In Orthodontics

Test your knowledge on Clinically Relevant Aspects of Dental Materials Science in Orthodontics (Graber 7th Edition) with these 35 high-yield MCQs.

---

1. The stiffness (elastic modulus) of a stainless steel bracket is clinically most relevant to which mechanic?

• A. Friction during sliding mechanics
• B. Bond strength to enamel
• C. Torque expression
• D. Corrosion resistance

Click to Reveal Answer

Correct Answer: C

Explanation: Stiffness determines the bracket's resistance to deformation. During torque application with rectangular wires, a bracket with low stiffness may experience elastic deformation (slot widening), which significantly reduces the effective torque delivered to the tooth.

2. Which feature of a stainless steel bracket slot surface has the greatest impact on increasing resistance to sliding (friction)?

• A. High polish
• B. Increased surface roughness
• C. Reduced slot depth
• D. Increased hardness

Click to Reveal Answer

Correct Answer: B

Explanation: While intuitive that smoother surfaces have more contact area, in clinical orthodontics, increased surface roughness creates mechanical interlocks and asperities that significantly increase the coefficient of friction and resistance to sliding.

3. Failure of the joint between the bracket base and wing in multi-piece stainless steel brackets is typically caused by:

• A. Galvanic corrosion
• B. Fatigue loading
• C. Thermal expansion mismatch
• D. Chemical degradation

Click to Reveal Answer

Correct Answer: B

Explanation: Multi-piece brackets are joined by welding or brazing. The repetitive cyclic loading of mastication causes fatigue failure at these joints, which is a common cause of breakage compared to single-piece MIM brackets.

4. Titanium brackets are primarily indicated for patients with:

• A. High aesthetic demands
• B. Severe crowding requiring superelastic brackets
• C. Documented nickel hypersensitivity
• D. Need for reduced friction

Click to Reveal Answer

Correct Answer: C

Explanation: Titanium brackets are biocompatible and nickel-free. They are the gold standard alternative for patients with a confirmed allergy to nickel, which constitutes ~8-12% of standard stainless steel.

5. Early generation polycrystalline ceramic brackets often failed due to:

• A. Low modulus of elasticity
• B. Low fracture toughness
• C. High water absorption
• D. Plastic deformation

Click to Reveal Answer

Correct Answer: B

Explanation: Ceramics are brittle materials characterized by low fracture toughness ($K_{IC}$). They lack the ability to plastically deform to blunt crack tips, leading to catastrophic failure (wing fracture) under stress.

6. 'Static fatigue' in ceramic brackets is a phenomenon caused by:

• A. Occlusal wear over time
• B. Stress-corrosion cracking in a wet environment
• C. Thermal cycling
• D. Ultraviolet light exposure

Click to Reveal Answer

Correct Answer: B

Explanation: Static fatigue refers to the delayed failure of ceramics under constant stress in the presence of water (saliva). Water molecules react with the strained bonds at the crack tip, lowering the energy required for crack propagation.

7. In polycrystalline ceramic brackets, a smaller grain size is associated with:

• A. Higher translucency and lower strength
• B. Lower translucency and higher strength
• C. Higher translucency and higher strength
• D. Lower translucency and lower strength

Click to Reveal Answer

Correct Answer: C

Explanation: Reducing grain size generally improves mechanical strength (Hall-Petch relation) and, if the grains are sufficiently small (nanoscale) to minimize light scattering at grain boundaries, can also improve translucency.

8. The primary risk during the debonding of ceramic brackets is:

• A. Bracket fracture leaving residue
• B. Enamel fracture
• C. Periodontal ligament trauma
• D. Pulp necrosis

Click to Reveal Answer

Correct Answer: B

Explanation: Because ceramic brackets are rigid and do not peel like metal brackets, debonding forces are transmitted directly to the adhesive-enamel interface. If the bond strength exceeds the cohesive strength of enamel (~14 MPa), enamel fracture can occur.

9. The 'unloading plateau' in a superelastic NiTi wire force-deflection curve represents:

• A. The force required to permanently deform the wire
• B. A region of constant force delivery during deactivation
• C. The breaking point of the wire
• D. The elastic limit of the austenitic phase

Click to Reveal Answer

Correct Answer: B

Explanation: The unloading plateau is the clinically desirable portion of the curve where the wire reverts from stress-induced martensite to austenite, delivering a nearly constant, physiologic force over a wide range of tooth movement.

10. Hysteresis in orthodontic archwires refers to:

• A. The energy lost during the loading and unloading cycle
• B. The wire's ability to resist corrosion
• C. The increase in stiffness with temperature
• D. The permanent deformation after bending

Click to Reveal Answer

Correct Answer: A

Explanation: Hysteresis is the difference between the energy required to activate the wire (loading) and the energy returned by the wire (unloading). Clinically, this means the activation force is higher than the force delivered to the tooth.

11. Compared to non-superelastic NiTi, superelastic NiTi wires are superior for:

• A. Detailed finishing bends
• B. Initial alignment of severe crowding
• C. Torque expression in rectangular slots
• D. Space closure with sliding mechanics

Click to Reveal Answer

Correct Answer: B

Explanation: Superelastic wires provide a light, continuous force over a large deflection range (crowding), whereas non-superelastic wires behave more linearly, exerting excessive force at large deflections.

12. The superelastic effect is due to a reversible phase transformation between:

• A. Alpha and Beta titanium
• B. Austenite and Martensite
• C. Ferrite and Cementite
• D. Amorphous and Crystalline phases

Click to Reveal Answer

Correct Answer: B

Explanation: Superelasticity is driven by a stress-induced transformation from the parent Austenite phase (stable at higher temps/low stress) to the Martensite phase (stable at lower temps/high stress).

13. Thermally activated NiTi wires are primarily in which phase at room temperature (below oral temp)?

• A. Austenitic
• B. Martensitic
• C. R-phase
• D. Work-hardened

Click to Reveal Answer

Correct Answer: B

Explanation: Thermal NiTi has a Transition Temperature Range (TTR) set near mouth temperature. At room temperature, they are Martensitic (soft, pliable), making ligation easy. They transform to Austenite (stiff) upon heating in the mouth.

14. Inadequate light intensity during bonding results in:

• A. Increased polymerization shrinkage
• B. Low degree of conversion and reduced bond strength
• C. Excessive heat generation
• D. Immediate adhesive discoloration

Click to Reveal Answer

Correct Answer: B

Explanation: Sufficient photon energy is required to activate the photoinitiator (Camphoquinone). Inadequate intensity leads to a low degree of conversion (fewer double bonds broken), resulting in weak mechanical properties and bond failure.

15. The main advantage of LED curing units over QTH (Halogen) units is:

• A. Wider spectral output
• B. Emission spectrum matched closely to Camphoquinone
• C. Higher heat generation for faster curing
• D. Ability to cure without photoinitiators

Click to Reveal Answer

Correct Answer: B

Explanation: LEDs emit a narrow band of blue light (approx. 450-470 nm) that peaks exactly where Camphoquinone absorbs energy, making them highly efficient compared to the broad, wasteful spectrum of Halogen lamps.

16. The 'Blue Light Hazard' associated with high-intensity curing lights refers to potential damage to the:

• A. Cornea
• B. Lens
• C. Retina
• D. Optic Nerve

Click to Reveal Answer

Correct Answer: C

Explanation: High-intensity blue light (400-500nm) can cause photochemical damage to the retinal pigment epithelium, potentially accelerating macular degeneration. Orange shields are mandatory.

17. Concerns regarding estrogenicity in orthodontic adhesives focus on the release of:

• A. TEGDMA
• B. Bis-GMA
• C. Bisphenol-A (BPA)
• D. Silica fillers

Click to Reveal Answer

Correct Answer: C

Explanation: BPA is a xenoestrogen. While not a direct ingredient in most modern adhesives, it can be present as an impurity or a degradation product of Bis-DMA or Bis-GMA precursors.

18. To mitigate aerosol hazards during adhesive grinding/debonding, the most effective measure is:

• A. Using a slow-speed handpiece
• B. High-volume evacuation (HVE) at the source
• C. Surgical masks
• D. Water irrigation alone

Click to Reveal Answer

Correct Answer: B

Explanation: Grinding generates respirable particulates (dust) containing resin and fillers. HVE significantly reduces the particulate load in the breathing zone of the patient and operator.

19. Light intensity decreases as the distance from the tip increases, following the:

• A. Linear law
• B. Inverse square law
• C. Exponential decay law
• D. Beer-Lambert law

Click to Reveal Answer

Correct Answer: B

Explanation: Intensity is inversely proportional to the square of the distance ($I \propto 1/d^2$). Moving the tip just a few millimeters away drastically reduces the energy reaching the adhesive.

20. Multistrand stainless steel wires are preferred for fixed retainers because they:

• A. Are stiffer than solid wires
• B. Allow physiologic tooth movement
• C. Bond chemically to composite
• D. Prevent calculus accumulation

Click to Reveal Answer

Correct Answer: B

Explanation: Solid wires are too rigid and can create stress concentrations that break the bond. Multistrand wires have a lower flexural modulus, allowing slight physiologic mobility of the splinted teeth.