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.

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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.

21. The composite resin used for fixed retainers should have:

• A. High flow and low wear resistance
• B. High wear resistance and appropriate viscosity
• C. Self-etching properties
• D. Opaque color for visibility

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Correct Answer: B

Explanation: The adhesive is often subject to occlusal or tongue abrasion. High wear resistance is crucial to prevent thinning of the coverage over the wire, which leads to detachment.

22. 'Stress relaxation' in thermoplastic aligner materials results in:

• A. The aligner becoming tighter over time
• B. A decrease in force delivery over time at constant deflection
• C. Increased brittleness
• D. Expansion of the material

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Correct Answer: B

Explanation: Viscoelastic materials do not maintain constant force under constant strain. The polymer chains rearrange to relieve stress, causing the force exerted on the tooth to drop significantly within hours.

23. Intraoral water absorption affects aligner materials by:

• A. Acting as a plasticizer and reducing mechanical properties
• B. Increasing stiffness
• C. Improving optical clarity
• D. preventing bacterial colonization

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Correct Answer: A

Explanation: Aligners absorb water from saliva. Water molecules insert themselves between polymer chains (plasticization), increasing flexibility but decreasing the force delivery capability and dimensional stability.

24. Composite attachments in aligner therapy function primarily by:

• A. Strengthening the aligner plastic
• B. Providing a surface for force application
• C. Improving esthetics
• D. Reducing treatment time chemically

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Correct Answer: B

Explanation: Aligners can slip over smooth tooth surfaces. Attachments provide a geometric undercut or vertical surface for the plastic to push against, essential for movements like extrusion, rotation, or root control.

25. Poor 'tracking' of an aligner is a clinical sign that:

• A. The material is too stiff
• B. The programmed movement has not been achieved
• C. The patient is allergic to the material
• D. The attachments are too large

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Correct Answer: B

Explanation: Tracking refers to the fit. An air gap (usually incisal) indicates the tooth has not moved as far as the aligner tray, implying the force system failed to express the desired movement.

26. Thermoplastic Polyurethane (TPU) aligners are generally preferred over PETG because TPU offers:

• A. Higher stiffness
• B. Greater toughness and resistance to cracking
• C. Zero water absorption
• D. Lower cost

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Correct Answer: B

Explanation: TPU is a block copolymer with soft and hard segments, giving it superior toughness, tear resistance, and elasticity compared to the more brittle PETG.

27. If a bracket slot widens (deforms) under torque, the clinical result is:

• A. Wire fracture
• B. Loss of torque expression
• C. Increased friction
• D. Bracket debonding

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Correct Answer: B

Explanation: Torque requires the wire to twist against the slot walls. If the walls yield (open up), the wire spins freely, and the effective torque couple on the tooth is lost.

28. Beta-titanium (TMA) wires generally exhibit higher friction than Stainless Steel wires because of:

• A. Surface roughness and chemical reactivity (stickiness)
• B. Low modulus of elasticity
• C. Cross-sectional shape
• D. Oxide layer thickness

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Correct Answer: A

Explanation: Beta-titanium is known for high surface friction ('stickiness') and a tendency for cold welding with the bracket, which can impede sliding mechanics.

29. Heat generation from LED curing units can potentially damage:

• A. Enamel prisms
• B. The dental pulp
• C. The bracket base
• D. The archwire

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Correct Answer: B

Explanation: The pulp is highly sensitive to temperature. A rise of just 5.5°C can cause irreversible pulpitis. High-power LEDs generate significant heat flux that must be managed.

30. An aligner material with a very high modulus of elasticity (stiff) will typically result in:

• A. High initial forces that decay rapidly
• B. Constant light forces
• C. Low initial forces
• D. Increased patient comfort

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Correct Answer: A

Explanation: Stiff materials generate high forces for a given deflection. However, they often have lower elastic limits and stress relax quickly, leading to a 'high force, then no force' cycle.

31. To reduce friction in ceramic brackets, manufacturers often:

• A. Use a metal slot insert
• B. Increase the grain size
• C. Make the slot narrower
• D. Use a rougher wire

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Correct Answer: A

Explanation: Metal slot inserts (usually SS or Gold) are used in some ceramic brackets to combine the aesthetics of ceramic with the low-friction mechanics of metal.

32. Which phase of NiTi is responsible for the shape memory effect?

• A. Austenite
• B. Martensite
• C. Ferrite
• D. Pearlite

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Correct Answer: A

Explanation: The Austenite phase is the 'remembered' shape. The wire is deformed in the Martensitic phase and returns to the Austenitic shape upon heating.

33. If the light guide tip is damaged or has resin cured on it, the light intensity:

• A. Remains the same
• B. Increases due to focusing
• C. Decreases significantly due to scattering
• D. Shifts to a red spectrum

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Correct Answer: C

Explanation: Debris or scratches on the tip scatter and block light, significantly reducing the irradiance reaching the adhesive and compromising the bond.

34. The oxygen-inhibited layer on the surface of cured adhesive:

• A. Is the strongest part of the bond
• B. Should be removed to prevent staining
• C. Is completely polymerized
• D. Prevents bracket drift

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Correct Answer: B

Explanation: Oxygen inhibits polymerization, leaving a sticky, uncured layer on the surface. This layer attracts plaque and stain and should be wiped off or minimized.

35. Acid etching of enamel creates microporosities that allow:

• A. Chemical bonding to hydroxyapatite
• B. Micromechanical retention via resin tags
• C. Covalent bonding to collagen
• D. Direct fusion of bracket to tooth

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Correct Answer: B

Explanation: The primary mechanism of orthodontic bonding is micromechanical. The resin flows into the acid-etched prisms, forms tags, and locks the bracket to the tooth.