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NZ Dental Technician REX practice questions

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Question 1Dental Anatomy, Oral Anatomy and Physiology

Which permanent tooth typically has the largest mesiodistal crown dimension?

  1. Maxillary central incisor
  2. Mandibular canine
  3. Mandibular first molar
  4. Maxillary first molar Correct answer.

Explanation

The maxillary first molar has the greatest mesiodistal crown width of any permanent tooth, a key landmark in arch-length and occlusion planning. Lab technicians use this when selecting moulds and arranging posterior teeth in denture setups.

Question 2Dental Anatomy, Oral Anatomy and Physiology

How many cusps does a typical permanent mandibular first molar possess?

  1. Five Correct answer.
  2. Four
  3. Six
  4. Three

Explanation

The mandibular first molar classically has five cusps: mesiobuccal, distobuccal, distal, mesiolingual and distolingual. This five-cusp pattern distinguishes it from the four-cusp mandibular second molar and guides occlusal wax-up.

Question 3Dental Anatomy, Oral Anatomy and Physiology

Which type of dentine forms first and lies immediately beneath the enamel during crown development?

  1. Mantle dentine Correct answer.
  2. Tertiary dentine
  3. Circumpulpal dentine
  4. Secondary dentine

Explanation

Mantle dentine is the first-formed dentine layer, deposited just beneath the enamel (and cementum) at the dentino-enamel junction. Circumpulpal dentine forms later and makes up the bulk of the tooth.

Question 4Dental Anatomy, Oral Anatomy and Physiology

Which periodontal ligament fibre group is principally responsible for resisting intrusive forces on a tooth?

  1. Oblique group Correct answer.
  2. Alveolar crest group
  3. Apical group
  4. Horizontal group

Explanation

The oblique fibres run apically from bone to root and are the most numerous group, converting axial (intrusive) loading into tension. They are the main reason a tooth resists being driven into its socket.

Question 5Dental Anatomy, Oral Anatomy and Physiology

Which nerve provides the primary sensory supply to the mandibular teeth?

  1. Maxillary division of the trigeminal nerve
  2. Inferior alveolar branch of the mandibular nerve Correct answer.
  3. Facial nerve
  4. Chorda tympani

Explanation

Mandibular teeth are innervated by the inferior alveolar nerve, a branch of the mandibular division of the trigeminal nerve. This is the target for inferior alveolar block anaesthesia in clinical practice.

Question 6Dental Anatomy, Oral Anatomy and Physiology

Which salivary gland secretes the greatest volume of saliva under resting conditions?

  1. Sublingual gland
  2. Minor palatal glands
  3. Submandibular gland Correct answer.
  4. Parotid gland

Explanation

At rest the submandibular glands contribute roughly two-thirds of total salivary flow, while the parotid dominates only under strong stimulation. This matters when assessing xerostomia and denture retention.

Question 7Dental Anatomy, Oral Anatomy and Physiology

In the FDI two-digit system, which tooth is designated 36?

  1. Maxillary right first molar
  2. Mandibular right first molar
  3. Mandibular left first molar Correct answer.
  4. Maxillary left first molar

Explanation

In FDI notation the first digit indicates quadrant and the second the tooth position. Quadrant 3 is the mandibular left, and position 6 is the first molar, so 36 is the mandibular left first molar.

Question 8Dental Anatomy, Oral Anatomy and Physiology

A crown is being designed for a maxillary first premolar. Which morphological feature most reliably distinguishes it from a maxillary second premolar?

  1. Two roots versus one Correct answer.
  2. A larger cervical cross-section
  3. A more rounded occlusal outline
  4. Presence of three cusps versus two

Explanation

The maxillary first premolar typically has two roots (buccal and palatal) and two cusps, whereas the second premolar usually has a single root. Root number is therefore the most dependable distinguishing feature in crown and core design.

Question 9Dental Anatomy, Oral Anatomy and Physiology

Which occlusal relationship describes the mesiobuccal cusp of the maxillary first molar occluding in the buccal groove of the mandibular first molar?

  1. Class III
  2. Class II division 1
  3. Class II division 2
  4. Class I Correct answer.

Explanation

Angle Class I is defined by the mesiobuccal cusp of the upper first molar lying in the buccal groove of the lower first molar. This normal anteroposterior relationship is the reference against which Class II and Class III are defined.

Question 10Dental Anatomy, Oral Anatomy and Physiology

During mastication, the mandible is depressed primarily by which muscle group?

  1. Temporalis and lateral pterygoid Correct answer.
  2. Buccinator and orbicularis oris
  3. Masseter and medial pterygoid
  4. Suprahyoid muscles acting alone

Explanation

Mandibular depression results from the combined action of the lateral pterygoid and the suprahyoid group, with the temporalis assisting. The masseter and medial pterygoid are elevators, so they oppose opening.

Question 11Dental Anatomy, Oral Anatomy and Physiology

Which structure marks the boundary between the attached gingiva and the alveolar mucosa?

  1. Base of the sulcus
  2. Gingival margin
  3. Free gingival groove
  4. Mucogingival junction Correct answer.

Explanation

The mucogingival junction separates the keratinised attached gingiva from the non-keratinised alveolar mucosa. Its position is assessed when planning the extent of a denture base or graft.

Question 12Dental Anatomy, Oral Anatomy and Physiology

Which enamel feature appears as a dark band in ground sections and represents a period of enamel formation disturbance?

  1. Hunter-Schreger bands
  2. Enamel tufts
  3. Striae of Retzius Correct answer.
  4. Perikymata

Explanation

Striae of Retzius are incremental growth lines reflecting rhythmic enamel deposition and occasional disturbances. Their surface manifestations are perikymata, while Hunter-Schreger bands reflect prism direction rather than growth disturbance.

Question 13Dental Anatomy, Oral Anatomy and Physiology

A technician is waxing a mandibular canine. Which characteristic best distinguishes it from a mandibular lateral incisor?

  1. A single cusp with a prominent lingual ridge Correct answer.
  2. A wider incisal edge relative to crown height
  3. A completely flat lingual surface
  4. Two distinct cusps

Explanation

The mandibular canine has a single pointed cusp with a well-developed lingual ridge dividing mesial and distal lingual fossae. The lateral incisor is more incisiform, with a flatter lingual surface and less prominent ridge.

Question 14Dental Anatomy, Oral Anatomy and Physiology

Which cranial bone forms the anterior part of the hard palate?

  1. Sphenoid bone
  2. Maxilla Correct answer.
  3. Palatine bone
  4. Vomer

Explanation

The hard palate is formed anteriorly by the palatine processes of the maxillae and posteriorly by the horizontal plates of the palatine bones. The palatine bone therefore contributes only the posterior portion.

Question 15Dental Anatomy, Oral Anatomy and Physiology

Which statement correctly describes the direction of enamel prism decussation in the inner enamel?

  1. Prisms reverse direction at the dentino-enamel junction and run cervically
  2. Prisms converge radially toward the cervical region only
  3. Prisms run in straight parallel lines from the dentino-enamel junction to the surface
  4. Prisms cross each other in alternating directions, forming Hunter-Schreger bands Correct answer.

Explanation

In the inner two-thirds of enamel, prism groups decussate, crossing at angles to one another and producing Hunter-Schreger bands in reflected light. This arrangement increases resistance to crack propagation under occlusal load.

Question 16Dental Anatomy, Oral Anatomy and Physiology

Which factor most directly determines the shape of the occlusal plane in a complete denture setup?

  1. The thickness of the denture base
  2. The colour of the denture teeth
  3. The curve of Spee and compensating curve Correct answer.
  4. The type of impression material used

Explanation

The occlusal plane is shaped by incorporating the anteroposterior curve of Spee and the mediolateral compensating curve so that balanced occlusion is achieved. These curves guide posterior tooth arrangement during setup.

Question 17Dental Anatomy, Oral Anatomy and Physiology

Which type of epithelium lines the base of the gingival sulcus and attaches to the tooth surface?

  1. Alveolar mucosa
  2. Masticatory mucosa
  3. Sulcular epithelium
  4. Junctional epithelium Correct answer.

Explanation

The junctional epithelium is a non-keratinised stratified squamous epithelium that forms the epithelial attachment to the tooth via hemidesmosomes. It seals the gingival sulcus from the connective tissue and is the first line of periodontal defence.

Question 18Dental Anatomy, Oral Anatomy and Physiology

Which primary tooth is usually the first to erupt in the oral cavity?

  1. Mandibular central incisor Correct answer.
  2. Maxillary canine
  3. Mandibular first molar
  4. Maxillary central incisor

Explanation

Eruption usually begins with the mandibular central incisors at around six to eight months of age. Maxillary centrals follow, and primary molars erupt later in the sequence.

Question 19Dental Anatomy, Oral Anatomy and Physiology

Which feature of the temporomandibular joint allows the mandible to perform a wide range of movements?

  1. A thick hyaline cartilage covering on both articular surfaces
  2. A fibrous articular disc dividing the joint into two compartments Correct answer.
  3. A synovial membrane enclosing the disc
  4. A bony hinge with no interposed tissue

Explanation

The TMJ is unusual in having a fibrocartilaginous disc that divides it into upper and lower compartments, permitting both rotation and translation. The articular surfaces are covered by fibrous tissue rather than hyaline cartilage.

Question 20Dental Anatomy, Oral Anatomy and Physiology

A crown preparation requires knowledge of the thickness of enamel at the cervical margin. Which statement is most accurate?

  1. Enamel is thickest at the cervical margin and thinnest at the incisal edge
  2. Enamel thickness is uniform across the crown
  3. Enamel is absent on the lingual surface
  4. Enamel is thickest at the incisal edge and tapers to a knife edge at the cervical margin Correct answer.

Explanation

Enamel reaches its maximum thickness over the cusps and incisal edges and thins progressively toward the cervical margin, where it ends as a fine edge. This is why cervical margins require care to avoid unsupported enamel during preparation.

Question 21Dental Materials Science and Biomaterials

Which property of a dental ceramic is most directly improved by glazing the outer surface of a metal-ceramic crown?

  1. Compressive strength of the core
  2. Surface smoothness and aesthetics Correct answer.
  3. Flexural strength of the metal substructure
  4. Coefficient of thermal expansion

Explanation

A glaze creates a smooth, vitreous outer layer that reduces surface roughness, improves aesthetics and limits plaque accumulation and wear of the opposing tooth. It does not meaningfully change the compressive strength of the core, the thermal expansion coefficient or the flexural strength of the metal framework, which are set by the bulk materials and their processing.

Question 22Dental Materials Science and Biomaterials

A technician is selecting an impression material for a crown preparation where dimensional stability over several days is required before pouring. Which material best meets this requirement?

  1. Polysulfide rubber
  2. Irreversible hydrocolloid (alginate)
  3. Reversible hydrocolloid (agar)
  4. Addition-cured silicone (polyvinyl siloxane) Correct answer.

Explanation

Addition-cured silicones set by a hydrosilation reaction that produces no by-product, so they show very low shrinkage and excellent dimensional stability, allowing pours or pours after storage. Alginate, agar and polysulfide all lose water or release by-products and change dimension relatively quickly.

Question 23Dental Materials Science and Biomaterials

Which type of gypsum product is produced by heating calcium sulfate dihydrate under steam pressure to form the alpha-hemihydrate, giving the densest, strongest set material?

  1. Model plaster
  2. Impression plaster
  3. Plaster of Paris
  4. Dental stone Correct answer.

Explanation

Dental stone (alpha-hemihydrate) is made by autoclaving under steam pressure, producing compact, regular crystals that require less water and set into a stronger, denser mass. Plaster of Paris and model plaster are beta-hemihydrate, made by dry heating, and require more water, giving a weaker set.

Question 24Dental Materials Science and Biomaterials

In a base metal casting alloy for removable partial denture frameworks, which element is primarily responsible for providing corrosion resistance?

  1. Cobalt
  2. Molybdenum
  3. Nickel
  4. Chromium Correct answer.

Explanation

Chromium forms a passive oxide layer on the alloy surface that protects against corrosion; cobalt-chromium and nickel-chromium alloys rely on sufficient chromium content for this. Cobalt and nickel provide strength and castability, and molybdenum contributes to strength and corrosion resistance but is not the primary passive-layer former.

Question 25Dental Materials Science and Biomaterials

What is the main purpose of adding a small amount of hydroquinone to an autopolymerising acrylic resin?

  1. To accelerate the initiator breakdown
  2. To act as an inhibitor preventing premature polymerisation Correct answer.
  3. To increase the translucency of the set resin
  4. To act as a plasticiser

Explanation

Hydroquinone is a free-radical scavenger that stabilises the monomer liquid, preventing spontaneous polymerisation during storage and giving a predictable working time. The initiator is activated by the tertiary amine, not by the inhibitor.

Question 26Dental Materials Science and Biomaterials

Which property of a dental amalgam is most improved by the presence of the gamma-2 phase in the set material?

  1. None; gamma-2 is the least corrosion-resistant phase Correct answer.
  2. Compressive strength
  3. Setting expansion
  4. Creep resistance

Explanation

Gamma-2 (Sn-Hg) is the weakest and most corrosion-prone phase in set amalgam, so high-copper alloys are designed to reduce or eliminate it. It does not confer improved strength, creep resistance or expansion; its presence is generally detrimental.

Question 27Dental Materials Science and Biomaterials

Which monomer is the principal liquid component of conventional heat-cured polymethyl methacrylate denture base resin?

  1. Methyl methacrylate Correct answer.
  2. Bisphenol A glycidyl methacrylate
  3. Urethane dimethacrylate
  4. Ethylene glycol dimethacrylate

Explanation

Conventional denture base resin uses methyl methacrylate monomer to polymerise with the PMMA powder. Ethylene glycol dimethacrylate is a cross-linking agent used in small amounts, while Bis-GMA and UDMA are dimethacrylates used in composite restorative materials.

Question 28Dental Materials Science and Biomaterials

Which statement correctly describes the setting reaction of a zinc phosphate cement?

  1. A free-radical addition polymerisation initiated by light
  2. An acid-base reaction between phosphoric acid liquid and zinc oxide powder Correct answer.
  3. A hydration reaction of calcium sulfate hemihydrate
  4. A chelation reaction between eugenol and zinc oxide

Explanation

Zinc phosphate cement sets by an acid-base reaction in which phosphoric acid attacks zinc oxide powder to form a zinc phosphate matrix. This is an exothermic, acidic reaction distinct from polymerisation, eugenol chelation and gypsum hydration.

Question 29Dental Materials Science and Biomaterials

A technician notices that a light-cured composite restoration is undercured at the bottom of a 2 mm increment. Which factor most likely explains this?

  1. Light attenuation and scattering reduce irradiance with depth Correct answer.
  2. The monomer has too low a viscosity
  3. The composite has too high a filler content
  4. The photoinitiator concentration is zero

Explanation

Both the filler and the resin absorb and scatter light, so irradiance falls with depth and the bottom of a thick increment may receive insufficient energy to reach adequate conversion. Filler content, viscosity and initiator level influence curing but the depth effect is specifically due to attenuation and scattering.

Question 30Dental Materials Science and Biomaterials

A crown is being cemented with a resin cement onto a zirconia coping. Which surface treatment of the zirconia is most appropriate to improve adhesion?

  1. Etching with hydrofluoric acid gel
  2. Airborne-particle abrasion with alumina and a phosphate-based primer Correct answer.
  3. Polishing the intaglio surface to a high gloss
  4. Application of a silane coupling agent alone

Explanation

Zirconia lacks a glass phase, so hydrofluoric acid etching does not create a retentive pattern and silane alone does not bond to it. Alumina abrasion increases surface roughness and a phosphate-based primer (such as MDP-containing) chemically bonds to the zirconia surface.

Question 31Dental Materials Science and Biomaterials

Which characteristic distinguishes a Type IV dental stone from a Type III stone?

  1. Lower compressive strength for mounting casts
  2. Larger particle size and higher water requirement
  3. Higher compressive strength and lower setting expansion for die use Correct answer.
  4. Ability to reproduce soft tissue detail for impressions

Explanation

Type IV (die stone) is a high-strength, low-expansion alpha-hemihydrate with a low water-to-powder ratio, giving the strength and dimensional accuracy needed for working dies. Type III (stone) is used for casts and has lower strength and higher expansion.

Question 32Dental Materials Science and Biomaterials

Why is a separating medium applied to the gypsum mould before packing heat-cured denture base resin?

  1. To increase the thermal expansion of the stone
  2. To accelerate the polymerisation of the resin
  3. To improve the colour stability of the resin
  4. To prevent the monomer from penetrating the stone and to allow separation Correct answer.

Explanation

Monomer can penetrate the porous stone and lock the resin to the mould, so a separating medium (such as sodium alginate solution) seals the stone surface and allows clean separation. It does not accelerate cure, change stone expansion or affect colour stability.

Question 33Dental Materials Science and Biomaterials

Which mechanism best explains the bonding of a conventional glass-ionomer cement to tooth structure?

  1. Micromechanical interlocking into acid-etched enamel only
  2. Covalent bonding to collagen in dentine
  3. Polymerisation shrinkage locked into undercuts
  4. Ionic interaction between polyacrylic acid and calcium in hydroxyapatite Correct answer.

Explanation

Glass-ionomer cements bond chemically through ionic interaction between the polyalkenoic acid and calcium ions in the tooth mineral, giving adhesion without separate etching. Micromechanical interlocking contributes to resin bonding, and covalent bonding to collagen is not the primary glass-ionomer mechanism.

Question 34Dental Materials Science and Biomaterials

A technician must choose a wax for building a pattern for a large removable partial denture framework. Which property is most important for this application?

  1. Low melting point with high flow
  2. High ash content after burnout
  3. Low thermal expansion and good dimensional stability Correct answer.
  4. High flow at room temperature

Explanation

Pattern waxes for large frameworks must hold their shape, so low thermal expansion and dimensional stability are critical to avoid distortion of the pattern. High flow, low melting point and high ash content are undesirable because they cause distortion and residue.

Question 35Dental Materials Science and Biomaterials

Which statement about the setting of an elastomeric impression material is correct for a material supplied as a two-paste system with base and catalyst?

  1. Setting is a reversible physical change on cooling
  2. Setting requires exposure to visible light
  3. Setting occurs by loss of water to the air
  4. The set is a cross-linking reaction whose rate is influenced by temperature Correct answer.

Explanation

Two-paste elastomers set by a chemical cross-linking reaction, and the reaction rate increases with temperature, shortening working time. They do not set by water loss, light exposure or reversible cooling.

Question 36Dental Materials Science and Biomaterials

Which factor most strongly influences the wear resistance of a dental composite restoration?

  1. The concentration of inhibitor
  2. The shade of the composite
  3. Filler content and particle size distribution Correct answer.
  4. The type of curing light used

Explanation

Wear resistance depends mainly on the amount, size and distribution of the inorganic filler, which bears the occlusal load and protects the softer resin matrix. Shade, inhibitor level and light type influence handling or cure but not wear resistance as directly.

Question 37Dental Materials Science and Biomaterials

What is the principal reason for using a noble alloy rather than a base metal alloy for a metal-ceramic crown?

  1. Lower cost than base metal alloys
  2. Good castability, corrosion resistance and a compatible oxide for ceramic bonding Correct answer.
  3. Greater tendency to form a thick oxide layer
  4. Higher modulus of elasticity than base metal alloys

Explanation

Noble alloys cast well, resist corrosion and form a thin, adherent oxide that bonds to porcelain. Base metal alloys have a higher modulus and lower cost, and excessive oxide formation is undesirable rather than a benefit.

Question 38Dental Materials Science and Biomaterials

Which property of a denture base acrylic resin is most affected by the presence of residual monomer?

  1. Colour of the packaging
  2. Setting time of the gypsum
  3. Melting point of the wax
  4. Biocompatibility and dimensional stability Correct answer.

Explanation

Residual monomer can leach from the resin, causing tissue irritation and allergic reactions, and its presence also affects dimensional stability and mechanical properties. Colour of packaging, gypsum setting and wax melting point are unrelated.

Question 39Dental Materials Science and Biomaterials

Which statement correctly describes the difference between a light-cured and a dual-cured resin cement?

  1. Light-cured cement sets without any light exposure
  2. Dual-cured cement sets only when light is applied
  3. Light-cured cement contains no photoinitiator
  4. Dual-cured cement contains a chemical initiator in addition to a photoinitiator Correct answer.

Explanation

Dual-cured cements contain both a photoinitiator system and a chemical (self-cure) initiator, so they set where light cannot reach. Light-cured cements rely on photoinitiation and do not set adequately without light, and they do contain photoinitiators.

Question 40Dental Materials Science and Biomaterials

A technician is investing a pattern for a base metal alloy crown. Which investment property is most critical for dimensional accuracy of the casting?

  1. Setting and thermal expansion matched to the alloy shrinkage Correct answer.
  2. High water-to-powder ratio
  3. High compressive strength at room temperature only
  4. Low permeability to allow gas escape

Explanation

The investment must expand sufficiently during setting and heating to compensate for the contraction of the cooling metal and for casting shrinkage, so its expansion must be matched to the alloy. Room-temperature strength alone, permeability and water ratio affect other aspects but not the primary compensation.

Question 41Dental Materials Science and Biomaterials

Which mechanism explains the strengthening of a glass-ceramic by controlled crystallisation compared with the parent glass?

  1. Crystals increase the solubility of the glass
  2. Crystals interrupt crack propagation and modify the thermal expansion mismatch Correct answer.
  3. Crystals increase the amount of glass phase
  4. Crystals lower the melting point of the material

Explanation

Controlled crystallisation produces a microstructure in which crystals deflect or interrupt cracks and thermal expansion mismatch creates beneficial residual stresses, raising strength and toughness. Crystals do not increase the glass phase, lower the melting point or increase solubility.

Question 42Dental Materials Science and Biomaterials

A cast post-and-core is being fabricated for a tooth with a short root. Which property of the post material is most important to consider for long-term success?

  1. Low melting point for easy casting
  2. High radiolucency to aid radiographic viewing
  3. High thermal expansion
  4. Modulus of elasticity similar to dentine to reduce stress concentration Correct answer.

Explanation

A post with a modulus close to that of dentine distributes stress more evenly and reduces the risk of root fracture. Radiolucency is undesirable, and low melting point and high thermal expansion are not relevant to clinical success.

Question 43Dental Materials Science and Biomaterials

Which statement correctly explains why a high-copper amalgam shows better corrosion resistance than a conventional low-copper amalgam?

  1. The setting reaction is slower
  2. The silver content is reduced
  3. The mercury content is increased
  4. The gamma-2 phase is largely eliminated and replaced by a copper-tin phase Correct answer.

Explanation

In high-copper amalgams the tin reacts preferentially with copper to form a Cu6Sn5 phase rather than gamma-2 (Sn-Hg), reducing the most corrosion-prone phase. Mercury, silver content and setting rate are not the reason for improved corrosion resistance.

Question 44Dental Materials Science and Biomaterials

A technician is comparing the polymerisation shrinkage stress of two composites: one with high filler loading and one with low filler loading, both with the same resin matrix. Which is expected?

  1. Both show identical stress because the matrix is the same
  2. The high-filler composite shows lower shrinkage stress because less resin is present Correct answer.
  3. The low-filler composite shows lower stress because it is softer
  4. The high-filler composite shows higher stress because it is stiffer

Explanation

Shrinkage arises from the resin matrix, so a higher filler fraction means less resin per unit volume and thus lower shrinkage and generally lower stress, although the higher modulus can offset some of this. The low-filler composite has more resin and greater shrinkage.

Question 45Dental Materials Science and Biomaterials

Which property of a dental wax most directly affects the accuracy of a cast metal framework when the wax pattern is invested?

  1. Taste and odour
  2. Thermal expansion and flow behaviour during burnout Correct answer.
  3. Colour of the wax
  4. Solubility in water

Explanation

Wax expansion and flow during handling and burnout can distort the pattern or create investment expansion mismatch, directly affecting casting accuracy. Colour, solubility and taste are not relevant to dimensional accuracy.

Question 46Dental Materials Science and Biomaterials

During the casting of a cobalt-chromium removable partial denture framework, which property of the investment material is most important for maintaining dimensional accuracy of the casting?

  1. Low thermal conductivity to slow cooling
  2. High green strength to resist pattern removal forces
  3. Thermal expansion that compensates for alloy solidification shrinkage Correct answer.
  4. High permeability to allow gases to escape

Explanation

The investment must expand sufficiently during heating to compensate for the contraction of the alloy as it solidifies and cools; this is achieved by a combination of setting and thermal expansion. Without adequate compensation, the casting will be undersized. Permeability and green strength are important for other reasons but do not directly compensate for shrinkage.

Question 47Dental Materials Science and Biomaterials

A technician is selecting a wax for a complex removable partial denture framework pattern. Which property is most critical to ensure the pattern maintains its shape during storage and investing?

  1. Low melting range
  2. High rigidity and dimensional stability Correct answer.
  3. High thermal expansion
  4. High flow at room temperature

Explanation

Pattern waxes must be rigid enough to resist deformation during handling and investing, and they must have low thermal expansion and contraction to maintain dimensional accuracy. High flow at room temperature would cause the pattern to slump, and low melting range or high expansion would compromise fit.

Question 48Dental Materials Science and Biomaterials

A technician pours a Type III dental stone model and a die stone die from the same impression. Which property difference between the two materials most directly explains why the die stone reproduces finer cavosurface detail?

  1. Longer final setting time
  2. Lower linear setting expansion
  3. Smaller particle size and lower water-to-powder ratio Correct answer.
  4. Higher compressive strength after full set

Explanation

Detail reproduction in gypsum products depends primarily on particle size and the volume of excess water needed for mixing. Die stones are milled finer and require a lower water-to-powder ratio, so the set mass is denser and flows into finer impression features. Strength and setting expansion are consequences of that same density, not the direct cause of fine detail capture.

Question 49Dental Materials Science and Biomaterials

A heat-cured PMMA denture base is processed and then found to have a slightly open posterior palatal seal area after deflasking. Which processing variable is the most likely cause of this dimensional inaccuracy?

  1. Removing the flask from the water bath while still hot and allowing it to cool in air Correct answer.
  2. Packing the resin dough at the dough stage rather than the sticky stage
  3. Excessive monomer added to the polymer during mixing
  4. Using a long, slow polymerisation cycle at low temperature

Explanation

Thermal shrinkage on cooling from processing temperature is the dominant cause of contraction in heat-cured PMMA, and it is greatest when the flask cools rapidly in air while the resin is still soft. Slow bench cooling in the flask allows stresses to relax and the resin to remain adapted to the cast. Monomer ratio and curing cycle influence residual monomer and porosity more than this gross adaptation loss.

Question 50Dental Materials Science and Biomaterials

A base metal (Ni-Cr) alloy and a high-noble alloy are both to be cast into the same refractory investment. The technician notes that the base metal alloy requires a higher casting temperature and shows poorer casting accuracy. Which explanation best accounts for the difference in casting accuracy?

  1. The base metal alloy has a higher coefficient of thermal expansion, so the investment mould expands more and distorts the casting.
  2. The base metal alloy has a wider melting range, so it stays partially solid during casting and cannot fill the mould.
  3. The base metal alloy oxidises heavily and its high melting temperature demands a larger burnout expansion, which the investment cannot match precisely, so mould expansion is harder to control. Correct answer.
  4. The base metal alloy has a lower modulus of elasticity, so the casting deforms on removal from the investment.

Explanation

Base metal alloys melt at much higher temperatures and form thick oxide layers, so they need investments with greater refractory content and higher burnout expansion to compensate for casting shrinkage. That expansion is more difficult to control consistently than the setting and thermal expansion used for noble alloys, so fit is less predictable. The problem is mould expansion control, not a wider melting range or a lower elastic modulus.