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Iran Dental Board - Prosthodontics study materials
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Iran Dental Board - Prosthodontics practice questions

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Question 1Fixed Prosthodontics

What is the primary function of a die spacer applied to a stone die before waxing a full-coverage crown?

  1. It compensates for shrinkage of the impression material
  2. It marks the finish line for the waxing technician
  3. It increases retention by roughening the axial walls
  4. It creates a uniform space for the luting agent Correct answer.

Explanation

A die spacer is painted onto the die to leave a controlled gap between the restoration and the prepared tooth. This gap allows the luting agent to flow and permits complete seating without hydraulic resistance. It is not used to roughen the die, compensate for impression shrinkage, or mark the margin.

Question 2Fixed Prosthodontics

Which finish line design is preferred on the facial aspect of a metal-ceramic crown because it provides a flat ledge of porcelain support?

  1. Chamfer margin
  2. Feather-edge margin
  3. Shoulder margin Correct answer.
  4. Knife-edge margin

Explanation

A shoulder is a flat, approximately 90-degree ledge that gives facial porcelain a definite stop and adequate bulk. Knife-edge and feather-edge margins lack this ledge, and a chamfer is rounded rather than flat. The shoulder therefore supports the porcelain at the esthetic facial margin.

Question 3Fixed Prosthodontics

Which pontic design is most appropriate for a mandibular posterior fixed partial denture when esthetics is not critical and the ridge is broad?

  1. Saddle pontic
  2. Modified ridge-lap pontic
  3. Ovate pontic
  4. Sanitary pontic Correct answer.

Explanation

A sanitary, or conical, pontic is indicated in posterior regions where esthetics is secondary because it clears the ridge and allows easy hygiene. Saddle and modified ridge-lap designs contact the ridge broadly and are harder to clean. An ovate pontic is used mainly in anterior esthetic zones.

Question 4Fixed Prosthodontics

A maxillary central incisor is prepared for a full-coverage crown and has a 2-mm circumferential ferrule of sound dentin. What is the main biomechanical advantage of this ferrule?

  1. It increases cement retention by enlarging surface area
  2. It resists wedging forces at the crown margin Correct answer.
  3. It improves the shade match of the ceramic crown
  4. It eliminates the need for a post and core in every case

Explanation

A ferrule is a circumferential band of dentin that grips the restoration and resists lever-like wedging forces at the margin. It improves fracture resistance of the restored tooth, especially when a post is present. It does not by itself eliminate the need for a core or affect the shade of the crown.

Question 5Fixed Prosthodontics

A single maxillary premolar crown preparation has a subgingival buccal margin with no bleeding. Which impression technique is most appropriate to record the margin accurately?

  1. Two-cord retraction with a two-step VPS impression Correct answer.
  2. A polyether impression taken after gingival curettage
  3. A monophase VPS impression made without any tissue displacement
  4. An alginate impression taken after air-drying the sulcus thoroughly

Explanation

Subgingival margins require temporary displacement of the gingiva so the impression material can capture the finish line. Two-cord retraction opens the sulcus, and a two-step vinyl polysiloxane technique allows the wash to record the margin. Alginate lacks the dimensional accuracy and detail needed for fixed prosthodontics.

Question 6Fixed Prosthodontics

A zirconia molar crown has short axial walls and limited retention. Which cementation protocol is most appropriate?

  1. Conventional resin cement used without any primer or surface treatment
  2. Glass ionomer cement applied after phosphoric acid etching of the zirconia
  3. Air-particle abrasion with an MDP-containing resin cement Correct answer.
  4. Zinc phosphate cement used without any surface treatment of the zirconia

Explanation

Zirconia is not a silica-based ceramic, so phosphoric acid etching and silane alone do not create a durable bond. Air-particle abrasion increases surface energy, and an MDP-containing primer or resin cement can chemically bond to zirconia. This approach is especially useful when retention is limited.

Question 7Fixed Prosthodontics

In a mutually protected occlusal scheme for a full-arch fixed reconstruction, what should occur during lateral mandibular excursion?

  1. The working-side premolars guide the lateral movement
  2. Only the nonworking posterior teeth make contact
  3. Anterior teeth disclude the posterior teeth on both sides Correct answer.
  4. All posterior teeth remain in contact on the working side

Explanation

Mutually protected occlusion means maximum intercuspation is protected by posterior contacts, while excursive movements are guided by anterior teeth that disclude the posterior teeth. This reduces lateral forces on posterior restorations. Working-side posterior contacts during excursion are not part of the scheme.

Question 8Fixed Prosthodontics

A long-span posterior provisional fixed partial denture must function for six months. Which property is most important in the provisional material?

  1. High translucency for matching the adjacent teeth
  2. Rapid polymerization for immediate finishing
  3. Adequate flexural strength and wear resistance Correct answer.
  4. Low coefficient of thermal expansion

Explanation

Long-term provisionals must withstand occlusal loading without fracture or excessive wear, so flexural strength and wear resistance are critical. Esthetics and handling matter, but they are secondary to mechanical durability over months. Low thermal expansion and rapid set do not ensure serviceability.

Question 9Fixed Prosthodontics

A three-unit metal-ceramic fixed partial denture replaces a mandibular first molar between a second premolar and a second molar. Which condition most increases the risk of connector fracture?

  1. A shallow occlusal anatomy on the molar pontic
  2. A long clinical crown on the mandibular second molar
  3. A slightly subgingival finish line on the premolar
  4. A connector with a small buccolingual cross section Correct answer.

Explanation

Connector strength depends mainly on cross-sectional area and the height available between the occlusal and gingival embrasures. A connector with a small buccolingual dimension has insufficient bulk and concentrates stress under molar occlusal loads. Shallow anatomy, crown length, and margin depth do not directly reduce connector area.

Question 10Fixed Prosthodontics

A three-unit metal-ceramic fixed partial denture seats completely on the stone dies but rocks on the prepared teeth. Which is the most likely cause?

  1. The framework was distorted during soldering or casting Correct answer.
  2. The die spacer was applied too thinly to the die
  3. The porcelain was fired at an excessively high temperature
  4. The impression was made with a stock metal tray

Explanation

If a framework fits the dies but not the mouth, the discrepancy usually arises after casting or soldering, such as distortion from heat treatment or an inaccurate solder index. Die spacer thickness affects cement space, not intraoral rocking. Porcelain firing and tray selection do not explain a framework that is stable on the dies.

Question 11Fixed Prosthodontics

According to Ante's law, a fixed partial denture has adequate abutment support when which condition is met?

  1. No more than one tooth is missing between the abutments.
  2. Each abutment has a crown-to-root ratio of at least 1:2.
  3. Each abutment has a uniform periodontal ligament width.
  4. Abutment root surface area is at least equal to the replaced teeth. Correct answer.

Explanation

Ante's law states that the total root surface area of the abutment teeth must equal or exceed the root surface area of the teeth being replaced. This guideline helps predict whether the abutments can withstand the added occlusal load of an FPD. Crown-to-root ratio, span length, and periodontal ligament uniformity are separate considerations and are not part of Ante's law.

Question 12Fixed Prosthodontics

When selecting the shade for a metal-ceramic crown, which color parameter should be matched first because it dominates the overall appearance?

  1. Chroma
  2. Value Correct answer.
  3. Translucency
  4. Hue

Explanation

Value describes the lightness or darkness of a color and is the most important shade parameter in restorative dentistry because a value mismatch is more noticeable than a hue or chroma mismatch. Hue is the basic color family, chroma is its intensity, and translucency is an optical property rather than a primary Munsell dimension.

Question 13Fixed Prosthodontics

What is the recommended total occlusal convergence for a full-coverage cast metal crown preparation?

  1. 2 to 5 degrees
  2. 10 to 20 degrees Correct answer.
  3. 25 to 35 degrees
  4. 40 to 50 degrees

Explanation

A total occlusal convergence of approximately 10 to 20 degrees provides a balance between retention, resistance, and the ability of the restoration to seat completely. Insufficient convergence can prevent complete seating, while excessive convergence reduces retention and resistance form.

Question 14Fixed Prosthodontics

A metal-ceramic crown margin is placed 2 mm subgingivally on a mandibular molar. Six weeks later, the patient has persistent gingival inflammation and bleeding. Which factor is the most likely cause?

  1. Use of a polyether impression
  2. Excess die spacer
  3. Violation of the biologic width Correct answer.
  4. Inadequate occlusal reduction

Explanation

Subgingival margins that encroach on the biologic width, which includes the junctional epithelium and connective tissue attachment, can cause chronic inflammation, bleeding, and bone loss. A 2-mm subgingival margin often violates this space unless the sulcus is unusually deep. Die spacer, occlusal reduction, and impression material do not directly cause this persistent soft-tissue response.

Question 15Fixed Prosthodontics

A maxillary premolar has a clinical crown height of 3 mm and requires a full-coverage crown. Which preparation modification best improves resistance form without compromising retention?

  1. Adding auxiliary grooves or a proximal box Correct answer.
  2. Reducing the occlusal surface by 0.5 mm
  3. Increasing the occlusal taper to 30 degrees
  4. Placing the finish line supragingivally

Explanation

Short clinical crowns lack resistance form, and auxiliary grooves, proximal boxes, or pins increase resistance to lateral dislodging forces. Increasing taper reduces both retention and resistance, a supragingival finish line does not add resistance, and insufficient occlusal reduction does not improve resistance form.

Question 16Fixed Prosthodontics

A provisional fixed partial denture is cemented with a eugenol-containing provisional cement. The definitive restoration will be cemented with a resin cement. What is the primary concern with this sequence?

  1. Residual eugenol inhibits resin cement polymerization Correct answer.
  2. Eugenol prevents the provisional from being removed
  3. Eugenol increases the setting expansion of the stone die
  4. Eugenol permanently stains the dentin

Explanation

Eugenol can penetrate dentin and remain in the smear layer, where it inhibits free-radical polymerization of resin cements and can reduce bond strength. Thorough cleaning of the preparation is recommended before definitive cementation. Eugenol does not permanently stain dentin, affect stone die expansion, or prevent removal of the provisional.

Question 17Fixed Prosthodontics

What is the primary purpose of a facebow transfer in fixed prosthodontics?

  1. To orient the maxillary cast to the hinge axis Correct answer.
  2. To measure the interocclusal distance
  3. To record centric relation
  4. To determine the vertical dimension of occlusion

Explanation

A facebow transfers the spatial relationship of the maxillary arch to the transverse hinge axis and a reference plane, allowing the maxillary cast to be mounted in the correct orientation on the articulator. Centric relation and vertical dimension are recorded separately with interocclusal records.

Question 18Fixed Prosthodontics

Which method is most appropriate for evaluating the internal fit of a cast metal crown on its stone die?

  1. Radiographic examination of the die
  2. Silicone disclosing media Correct answer.
  3. Visual inspection with magnification only
  4. Probing the margin with a periodontal probe

Explanation

A silicone disclosing medium or fit checker reveals internal and marginal discrepancies by showing areas of contact and pressure on the die. Visual inspection alone is insufficient, radiographs cannot clearly show the die interface, and a periodontal probe is too coarse for evaluating internal fit.

Question 19Fixed Prosthodontics

In a five-unit fixed partial denture, a lone abutment lies between two pontics and functions as a pier abutment. Which design feature is most important to reduce stress on this abutment and the cement seals?

  1. A nonrigid connector at the pier abutment Correct answer.
  2. A narrower connector at the anterior abutment only
  3. A cantilever extension from the terminal abutment
  4. Rigid connectors at all abutments

Explanation

A pier abutment acts as a fulcrum between two segments of an FPD, so a nonrigid connector at the pier allows independent movement and reduces torque on the abutment and cement seals. Rigid connectors across the pier increase stress, cantilevers do not address the fulcrum effect, and isolated narrow connectors do not solve the problem.

Question 20Fixed Prosthodontics

When designing a three-unit metal-ceramic fixed partial denture to replace a mandibular first molar, which connector dimension is most appropriate for the posterior region?

  1. 2 mm height by 2 mm width
  2. 1 mm height by 1 mm width
  3. 6 mm height by 6 mm width
  4. 4 mm height by 4 mm width Correct answer.

Explanation

Posterior metal-ceramic connectors should have a cross-sectional area of approximately 4 mm by 4 mm to resist occlusal forces and avoid fracture or flexure. Anterior connectors can be smaller, around 2 to 3 mm. A 1-mm or 2-mm connector is too thin, while a 6-mm connector is unnecessarily bulky and difficult to clean.

Question 21Fixed Prosthodontics

During fabrication of a cast metal crown, what is the primary function of the sprue former attached to the wax pattern?

  1. It compensates for casting shrinkage by expanding the investment.
  2. It creates a channel for wax elimination and for entry of molten alloy. Correct answer.
  3. It supplies extra investment to support the wax pattern in the ring.
  4. It controls the thickness of die spacer applied to the stone die.

Explanation

The sprue former connects the wax pattern to the crucible former and, after burnout, leaves a channel through which molten alloy enters the mold. It also provides a path for wax to escape during heating. Casting shrinkage is compensated by investment expansion and alloy selection, not by the sprue itself.

Question 22Fixed Prosthodontics

For a metal-ceramic crown on a mandibular molar, what is the minimum recommended occlusal reduction over the functional cusp?

  1. 1.5 mm Correct answer.
  2. 1.0 mm
  3. 3.0 mm
  4. 0.5 mm

Explanation

A metal-ceramic restoration needs space for both the metal substructure and the veneering porcelain. Over a functional cusp, about 1.5 to 2.0 mm of occlusal reduction is recommended to avoid thin porcelain and fracture. Reduction of only 1 mm or less leaves insufficient porcelain thickness, whereas 3 mm unnecessarily removes tooth structure.

Question 23Fixed Prosthodontics

On a full-coverage cast metal crown, what is the main purpose of a bevel at the gingival finish line?

  1. It lets the thin metal margin be burnished and adapted. Correct answer.
  2. It provides space for die spacer on the working cast.
  3. It creates a flat butt joint that resists cement washout.
  4. It increases the surface area available for porcelain bonding.

Explanation

A bevel produces a thin, flexible metal edge that can be burnished against the prepared tooth, improving marginal adaptation and reducing the cement line. Burnishing also helps close microscopic gaps at the margin. A butt joint, by contrast, is a shoulder or chamfer without a bevel.

Question 24Fixed Prosthodontics

A metal-ceramic crown on a maxillary central incisor has a 1-mm facial metal collar. The patient complains of a dark line at the gingival margin. Which modification is most appropriate?

  1. Place the facial margin 1 mm subgingivally without changing the collar.
  2. Widen the facial metal collar to 2 mm to mask the dark line.
  3. Use a porcelain labial margin with a porcelain butt shoulder. Correct answer.
  4. Change the facial finish line to a chamfer and retain the metal collar.

Explanation

A porcelain labial margin replaces the visible metal collar with porcelain, so no dark metal line shows through the gingiva. It requires a shoulder finish line and adequate labial space for porcelain. Simply moving the metal margin subgingivally can cause inflammation and may still show gray.

Question 25Fixed Prosthodontics

Before placing a cast post-and-core in a previously endodontically treated maxillary premolar, at least how much apical gutta-percha should remain to maintain the apical seal?

  1. 2 mm
  2. 3 mm
  3. 4 mm Correct answer.
  4. 1 mm

Explanation

At least 4 to 5 mm of apical gutta-percha should be retained to preserve the apical seal after post space preparation. Removing more can compromise the seal and increase the risk of apical leakage. The post space should also leave enough dentin for a ferrule.

Question 26Fixed Prosthodontics

For a patient with a stable and reproducible maximum intercuspation, which technique is most appropriate for recording the maxillomandibular relationship before fabricating a single posterior metal-ceramic crown?

  1. Record maximum intercuspation with a rigid polyvinyl siloxane bite registration. Correct answer.
  2. Use a facebow transfer alone, without an interocclusal record.
  3. Record the vertical dimension of rest with a Willis gauge.
  4. Record centric relation with a wax wafer while the patient is supine.

Explanation

When maximum intercuspation is stable and reproducible, a single posterior crown is usually constructed to that position. A rigid elastomeric bite registration captures the existing occlusal relationship accurately. A facebow alone does not record the maxillomandibular relationship.

Question 27Fixed Prosthodontics

A resin-bonded fixed partial denture has debonded twice. Which framework modification is most likely to improve long-term retention?

  1. Extend the retainer to wrap more than 180 degrees around the abutment. Correct answer.
  2. Confine the retainer to a small lingual enamel plate.
  3. Increase the thickness of the pontic to improve rigidity.
  4. Use a nonprecious alloy without air abrasion or etching.

Explanation

Retention of resin-bonded FPDs depends on broad, rigid coverage of the abutment and adhesion to prepared enamel. A wraparound design covering more than 180 degrees increases resistance to dislodgment. Air abrasion and adhesive resin cement are also essential.

Question 28Fixed Prosthodontics

A metal-ceramic FPD has a chipped porcelain area exposing metal on a maxillary premolar. Which intraoral repair sequence is most appropriate?

  1. Polish the exposed metal and leave the area without repair.
  2. Air-abrade metal, prime it, silanate porcelain, then bond composite. Correct answer.
  3. Etch the metal with phosphoric acid, apply bonding agent, and place composite.
  4. Cement a new crown immediately without any repair.

Explanation

Intraoral porcelain repair requires mechanical roughening of the metal, air abrasion, and a metal primer to bond composite to the alloy. The porcelain portion is etched and silanated to improve adhesion. Phosphoric acid alone does not etch base-metal alloys.

Question 29Fixed Prosthodontics

During casting of a base-metal crown, rounded porosity is found at the surface farthest from the sprue, and the pore surface appears smooth. Which factor is the most likely cause?

  1. Too much die spacer on the stone die.
  2. Incomplete wax elimination from the mold.
  3. Back pressure from inadequate venting of the investment. Correct answer.
  4. Excessive expansion of the wax pattern.

Explanation

Back-pressure porosity occurs when air in the mold cannot escape as molten alloy enters, especially in areas farthest from the sprue. It produces rounded, smooth-walled pores. Adequate venting, proper investment mixing, and correct burnout reduce this defect.

Question 30Fixed Prosthodontics

A three-unit FPD is planned to replace a maxillary first molar, using the second premolar and second molar as abutments. The premolar is tilted mesially by 20 degrees. Which step is most important before tooth preparation to establish a common path of insertion?

  1. Reduce the occlusal surface of the premolar by 3 mm.
  2. Use a flexible impression material to accommodate the tilt.
  3. Prepare the premolar with a 30-degree mesial taper to match its tilt.
  4. Survey the diagnostic cast and plan a common path of insertion. Correct answer.

Explanation

Tilted abutments can prevent a single rigid FPD from seating on a common path. A diagnostic cast and survey allow the path of insertion to be planned and show whether the tilt must be corrected or the preparation modified. Overtapering the tilted tooth reduces retention and does not solve the alignment problem.

Question 31Fixed Prosthodontics

Which finish line design creates a flat, 90-degree ledge between the axial wall and the unprepared tooth surface, and is therefore chosen on the facial surface of an anterior metal-ceramic crown where porcelain bulk is required?

  1. Knife-edge finish line
  2. Sloped bevel finish line
  3. Shoulder finish line Correct answer.
  4. Chamfer finish line

Explanation

A shoulder is prepared perpendicular to the path of insertion, producing a flat ledge with a distinct stop and a sharp internal line angle. That geometry gives the laboratory room to place opaque and dentine porcelain without overcontouring the facial profile, which is why it is preferred where esthetics and porcelain thickness matter. Rounded and feathered designs sacrifice this space.

Question 32Fixed Prosthodontics

The edentulous ridge has been shaped so that a pontic can be pressed into a shallow concavity, giving the restoration the emergence profile of a natural tooth. Which pontic design is being described?

  1. Saddle (ridge lap) pontic
  2. Ovate pontic Correct answer.
  3. Sanitary (hygienic) pontic
  4. Modified ridge lap pontic

Explanation

An ovate pontic has a convex, egg-shaped tissue surface that is seated into a prepared soft-tissue socket, so the restoration appears to emerge from the ridge. It is the design of choice in the anterior and visible premolar region when the ridge can be conditioned. The tissue contact must be cleanable and non-inflamed for the design to succeed.

Question 33Fixed Prosthodontics

Before waxing a crown on a gypsum die, the technician applies two coats of die spacer. What is the principal purpose of this step?

  1. To create uniform space for the luting agent Correct answer.
  2. To transfer the surface texture of the intaglio to the wax pattern
  3. To harden the die surface before wax carving
  4. To increase retention of the casting on the die

Explanation

Die spacer is painted onto the axial walls and occlusal surface of the die to reserve a controlled film of cement, typically in the range of 25 to 50 micrometres per coat. Without this relief the casting would seat on the die but stand proud intraorally once cement is introduced. The spacer stops short of the finish line so that the margin remains adapted.

Question 34Fixed Prosthodontics

Which patient finding is the clearest contraindication to using a gingival retraction cord impregnated with racemic epinephrine?

  1. Type 2 diabetes with an HbA1c of 6.5%
  2. Well-controlled hypothyroidism
  3. A history of penicillin allergy
  4. Uncontrolled hypertension with a resting blood pressure of 180/110 mmHg Correct answer.

Explanation

Epinephrine in retraction cord is absorbed through the sulcular epithelium and can produce systemic pressor and arrhythmogenic effects. Severe uncontrolled hypertension, unstable angina, recent myocardial infarction and significant arrhythmias are the accepted contraindications. In such patients a plain cord, a mechanical technique, or an aluminium chloride or ferric sulphate impregnated cord should be used instead.

Question 35Fixed Prosthodontics

A zirconia crown is planned for a maxillary premolar with only 3 mm of axial wall height. Which luting approach offers the most dependable retention?

  1. Zinc oxide-eugenol cement
  2. Zinc phosphate on the untreated intaglio
  3. Airborne-particle abrasion plus MDP-containing resin cement Correct answer.
  4. Conventional glass ionomer alone

Explanation

Zirconia cannot be etched to form a micromechanical bond, so adhesion depends on surface roughening plus chemical affinity between the primer and the polycrystalline surface. Airborne-particle abrasion with alumina increases the surface area, and cements containing the phosphate monomer MDP (10-methacryloyloxydecyl dihydrogen phosphate) bond chemically to zirconia. On a short preparation this adhesive contribution compensates for the limited axial wall area.

Question 36Fixed Prosthodontics

A cast metal crown seats fully on its stone die with closed margins, but intraorally it stops about 1 mm short of the finish line and rebounds when finger pressure is released. Which is the most likely explanation?

  1. A tight proximal contact with the adjacent tooth Correct answer.
  2. Incomplete setting of the impression material
  3. A die spacer layer that is too thin
  4. A stone die that was overtrimmed at the margin

Explanation

Elastic rebound on release of pressure is the classic sign of a contact interference, and the adjacent proximal surface is the usual site in a single crown. The correspondence between die and mouth for the intaglio surface excludes laboratory causes. The contact should be identified with shim stock or articulating film and relieved in small increments, then repolished.

Question 37Fixed Prosthodontics

Two prepared premolars have identical total occlusal convergence, but one has 6 mm of axial wall height and the other only 3 mm. Why does the taller preparation retain a cast crown more effectively?

  1. It shortens the path of insertion
  2. It changes the finish line circumference relative to the axial area
  3. It increases the total cemented area of the axial walls Correct answer.
  4. It produces a thinner cement film

Explanation

Retention of a cemented casting depends on the convergence angle, the roughness of the surfaces and the total area of the prepared walls in intimate contact with cement. Doubling axial wall height while holding taper constant roughly doubles the cemented surface area and therefore the frictional and adhesive contribution. Where height is deficient, auxiliary retentive features such as grooves or boxes are needed.

Question 38Fixed Prosthodontics

Compared with a soldered fixed partial denture, what is the principal advantage of a one-piece cast framework?

  1. It restricts the framework to base-metal alloys
  2. It avoids the dimensional changes introduced by soldering Correct answer.
  3. It removes the need for a common path of insertion
  4. It allows a thinner connector than a soldered joint

Explanation

Soldering introduces a second heating cycle, investment expansion and alloy contraction that can distort the relationship between retainers and prevent simultaneous seating. Casting the framework in one piece eliminates that variable and produces a joint of the same alloy as the retainers. The one-piece approach also removes the risk of a weak or porous solder joint at the connector.

Question 39Fixed Prosthodontics

A metal-ceramic crown on a maxillary canine is functionally satisfactory, but a grey-blue halo is visible along the incisal edge where the porcelain looks translucent. What is the most likely cause?

  1. Excess opaque porcelain was fired over the facial surface
  2. Porcelain thickness over a framework that extends too far incisally is inadequate Correct answer.
  3. The crown was cemented with zinc phosphate cement
  4. The glaze was fired below the recommended temperature

Explanation

Porcelain is translucent, so the dark metal beneath it becomes visible whenever the ceramic layer is thinner than about 1 mm or the framework is carried too close to the incisal edge. The design should place the metal-ceramic junction lingually or well away from the incisal third, with enough dentine and enamel porcelain to mask the opaque layer. A thick opaque layer, by contrast, produces a chalky, over-opaque appearance rather than a grey halo.

Question 40Fixed Prosthodontics

A patient reports a sharp metallic taste and a brief electric sensation whenever a newly cemented nickel-chromium crown in the mandibular arch contacts an existing amalgam restoration in the opposing arch. Which mechanism best explains this?

  1. Pulpitis of the freshly prepared abutment
  2. A galvanic current generated between dissimilar metals in saliva Correct answer.
  3. Allergic contact stomatitis to nickel
  4. Cement-induced thermal sensitivity

Explanation

Two dissimilar metals in contact through an electrolyte such as saliva form a galvanic cell, and the resulting current is perceived as a sharp metallic taste or sting. The effect is transient in many patients as the surface passivates, but persistent symptoms are managed by replacing one of the metals with a compatible alloy or an all-ceramic restoration. Allergy and pulpal disease produce different, typically delayed, presentations.

Question 41Fixed Prosthodontics

A working cast for a three-unit fixed partial denture must provide dies that resist abrasion during waxing and have minimal setting expansion. Which material is specified for this purpose?

  1. Type IV stone Correct answer.
  2. Type I plaster
  3. Type II plaster
  4. Type III stone

Explanation

Type IV is a high-strength, low-expansion die stone. Its low setting expansion and high compressive strength and abrasion resistance allow the technician to carve wax and adapt margins repeatedly without wearing or distorting the die, which is essential for marginal accuracy of the cast framework.

Question 42Fixed Prosthodontics

A mandibular first molar is to be replaced with a metal-ceramic fixed partial denture, and the residual ridge is broad and flat with good access for oral hygiene aids. Which pontic design is most appropriate?

  1. Saddle pontic
  2. Sanitary (hygienic) pontic Correct answer.
  3. Ovate pontic
  4. Modified ridge-lap pontic

Explanation

The sanitary or hygienic pontic has a convex, ovoid tissue surface that leaves a 2-3 mm space between the restoration and the mucosa, so floss or an interdental brush can pass beneath it. It is the design of choice for mandibular posterior teeth where tissue contact is not needed for appearance and cleansability is the priority.

Question 43Fixed Prosthodontics

A custom impression tray is fabricated with 2-3 mm of space between the tray and the prepared teeth. What is the principal reason for this space?

  1. It provides a uniform, controlled thickness of impression material Correct answer.
  2. It allows the material to be mixed with less catalyst and set more slowly
  3. It reduces the total amount of impression material required
  4. It permits border molding of the tray after the material has set

Explanation

A uniform, controlled bulk of impression material minimizes the effect of polymerization shrinkage and thermal contraction, because distortion is proportional to the volume of material. The tray-space also ensures that the material is not so thin that it tears or so thick that it shrinks unevenly around the preparations.

Question 44Fixed Prosthodontics

A five-unit mandibular metal-ceramic fixed partial denture replaces the first premolar and the first molar, so the second premolar becomes a pier abutment between two pontics. Where should a nonrigid connector be placed?

  1. At the distal surface of the pier abutment Correct answer.
  2. At the distal surface of the molar retainer
  3. At the mesial surface of the pier abutment
  4. At both surfaces of the pier abutment

Explanation

A pier abutment acts as a fulcrum, so forces applied on one segment tend to lift the retainer on the opposite side. Placing a nonrigid connector on the distal side of the pier abutment divides the long span into two shorter segments and lets the posterior segment move slightly without displacing the pier retainer.

Question 45Fixed Prosthodontics

During tissue displacement for an impression of a maxillary central incisor preparation, a thin cord is placed in the sulcus and a second, thicker cord is placed over it. What is the purpose of leaving the inner cord in place while the outer cord is removed?

  1. The inner cord expands osmotically and pushes the gingiva away
  2. The inner cord stays in place to keep the sulcus open and dry Correct answer.
  3. The inner cord releases a vasoconstrictor that stops sulcular bleeding
  4. The inner cord prevents the outer cord from being displaced apically

Explanation

In the two-cord technique the thin inner cord remains in the sulcus after the outer cord is removed. It keeps the gingival cuff retracted and the sulcular floor dry at the moment the impression is made, preventing the tissue from collapsing back onto the finish line and contaminating the impression with fluid.

Question 46Fixed Prosthodontics

A final impression of multiple prepared teeth is needed in a mouth where the gingival margins cannot be kept perfectly dry. Which property of polyether makes it preferable to a vinyl polysiloxane in this situation?

  1. It stays elastic indefinitely, so dies can be poured weeks later
  2. It has very low tear strength, so it releases easily from undercuts
  3. It sets without any dimensional change after removal from the mouth
  4. It wets a slightly moist preparation better than a vinyl polysiloxane Correct answer.

Explanation

Polyether is more hydrophilic than vinyl polysiloxane and displaces moisture on the prepared surface more readily, giving better detail reproduction when hemostasis is imperfect. This wettability is the property that justifies its selection, although it is stiffer and less tear-resistant than vinyl polysiloxane.

Question 47Fixed Prosthodontics

A previously endodontically treated tooth is to receive a cast post-and-core and a full-coverage crown. What amount of sound, 360-degree cervical dentin should remain coronal to the preparation finish line to provide a ferrule?

  1. 0.5 mm
  2. 1.0 mm
  3. 1.5 to 2.0 mm Correct answer.
  4. 3.0 to 4.0 mm

Explanation

A ferrule of approximately 1.5-2.0 mm of sound dentin encircling the tooth circumferentially resists functional and parafunctional forces and reduces the risk of root fracture or crown dislodgement. It should be created in dentin, not in core build-up material or in the post, for the effect to be reliable.

Question 48Fixed Prosthodontics

A cantilever fixed partial denture is planned to replace a maxillary lateral incisor, with the canine as the only abutment. Which design change most effectively reduces the tipping moment on the canine?

  1. Placing a nonrigid connector between the retainer and the pontic
  2. Widening the pontic occlusal table to spread the load over a larger area
  3. Raising the pontic slightly out of contact in excursive movements
  4. Adding a second, splinted abutment tooth to the framework Correct answer.

Explanation

The tipping moment equals the force multiplied by the length of the cantilever arm, and the stress is borne by a single abutment. Splinting a second abutment to the framework distributes that moment over a larger periodontal root surface area and shortens the effective lever, which is the most reliable way to reduce the load.

Question 49Fixed Prosthodontics

A three-unit metal-ceramic fixed partial denture replacing a maxillary first molar fractures repeatedly at the connector between the second premolar and the pontic. Which factor is the most likely cause?

  1. The pontic was designed with a 2-mm space for cleaning
  2. The porcelain was fired at a slightly reduced vacuum level
  3. The restoration was cemented with a resin-modified glass ionomer
  4. Insufficient connector bulk, with porcelain extending deep into the joint Correct answer.

Explanation

Connectors concentrate stress and should be built in metal, since porcelain is weak in tension. If the wax connector is too thin or if porcelain is carried deep into the joint, the metal support is lost and the connector becomes the weakest point of the framework under occlusal load.

Question 50Fixed Prosthodontics

A monolithic zirconia fixed partial denture is to be adhesively luted. Which surface conditioning produces the most durable bond to the zirconia intaglio surface?

  1. Airborne-particle abrasion with 50-µm alumina plus an MDP primer Correct answer.
  2. Applying a silane coupling agent alone
  3. Applying 1.23% acidulated phosphate fluoride gel for 4 minutes
  4. Etching with 9.5% hydrofluoric acid for 60 seconds, then silane

Explanation

Zirconia has no glass phase, so hydrofluoric acid etching produces no retentive pattern and silane has nothing to bond to. Airborne-particle abrasion increases surface roughness and reactivity, and a phosphate monomer such as MDP forms a chemical bond with zirconium oxide, giving a more durable resin bond.