Restoring a broken-down tooth

Correct answer: B. Inadequate ferrule height

• A. Shade is an esthetic consideration and is not the structural concern that governs whether this tooth is restorable.
• B. Correct. A ferrule requires at least 1.5 to 2 mm of sound circumferential dentin coronal to the finish line, encircled by the crown. That band resists the wedging force of a post and protects against vertical root fracture. At 0.5 mm there is effectively no ferrule, and crown lengthening or orthodontic extrusion is needed before restoring.
• C. The problem is too little remaining tooth structure, not too much reduction.
• D. Pulpal protection is not the issue in a tooth this broken down, which will be endodontically treated in any case.

Correct answer: C. Proximal grooves and boxes placed in sound axial walls

• A. Deep subgingival margins violate the biologic width, cause chronic inflammation and make impressions and finishing harder. They do not add retention.
• B. Greater taper reduces retention. The ideal convergence is about 6 degrees total, and retention falls sharply as taper widens.
• C. Correct. When axial wall height is limited, auxiliary retentive features restore resistance form. Grooves and boxes in sound dentin resist rotational and lateral displacement, and they work by limiting the paths along which the restoration can be dislodged.
• D. Retention increases with axial wall height and surface area, so shortening the walls makes matters worse.

Note: The recall recorded the option as lingual retention grooves. Grooves are conventionally placed on the proximal walls, which resist rotation most effectively, though additional lingual features can be used. The concept being tested is auxiliary retention.

Correct answer: A. Remake the impression

• A. Correct. The margin is the one part of the preparation that cannot be guessed. If it is not recorded cleanly the crown will not seat accurately, and an inaccurate margin means plaque retention, recurrent caries and periodontal inflammation for the life of the restoration. Retake the impression and address the cause, which is usually moisture control or inadequate tissue displacement.
• B. Waxing a cast invents anatomy that was never recorded. The result is a crown fitted to a fiction.
• C. Asking a technician to guess at a margin transfers a clinical error to someone with no access to the patient.
• D. Chairside adjustment cannot compensate for a margin that was never captured.

Correct answer: A. Value

• A. Correct. Value is lightness or brightness and is the dimension the eye detects first and finds most objectionable when it is wrong. A crown that reads as too white against yellow neighbours is too high in value. Value is also the hardest error to correct, which is why it is assessed first and under multiple light sources.
• B. Chroma is the saturation or intensity of the hue. It matters, but a mismatch in chroma is far less conspicuous than one in value.
• C. Hue is the basic colour family, in dentistry mainly within the yellow to yellow-red range, and it varies least between natural teeth.
• D. Translucency affects the vitality of a restoration, particularly at the incisal third, but it is not the dimension responsible for a crown reading as simply too white.

Correct answer: A. Conventional glass ionomer

• A. Correct. Conventional glass ionomer releases the most fluoride, with a large initial burst followed by sustained release, and it recharges from topical fluoride exposure. This is why it is chosen for high caries risk patients, root surface lesions and atraumatic restorative treatment.
• B. Resin-modified glass ionomer releases substantial fluoride but less than the conventional material, trading some release for improved strength and esthetics.
• C. Fluoride-containing composites release the least, at levels of doubtful clinical significance.
• D. Compomers release considerably less than either glass ionomer.

Correct answer: A. Polymethyl methacrylate

• A. Correct. Polymethyl methacrylate, the acrylic of denture bases and provisional restorations, is composed of low atomic number elements and is essentially radiolucent. This is clinically important because an aspirated or swallowed acrylic fragment may not be visible on a radiograph.
• B. Amalgam is highly radiopaque because of its silver, tin and mercury content.
• C. Gold alloy is among the most radiopaque of dental materials.
• D. Zirconia is markedly radiopaque, appearing similar to or denser than metal on radiographs.

Correct answer: A. A shoulder or heavy chamfer

• A. Correct. Porcelain is brittle and needs bulk to resist fracture. A shoulder or heavy chamfer of roughly 1 mm at the incisal provides that bulk and creates a definite, readable finish line for both the technician and the seating of the restoration.
• B. A knife edge leaves porcelain feathered to nothing at the point of greatest load, and it fractures.
• C. A feather edge has the same problem and additionally makes the margin impossible to locate accurately on the die.
• D. A bevel thins the porcelain at the margin and is used with metal restorations, not with ceramics.