Crown Preparations and Prosthodontics Review

Flashcards testing concepts of fixed prosthodontics including full metal crowns, metal-ceramic crowns, all-ceramic crowns, partial veneer crowns, and laminate veneers.

What is the primary function of a cast-metal full veneer crown?

It covers all of the clinical crown and reproduces the morphology and contours of the damaged coronal portions of a tooth.

What are the main indications for a full metal crown?

Extensive destruction of tooth structure from caries or trauma, endodontically treated teeth, need for maximum retention and strength, necessity for recontouring the clinical crown (e.g., for an RPD), and correction of the occlusal plane.

What are the primary contraindications for a cast full metal crown?

High esthetic demands and situations where less than maximum retention is required.

What is the required occlusal reduction on functional versus non-functional cusps for a cast metal crown?

Functional cusps require $1.5\,mm$ of reduction, while non-functional cusps require $1.0\,mm$ of reduction.

What are the dimensions and angulation required for a functional cusp bevel in a cast metal crown preparation?

It should be $1.5\,mm$ deep on the functional cusps and form a $45^\circ$ angle with the axial wall.

What clinical problems result from omitting the functional cusp bevel during preparation?

It can lead to a thin area in the crown that may perforate during function, or cause an overcontoured restoration and poor occlusion.

What total convergence angle (taper) is recommended for axial wall preparation?

A taper of $6^\circ$ (equivalent to $3^\circ$ axial inclination per wall).

What are the effects of incorrectly tilting the bur away from or toward the tooth surface during axial reduction?

Tilting the bur away creates an undercut that prevents crown seating, while tilting toward the tooth causes excessive convergence, resulting in excessive tooth destruction and loss of retention.

What methods or tools can be used to evaluate the amount of occlusal reduction achieved?

Dark-colored utility wax measured with a wax caliper, fast-set rubber base material, calibrated instruments (e.g., PrepSure), or flexible clearance tabs (PrepCheck).

Why is a two-plane facial reduction required for an anterior metal-ceramic (PFM) crown?

The cervical plane determines the path of withdrawal, while the incisal plane provides necessary space for porcelain to achieve esthetics without endangering the pulp.

What facial reduction depths are required for PFM crowns based on the alloy type?

$1.2\,mm$ for base metal alloys and $1.5\,mm$ for noble metal alloys.

What functions does the wing-like structure serve in a standard anterior PFM preparation?

It allows adequate porcelain bulk in interproximal areas for esthetics, enhances resistance form, preserves tooth structure, and adds torque resistance.

How does a wingless metal-ceramic preparation differ from a standard wing preparation?

It utilizes a continuous deep chamfer finish line rather than transitioning from a facial shoulder to a lingual chamfer.

Where should centric occlusal contacts be located on a posterior PFM crown relative to the porcelain-metal junction?

Centric contacts must be located entirely on metal or entirely on porcelain, at least $1.5\,mm$ away from the porcelain-metal junction.

What are the main contraindications for an all-ceramic crown?

Requirement for superior strength, insufficient coronal structure for support, thin teeth faciolingually, or unfavorable occlusal loads (edge-to-edge occlusion, bruxism, or contact on the cervical fifth of the lingual surface).

What type of restoration failure occurs if the $45^\circ$ labiolingual incisal bevel is omitted in an all-ceramic preparation?

A facial half-moon fracture of the restoration caused by excessive stress concentration at the shoulder finish line.

What are the four main processing categories for ceramic crowns?

Castable ceramic (e.g., Dicor), Heat-pressed casting (e.g., IPS Empress), Machinable ceramic (CAD-CAM), and Core strengthening ceramics (e.g., Hi-Ceram, In-Ceram).

What are the three common techniques for porcelain condensation?

Vibration, spatulation, and brush (capillary attraction).

What is the specific role of opaque porcelain in metal-ceramic restorations?

It masks the color of the underlying metal alloy and establishes the chemical bond between the metal and porcelain.

What is the primary advantage of partial veneer crowns over complete coverage crowns?

Conservation of tooth structure, reduced pulpal and periodontal trauma, supragingival margin access for oral hygiene, easy cement escape during seating, and feasible vitality testing.

What is the function of the occlusal offset in a posterior $3/4$ crown

To connect the proximal grooves together, enhancing structural durability and increasing metal bulk in that area.

When is a proximal half crown specifically indicated?

As an FPD retainer on a tilted mandibular molar with good oral hygiene, low caries risk, no marginal ridge discrepancy, and a tilt exceeding $25^\circ$.

What are the standard reduction depth guidelines for a porcelain laminate veneer preparation?

$0.5\text{--}0.7\,mm$ for facial reduction and $0.3\text{--}0.5\,mm$ at the gingival margin.

What are the contraindications for porcelain laminate veneers?

Tooth wear from bruxism, short teeth, severe abrasion (insufficient enamel), oral habits like nail or pencil biting, and existing large restorations or endodontically treated teeth with minimal remaining tooth structure.