BDS Prosthodontics and Crown & Bridge Syllabus

How much occlusal/incisal reduction is generally required for a metal-ceramic (PFM) crown versus an all-ceramic crown?

PFM: approximately $1.5$–$2.0$ mm occlusal reduction (to accommodate metal + porcelain). All-ceramic: approximately $1.5$–$2.0$ mm incisal and $1.0$–$1.5$ mm axial reduction depending on the ceramic system.

Classify dental impression materials by their setting mechanism.

Reversible (physical set): agar hydrocolloid, impression compound. Irreversible (chemical set): alginate, elastomers (polysulfide, condensation silicone, addition silicone/PVS, polyether), zinc oxide eugenol, and impression plaster.

Which elastomeric impression material has the best dimensional accuracy and stability, and what is its main byproduct concern?

Addition silicone (polyvinyl siloxane/PVS) has excellent accuracy and dimensional stability. Its concern is hydrogen gas release during setting (may cause pitting if poured too soon) and inhibition of set by latex/sulfur contamination.

Describe the putty-wash (two-stage) and single-stage impression techniques.

Putty-wash: a putty is used to make a preliminary impression, then a low-viscosity wash captures fine detail in a second stage (or single mix). Single-stage/dual-viscosity: heavy body in the tray and light body around the preparation are set simultaneously in one step.

What is gingival retraction and name common methods used before impression making.

Gingival retraction is temporary lateral and vertical displacement of gingival tissue to expose the finish line and control fluids. Methods: mechanical (retraction cord), chemico-mechanical (medicated cord with astringents like aluminum chloride or ferric sulfate), surgical (electrosurgery, rotary curettage), and cordless retraction pastes.

Define centric relation (CR) per current prosthodontic understanding.

Centric relation is the maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective discs, with the complex in the anterosuperior position against the articular eminences — a reproducible, bone-braced reference position independent of tooth contact.

Differentiate centric relation (CR) from centric occlusion / maximum intercuspation (MICP).

Centric relation is a jaw-to-jaw bone relationship (condylar position). Centric occlusion/maximum intercuspation is a tooth-to-tooth relationship of maximum cuspal interdigitation. When CO coincides with CR it is termed a 'point centric' or centric relation occlusion.

What is a face-bow and its purpose in prosthodontics?

A face-bow is a caliper-like device that records the spatial relationship of the maxillary arch to the transverse horizontal (hinge) axis of the condyles and transfers it to an articulator, allowing the maxillary cast to be mounted in the same orientation as in the patient.

Classify dental articulators.

Class I: simple non-adjustable holders. Class II: allow horizontal/vertical motion but not oriented to condyles (mean-value). Class III: semi-adjustable arcon and non-arcon types accepting face-bow and interocclusal records. Class IV: fully adjustable, accept 3D records and set to individual condylar paths.

Distinguish arcon from non-arcon articulators.

Arcon (articulator + condyle): condylar spheres are on the lower member and the condylar guidance (fossae) is on the upper member — mimicking natural anatomy. Non-arcon: the condylar guidance is on the lower member and condylar ball on the upper; the relationship changes as arms open.

State Hanau's Quint (the factors of balanced occlusion).

The five interdependent factors: (1) condylar guidance (inclination), (2) incisal guidance, (3) occlusal plane orientation, (4) cusp height/inclination, and (5) compensating curve. Hanau's formula: $K = \frac{OP \times CC}{CG \times IG}$ relating these factors.

Define the different types of occlusal schemes: balanced, group function, and canine-guided (mutually protected) occlusion.

Balanced occlusion: simultaneous bilateral contacts in centric and all excursions (used for complete dentures). Group function: multiple posterior teeth on the working side contact in lateral excursion. Canine-guided (mutually protected): the canine disoccludes posteriors in lateral excursion; posteriors protect the anteriors in centric.

During bridge try-in, list the sequence of checks performed before cementation.

Check marginal fit/seating, proximal contacts, occlusion in centric and excursions, pontic-tissue relationship, esthetics (shade and contour), and interproximal/embrasure form for cleansability.

What is 'seating' and how is marginal adaptation verified clinically at try-in?

Seating is complete passive placement of the restoration to its finish line. Marginal adaptation is verified with an explorer (for ledges/gaps), disclosing media (fit-checker/silicone), radiographs for proximal margins, and visual inspection for open or overhanging margins.

Classify dental cements used for luting fixed prostheses.

Zinc phosphate, zinc polycarboxylate, glass ionomer (GIC), resin-modified glass ionomer (RMGIC), and resin cements (adhesive). Provisional cements include zinc oxide eugenol (ZOE) and non-eugenol ZOE.

Which luting cement is recommended for all-ceramic and resin-bonded restorations, and why?

Resin cements — they provide high bond strength via micromechanical and chemical adhesion, low solubility, and strengthen the restoration by reinforcing the ceramic; essential for bonding etchable glass-ceramics and resin-bonded (Maryland) bridges.

Describe the two functions/purposes of provisional (temporary/interim) restorations.

Biologic: protect pulp, maintain periodontal health, and preserve tooth position. Mechanical: maintain occlusal function and prevent tooth migration. Esthetic and diagnostic: preview form/esthetics, allow evaluation of occlusion before the definitive restoration; also maintain patient comfort and phonetics.

Name common materials used for fabricating provisional crowns.

Polymethyl methacrylate (PMMA), polyethyl methacrylate, bis-acryl composite resins, and light-cured urethane dimethacrylate resins; preformed polycarbonate, aluminum, or stainless-steel crown forms are also used.

Compare the two basic techniques for fabricating provisional restorations: direct and indirect.

Direct: the provisional is made intraorally directly on the prepared tooth (fast, but exothermic and monomer contact). Indirect: made on a cast/die outside the mouth (better fit, marginal accuracy, less tissue irritation). An indirect-direct combination is also used.

What is the purpose of a metal coping/framework in a metal-ceramic restoration and its ideal thickness?

The metal coping provides a rigid substructure to support and bond the porcelain, resisting fracture. Ideal cast metal coping thickness is approximately $0.3$–$0.5$ mm; porcelain veneer thickness is ideally $1.0$–$1.5$ mm for esthetics and strength.

Explain the lost-wax casting technique used in crown & bridge fabrication.

A wax pattern of the restoration is invested in a refractory mold, the wax is eliminated by burnout leaving a cavity, and molten alloy is cast into the mold under centrifugal or vacuum-pressure force. After cooling, the casting is divested, finished, and polished.

What is the function of the sprue, and why is a casting ring given a compensating expansion?

The sprue forms the channel through which molten metal flows into the mold and acts as a reservoir for solidification shrinkage. Investment expansion (setting + thermal/hygroscopic) is designed to compensate for the casting shrinkage of the solidifying alloy, ensuring accurate fit.

How should a fixed partial denture be cleaned by the patient to maintain periodontal health?

Using dental floss with a floss threader or Superfloss passed under the pontic and through connectors, interdental brushes, water irrigators, and routine brushing — cleansing the pontic-tissue junction to prevent plaque accumulation and gingival inflammation.

List common causes of failure requiring repair of fixed and removable prostheses.

Loss of retention (decementation), porcelain fracture/chipping, recurrent caries at margins, periodontal breakdown of abutments, connector fracture, occlusal wear, and mechanical failure of the framework or acrylic (for dentures).

Define an osseointegrated dental implant (Brånemark's concept).

Osseointegration is a direct structural and functional connection between ordered, living bone and the surface of a load-bearing implant, without intervening fibrous connective tissue, observed at the light-microscopic level.

Name the components of a typical two-piece endosseous dental implant system.

The implant fixture (endosseous body placed in bone), the abutment (connects fixture to prosthesis), the abutment/prosthetic screw, a healing abutment/cover screw, and the prosthesis (crown/bridge/denture).

Differentiate one-stage (non-submerged) from two-stage (submerged) implant surgical protocols.

Two-stage: the implant is placed and covered by a cover screw beneath the mucosa; after osseointegration a second surgery uncovers it to place a healing abutment. One-stage: a healing abutment/transmucosal collar is placed at surgery so the implant heals exposed to the oral cavity, avoiding a second surgery.

What is the typical healing (osseointegration) period before loading implants in the mandible versus the maxilla?

Conventional loading: approximately $3$ months for the mandible and $6$ months for the maxilla, due to the lower bone density and quality of the maxilla. Immediate and early loading protocols also exist under favorable conditions.

During the implant prosthetic phase, differentiate a screw-retained from a cement-retained implant prosthesis.

Screw-retained: the prosthesis is secured by a screw into the abutment/fixture — retrievable, no residual cement, but has an access hole. Cement-retained: the crown is cemented onto the abutment — better esthetics/occlusion and passive fit, but risk of retained subgingival cement causing peri-implantitis and less retrievability.

Why must an implant prosthetic framework achieve 'passive fit,' and what problems arise from a non-passive fit?

Because implants lack a periodontal ligament and cannot move to accommodate strain, a non-passive framework transmits static preload stress to the bone and components, causing screw loosening/fracture, framework or porcelain fracture, marginal bone loss, and potential loss of osseointegration.