16.1 Fixed Prosthodontics: Crown/Bridge Preparation, Design, and Cementation

Fixed Prosthodontics: Crown & Bridge Principles

Fixed prosthodontics involves the restoration of damaged teeth or replacement of missing teeth using non-removable indirect restorations. Success depends on adhering to rigorous biological, mechanical, and esthetic principles during tooth preparation, impression taking, provisionalization, and final cementation.

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Principles of Tooth Preparation

Tooth preparation requires balancing the preservation of tooth structure (biological) with providing adequate space and resistance for the restoration (mechanical).

Retention and Resistance Form

• Retention Form: The quality of the preparation that prevents removal of the restoration along its path of insertion or long axis. Primary retention is governed by the parallelism of opposing axial walls and total surface area.
• Resistance Form: The quality that prevents dislodgement of the restoration by oblique, lateral, or occlusal forces. Resistance is enhanced by longer axial walls, narrower tooth width, and auxiliary features such as grooves, boxes, and pinholes.
• Total Occlusal Convergence (TOC): The angle formed between two opposing axial preparation walls. The ideal taper is 6° to 10° (3° to 5° per wall). Convergence angles exceeding 12° drastically decrease retention and resistance, increasing the risk of crown decementation.

Occlusal & Axial Reduction Depths

Inadequate tissue reduction leads to over-contoured restorations, compromised esthetics, or structural failure of the material due to thin cross-sectional bulk.

• Porcelain-Fused-to-Metal (PFM) Crowns:
  • Functional Cusp Reduction: 2.0 mm (allows space for 0.3–0.5 mm metal coping, 0.2 mm opaque porcelain, and 1.0–1.2 mm body/incisal porcelain).
  • Non-Functional Cusp Reduction: 1.5 mm.
  • Axial Reduction: 1.2 mm to 1.5 mm.
  • Functional Cusp Bevel: Placed on the palatal cusps of maxillary molars/premolars and buccal cusps of mandibular molars/premolars to ensure adequate material thickness without creating hyper-occlusion.
• All-Ceramic Crowns (Zirconia / Lithium Disilicate):
  • Occlusal / Incisal Reduction: 1.5 mm to 2.0 mm.
  • Axial Reduction: 1.0 mm to 1.5 mm.
  • Internal Line Angles: Must be rounded to eliminate stress concentration points that cause ceramic fracture.
• Full Cast Gold Crowns:
  • Functional Cusp Reduction: 1.5 mm.
  • Non-Functional Cusp Reduction: 1.0 mm.
  • Axial Reduction: 0.5 mm to 1.0 mm.

Margin Geometries & Indications

⚠️ Exam Trap: A bevel is placed on full gold cast restorations to allow burnishing of the metal margin. Never place a bevel on all-ceramic preparations, as thin porcelain margins will chip and fracture during seating.

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Tissue Management & Gingival Displacement

Accurate elastomeric impressions require complete exposure of the subgingival preparation margin and a dry field.

Cord Packing Techniques

• Single-Cord Technique: Used for supragingival or flush gingival margins with healthy, minimally creased tissue.
• Double-Cord Technique: The gold standard for subgingival margins and shallow sulci.
  • First Cord (#000 or #00): Placed deeply into the base of the sulcus to control sulcular fluid exudate and apical tissue seepage.
  • Second Cord (#0 or #1): Impregnated with hemostatic solution and placed above the first cord to displace the free gingival margin laterally (minimum 0.2 mm lateral displacement required for impression material bulk).
  • The second cord is removed immediately prior to injecting light-body impression material, while the first cord remains in the sulcus.

Hemostatic Chemical Agents

• Aluminum Chloride (15%–25%): Non-staining hemostatic agent (e.g., ViscoStat Clear). Causes vasoconstriction and protein precipitation without tissue discoloration. Preferred agent in the esthetic zone.
• Ferric Sulfate (13%–20%): Highly effective coagulant but causes transient dark brown/black tissue staining and inhibits resin cement polymerization if not thoroughly rinsed.
• Epinephrine Cords: Impregnated with racemic epinephrine. Contraindicated in patients with cardiovascular disease, uncontrolled hypertension, hyperthyroidism, or taking non-selective beta-blockers, due to the risk of epinephrine syndrome (tachycardia, tachypnea, hypertension, and anxiety).

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Provisional (Temporary) Restorations

Provisional crowns protect pulpal tissue, maintain periodontal health, preserve tooth position (preventing mesial drift or super-eruption), and provide functional evaluation before final delivery.

Bis-Acryl Composite vs. Polymethyl Methacrylate (PMMA)

• Bis-Acryl Composite (e.g., Protemp, Integrity):
  • Advantages: Low exothermic reaction during setting, minimal volumetric polymerization shrinkage, high flexural strength, auto-mix delivery, excellent marginal adaptation.
  • Disadvantages: Brittle in thin sections, more difficult to repair with flowable resin.
• Polymethyl Methacrylate - PMMA (e.g., Jet, Lang):
  • Advantages: Inexpensive, highly repairable, customizable shade layering, high wear resistance.
  • Disadvantages: High exothermic heat reaction (can cause pulpal necrosis if left to set intraorally on vital dentin), high volumetric shrinkage (Up to 8%), strong free-monomer odor, irritating to unattached mucosa.

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Dental Cements & Luting Agents

Dental cements fall into two categories: conventional non-adhesive luting agents (mechanical retention) and adhesive resin cements (chemical/micromechanical bonding).

                    ┌─────────────────────────────────────────┐
                    │          Dental Luting Cements          │
                    └────────────────────┬────────────────────┘
                                         │
               ┌─────────────────────────┴─────────────────────────┐
               ▼                                                   ▼
   ┌──────────────────────┐                            ┌──────────────────────┐
   │ Conventional Luting  │                            │   Resin-Based Luting │
   └──────────┬───────────┘                            └──────────┬───────────┘
              │                                                   │
     ┌────────┴────────┬──────────────────┐              ┌────────┴────────┐
     ▼                 ▼                  ▼              ▼                 ▼
Zinc Phosphate   Glass Ionomer        RMGI          Self-Adhesive    Adhesive Resin
(Exothermic)    (Calcium Bond)  (Expansion Risk)   Resin (Zirconia)   (Veneers/PFM)

Cement Classifications & Clinical Characteristics

1. Zinc Phosphate Cement:
   • Mechanism: Pure mechanical interlocking.
   • Properties: Mixing produces a highly exothermic reaction (requires mixing over a wide area on a cold glass slab in incremental portions). Contains phosphoric acid, yielding an initial pH of ~2.0, which can induce pulpal irritation if dentinal tubules are unsealed.
2. Glass Ionomer (GI) Cement:
   • Mechanism: True chemical adhesion via chelation of carboxyl groups to calcium ions in hydroxyapatite.
   • Properties: Releases fluoride over time; coefficient of thermal expansion matches tooth structure. Moisture sensitive during initial 10-minute setting phase.
3. Resin-Modified Glass Ionomer (RMGI) Cement (e.g., RelyX Luting):
   • Mechanism: Dual-cure (acid-base GI reaction + radical resin polymerization).
   • Properties: Higher flexural strength and lower solubility than traditional GI; releases fluoride.
   • Contraindication: Contraindicated for all-ceramic crowns with low flexural strength (e.g., feldspathic porcelain or leucite-reinforced glass-ceramics) due to hygroscopic water absorption and expansion, which can crack the ceramic crown post-cementation.
4. Self-Adhesive Resin Cement (e.g., RelyX Unicem):
   • Mechanism: Contains acidic phosphate monomers (e.g., MDP) that etch dentin/zirconia and bond chemically without separate etching/bonding steps.
   • Indications: High-strength monolithic zirconia crowns, PFM restorations, and post cementation.

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Pontic Designs for Fixed Partial Dentures (FPDs)

Pontic selection requires balancing esthetics, tissue health, and cleanability. Pontics should exert zero pressure on the underlying mucosa (passively touch or hover).

  [ Hygienic / Sanitary ]        [ Modified Ridge Lap ]           [ Ovate Pontic ]
     ┌───────────────┐              ┌───────────────┐           ┌───────────────┐
     │  Pontic Body  │              │  Pontic Body  │           │  Pontic Body  │
     └───────┬───────┘              └───────┬───────┘           └───────┬───────┘
             │ (3mm Gap)                    │ (T-Contact)               │ (In Socket)
     ~~~~~~~~~~~~~~~~~              ┌───────┴───────┐           └───┬───┬───┬───┘
       Tissue Ridge                 │  Ridge Tissue │               │ Socket │
                                    ~~~~~~~~~~~~~~~~~               ~~~~~~~~~~

• Hygienic / Sanitary Pontic:
  • Leaves a 3 mm vertical clearance between the tissue surface of the pontic and the residual ridge.
  • Convex in all directions to allow easy cleaning with interdental brushes.
  • Indication: Mandibular molar region (non-esthetic zone).
• Modified Ridge Lap Pontic:
  • Gold standard for esthetic zones (anterior teeth and maxillary premolars).
  • Possesses a convex tissue-facing surface that makes contact with the ridge only on the facial aspect (T-shaped contact line), avoiding lingual concavities.
• Ovate Pontic:
  • Requires a pre-existing extraction socket or surgical creation of a concave tissue bed (1.0–1.5 mm deep).
  • The tissue surface is hyper-convex and rests inside the tissue depression, giving the illusion that the tooth emerges naturally from the gingiva.
  • Requires flawless oral hygiene with superfloss.
• Saddle / Ridge Lap Pontic:
  • Wraps around both facial and lingual aspects of the residual ridge, forming a concave inner surface.
  • STRICTLY CONTRAINDICATED in modern dentistry: forms an uncleansable tissue trap leading to severe plaque accumulation, chronic inflammation, and fetid breath.

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Ante's Law & Structural Mechanics of FPDs

Fixed partial denture design must respect periodontal support and material flexural physics.

Ante's Law

Ante's Law Statement: The combined root surface area (pericemental area) of all abutment teeth supporting a fixed partial denture must be equal to or greater than the root surface area of the teeth being replaced by pontics.

• Clinical Impact: Replacing two missing molars using only the first premolar and third molar as abutments violates Ante's Law, leading to periodontal ligament overload, mobility, and structural failure.

Bending Deflection (Span Length Physics)

Deflection (flexure) of a fixed partial denture under occlusal loading varies directly with the cube of the span length ($D \propto L^3$) and inversely with the cube of the occluso-gingival height of the connector ($D \propto \frac{1}{H^3}$):

• A 2-pontic span deflects 8 times ($2^3 = 8$) as much as a single-pontic span under the same force.
• A 3-pontic span deflects 27 times ($3^3 = 27$) as much as a single-pontic span.
• Doubling the connector height reduces deflection to $\frac{1}{8}$th ($2^3 = 8$) of its original value.