3.2 Fixed Prosthodontics
Key Takeaways
- Biological width (Gargiulo 1961) averages 2.04 mm (0.97 mm junctional epithelium + 1.07 mm connective tissue attachment); a minimum of 3 mm is required between the restorative margin and the alveolar crest to avoid violation
- A ferrule of at least 1.5-2 mm of circumferential tooth structure above the preparation margin is the single most important factor in preventing fracture of endodontically treated teeth restored with a post crown
- Ante's law (1926) states the combined periodontal ligament area of abutments should equal or exceed that of the replaced teeth, but modern evidence shows FDPs can succeed when this ratio is not met if tissues are healthy
- Resin-bonded (Maryland) bridges rely on electrolytically etched or sandblasted metal retainer bonding to enamel; ideal for single anterior pontics in young patients with intact abutments and minimal caries
- Biological width violation presents as persistent gingival inflammation, bone loss, and gingival recession around the restoration margin
Indications and Types of Fixed Restorations
Fixed prosthodontics restores form, function, and aesthetics using cemented restorations that the patient cannot remove. The main types are:
- Crowns: Full coverage (e.g., metal-ceramic, all-ceramic, full gold), partial coverage (3/4 and 7/8 crowns). Indicated for extensively broken-down teeth, root-filled teeth, or to alter shape/colour.
- Bridges (fixed dental prostheses, FDPs): Replace one or more missing teeth by relying on abutment teeth. Conventional bridges require preparation of abutments; resin-bonded bridges are minimally invasive.
- Veneers: Thin bonded ceramic facings for anterior aesthetic improvement when underlying tooth is largely intact.
- Inlays/onlays: Indirect intracoronal restorations for moderate cavities where a direct restoration would be inadequate but a full crown is excessive. Onlays cover one or more cusps.
Contraindications
- Active caries or periodontal disease (must be controlled first)
- Poor oral hygiene and unmotivated patient
- Insufficient tooth structure for retention (unless post-and-core or implant considered)
- Unfavourable occlusion (e.g., severe bruxism contraindicates all-ceramic anterior restorations)
- Young patients with large pulps (risk of pulpal exposure during preparation)
Principles of Tooth Preparation
Every preparation must provide retention form (resistance to dislodgement along the path of insertion) and resistance form (resistance to dislodgement under lateral forces).
Retention Form
- Taper: Optimal convergence angle is 3-6 degrees per wall (total 6-12 degrees). Excessive taper reduces retention; too parallel risks under-preparation and fracture of the die.
- Height: Greater height improves retention. A minimum preparation height of 3-4 mm is recommended for anterior teeth and 4-5 mm for posterior teeth.
- Surface area: More surface area = more retention. A full crown has more retention than a partial coverage crown.
- Path of insertion: Must be single and unambiguous; undercuts must be eliminated.
Resistance Form
- Provided by axial walls that resist lateral force by creating a frictional lock.
- Proportional to preparation height and diameter: short, wide preparations (e.g., a lower molar with little height) have poor resistance form.
- Additional features: grooves, boxes, pins, and axial wall modification can improve resistance.
Margin Design
| Margin type | Indication | Notes |
|---|---|---|
| Chamfer | Metal-ceramic (buccal), all-ceramic | Gentle slope; distinct margin on die |
| Shoulder | All-ceramic crowns | 90-degree for strength; may be bevelled |
| Shoulder with bevel | Metal-ceramic (metal margin) | Bevel improves marginal fit |
| Knife-edge (feather) | Full metal crowns | Minimal tooth reduction; harder to read on die |
Ferrule Effect
A ferrule is a circumferential band of tooth structure (enamel and/or dentine) that encircles the coronal aspect of a root-filled tooth and is gripped by the crown. A minimum of 1.5-2 mm of tooth structure above the margin is required. The ferrule splints the tooth and prevents vertical root fracture. It is the single most important factor in the survival of post-retained crowns, more important than the post material or design.
Biological Width
Biological width (Gargiulo, Wentz and Orban, 1961) is the dimension of the dentogingival junction that must be preserved to maintain periodontal health. From a study of 287 autopsy teeth, the mean dimensions are:
| Component | Mean |
|---|---|
| Sulcus depth | 0.69 mm |
| Junctional epithelium | 0.97 mm |
| Connective tissue attachment | 1.07 mm |
| Biological width (JE + CT) | 2.04 mm |
Clinically, a minimum of 3 mm is recommended between the restorative margin and the alveolar crest (2 mm biological width + 1 mm sulcus depth). Violation of biological width, by placing a margin too far subgingivally, causes persistent gingival inflammation, bone loss (to re-establish the dimension), and gingival recession. Management requires surgical crown lengthening or orthodontic extrusion before definitive restoration.
Bridge Designs
Conventional Bridges
| Design | Description | Indications | Drawbacks |
|---|---|---|---|
| Fixed-fixed | Two abutments rigidly connected by a pontic | Short spans, robust abutments | Both abutments must be parallel; high stress on abutments |
| Cantilever | One abutment supports a pontic at one end | Single pontic with strong abutment, usually anterior | Rotational forces on abutment; not for posterior long spans |
| Spring cantilever | Pontic connected to a distal abutment via a long curved connector running along the palate | Replacing an upper anterior tooth when abutments are unsuitable as immediate neighbours | Bulky palatal bar; hygiene challenge |
| Fixed-movable | One end fixed, the other with a movable joint (non-rigid connector) | Tilted abutments, long spans | Movable end requires a precision attachment or stress breaker |
Resin-Bonded (Maryland) Bridges
- Retainer is a thin metal casting bonded to etched enamel of the abutment(s).
- Minimal tooth preparation (slight retentive grooves, enamel-only).
- Ideal for single anterior pontics in young patients with intact abutments, minimal caries, and favourable occlusion.
- De-bonding is the commonest mode of failure; survival is improved with adequate bonding area, occlusal clearance, and avoidance of heavy occlusal contacts.
Abutment Selection and Ante's Law
Ante's law (1926) states: the total periodontal membrane (ligament) area of the abutment teeth must equal or exceed that of the teeth being replaced. It guided abutment selection and popularised double abutments (splinting a second tooth to increase root surface area). However, systematic reviews (Lulic et al., 2007) show FDPs on severely reduced but healthy periodontal support achieve 10-year survival of ~93%, challenging the law. It remains a historical reference, not a strict clinical requirement.
Practical Abutment Selection Criteria
- Periodontal health: Abutments must have treatable, stable periodontium.
- Root form and length: Long, conical roots with good bone support are preferred.
- Crown-to-root ratio: A favourable ratio is generally >=1:1 (clinician judgement applies).
- Pulp status: Vital teeth are preferred; root-filled teeth require a post and ferrule.
- Crown length: Sufficient coronal tooth structure for retention.
Pontic Design, Provisionals, Cementation, and Failures
Pontic Design
The pontic is the suspended artificial tooth. Its tissue surface must be cleansable and non-irritating.
| Pontic type | Description | Indication |
|---|---|---|
| Ridge lap (saddle) | Concave underside contacts a broad area of ridge | Aesthetic anterior; criticised for plaque retention |
| Modified ridge lap | Contacts ridge only on the buccal/lingual aspect, open embrasures gingivally | Standard for anterior and posterior; cleansable |
| Hygienic (sanitary) | Bullet-shaped, no tissue contact; gap below | Posterior, non-aesthetic zones; easy to clean |
| Conical (spheroidal) | Point contact with ridge | Narrow ridges; posterior |
Modified ridge lap is the most widely used in modern practice because it balances aesthetics with cleansability.
Provisional Restorations
Provisionals protect prepared dentine, maintain aesthetics, prevent tooth migration, and test occlusion and aesthetics before the definitive restoration. Requirements:
- Marginal fit to prevent leakage and sensitivity
- Adequate strength for the interim period
- Correct contour and contact points to maintain gingival health
- Occlusal harmony
Materials: methyl methacrylate (strong, aesthetic, exothermic), bis-acryl (less exothermic, common in practice), polycarbonate (preformed, anterior).
Cementation
| Cement | Use | Notes |
|---|---|---|
| Zinc phosphate | Metal and metal-ceramic crowns | Traditional; non-adhesive; low film thickness |
| Zinc polycarboxylate | Metal crowns; vital teeth | Bonds to tooth; kinder to pulp |
| Glass ionomer (GIC) | Metal, some ceramic | Fluoride release; chemical bond to tooth |
| Resin-modified GIC | Metal-ceramic, ceramic | Combines chemical and micromechanical bond |
| Resin cement | All-ceramic, veneers, resin-bonded bridges | Adhesive; requires isolation; highest retention |
Common Failures
- De-bond / loss of retention: From inadequate taper, moisture contamination, or insufficient occlusal clearance.
- Caries at the margin: From poor marginal fit or inadequate oral hygiene.
- Pulpal death: From over-preparation, inadequate provisional, or thermal injury.
- Periodontal inflammation: From biological width violation, overhanging margins, or poor pontic cleansability.
- Ceramic fracture: From inadequate occlusal clearance or parafunction.
- Root fracture (post crowns): From inadequate ferrule or oversized post preparation.
A lower first premolar requiring a full gold crown has 4 mm of clinical crown height. The dentist prepares a near-parallel taper of 6 degrees total. Which factor most limits retention and resistance form, and what additional preparation feature is most appropriate?
An upper central incisor is root-filled and requires a post crown. The remaining tooth structure is 1 mm above the gingival margin circumferentially. What is the most critical factor for long-term survival, and what minimum dimension is required?
A 22-year-old patient with excellent oral hygiene loses an upper lateral incisor through trauma. The adjacent canine and central incisor are sound and unrestored. Which bridge design is most appropriate to avoid compromising intact abutments?
A crown margin is placed 1 mm below the alveolar crest. Six months later the patient has persistent gingival inflammation and radiographic crestal bone loss adjacent to the restoration. What is the diagnosis and the definitive management?
A patient has a missing upper first premolar with sound, vital, well-supported canine and second premolar abutments. According to Ante's law, what should guide abutment selection, and how should this be interpreted in modern practice?