2.3 Operative Dentistry & Cavity Preparation

Black's Classification of Cavities

G.V. Black (1908) classified carious lesions by the tooth surfaces they affect. The system remains a universal descriptive framework, though modern minimally invasive practice has modified the original preparation design philosophy.

Class VI was later added for cavities on the incisal edges or cusp tips of teeth, not covered by Black's original scheme.

Modifications of Black's Principles

Black advocated extension for prevention — extending cavity margins to self-cleansing areas to prevent recurrent caries. Modern adhesive dentistry has modified this:

• Minimally invasive preparations preserve sound tooth structure; margins need not extend to self-cleansing areas when adhesive materials provide a marginal seal
• Preventive resin restorations for Class I lesions involve only the carious fissure, leaving sound fissures sealed rather than cut
• Tunnel and slot preparations for Class II lesions preserve the marginal ridge where possible

Cavity Preparation Principles

Stages of Cavity Preparation

1. Outline form — establish the external shape of the cavity, extending only to the extent of the carious lesion and any unsupported enamel
2. Resistance form — design the cavity to withstand occlusal forces (flat floors, rounded internal angles, adequate bulk of restorative material)
3. Retention form — provide features that prevent dislodgement of the restoration (dovetails, undercuts, or adhesive bonding)
4. Convenience form — allow access for instrumentation and placement of the restorative material
5. Removal of caries — excavate infected dentine (soft, wet, infected) while preserving affected dentine (firm, demineralised but not infected)
6. Finishing of enamel margins — create a clean, distinct cavosurface angle for the restorative material
7. Toilet of the cavity — remove all debris and moisture before restoration

Infected vs Affected Dentine

A critical distinction for the MFDS candidate:

Liners and Bases

Liners are thin (0.5 mm) materials placed on the pulpal floor or axial wall to protect the pulp or provide a therapeutic effect. Bases are thicker (1-2 mm) materials that provide thermal insulation and mechanical support under deep restorations.

Matrices and Interproximal Contacts

Matrix systems are essential for Class II and Class IV restorations to restore interproximal anatomy and contact points.

• Sectional matrices (e.g., Palodent, Triodent) with a separation ring are the standard for posterior composites, providing tight contact points
• Circumferential matrices (e.g., Tofflemire) are used for amalgam restorations and larger cavities
• Transparent matrices are used with light-cured composites to allow light transmission through the matrix
• Wedges (wooden or plastic) are placed interproximally to adapt the matrix to the cervical margin and prevent overhangs

Isolation and Moisture Control

Rubber Dam

Rubber dam isolation is the gold standard for moisture control and soft-tissue protection. Its advantages include:

• Dry field — essential for adhesive bonding and moisture-sensitive materials
• Soft-tissue retraction and protection — prevents trauma to tongue, cheeks, and gingiva
• Aerosol control — reduces bacterial and particulate contamination
• Patient comfort — prevents swallowing of debris and irrigation fluids
• Improved visibility and access

Contraindications and Limitations

• Patients with compromised nasal breathing
• Some third molar restorations where access is difficult
• Patients with severe latex allergy (use non-latex dam: nitrile or silicone)

Alternatives

Where rubber dam is not feasible, cotton roll isolation with high-volume evacuation and retraction cord can provide acceptable moisture control for simple restorations.

Deep Caries Management: Pulp Capping

When caries excavation approaches the pulp, the clinician must decide between indirect pulp capping, direct pulp capping, and pulpotomy.

Indirect Pulp Cap

• Indication: Deep caries with no clinical or radiographic signs of irreversible pulpitis; a thin layer of affected dentine is left over the pulp
• Material: Calcium hydroxide or MTA/Biodentine, followed by a definitive restoration
• Rationale: Stimulates reparative (tertiary) dentine formation and allows the affected dentine to remineralise

Direct Pulp Cap

• Indication: Small mechanical or traumatic pulp exposure (<1 mm) in a vital, asymptomatic tooth with no signs of irreversible pulpitis
• Materials:

Evidence: Multiple randomised controlled trials and systematic reviews demonstrate that MTA and Biodentine achieve significantly higher pulp survival rates than calcium hydroxide for direct pulp capping in permanent teeth, owing to their superior sealing ability and biocompatibility.

Restorability Assessment

Before committing to a complex restoration (or root canal treatment followed by restoration), the clinician must assess whether the tooth is restorable.

Key Factors

1. Remaining tooth structure — Is there sufficient sound dentine and enamel to support a restoration? Teeth with subgingival margins or minimal coronal tooth structure may be unrestorable
2. Ferrule effect — For teeth requiring crowns after endodontic treatment, a minimum of 1.5-2 mm of sound tooth structure above the finish line (the ferrule) significantly improves fracture resistance
3. Pulp status — Is the pulp vital, reversibly inflamed, necrotic, or previously treated?
4. Occlusal factors — Heavy occlusion, parafunctional habits, and bruxism increase the risk of restoration failure
5. Periodontal status — Adequate bone support and attachment are prerequisites for restoration
6. Patient factors — Caries risk, oral hygiene, and motivation to maintain the restoration

The Restorability Decision Tree

Is there sufficient sound tooth structure for a restoration?
  ├── Yes → Is the pulp vital and healthy?
  │     ├── Yes → Restore
  │     └── No → Endodontic treatment, then restore (assess ferrule)
  └── No → Consider crown lengthening, orthodontic extrusion, or extraction

A tooth with insufficient ferrule after endodontic treatment is at high risk of vertical root fracture and may be better managed with extraction and replacement (implant, bridge, or partial denture).