1.4 Indirect Inlays and Onlays, Minimally Invasive Restorations, Finishing and Repair
Key Takeaways
Cusp coverage (an onlay) is indicated when the preparation isthmus exceeds about half to two-thirds of the intercuspal distance or a cusp is undermined, because unsupported cusps fracture under load.
Bonded ceramic inlays and onlays need divergent walls, rounded internal angles, butt-joint margins without bevels and about 1.5-2.0 mm occlusal clearance, whereas cast gold uses beveled margins that can be burnished.
Resin infiltration treats non-cavitated proximal lesions confined to enamel or the outer third of dentin by etching with 15% hydrochloric acid, drying with ethanol and infiltrating a low-viscosity resin.
Selective caries removal leaves firm (shallow and moderate lesions) or soft (deep lesions) dentin over the pulp and is preferred to non-selective removal to hard dentin, which risks pulp exposure.
Repairing a localized defect (marginal ditch, small chip, adjacent caries) is preferred to replacing the whole restoration because each replacement enlarges the cavity and shortens tooth survival.
Direct composite and glass ionomer handle most lesions, but SDLE vignettes regularly ask when an indirect restoration is better, how its preparation differs by material, and which minimally invasive option fits an early lesion. The blueprint for operative dentistry explicitly includes fabricating and cementing inlays and onlays, finishing and polishing, post-operative maintenance, and preventive and minimally invasive procedures.
Direct Versus Indirect Restorations
| Factor | Favors direct composite | Favors indirect inlay/onlay |
|---|---|---|
| Lesion size | Small to moderate | Wide isthmus, lost cusps |
| Contacts and contours | Easy to achieve | Large proximal boxes where direct contacts are unreliable |
| Polymerization stress | Low C-factor cavities | Large volumes where shrinkage stress is high |
| Wear and occlusion | Normal loading | Heavy occlusion, need to restore occlusal scheme |
| Visits and cost | One visit, lower cost | Two visits (or same-day CAD/CAM), higher cost |
Inlays restore the occlusal and proximal surfaces within the cusps. Onlays cap one or more cusps. Cusp coverage is indicated when the isthmus exceeds about one-half to two-thirds of the intercuspal distance, when a cusp is undermined or cracked, or after endodontic treatment of a posterior tooth.
Preparation Rules by Material
| Feature | Cast gold inlay/onlay | Bonded ceramic or composite inlay/onlay |
|---|---|---|
| Wall divergence | About 2-5 degrees per wall for retention | About 6-10 degrees per wall to seat a brittle restoration |
| Internal angles | Defined | Rounded to avoid stress concentration |
| Cavosurface margin | Beveled (gingival bevel and occlusal bevels) for burnishing | Butt joint, no bevel (a bevel creates thin, fragile ceramic) |
| Cusp reduction | About 1.5 mm functional, 1.0 mm non-functional cusp | About 1.5-2.0 mm for adequate ceramic thickness |
| Retention | Mechanical (parallel walls, grooves) | Adhesive (resin cement) |
| Undercuts | Removed by preparation | Blocked out with bonded resin rather than cutting tooth |
For bonded indirect restorations, immediate dentin sealing (bonding the fresh dentin before the impression) improves bond strength and reduces sensitivity. Luting follows the ceramic chapter: hydrofluoric acid etching and silane for glass ceramics, then adhesive resin cement under rubber dam.
Chairside CAD/CAM and Laboratory Workflow
Indirect restorations can be made in the laboratory from an impression or milled chairside from an intraoral scan. Chairside CAD/CAM (for example milled lithium disilicate or resin-ceramic blocks) allows a single-visit onlay with no provisional, which removes the risk of provisional leakage and dentin contamination. The scan must capture every margin, so subgingival margins are often raised with deep margin elevation (a bonded composite layer placed under rubber dam) before scanning. Milled glass ceramics are crystallized or glazed in a furnace before bonding, and the occlusion is checked after cementation because a bonded ceramic should not be adjusted heavily before it is luted.
Selective Caries Removal
Current consensus (International Caries Consensus Collaboration and European Society of Endodontology statements) favors selective removal:
- Remove carious tissue completely from the periphery so the margins sit on sound enamel and dentin for a sealed restoration.
- Over the pulp, leave firm, leathery dentin in shallow or moderate lesions and soft dentin in deep lesions in vital, asymptomatic teeth.
- Seal with a well-adapted restoration; sealed residual bacteria become inactive.
Non-selective removal to hard dentin is now considered over-treatment because it raises the risk of pulp exposure. Stepwise removal (two visits) is an option for very deep lesions but has a re-entry risk.
Minimally Invasive and Preventive Options
| Technique | Indication | Key steps |
|---|---|---|
| Pit and fissure sealant | Caries-susceptible pits and fissures, ICDAS 1-2 lesions in at-risk patients | Clean, isolate, etch 30-60 s (resin), rinse, dry, apply, cure; glass ionomer sealant when isolation is poor |
| Preventive resin restoration | Minimal fissure caries into dentin | Remove only carious tissue with a small bur, restore with composite, seal the remaining fissures |
| Resin infiltration | Non-cavitated proximal lesion to the outer third of dentin; white-spot lesions | Etch 15% HCl about 2 min, ethanol dry, apply low-viscosity infiltrant, cure |
| Atraumatic restorative treatment (ART) | Field settings, anxious or young patients | Hand excavation, restore with high-viscosity glass ionomer |
| Fluoride varnish and remineralization | Active non-cavitated enamel lesions | 5% NaF varnish every 3-6 months, diet control |
Finishing, Polishing and Post-Operative Care
- Composite: contour with fine diamonds or multifluted carbide burs, then sequential discs and strips, then polishing pastes or rubber points. A smooth surface reduces plaque retention and staining.
- Amalgam: carve at the initial set, burnish, and polish after at least 24 hours when the alloy has fully set.
- Check occlusion in maximum intercuspation and excursions with articulating paper; high spots on a new restoration cause pain on biting and cusp fracture.
- Give instructions: no chewing on numb soft tissue, expect short-lived cold sensitivity, maintain fluoride toothpaste and interdental cleaning.
Repair Versus Replacement
Every replacement removes more sound tooth (the "restorative cycle"). When a restoration has a localized defect, such as a marginal ditch, a small fracture, a stained margin or a small adjacent carious lesion, repair is preferred: roughen the existing material (bur or air abrasion), condition it (silane for composite and ceramic, metal primer for alloy), bond, and add new composite. Replace the whole restoration when there is extensive secondary caries, poor overall fit, or a fracture that compromises retention.
Exam Traps
- A bevel on a ceramic onlay margin is wrong; a bevel on a gold onlay is correct.
- Resin infiltration is for non-cavitated lesions; a cavitated proximal lesion needs a restoration.
- Sealing over caries is acceptable for early non-cavitated fissure lesions when the sealant is monitored.
A 25-year-old has a non-cavitated radiolucency on the distal of tooth 15 extending to the outer third of dentin on a bitewing. Cold testing is normal and the marginal ridge is intact. Which treatment best preserves tooth structure?
Resin infiltration after hydrochloric acid etching and ethanol drying of the lesion
A ceramic onlay covering the buccal and palatal cusps of tooth 15 to protect the weakened tooth
A conventional Class II box preparation restored with bulk-fill composite resin
No treatment and no review, because outer-dentin lesions never progress further
Which preparation feature is correct for a bonded lithium disilicate onlay on tooth 46 but would be wrong for a cast gold onlay?
Near-parallel walls of 2 to 5 degrees for mechanical retention
Sharp internal line angles to resist rotational displacement
Butt-joint margins with no bevel and rounded internal line angles
A long gingival bevel so the thin margin can be burnished closed against the tooth
A 10-year-old composite on tooth 26 has a 1 mm marginal ditch at the distal margin without caries; the rest of the restoration is sound. What is the most appropriate management?
Remove the whole restoration and place a new composite to restart its lifespan
Repair the defect by roughening, conditioning and bonding new composite
Crown the tooth because ditched margins indicate imminent cusp fracture
Apply fluoride varnish only, because ditching never allows plaque retention
Sections you finish are checked off in the contents.