M6 Onlays

Partial coverage restorations¹

Info

This lecture covers partial coverage restorations, with a specific focus on inlays and onlays. The content is designed to provide a comprehensive understanding of their application, from definitions and material science to clinical preparation techniques. It is recommended to supplement this lecture with the provided reference papers to expand knowledge on the topic.

In

Inlay and onlay²

Indirect dental restorations

• Dental restoration made outside of the mouth to correspond to the form of the prepared tooth.
• Cemented or bonded onto the tooth.

At least 2 appointments required:

• 1^st Tooth preparation + impression/scan + temporarization

• 2^nd Try-in, Insertion

• ==Appointment 1:==

  • Tooth investigation and treatment planning (removing old fillings, assessing remaining structure).
  • Tooth preparation.
  • Impression or digital scan.
  • Bite registration.
  • Placement of an interim (provisional) restoration.

• ==Appointment 2:==

  • Try-in of the final restoration.
  • Cementation or adhesive bonding of the restoration.

Inlay and onlay definitions³

Inlay

• Fixed intracoronal restorations that fit within the anatomic contour of the clinical crown.
• Relies on the strength of the remaining tooth structure for support and retention.
  • the cusps must be robust enough to bear occlusal loads independently
  • the inlay itself does not protect the cusps from fracture

Onlay⁴

• Partial-coverage restoration that restores one or more cusps and adjoining occlusal surfaces; or the entire occlusal surface.

• Retained by mechanical and/or adhesive means.

• ==Terminology: When an onlay covers the entire occlusal surface, it may be referred to as an overlay or tabletop.==

• ==Function: Onlays are designed to protect weakened cusps from occlusal forces, preventing cracks and fractures. This is known as cuspal coverage.==

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The Extent of Tooth Structure Loss requiring replacement will define the need for an INLAY or ONLAY or a Full crown.

ONLAY - the need for tooth protection with CUSPAL COVERAGE whilst trying to avoid a traditional full crown.

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Indications and contraindications⁵⁶

Inlay

Indications

• Moderate-sized cavities confined within the cusps.
• Teeth with at least one sound marginal ridges.
• Replacement of small defective restorations (amalgam or composite) in the occlusal area.
• Restorations requiring better marginal fit and durability than direct composite or amalgam.
• Cases where occlusal anatomy and contact points need precise reproduction.

, especially for deep proximal boxes where direct placement is challenging and to ensure occlusal stability and prevent future periodontal issues.

Contraindications

• Extensive tooth destruction involving cusps.
• Poor isolation or moisture control for bonding.
• MOD cavities with weak cusps.

Onlay⁷

Indications

• Large cavities involving one or more cusps, but enough tooth structure (enamel) remains for bonding.
• Teeth with fractured or weakened cusps that need protection.
• Replacement of defective large restorations.
• Restorations of endodontically treated teeth when sufficient tooth structure is still present.
  • which are structurally compromised due to access cavity preparation and previous decay. Onlays or full crowns are necessary to protect the remaining cusps.
• Situations where full crowns are not indicated to preserve tooth structure (conservative preparation).
• such as when preparing axial walls would leave them excessively thin (e.g., less than 1mm). An onlay preserves more tooth structure in the cervical area.
• Replacement of a large MOD amalgams due to poor aesthetics.

Contraindications

• Teeth with severely compromised - lack of sound enamel for a predictable bonding.
  • Bonding to dentin and cementum is less predictable. In such cases, a restoration with mechanical retention (like a full crown) is preferred.
• Severe parafunctional habits.
  • (e.g., bruxism), as the high forces can fracture ceramic restorations. A stronger material like monolithic zirconia in a full crown design may be a better option.

Tooth structure removal in preparation designs⁸

• Onlays require an intermediate amount of tooth structure removal (falling between 27.2% for inlays and 67.5-75.6% for full crowns).

Info

This data highlights the importance of choosing a conservative design like an onlay whenever clinically appropriate to preserve natural tooth structure.

Considerations for inlay vs. onlay⁹

Extension of the restoration

• When the width of an intracoronal cavity exceeds 1/2 the distance of the cusp tips, an onlay or crown is probably more suitable.

Info

If the width of the occlusal isthmus is greater than half the distance between the cusp tips, the cusps are likely undermined and require onlay coverage for protection.

• Cuspal coverage is needed for endodontically-treated tooth.

Info

These teeth should receive either an onlay or a full crown for cuspal protection. A small inlay is not justified, as a direct composite is more conservative if the tooth structure is sufficient.

Things to consider

• enamel bonding vs dentin bonding
• variance in bonding to different qualities of dentin
• ==Bonding to sclerotic dentin (often found under old amalgam fillings) is less predictable due to calcified tubules, resulting in lower bond strength. If bonding is compromised, the preparation may require more retentive features.==
• possibility of limited retentive form

Indications for Cuspal coverage¹⁰

• A) Tooth structure less than 2 mm
  • If a cusp wall is less than 2mm in width, it is considered weak and should be covered with an onlay.
• B) Low quality of the enamel covering the cusp - unsupported enamel are likely to fracture under occlusal load
• C) Occlusal contacts on the margin of the preparation.

Info

The margin of the restoration should not be placed directly at the occlusal contact point. The contact should ideally be on either the restoration or the natural tooth, away from the interface, to prevent fracture.

Clinical rationale¹¹

Cavity Preparation Type	Recommended Restoration(s)
Class I	Composite, Amalgam
Class II MO, DO	Composite, Amalgam
Class II MOD	Composite, Amalgam, Indirect restoration
Class II MODL, MODB	Amalgam, Indirect restoration
Multiple cusps missing	Indirect restoration

Longevity of restorations in posterior teeth and reasons for failure¹²

Annual failure rates in posterior stress-bearing restorations

Restoration Type	Annual Failure Rate
Glass ionomers and derivatives	~14.00%
Direct composites	~8.00%
Ceramic restorations	~7.00%
Amalgam	~6.00%
Gold inlays and onlays	~5.00%
CAD/CAM ceramic restorations	~4.00%

Gold Standard?

CAD/CAM Ceramics are considered a potential gold standard due to the high quality of modern materials and the precision of digital fabrication, which reduces technical errors.

Main reasons for failure: secondary caries, fracture, marginal deficiencies, wear, and postoperative sensitivity.

Survival rate vs. success rate

Info

It is important to distinguish between two key metrics in clinical studies:

• ==Survival Rate: The restoration is still in place and functioning, but may have technical issues like chipping, wear, or marginal staining.==
• ==Success Rate: The restoration is in place, functioning, and has no technical problems or failures.==

Material & fabrication methods¹³

• Precious alloys: Gold / Cast

• Indirect composite resin (polymer-based): CAD-CAM (chair side or lab)

• Ceramic:

  • Feldspathic, Leucite-reinforced ceramic, Lithium disilicate, Polymer-infiltrated ceramic, Alumina-based and Zirconia / Sintering, hot pressing, chairside CAD/CAM and laboratory CAD/CAM)

• Survival rate: similar

• Deterioration: Gold < Ceramic < Composite

Longevity of ceramic onlays: a systematic review¹⁴

Abduo et al. 2018

• No indication that one ceramic material performs better than another, and the fabrication methods appear to minimally influence the ceramic onlay performance.

Caveat

This conclusion should be interpreted with caution, as long-term, well-controlled clinical studies in prosthodontics are difficult to conduct and limited in number.

• Survival rate:
  • medium-term studies (2–5 years) - 91–100%
  • long-term studies (more than 5 years) - 71–98.5%
• Common cause of failure:
  • 1^st Ceramic fracture
  • 2^nd Debonding
  • 3^rd Caries
• Deterioration of ceramic onlays: marginal integrity, margin discoloration, surface roughness, color match and anatomical form
  • Most common: loss of marginal integrity
  • Second: margin discoloration

Material comparisons¹⁵

CAD/CAM polymer-based material – indirect composite restorations

• Indirect CAD/CAM resin composite materials - higher edge stability than ceramics, permitting restorations with very thin margins.
• CAD/CAM composites: more homogenous and more abrasion-resistant restorations than direct composite resins.
• Ceramics are generally superior to CAD/CAM polymer-based materials in terms of flexural strength, abrasion resistance and discoloration rates, whereas CAD/CAM polymer-based materials are more antagonist-friendly.
  • meaning they cause less wear to opposing teeth than harder ceramics. They are also easily repairable in the mouth.

Pressable lithium disilicate ceramic versus CAD/CAM resin composite restorations in patients with moderate to severe tooth wear: clinical observations up to 13 years

• Pressed LD single-tooth restorations showed lower long-term failure rates than RC.

• CAD/CAM RC restorations showed higher abrasion and significantly higher discoloration rates.

• RC restorations exhibited more material fractures.

• Despite the technical problems, the survival rate for both was 100%.

• ==While the survival rate was similar, the pressable lithium disilicate had a lower long-term failure rate and was more resistant to abrasion.==

Direct vs. indirect composite restorations¹⁶¹⁷

Direct (posterior composites)

Pros

• Restores aesthetic and functionality
• More affordable - single appointment
• Maximum preservation of tooth structure (minimally-invasive dentistry)

Cons

• Relatively short lifespan: microleakage, secondary caries, staining, factures
• Relies on remaining tooth structure for strength and integrity of restoration
• Incomplete polymerization
• Polymerisation shrinkage
• Degrade chemically in the oral environment*
• Lower wear resistance
• Contacts & contours technique sensitive
  • especially for establishing tight proximal contacts and ideal occlusal anatomy in large restorations.

* Chemical agents found in saliva, food and beverages. Intermittent exposure occurs during eating or drinking until teeth are cleaned. Continuous exposure may occur as chemical agents can be absorbed by adherent debris (such as calculus or food particles) at the margins of restorations or be produced by bacterial decomposition of debris. Yep 2008 Chemical degradation of composite restoratives

Indirect (composite)

Pros

• Complete polymerization

  • Fabricated from a fully polymerized block, resulting in complete polymerization, less porosity, and no clinical shrinkage stress.

• Less porosity

• Better control of contours & contacts

• Better Marginal Adaptation and bond strength

• Easier to repair (vs ceramic)

Cons

• Less conservative (vs direct)
• More expensive (vs direct)
• Unpredictable color stability (vs ceramic)
• Inferior mechanical properties (vs ceramic)
• More wear and risk of fracture (vs ceramic)

Ceramic onlay/inlay¹⁸

Advantages

• Superior aesthetics – excellent translucency, shade stability, and enamel-like appearance.
• High wear resistance – maintains anatomical form over time.
• Excellent biocompatibility – inert and tissue-friendly.
• Resists surface degradation and staining.
• High compressive strength – suitable for posterior load-bearing areas when properly bonded.
• Ability to bond to tooth structure, reinforcing weakened cusps.

Disadvantages

• Brittleness – prone to fracture under high occlusal stress if preparation or bonding is inadequate.
  • More abrasive to opposing natural dentition compared to composites or gold.
• Irreparable – if fractured, usually requires replacement.
  • as intraoral repair would require etching with hydrofluoric acid, which is not used clinically in the patient's mouth.
• More invasive preparation – requires slightly more tooth reduction to allow adequate ceramic thickness.
• Technique-sensitive bonding – requires resin cementation.
• Higher cost – material and laboratory fees are more expensive.

Clinical long-term study: Cerec restorations¹⁹

Results

• Probability of survival decreased to 90% after 10 years and 84.9% after 11.8 years.

Info

This is considered a very good long-term outcome for large restorations.

• Restoration size and outline did not affect success rate.

• Premolars better than molars.

  • (due to lower occlusal forces).

• Vital teeth better than non vital.

Tooth preparation²⁰

Two main philosophies exist for onlay preparation:

1. (Old) Conventional Preparation:
   • ==Characterized by boxes, sharp angles, and retentive grooves.==
   • ==Designed for mechanical retention of restorations like cast gold onlays.==
   • ==This design creates stress concentrations and is unfavorable for brittle ceramic materials.==
2. Morphology Driven Preparation:
   • ==Characterized by smooth, flowing curves, rounded internal angles, and simple geometry.==
   • ==Designed for adhesively bonded restorations (ceramics, indirect composites).==
   • This design minimizes stress concentration and allows for more even stress distribution, which is ideal for ceramics.

*P

Principles of preparation for ceramic restorations²¹

Fundamental Principle

The fundamental principle is that ceramics perform well under compression but fail under tension. Preparation design must accommodate this property.

Favorable designs

• Sufficient and even material thickness to distribute stress uniformly.

• Preparation follows the existing defect or restoration, removing only compromised tooth structure.

• Compressive stress

• Curved transition

• Simple geometry

• Gradual cross sectional transition

Unfavorable designs²²

• Tensile stress

• ** sharp edges**

• complex geometry**

• abrupt cross sectional transition

• Box style preparation tends to produce tensile stresses on the internal surface.

• Curved preparation tends to produce compressive stresses on internal surface.

Key preparation features for ceramic onlay and inlay²³²⁴²⁵

• No sharp internal line angles or points, only curved transitions.

• The prep extension depends on the extension the previous restoration or the caries lesion.

• All cavo-surface angles need to be sharp, well defined.

  • ==Avoid bevels, especially in occlusal contact areas. A beveled or undefined margin makes it impossible for the lab to know where the restoration should end, leading to over-extension and thin, chippable ceramic edges.==

• Butt joint (90°)

• Greater than or equal to 10 degrees of divergence on buccal and lingual walls.

• • 10 to 12 degrees of axial wall convergence.
• • Avoid complex internal geometry to the preparation.

• Transition of material thickness should be gradual not abrupt.

• No acute angles within preparation

  • Milling machines use burs with a minimum diameter (e.g., 1.0-1.5 mm) and cannot physically mill a sharp internal corner. The machine will compensate by over-milling the area, creating a void that will be filled with a thick layer of cement, compromising the fit and stress distribution of the final restoration.
• • Smooth flowing margins

• No additional retention features

• No tooth structure without support

• Obtuse external line angles

Path of Insertion

The preparation must have a single path of insertion with no undercuts to allow for passive seating.

• ==The main buccal and lingual walls should be divergent towards the occlusal surface.==
• ==The axial walls of any proximal boxes (mesial or distal) must be convergent towards the occlusal surface. This is a critical feature to ensure a single path of draw.==
• ==The traditional recommendation for the buccal-lingual width of the isthmus is a minimum of 2mm.==

Position of buccal margin^2627282930

Avoid contact points

• (a) Ultraconservative buccal cusp coverage.
  • restoration margin is on the incisal third
• (b) Conventional buccal cusp coverage
  • the restoration margin is in the middle third .
• (c) Full buccal cusp coverage (VONLAY).
  • restoration margin is on the cervical third

For aesthetics: the simplest and most ideal situation is for the restoration margins to be located in the incisal or cervical thirds.

Proximal margins and emergence profile³¹

• make sure there is an Absence of contact between the preparation and the adjacent teeth.
• Drop down the margins in order to obtain an optimal, natural proximal emergence profile of the future restoration.

Margins³²³³

• No aspect of the margin should be located in an undercut and no unsupported enamel.

Accepted Prep

• Deep Chamfer, Modified Shoulder
  • Supra or equigingival

Unacceptable Prep

• Feather edge
  • Subgingival
• “Gutter” margin
  • preparation

Path of insertion³⁴³⁵³⁶

• make sure that neighboring teeth are not blocking the path of insertion!

Preparation techniques³⁷³⁸ iothis

Morphology driven preparation

• Bonded restorations
• Immediate dentin sealing and cavity design optimization (after tooth preparation):
  • avoid unnecessary removal of tooth structure
  • protect the pulpodentinal structures from any contamination/disturbance during temporarization
  • stabilize and improve the adhesive interface quality.
• Deep Margin Elevation

(Old) conventional preparation

• Cemented or bonded
• Deep occlusal box
• Width of occlusal isthmus ≥ 2 mm
• Geometrical reduction
• More suitable for indirect non-adhesive restorations.

MR6 Onlays, p.39

Morphology driven of onlays preparation details³⁹

Checklist before impression or scanning⁴⁰

1. Well defined sharp margins
2. No undercuts, no sharp internal line angles
3. Smooth surface
4. Accessibility to all margins, especially subgingival margins
5. Absence of contact between the prep margin and the adjacent tooth.
6. Adequate interocclusal space.

Interim restoration⁴¹

Non-cemented “semi-rigid” light-curing resin

Examples:

• Telio
• Ivoclar

Prep isolated with Vaseline at the periphery and over the axial walls before applying the material.

Self-curing material⁴²

• such as Cavit W

Direct technique⁴³

Bis-Acryl composite

References⁴⁴

• Abduo and Sambrook. Longevity of ceramic onlays: A systematic review J Esthet Restor Dent. 2018 May;30(3):193-215.
• Edelhoff et al. Pressable lithium disilicate ceramic versus CAD/CAM resin composite restorations in patients with moderate to severe tooth wear: Clinical observations up to 13 years. J Esthet Restor Dent. 2023;35:116-128.
• Hickel and Manhart. Longevity of restorations in posterior teeth and reasons for failure J Adhes Dent. 2001 Spring;3(1):45-64.
• Reiss and Walther. Clinical long-term results and 10-year Kaplan-Meier analysis of Cerec restorations. Int J Comput Dent. 2000 Jan;3(1):9-23.
• Arnetzl & Arnetzl. Biomechanical examination of inlay geometries—is there a basic biomechanical principle?. Int J Comput Dent. 2009;12(2):119-30.
• Edelhoff & Sorensen. Tooth structure removal associated with various preparation designs for posterior teeth. Int J Periodontics Restorative Dent. 2002 Jun;22(3):241-9.
• Hopp & Land. Considerations for ceramic inlays in posterior teeth: a review. Clinical, Cosmetic and Investigational Dentistry 2013:5 21-32
• Rocca et al. Evidence-based concepts and procedures for bonded inlays and onlays. Part II. Guidelines for cavity preparation and restoration fabrication. Int J Esthet Dent. 2015;10(3):392-413.
• Veneziani. Posterior indirect adhesive restorations: updated indications and the Morphology Driven Preparation Technique. INT J ESTHETIC DENT 12:2 2017
• Edelhoff et. All. Clinical performance of occlusal onlays made of lithium disilicate ceramic in patients with severe tooth wear up to 11 years. Dental Materials. 35:9, 2019, Pages 1319-1330

Footnotes

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