What are they?

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Inlays & Onlays, the unsung heroes

What are they?

Inlay: An indirect intracoronal restoration made to strengthen and repair posterior teeth

Onlay: Same as an inlay but including cuspal coverage

A history lesson

1835: The first porcelain inlay credited to John Murphy

1880: ‘Burnish foil technique’ used for inlays by Ames and Swa-sery

1897: The first cast inlay by Phil-brook

1907: The year everyone remembers as when Taggart introduced the lost wax technique to dentistry

1980: Mormann and Brandestini developed chairside CAD/CAM system

1985: Alumina infiltrated glass ceramics developed by Sadoun

1990: Technique of pressed glass reported on by Wohlvend and Scharer

Bad Press

Early ceramic inlays weren’t very successful due to problems relating to marginal integrity, lack of bond between restoration and tooth tissue, ‘wash-out’ of luting cements and; discolouration, marginal openings and secondary caries.

The teaching of inlays & onlays has also been seen as a side thought with the emphasis put on direct restorations or indirect full coverage.

Why use them?

Alternatives contraindicated:

  • Amalgam...need I say more.

  • Resin composite not indicated (Large cavity, previous failure restoration, aesthetic considerations)

  • Full coverage crowns cause unnecessary loss of tooth substance

Other considerations: Risk of cuspal failure (cavity size, unsupported cusp, parafunction, etc.), altering tooth morphology (e.g. cuspal guidance, removal prosthetic components), and many more.

*Sidenote: Use in parafunction is controversial and parafunction should be resolved prior to advanced restorations*




Direct Resin

Indirect Resin





May lack translucency





Clinical Performance


Single visit.

Poor margins.

Still being proved.

Similar hardness to tooth structure.

Proven good marginal fit.

Does not wear opposing teeth.

Still being proved.

Similar hardness to tooth structure.

One visit.

No stress on


Still being proved.

No stress on tooth.

Similar hardness to tooth structure.

Short Chairside time.

Still being proved.

Clinical Weakness


Mercury toxicity.

Large tooth structure required.

Polymerization shrinkage can stress tooth.

Long Chairside time.

Meticulous technique.


Weakens tooth.

Two visits.

Tooth preparation critical.

Finishing of margin time-consuming.

Requires good margins.

Two visits

Laboratory performs all

Construction work.

Requires good margins.

One/two visits.

Generally wear opposing teeth.

Laboratory Weakness

No laboratory cost.

No laboratory cost.

Requires good margins.

Lost wax technique is time-consuming.

Material is expensive.

No laboratory cost.

Needs finishing.

Lost wax technique is time-consuming.

Expensive outlay.

Table 1 - A Comparison of some materials used for posterior restorations (M Abdul Razak. The Tooth-Coloured Inlay/Onlay Restorations Annals Dent Univ Malaya 1998 5: 7-23)



Once considered ‘The Gold Standard’.

Advantages: Excellent physical and mechanical properties, corrosion resistant and excellent survival rates.

Disadvantages: Not very aesthetic (personal opinion) and wedge effect on inlays (increased stress in wide/deep inlay preparations).

Adhesive Systems

The advent of acid etch (Buonocore 1955) allowed adhesive restorations to become a viable alternate.


  • Conventional ceramics

    • As mentioned earlier, conventional ceramics were problematic due to: Material weakness, Poor marginal integrity and Lack of adequate cement.

    • Advantages: Aesthetic, Decreased marginal leakage associated with polymerisation shrinkage and Less wear and discoloration

    • Disadvantage: Technique sensitive, Moisture sensitive, Weak until bonded and can fracture during try-in, Expensive, Unable to adjust.

  • Glass Ceramics

    • Castable (Dicor)

      • Uses process similar to lost wax technique for processing

      • Suitable for single units

      • Advantages: Far superior fit (allowing for less resin luting cement, decreasing the possibility for ditching), Less wear to opposing dentition, Similar thermal cycling properties to enamel, Greater mechanical properties.

      • Disadvantages: Adjustment or wear results in loss of surface stain.

    • Hot-Pressed Leucite-Reinforced

      • IPS Empress

        • All-ceramic restoration using pressing of pre-cerammed ceramics under high temperature (using hydrostatic pressure) within a vacuum. Layered with leucite for aesthetics.

        • Advantages: Dimensionally accurate (lost wax procedure), Even greater mechanical properties (dense micro-crystals of leucites), Very esthetic

      • IPS Empress II

        • Uses lithium disilicate glass ceramic rather than leucite (greater mechanical properties compared with leucite) & layered with a flouroapatite glass ceramic (apatite relates to natural tooth structure allowing for a more natural appearance)

    • CAD-CAM

      • CAD-CAM uses a pre-manufactured single unit of ceramic that is milled by a machine.

      • Can be:

        • ‘In-house’ units. Information from intra-oral camera is sent directly to milling machine within practice.

        • Lab made units. Information sent to lab to fabricate coping +/- separate porcelain veneer.

      • Advantages: No need for lab (if milled in-house), greater dimensional accuracy (no impressions), Materials less porous and even greater mechanical properties, Increased fracture resistance (single homogeneous block material), Improved aesthetics.

      • Disadvantages: Expense

Indirect Resin Composites

There are three main choices of Indirect resin composites: Hybrid composites, Microfilled resins, Ceramic optimised resins (Ceromer).

Advantages: High esthetics, Easier to adjust or repair, Less abrasive to opposing dentition, Good marginal integrity (polymerisation shrinkage occurs prior to cementation), Can be completed chairside (Indirect immediate) and most importantly they are Cheaper!!!

Disadvantages: Increased wear (restoration worn rather than opposing dentition), Less dimensionally stable, Poorer mechanical properties, Uncured cement if >2mm thickness.

Which should I use?

Rule of thumb: Unfortunately there isn’t one!

Each case has to be decided on its merits. But I have listed a few of the key factors to consider.

Inlay or Onlay?

  • Size of Cavity (If the isthmus is <1/4 width tooth do a direct restoration, if it is between 1/4 -1/3 choose a direct or inlay, if it is >1/3 do an onlay- as cuspal coverage should be required)

  • Weakened cusp (By checking occlusal contacts you can identify if excessive force is being transferred through a cusp – if so an onlay is indicated. See Fig 1)

Figure 1. Wedging stresses due to inadequate design considerations (Fisher et al. Photoelastic analysis of inlay and onlay preperations J Proshetic Dentistry,1975 33:47)

How do I do it?

There are specific guidelines depending on the material being used.

General rules

  • Box shaped cavities

  • Slightly divergent walls to allow for path of insertion (undercuts may require blocking out with adhesive restorative material)

  • Avoid occlusal contacts on margins (lead to deterioration and leakage)

  • Floor cavity should be flat or concave, following occlusal form

  • Rounded line angles (Ceramic & Composite)

  • Rounded shoulder or Butt join margin (Ceramic & Composite)

  • Onlay: 1.5-2mm occlusal reduction of cusps

Some pictures of preparations:http://www.dentalorg.com/wp-content/uploads/2010/09/inlay-and-onlay-pic7.jpg


Figure 2 – Inlay and Onlay preparations


Figure 3 – Onlay preparation

Figure 4 – Inlay preparation (http://www.kaylordental.com/ips-empress.htm)


al-Wahadni A. History, development and clinical success of porcelain inlays. J Dent Assoc. 2000; 46(2): 49-54

Christensen OJ. A look at the state-of-the-art tooth-coloured inlays and onlays. Am Dent Assoc 1992; 123:66-70.

Dejak B. Strength estimation of different designs of ceramic inlays and onlays in molars based on the Tsai-Wu failure criterion The Journal of Prosthetic Dentistry 2007;98(2):89–100

Dickerson W. Indirect resin restorations: All the benefits without the disadvantages. Dent Today 1991; 10:32-36.

Fisher et al. Photoelastic analysis of inlay and onlay preparations J Prosthetic Dentistry, 1975 33:47

Inlay & Onlay preparations: http://www.kaylordental.com/ips-empress.htm

Jackson RD. Esthetic inlays and onlays. Current Opinion in Cosmetic Dentistry. 2nd Edition, Current Science. 1994; 30-39.

M Abdul Razak. The Tooth-Coloured Inlay/Onlay Restorations Annals Dent Univ Malaya 1998 5: 7-23

Ritter AV, Nunes MF. Longevity of ceramic inlays/onlays: Part II. J Esth Restor Dent.2003;15(1):60-3

Ruyter E. Types of resin-based inlay materials and their properties. Int Dent J 1992; 42: 139-144.

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