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:
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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*
Aesthetics
Amalgam
Composite
Gold
Direct Resin
Indirect Resin
Ceramic
Poor
Excellent.
May lack
translucency
Still being proved.
Similar hardness to
tooth structure.
Poor
Excellent
Excellent
Good/Excellent
Proven good
marginal fit.
Does not wear
opposing teeth.
Metallic.
Weakens tooth.
Two visits.
Tooth preparation
critical.
Finishing of margin
time-consuming.
Requires good
margins.
Lost wax technique
is time-consuming.
Material is
expensive.
Still being proved.
No stress on tooth.
Similar hardness to
tooth structure.
Short Chairside
time.
Two visits
Laboratory
performs all
Construction work.
Still being proved.
Polymerization
shrinkage can
stress tooth.
Long Chairside
time.
Meticulous
technique.
No laboratory cost.
Still being proved.
Similar hardness to
tooth structure.
One visit.
No stress on
tooth.
Requires good
margins.
No laboratory cost.
Needs finishing.
Lost wax technique
is time-consuming.
Expensive outlay.
Clinical
Performance
Proven.
Single visit.
Poor margins.
Clinical
Weakness
Metallic.
Mercury toxicity.
Large tooth
structure required.
Laboratory
Weakness
No laboratory cost.
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)
Requires good
margins.
One/two visits.
Generally wear
opposing teeth.
Materials
Gold
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.
Ceramics
 Conventional ceramics
o As mentioned earlier, conventional ceramics were problematic due to: Material
weakness, Poor marginal integrity and Lack of adequate cement.
o Advantages: Aesthetic, Decreased marginal leakage associated with polymerisation
shrinkage and Less wear and discoloration
o Disadvantage: Technique sensitive, Moisture sensitive, Weak until bonded and can
fracture during try-in, Expensive, Unable to adjust.
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Glass Ceramics
o 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.
o 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)
o
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?
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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
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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:
Figure 2 – Inlay and Onlay preparations
Figure 3 – Onlay preparation
Figure 4 – Inlay preparation (http://www.kaylordental.com/ips-empress.htm)
References
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.