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What are Onlays?

Core buildup with pins used to rebuild a badly damaged or decayed molar tooth prior to placing a crown.

Teeth which have sustained heavy damage to the visible part of a tooth (natural clinical crown) can often be repaired with filling material and placement of structural pins. The combination of modern dental adhesives and pin retention can produce a durable restoration; however ideal tooth shape and improved strength, and superior fracture resistance may be achieved by placing a crown or onlay on the tooth.

An onlay is a type of cusp-covering dental restoration that is made in a laboratory by a skilled technician, or by a computer controlled milling machine. Onlays can be made of ceramic, composite resin, gold, titanium or other metals. Metallic onlays are generally cemented (glued) onto the tooth, at a separate appointment from the tooth preparation appointment.

Ceramic and composite resin onlays are generally bonded (fused with adhesive) onto the tooth, either at the preparation appointment (if CAD/CAM is available for ceramic onlay production) or at a second appointment (more commonly the case). If you need two appointments, it will often mean having to numb the tooth at both appointments, and having a temporary restoration placed between appointments.

Onlays are like crowns, in that they afford protection against cracking to the tooth; but they’re more conservative in the amount of natural tooth structure that needs to be removed to make room for them. They’re also similar to inlays, except that they cover at least one of the pointed chewing cusps of the tooth.

The process of creating and placing Onlays

Before the procedure is started

When you have restorative dental procedures like onlays done, the dentist will review your health history. If you have replacement joints (for example, total knee replacement, hip replacement, etc.), you may be pre-medicated with antibiotics for the procedure. If you have certain types of heart murmurs or replacement heart valves, you may also need to take an antibiotic pre-medication prior to the procedure.

If you are anxious about dental procedures, your dentist may recommend sedating you for the procedure. There are several methods of relaxing patients for dental treatment, including oral anti-anxiety pills like ValiumĀ®; inhaled anti-anxiety medication like nitrous oxide; and intravenous anti-anxiety medication, such as VersedĀ®. Your dental plan may not pay benefits toward sedation.

The following describes the typical onlay/partial crown preparation and laboratory process in detail. Your procedure may vary a bit from the procedure described.


The tooth to be restored is usually numbed by injecting local anesthetic around the nerve(s) that supply sensation to the tooth. Discomfort from the injection can be minimized by applying a topical numbing gel for a minute or two prior to the injection.


Frequently, a preliminary impression (mold) is made of the teeth before they are altered. The material used most for onlay/partial crown impressions is polyvinyl siloxane, a dimensionally stable and extremely accurate elastomer (meaning it’s stretchy, but returns to the shape it takes when it cures after a minute or two). Other materials may be used. The impression can be used to make a temporary onlay for the tooth if the final restoration is being made in a laboratory (a process that can take a couple of weeks).

Shade Matching

If the tooth is to be restored with a tooth-colored onlay/partial crown, a shade matching guide will be used to determine the shade of your natural teeth. The shade is generally matched in natural (full spectrum) lighting. Fluorescent lights can make teeth appear blue to grey; incandescent lights can make them appear more yellow. Dental porcelains and resins available today can produce a stunningly precise match for the shade and optical properties of your natural tooth enamel, often allowing a single restoration to be made that matches your teeth nearly imperceptibly.


The tooth is isolated from mouth structures like the tongue and cheeks to prevent injuries from instrumentation used to prepare the tooth. An isolation barrier known as a rubber dam or dental dam is frequently used, but there are other retraction devices in use. Some dentists may simply use cotton rolls and cheek shields.

Core Preparation (core buildup)

Molar tooth crown preparation showing high speed handpiece, cooling spray, metal core material and crown margin.

The preparation margin is a ledge that is shaped into the tooth, against which the crown (or onlay) will seal.

The tooth is prepared by removing old restorative materials (if necessary), removing any decay, and (if necessary) filling in any deep holes or missing corners of the tooth. It may be necessary to place small metal (normally titanium) pins in the tooth to rebuild the portion of the tooth that will be covered with the onlay/partial crown. The dentist may use any of a variety of filling materials to rebuild the tooth, including composite resin, glass ionomer, and silver amalgam.

Core buildups are considered a separate, billable procedure from onlays/crowns. A distinction is made between buildups that involve all or a substantial portion of the core of the tooth, and those that require only a small amount of filling material in the core to build out a minor chip. It’s a good idea to discuss with your dentist in advance whether a complete core build-up is going to be required or not to avoid unexpected costs.

Onlay/Partial Crown Preparation

Preparation of a damaged tooth for a crown or onlay is done with a tapered diamond bur using large amounts of cooling spray.

Preparation of a badly damaged or decayed tooth for a crown or onlay is done with a tapered diamond bur (shaping point), which spins in a high speed dental handpiece using large amounts of cooling spray to avoid overheating the tooth.

The outer surface of the tooth is reduced in all dimensions (biting surface and sides) by 0.75mm to 3mm to make room for the material that will be placed on the tooth. The walls of the preparation are tapered to allow the onlay/partial crown to be slipped down over the tooth. Sometimes internal walls will be prepared in the tooth to provide inlay retention. A ledge (margin) is created around the circumference of the preparation against which the crown/onlay will be tightly sealed.

Impression of Prep

An impression of the prepared tooth and the teeth that bite against it is made, taking care to gently reflect the gum tissue away from the prepared tooth. Often, the dentist will place “retraction cord” in the trough between the gums and prepared tooth prior to making the impression. This clearly exposes the preparation margins. The cord will be removed after the impression is made.

Temporary Onlay/Partial Crown

If the final onlay or crown will be made in a laboratory, the impression that was made prior to preparing the tooth can now be used to make a temporary onlay/partial crown. This is done by filling the pre-impression with a gooey tooth-colored resin material, having the consistency of thick syrup, and placing it over the prepared tooth. The resin material will gel in about a minute, and the impression can be removed from the mouth. The material will completely harden in another minute or two. Once hardened, the temporary restoration will be trimmed to proper fit, polished, and cemented onto the tooth with temporary cement.

Labwork and Final Onlay/Partial Crown Seat

  • Cast Fabrication: the impression of the prepared tooth and the teeth that oppose it is poured with dental stone and allowed to harden into a cast of the teeth.
  • Articulation: the casts of the teeth are assembled into a hinged jaw simulation device known as an articulator in their proper bite relationship.
  • Die preparation and fabrication of a wax pattern: The cast of the prepared tooth (working die) is inspected closely for undercuts and any other irregularities. The technician will create a crown from wax using sculpture techniques. This wax pattern will be used in any of a variety of ways to produce the final restoration, depending on whether the onlay/partial crown is to be made of ceramic, metal, or a combination of those.
  • Production of the actual onlay/partial crown: Depending on which type of onlay/partial crown is to be fabricated, the restoration may be cast from a variety of metal alloys, pressed from ceramic, or made of ceramic fused to the metal alloy.
  • Finishing: Depending on which type of restoration is made, it may require metal finishing and polishing or (if ceramic) other staining and glazing procedures to make the tooth match the patient’s natural teeth as closely as possible.

Delivery of Final Onlay/Partial Crown

When your final onlay/partial crown is ready, a second visit is necessary to remove the temporary restoration and replace it with the permanent one. About half of the time, a patient will ask to be numb for the second visit, to avoid any discomfort associated with removing the temporary onlay and cleaning the temporary cement from the prepared tooth.

Teeth which have been endodontically treated (i.e. root canal) generally do not need to be anesthetized for the delivery of a permanent onlay/partial crown, although many dentists prefer the patient to be numb for the tooth preparation due to the potential for discomfort associated with soft tissue management (gum retraction, etc.).

Adjust and Polish

The functional biting relationship (occlusion) of the onlay/partial crown may need to be adjusted slightly, and the restoration repolished. This should take just a minute or two under normal circumstances.

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Author: Thomas J. Greany, D.D.S. / Editor: Ken Lambrecht

This page was last updated on December 17, 2018.

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