What are Onlays?
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 Valium®; inhaled anti-anxiety medication like nitrous oxide; and intravenous anti-anxiety medication, such as Versed®. 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).
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)
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
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.
Some advantages and benefits of Onlays
- Since they are made outside of the mouth, it is easier to rebuild the ideal contours of the natural tooth than it is with large fillings.
- Onlays and crowns can prevent cracks from spreading through a tooth.
- Porcelain onlays and crowns can restore the tooth to its natural contour, function, and appearance.
- Metal onlays and crowns will not generally chip, and provide the tooth with a surface that is durable.
- Onlays have the advantage over crowns of leaving more natural tooth structure intact.
Potential disadvantages and risks of Onlays
- Compared to fillings, onlays and crowns are relatively expensive. However, they generally afford better protection against tooth fractures, and can make it more predictable to achieve contact with adjacent teeth (versus leaving a gap). It may also be easier to sculpt an ideal tooth shape working outside the mouth.
- Traditionally, onlays and crowns require two visits to complete. Often, this means being “numb” at both appointments. Computer-milled inlays, onlays and crowns, which are delivered the same day the tooth is prepared, eliminate the need for a second appointment; however, these may not be appropriate for all teeth, and are still not widely available.
- Preparing a tooth for an onlay or crown involves reducing the sides of a tooth and its biting surface. Depending on how thick the remaining walls of the prepared natural stump are, the tooth itself can be weakened, reducing its long-term prognosis.
- Inflammation and/or infection from the preparation procedures can occur, and may be more likely with ceramic onlays and crowns, because it is necessary to remove more natural tooth structure than for metal ones. This is because porcelain must be thicker than metal to have comparable strength. Studies aiming to quantify the risk of tooth abscess following crown preparation have shown an abscess rate of about 10%.
- If the tooth being prepared is vital (“alive”), it will normally be numbed with local anesthetic. Local anesthetic has disadvantages of its own, which are generally considered minor compared to the advantages of having the procedure performed comfortably.
- Fillings, inlays, onlays and crowns can fall out and be lost, requiring replacement. Modern bonding adhesives and cements used in dentistry have reduced this risk significantly.
- Ceramic and composite resin inlays, onlays and crowns can chip, requiring repair or replacement. The risk is significantly greater in patients who grind their teeth (brux) or use them inappropriately (chewing ice, popcorn kernels, etc.).
- Leakage can occur along the interface between an inlay, onlay or crown over time, and the prepared tooth (margin), leading to sensitivity and decay. Poor oral hygiene is often a significant factor in such cases.
- All dental procedures can produce lip dryness, chapping and cracking. Some patients develop cold sores following dental treatment. Some degree of post-operative discomfort frequently accompanies tooth preparation, including tenderness to biting and cold which is normally temporary. Some studies have shown prolonged sensitivity over a year in certain patients, although such cases are relatively rare.
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