Download What is a crown or onlay

INFORMATION ABOUT CROWN AND ONLAYS
What is a crown or onlay?
A crown or onlay is a man-made covering for a tooth which is usually fabricated in a
dental laboratory. It can be made of gold or some other metal; or one of several types of
porcelain (which is a type of glass); or a special metal called zirconium (similar to cubic
zirconium used to made jewelry); or a combination of metal and porcelain or zirconium
and porcelain. A crown covers the entire tooth, or a substantial portion of the tooth,
usually at least 3 full sides of the tooth. Onlays cover only those parts of the tooth which
are damaged, making them a more conservative, tooth-sparing alternative to crowns.
Whether to use a crown or onlay is a decision made by the dentist, in concert with the
patient. My personal preference is to use onlays wherever possible and unless a crown
offers significant advantages. By virtue of their being less invasive, onlays are less likely
to irritate the nerve or cause damage (reversible or irreversible) to the nerve. They are
approximately the same cost in this office, so cost does not usually enter into the decision
making process.
Avoiding confusions with the word “crown”
In dentistry, the word “crown” refers to two different things. It can refer to an artificial
crown made by a dentist, which is what we are talking about here. But it can also refer to
the anatomical part of a tooth that sticks up out of the gums and that you see when you
smile. This is called an “anatomical crown”. But often, the shortened version “crown” is
used. Which crown is being referred to is determined by the context. Just to make
things a little more confusing, there is a similar term, “clinical crown”. To keep things as
simple as possible, we won’t get in to that here.
Why is a crown or onlay necessary?
When a tooth is too badly damaged from decay (cavities); fractures; trauma; previous
restorations; wear; grinding habits; or any other reason, it is necessary to place a crown or
onlay on the tooth to protect it and keep the tooth from being further damaged. A crown
can also be used to attach to and cover an implant. Crowns and onlays help to restore the
form, function, and esthetics of the tooth.
How are crown or onlays made?
The process used to fabricate a crown or onlay depends on the material it’s made of.
Crowns or onlays are generally made of gold or some other metal, or porcelain (which is
a type of glass), or a combination of the two. Gold crowns are made from casting molten
gold into a form made by the laboratory technician from a wax pattern made using a
model of the patient’s teeth sent from the dental office. Porcelain-and-gold crowns are
made similar to all-gold crowns or onlays, except a layer of porcelain is baked onto the
gold to give it a life-like color. All-porcelain crowns or onlays are made either by
heating a powder and liquid until it liquefies and becomes a glass (picture making
something out of beach sand and then turning it to glass); pressing softened molten
porcelain glass into a mold of the tooth at very high temperatures and pressure, or
digitally manufacturing the crown or onlay using rapid a prototyping technology called
CAD-CAM. In this process the crown or onlay is shaped mechanically from a solid
block of the material.
The dentist’s skill and judgment in preparing the tooth for the crown or onlay, and the
skill and experience of the dental technician will help to determine the final outcome and
hence the longevity of the crown or onlay.
What are Post-and-cores and build-ups?
If so much tooth structure is lost that not enough tooth structure is left to hold on the
crown or onlay, a sub-structure of artificial tooth may need to be done first before the
crown or onlay is placed. This is done in one of two ways:
1. If the tooth has been root-canal -treated, a post-and-core may be necessary.
2. If the tooth is still vital (still has a live nerve) a build-up may be necessary.
Whichever is done, enough ferrule (see “ferrule” below”) needs to be present, and if it
isn’t, crown lengthening (see “crown lengthening” below) needs to be done or
alternatives other than restoring the tooth explored.
The dentist’s judgment, experience, and skill will play a major role in whether these
procedures will be successful.
What is a ferrule and what is periodontal and orthodontic crown lengthening?
For a crown or onlay to stay on there must be a minimum amount of tooth structure
visible above the gum line. Authorities disagree on exactly how much this should be, but
everyone does agree that it’s between 2mm and 4mm. Less than this and you can usually
count on the crown or onlay coming off or breaking off. Ferrule is the term used to refer
to the amount of real tooth structure (as opposed to man made materials comprising a
post and core or build-up described above) that holds on the crown or onlay. I prefer 34mm of ferrule. In most cases there is adequate ferrule and this is not a concern.
However, if the tooth is badly damaged, or has broken off near the gums, there may not
be enough tooth above the gum line (remember we need at least 3 or 4mm) to hold on a
crown or onlay. There are two ways to get more ferrule:
1. Surgical crown lengthening
Imagine that your tooth is like a post in the ground. What holds the post up is the
part buried in the earth. What holds your tooth up is the part that’s buried in the bone
below the gums (the roots). If we wanted more of the post to stick up out of the
ground, we could remove some of the earth from around the bottom of the post. With
surgical crown lengthening, we remove some of the bone around the bottom of the
tooth, i.e., around the roots, to make it appear that more tooth is sticking up out of the
gums and bone.
2. Orthodontic crown lengthening
Again, using our wooden post analogy, we could also pull more of the post out of the
earth to make it look longer. With orthodontic crown lengthening, we pull the tooth
with orthodontics (braces) out of its socket until enough is sticking up above the gum
line to provide enough natural tooth structure (ferrule) to hold on the crown or onlay.
Orthodontic crown lengthening may sound painful, but it isn’t at all. In most cases
some surgical crown lengthening and recontouring of the gums will also be
necessary. Since the bone and gums follow the tooth as it is pulled up, there may end
up being too much bone around the root, and some of the excess may need to be
trimmed away.
Crown lengthening, either surgical or orthodontic, is done for other reasons too, such as
to gain access to a cavity or defect that is below the gum line so that it can be repaired.
The dentist’s experience and judgment, and the skill with which these procedures are
done, will contribute to the success of the final crown or onlay.
What takes place during a crown or onlay appointment?
1. The tooth is reexamined and pre-operative photos may be taken.
2. Any additional necessary radiographs are taken.
3. So that the injection of local anesthetic will be comfortable, a topical anesthetic
gel is placed on the tissues to anesthetize the surface
4. The tooth is anesthetized with a local anesthetic (usually painless).
5. A rubber dam is placed. This is a piece of latex rubber or non-latex rubber that is
placed around the tooth to keep it dry and free of saliva.
6. Any old crown or onlays or fillings and any other man-made materials are
removed. This may not be standard treatment in all offices but is in mine. This is
because I have found that about 60-80% of old fillings have decay under them,
even if it does not show up on the x-ray.
7. All decay is removed.
8. In order to help the crown stay on the tooth better, a post and core or build-up
may be placed. Whether this is necessary is sometimes known before starting;
other times it is not known until the time of the tooth preparation where the tooth
is cleaned free of decay and man made materials.
9. The tooth is shaped to receive the crown or onlay. This shape helps determine
how well the crown or onlay will stay on, and the fit of the crown or onlay. So it
is a vitally important step.
10. The tooth is pre-bonded if the final restoration is to be bonded on, which seals the
nerve endings on the tooth.
11. A thin thread is placed around the tooth slightly under the gum tissue, to keep the
gum tissue out of the way.
12. An impression is taken.
13. A bite registration is taken so the laboratory technician knows exactly how the
teeth fit together.
14. A temporary crown or onlay is made and cemented on.
15. A shade of the adjacent teeth is taken to match the color, and photographs of the
tooth, adjacent teeth, and shade selection are taken to send to the laboratory
technician, so he has a precise picture of tooth. Teeth are actually multiple colors,
and there is an art to make it match and appear natural.
16. The final crown or onlay will be cemented on approximately 3 weeks later.
After the appointment:
17. The impression is sterilized and a model is made from the impression.
18. The tooth to be crowned (die) is marked by the dentist where the crown will start
an stop under a microscope.
19. The models, bite, die, photos and drawings are sent to the laboratory technician.
How well each of these 19 steps is done will determine the comfort of the procedure, the
final quality of the crown or onlay, and play a vital role in determining how long the
crown or onlay will last before it needs to be replaced.
Frequently asked Questions and Answers about crown or onlays and onlays and
onlays
1. Q. If I need a crown or onlay, does that mean I’ll also need a root canal
treatment?
A. Not usually. It depends on how damaged the nerve has been from all the
previous restorations and how deep and how big the decay was.
2. Q. Are crowns or onlays expensive?
A. Crown or onlays are more expensive than fillings, but over the long run may
be less, as they usually, with good home care, will last much longer than fillings.
3. Q. How many visits are required?
A. Ordinarily, two visits are required.
4. Q. Are the visits long?
A. Because multiple procedures are necessary to get the tooth ready for the
crown or onlay, and provide the necessary models and information for the
laboratory technician, a crown or onlay appointment will take longer than a filling
or other simpler appointments.
5. Q. What holds a crown or onlay on?
Because the walls of the prepared tooth are nearly vertical (without taper) the
crown or onlay stays on and is not easily dislodged. The crown or onlay is
cemented on with special cement to fill the minute space between the crown or
onlay and the tooth, and to help hold it on.
Bonded crowns or onlays are held on by a special process and special cement
which enhances the bond of the restoration to the tooth to the extent that it is
difficult or impossible to dislodge without the tooth itself breaking.
The skills of the dentist and laboratory technician are vital parts of how well the
crown or onlays will stay on and how long it will last.
6. Q. What determines the cost of the crown or onlay?
A. The cost of a crown or onlay is determined by the care, skill, judgment,
experience, time and training of the dentist; and by the cost of the laboratory
technician’s services, which in turn is determined by his care, skill, judgment,
experience, time and training, and by the cost of materials and precious metals
(primarily gold, platinum, and palladium).