How is a post crown placed?
What is a Post Crown?
A post crown is a combination of a post (that extends down into the root canal of your tooth) and a dental crown.
There is actually a third element called the ‘core’. So sometimes, you will hear it referred to as a post and core, and crown- as generally this is how the components are split up when the dentist makes up your bill.
There are always these three elements, it’s just that sometimes the core is incorporated into the post (custom made) and at other times, it is built up on top of the post, in filling material. This happens after the post has been cemented. The latter is much more common nowadays.
A post crown always requires a root canal to have been completed before it can be carried out. That’s because the post extends deep down into the root canal of the tooth in order to get extra grip for the core.
This is unlike a pin, which simply screws a couple of millimetres into the dentine of the tooth to help hold a filling.
When Would I Need a Post and Core?
A crown requires a solid core if it is to be successful and resist all the forces that will be placed on it for many years to come.
If sufficient good tooth remains, a filling (either composite or amalgam) is all that is needed to provide a suitable core. However, if very little tooth remains and the core essentially consists of just one huge filling, it’s going to be a lot weaker. The risk is that this will fracture and come off in the crown- a situation that dentists really hate to see. The reason is, it can’t just be stuck on and any decent bit of tooth to help hold the core in place, has broken away- now, we have to rely solely on a post placed in the root canal to retain it.
It makes a lot more sense to place the post initially- strengthening the whole structure and letting us make use of any good remaining tooth for the filling material to bond to.
Sometimes the tooth will have already had a root canal, in which case, we can go straight ahead and place a post in the most suitable canal and build up a core for the new crown. If it hasn’t, then we will need to do a root canal treatment first; the only other option is that we extract the tooth and think about a replacement for the missing tooth.
There are two main circumstances in which posts are used:
If you have had a root canal treatment and the remaining tooth is weak, a post will help to re-inforce this structure providing a more solid core for your crown. This is the most common use and avoids the situation that I described above; the crown breaking off with the core inside.
If a tooth has fractured off at the gum level, so there is nothing but a root stub remaining. By sticking a post into the root canal, we can get extra grip to help hold the core for a crown. This is commonly seen and successfully carried out with anterior (front) teeth as they don’t take the same pressures as your back teeth during chewing. A back tooth may require crown lengthening, in order to expose a bit more tooth for extra stability.
There are two different basic types of post:
Custom made. The lab make a post to match the impression of the inside of the canal . It will have a core included on top.
Pre-fabricated. These come in different shapes and sizes and are placed directly by the dentist into the canal. The two most common materials used, are a carbon- fibre post and a stainless steel post. They can be parallel-sided, tapered and smooth or serrated. Threaded posts which are screwed in are no-longer used, as they set up a lot of stress in the root and make it prone to fracture. The other systems are really a matter of dentist preference.
What is the Procedure for a Post and Core?
They all follow a similar procedure, in which special drills are used to prepare a space down the widest canal in the tooth, (if there is more than one!) The longer the post i.e. the further it goes down, the more retention there is for the filling on top, but it’s important to leave at least 5mm of root filling (Gutta Percha) at the bottom in order to keep the canal sealed.
The dentist will take an X-ray to see how far down the post is and to check the post is in-line with the canal. Once satisfied, the post will be stuck in place and the filling material built up around it- if it is of the prefabricated type (usually composite). When the dentist does this- it is called a direct post and core because they complete it directly there and then.
The dentist may decide that a custom- made post and core is best; it may just be their preference or because of a very wide or awkward shaped canal. This is an indirect method and involves taking an impression of the canal space using an impression post with impression material over the top and sending it off to the lab.
The lab will then construct your custom-post, meaning it is custom- made for your particular canal (either in wrought metal or cast metal). They will then return it to the dentist for sticking in. When made in this manner, there is no need to build up filling material on the top before placing the crown, as the post and the core come as one piece.
Sometimes the crown will also be made at that appointment and everything can be cemented in at the next visit, or maybe, the post will be cemented and a new impression taken- on which the crown will be made. The latter, has given a more consistent result to me personally, but it does mean an additional appointment will be needed to get the treatment completed.
Any tooth having a post, will need a crown on top to protect it.
What Problems can I get with Post Crowns?
The following are some of the problems you can get with teeth that have had post crowns:
Fracture. Done correctly posts can strengthen teeth considerably; done incorrectly they can actually increase the likelihood of fracture. Because of improved bonding techniques; where a good amount of strong tooth remains, posts are not routinely needed. Smaller, thinner roots, such as those of your upper lateral incisors are more at risk. If a fracture happens, most of the time the tooth will need to be extracted.
Loss of post. If the post for some reason is quite short or too tapered, it may simply weaken over time with the stresses placed on it and come out. Providing the tooth has fallen out because the cement had failed (and not because of a root fracture), then it can be just stuck back in- though if the post is short, it may come out again unless a new longer post is placed to try and stop this from happening.
Failed root canal. If the root canal fails for whatever reason- and you continue to get pain or persistent infection, repeating a root canal treatment with a post is very difficult. Sometimes they can be removed, but often it is impossible without risking the fracture of the tooth. Then, either a apicoectomy (if possible) or an extraction may be required.
Perforation. A potential complication of placing the post is perforation; meaning the post comes out of the side of the root. If this happens, it may still be possible to save the tooth and repair the perforation. Some roots can be quite curved and posts are very straight, so careful selection of appropriate canals is important to prevent this problem.