Do you need to get a new crown or an old one replaced? If so, you may have heard your dentist talking about getting a good margin line. In dental anatomy, the margin of the tooth is the junction where the tooth crown and tooth roots meet. When your dentist places a restoration, like a zirconia crown, the margin line is the perimeter of your shaped enamel after your dentist removes decay. Read on to learn why margin lines are so important and how your dentist gets good margins for your restorations.
What if the Crown Doesn't Seat Fully on the Margin Line?
In an ideal procedure, the crown should fit snugly against the margin line (called a closed margin). Sometimes the margin line is uneven and doesn't match the crown well, which means that it cannot fit snugly. This leaves an open margin where it's easy for bacteria to get underneath the crown and destroy your remaining enamel. Because you cannot brush underneath the crown, it's easy for decay to take hold. These types of cavities can only be spotted on X-rays or if the crown is removed. Besides increasing the risk of decay, poor margin closure makes it easier for the crown to wear down and break since chewing forces won't be distributed evenly. Fortunately, there are lots of things your dentist can do to prevent these problems.
How Does Your Dentist Get a Good Margin Line?
Here are just a few things your dentist can do to shape a good margin line and make sure that it fits correctly with the final crown.
1. Creates the Right Shape for the Material
There are actually different recommendations for margin contours and slopes depending on the adjoining crown material. For instance, if you are getting a metal-ceramic restoration, a chamfer margin can be a good fit (similar to carpentry, a chamfer is a right-angled edge). If you are getting an all-ceramic crown, the material is weaker and will need better support, so your dentist will use a dental drill to make a shoulder margin, which has a wider ledge for the crown to rest on. By shaping the margin to the appropriate material, the crown should seat better and last longer.
2. Uses Retraction Cords
A retraction cord looks similar to floss, but its main purpose is to push gum tissue out of the way so your dentist can not only better shape the margin line, but so that he or she can take a good dental impression. You could have a great margin line, but the dental laboratory wouldn't know unless you had a good impression taken.
3. Takes Detailed Impressions
While your dentist can take some impressions with alginate, this material tends to warp and shrink when it dries out. As you can imagine, a warped impression could hinder crown fabrication and distort the margin line. To mitigate this issue, a dentist can use a light-body impression material with low viscosity, like a vinyl polysiloxane. This impression material can capture tiny details of the margin line and it won't warp like other materials.
4. Uses Articulating Paper
Once your dentist has the final crown, they'll try to firmly seat it against the margin line. To check if the crown is fully seated, your dentist may have you bite on articulating paper. This paper is a diagnostic tool that places a temporary dye on your teeth to show where most of the biting forces are distributed. For instance, if one area is getting a lot of biting force, then there will be darker levels of dye in those areas. Your dentist can readjust the crown with a dental drill to remove uneven areas of force before reseating the crown.
If the crown isn't seating well despite these efforts, your dentist can retake an impression and send it to a laboratory so they can make a new crown. Reach out to a restorative dentistry like AQ Denture and Dental Implant Center for more information about fitting your crown.