Advantages of porcelain fused to metal crowns
- The underlying metal fused with the porcelain provides stability and strength to the crown.
- It has great aesthetic appeal and is most commonly preferred as it gives the restored tooth a natural look.
- They have a very high rate of success in the long run.
- The cosmetic appearance of these crowns is commendable but they are less appealing than other types.
- While the appearance of a PFM crown can be excellent, it's often difficult to create one that's as natural-looking as the best all-ceramic ones. For the most part, this difficulty stems from the crown's metal substructure and how it must be masked by covering it with relatively opaque (less translucent and therefore less natural-looking) porcelain. And while this doesn't create a problem in all cases, it frequently involves challenges or compromises.
- They make the gums more vulnerable to gingival inflammation as compared to porcelain fused to gold and porcelain crowns.
- Restoring tooth with these crowns requires a large part of the tooth structure to be removed.
- Porcelain from the crown can easily break or chip off. If porcelain breakage does occur, it's very difficult to make a lasting repair. The most predictable solution typically involves making a new crown. As a compromise, some minor chipping may just be smoothed over or polished.
- PFM crowns may wear opposing teeth.
- The porcelain surface of a porcelain-fused-to-metal crown can create (possibly significant) wear on those teeth that it bites on or rubs against. (Many types of all-metal or all-ceramic crowns are more bio-compatible in this regard.) This issue might be especially important for people who brux (clench and grind) their teeth.
- This potential is greatest in cases where during placement the crown's biting surface needed to be trimmed and it was not subsequently re-glazed (given a glass-like finish in a high-heat oven), or at least thoroughly smoothed and polished.
- Increased cost becomes a major drawback when precious metals are fused.
- The "dark line" phenomenon. The metal that lies underneath the crown's porcelain surface can sometimes be seen as a dark line right at the crown's edge. A dentist will usually try to position the edge of a PFM crown just underneath the tooth's gum line; but, if a person's gums happen to recede, this dark line can show, thus spoiling the crown's appearance.
Pressed-to-metal crowns (PTM's)
Newly developed fabrication methods along with the use of modern high-strength ceramics has resulted in the development of a new type of porcelain-metal crown. This type of restoration is referred to as the pressed-to-metal (PTM), pressed-on-metal or pressed-over-metal (POM) crown.
Similarities to traditional porcelain-fused-to-metal restorations
In terms of construction, PTM/POM's are essentially the same as PFM's (a ceramic outer layer encasing an underlying shell of metal). But instead of traditional porcelain, an engineered (synthetic) high-strength dental ceramic is used instead (lithium disilicate is common).
Advantages of pressed-to-metal crowns
PTM's seem to offer a solution for most of the disadvantages associated with PFM's.
- The ceramic used has greater strength, chip resistance and a monolithic (single piece) construction, all of which makes the crown's outer covering less likely to fracture.
- Some feel the optical properties of the ceramic used gives a superior esthetic result as compared to traditional porcelain.
- The physical characteristics of the ceramic may cause less tooth wear of opposing teeth.
- Due to the ceramic's great strength, the front side of a PTM crown can be given a fully ceramic edge, thus side stepping the "black line" difficulty associated with PFM's (see picture above).
It's important to point out that despite these apparent advantages, pressed-to-metal (pressed-over-metal) crowns are a much newer type of restoration than PFM's and therefore do not share their long, proven track record of durability and success. You'll need to consult with your dentist about the prudence of