Dental Bonding

- Applications for back teeth. / Advantages & disadvantages over amalgam. / Longevity comparisons.

Dental composite (the white plastic compound utilized with tooth-bonding technique) is routinely used to make fillings for back teeth. In fact, it's likely that slightly over half of all posterior restorations placed are composite ones.

But due to factors related to its physical properties and the technique needed to place it, it doesn't always make a better choice than an old-fashioned "silver" filling (dental amalgam).

This page outlines the types of issues your dentist must take into consideration when deciding which of the two makes the more appropriate choice for your tooth.

It also discusses the general advantages and disadvantages of one over the other. As well as comparisons regarding how long each can last.

Composite vs. Silver (amalgam) fillings -

Application considerations for back teeth.

Large composite and amalgam fillings.

Both composite and amalgam can make a reasonable choice for large fillings, like those that involve replacing a tooth cusp.

A) Filling size.

The general consensus used to be that dental composite didn't have the combined strength and wear characteristics of dental amalgam and therefore wasn't the best choice for comparatively larger fillings for back teeth.

Today, with improvements in the composition of bonding materials and the techniques used to place them; this possibly isn't a major consideration any more.

Some recent studies have reported that they have found no significant differences between the two, even when they were used to create large fillings. (See Longevity section below).

B) Exposure to excessive forces.

One of the main causes of bonded filling failure is fracture. That means dental composite doesn't make a good choice for fillings that will routinely be exposed to excessive forces. For example, fillings for people who clench and grind their teeth. (See Longevity section below).

C) High decay rates.

Dentists use the term "secondary" decay to refer to cavities that have formed adjacent to existing fillings.

This type of cavity is more likely to form in situations where micro gaps exist between a filling and its tooth. (The gaps accumulate dental plaque, which in turn causes the cavity.)

Amalgam vs. composite.

1) By nature, dental amalgam fillings are self-sealing. Changes take place with the metal itself that fill in any micro spaces that exists between it and the tooth.

2) Bonded restorations shrink slightly when they are cured. And for this reason it's always possible that some micro gaps exist.

For this reason, for people who already have a high decay rate, a dental amalgam filling may make the better choice.

D) How visible is the filling?

Bonded fillings are white.

Dental composite comes in a range of tooth-colored shades, whereas dental amalgam is silver in color.

There is no question that patient's strongly prefer white fillings. And in situations where either type could suffice, this is often the deciding factor.

Tooth stain caused by a dental amalgam filling.

This amalgam filling both shows prominently and has stained its tooth.

Other color issues.

Besides being directly visible, there are other ways a silver filling might cause a cosmetic problem.

Its tint may be visible through the tooth structure that surrounds it. And this stain may become more pronounced over time as the restoration ages.

With many back teeth, this may not be much of a concern. But it can be for upper and lower premolars (bicuspids) and even some upper molars.

E) Does the patient want to avoid the use of amalgam?

One of the components of dental amalgam is mercury. And over the past few decades there has been some debate (more so in the popular press than in bona fide dental research) as to its ill effects on a person's health.

In defense of amalgam, we would like to bring to your attention that it has been in use in dentistry for more than 150 years. And during this time period literally billions of amalgam fillings have been placed.

One would expect that if a significant problem did exist that by now there would be a substantial amount of research documenting it. To the contrary, however, there is not.

Several national and international health organizations (U.S. Public Health Service, The American Dental Association, Food and Drug Administration, World Health Organization, FDI World Dental Federation) have gone on record as stating that they have reviewed the scientific literature and see no health concerns.

Despite this, there are people who have decided that they do not want to have their teeth restored with dental amalgam. If so, then a composite filling may be their only cost-effective choice.

F) Metal allergies.

While rare, people can have an allergic reaction to the metals contained in dental amalgam. So, in these cases, a dental composite restoration may be the only reasonable choice. (In dental literature, there are fewer than 100 documented cases of localized allergic reactions associated with the use of dental amalgam.)

Comparative advantages and disadvantages of both materials.

1) Advantages of bonded fillings for back teeth.

A) Less thermal sensitivity.

Since dental amalgam is a metal, it's a good thermal conductor. And when it's used to make fillings (especially relatively large ones) it's not uncommon that a person will experience some degree of thermal sensitivity with their tooth when it's exposed to hot and cold foods and beverages.

In most cases, this sensitivity will subside as days and weeks pass. But for some people it will persist and be a nuisance, especially when very cold items (like ice cream) are consumed.

Since dental composite is a plastic, and therefore a comparatively poor thermal conductor, this issue is much less of a concern.

B) `Immediate use.

Once placed, a dental amalgam filling requires several hours before it has substantially hardened and is ready to be put into service.

In comparison, a bonded filling is fully cured during its placement process. That means once the procedure has been completed the tooth and its new filling are ready for use.

C) Repair may be possible.

If a defect is found in a dental composite restoration, it may be possible for the dentist to just make a repair rather than replace the whole thing. That's because the newly added composite will create a bond with both the tooth and the existing filling.

Dental amalgam is a different story. A new restoration will not fuse with an existing one. That means it's always better to take an old filling out and replace it with an entire new one.

Because each time a filling is replaced the hole in the tooth becomes larger, the possibility of repair is an attractive feature of bonded fillings.

D) Composite fillings are white.

Beyond the issue where a silver filling might be easily seen, some people feel that color is an important issue even for teeth all of the way in the back of their mouth. If so, dental composite offers this advantage.

Composite fillings can often be smaller than amalgam ones.

The overall size of a composite filling is often smaller than an equivalent amalgam one.

E) Bonded filling tend to be smaller.

The adhesive nature of bonded fillings allows a dentist to perform what is termed "minimally invasive" dentistry.

1) Cavity preparation for amalgam.

With metal restorations, an important factor in the long-term success of the filling is the shape of the hole in which it's placed. It must be designed properly, otherwise the restoration will likely fail prematurely.

Meeting these design needs usually means that the extent of the hole will need to extend beyond just that immediate area where the tooth's problem (cavity, etc...) lies.

2) Cavity preparation for dental composite.

With bonded fillings, the rules regarding shape are more lax. Certain design considerations still need to be followed but, to a great degree, the restoration can just be placed in that local area where the tooth's problem lies. That means, overall, a white filling can often be smaller, possibly much so.

F) Tooth reinforcement.

With a silver filling, it's the shape of the hole it fills that keeps it in place. In comparison, with dental composite the filling itself actually creates a bond with the tooth. Knowing this difference, one might conclude that placing a bonded restoration actually helps to strengthen a tooth.

There seems to be some evidence of this (Soares 2008, Herrmann 2010) [References for this page.], although creating this strengthening effect may be relatively technique sensitive. And while this effect may be a reason to choose a composite restoration over an amalgam one, if a substantial strengthening effect is needed a dental crown would probably make the better choice.

2) Disadvantages of white fillings vs. dental amalgam.

A) Amalgam is less expensive.

The cost of a silver filling is usually about 25 to 30% less than that for a comparable dental composite one. This difference reflects the relative cost of the materials involved and the relatively greater amount of time that it takes the dentist to place a white filling.

If you have dental insurance, you should be sure to ask your dentist's office staff about the specific details of your coverage. Some plans will only cover the cost of bonded fillings up to that amount an equivalent amalgam one would cost. This means that you, the dental patient, would pay the difference.

B) Longevity.

There have been a number of studies that have evaluated the relative longevity of different types of dental fillings placed in back teeth.

1) Studies that favor amalgam.

1) Van Nieuwenhuysen (2003) evaluated more than 100 dental composite and 700 dental amalgam restorations. It found an average life span of 12.8 years for the amalgams and 7.8 years for the composites.

2) Another study (Forss, 2001) calculated an average life span of 12 years for amalgam fillings and about 5 years for composite ones.

3) An evaluation of a dental insurance claims database (Bogacki, 2002) concluded that composite restorations in back teeth do not last as long as their metal counterparts.

2) More recent studies.

The findings of more recent studies, including those that have evaluated larger-sized fillings, are more varied.

1) Some report greater longevity for dental amalgam restorations (Bernardo 2007, Antony 2008).

2) Others have found the longevity of fillings formed from either material to be essentially the same (Heintze 2012, Kim 2013, Rho 2013, Opdam 2010).

The emphasis of more modern studies seems to be on correlating the lifespan of composite restorations to factors such as proper patient selection (see below), the specific protocol used (which continues to evolve), the materials used (which are constantly improving) and clinician skills.

If protocol and materials issues are key factors, one could reasonably conclude that over time the longevity of composite restorations will only continue to improve.

One final point about this comparison.

It should be noted that in regard to longevity, the issue at hand isn't simply if dental composite makes a suitable filling material for back teeth. Instead, it's if it's suitable for use for restorations that must function under load (significant biting and chewing pressure), like many placed in back teeth must.

In regard to this matter, the Rho (2013) study mentioned above found that:

  • The longevity of dental amalgam was superior to composite when measured from the point of initial placement. (8.7 years vs. 5.0 years).
  • But in terms of working restorations (fillings that had already survived in the oral environment past an initial period), the survival rate for both was the same.

Those findings suggest that:

  • Any new composite filling should have periodic follow-up evaluation so repair, if needed, can be made in a timely fashion.
  • Statistically speaking, your tooth should require less overall attention if amalgam is chosen initially.
  • But if having a white restoration in a back tooth is important to you, dental composite can make a reasonable choice.

C) Bonded restorations take longer to place.

The process of placing a composite filling is much more technique intense than placing a silver one. And because of this they take longer to place.

The amount of time difference involved will depend on several factors but as a ballpark estimate it might take your dentist only about half as long to place a dental amalgam restoration.

D) Bonding is more technique sensitive.

As compared to dental amalgam, the process of placing composite fillings is very technique sensitive. For example, there are steps that can only be completed while the tooth is kept saliva-free.

There can be places on a tooth (like at and below the gum line) or locations in the mouth where meeting this condition is essentially impossible. In these cases, dental amalgam can often be placed to create a serviceable filling whereas dental bonding cannot.

E) Post-operative sensitivity.

People who have bonded fillings placed sometimes experience post-operative tooth sensitivity. While the exact cause is often impossible to determine, it's often attributed to clinician error (in appropriate selection of materials, failure to comply strictly with placement protocol).

This issue seems less of a factor with more modern techniques and materials. The sensitivity usually disappears over time.

F) White fillings will tend to discolor.

It's the nature of dental composite to stain over time. Of course, it's unlikely that one would ever become as dark as a silver filling.


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