Regulators eye link between denture cream and zinc toxicity

Regulators eye link between denture cream and zinc toxicity

Regulatory authorities and dental associations are examining reports that overuse of zinc-based denture adhesives may cause neurological damage. While a direct causal link has not yet been established, dentists are being advised to counsel denture-wearing patients to ensure they aren't using too much adhesive.

The issue has been percolating for several years, but received renewed attention this month following a segment that appeared May 5 on "Good Morning America." In addition, the U.S. FDA has received adverse event reports claiming that cases of zinc toxicity were caused by denture cream overuse, and the agency has confirmed that it has formed a working group to examine the issue.

At issue is the effect that excessive ingestion of zinc over a long period of time can have in depleting the amount of copper in the human body, as copper deficiency can lead to the impairment of various neurological processes. In fact, copper deficiency is a well-established and increasingly recognized cause of neurologic and hematologic disease, and excess zinc has been identified as a source of copper deficiency (Journal of Neurology, January 2004, Vol. 251:1, pp. 111-114).

This is where denture adhesives come in -- some, anyway. While not all denture creams contain zinc, two of the most popular are GlaxoSmithKline's (GSK) Super PoliGrip and Procter and Gamble's (P&G) Fixodent, both of which are zinc-based (GSK also offers a zinc-free version of PoliGrip). The zinc acts as a binding agent that increases the product's hold, according to Jennifer May, a spokesperson for over-the-counter products at GSK.

While zinc is an essential mineral naturally present in many protein-rich foods such as beef, chicken, and nuts, and many supplements and other over-the-counter products such as Zicam (a cold medicine), there are limits to how much a person should ingest on a daily basis, especially with long-term use. The acceptable daily intake is 8 mg for women and 11 mg for men, and more than 40 mg constitutes zinc poisoning, according to the U.S. National Academy of Sciences.

A 2.4 oz (68 gram) tube of Super PoliGrip contains 38 mg of zinc per gram of product, and GSK recommends that a single tube should last approximately six weeks. According to the company's Web site:

You should not use more than the indicated quantity of Super PoliGrip, or use it more often than once per day, to compensate for poorly fitting dentures. You should consult your dentist regularly to maintain proper-fitting dentures.

The Super PoliGrip packaging carries these same recommendations, along with the list of ingredients (topping the list is "poly(methylvinylether/maleic acid) sodium-magnesium-zinc mixed partial salt").

Language on the Fixodent packaging is less explicit; under "Helpful Hints," a highlighted section reads "Don't use too much product at first. It may take a few tries to find the right amount for you!" and "Consult your dentist regularly to ensure proper fitting dentures." There is no list of ingredients on either the Fixodent box or tube; because the FDA has classified denture cream as a Class I device, the manufacturers are not required to list the ingredients.

However, the Fixodent Web site does feature a letter from J. Leslie Winston, D.D.S., Ph.D., P&G's director of professional and scientific relations for North America, which states in part:

A small amount of zinc is used in Fixodent to provide denture hold. This helps the denture stay in place securely so our consumers can eat, chew, and talk more confidently. All Fixodent products undergo rigorous scientific evaluations and safety testing before appearing on shelf and we continue to monitor them once in market. ... The average amount of zinc an average denture adhesive user would ingest from daily usage of Fixodent is less than the amount of zinc in most daily multivitamins and comparable to 6 ounces of ground beef.

Apex Dental Materials

Apex Dental Materials

Another problem with the newer systems is that combining primer and adhesive or primer and etch results in primer being applied to enamel. Surpass (Apex Dental Materials) and All-Bond SE (Bisco) are the only newer-generation products Dr. Brucia has found that have no solvent in the adhesive. "I absolutely would never want primer on enamel," he said. "Dentin primer does nothing but get in your way."

The solvent in the primer doesn't allow adhesives to cure. With fourth-generation products, in which the primer, etch, and adhesive are separate, you can remove the solvent by air thinning the primer until the movement of fluid stops. "When you mix primer and adhesive together, no matter how good your technique is, 25% of what you have left is solvent," Dr. Brucia said.

Light also helps remove the solvent, he noted. "If you light assist the evaporation of your primer, your bond strength will routinely go up 20%. I have tested this for years. The added amount of time and heat will give you a much better adhesion," he said.

A problem with sixth-generation products (in which the primer and etch are in one bottle, the adhesive in another) is that the acids aren't as effective for etching, Dr. Brucia said. Five-amino salicylic acid, citric acid, and nitric acid are used because they neutralize themselves in the mouth, eliminating the need for rinsing.

But phosphoric acid is better for etching, Dr. Brucia said. "I use phosphoric acid on everything. If you were to take my phosphoric acid away from me, you might lose a finger."

The differences add up starkly in bench tests, he said. With his fourth-generation OptiBond, PermaQuik, Scotchbond and All-Bond, he can instantly achieve 40 megapascals (MPa) of bond strength, and 72 MPa after 24 hours. By contrast, with the newest products, he can only achieve 10-15 MPa.

As further evidence that fourth-generation products last longer, Dr. Brucia cited an article in Dental Materials (July 2000, Vol. 16:4, pp. 285-291) that found that the "three-step resin adhesive and the RMGIC [resin-modified glass ionomer cement) showed clinically acceptable retention rates, while a high failure rate was registered for the one-bottle adhesive."

"I would still rather have a 12-year-old bonding agent and a 20-year-old composite in my inventory than the newer systems today," Dr. Brucia concluded.

Not everyone agrees that bonding strength has declined with the newer systems. Junichi Ohtsuki, product manager for Kuraray America, acknowledged that seventh-generation adhesives "are not perfect yet." But he insisted that "The sixth generation is the best bonding agent for relieving sensitivity." The fourth generation, he said, "is similar in strength, but it's technique-sensitive."

Asked for evidence to back up his claims, Ohtsuki provided DrBicuspid.com with three articles from peer-reviewed journals. Of theses, one dealt only with the consequence of mistakes in technique. The other two actually supported Dr. Brucia's position. For example, a September 2005 review by Peumans et al in Dental Materials concluded that, compared to fourth generation systems, "The clinical effectiveness of two-step etch-and-rinse adhesives was less favorable, while an inefficient clinical performance was noted for the one-step self-etch adhesives" (September 2005, Vol. 21:9, pp. 864-881).

Ohtsuki also provided a symposium paper speculating on hypothetical advantages of "self-etch" (later than fourth-generation) adhesives. Finally, he supplied two articles from Gordon Christensen's CRA Newsletter, but only one (November/December 2003, Vol. 27:11/12, pp. 1-5) reported tests of a fourth-generation product: All-Bond 2 (Bisco). The anonymous authors gave this product a bond strength of 26.5 ± 6.3 MPa on enamel and 36.1 ± 4.9 MPa on dentin.

These researchers found several other recent-generation products stronger. They rated two fifth-generation products the highest in strength. Prime & Bond NT (Caulk/Dentsply) got the highest enamel bond strength rating (46.3 ± 9.8 MPa), while OptiBond Solo Plus SEP got the highest dentin bond strength rating (59.5 ± 6.9 MPa).

On the other hand, Douglas J. Brown, D.D.S., senior manager for clinical affairs at Bisco, acknowledged that the fourth-generation systems have clear advantages. "The evidence has shown that they are stronger and longer-lasting than the self-etches," he said.

The only advantages to the newest products -- particularly the one-step systems -- are speed and convenience. In some instances, such as placing a restoration in an impatient child, that speed might be worth the loss of durability, Dr. Brown argued.

And even Dr. Brucia acknowledged that in situations in which strength is not at a premium, the newest systems might be useful. Overall, though, he warned that many dentists would pay the price for taking the easy route.

"I'm too busy to have sensitivity," he said. "I'm too busy to have failures."