Older dental fillings contain form of mercury unlikely to be toxic

A new study, "The Chemical Forms of Mercury in Aged and Fresh Dental Amalgam Surfaces," on the surface chemistry of silver-colored, mercury-based dental fillings suggests that the surface forms of mercury may be less toxic than previously thought. It appears online in ACS' journal Chemical Research in Toxicology.




In the study, Graham George and colleagues note that mercury-based fillings, also called amalgams, have been used by dentists to repair teeth for well-over a century. In recent decades their use has become controversial because of concerns about exposure to potentially toxic mercury. However, mercury can potentially exist in several different chemical forms, each with a different toxicity. Prior to this report, little was known about how the chemical forms of mercury in dental amalgam might change over time.

Using a special X-ray technique, the scientists analyzed the surface of freshly prepared metal fillings and compared these with the surface of aged fillings (about 20 years old) from a dental clinic. Fresh fillings contained metallic mercury, which can be toxic. Aged fillings, however, typically contain a form of mercury, called beta-mercuric sulfide or metacinnabar, which is unlikely to be toxic in the body. The scientists found that the surfaces of metal fillings seem to lose up to 95 percent of their mercury over time. Loss of potentially toxic mercury from amalgam may be due to evaporation, exposure to some kinds of dental hygiene products, exposure to certain foods, or other factors. The scientists caution that "human exposure to mercury lost from fillings is still of concern."

Studies advise on fluoridated toothpaste use in children

Parents should use toothpastes that contain fluoride with a minimum concentration of 1,000 parts per million to prevent tooth decay in their children, says a new report. Preventing tooth decay can help reduce the need for extensive and costly dental treatments, including extractions.

But the authors, in a second related study, suggest that parents concerned about the risk of fluorosis – the discolouration or mottling of the teeth caused by excessive fluoride ingestion – should consult their dentist to discuss the benefits and risks.

Researchers for the Cochrane Oral Health Group, based at the School of Dentistry, The University of Manchester, have previously shown that fluoride toothpastes reduce dental decay by 24% on average compared to non-fluoride products.

The group's latest research, which involved 79 trials on 73,000 children worldwide, examined the effect of different children's toothpastes and found that those with fluoride concentrations less than 1,000 parts per million were only as effective as non-fluoride toothpastes at preventing tooth decay. Children's toothpastes range from 100ppm to 1,400ppm fluoride concentration.

The report suggests that brushing a child's teeth with a toothpaste containing fluoride before the age of 12 months may be associated with an increased risk of developing mild fluorosis. Swallowing large amounts of toothpaste may still cause fluorosis in children up to the age of six years when the permanent teeth are still developing, but using a small amount, carefully, will reduce these risks. After the age of six years, the teeth are fully developed and toothpaste can be used without fear of fluorosis.

Dr Anne-Marie Glenny, an author on the review, said: "It is very confusing for parents to know how to strike the right balance, which isn't helped by the fact that different companies use different concentrations of fluoride in their toothpastes aimed at children.

"From a public health point of view, the risk of tooth decay and its consequences such as pain and extractions is greater than the small risk of fluorosis. Children would have to swallow a lot of toothpaste over a long period of time to get the severe brown mottling on the teeth, as opposed to the more typical mild white patches.

"For children that are considered to be at a high risk of tooth decay by their dentist, the benefit to health of preventing decay is likely to outweigh the risk of fluorosis. In such cases, careful brushing of their children's teeth by parents with a small amount of toothpaste containing higher levels of fluoride would be beneficial. If in any doubt, we would advise parents to speak to their family dentist."

Dental Implants -- a Permanent Fix for Missing Teeth

Dental implants, rather than a fixed bridge or removable dentures, are an increasingly popular method of replacing teeth lost to an accident, gum diseases or tooth decay.

In an interview in the January issue of Mayo Clinic Women’s HealthSource, Sreenivas Koka, D.D.S., Ph.D., chair of Mayo Clinic Department of Dental Specialties, discusses the advantages of dental implants and what’s involved.

To place an implant, an oral surgeon or periodontist cuts open the gum to expose the jawbone and then drills a small hole in the bone for the metal cylinder that serves as the implant. “It’s almost like drilling a screw into the wall,” says Dr. Koka. The patient is under anesthesia.

Over the next three months, the area heals and the implant fuses with the jawbone. Patients may undergo a second procedure in which a post, called an abutment, is attached or screwed down into the implant. This can be done at the same time the implant cylinder is put in or after the area has healed.

In the final step, the dentist attaches a realistic-looking artificial tooth to the implant or to the post. The entire process takes about four months.

A big advantage, says Dr. Koka, is that an implant acts as a substitute for the roots of a natural tooth. “If you are missing a single tooth, this allows you to leave the other teeth around it alone,” says Dr. Koka. “With a bridge, you have to cut down the teeth on each side of the empty space so that a false tooth can be held in place by two crowns.”

Almost any adult in reasonably good health is a candidate for dental implants.

Dr. Koka says that patients often ask if osteoporosis would prevent them from getting dental implants. It doesn’t. Mayo Clinic research has shown that patients with osteoporosis or those taking oral bisphosphonates used to treat osteoporosis have about the same success rate as other patients. And the success rates for dental implants are high -- 90 to 96 percent.

Dental Enamel: Ways to Give it a Boost

Tooth enamel is tough, harder even than skeletal bone. But even with good dental hygiene, the enamel protecting teeth may show signs of decay in older adults. The January issue of Mayo Clinic Health Letter covers what to look out for and what to do to promote an enamel-friendly setting in the mouth. According to the newsletter, two significant contributors to tooth decay are too little saliva and inadequate fluoride.

Too little saliva: Saliva helps repair the earliest stages of tooth decay. It supplies high levels of calcium and phosphate particles that enhance protection of the tooth’s enamel surface. In addition, saliva protects the mouth by washing away food and the sticky film of acid-producing plaque that can cling to teeth.

Because of saliva’s importance, dry mouth symptoms should be evaluated by a care provider. Dry mouth can be caused by medical conditions or their treatments. Examples include Sjogren’s syndrome, an autoimmune disorder, diabetes and HIV or AIDS. Dry mouth is a common side effect of many prescription and nonprescription medications, including some antidepressants and anti-anxiety medications, antihistamines, medications for high blood pressure, anti-diarrheals, muscle relaxants and medications for urinary incontinence and Parkinson’s disease.

For some people, medications or doses can be changed to minimize this side effect. When that’s not possible, chewing sugar-free gum sweetened by a naturally occurring sugar substitute called xylitol can help. Studies have shown that the frequent use of gum with high levels of xylitol can prevent cavities and even harden a tooth’s surface where a cavity has begun. Chewing gums that contain Recaldent also can help. This ingredient is a form of calcium phosphate that penetrates and binds to the tooth enamel. It’s found in Trident Xtra Care gum.

Inadequate fluoride: This mineral is an important enamel ally that can make teeth stronger and can enhance saliva’s remineralizing, anti-decay properties. Most people get adequate fluoride exposure through drinking fluoridated water and brushing twice daily with fluoride toothpastes.

But people who drink primarily bottled or filtered water may not be getting adequate fluoride. Even with adequate fluoride, dry mouth can throw off the normal balance teeth need to stay healthy.

A dentist may recommend fluoride treatment to protect and strengthen tooth enamel. Fluoride can be applied two to four times a year. Another option is nonprescription fluoride rinses available in drugstores.

Also helpful is limiting tooth enamel contact with acids that come from many foods and beverages. Examples include citrus fruits, tart candies, diet and regular sodas or sports drinks, fruit juices and wine.

Origins of Cavity-Causing Bacteria

Researchers have uncovered the complete genetic make-up of the cavity-causing bacterium Bifidobacterium dentium Bd1, revealing the genetic adaptations that allow this microorganism to live and cause decay in the human oral cavity. The study, led by Marco Ventura's Probiogenomics laboratory at the University of Parma, and Prof. Douwe van Sinderen and Dr Paul O'Toole of the Alimentary Pharmabiotic Centre at University College Cork, is published December 24 in the open-access journal PLoS Genetics.

Bifidobacteria, largely known as long-term beneficial gut bacteria, are often included as probiotic components of food to aid digestion and boost the immune system. However, not all species within the genus Bifidobacterium provide beneficial effects to the host's health. In fact, the Bifidobacterium dentium species is an opportunistic pathogen since it has been linked to the development of tooth decay. The genome sequence of B. dentium Bd1 reveals how this microorganism has adapted to the oral environment through specialized nutrient acquisition features, acid tolerance, defences against antimicrobial substances and other gene products that increase fitness and competitiveness within the oral niche.

This report identifies, through various genomic approaches, specific adaptations of a Bifidobacterium taxon to a lifestyle as a tooth decay-causing bacterium. The data in this study indicate that the genome of this opportunistic pathogen has evolved through only a small number of horizontal gene acquisition events, highlighting the narrow boundary that separates bacteria that are long-term residents on or in the human body from opportunistic pathogens.

Cracking The Root Of Tooth Strength

After years of biting and chewing, how are human teeth able to remain intact and functional? A team of researchers from The George Washington University and other international scholars have discovered several features in enamel—the outermost tooth tissue—that contribute to the resiliency of human teeth.



A human molar compared to a sea otter molar. (Credit: The George Washington University)

Human enamel is brittle. Like glass, it cracks easily; but unlike glass, enamel is able to contain cracks and remain intact for most individuals’ lifetimes. The research team discovered that the major reason why teeth do not break apart is due to the presence of tufts—small, crack-like defects found deep in the enamel. Tufts arise during tooth development, and all human teeth contain multiple tufts before the tooth has even erupted into the mouth.

Many cracks in teeth do not start at the outer surface of the tooth, as has always been assumed. Instead cracks arise from tufts located deep inside the enamel. From here, cracks can grow towards the outer tooth surface. Once reaching the surface, these cracks can potentially act as sites for dental decay. Acting together like a forest of small flaws, tufts suppress the growth of these cracks by distributing the stress amongst themselves.

“This is the first time that enigmatic developmental features, such as enamel tufts, have been shown to have any significance in tooth function” said GW researcher Paul Constantino. “Crack growth is also hampered by the “basket weave” microstructure of enamel, and by a ‘self-healing’ process whereby organic material fills cracks extended from the tufts, which themselves also become closed by organic matter. This type of infilling bonds the opposing crack walls, which increases the amount of force required to extend the crack later on.”

This research evolved as part of an interdisciplinary collaboration between anthropologists from The George Washington University and physical scientists from the National Institute of Standards and Technology in Gaithersburg, Md. The team studied tooth enamel in humans and also sea otters, mammals with teeth showing remarkable resemblances to those of humans.

Proline Repeats Help Protein Grow Tooth Enamel

A simple amino acid that is repeated in the center of proteins found in tooth enamel makes teeth stronger and more resilient, according to new research at the University of Illinois at Chicago.

Researchers compared proline repeats in amphibian and animal models and discovered that when the repeats are short, such as in frogs, teeth will not have the enamel prisms that are responsible for the strength of human enamel. In contrast, when the proline repeats are long, they contract groups of molecules that help enamel crystals grow.

The findings will be published in the December 21 online version of the Journal PLoS Biology.

"Proline repeats are amazing," said Tom Diekwisch, professor and head of oral biology in the UIC College of Dentistry and lead researcher on the study. "They hold the key to understanding the structure and function of many natural proteins, including mucins, antifreeze proteins, Alzheimer amyloid, and prion proteins.

"We hope that our findings will help many other important areas of scientific research, including the treatment of neurodegenerative diseases."

When tooth enamel is grown it is bathed in bubble-shaped groupings of proteins, Diekwisch said. The size of the protein bubbles varies in different animals, from 5 nanometers in cows to 20 nanometers in mice and 40 nanometers in frogs.

Diekwisch's team discovered that the longer the stretch of proline repeats, the more the protein bubbles contracted. The study also showed that the smaller protein bubbles were associated with longer enamel crystals, he said.

The new discovery, Diekwisch said, will give new clues to engineer tooth enamel.

"We hope that one day these findings will help people replace lost parts of the tooth with a healthy layer of new enamel."