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What Causes Missing Teeth?
Missing teeth are one of the most common developmental problems in children. Nearly 20% of the U.S. population has congenitally missing third molars, 3.4% has missing second premolars, and 2.5% has missing upper lateral incisors. The adult teeth are more frequently affected than the baby teeth. Absence of baby teeth occurs in 0.5% to 0.9% of the population. As a rule, when a baby tooth is missing, its permanent counterpart will also be absent.

Missing teeth (tooth agenesis) can occur in an isolated fashion, or as part of a syndrome. Isolated cases of missing teeth can be familial or sporadic in nature. Familial tooth agenesis is transmitted as an autosomal dominant, autosomal recessive, or X-linked genetic condition. In addition, there are more than 49 syndromes which are associated with tooth agenesis.

Specific terms are used to describe the nature of tooth agenesis. Oligodontia is the lack of tooth development of 6 or more permanent teeth, without an associated systemic disorder. Hypodontia is the absence of 6 or less teeth, but is usually part of a more complex set of developmental problems. Most cases of tooth agenesis involve hypodontia.

Researchers have recently discovered a number of genes and gene products which control communication between and within the cells that are necessary for tooth formation. These gene products are chemicals (proteins) which either affect the DNA on the chromosomes, or function as extracellular messengers. Examples of DNA control genes are MSX-1 and MSX-2. Examples of extracellular signals are bone morphogenetic proteins and fibroblast growth factors.

Genes are the molecular code of life. The 46 human chromosomes contain approximately 100,000 genes. Three billion information bases make up these 100,000 genes. Genetic mistakes may occur due to "misspellings" in the genetic code. These genetic mistakes may then be passed on from generation to generation as mendelian inheritance traits. Familial tooth agenesis can be transmitted as an autosomal dominant , autosomal recessive, or X-linked condition.

Mistakes (mutations) in the genes which help produce teeth can cause missing teeth. Recent studies have found that mutations in two regulatory genes, MSX-1 and PAX9, cause tooth agenesis. The mutations causing tooth agenesis fall into one of three categories: point mutations, frameshift mutations, or large DNA deletions.

One study found that certain families with congenitally missing teeth had a point mutation of the MSX-1 gene. The MSX-1 gene is a homeobox gene, and is required for tooth development. MSX-1 is found on the short arm of chromosome 4 at location 4p16.1 . The point mutation of MSX-1 involved a "missense" substitution of proline in the place of arginine. This particular genetic mutation is transmitted as an autosomal dominant trait.

Another study found that three generations of a particular family had congenitally missing molars, and this was caused by a mutation of the PAX9 gene. The PAX9 gene is involved in the formation of the eyes, palate, teeth, and thyroid gland. An analysis of the gene indicated that a frameshift mutation had taken place, and an extra guanine base was inserted at nucleotide 219 of chromosome 14. The end result of this mutation was the production of a smaller, and less effective, protein required for normal tooth development. In this study, the gentic mutation was transmitted as an autosomal dominant trait.

A dental journal article discusses the genetic basis of dental disorders:
Slavkin HC: Entering the era of molecular dentistry. JADA 130:413-417,1999.



Are Dental Sealants Safe For Children?
Sealants are a protective covering which are applied to the back teeth and help prevent dental caries, especially on the chewing surfaces. One research team in Spain believes that sealants release an estrogenic compound, bisphenol A, into the oral environment. Most U.S. research teams, however, believe that American-made sealants do not release any detectable estrogenic compounds, and are therefore safe for use in children.

Estrogens are steroid hormones which are responsible for the development of genitalia, ovaries, breasts, and other secondary sex characteristics. Xenoestrogens are estrogen-mimics which have disruptive effects on cells with high levels of estrogen receptors. Although xenoestrogens are chemically unrelated to natural estrogen, they can bind to the body's estrogen receptors and cause inappropriate signals to be transmitted. Xenoestrogens compete with natural estrogen, 17B-estradiol, for hormone binding sites in certain cells. Major sources of xenoestrogens include pesticides, polycarbonate lining of food and soda cans, and plastic food containers.

Bisphenol A is a starting raw chemical found in unprocessed (unpolymerized) dental sealant material, and it has proliferative effects on cells with high levels of estrogen receptors. When you take your child to the dentist to get sealants, the raw sealant material is very briefly placed on the biting surface of the tooth, after which it is quickly and thoroughly cured (polymerized) and hardened. The finished polymerized sealant product is a hard protective coating, with a different chemical structure than the raw components.

The main issue is whether or not dental sealants have estrogenic effects. Based on the research material available today, dental sealants probably have no estrogenic effects.

A recent dental journal article reviews this topic:
Schafer TE. Lapp CA. Hanes CM. Lewis JB: What parents should know about estrogen-like compounds in dental materials. Pediatric Dentistry 22:75-76, 2000.



What Is A Double Tooth?
A double tooth is a tooth with a double crown. It may have one or two roots, depending on the developmental cause of the problem. A double tooth is more cavity-prone than other teeth, due to its unusual shape. In some cases, a double baby tooth is associated with a missing adult tooth.

Possible causes for double teeth include: environmental factors, crowding of tooth germs, trauma, systemic disease, and hereditary factors. Double teeth appear more frequently in children than adults. Approximately 0.5% of children have them. The front teeth are most likely to be affected by this problem. Double teeth run in families, and follow a hereditary pattern - usually autosomal dominant or autosomal recessive.

Double teeth are the result of disturbances during embryological tooth formation. Since it is not possible to know what caused the embryonic disturbance, it is not appropriate to use terms such as gemination and fusion when talking about a diagnosis.

One of the embryological causes of double teeth is gemination. Gemination is an attempted division of a tooth germ, resulting in a tooth with a bifid crown and a single root. The error occurs at the proliferative stage of tooth development.

Fusion occurs when two or more tooth germs form a single tooth. In cases of complete fusion, the tooth germs unite before the calcification process begins, and it results in a fused tooth with only one root canal. In a variant of fusion, known as concrescence, the fusion occurs at the embryonic stage when the cementum layer of the root is forming. Cases of fusion are mostly seen in the lower incisor and canine regions, and have a 78% chance of resulting in a missing adult lateral incisor.

Double teeth often have problems such as poor esthetics, dental caries, arch-length problems, and/or periodontal disease. Fused or geminated teeth usually have a vertical groove on the front and back of the teeth. These grooves may be very difficult to clean, leading to dental stains and caries. This problem can be corrected by placing a sealant or composite material into the grooves to decrease the risk of caries.

A recent dental journal article presents a clinical case of fused teeth:
Mochizuki K, Yonezu T, Yakushiji et al: The fusion of three primary incisors: Report of a case. Journal of Dentistry for Children, 66:421-425,1999.


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