present and the critical ph is reached, the hydroxyapatite crystallites of enamel demineralize, allowing for greater bacterial invasion deeper into the tooth. the most important bacterium involved with tooth decay is streptococcus mutans, but the number and type of bacteria varies with the progress of tooth destruction. furthermore, tooth morphology dictates that the most common site for the initiation of dental caries is in the deep grooves, pits, and fissures of enamel. this is expected because these locations are impossible to reach with a toothbrush and allow for bacteria to reside there. when demineralization of enamel occurs, a dentist can use a sharp instrument, such as a dental explorer, and "feel a stick" at the location of the decay. as enamel continues to become less mineralized and is unable to prevent the encroachment of bacteria, the underlying dentin becomes affected as well. when dentin, which normally supports enamel, is destroyed by a physiologic condition or by decay, enamel is unable to compensate for its brittleness and breaks away from the tooth easily. the effects of bruxism on an anterior tooth, revealing the dentin and pulp which are normally hidden by enamel the extent to which tooth decay is likely, known as cariogenicity, depends on factors such as how long the sugar remains in the mouth. contrary to common belief, it is not the amount of sugar ingested but the frequency of sugar ingestion that is the most important factor in the causation of tooth decay. when the ph in the mouth initially decreases from the ingestion of sugars, the enamel is demineralized and left vulnerable for about 30 minutes. eating a greater quantity of sugar in one sitting does not increase the time of demineralization. similarly, eating a lesser quantity of sugar in one sitting does not decrease the time of demineralization. thus, eating a great quantity of sugar at one time in the day is less detrimental than is a very small quantity ingested in many intervals throughout the day. for example, in terms of oral health, it is better to eat a single dessert at dinner time than to snack on a bag of candy throughout the day. in addition to bacterial invasion, enamel is also susceptible to other destructive forces. bruxism, also known as clenching of or grinding on teeth, destroys enamel very quickly. the wear rate of enamel, called attrition, is 8 micrometers a year from normal factors. a common misconception is that enamel wears away mostly from chewing, but actually teeth rarely touch during chewing. furthermore, normal tooth contact is compensated physiologically by the periodontal ligaments (pdl) and the arrangement of dental occlusion. the truly destructive forces are the parafunctional movements, as found in bruxism, which can cause irreversible damage to the enamel. other nonbacterial processes of enamel destruction include abrasion (involving foreign elements, such as toothbrushes), erosion (involving chemical processes, such as dissolving by soft drinks or lemon and other juices), and possibly abfraction (involving compressive and tensile forces). though enamel is described as tough, it has a similar brittleness to glass, making it, unlike other natural crack-resistant laminate structures such as shell and nacre, vulnerable to fracture. in spite of this it can withstand bite forces as high as 1,000 n many times a day during chewing. this resistance is due in part to the microstructure of enamel which contains enamel tufts that stabilize such fractures at the dentinoenamel junction. the configuration of the tooth also acts to reduce the tensile stresses that cause fractures during biting. gastroesophageal reflux disease can also lead to enamel loss, as acid refluxes up the esophagus and into the mouth, occurring most during overnight sleep. oral hygiene main article: oral hygiene because enamel is vulnerable to demineralization, prevention of tooth decay is the best way to maintain the health of teeth. most countries have wide use of toothbrushes, which can reduce the number of dental biofilm and food particles on enamel. in isolated societies that do not have access to toothbrushes, it is common for those people to use other objects, such as sticks, to clean their teeth. in between two adjacent teeth, floss is used to wipe the enamel surfaces free of plaque and food particles to discourage bacterial growth. although neither floss nor toothbrushes can penetrate the deep grooves and pits of enamel, good general oral-health habits can usually prevent enough bacterial growth to keep tooth decay from starting. structural integrity of the enamel is genetic, and so is its predisposition to demineralization or attack from bacteria. fluoride remineralization fluoride catalyzes the diffusion of calcium and phosphate into the tooth surface, which in turn remineralizes the crystalline structures in a dental cavity. the remineralized tooth surfaces contain fluoridated hydroxyapatite and fluorapatite, which resist acid attack much better than the original tooth did. fluoride therapy is used to help prevent dental decay. common dentistry trays filled with fluoride foam fluoride ions, as an antimicrobial, may activate bacterial genes associated with fluoride riboswitches. the combination of fluoride ions and qas (quaternary ammonium salts) was found to have a stronger antimicrobial effect on many oral bacteria associated with dental decay, including s. mutans. fluoride in drinking water most dental professionals and organizations agree that the inclusion of fluoride in public water has been one of the most effective methods of decreasing the prevalence of tooth decay. fluoride can be found in many locations naturally, such as the ocean and other water sources. the recommended dosage of fluoride in drinking water does not depend on air temperature. some groups have spoken out against fluoridated drinking water, for reasons such as the neurotoxicity of fluoride or the damage fluoride can do as fluorosis. fluorosis is a condition resulting from the overexposure to fluoride, especially between the ages of 6 months and 5 years, and appears as mottled enamel. consequently, the teeth look unsightly, although the incidence of dental decay in those teeth is very small. where fluoride is found naturally in high concentrations, filters are often used to decrease the amount of fluoride in water. for this reason, codes have been developed by dental professionals to limit the amount of fluoride a person should take. these codes are supported by the american dental association and the american academy of pediatric dentistry. furthermore, whereas topical fluoride, found in toothpaste and mouthwashes, does not cause fluorosis, its effects are now considered more important than those of systemic fluoride, such as when drinking fluorinated water. however, systemic fluoride works topically as well with fluoride levels in saliva increase also when drinking fluoridated water. lately, dental professionals are looking for other ways to present fluoride (such as in varnish) or other mineralizing products such as amorphous calcium phosphate to the community in the form of topical procedures, either done by professionals or self-administered. mineralization of the incipient lesion instead of restoration later is a prime goal of most dental professionals. dental procedures an x-ray showing enamel and dentin replaced by an amalgam restoration dental restorations most dental restorations involve the removal of enamel. frequently, the purpose of removal is to gain access to the underlying decay in the dentin or inflammation in the pulp. this is typically the case in amalgam restorations and endodontic treatment. nonetheless, enamel can sometimes be removed before there is any decay present. the most popular example is the dental sealant. in the past, the process of placing dental sealants involved removing enamel in the deep fissures and grooves of a tooth, followed by replacing it with a restorative material. presently, it is more common to only remove decayed enamel if present. in spite of this, there are still cases where deep fissures and grooves in enamel are removed in order to prevent decay, and a sealant may or may not be placed depending on the situation. sealants are unique in that they are preventative restorations for protection from future decay, and have been shown to reduce the risk of decay by 55% over 7 years. aesthetics is another reason for the removal of enamel. removing enamel is necessary when placing crowns and veneers to enhance the appearance of teeth. in both of these instances, when unsupported by underlying dentin, that portion of the enamel is more vulnerable to fracture. acid-etching techniques invented in 1955, acid-etching employs dental etchants and is used frequently when bonding dental restoration to teeth. this is important for long-term use of some materials, such as composites and sealants. by dissolving minerals in enamel, etchants remove the outer 10 micrometers on the enamel surface and make a porous layer 5–50 micrometers deep. this roughens the enamel microscopically and results in a greater surface area on which to bond. the effects of acid-etching on enamel can vary. important variables are the amount of time the etchant is applied, the type of etchant used, and the current condition of the enamel. there are three types of patterns formed by acid-etching. type 1 is a pattern where predominantly the enamel rods are dissolved; type 2 is a pattern where predominantly the area around the enamel rods are dissolved; and type 3 is a pattern where there is no evidence left of any enamel rods. besides concluding that type 1 is the most favorable pattern and type 3 the least, the explanation for these different patterns is not known for certain but is most commonly attributed to different crystallite orientation in the enamel. tooth whitening the discoloration of teeth over time can result from exposure to substances such as tobacco, coffee, and tea. the staining occurs in the interprismatic region internally on the enamel, which causes the tooth to appear darker or more yellow overall. in a perfect state, enamel is colorless, but it does reflect underlying tooth structure with its stains since light reflection properties of the tooth are low. tooth whitening or tooth bleaching procedures attempt to lighten a tooth's color in either of two ways: by chemical or mechanical action. working chemically, a bleaching agent is used to carry out an oxidation reaction in the enamel and dentin. the agents most commonly used to intrinsically change the color of teeth are hydrogen peroxide and carbamide peroxide. oxygen radicals from the peroxide in the whitening agents contact the stains in the interprismatic spaces within the enamel layer. when this occurs, stains will be bleached and the teeth now appear lighter in color. teeth not only appear whiter but also reflect light in increased amounts, which makes the teeth appear brighter as well. studies show that whitening does not produce any ultrastructural or microhardness changes in the dental tissues. studies show that patients who have whitened their teeth take better care of them. however, a tooth whitening product with an overall low ph can put enamel at risk for decay or destruction by demineralization. consequently, care should be taken and risk evaluated when choosing a product which is very acidic. tooth whiteners in toothpastes work through a mechanical action. they have mild abrasives which aid in the removal of stains on enamel. although this can be an effective method, it does not alter the intrinsic color of teeth. microabrasion techniques employ both methods. an acid is used first to weaken the outer 22–27 micrometers of enamel in order to weaken it enough for the subsequent abrasive force. this allows for removal of superficial stains in the enamel. if the discoloration is deeper or in the dentin, this method of tooth whitening will not be successful. related pathology irreversible enamel defects caused by an untreated celiac disease. they may be the only clue to its diagnosis, even in absence of gastrointestinal symptoms, but are often confused with fluorosis, tetracycline discoloration, or other causes. the national institutes of health include a dental exam in the diagnostic protocol of celiac disease. there are 14 different types of amelogenesis imperfecta. the hypocalcification type, which is the most common, is an autosomal dominant condition that results in enamel that is not completely mineralized. consequently, enamel easily flakes off the teeth, which appear yellow because of the revealed dentin. the hypoplastic type is x-linked and results in normal enamel that appears in too little quantity, having the same effect as the most common type. chronic bilirubin encephalopathy, which can result from erythroblastosis fetalis, is a disease which has numerous effects on an infant,