Functional Role of Inorganic Trace Elements on Enamel and Dentin Formation: A Review

Abstract

Calcium and phosphate are the major components of hydroxyapatite crystals that form the inorganic portion of the teeth. Apart from these, certain elements are present in little amounts in enamel and dentin of the human teeth. Although they are required in minute quantities, their absence may alter healthy development of enamel and dentin and may result in developmental tooth defects as well as dental caries. Furthermore, excessive intake of some trace elements may inversely affect tooth development and health. The exact of effects that trace elements have on teeth and oral health is still an unexplored territory. The present paper reviews the presence of trace elements in teeth and their role in tooth health and development.

INTRODUCTION

The human body's various functions depend substantially on various nutritional elements.[1] There are two types of nutritional elements required by the human body, namely macronutrients and micronutrients. Macronutrients include carbohydrates, fats, and proteins whereas micronutrients comprise vitamins and minerals. Macronutrients are required in larger amounts in the body as compared to micronutrients. The mineral component of the micronutrients is composed of macrominerals and microminerals.[2] The macrominerals required in smooth physiological functioning of the body include sodium, potassium, calcium, phosphorus chloride, and sulfur.[3] In a healthy adult, >100 mg/day of macrominerals is required. According to Frieden, microminerals include iron, copper, zinc, magnesium, cobalt, nickel, iodine, fluorine, vanadium, selenium, chromium, molybdenum, tin, and silicon.[3,4] These microminerals can be further subdivided into trace elements and ultratrace elements. Both trace and ultratrace elements are required in small amounts in the body. Trace elements are needed up to 1 mg/kg body weight while ultratrace elements are needed <1 mg/kg body weight in a healthy human adult. Even though trace elements are required in small amounts by the body, their deficiency can have serious implications.[1,5]

Trace elements in enamel

The hardest tissue in the entire human body is the enamel of our teeth. Dental enamel is greatly mineralized in its composition with approximately 92%–96% of inorganic matter, 1%–2% organic matter, and 3%–4% of water, weight/weight (w/w). Such high percentage of inorganic matter is what makes the dental enamel brittle and nonvital. The smaller organic part of dental enamel comprises mostly proteins which include amelogenins, tuftelins, and aminoglycans.[6] The larger inorganic part of dental enamel is mostly composed of hydroxyapatite crystals. These hydroxyapatite crystals are arranged as prisms and interprisms of 70 nm width and 30 nm thickness.[7,8] Hydroxyapatite crystals also contain numerous trace elements. These trace elements are mostly incorporated within the dental enamel during tooth mineralization and maturation.[9,10] Shashikiran et al.[11] have enumerated the presence of eighteen trace elements in dental enamel by atomic absorption spectrometry. They found the presence of fluorine (F), strontium (Sr), potassium (K), aluminum (Al), silicon (Si), nickel (Ni), boron (B), iron (Fe), copper (Cu), chromium (Cr), zinc (Zn), manganese (Mn), cobalt (Co), selenium (Se), lead (Pb), molybdenum (Mo) and vanadium (V) in sound human dental enamel samples. They also found differences in the concentrations of certain trace elements in primary and permanent teeth. They discovered that K, F, and Sr concentrations were higher in the enamel of permanent teeth as compared to primary teeth. On the other hand, concentrations of Cu, Al, and Si were more in dental enamel of primary teeth as against the dental enamel of permanent teeth.[11] Ghadimi et al.[12] in their study postulated that trace elements may have a significant effect on the size of hydroxyapatite crystals which constitute a major part of dental enamel. Some trace elements in the form of ions such as iron in ferrous (Fe²⁺) and ferric (Fe³⁺) form, strontium (Sr²⁺), and zinc (Zn²⁺) (with molar fraction >10%) ions can expand the crystal lattice of hydroxyapatite crystals along the a-axis, whereas carbonate (CO₃²⁻), silicate (SiO₄⁴⁻), magnesium (Mg²⁺), titanium (Ti⁴⁺) and zinc (Zn²⁺) (with molar fraction <10%) ions have the potential to shrink the lattice along the a-axis. Along the c-axis, silicate (SiO₄⁴⁻), carbonate (CO₃²⁻), ferrous (Fe²⁺), ferric (Fe³⁺), zinc (Zn²⁺), and strontium (Sr²⁺) ions can expand the hydroxyapatite crystal lattice and magnesium (Mg²⁺), chromic (Cr³⁺), titanium (Ti⁴⁺), nickelous (Ni²⁺), and cobaltous (Co²⁺) shrink the hydroxyapatite crystal lattice. Therefore, trace elements can affect the average size of the inorganic component of the dental enamel, i.e., the hydroxyapatite crystals.

Trace elements in dentin

In a healthy human tooth dentin is sealed from the environment since it is encased by enamel and cementum.[13] Another major difference from enamel is that physiological exchange of elements occurs in tooth dentin even after mineralization unlike dental enamel.[14] After the metabolism of elements in dentin formation completes it becomes inactive, which leads to accumulation of these elements.[15,16] Kumagai et al.[17] postulated that the concentrations of trace elements in human tooth dentin increase with age. They reported increased concentrations of ten trace elements, i.e., B, Co, Cr, Mn, Zn, Sr, Mo, Cd (cadmium), Rb (rubidium), and Pb (lead).[17] It may be due to abundance of collagen fibers in dentin. It is believed that certain elements have a propensity to accumulate in the collagen fibers present in the tooth. Therefore, such elements increase in concentrations over in the collagen-rich tooth dentin.[18] Fernández-Escudero et al.[18] investigated the concentration of trace elements in human coronal dentin with age using inductively coupled plasma-mass spectrometry. They found that 12 elements, namely Sr, Mg, S (sulfur), K, Zn, Ba (barium), B, Co, V, Li (lithium), Sn (tin), and Pb, had a significant correlation with age. Out of these 12 elements, Li, Sn, and Pb have been considered potentially toxic to the body. They also suggested that the increase in concentration of Pb with age may be due to its tendency to be attracted to collagen fibers.[18] Skalnaya et al. have reported that Li and Sn concentrations increase with age in human hair also.[19] Li, though used in the treatment of mood disorders, has been found to increase the risk of hypothyroidism, hyperparathyroidism, reduced urinary concentration, and weight gain. Sn intoxication has been seen to affect the metabolism of other elements in human tissues like Fe, Cu, and Zn in human tissues. In addition, Pb has been considered an environmental pollutant and increase in its concentration may also inversely affect the concentrations of Fe, Cu, and Zn in dentin. Mg has been regarded as a calcium antagonist and regulates many cell functions. Its deficiency may result in several pathological conditions.[20] Previous studies indicate a positive correlation between decreased levels of Mg, periodontitis, and heart disease.[21] Boron (B) has been actively derived from food and water contaminated with borate-containing fertilizers. It has been reported to have favorable effects in thyroid and lipid metabolism and in obesity.[22] Cariostatic effects of boron have also been reported in the literature.[23] Strontium is known to accumulate in bones and result in hypocalcemia.[15] It has been reported to have anticariogenic effects by substitution of calcium in the hydroxyapatite crystals.[24] Strontium is considered as an essential trace element.[24] Strontium accumulates in the bones and its excessive deposition can result in disturbances in metabolism and mineralization of bones which in turn results in loss of calcium from bones resulting in hypocalcemia.[15] It is also believed to possess anticariogenic properties.[25] It can be derived from plants and animals and is stored in human bones and teeth.[26] Recent studies have reported that strontium is essential for bone and teeth health and that its deficiency leads to defective mineralization of bones and teeth. Furthermore, a supplement of strontium, strontium ranelate, has been shown to be effective in osteoporosis among women by decreasing bone resorption and increasing osteoblastic activity.[27] Boron deposits in teeth in the form of calcium borate instead of calcium phosphate and helps in making them caries resistant.[26] It can be derived from food and water esp. contaminated with borate-containing fertilizers.[22] It improves lipid metabolism and is helpful in obesity and thyroid metabolism.[23] Barium reportedly replaces calcium in dentin hydroxyapatite, but its excessive consumption may lead to acute intoxication.[15] Barium accumulates largely in bone, but small amounts may also be found in skin, fat, and muscular tissues.[26] Furthermore, its accumulation in bones and teeth increases with age and is also found to have some caries-reducing properties.[18,26,28]

Anticariogenic effect of trace elements

Fluoride

The anticariogenic potential of fluoride has been well known in dentistry for many decades. The milestone shoe leather survey by Petersen et al. in the early 1970s paved the way for further studies on the effect of fluoride on teeth. The caries preventive effects of fluoride have been so well-documented that most toothpaste today come with fluoride incorporated in them and so much so that the World Health Organization recommends community water fluoridation in areas with high caries prevalence.[29] Fluorine in the form of fluoride (F⁻) possesses the potential to replace the hydroxyl ion (OH⁻) in the hydroxyapatite crystals of dental enamel. The newly formed fluorohydroxyapatite is less soluble than hydroxyapatite and also more acid resistant. Thus, fluorohydroxyapatite has superior caries and dental erosion resistance than hydroxyapatite. It has been speculated that this substitution of OH⁻ with F⁻ stabilizes the crystal structure of the hydroxyapatite.[30,31] It is also believed that fluoride hastens post-eruptive maturation and helps in the remineralizing incipient carious lesions.[32]

Molybdenum

Molybdenum has been reported to have a cariostatic effect. Adler and Straub[33] in their study reported decreased prevalence of dental caries in Hungarian children residing in areas with molybdenum content in drinking water as compared to those living in areas without the presence of any molybdenum in water.[34] Hewat and Eastcott[34] also reported a lower prevalence of dental caries in children that consumed vegetables derived from areas with soil containing molybdenum as opposed to children living in a nearby area which did not have any molybdenum content in its soil.[34]

Strontium

Increased strontium ion (Sr²⁺) concentrations in dental enamel have also been reported to have an anticariogenic effect. Its affinity to exchange calcium makes teeth somewhat caries protected by replacing the calcium in hydroxyapatite crystals.[25] Deficiency of strontium may lead to defective mineralization of bones and teeth.[27] Qamar et al. reported that the strontium content in teeth decreases with age.[1]

Lithium

Lithium has been reported to have anticariogenic effect on teeth. It is also used in treatment of bipolar mental disorder.[1,35]

Trace elements promoting dental caries

Selenium

Selenium has been found to increase the incidence of caries in humans. Previous studies have found an increased incidence of dental caries in children with selenium present in their urine.[1,32] It is believed that selenium induces structural changes in the dentin of teeth.[36]

Copper

Copper is considered to be a caries promoting trace element.[37,38] Increased occurrence of caries has been found to be associated with the presence of copper in food and water.[39]

Cadmium

Cadmium has been linked to increased prevalence of dental caries in teeth.[38] Despite that, some studies claim that cadmium when incorporated in teeth postdevelopment does not have caries promoting activity but while not excluding previous fluoride exposure.[40]

Lead

It is believed that lead ions substitute calcium and calcium and phosphorus in the crystals of bone minerals causing hypercalcemia and hyperphosphatemia.[41] It is considered as a caries promoting element. A direct relationship has been found between enamel hypoplasia and lead exposure in children.[42] Needleman et al. in their study found an increased incidence in deciduous teeth of American suburban children exposed to lead.[43] Furthermore, a direct link has been found between development of early childhood caries in children and lead exposure.[44]

CONCLUSION

Many trace elements are required for proper development of the body and our teeth. They may play a vital role in the development of teeth, including that in amelogenesis as well as dentinogenesis. Further studies should be done in this field to accurately determine the role of various trace elements on enamel and dentin formation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.