Biominerals in restorative dentistry

Review Article

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Biominerals in restorative dentistry Neeta Shetty, M. Kundabala Departments of Conservative Dentistry and Endodontics, Manipal College of Dental Science, Manipal University, Mangalore, Karnataka, India

Address for correspondence: Dr. Neeta Shetty, E‑mail: [email protected]

ABSTRACT Restorative treatment strategies are being developed to repair and replace lost tooth structures and surrounding bone. The teeth under goes a constant cycle of demineralization and remineralization, but this natural remineralization process is inadequate to prevent progression of dental caries. Hence there is a need to supplement the tooth with a biomaterial which is bio inert or bioactive to remineralize, repair or regenerate the tissues of tooth. Calcium hydroxide is considered the gold standard material for repair of dentin, which is presently being replaced by materials with superior properties such as mineral trioxide aggregate. Biomaterials such as calcium phosphate cements are been advocated as bone substitute material because of properties such as biocompatibility, osteoconductivity and moldability. This review deals with the physiochemical properties of some of the biomineral based biomaterials which are currently used for repair, replacement or regeneration of hard tissues of teeth and bone.

CLINICAL RELEVANCE TO INTERDISCIPLINARY DENTISTRY • • •

ynthetic biomaterials containing biominerals are used in dentistry to repair and regenerate hard tissues of the S teeth and bone. Biomaterials containing biominerals are routinely used by specialist from various fields of dentistry such as restorative dentistry, periodontics and oral surgery. Biomineral based biomaterials are used for direct and indirect pulp capping procedures, as an intracanal medicament in root canals, root perforation repair, periapical surgeries, repair of bony defects.

Key words: Biomaterials, biominerals, calcium hydroxide, mineral trioxide aggregate, remineralization

B

INTRODUCTION

ones and teeth are biocomposites that require controlled mineral deposition during their self‑assembly to form tissues with unique mechanical properties.[1] Biomineral refers not only to the minerals produced by organisms, but also to the fact that almost all of these mineralized products are composite materials comprised of both mineral and organic components. [2] Biomineralization is the process by which living forms influence the precipitation of minerals which is a natural evolution process that creates heterogeneous accumulations, composed of organic and inorganic compound products that are created and maintained during the life by dynamic metabolism. It is a physical process Access this article online Quick Response Code: Website: www.jidonline.com

DOI: 10.4103/2229-5194.126858

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or a guided biological process and there are proteins that accelerate or inhibit biomineral formation such as Osteopontin, that can turn on mineral formation or inhibit it.[3,4] Biominerals such as calcium and phosphate synthetically produced or obtained from natural sources thus has an important function in the preventing demineralization and encouraging remineralization of hard tissues of the tooth along with the preservation and maintenance of the health of the pulp. Remineralization is defined as the process whereby calcium and phosphate ions are supplied from a source external to the tooth to promote ion deposition into crystal voids in demineralized enamel, to produce net mineral gain.[5] The present concept of remineralization is that the fluoride ions promote the formation of fluorapatite in enamel in the presence of calcium and phosphate ions produced during enamel demineralization by plaque bacterial organic acids, but the ready availability of calcium and phosphate from saliva and plaque could be a hindering factor in this process.[6] Hence the need for supplementation of these minerals may be a necessity for remineralization of enamel.

Journal of Interdisciplinary Dentistry / May-Aug 2013 / Vol-3 / Issue-2


Shetty and Kundabala: Biominerals in restorative dentistry

Application of calcium and phosphate ions for remineralization has not been successful due to the low solubility of calcium phosphates; insoluble calcium phosphates do not localize effectively at the tooth surface and require acid for solubility to produce ions capable of diffusing into enamel subsurface lesions. Salivary remineralization of enamel promoted by topical fluoride has been shown to give rise to predominantly surface remineralization which does not improve the esthetics and structural properties of the deeper lesion.[5,7] Hence the focus is on those biomaterials, which will promote subsurface mineral gain rather than deposition only in the surface layer. At present, there are technologies developed to stabilize the amorphous calcium phosphate (ACP). The therapeutic effect of materials such as calcium hydroxide Ca(OH)2 and mineral trioxide aggregate (MTA) may be due to their extraction of growth factors from the dentin matrix.[8] The release of growth factors due to dentin injury acts as a signal to stimulate the dental pulp for cell differentiation, proliferation, chemotaxis, extracellular remodeling and the secretion of tertiary dentin matrix. Growth factors, mainly transforming growth factor beta‑1, are important for the stimulation of dentin matrix secretion.[8,9] Biominerals play an important role in remineralization of enamel, regeneration of dentin. Biomineral substrates are also implicated to play major roles in bone development and regeneration [Table 1].[10] Calcium phosphate, hydroxyapatite (HA), calcium silicate, calcium carbonate and calcium sulfate, etc., are important materials and have been widely used in biomedical fields such as bone cements. At present, calcium‑based inorganic biodegradable nanomaterials which have excellent osteoconductivity, biocompatibility, bioactivity, biodegradability, chemical stability and mechanical strength are been experimentally used as bone graft materials.[11]

CLASSIFICATION OF BIOMINERAL BASED BIOMATERIALS Biomaterials for remineralization and regeneration tooth can be classified into [Figure 1].

Biominerals based biomaterials for remineralization of enamel Calcium phosphate In 1920’s Albee reported the use of calcium phosphate known as “triple calcium phosphate” in a bony defect to promote osteogensis or new bone formation.[12] Further research lead to the development of a calcium‑ and‑phosphate‑containing glass ceramic, known as Bioglass, which chemically bonded to the bone.[13] In 1987, Brown and Chow described self‑setting calcium phosphate cement (CPC) containing tetracalcium phosphate (TTCP) and either dicalcium phosphate anhydrous (DCPA) or dicalcium phosphate dihydrate which when mixed with water hardened into cement with a chemical composition and crystal structures similar to tooth and bone.[14] Tricalcium phosphate (TCP) has been added to oral hygiene products to remineralize white spot lesions. A study by Yoshiba et al. suggested that α‑TCP containing Ca(OH)2, induces consistent hard tissue formation, without excessive destruction of underlying pulp tissue and can be used as a pulp capping agent.[15] β‑TCP, is the most commonly used as synthetic bone substitutes. Calcium phosphate fillers are recently been incorporated in composite resins and shown to released calcium (Ca) and phosphate (PO4) ions to supersaturated levels for apatite precipitation and effectively remineralized tooth lesions in vitro.[16] ACP It is the initial solid phase that precipitates from a highly supersaturated calcium phosphate solution and can convert readily to stable crystalline phases such as octacalcium phosphate or apatite products. It is osteoconductive, bioactive, biodegradable, non – cytotoxic.[17] ACP acts as a precursor to bioapatite, used as filler in glass ionomer cements. [18] ACP has been added to composites as fillers which release Ca and PO ions into saliva, which are deposited on tooth structures as apatite minerals. Furthermore, nanoparticles of calcium phosphates have been synthesized and incorporated into dental resins. The high surface area of the nanoparticles, along with

Table 1: Clinical restorative application of biomineral based biomaterials Clinical application of biomaterials Calcium phosphate cement

✓

Dentinogensis -

✓

Apexification -

✓

Repair of resorption defects -

β-tricalcium phosphate Bioactive glass

✓

-

✓

-

✓

-

✓

-

-

-

-

-

Synthetic hydroxyapatite

✓ -

-

✓ -

-

-

-

✓

✓

✓

✓

Portland cement

-

✓

✓ -

✓ -

✓

✓

✓

-

Mineral trioxide aggregate

-

✓

✓

✓

✓

✓

✓

✓

✓

✓

✓

✓

✓ -

Calcium hydroxide

Biodentine

Enamel remineralization

Cement-ogensis


Perforation repair

Repair of bony defects ✓

✓

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Figure 1: Classification of biominerals based biomaterials

strong reinforcement fillers, resulted in composites with stress‑bearing and Ca and PO4 releasing capabilities.[19,20] Various technologies developed to stabilize calcium phosphate are:[21] • Casein phosphopeptide (CPP) stabilized ACP • Unstabilized ACP (EnamelonTM) • NovaMinTM technology. CPP‑ACP CPP‑ACP is ACP complexed with the milk protein, CPP. On acid challenge, the attached CPP‑ACP releases calcium and phosphate ions, thus maintaining a supersaturated mineral environment and thereby reducing demineralization and enhancing remineralization of enamel.[22,23] The rationale for the use of ACP‑CPP is that the remineralized enamel is generally more resistant to decalcification than untreated enamel.[24] Therefore CPP‑ACP is being incorporated into chewing gums, toothpaste, mouth rinses etc., for preventive treatment of dental caries. The ability to stabilize calcium phosphate and thereby enhance mineral solubility and bioavailability, confers upon the CPP the potential to be biological delivery vehicles for calcium and phosphate.[25,26] The acid resistance of enamel exposed to CPP‑ACP is increased by the addition of fluoride.[27] Since CPP‑ACP are natural derivatives of milk and readily available in dairy products, they could be added as a food additive and aid in prevention of caries.[28]

developed and patented by NovaMin Technology, Inc., for the treatment of hypersensitivity by physical occlusion of dentinal tubules, when it is incorporated into a dentifrice, these particles are deposited onto dentin surfaces and mechanically occlude dentinal tubules.[29] Bioglass reacts when it comes in contact with water, saliva or other body fluids. This reaction releases calcium, phosphorus, sodium and silicon ions, which result in the formation of crystalline HCA layer that is structurally and chemically similar to natural tooth mineral.[30] The material behaves as a biomimetic mineralizer. Bioactive glass when used as prophy powder could induce immediate remineralization of dentin by its ability to precipitate HA.[31] Synthetic hydroxapatite Yamagishi et al. developed a white crystalline paste of modified HA, which chemically and structurally resembles natural enamel and used it to repair an early caries lesion.[32] Dental enamel‑like structure of HA was achieved through a solution mediated solid‑state conversion process with organic phosphate surfactant and gelatin as the mediating agent. HA by itself can be prepared artificially using a variety of methods such as precipitation reactions and sol‑gel synthesis. However, reproducing various parameters such as solubility, super saturation and energetics is challenging.[33] Biomineral based biomaterial for repair and replacement of dentin

Bioactive glass

Ca(OH)2

Bioactive glass containing calcium sodium phosphosilicate originally developed as bone‑regenerative material which was reactive when exposed to body fluids and deposited hydroxycarbonate apatite (HCA). NovaMin in oral care was

Ca(OH)2 was introduced to the dental profession in 1921, Hermann demonstrated the formation of dentinal bridge in an exposed pulpal surface and it is now considered the “gold standard” for direct pulp capping agents.[34] The

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calcium oxide (CaO) known as “quicklime” contacts water, the following reaction occurs: CaO + H2 —> Ca(OH)2. CH is a white odorless powder with a molecular weight of 74.08. The material is chemically classified as a strong base with a high pH (12.5) and is only slightly soluble in water with a solubility of 1.2 g/l, at a temperature of 25˚C.[35] The of dissociating of Ca(OH)2 into calcium and hydroxyl ions results in increased pH locally. Ca(OH)2 high pH causes irritation of the pulp tissue, which stimulates repair of dentin by the release of bioactive molecules such as Bone Morphogenic Protein and Transforming Growth Factor‑Beta One.[34,36] The induction of mineralization seems to be the result of the highly alkaline pH of CH and its antimicrobial activity is due to the hydroxide ions which promote enzymatic inhibition of microorganisms.[37] Draw backs of CH as remineralization agent are:[38] • Inadequate strength • Long‑term solubility • Lack of chemical and mechanical adhesion to the surrounding hard tissues • Accelerated degradation after being acid etched during bonding procedures • Tunnel formation seen in dentinal bridge formed. Tricalcium silicate (Ca3SiO5) Ca3SiO5 is a major component of MTA and has been used on its own or with additives. The Ca3SiO5 exhibited adequate physical properties and induces cell growth, differentiation and the deposition of HA on its surface. It can induce HA formation and dissolve slowly in simulated body fluids. Peng et al. in their study suggested that Ca3SiO5 can induce the proliferation and odontogenic differentiation of human dental pulp cells in vitro.[39] Portland cement (PC) PC is construction cement with great similarity to MTA. It is a fine powder composed of 65% lime, 20% silica, 10% alumina and ferric oxide and 5% other compounds. The setting reaction of PC is similar to that of MTA.[40] Part of the end product of the setting reaction is Ca(OH) 2 which is enclosed in the form of complex gels or crystalline substances.[41] PC has been reported to have intrinsic radiopacity values ranging from 0.86 to 2.02‑mm aluminum (Al), which is a drawback. [42] The primary differences between both (grey and white) types of MTA and PC is lack of potassium and the presence of bismuth oxide.[43] PC contains arsenic which is an impurity of limestone used to manufacture PC, but studies have shown that the arsenic level release is low and unable to cause toxic effects.[44] According to Camilleri et al. the commercial versions of MTA were shown to have broadly similar constitution to ordinary PC except for the addition of bismuth compounds and white MTA did not contain iron.[40] The disadvantage of PC is that it has lower Journal of Interdisciplinary Dentistry / May-Aug 2013 / Vol-3 / Issue-2

radio‑opacity and the main advantage is its very low cost. If PC is intended for clinical use it should be tested for any pollution effects by heavy metal ions, should be sieved to unique particle size and sterilized.[41]

MTA MTA was developed by Tirebinejad (1995) at Loma Linda University as a root‑end filling material. In 2002, white mineral trioxide aggregate (WMTA) was introduced to overcome esthetic concerns.[45] The MTA patent shows that it contains CaO and silicon (SiO). The major component is a mixture of dicalcium silicate, Ca3SiO5, tricalcium aluminate, tetracalcium aluminoferrite and trace amounts of SiO2, CaO, MgO, K2SO4 and Na2SO4. Grey mineral trioxide aggregate basically consists of dicalcium and Ca3SiO5 and bismuth oxide, whereas WMTA is primarily composed of Ca3SiO5 and bismuth oxide.[40] When MTA powder is mixed with water, Ca(OH)2 and calcium silicate hydrate are initially formed and eventually transform into a poorly crystallized and porous solid gel.[46] MTA powder is mixed with a vehicle such as sterile water in a 3:1 powder/liquid ratio. A moist cotton pellet is placed in direct contact with the material, which facilitates its setting. Upon hydration the material forms a colloidal gel that solidifies to a hard structure in 3‑4 h.[47] The initial pH of the mixed material is 10.2 which rise to 12.5 after 3 h, which is considered as the setting time for MTA. It is considered a bioactive material, it is hard tissue conductive, hard tissue inductive and has the potential to interact with the natural fluids present in tissues, also it is proven to be non‑mutagenic and non‑cytotoxic.[48,49] Compared to Ca(OH)2 cement, MTA has demonstrated a greater ability to maintain the integrity of pulp tissue. Histological evaluations of exposed pulp tissue from animals capped with MTA have shown the formation of a thicker dentinal bridge, with low inflammatory response, hyperemia and pulpal necrosis compared with Ca(OH)2 cement.[50,51] Analysis of clinical treatment by Mente et al. concluded that MTA appears to be more effective than Ca(OH)2 for maintaining long‑term pulp vitality after direct pulp capping.[52] Biodentin Biodentin is projected as dentin substitute by the manufacturers since it stimulates teriatary dentin formation. It is a calcium silicate – based restorative cement with dentin‑like mechanical properties.[53] The powder mainly contains tricalcium, dicalcium silicate, calcium carbonate and zirconium dioxide as contrast medium. The liquid consists of calcium chloride in aqueous solution with an admixture of polycarboxylate. The powder is dispensed in a capsule which is mixed with the liquid in a triturator for 30 s. Biodentine sets in approximately 10 min. The material can be applied directly in the restorative cavity with a spatula as a bulk dentin substitute without any 67

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conditioning treatment Ca(OH2) is formed during the setting of the cement.[54] Calcium enriched mixture In the study by Asgary et al., introduced new endodontic cement known as CEM which is alkaline cement which releases CH during and after setting. [55] The major components of the powder are 51.75% wt CaO, 9.53% wt SO3, 8.49% wt P2O5, 6.32% wt SiO2 respectively. When mixed with the water‑based solution, a bioactive calcium and phosphate enriched material forms. It has the advantage of shorter setting time (