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Chemistry Of Hydroxyapatite By Dania Al-zubaidyEnamel Defects • May be of genetic and/or environmental origins • Disturbances in initial matrix deposition result in hypoplasia : manifesting as pits & grooves on enamel surface • Defects in enamel maturation result in hypomineralisation / hypomaturation : largely intact surface but opaque rather than translucent appearance and softWhat is Hydroxyapatite? • The main inorganic component found in dental enamel is HAP which is a mineral composed of calcium and phosphate ions. HAP crystals are highly organized to form compact enamel rods which extend from the dentin enamel junction to the outer surface of tooth enamel which are surrounded by enamel interrod . • The formula is : Ca5(PO 4 3OH) • The OH ion can be replaced by fluoride or chloride, producing fluorapatite or chlorapatite.Definitions: The term "hypoplastic" means less quantity of enamel, and the term "hypomineralised" means less mineralised tissue. • Enamel Hypoplasia= enamel hypoplasia is a quantitative defect of the teeth, which results in small or irregular shaped teeth. It occurs in initial matrix deposition. • Enamel Hypomineralisation=Enamel hypomineralisation is a qualitative defect, with reduced mineralisation resulting in more opaque enamel, creamy/white or have yellow/brown discolourations, the opacities may be diffused . This occurs when there is defects during enamel maturation. -The most common hypomineralisation defect seen in children is Molar incisor hypomineralisation (MIH), which affects the first permanent molar teeth and often the permanent incisors.How is surface enamel different from core enamel? • Surface enamel differs both physically and chemically from underlying core enamel as it is harder, less porous and less solubleCharacteristics of the enamel:Amelogenesis Imperfecta • Inherited defect of dental enamel, affects quality and/or quantity of enamel : • It is a x-linked disorder. • DEJ defects result in an enamel layer that shears easily • Secretory stage defects result in insufficient crystal elongation & leave enamel layer very thin & disorganised • Maturation stage defects i.e deficient matrix degradation, produce thick but soft enamel • Current estimates suggest that FAM83H, FAM20A, ENam & AMELX mutations account for 60% of disease burden • Affected male member of family with X-linked AI presents as brown-mottled hypo mature enamel • normal and hypomature enamel (have 2 X’s, chromosome masking)nating areas ofEnvironmental Defects of AI Frequently Relate T o… • Infectious diseases of childhood such as measles occurring during tooth development • Fluorosis as a result of prolonged 5ppmstion of fluoride in excess of • Incorporation of ingested supplements into mineralised matrix eg tetracycline • Dietary deficiencies Enamel PH during the maturation stage • During enamel maturation pH oscillates between acidic & neutral • For every unit cell of HAP deposition 8H+ ions are released —> acidification of environment • Carbonic anhydrase II expression by smooth stage and continues through maturation stage - this enzyme produces HCO3 to neutralise the occurs while ruffle ended ameloblasts produce an extracellular pH of 5-6. • Ameloblasts pump H+ generated by Ca II into enamel space using vacuolar H+-ATPase and membranetransporter relocates to the basolateral Chemical Composition of the Mineral Phase of Enamel • Bundles of >1000 crystallites form an enamel rod • Crystallites are long apatite crystals with a thickness of about 25nm extending from DEJ to enamel surface • Hydroxyapatite is represented by the stoichiometric formula : Ca10(PO4)6(OH)2 or Ca5(PO4)3OH • Apatite growth on crystal surfaces progresses in 2 discrete manners : • Rapid growth in epitaxial direction (along c-axis) which gives rise to elongated hexagonal crystals • Thickening on side prism planes occurs at slow rate by successively accreting one or several unit-cell-thick layers Substitutions • F- for OH- : stabilising effect - forms fluoroapatite which is less soluble to acid • F- is concentrated at enamel surface and falls dramatically toward tissue interior • caries prevention • CO32- for OH- or PO43- : has a destabilising effect • Mg2+ for Ca2+ : has a destabilising effect -This is very rare/limited, approx. up to 0.3% substitution • other ions such as Sodium or Lead can also occasionally substitute for Ca2+ • Fluoride ion fits more closely within calcium triangle : high charge density & symmetry of fluoride ion reduces lattice energy • Such defects & substitutions do have profound effect on behaviour of apatite, especially with regard to solubility at low pH • The youngest secretory enamel is the most soluble & the solubility decreases w/ advancing developmental stagesAcid Etching • Limited acid treatment of enamel generates ‘rough’ surface : • dependant on nature & conc of acid • etching pattern depends on orientation of crystals • Interlocking enhances bonding with material used in restorations - e.g. 37% phosphoric acid is in ESPE scotchbond. • Crystal formation is a slow process usually involving different intermediates i.e the structural arrangement and stoichiometry of formed crystall solid precipitated is different from one in ultimately • Several different forms of calcium phosphate mineral can be present in enamel & ostacalcium phosphate crystals are thought to be precursor to ultimately formed substituted HAP crystallitesEnamel Formation 1.Secretory ameloblasts (Tomes’ process) & DEJ delineate/indicate the position of the enamel space 2.frameworkamelogenin assembles, forming supramolecular structural 3.crystal nucleation by pre-existing dentine or non-amelogenin matrix molecules 4.Control of crystal growth, morphology & orientation by matrix i.e amelogenin nanospheres 5.Cessation of initial crystallite growth by degradation of matrix ; and final maturation involves rapid crystal growth concomitant with controlled protein processing, degradation & lossHow is acidity neutralised in the enamel maturation stage? • A- Carbonic anhydrase II expression by smooth ended ameloblasts starts , producing HCO3- to neutralise the acidity generated • B- ruffle ended ameloblasts secreting carbonic anhydrase which produces HCO3-. • C- It is not neutralised • D- along the c-axisHow is acidity neutralised in the enamel maturation stage? • A- Carbonic anhydrase II expression by smooth ended ameloblasts starts , producing HCO3- to neutralise the acidity generated • B- ruffle ended ameloblasts secreting carbonic anhydrase which produces HCO3-. • C- It is not neutralised • D- along the c-axisWhich of these statements is correct? • A- CO32- substitutes for OH-, or PO43- and Mg2+ substitutes for Ca2+ =has a destabilising effect. • B-Mg- substitutes for OH- or PO43, and, CO32+ substitutes for Ca2+ =has a destabilising effect. • C-CO32- substitutes for OH- or PO43- and Mg2+ substitutes for Ca2+ =has a stabilising effect.Which of these statements is correct? • A- CO32- substitutes for OH- or PO43- and Mg2+ substitutes for Ca2+ has a destabilising effect. • B-Mg- substitutes for OH- or PO43, and, CO32+ substitutes for Ca2+ has a destabilising effect. • C-CO32- substitutes for OH- or PO43- and Mg2+ substitutes for Ca2+ has a stabilising effect.