Calcium Phosphates in Dentistry – Condensed Exam Notes

Calcium & Phosphorus in the Body

  • ≈99%\approx 99\% of body Ca in hard tissues (bone, dentine, cementum, enamel); ≈1%\approx 1\% in extracellular fluids
  • Saliva is naturally supersaturated with Ca2+Ca^{2+} and PO43−PO_4^{3-}
  • Optimal Ca absorption needs Vitamin D; low serum Ca triggers bone demineralisation (osteoporosis)
  • Hypocalcaemia → delayed tooth eruption, poor enamel/dentine calcification
  • Kidney stones: calcium phosphate / calcium oxalate (often with hypercalciuria)

Regulation of Ca & P

  • Low serum Ca2+Ca^{2+} → Parathyroid Hormone (PTH) release → conversion to calcitriol (active Vit D)
  • Calcitriol ↑ intestinal absorption of Ca2+Ca^{2+} & PO43−PO_4^{3-}; PTH + calcitriol mobilise bone minerals
  • Phosphorus reduces renal Ca excretion; ~85%85\% stored in bone & teeth

Calcium Phosphate Phases

  • Apatite type: generic Ca<em>10(PO</em>4)<em>6X</em>2Ca<em>{10}(PO</em>4)<em>6X</em>2 (X = OH−OH^-, F−F^-, Cl−Cl^-)
    • Hydroxyapatite (HAP) – main mineral of enamel/dentine
    • Fluorapatite – more acid-resistant
  • Glaserite type: includes Tricalcium Phosphate (TCP) Ca<em>3(PO</em>4)2Ca<em>3(PO</em>4)_2 (major in milk)
  • Ca–PO\textsubscript{4} sheet compounds: Brushite CaHPO<em>4⋅2H</em>2OCaHPO<em>4\cdot2H</em>2O (kidney stones, calculus)

Calcium Phosphates in the Oral Environment

  • Saliva supersaturation promotes enamel remineralisation
  • Hard-tissue mass of vertebrates ≈1–2%1–2\% body weight in calcium phosphates

Dental Calculus

  • Inorganic: 40–60%40–60\% Ca-phosphates (HAP, whitlockite, octacalcium phosphate, brushite)
  • Organic: bacteria, yeast, salivary proteins, gingival crevicular fluid, diet remnants

Solubility & pH

  • At pH>5: stability order → CaF2CaF_2 (least soluble) > HAP > Dicalcium phosphate anhydrous (DCPA) > Octacalcium phosphate (OCP)
  • pH shifts drive phase conversions and de/re-mineralisation

Amorphous Calcium Phosphate (ACP)

  • Formula Ca<em>3(PO</em>4)<em>2 nH</em>2OCa<em>3(PO</em>4)<em>2\,nH</em>2O; lacks crystal lattice; transient biomineral phase
  • Quickly transforms to crystalline apatite unless stabilised (salivary statherin, CPP-ACP products)
  • Dental use: hypersensitivity relief, remineralising creams/pastes

Crystallisation & Precursors

  • Crystallisation passes through precursor phases: ACP → Octacalcium phosphate (OCP) → HAP (thermodynamically stable)
  • Epitaxy: organic matrices guide heterogenous nucleation; exploited on bone grafts/Ti implants coated with OCP

Hydroxyapatite Structure

  • Unit cell: Ca<em>10(PO</em>4)<em>6(OH)</em>2Ca<em>{10}(PO</em>4)<em>6(OH)</em>2; hexagonal Ca ring enclosing PO43−PO_4^{3-}
  • Ion substitutions: CO32−,Mg2+,Na+,F−,Cl−,Pb2+,Zn2+CO_3^{2-}, Mg^{2+}, Na^{+}, F^{-}, Cl^{-}, Pb^{2+}, Zn^{2+} etc.—modify solubility & hardness
  • Crystal domains: interior, surface, hydration shell (water layer)

Enamel vs Dentine

  • Enamel: 95–96%95–96\% mineral (vol 88–90 %), minimal protein/water; hardest tissue but brittle; modulus high, tensile strength low
  • Dentine: ~40%40\% organic (mainly collagen); collagen rods reinforce HAP → resilience & shock absorption
  • Enamel crystallites: ∼35 nm\sim 35\,\text{nm} wide, ∼10 nm\sim10\,\text{nm} thick; extend full tissue width; core richer in Mg2+Mg^{2+} & CO32−CO_3^{2-} (more soluble)
  • Dentine crystallites smaller (≈26×68 nm26\times68\,\text{nm})

Amelogenesis & Amelogenin

  • Stage 1: secretory enamel ≈30%30\% mineral; ameloblasts secrete matrix (amelogenins, enamelins)
  • Stage 2: maturation—protein & water removed, mineral influx → ≈96%\approx 96\% HAP
  • Amelogenins: small globular proteins; self-assemble into nanospheres → chains → microribbons → scaffold
    • Negatively charged C-termini attract Ca2+Ca^{2+} from Ca-phosphate solution, nucleating crystals along ribbons
    • Proteolytic cleavage removes scaffold as enamel matures

Clinical / Product Applications

  • CPP-ACP pastes maintain supersaturated saliva, aiding remineralisation & sensitivity control
  • OCP/HAP coatings improve osseointegration of implants and fill enamel defects
  • DCPD (brushite) used as abrasive in toothpastes

Key Recall Points

  • Generic apatite formula: Ca<em>10(PO</em>4)<em>6X</em>2Ca<em>{10}(PO</em>4)<em>6X</em>2
  • Precursor to HAP: Octacalcium phosphate (OCP)
  • Salivary protein maintaining ACP: statherin
  • Three crystal layers: interior, surface, hydration shell
  • Epitaxy: crystal growth induced on solid (organic) substrate