21.1 Caries Pathogenesis and the Stephan Curve

Caries Pathogenesis & The Demineralisation-Remineralisation Equilibrium

Dental caries is defined as a biofilm-mediated, diet-modulated, multifactorial, non-communicable dynamic disease resulting in net mineral loss from dental hard tissues. The modern scientific paradigm has transitioned entirely away from outdated specific infection hypotheses (which blamed solitary pathogens such as Streptococcus mutans) toward the Ecological Plaque Hypothesis (Marsh, 1994).

                                Ecological Plaque Hypothesis
                                             │
                   ┌─────────────────────────┴─────────────────────────┐
                   ▼                                                   ▼
       Homeostatic Biofilm                                   Dysbiotic Biofilm
  • Dynamic balance (pH neutral)                       • Frequent fermentable sugars
  • Low numbers of aciduric taxa                       • Sustained low pH microenvironment
  • Net mineral remineralisation                       • Selection for acidogenic/aciduric taxa
    (Salivary Ca²⁺, PO₄³⁻, F⁻)                           (S. mutans, Lactobacilli, Bifidobacteria)
                                                       • Net mineral demineralisation

The Physicochemical Dynamic Equilibrium

Dental hard tissues exist in a continuous thermodynamic equilibrium with the surrounding oral fluid (saliva and dental plaque biofilm fluid). Hydroxyapatite, the primary mineral component of enamel ($96%$ by weight), dentine ($70%$ by weight), and cementum ($45%$ by weight), undergoes reversible dissociation:

$\mathrm{Ca_{10}(PO_4)_6(OH)_2 \rightleftharpoons 10\,Ca^{2+} + 6\,PO_4^{3-} + 2\,OH^-}$

1. Demineralisation Drive:

   • When acidogenic microorganisms metabolise dietary fermentable carbohydrates (primarily sucrose, glucose, and fructose), they excrete organic acids—predominantly lactic acid ($pK_a = 3.86$), alongside acetic, propionic, and formic acids.
   • The liberated hydrogen ions ($H^+$) react with free phosphate and hydroxyl ions in the plaque fluid, forming hydrogen phosphate ($HPO_4^{2-}$), dihydrogen phosphate ($H_2PO_4^-$), and water ($H_2O$).
   • By depleting free $PO_4^{3-}$ and $OH^-$, the ion activity product (IAP) drops below the solubility product constant ($K_{sp}$) of hydroxyapatite. To restore thermodynamic equilibrium, calcium and phosphate ions dissolve out of the tooth crystal lattice.

2. Remineralisation Drive:

   • As the pH rises back toward neutrality, saliva—which is supersaturated with calcium and phosphate ions—drives mineral precipitation back into the porous apatite lattice.
   • In the presence of trace fluoride ions ($F^-$), remineralisation is accelerated through the formation of fluorapatite or fluorhydroxyapatite:

$\mathrm{10\,Ca^{2+} + 6\,PO_4^{3-} + 2\,F^- \rightleftharpoons Ca_{10}(PO_4)_6F_2}$

Fluorapatite incorporates into the crystal lattice, replacing hydroxyl groups. Because fluorapatite has a substantially lower solubility product constant than pure hydroxyapatite, it exhibits superior thermodynamic stability against subsequent acid challenges.

Stephan Curve Dynamics & Salivary Buffering

First documented by Robert Stephan in 1944, the Stephan curve describes the rapid drop and subsequent gradual recovery of dental plaque pH following exposure to fermentable carbohydrates.

  Plaque pH
    7.0 ─┐  Baseline resting pH (~6.8)
         │   ╲
    6.5 ─┤    ╲                       Dentine/Cementum Critical pH (6.2–6.7)
         │     ╲ ─────────────────────────────────────────────────────────────
    6.0 ─┤      ╲
         │       ╲                    Enamel Critical pH (5.5)
    5.5 ─┤- - - - ╲ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    5.0 ─┤         ╲       ▲ Clearance & Salivary Bicarbonate Buffering
         │          ╲     ╱
    4.5 ─┤           └───┘ Minimum pH reached in 2–5 min (Demineralisation Zone)
    4.0 ─┴──────────┬───────────┬───────────┬───────────┬───────────┬────────►
         0          5          10          20          30          40   Time (min)
                    Sugar Rinse

Critical pH Discrepancy: Enamel vs Dentine/Cementum

The critical pH is the specific environmental pH at which the plaque fluid is exactly saturated with respect to the dental hard tissue mineral. At any pH below this threshold, the fluid is undersaturated, driving passive mineral dissolution:

• Enamel Critical pH: Approximately 5.5 (varies slightly from 5.2 to 5.7 depending on the local concentrations of calcium and phosphate in saliva).
• Dentine and Cementum Critical pH: Approximately 6.2 to 6.7.

[!IMPORTANT] Clinical Examination Milestone — Root Caries Susceptibility: Dentine and cementum begin to dissolve at a much higher pH (6.2–6.7) than enamel (5.5). This physiological vulnerability stems from dentine's lower inorganic mineral density ($70%$ vs $96%$), smaller apatite crystallites (providing greater relative surface area for acid attack), and higher structural carbonate content ($4-5%$ vs $2%$ in enamel). Carbonate ions substitute for phosphate or hydroxyl groups, distorting the crystal lattice and rendering it drastically more acid-soluble. Consequently, elderly patients or periodontally compromised patients with gingival recession develop rapid root surface demineralisation under dietary conditions that leave coronal enamel unaffected.

Kinetic Phases of the Stephan Curve

1. Acidogenesis Phase (0 to 5 minutes): Unstimulated resting plaque pH sits around 6.5–7.0. Within 2 to 5 minutes of exposing plaque to fermentable carbohydrates (especially sucrose), acidogenic bacteria rapidly transport and metabolise the sugars via glycolysis, precipitating a sharp pH drop below the critical threshold.
2. Trough Phase (5 to 10 minutes): Plaque pH reaches its minimum nadir (often falling to pH 4.0–4.5), producing rapid outward flux of calcium and phosphate.
3. Recovery Phase (20 to 40 minutes): The pH slowly returns to resting levels over 20 to 40 minutes (extending up to 60+ minutes in xerostomic patients). Recovery is mediated by:
   • Salivary Clearance: Mechanical washing away of fermentable carbohydrates and unattached organic acids.
   • Carbonic Acid–Bicarbonate Buffer System: Stimulated parotid saliva increases its bicarbonate ($HCO_3^-$) concentration from ~5 mmol/L (unstimulated) to over 30–60 mmol/L, neutralising $H^+$:

$\mathrm{H^+ + HCO_3^- \rightleftharpoons H_2CO_3 \rightleftharpoons H_2O + CO_2 \uparrow}$

• Basic Peptides & Ammonia Production: Bacterial metabolism of salivary urea (via urease) and arginine-rich peptides (sialin, via the arginine deiminase pathway) generates alkaline ammonia ($NH_3 + H^+ \rightarrow NH_4^+$), aiding pH restoration.

Dietary Frequency vs Total Sugar Intake

The Vipeholm and Hopewood House clinical trials proved that the frequency of fermentable carbohydrate intake is far more damaging than the total quantity consumed. Frequent snacking keeps the plaque pH below 5.5 for cumulative hours each day, preventing salivary remineralisation and causing cavitated breakdown.