3.2 Physics & Chemistry of Oral Biology: pH, Diffusion & Demineralization
Key Takeaways
- The critical pH for enamel hydroxyapatite is approximately 5.5, while root dentin and cementum demineralize at a higher pH of roughly 6.2 to 6.7
- Fluorapatite has a critical pH near 4.5, which is why fluoride exposure shifts the demineralization threshold rather than simply hardening enamel
- The Stephan curve shows salivary pH falling within 2 to 5 minutes of a sugar challenge and returning to baseline over roughly 20 to 40 minutes through salivary buffering and clearance
- Bicarbonate is the dominant salivary buffer at high flow rates, while phosphate dominates at low flow rates and urea contributes through ammonia release
- Dentin hypersensitivity is explained by the hydrodynamic theory, in which fluid movement in patent dentinal tubules deforms odontoblastic processes and A-delta fibers
Physics & Chemistry of Oral Biology: pH, Diffusion & Demineralization
Why this matters on the INBDE: FK2 carries 7% of examination items, and it is the science that makes caries, erosion, sensitivity, and material selection make sense. Items in this area rarely announce themselves as chemistry — they appear as a patient whose lesions are all on exposed root surfaces, or a patient whose reflux is dissolving palatal enamel.
Acid-Base Fundamentals in the Oral Cavity
pH is the negative logarithm of hydrogen ion concentration, so a drop of one pH unit is a tenfold increase in acidity. A plaque pH of 4.5 is ten times more acidic than 5.5 and one hundred times more acidic than 6.5. That logarithmic scale explains why small pH changes produce dramatic changes in mineral loss.
The Henderson-Hasselbalch relationship — pH equals pKa plus the log of the ratio of conjugate base to acid — governs three separate dental problems:
- Salivary buffering. The bicarbonate/carbonic acid pair (pKa about 6.1) resists pH change.
- Local anesthetic onset. Anesthetics are weak bases; the higher the pKa, the smaller the uncharged free-base fraction at tissue pH and the slower the onset. This is why inflamed, acidic tissue anesthetizes poorly.
- Drug absorption and ion trapping. Weak acids are absorbed better from an acidic stomach; weak bases from the more alkaline intestine.
Salivary buffering systems
| Buffer | Effective range | When it dominates |
|---|---|---|
| Bicarbonate | pKa ~6.1 | Stimulated (high) flow — bicarbonate concentration rises sharply with flow rate and provides most buffering capacity during and after eating |
| Phosphate | pKa ~6.8 | Unstimulated (low) flow — resting saliva; concentration falls as flow rises |
| Protein / urea | Broad | Urea is hydrolyzed by bacterial urease to ammonia, raising plaque pH; salivary proteins provide additional buffering and pellicle formation |
A patient with xerostomia loses all three simultaneously, which is why radiation- and medication-induced dry mouth produces rampant, rapidly progressing cervical caries.
Critical pH and the Demineralization Equilibrium
Enamel is roughly 96% inorganic by weight, essentially carbonated hydroxyapatite. Dissolution follows a solubility equilibrium: when saliva or plaque fluid becomes undersaturated with respect to a mineral phase, that phase dissolves; when it is supersaturated, mineral precipitates.
| Mineral / substrate | Approximate critical pH | Clinical meaning |
|---|---|---|
| Enamel hydroxyapatite | 5.5 | Standard threshold for coronal caries |
| Root dentin and cementum | 6.2 – 6.7 | Root surfaces demineralize at a less acidic pH, so exposed roots in an elderly or periodontally treated patient are at far greater risk from the same diet |
| Fluorapatite | ~4.5 | Fluoride substitution lowers the threshold, so a fluoride-rich surface tolerates acid challenges that would dissolve hydroxyapatite |
High-yield reasoning: a 78-year-old with generalized recession, three medications causing xerostomia, and a habit of sipping sweetened coffee has root caries risk that cannot be explained by the pH 5.5 number alone. Root surfaces are dissolving at every pH excursion below about 6.5.
Fluoride chemistry
Fluoride acts through three distinct mechanisms, and the exam expects all three:
- Inhibits demineralization by adsorbing to the crystal surface and reducing dissolution.
- Enhances remineralization by drawing calcium and phosphate back into the lattice, forming a more acid-resistant fluorapatite-like surface (the common ion effect shifts equilibrium toward precipitation).
- Inhibits bacterial metabolism — at low pH, HF crosses the bacterial membrane, dissociates intracellularly, and inhibits enolase in glycolysis and the proton-pumping ATPase.
The topical effect is primary for erupted teeth; the systemic (pre-eruptive) effect is real but secondary and carries fluorosis risk during enamel maturation.
The Stephan Curve
After a fermentable carbohydrate exposure, plaque pH:
- Falls steeply within 2 to 5 minutes as acidogenic bacteria (chiefly Streptococcus mutans and lactobacilli) ferment sugar to lactic, acetic, propionic, and formic acids.
- Reaches a minimum that may fall below the critical pH.
- Recovers over roughly 20 to 40 minutes through salivary buffering, clearance, and bacterial base production.
The area of the curve below critical pH — not the peak depth alone — determines net mineral loss. This is the quantitative reason that frequency of exposure matters more than total quantity. Six small sugar exposures spread across a day drop plaque pH below critical six times; the same sugar consumed at one sitting does so once.
A patient with reduced salivary flow shows a deeper trough and a much slower recovery limb, and a patient with a fluoride-treated surface tolerates the same trough with far less mineral loss.
Erosion vs Caries: Different Chemistry
| Dental caries | Erosion | |
|---|---|---|
| Acid source | Bacterial fermentation within plaque | Non-bacterial — dietary (citrus, carbonated drinks, wine), gastric (GERD, eating disorders), occupational |
| Location | Plaque stagnation areas: pits, fissures, interproximal, cervical | Smooth, plaque-free surfaces — palatal of maxillary anteriors in regurgitation, facial in dietary acid |
| Appearance | Opaque white spot, then cavitation | Broad, shallow, glossy concavities; restorations left standing proud of surrounding tooth |
| First countermeasure | Plaque control, fluoride, diet frequency | Remove the acid source, neutralize (rinse with water or bicarbonate), do not brush immediately, fluoride and sensitivity control |
Diffusion, Osmosis, and Fluid Movement
Diffusion moves solute down a concentration gradient; osmosis moves solvent across a semipermeable membrane toward higher solute concentration. Both operate constantly in the oral environment.
- Dentinal tubules are fluid-filled channels running from pulp to the dentinoenamel junction. They are wider and more numerous near the pulp, which is why deep preparations are more sensitive and more permeable than shallow ones.
- Hydrodynamic theory of dentin hypersensitivity: a stimulus — cold, air blast, evaporation, or a hypertonic sugar solution — causes rapid fluid movement within patent tubules, mechanically deforming odontoblastic processes and stimulating A-delta fibers in the pulp, producing sharp, brief pain. Note that hypertonic sugar draws fluid outward and hurts, exactly as cold does; heat drives fluid inward and typically hurts less.
- Desensitizing agents therefore work by occluding tubules (oxalates, calcium phosphates, varnishes, glass ionomer, bonding agents) or by depolarizing the nerve (potassium nitrate, which raises extracellular potassium and blocks repolarization).
- Smear layer occludes tubules after preparation; acid etching removes it, which increases permeability and postoperative sensitivity if the hybrid layer is not properly sealed.
Physical Properties That Drive Clinical Decisions
| Property | Definition | Clinical consequence |
|---|---|---|
| Coefficient of thermal expansion | Fractional dimensional change per degree | Composite and acrylic expand far more than tooth structure; mismatch drives percolation and microleakage at margins |
| Thermal conductivity / diffusivity | Rate of heat transfer | Metals conduct readily, so deep amalgam and gold restorations need a base or liner; composite and cement insulate |
| Galvanism | Current flow between dissimilar metals in an electrolyte | A new amalgam contacting an existing gold crown produces a sharp, transient shock |
| Surface tension and wetting (contact angle) | Tendency of a liquid to spread on a solid | Low contact angle equals good wetting — essential for sealants penetrating etched enamel and for impression materials recording detail. Hydrophilic (polyether, addition silicone with surfactant) materials wet moist tissue better |
| Viscosity, pseudoplasticity, thixotropy | Resistance to flow and its change under stress | Pseudoplastic (shear-thinning) impression materials flow under syringe pressure and stay put at rest |
| Polymerization shrinkage and C-factor | Volumetric contraction on curing; ratio of bonded to unbonded surfaces | High C-factor cavities (Class I, Class V) generate the greatest shrinkage stress, favoring incremental placement |
| Solubility and disintegration | Loss of material into oral fluids | Explains why zinc phosphate and especially conventional glass ionomer luting agents must be protected from early moisture, and why exposed cement margins wash out |
Chairside example: A patient reports a brief sharp pain to cold on a newly placed deep Class II composite. The physics is the sequence — etching removed the smear layer, an incompletely sealed hybrid layer left tubules patent, and polymerization shrinkage in a high C-factor box opened a marginal gap. The fix is sealing the dentin, not prescribing an analgesic.
A 76-year-old patient with generalized gingival recession takes three xerostomic medications and sips sweetened tea throughout the day. Which pH threshold best explains why this patient develops rapid root-surface lesions even when plaque pH never falls below 5.5?
A patient reports sharp, brief pain when air is blown on an exposed cervical root surface. Which mechanism best explains the sensation?
Which salivary buffer system provides most of the buffering capacity at high, stimulated flow rates?
A patient with gastroesophageal reflux shows broad, glossy concavities on the palatal surfaces of the maxillary anterior teeth, with amalgam restorations standing proud of the adjacent tooth structure. What is the immediate post-episode instruction?