Basic and Applied Dental Sciences
Hard- and soft-tissue fundamentals
Enamel is about 96% mineral by weight (hydroxyapatite) arranged in rods with a keyhole cross-section; it cannot repair itself, so any true defect is permanent. Dentin is roughly 70% mineral, 20% type I collagen and 10% water, and its tubules (about 1–2.5 µm wide, 20,000–45,000/mm²) make it permeable - the anatomical basis of sensitivity, hybrid-layer bonding and bacterial access to the pulp. Cementum, the periodontal ligament (about 0.2 mm) and alveolar bone form the attachment apparatus, with Sharpey's fibres inserting into acellular extrinsic-fibre cementum.
Microbiology and pharmacology
Caries is a sucrose-driven biofilm disease: Streptococcus mutans uses glucosyltransferase to build glucans and lower plaque pH below the critical ~5.5. Chronic periodontitis is associated with the red complex - Porphyromonas gingivalis, Tannerella forsythia and Treponema denticola. For local anaesthesia, only the uncharged base crosses the nerve membrane; inflamed tissue is acidic, so more drug remains ionised and anaesthesia fails - the reason for a supplementary block or intraseptal injection. Epinephrine 1:100,000 equals 0.01 mg/mL and prolongs duration while reducing systemic absorption.
Worked example
A 60 kg patient needs lidocaine 2% with 1:100,000 epinephrine. Maximum dose with vasoconstrictor is about 7 mg/kg (4.4 mg/kg for plain solution): 7 × 60 = 420 mg. Each 1.8 mL cartridge contains 36 mg, so 420 ÷ 36 ≈ 11.6 cartridges. In practice the limiting factor is usually epinephrine (about 0.2 mg in a healthy adult, 0.04 mg in cardiac disease) or patient tolerance, not the lidocaine figure.
Traps
- Swapping the mineral percentages for enamel and dentin.
- Believing inflamed, acidic tissue improves local anaesthesia.
- Quoting a cartridge volume of 2.2 mL instead of 1.8 mL.
- Forgetting that local anaesthetic toxicity presents first with CNS excitation, then depression.