Basic Biomedical Sciences
Basic science questions are integrated rather than isolated: anatomy used during an injection, pharmacology explaining a failed block, microbiology explaining an infection.
Anatomy and histology
- Trigeminal: V2 exits the foramen rotundum, V3 the foramen ovale; the inferior alveolar nerve enters the mandibular foramen and exits at the mental foramen; the lingual nerve carries chorda tympani fibres for submandibular and sublingual secretion.
- Enamel is about 96% mineral, acellular and irreplaceable; dentine is type I collagen with odontoblast processes in tubules; cementum anchors Sharpey fibres; the periodontal ligament provides proprioception.
Physiology and biochemistry
- Saliva: 1-1.5 litres daily, mostly unstimulated from submandibular glands; IgA is the dominant immunoglobulin; pH about 6.7-7.4.
- Calcium homeostasis: parathyroid hormone raises serum calcium, calcitriol increases intestinal absorption, calcitonin lowers calcium.
Pharmacology
- Local anaesthetics are weak bases; only the un-ionised form crosses the membrane. A pKa closer to tissue pH gives faster onset; inflamed tissue is acidic and traps the base, so infiltration fails in acute pulpitis.
- Amides are metabolised in the liver, esters by plasma esterases. Lidocaine 2% with 1:100,000 contains 36 mg per cartridge and the maximum is about 7 mg/kg.
Microbiology and immunology
- Caries: mutans streptococci and lactobacilli; periodontitis: Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola.
- Hypersensitivity types I-IV; IgA dominates secretions, IgG dominates serum.
Worked example
A mandibular molar with acute pulpitis is not numb after a standard inferior alveolar injection. Tissue pH is low, so less anaesthetic exists as free base and less crosses the nerve membrane. The reasoning justifies a supplementary intraligamentary or buccal infiltration rather than repeating the same volume.
Traps
Assuming a higher pKa means faster onset; confusing the rotundum and ovale foramina; forgetting maximum dose by body weight; naming red complex organisms as caries pathogens.