Basic Biomedical Sciences (Anatomy, Physiology, Biochemistry, Microbiology, Pathology, Pharmacology)
Head and neck anatomy
The trigeminal nerve carries all tooth sensation. V2 supplies the superior alveolar nerves, greater palatine and nasopalatine nerves; V3 supplies the inferior alveolar, lingual, buccal and mylohyoid nerves. The lingual nerve carries chorda tympani fibres - taste from the anterior two-thirds of the tongue and secretomotor supply to the submandibular and sublingual glands. The facial nerve supplies muscles of facial expression through temporal, zygomatic, buccal, marginal mandibular and cervical branches. Know the lingual nerve's proximity to the retromolar triangle and the mental nerve's exit between the premolars.
Physiology and biochemistry
Saliva (about 0.5–1 L/day) is hypotonic and bicarbonate-buffered, containing amylase and secretory IgA; parasympathetic stimulation produces copious watery saliva, sympathetic stimulation produces small volumes rich in protein. PTH raises serum calcium by bone resorption, renal tubular reabsorption and activation of renal 1-alpha-hydroxylase. Vitamin C is required to hydroxylate proline and lysine during collagen synthesis - deficiency impairs wound healing and produces bleeding gums.
Pharmacology of local anaesthetics
Local anaesthetics are weak bases. Only the uncharged base crosses the nerve membrane; the charged cation blocks the sodium channel from inside. Inflammation lowers tissue pH, trapping more drug in the charged form - slow onset, poor depth, shorter duration. For lidocaine (pKa 7.9) at tissue pH 7.4, the fraction of base is 1/(1+10^0.5) ≈ 24%.
Worked example
A 1.8 mL cartridge of 2% lidocaine with 1:100,000 epinephrine contains 36 mg lidocaine and 18 µg epinephrine. At the 4.4 mg/kg limit, a 60 kg adult may receive about 264 mg - roughly 7 cartridges. Traps: mixing mg/mL with %, and forgetting that epinephrine (not the anaesthetic) sets the cartridge limit in cardiac patients.