Basic Sciences: Anatomy, Physiology, Biochemistry and Pharmacology
Basic Sciences
This domain tests the basic science substrate that explains clinical dentistry: cranial nerve anatomy, calcium homeostasis, collagen biochemistry, and drug mechanisms relevant to dental practice.
Core rules
- Trigeminal nerve: ophthalmic (V1), maxillary (V2) and mandibular (V3) divisions; V3 is motor to muscles of mastication and carries the inferior alveolar, lingual and buccal nerves. Facial nerve (VII) supplies muscles of facial expression - a lesion shows forehead involvement in lower motor neuron palsy but spares the forehead in an upper motor neuron lesion.
- Muscles of mastication: masseter, temporalis, medial and lateral pterygoid, all V3; lateral pterygoid is the only depressor and the only one opening the mouth along with suprahyoids.
- Calcium homeostasis: parathyroid hormone raises serum calcium by bone resorption and renal reabsorption; calcitonin lowers it; vitamin D increases intestinal absorption.
- Collagen requires vitamin C for hydroxylation of proline and lysine; deficiency causes scurvy with bleeding, swollen gingiva and poor wound healing.
- Local anaesthetics are weak bases that block voltage-gated sodium channels; inflamed tissue lowers pH, reducing the unionised fraction and explaining failed anaesthesia in acute infection.
- NSAIDs inhibit cyclo-oxygenase; amoxicillin-clavulanate covers beta-lactamase producers; metronidazole is the key agent for anaerobic dental infection.
Worked example
A patient with acute apical abscess reports that the anaesthetic did not work. The reason is that inflammation lowers tissue pH, so more of the local anaesthetic remains in the ionised form and crosses the nerve membrane poorly; the pKa of lidocaine is 7.9, so small pH falls sharply reduce the unionised fraction. Management is to achieve drainage and reduce inflammation, use a nerve block rather than infiltration, and consider a different agent - increasing the volume without addressing pH is the common error. Note that once the abscess is drained, the same dose usually works, confirming the mechanism.
Traps
- Attributing forehead sparing to a facial nerve lesion: that indicates an upper motor neuron lesion.
- Naming the medial pterygoid as a depressor.
- Blaming 'weak anaesthetic' for failure instead of pH and anatomical variation.
- Confusing the roles of PTH and calcitonin.