Oral Pathology, Oral Diagnosis and Oral Radiology
Oral Pathology
Cysts and tumours
Rank the common lesions: radicular (periapical) cyst is the most common odontogenic cyst, dentigerous cyst second, and odontogenic keratocyst the most common developmental cyst - recognised by a thin parakeratinized lining of 6–8 cells, palisaded hyperchromatic basal cells and satellite cysts, with PTCH1 mutation and Gorlin syndrome association. Odontoma is the most common odontogenic tumour overall (a hamartoma), while ameloblastoma is the most clinically significant: locally invasive, multilocular 'soap-bubble' radiolucency, posterior mandible.
Mucosal disease
Oral lichen planus shows Wickham striae, band-like lymphocytic infiltrate, hydropic degeneration of the basal layer and saw-tooth rete ridges. Pemphigus vulgaris is intraepithelial (suprabasal acantholysis, IgG against desmoglein 3, positive Tzanck smear), whereas mucous membrane pemphigoid is subepithelial with linear IgG and C3 along the basement membrane. Recurrent aphthous ulcers are not vesicular and heal without scarring (except major aphthae).
Malignancy
Squamous cell carcinoma is by far the most common intraoral malignancy, with floor of mouth and lateral tongue as the classic sites. Erythroplakia carries a higher malignant transformation risk than leukoplakia. Any ulcer persisting beyond two to three weeks without an identifiable cause, any induration, and any lesion at a high-risk site must be biopsied.
Rule of thumb
Midline or floor-of-mouth ulcers are guilty until proven innocent. Trap: the question may ask for 'most common odontogenic tumour' (odontoma) but describe an aggressive lesion - read the stem, not the category.
Oral Diagnosis and Oral Radiology
The diagnostic sequence
History (complaint, medical history, medications, allergies, habits), then extraoral examination (lymph nodes, TMJ, salivary glands, facial symmetry), then intraoral examination (soft tissues, periodontal charting, vitality, percussion, palpation, transillumination). Vitality tests assess nerve response, not blood supply - a necrotic pulp can occasionally respond, and a traumatised tooth may be temporarily unresponsive. Select biopsy type by size: excisional for small, accessible lesions (complete removal with a margin), incisional for large or suspicious lesions, aspiration for fluid-filled swellings, and cytology as an adjunct only.
Imaging selection
Bitewings detect interproximal caries and crestal bone loss; periapicals show the whole tooth and periapex; panoramic images survey broadly but with lower resolution; CBCT gives three-dimensional bone information at a higher dose. Apply ALARA: rectangular collimation, thyroid shield, lead apron, film-holding devices, and digital receptors. Know that kVp governs contrast (and thus the scale of grey), while mA and exposure time govern density/quantity.
Localisation
Use the SLOB rule with horizontal tube shift: the object that moves in the SAME direction as the tube head is LINGUAL; the object moving opposite is buccal. Vertical shift localises in the third dimension (objects move opposite to the tube's vertical movement when they lie above the focal trough).
Traps
Radiographs cannot show early caries - roughly 30–40% demineralisation is needed before a radiolucency appears. Radiographic findings must always be correlated clinically. For radiation risk, deterministic effects have a threshold; stochastic effects (cancer, heritable) are assumed to have no safe threshold.