BDS Dental Anatomy Syllabus

Define heterodont and diphyodont dentition, and which applies to humans.

Heterodont means having teeth of different morphological types (incisors, canines, premolars, molars). Diphyodont means having two successive sets of teeth (deciduous then permanent). Humans are both heterodont and diphyodont.

Contrast the human dentition (brachydont) with the hypsodont dentition seen in some herbivores.

Brachydont (low-crowned) teeth, as in humans, have a short crown, well-developed roots, and stop growing once formed. Hypsodont (high-crowned) teeth, as in horses and cattle, have long crowns extending well beyond the gum line with continuous or delayed root formation to compensate for heavy abrasive wear.

What are the two main basic occlusal concepts: centric occlusion and centric relation?

Centric occlusion (maximum intercuspation) is the occlusion with the most complete interdigitation of opposing teeth, independent of condylar position. Centric relation is the most retruded, unstrained physiological position of the mandibular condyles in the glenoid fossae, from which lateral movements can be made — a reproducible bone-to-bone relationship.

In Angle's classification, what defines a Class I (normal) molar relationship?

In Angle's Class I, the mesiobuccal cusp of the maxillary permanent first molar occludes in the mesiobuccal (buccal) groove of the mandibular permanent first molar. This is the key of occlusion.

Describe Angle's Class II and Class III malocclusions relative to the first molars.

Class II (distocclusion): the mandibular first molar is positioned distally relative to the maxillary first molar (maxillary mesiobuccal cusp occludes anterior to the mandibular buccal groove). Class III (mesiocclusion): the mandibular first molar is positioned mesially (maxillary mesiobuccal cusp occludes posterior to the buccal groove) — a prognathic profile.

What is the Curve of Spee?

The Curve of Spee is the anteroposterior (sagittal) curvature of the occlusal plane, an imaginary curved line following the buccal cusp tips of the mandibular posterior teeth from the canine to the last molar, concave for the mandibular arch (curving upward posteriorly).

What is the Curve of Wilson?

The Curve of Wilson is the mediolateral (transverse/frontal-plane) curvature of the occlusal plane. It is formed by the inward inclination of the mandibular posterior teeth, so a line connecting the buccal and lingual cusp tips of right and left molars is concave (upward), reflecting the lingual tilt of the teeth.

What is the Sphere of Monson?

The Sphere of Monson is a theoretical concept in which the curves of Spee and Wilson are described as part of the surface of an imaginary sphere approximately $8$ inches (about $4$ inch radius) in diameter, with its center in the region of the glenoid fossa/orbit, upon which the cusp tips and incisal edges lie.

What are cusp-fossa and cusp-marginal ridge relationships in functional occlusion?

These are the two schemes of intercuspation: in a cusp-to-fossa relationship a supporting cusp seats into the central fossa of the opposing tooth (one-tooth-to-one-tooth), while in a cusp-to-marginal-ridge relationship a supporting cusp contacts the embrasure/marginal ridges of two opposing teeth (one-tooth-to-two-teeth), which is the more common natural arrangement.

Which cusps are the supporting (centric/functional) cusps in normal occlusion?

The supporting (stamp/centric) cusps are the maxillary palatal (lingual) cusps and the mandibular buccal cusps. They occlude into opposing fossae/marginal ridges and bear the vertical load of occlusion; the maxillary buccal and mandibular lingual cusps are the non-supporting (guiding/shearing) cusps.

What are canine (cuspid) guidance and group function during lateral movement?

Canine guidance (mutually protected occlusion) is where, during lateral excursion, only the canines on the working side contact and disocclude (separate) all posterior teeth. Group function is where several teeth on the working side (canine plus premolars/molars) share contact simultaneously during lateral movement, distributing the load.

What is the chemical composition of enamel by weight?

Enamel is approximately $96\%$ inorganic material (mainly hydroxyapatite, $\ce{Ca10(PO4)6(OH)2}$) and about $4\%$ organic matrix and water by weight. It is the hardest and most highly mineralized tissue in the human body.

What is the structural (histological) unit of enamel, and how many are approximately present?

The structural unit of enamel is the enamel rod (enamel prism), which extends from the dentinoenamel junction to the enamel surface. Roughly $5$ million (in a lower lateral incisor) to $12$ million (in an upper first molar) rods are present in a tooth, each with a characteristic keyhole/fish-scale cross-section.

Name four surface/incremental structures of enamel.

Enamel structures include: the striae of Retzius (incremental growth lines), Hunter-Schreger bands (optical bands from prism direction change), enamel spindles, enamel tufts, enamel lamellae (hypomineralized defects), and perikymata (external surface manifestation of striae of Retzius).

Compare the mineral and organic content of dentin with that of enamel.

Dentin is approximately $70\%$ inorganic (hydroxyapatite), $20\%$ organic (mainly type I collagen), and $10\%$ water by weight. It is less mineralized and softer than enamel ($96\%$ inorganic) but harder than bone and cementum, and unlike enamel it is capable of ongoing formation and repair.

What are dentinal tubules and the odontoblast process?

Dentinal tubules are microscopic S-shaped (primary curvature) channels that run through the entire thickness of dentin from the pulp to the dentinoenamel/dentinocemental junction. Each tubule houses an odontoblast process (Tomes' fiber), the cytoplasmic extension of the odontoblast whose cell body lies at the pulpal surface, making dentin a living, sensitive tissue.

Distinguish primary, secondary, and tertiary (reparative) dentin.

Primary dentin forms before root completion (the bulk of the tooth, with mantle and circumpulpal layers). Secondary dentin forms slowly after root completion throughout life, gradually reducing pulp chamber size. Tertiary (reparative/reactionary) dentin is localized dentin formed in response to stimuli such as caries, attrition, or cavity preparation.

What is the composition and function of cementum?

Cementum is a bone-like calcified connective tissue covering the root, approximately $45$–$50\%$ inorganic (hydroxyapatite) and $50$–$55\%$ organic matrix and water. Its main function is to anchor the collagen (Sharpey's) fibers of the periodontal ligament to the root, attaching the tooth to the alveolar bone.

Differentiate acellular (primary) cementum from cellular (secondary) cementum.

Acellular (primary) cementum forms first, contains no embedded cementocytes, is found mainly on the coronal two-thirds/cervical portion of the root, and is the principal anchoring cementum. Cellular (secondary) cementum contains cementocytes in lacunae, forms later and faster, and is found mainly on the apical third of the root and in furcation areas, functioning in adaptation and repair.

What are the three types of cementoenamel junction (CEJ) relationships and their approximate frequencies?

At the CEJ: (1) cementum overlaps enamel in about $60\%$ of cases, (2) cementum meets enamel edge-to-edge in about $30\%$, and (3) a gap exists exposing dentin (cementum and enamel do not meet) in about $10\%$. This variability has clinical significance for dentin sensitivity.

What are the four main functions of the dental pulp?

The four functions of the dental pulp are: (1) formative/developmental — odontoblasts form dentin; (2) nutritive — supplies nutrients to dentin via odontoblasts; (3) sensory/protective — mediates pain sensation and dentin sensitivity; and (4) defensive/reparative — forms tertiary dentin and mounts immune responses to injury.

Describe the histological zones of the pulp from the dentin inward.

From the periphery inward: (1) the odontoblastic layer (odontoblast cell bodies lining the predentin), (2) the cell-free zone of Weil (poor in cells, rich in nerve and capillary plexuses), (3) the cell-rich zone (high density of fibroblasts and undifferentiated mesenchymal cells), and (4) the pulp core/proper (central connective tissue with major vessels and nerves).

Give examples of developmental anomalies of tooth number and define them.

Anodontia is the total congenital absence of teeth; hypodontia (oligodontia) is the congenital absence of one or a few teeth (common with third molars and maxillary lateral incisors). Hyperdontia is the presence of supernumerary (extra) teeth, e.g., a mesiodens between the maxillary central incisors.

Define microdontia and macrodontia.

Microdontia is an anomaly in which teeth are smaller than normal (generalized, relative, or localized — e.g., peg-shaped maxillary lateral incisor). Macrodontia is an anomaly in which teeth are larger than normal (generalized or localized), sometimes associated with pituitary gigantism.

Define gemination, fusion, and concrescence (anomalies of tooth shape/union).

Gemination is an attempt of a single tooth germ to divide, producing a bifid crown on a single root (tooth count usually normal). Fusion is the union of two separate tooth germs by dentin, giving a single large tooth (count usually reduced by one). Concrescence is the union of two adjacent fully formed teeth by cementum only, without dentin fusion.

What are dens invaginatus (dens in dente) and dens evaginatus?

Dens invaginatus (dens in dente, 'tooth within a tooth') is an infolding of the enamel/outer surface into the interior before mineralization, most common in maxillary lateral incisors, predisposing to caries and pulp infection. Dens evaginatus is an accessory tubercle/cusp of enamel and dentin projecting from the occlusal or lingual surface (e.g., talon cusp on anterior teeth, Leong's premolar).

Define taurodontism and dilaceration.

Taurodontism is a morphological anomaly of multi-rooted teeth in which the pulp chamber is enlarged vertically (apically displaced furcation) giving a 'bull-like' elongated body and short roots. Dilaceration is an abnormal angulation or sharp bend in the root or crown, usually due to trauma to the developing tooth.

Compare amelogenesis imperfecta and dentinogenesis imperfecta (structural anomalies).

Amelogenesis imperfecta is a hereditary defect of enamel formation (hypoplastic, hypomaturation, or hypocalcified types) affecting enamel quantity/quality while dentin is normal. Dentinogenesis imperfecta is a hereditary defect of dentin formation producing opalescent, discolored teeth with bulbous crowns, cervical constriction, and obliterated pulp chambers; it can be associated with osteogenesis imperfecta.

What is enamel hypoplasia and how does it differ from enamel hypocalcification?

Enamel hypoplasia is a quantitative defect — incomplete or deficient formation of the enamel matrix during the apposition stage, producing pits, grooves, or thin/missing enamel. Enamel hypocalcification is a qualitative defect — deficient mineralization of a normally formed matrix, producing soft, opaque, chalky enamel. Fluorosis and dental caries can involve hypomineralization.

In tooth dimensions, what do the terms crown length and root length refer to, and how is the crown/root ratio used?

Crown length is the vertical (cervico-incisal/occlusal) height of the anatomic crown from the CEJ to the incisal edge or cusp tip; root length is measured from the CEJ to the root apex. The crown-to-root ratio compares these; a favorable (smaller) ratio, where root length exceeds crown length, provides better periodontal support and prognosis.

What is meant by tooth proportion in terms of the mesiodistal versus faciolingual and cervical measurements?

Tooth proportion describes the ratios among a tooth's key dimensions — mesiodistal (M-D) crown width, faciolingual (labiolingual/buccolingual) crown width, and the corresponding cervical (root) measurements. For anterior teeth an esthetic guideline is a width-to-length crown ratio of roughly $75$–$85\%$ ($\frac{\text{width}}{\text{length}} \approx 0.8$), and posterior teeth are generally wider faciolingually than anterior teeth to withstand grinding forces.