Anatomy: Dental and Head & Neck
Cranial Nerves · Muscles of Mastication · Salivary Glands · Blood Supply · Tooth Morphology
TL;DR
Dental anatomy and head and neck anatomy form the structural foundation of clinical dentistry. For board examinations, the highest-yield topics are: the trigeminal nerve (CN V) and its branches — understanding which nerve supplies which area determines local anaesthesia technique; muscles of mastication — origin, insertion, and action of the four main muscles; salivary glands — the three major glands, their ducts, and their associated nerves; blood supply — the external carotid artery branches; and dental anatomy — tooth morphology, cusp identification, and clinically significant anatomical variations.
- The trigeminal nerve (CN V) provides all general somatic afferent innervation to the teeth, periodontium, oral mucosa, and most of the facial skin: CN V is the largest cranial nerve and is the primary sensory nerve of the face and oral cavity. It has three divisions — V1 (ophthalmic), V2 (maxillary), and V3 (mandibular). V1 exits through the superior orbital fissure and supplies the forehead, scalp, upper eyelid, nose tip (external nasal nerve), and cornea. V2 exits through the foramen rotundum into the pterygopalatine fossa and supplies the maxillary teeth (via posterior superior alveolar [PSA], anterior superior alveolar [ASA], middle superior alveolar [MSA] branches), maxillary sinus, palate (greater palatine nerve supplies posterior two-thirds; nasopalatine nerve through incisive foramen supplies anterior hard palate), cheek (infraorbital nerve), and nasal mucosa. V3 exits through the foramen ovale and has both sensory and motor components — it supplies the mandibular teeth (inferior alveolar nerve [IAN]), anterior two-thirds of tongue (lingual nerve — general sensation only, not taste), cheek mucosa (long buccal nerve), chin and lower lip (mental nerve — terminal branch of IAN), and all four muscles of mastication (motor root of V3 via masseteric nerve to masseter and temporalis, medial and lateral pterygoid nerves to pterygoids).
- The four muscles of mastication are masseter, temporalis, medial pterygoid, and lateral pterygoid — all supplied by CN V3 (mandibular nerve): The masseter arises from the zygomatic arch and inserts on the lateral surface of the ramus and angle of the mandible — it elevates the mandible (closes the jaw) and protrudes slightly. The temporalis arises from the temporal fossa (fan-shaped) and inserts on the coronoid process of the mandible via a thick tendon — it elevates the mandible (the posterior fibres also retract the mandible — important for condyle repositioning after protrusion). The medial pterygoid arises from the medial surface of the lateral pterygoid plate and the pterygoid fossa, and inserts on the medial surface of the angle/ramus of the mandible — it elevates, protrudes, and provides lateral excursion. The lateral pterygoid is the only muscle of mastication that depresses the mandible (opens the jaw) and is the only one with two heads: the upper head (sphenoid greater wing → articular disc and condylar neck — stabilises the disc during closure) and the lower head (lateral pterygoid plate → pterygoid fovea of the condyle — translates the condyle anteriorly for opening and lateral excursion). The lateral pterygoid is exclusively innervated by the lateral pterygoid nerve (a branch of V3). All four muscles of mastication are derived embryologically from the first pharyngeal arch (Meckel’s cartilage arch).
- The three major salivary glands each have a distinct duct, nerve supply relationship, and stone risk profile: The parotid gland is the largest major salivary gland. It is a serous gland (produces thin, enzyme-rich saliva — the primary source of salivary amylase). Its duct — Stensen’s duct (parotid duct) — passes over the masseter, pierces the buccinator, and opens into the oral vestibule opposite the upper second molar. The facial nerve (CN VII) passes through the parotid gland (it does not originate from the gland — it passes through and divides into five terminal branches: temporal, zygomatic, buccal, marginal mandibular, and cervical) — this is why parotid tumours, parotid surgery, or parotitis can cause facial nerve dysfunction. The submandibular gland is a mixed gland (predominantly serous). Its duct — Wharton’s duct — runs forward in the floor of the mouth between the mylohyoid and hyoglossus muscles and opens on the sublingual papilla (caruncle) beside the lingual frenulum. The submandibular gland is the most common site of salivary calculi (sialolithiasis) — 80–90% of salivary calculi occur here, because Wharton’s duct is long, wide, and runs against gravity. The lingual nerve (V3 branch) and the chorda tympani (CN VII branch — carrying parasympathetic fibres to both submandibular and sublingual glands) are closely related to the submandibular gland — lingual nerve injury risk during submandibular gland surgery. The sublingual gland is the smallest major salivary gland. It is a mixed gland but predominantly mucous (secretes thick, viscous saliva). Unlike the other two major glands, the sublingual gland has multiple small ducts — the ducts of Rivinus — that drain directly into the floor of the mouth; some may join to form a common duct (duct of Bartholin) that opens near Wharton’s duct opening.
- The maxillary artery (a terminal branch of the external carotid artery) supplies most of the deep face and oral cavity: The external carotid artery (ECA) has eight named branches — the mnemonic SALFOPSM helps: Superior thyroid, Ascending pharyngeal, Lingual, Facial, Occipital, Posterior auricular, Superficial temporal (terminal), Maxillary (terminal). The maxillary artery is the larger terminal branch of the ECA. It has three parts: the first (mandibular) part lies posterior to the neck of the mandible and gives off: deep auricular artery, anterior tympanic artery, middle meningeal artery (exits via foramen spinosum — epidural haematoma from rupture in temporal bone trauma), inferior alveolar artery (enters mandibular foramen → supplies mandibular teeth and bone → gives off mental artery at mental foramen → labial branch to lower lip skin). The second (pterygoid) part passes through or around the lateral pterygoid muscle and gives off branches to the pterygoids and the masseteric artery. The third (pterygopalatine) part enters the pterygopalatine fossa and gives off: posterior superior alveolar artery (maxillary molar/premolar region), infra-orbital artery (anterior superior alveolar artery to maxillary incisors/canines + canine), descending palatine artery (→ greater and lesser palatine arteries), sphenopalatine artery (nasal cavity — final branch of the maxillary artery; the most common site of posterior epistaxis — Kiesselbach’s plexus in Little’s area of the nasal septum is the most common site of anterior epistaxis).
- The permanent molar cusp ridges and fossa anatomy determine both occlusal function and caries susceptibility: Upper first molars have four main cusps (mesiobuccal, distobuccal, mesiopalatal, distopalatal) plus the cusp of Carabelli (fifth cusp on the mesiopalatal cusp — a non-functional cusp). Lower first molars have five cusps (mesiobuccal, distobuccal, mesiolingual, distolingual, and the distal cusp). Upper first molars have three roots: mesiobuccal (MB), distobuccal (DB), and palatal (P) — the palatal root is the largest and longest. Lower first molars have two roots (mesial and distal) with two canals in the mesial root (MB and ML) in ~70–75% of cases — important for endodontics. The MB root of upper molars has two canals (MB1 and MB2) in ~60–70% of cases — the MB2 canal is frequently missed in root canal treatment. Key groove patterns: upper molars have an H-shaped groove pattern; lower molars have a Y-shaped groove pattern (or +). Fossa and ridge anatomy: marginal ridges (mesial and distal) are the transitional anatomy between the proximal surface and the occlusal surface — higher in posterior teeth, they are important for food deflection in occlusion. Oblique ridges connect the distal cusp ridge of the mesiobuccal cusp to the mesial cusp ridge of the distopalatal cusp in upper first molars — the oblique ridge divides the occlusal surface into a central fossa (mesial) and a distal fossa.
Key Facts
What Is Dental Anatomy?
Dental anatomy is the study of the structure, form, and function of individual teeth and the supporting tissues, while head and neck anatomy provides the broader structural context for clinical dental practice — including the neural pathways essential for local anaesthesia, the vascular architecture critical for surgical and emergency management, the muscular and joint anatomy relevant to occlusal and TMD assessment, and the glandular anatomy relevant to salivary disease and surgical risk. Together, dental anatomy and head and neck anatomy form a continuously applied knowledge base: the dentist uses this knowledge during every injection (selecting the correct nerve block), every surgical procedure (identifying and protecting nerves and vessels), every occlusal analysis (understanding muscle function), and every diagnosis (recognising pathology within the anatomical context).
Why It Matters
Board examinations test anatomy extensively, with a consistent focus on: nerve branch identification (which branch supplies which specific area — tested through clinical scenarios of anaesthesia failure or nerve injury); muscle origin/insertion/action (clinical scenarios of TMD, mandibular dysfunction); salivary gland anatomy (sialolithiasis, infection spread, surgical risk, duct anatomy); blood supply (epistaxis, surgical bleeding, anatomical variants); and tooth morphology (crown and root anatomy, cusp patterns, root canal anatomy). Clinically, every inferior alveolar nerve block injection is an exercise in applied anatomy, and the most common cause of anaesthesia failure is anatomical variation or inaccurate landmark identification.
Cranial Nerves Relevant to Dentistry
Trigeminal Nerve (CN V)
The trigeminal nerve is the fifth cranial nerve and the principal sensory nerve of the face. The cell bodies of the sensory neurons of CN V are in the trigeminal (Gasserian/semilunar) ganglion, located in Meckel’s cave on the petrous apex of the temporal bone. The sensory root passes to the pons (entering the lateral pons at the level of the middle cerebellar peduncle). The motor root arises from the motor nucleus of the trigeminal nerve (in the upper pons) and passes out exclusively with V3.
| Division | Exit Foramen | Named Branches (Dental Relevance) | Area Supplied |
|---|---|---|---|
| V1 — Ophthalmic | Superior orbital fissure | Lacrimal, frontal (supraorbital + supratrochlear), nasociliary (external nasal, anterior ethmoidal) | Forehead, scalp, upper eyelid, conjunctiva, cornea (absent corneal reflex = V1 damage), dorsum of nose, tip of nose (external nasal nerve). Dental relevance: zygomatico-temporal nerve (V1/V2 overlap) — may need to be blocked for upper third molar anaesthesia |
| V2 — Maxillary | Foramen rotundum → pterygopalatine fossa | Posterior superior alveolar (PSA); anterior superior alveolar (ASA — branch of infraorbital); middle superior alveolar (MSA — absent in ~28%); greater palatine; lesser palatine; nasopalatine; infraorbital; zygomaticotemporal; zygomaticofacial | Maxillary teeth + PDL + alveolar bone. Maxillary sinus. Anterior hard palate (nasopalatine). Posterior hard palate + soft palate (greater/lesser palatine). Cheek skin + upper lip (infraorbital) |
| V3 — Mandibular | Foramen ovale (sensory + motor roots join here) | Inferior alveolar nerve (IAN → incisive + mental branches); lingual nerve; long buccal (buccal nerve); auriculo-temporal nerve; masseteric nerve; deep temporal nerves; medial pterygoid nerve; lateral pterygoid nerve; nerve to mylohyoid (branch of IAN before entering mandibular foramen) | Mandibular teeth + PDL (IAN); anterior 2/3 tongue general sensation (lingual nerve); buccal mucosa + buccal gingiva of lower molars (long buccal); chin + lower lip (mental nerve); TMJ + parotid gland (auriculo-temporal); masseter, temporalis, medial and lateral pterygoids (motor) |
Facial Nerve (CN VII)
The facial nerve is primarily a motor nerve (to the muscles of facial expression — all derived from the second pharyngeal arch) with several important additional functions: taste from the anterior two-thirds of the tongue (via chorda tympani → lingual nerve → geniculate ganglion → nucleus tractus solitarius); parasympathetic secretomotor fibres to the submandibular and sublingual glands (chorda tympani → lingual nerve → submandibular ganglion) and the lacrimal and nasal glands (greater petrosal nerve → pterygopalatine ganglion). The chorda tympani is the branch of CN VII most relevant to dentistry — it travels with the lingual nerve through the infratemporal fossa and enters the floor of the mouth. Chorda tympani damage (e.g., from an inferior alveolar nerve block or parotid surgery) causes: loss of taste on the anterior two-thirds of the tongue (ipsilateral) + reduced submandibular and sublingual saliva production. The five terminal motor branches of CN VII in the face: Temporal, Zygomatic, Buccal, Marginal mandibular, Cervical (mnemonic: “Two Zebras Bit My Cat” or “To Zanzibar By Motor Car”). The marginal mandibular branch is the most commonly injured branch in facial surgery — it runs along the inferior border of the mandible and supplies the muscles depressing the lower lip and corner of the mouth.
Other Relevant Cranial Nerves
| Cranial Nerve | Name | Dental / Clinical Relevance |
|---|---|---|
| CN IX | Glossopharyngeal | General sensation to posterior one-third of tongue + soft palate + pharynx (gag reflex — afferent limb); taste from posterior one-third of tongue; parasympathetic secretomotor to parotid gland via lesser petrosal nerve → otic ganglion → auriculo-temporal nerve (CN V3 branch). Tympanic branch (Jacobson’s nerve) → otic ganglion → parotid. Damage causes loss of taste on posterior tongue + reduced parotid secretion |
| CN X | Vagus | Motor to soft palate (except tensor veli palatini — CN V3), pharynx (with CN IX), and larynx. Gag reflex afferent (CN IX) + efferent (CN X). Dental relevance: vasovagal syncope (fainting) — the most common medical emergency in a dental setting — is vagal-mediated (parasympathetic cardioinhibitory response to pain/anxiety → bradycardia + hypotension) |
| CN XII | Hypoglossal | Motor to all intrinsic and most extrinsic muscles of the tongue (except palatoglossus — CN X). Damage → tongue deviation toward the lesion side (ipsilateral deviation — the intact side pushes the tongue toward the damaged side). Relevant to floor of mouth surgery, submandibular gland removal, and hypoglossal nerve stimulators for obstructive sleep apnoea |
Muscles of Mastication
All four muscles of mastication are innervated by the motor root of CN V3 and all are derived from the first pharyngeal arch mesoderm. They move the mandible and are therefore directly involved in jaw opening, closing, chewing, and lateral excursion. Understanding their actions is essential for interpreting TMD, mandibular fractures, and splint therapy.
| Muscle | Origin | Insertion | Main Action(s) | Clinical Note |
|---|---|---|---|---|
| Masseter | Inferior surface and medial aspect of zygomatic arch | Lateral surface of ramus and angle of mandible (superficial fibres); posterior ramus and coronoid (deep fibres) | Elevation (jaw closure); slight protrusion | Masseter hypertrophy is visible as fullness at the angle of the jaw — associated with bruxism; treated with botulinum toxin injection for masseteric hypertrophy or bruxism management |
| Temporalis | Temporal fossa (fan-shaped — fills the temporal fossa from the temporal line superiorly to the infratemporal crest inferiorly) | Coronoid process of mandible + anterior border of ramus (via a thick tendon that passes under the zygomatic arch) | Elevation (primarily, via anterior fibres); retraction of the mandible (posterior fibres — fibres run horizontally) | Tenderness on palpation of the temporalis (temporal headache) is a common feature of TMD and bruxism. The tendon of the temporalis can be palpated internally (coronoid notch) during large mouth opening |
| Medial pterygoid | Deep head: medial surface of the lateral pterygoid plate and pyramidal process of palatine bone. Superficial head: tuberosity of maxilla | Medial surface of ramus and angle of mandible (mirrors the masseter — together they form a “sling” around the angle of the mandible) | Elevation; protrusion; lateral excursion (ipsilateral side moves toward opposite side) | The medial pterygoid muscle sling with the masseter provides the primary closure force of the mandible. Trismus after IANB or lower third molar surgery is often due to haematoma in or spasm of the medial pterygoid |
| Lateral pterygoid | Upper head: greater wing of sphenoid (infratemporal surface). Lower head: lateral surface of lateral pterygoid plate | Upper head: articular disc and capsule of TMJ. Lower head: pterygoid fovea (depression on the anterior aspect of the condylar neck) | Bilateral: protrusion + depression (jaw opening — the lower head translates the condyle anteriorly; the upper head stabilises/pulls the disc anteriorly during opening). Unilateral: lateral excursion to opposite side (contralateral excursion) | The upper head is thought to be active during jaw closure (stabilising the disc against the condyle). The lower head activates during jaw opening. In anterior disc displacement, the superior head may be malfunctioning and allowing the disc to slip anteriorly. The lateral pterygoid is the only masticatory muscle that opens the jaw |
Temporomandibular Joint (TMJ)
The TMJ is a synovial joint between the condyle of the mandible and the mandibular fossa/articular eminence of the temporal bone. It is unique among synovial joints: (1) the articular surfaces are covered by fibrocartilage (not hyaline cartilage — this is clinically important because fibrocartilage has greater capacity for remodelling under mechanical stress); (2) the joint is divided into upper and lower compartments by the articular disc (meniscus) — the upper compartment allows translation (gliding — condyle moves forward onto the eminence during wide opening); the lower compartment allows rotation (condyle rotates during early opening). Normal maximum interincisal opening is approximately 35–50mm (commonly cited as ≥40mm in adults as normal). Ligaments of the TMJ: (1) Temporomandibular (lateral) ligament — the primary ligament; outer oblique portion prevents excessive retrusion; inner horizontal portion limits condylar translation; (2) Sphenomandibular ligament — a remnant of Meckel’s cartilage; runs from the spine of the sphenoid to the lingula of the mandible (where the inferior alveolar nerve and vessels enter the mandibular foramen — the lingula is the bony projection adjacent to the mandibular foramen, and the IAN passes medial to it). The sphenomandibular ligament is medial to the TMJ and does not directly restrict movement; (3) Stylomandibular ligament — from the styloid process to the angle of the mandible; limits excessive protrusion.
Salivary Glands
Saliva is critical for oral health: it lubricates the oral mucosa, initiates starch digestion (amylase), provides antimicrobial proteins (IgA, lysozyme, lactoferrin, mucins), buffers acid (bicarbonate, phosphate), and provides mineral ions (calcium, phosphate, fluoride) for remineralisation. Daily saliva production is approximately 0.5–1.5 litres. Unstimulated (resting) saliva: ~70% from submandibular glands. Stimulated saliva: ~50% parotid (the parotid is the dominant gland during eating — it produces thin, enzyme-rich serous saliva that initiates digestion).
| Gland | Duct | Opening | Type | Nerve Supply (Parasympathetic) | Stone Risk |
|---|---|---|---|---|---|
| Parotid | Stensen’s duct (parotid duct) | Buccal mucosa, opposite upper 2nd molar | Purely serous | CN IX → lesser petrosal nerve → otic ganglion → auriculotemporal nerve (V3) | Low (15–20% of stones) |
| Submandibular | Wharton’s duct | Sublingual papilla (caruncle) at base of lingual frenulum — opens bilaterally | Mixed: predominantly serous (60% serous, 40% mucous) | Chorda tympani (CN VII) → lingual nerve (V3) → submandibular ganglion | High (80–90% of all salivary stones) |
| Sublingual | Ducts of Rivinus (10–12 small ducts); some unite as Bartholin’s duct joining Wharton’s duct | Directly into floor of mouth (sublingual fold); Bartholin’s duct → sublingual papilla | Mixed: predominantly mucous | Chorda tympani → lingual nerve → submandibular ganglion (shares with submandibular) | Very low (<5% of stones) |
Blood Supply to the Oral and Maxillofacial Region
The common carotid artery bifurcates at the level of the upper border of the thyroid cartilage (C3–C4 vertebral level) into the internal carotid artery (ICA — supplies the brain; has no branches in the neck) and the external carotid artery (ECA — supplies most structures of the head and neck). The ECA gives off eight branches (mnemonic SALFOPSM — see Key Facts above). Of these, the facial artery and lingual artery are the primary arterial supply to the oral soft tissues: the lingual artery (arises from ECA just above the greater cornu of the hyoid) passes deep to the hyoglossus muscle to supply the tongue (deep lingual artery — terminal branch) and floor of mouth (sublingual artery — branch of lingual artery, supplies sublingual gland and floor of mouth). The facial artery crosses the lower border of the mandible just anterior to the masseter (where it can be palpated as the facial artery pulse) and supplies the lips (inferior and superior labial arteries) and the nose (angular artery — terminal branch, anastomoses with the dorsal nasal branch of the ophthalmic artery). The inferior alveolar artery (branch of the first part of the maxillary artery) accompanies the IAN through the mandibular foramen and runs in the mandibular canal to supply the mandibular teeth, alveolar bone, and buccal gingiva. The mental artery is a terminal branch of the IAN/IVA that exits the mental foramen and supplies the chin and lower lip skin.
Dental Anatomy
Tooth Structure
Each tooth consists of: Enamel — the hardest substance in the human body (96% hydroxyapatite); formed by ameloblasts (lost at eruption — enamel cannot regenerate after eruption). Dentine — the bulk of the tooth (70% inorganic hydroxyapatite; 30% organic collagen + water); formed by odontoblasts throughout life (primary dentine, secondary dentine, reactionary/reparative dentine). Cementum — covers the root surface (50% inorganic); formed by cementoblasts; contains Sharpey’s fibres (the extrinsic fibres of the periodontal ligament that insert into cementum). Pulp — the soft connective tissue core of the tooth; contains odontoblasts (peripherally), fibroblasts, macrophages, undifferentiated mesenchymal cells, blood vessels, and nerves (sensory: CN V; sympathetic: from the superior cervical ganglion); functions: formative (odontoblasts produce dentine), nutritive (blood supply), sensory (pain — all stimuli to the pulp are perceived as pain), defensive (inflammatory response, reactionary dentine). The periodontal ligament (PDL) connects cementum to alveolar bone via Sharpey’s fibres; PDL space is ~0.2–0.3mm; contains proprioceptors (the most precise sensory feedback mechanism in the dentition — allows discrimination of objects as thin as 25µm between the teeth).
Cusp Anatomy and Root Canal Configuration
| Tooth | Crown Cusps / Fossae | Roots | Root Canals | Clinical Notes |
|---|---|---|---|---|
| Upper central incisor | No cusps; shovel-shaped cingulum; mesial marginal ridge; incisal edge | Single conical root | 1 (single) | Widest mesiodistally of upper anteriors; dens invaginatus (dens in dente) most common here; talon cusp (mesiopalatal extension of cingulum) |
| Upper first premolar | 2 cusps (buccal + palatal; buccal larger and longer); mesial and distal marginal ridges; central fossa; mesial root concavity (important for endo access) | Usually 2 (buccal + palatal); sometimes 1 or 3 | 2 (buccal + palatal) in ~72%; 1 in ~26%; 3 in ~2% | Mesial root concavity is the most pronounced of any premolar — perforations during root canal preparation common here |
| Upper first molar | 4 main cusps (MB, DB, MP [mesiopalatal — largest], DP) + cusp of Carabelli (5th, on MP cusp). Oblique ridge connects DB cusp to MP cusp. Central fossa (mesial) + distal fossa (distal to oblique ridge) | 3 roots: MB, DB, and palatal (palatal root is longest, largest, most divergent) | 3–4: MB1, MB2 (present in ~60–70%); DB (1); palatal (1 — largest diameter) | MB2 is most frequently missed canal in root canal treatment. Cusp of Carabelli: non-functional supernumerary cusp; familial trait; clinically important as a landmark for identification |
| Lower first molar | 5 cusps (MB, DB, ML, DL, distal cusp — distal cusp smallest; sometimes absent → 4 cusps). Y-shaped or + groove pattern. Central fossa (primary fossa) | 2 roots: mesial (broader, with 2 canals in ~70–75%) + distal (often 1 canal, occasionally 2) | 3 (MB, ML, distal) commonly; up to 4 (MB, ML, DB, distal); radix entomolaris: 3rd root on the distal-lingual found in ~3.5% of Caucasians but up to 40% in some Asian and African populations | Lower first molar most commonly extracted tooth (followed by upper first molar). ML canal of mesial root is most common missed canal |
| Lower third molar | Variable — 4–5 cusps; often irregular or fused cusps. Groove pattern variable | 2 fused or separate roots; variable; may have 3 roots; fused conical roots common | 2–4; variable; commonly 3 canals; fused tapered canals at apex | Root proximity to IAN varies widely — CBCT is gold standard for surgical planning. Dilaceration (abrupt root curvature) most common in third molars |
Clinically Significant Anatomical Variations
Dens invaginatus (dens in dente): an anomaly caused by invagination of the enamel organ into the dental papilla before calcification — results in an enamel-lined “invagination” within the crown (and sometimes root) of the tooth. Most common in the upper lateral incisor. Clinically: can present as an “extra” or duplicated lumen on periapical radiograph (the “tooth within a tooth” appearance); the invagination communicates with the oral environment and is a portal of entry for bacteria → early pulp necrosis despite an apparently intact crown. Management: flowable composite into the invagination if caught early; root canal treatment when necrosis occurs.
Taurodontism: an anomaly where the body of the tooth is enlarged apically and the furcation is displaced towards the root apices (pulp chamber elongated, roots shortened). Associated with various syndromes (Klinefelter’s, tricho-dento-osseous syndrome). Root canal treatment is challenging due to the displaced furcation.
Dilaceration: an abrupt angulation or bend in the root of a tooth, most often the result of a traumatic event during tooth development displacing the tooth germ. Most common in the upper central incisor (trauma to primary predecessor during development) and the lower third molar. Dilacerated roots complicate extraction (risk of root fracture) and root canal treatment.
Gemination vs. Fusion: Gemination = one tooth bud attempts to divide → one tooth with bifid crown and one root (tooth count is normal). Fusion = two adjacent tooth buds fuse → one tooth with combined crown and one or two roots (tooth count is one fewer than expected). Clinical distinction: count the teeth — if total count is normal, it is gemination; if total count is reduced by one, it is fusion.
Maxillary Sinus
The maxillary sinus (antrum of Highmore) is the largest of the paranasal sinuses. It is a pyramidal air-filled space within the body of the maxilla. Important clinical relationships: (1) The floor of the maxillary sinus is intimately related to the roots of the maxillary posterior teeth, particularly the upper first and second molars (the palatal root of the upper first molar has the closest relationship in most patients) and the upper second premolar. In some patients, the roots of these teeth project directly into the sinus floor — making extraction difficult without oro-antral communication (OAC), and making periapical infections capable of causing secondary sinusitis (dentoalveolar sinusitis). (2) The Schneiderian membrane is the thin mucoperiosteal lining of the maxillary sinus — it is continuous with the periosteum of the sinus walls. It must be preserved intact during implant-related sinus lift surgery (lateral window sinus augmentation and transcrestal/osteotome sinus elevation). Membrane perforation during sinus lift requires either repair with a collagen membrane and continuation of surgery, or termination of the procedure and rescheduling. (3) Drainage: the maxillary sinus drains through the ostium (located high on the medial wall of the sinus, near the roof) into the middle meatus of the nasal cavity (under the middle turbinate). The high position of the ostium relative to the sinus floor is clinically problematic — the sinus does not drain by gravity, and ciliary transport is required to move mucus up to the ostium. When the ostium is blocked (rhinitis, polyps, deviated septum), the sinus retains secretions → sinusitis.
Clinical Considerations
- The long buccal nerve must be blocked separately for lower molar soft tissue procedures: The inferior alveolar nerve block anaesthetises the mandibular teeth, periodontal ligament, alveolar bone, and the lingual and buccal gingiva of the anterior teeth (via the mental nerve). However, the buccal gingiva and mucosa of the lower molar region is supplied by the long buccal nerve (a separate branch of V3 that does not pass through the mandibular foramen). The IANB does not block the long buccal nerve. If a lower molar procedure involves buccal soft tissue manipulation (subgingival scaling, flap surgery, or extraction), a separate long buccal nerve block injection (small volume of LA into the buccal mucosa at the level of the crown of the lower second molar) is required.
- Tongue deviation on protrusion indicates CN XII (hypoglossal) damage on the side toward which it deviates: The genioglossus muscle (the primary tongue protrusor, innervated by CN XII bilaterally) protrudes the tongue when contracting bilaterally. When one CN XII is damaged, the intact contralateral genioglossus is unopposed and pushes the tongue toward the ipsilateral (damaged) side. A simpler way to remember: the tongue deviates toward the lesion (toward the damaged side). This is tested clinically in head and neck examination and is relevant whenever a floor of mouth, submandibular, or hypoglossal nerve stimulator procedure is planned.
- The submandibular gland is the most common site of salivary calculi — and the anatomy explains why: Wharton’s duct is long (~5cm), relatively wide in calibre, and runs in an upward direction from the hilum of the gland at the back of the floor of the mouth to its opening at the sublingual papilla anteriorly. This means saliva must flow against gravity from the gland to the duct opening. The submandibular gland also produces a more viscous, calcium-rich secretion than the parotid. These two factors (long upward-running duct + viscous calcium-rich saliva) explain the high rate of calculus formation here. Clinically: pain and swelling of the submandibular gland that is worst just before and during meals (anticipatory pain — the gland fills with saliva but cannot drain past the stone) is diagnostic. Bimanual palpation (one finger intraorally in the floor of the mouth, one extraorally below the mandible) can often palpate the stone.
- Chorda tympani damage causes ipsilateral loss of taste from the anterior two-thirds of the tongue plus reduced submandibular/sublingual saliva: The chorda tympani is a branch of CN VII that exits the skull through the petrotympanic fissure, joins the lingual nerve (CN V3 branch) in the infratemporal fossa, and travels with it to the floor of the mouth. It carries: (a) special visceral afferent (SVA) fibres for taste from the anterior two-thirds of the tongue (cell bodies in the geniculate ganglion); (b) preganglionic parasympathetic efferent fibres to the submandibular ganglion (which then relays to the submandibular and sublingual glands). Damage during an inferior alveolar nerve block (the lingual nerve and chorda tympani run together) or during floor of mouth surgery produces: metallic/altered taste on the ipsilateral anterior tongue + dry mouth on that side. Most chorda tympani injuries from nerve blocks are temporary (neuropraxia — resolves in weeks to months).
- The cusp of Carabelli on the upper first molar is a valuable aid in tooth identification — but has no occlusal function: The cusp of Carabelli (fifth cusp) is located on the mesiopalatal (mesiolingual) aspect of the upper first molar — specifically on the mesiopalatal cusp. It ranges from a faint pit or groove (in its mildest expression) to a full accessory cusp. It is a familial trait (autosomal dominant with variable expressivity) and is more commonly expressed in Caucasian populations. On a periapical radiograph, it can appear as an additional enamel protrusion. It may present a caries risk if its groove is deep and fissured. Most importantly for board examinations: the cusp of Carabelli is on the upper first molar, mesiopalatal cusp — not on any other tooth or cusp.
Common Mistakes & Misconceptions
- Misconception: “The lingual nerve provides taste sensation to the anterior two-thirds of the tongue.”
Correction: The lingual nerve (a branch of CN V3) provides only general somatic sensation (touch, temperature, pain) to the anterior two-thirds of the tongue — NOT taste. Taste from the anterior two-thirds of the tongue is carried by the chorda tympani (a branch of CN VII) which joins the lingual nerve in the infratemporal fossa and travels with it. Taste from the posterior one-third of the tongue is carried by the glossopharyngeal nerve (CN IX). This distinction is clinically important: if a patient has lost taste on the anterior tongue after a dental injection, the chorda tympani (not just the lingual nerve) has been affected. - Misconception: “The lateral pterygoid muscle closes the jaw.”
Correction: The lateral pterygoid is the only muscle of mastication that opens the jaw (depresses the mandible), along with the suprahyoid muscles. Specifically, the lower head of the lateral pterygoid contracts during opening to translate the condyle anteriorly along the articular eminence. All other muscles of mastication (masseter, temporalis, medial pterygoid) are jaw-closing (elevating) muscles. The lateral pterygoid also protrudes the mandible and produces lateral excursion to the contralateral side when contracting unilaterally. - Misconception: “Parotid gland stones are the most common salivary calculi.”
Correction: The submandibular gland is the site of approximately 80–90% of all salivary calculi — not the parotid. The parotid accounts for only ~10–15% of salivary stones. The anatomy explains this: Wharton’s duct (submandibular) is long and runs against gravity; the submandibular gland secretes viscous, more alkaline, calcium-rich saliva. The parotid produces thin serous saliva and its duct (Stensen’s) runs anteriorly with gravity assistance. - Misconception: “The hypoglossal nerve supplies all tongue muscles.”
Correction: The hypoglossal nerve (CN XII) supplies all intrinsic tongue muscles and most extrinsic tongue muscles — but NOT the palatoglossus. The palatoglossus (elevates the back of the tongue and depresses the soft palate — important for swallowing initiation) is innervated by the vagus nerve (CN X) via the pharyngeal plexus. All other extrinsic tongue muscles (genioglossus, hyoglossus, styloglossus) are innervated by CN XII. - Misconception: “Gemination produces a tooth count that is one fewer than normal.”
Correction: It is fusion (not gemination) that produces one fewer tooth in the count. Gemination occurs when one tooth bud attempts to divide (and usually fails completely), resulting in a double or bifid crown on a tooth with one root — the total tooth count is normal (because one bud produced one — albeit double — tooth). Fusion results from two adjacent tooth buds fusing — the total count is one fewer than normal (because two buds produced one tooth). The clinical test: count the teeth. Normal count → gemination. One fewer tooth → fusion.
Related Topics
References & Sources
- Fehrenbach MJ, Herring SW (2021). Illustrated Anatomy of the Head and Neck, 6th ed. Elsevier. [Primary reference for dental anatomy of the head and neck — cranial nerves, muscles, salivary glands, blood supply]
- Ash MM, Nelson SJ (2003). Wheeler’s Dental Anatomy, Physiology, and Occlusion, 8th ed. Saunders. [Classic reference for crown and root morphology, pulp anatomy, cusp anatomy]
- Nanci A (2017). Ten Cate’s Oral Histology: Development, Structure, and Function, 9th ed. Elsevier. [Tooth structure — enamel, dentine, cementum, pulp, periodontal ligament — histology and clinical correlates]
- Standring S (ed) (2020). Gray’s Anatomy: The Anatomical Basis of Clinical Practice, 42nd ed. Elsevier. [Comprehensive head and neck anatomy — cranial nerves, muscles, vessels, spaces, TMJ]
- Vertucci FJ (1984). Root canal anatomy of the human permanent teeth. Oral Surgery, Oral Medicine, Oral Pathology, 58(5):589–599. [Classic study on root canal configuration — Vertucci classification; MB2 prevalence in upper molars]
- Weiglein AH, Moriggl B, Michelmayr K, Maurer H (1997). Pterygomandibular region and infratemporal fossa — cadaveric study. Surgical and Radiologic Anatomy. [Dissection-based study — chorda tympani and lingual nerve relationship in the infratemporal fossa]
- Geist JR, Katz JO (1990). The frequency and distribution of idiopathic osteosclerosis. Oral Surgery, Oral Medicine, Oral Pathology, 69(3):388–393. [Dental anatomical variants and their radiographic recognition]
- McGurk M, Escudier MP, Brown JE (2005). Modern management of salivary calculi. British Journal of Surgery, 92(1):107–112. [Salivary calculi — prevalence, site distribution, management options including sialendoscopy]
Summary
Dental and head and neck anatomy provides the essential structural context for every clinical procedure in dentistry. The trigeminal nerve (CN V) is the master sensory nerve of the face and oral cavity — V1 supplies the forehead and eye region; V2 supplies the maxillary teeth, palate, and cheek; V3 (the only division with a motor root) supplies the mandibular teeth, tongue sensation, and all four muscles of mastication. The four muscles of mastication — all innervated by CN V3 and all derived from the first pharyngeal arch — are masseter, temporalis, and medial pterygoid (all jaw closers), and the lateral pterygoid (the only jaw opener, with two distinct heads). The three major salivary glands are: parotid (largest; serous; Stensen’s duct; CN IX parasympathetics via auriculotemporal nerve; CN VII passes through it), submandibular (mixed; Wharton’s duct; CN VII chorda tympani parasympathetics via lingual nerve; most common site for calculi), and sublingual (smallest; mucous; ducts of Rivinus). Blood supply is primarily from the external carotid artery — with the maxillary artery (its larger terminal branch) supplying the deep face, mandibular and maxillary teeth, and palate. Dental anatomy highlights include: the cusp of Carabelli on the upper first molar mesiopalatal cusp, the oblique ridge of the upper first molar, the MB2 canal in the mesiobuccal root of upper molars (present in ~60–70%), and the five-cusp pattern of the lower first molar.
Key Takeaways
- Trigeminal nerve: V1 = forehead/eye; V2 = maxillary teeth/palate/cheek (foramen rotundum); V3 = mandibular teeth/tongue sensation/muscles of mastication (foramen ovale). Lingual nerve = touch only; chorda tympani (CN VII) = taste anterior 2/3 tongue.
- Muscles of mastication: All from CN V3 + first pharyngeal arch. Elevators (close jaw): masseter, temporalis, medial pterygoid. Opener/protruder: lateral pterygoid (two heads; only muscle that depresses mandible).
- Salivary glands: Parotid (serous; Stensen’s duct; CN VII passes through; CN IX supplies). Submandibular (mixed; Wharton’s duct; most common calculi site 80–90%). Sublingual (mucous; ducts of Rivinus). Parasympathetics submandibular + sublingual: chorda tympani (CN VII) → lingual nerve.
- ECA terminal branch — maxillary artery: Middle meningeal (foramen spinosum — epidural haematoma); inferior alveolar (mandibular teeth); PSA (upper molars); infraorbital/ASA (upper incisors/canines); descending palatine; sphenopalatine (posterior epistaxis).
- Dental anatomy: Upper first molar: 4 main cusps + cusp of Carabelli (on mesiopalatal cusp); oblique ridge; MB2 canal in ~60–70%. Lower first molar: 5 cusps; 2 roots; mesial root has 2 canals in ~70–75%. Gemination = normal tooth count; fusion = one fewer tooth.
About the Author
