Skull and skull base foramina
Skull, facial bones, mandible and teeth; skull base foramina
The skull is read on plain films, panoramic radiographs and thin-section CT, and most questions ask you to match an opening to what passes through it, or a fracture pattern to the nerve or muscle it disturbs. Learn the foramina as a map of the cranial fossae, then learn how the facial skeleton and mandible break.
Skull base foramina and their contents
| Opening | Fossa and bone | Main contents | Lesion or imaging point |
|---|---|---|---|
| Cribriform plate | Anterior; ethmoid | Olfactory nerve filaments, anterior ethmoidal nerve and vessels | Fracture gives anosmia, CSF rhinorrhoea and bilateral periorbital bruising |
| Optic canal | Middle; lesser wing of sphenoid | Optic nerve, ophthalmic artery | Narrowed by fibrous dysplasia |
| Foramen rotundum | Greater wing | V2 only | Numb cheek, upper lip and upper teeth; chewing normal (V2 has no motor fibres) |
| Foramen ovale | Greater wing | V3, lesser petrosal nerve, accessory meningeal artery | Route for perineural tumour spread from the masticator space |
| Foramen spinosum | Greater wing | Middle meningeal artery, meningeal branch of V3 | Smallest of the three greater-wing foramina; lies posterolateral to ovale |
| Foramen lacerum | Petrous apex, sphenoid and basiocciput | Closed by cartilage in life; ICA crosses over its upper end | Nothing major passes vertically through it |
| Pterygoid (vidian) canal | Sphenoid body, inferomedial to rotundum | Nerve of the pterygoid canal | Both canals open into the pterygopalatine fossa |
| Internal acoustic meatus | Posterior; petrous temporal | VII, VIII, labyrinthine artery | Widened by vestibular schwannoma |
| Jugular foramen | Posterior; between temporal and occipital | IX, X, XI, sigmoid sinus becoming the IJV, inferior petrosal sinus | Glomus tumour: pulsatile tinnitus with IX–XI palsy |
| Hypoglossal canal | Occipital condyle | XII | Adding XII to a jugular foramen lesion means spread here |
| Foramen magnum | Occipital | Medulla, vertebral arteries, spinal roots of XI ascending | Cerebellar tonsils herniate here (Chiari I) |
Pterygopalatine fossa
- Small fat-filled space behind the maxilla, containing the third part of the maxillary artery, V2 and the pterygopalatine ganglion.
- It opens into the middle cranial fossa (rotundum), nasal cavity (sphenopalatine foramen), orbit, infratemporal fossa and palate (greater palatine canal).
- On CT and MRI, loss of its normal fat is the sign of perineural spread, for example from an adenoid cystic carcinoma of the palate.
Vault, sutures and fontanelles
- The pterion (frontal, parietal, squamous temporal and greater wing of sphenoid meet) overlies the anterior branch of the middle meningeal artery. A blow here gives an extradural haematoma: biconvex and stopped at the sutures, because the dura is firmly fixed at suture lines.
- A subdural haematoma is crescentic and crosses sutures (torn bridging veins).
- Anterior fontanelle closes by about 18 months; the posterior by about 2–3 months.
- Craniosynostosis: sagittal (commonest) gives a long narrow head; unilateral coronal gives a flat forehead and a raised orbital roof (harlequin orbit on the frontal film); metopic gives a triangular forehead.
- Cleidocranial dysplasia (RUNX2): open fontanelles, many wormian bones, absent or small clavicles, delayed and supernumerary teeth.
- Fracture or groove? A fracture is a sharp, straight, lucent line with no sclerotic rim; vascular grooves taper and branch with corticated margins. Sutures and synchondroses (such as the spheno-occipital synchondrosis, which fuses in adolescence) are smooth and corticated.
Facial fractures
| Pattern | Line of fracture | Clue |
|---|---|---|
| Le Fort I | Horizontal through the maxilla above the teeth | Mobile palate and teeth |
| Le Fort II | Pyramidal: nasal bones, medial orbital walls, orbital floors, zygomaticomaxillary sutures | Cheek numbness (infraorbital nerve) |
| Le Fort III | Nasofrontal, lateral orbital walls, zygomatic arches | Whole face separated from the skull |
- All three Le Fort fractures involve the pterygoid plates.
- An isolated zygomatic arch fracture is V-shaped on the submentovertex view; the depressed fragment hits the coronoid process and causes trismus.
Mandible and temporomandibular joint
- The mandible is a ring, so look for a second break: parasymphyseal fracture with a contralateral condylar neck fracture is classic.
- Angle fracture: the posterior fragment is pulled up and medially by masseter and medial pterygoid (and temporalis). A condylar neck fragment is pulled forwards and medially by lateral pterygoid.
- A fracture through the mandibular canal injures the inferior alveolar nerve: numb lower lip, chin and lower teeth. The mental foramen lies below the second lower premolar.
- TMJ: synovial joint with an articular disc. On closed-mouth sagittal MRI the posterior band sits directly above the condyle. The superior head of lateral pterygoid pulls the disc forwards; anterior disc displacement with reduction gives a reciprocal click.
- Childhood condylar injury can cause ankylosis with loss of condylar growth: small receding mandible.
Teeth on radiographs
- Enamel is the most radiopaque tissue in the body (about 96% hydroxyapatite); dentine is less dense; the pulp is lucent.
- The lamina dura is the thin white cortical line of the socket. Generalised loss of lamina dura suggests hyperparathyroidism.
- Teeth sit in a fibrous joint, the gomphosis. The first permanent molar erupts at about 6 years, behind the primary second molar.
| Jaw lesion | Imaging and key feature |
|---|---|
| Radicular (periapical) cyst | Round corticated lucency at the apex of a non-vital tooth; the commonest jaw cyst |
| Dentigerous cyst | Unilocular lucency around the crown of an unerupted tooth, attached at the cemento-enamel junction (often lower third molar) |
| Odontogenic keratocyst | Expands along the bone; multiple in basal cell naevus syndrome with falx calcification |
| Fibrous dysplasia | Expanded bone with ground-glass matrix; activating GNAS mutation |
Remember Along the greater wing, from front to back: rotundum (V2), ovale (V3), spinosum (middle meningeal artery).
Exam trap The ICA does not run up through the foramen lacerum; it crosses over the cartilage that fills it.
Test yourself on this topic
5 of the 61 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 50-year-old woman with an adenoid cystic carcinoma of the hard palate develops numbness of her cheek. The tumour has spread backwards along the greater palatine nerve. Which space does the greater palatine canal lead into?
Answer: Pterygopalatine fossa
The greater palatine canal runs from the palate up to the pterygopalatine fossa, carrying the greater palatine nerve and vessels. Tumour reaching the fossa can then follow the maxillary nerve, which passes through the fossa, back through the foramen rotundum to the cavernous sinus; involvement of the maxillary nerve explains the cheek numbness. Adenoid cystic carcinoma is the salivary tumour best known for perineural spread.
Pterygopalatine fossa spread
- Palate to fossa via greater palatine canal
- Fossa contains the maxillary nerve and ganglion
- Adenoid cystic carcinoma spreads along nerves
- Onward via foramen rotundum to cavernous sinus
Remember Palate to fossa to rotundum to cavernous sinus.
Exam trap Perineural spread can be present with a small primary, so cranial nerve symptoms should never be dismissed.
Why the other options are wrong
- Infratemporal fossa Lies lateral to the fossa, through the pterygomaxillary fissure; not the direct target of the palatine canal.
- Cavernous sinus Reached later, via the foramen rotundum.
- Parapharyngeal space Lies medial to the masticator space; not connected to the palatine canal.
- Sphenoid sinus Lies above and medial; tumour reaches it only by bone destruction.
A week after an untreated boil on the scalp, a 30-year-old man develops thrombosis of the superior sagittal sinus. Through which channels did the infection reach the sinus?
Answer: Emissary veins
Emissary veins are valveless veins that pass through foramina in the skull, such as the parietal and mastoid foramina, joining the veins of the scalp to the dural venous sinuses. Because they have no valves, blood and infection can flow in either direction. They cross the loose areolar layer of the scalp, which is why infection in that layer is dangerous. Infected thrombus can then spread into the sinus and cause sinus thrombosis or meningitis.
Emissary veins
- Valveless veins through the skull
- Join scalp veins to dural sinuses
- Cross the loose areolar layer
- Carry infection to sinuses and meninges
Remember Scalp infection reaches the brain through emissary veins.
Exam trap The danger layer is the loose areolar tissue, but the channel into the skull is the emissary vein.
Why the other options are wrong
- Arachnoid granulations Project into the sinuses to return CSF; they do not drain the scalp.
- Scalp lymphatic vessels Drain to cervical nodes, away from the skull.
- Branches of occipital artery Arterial flow runs towards the scalp, not into the sinuses.
- Middle meningeal veins Lie between dura and bone, draining the meninges, not the scalp.
A 35-year-old man has a grossly carious lower first molar that does not respond to pulp testing. A cyst attached to its root apex is removed; it is lined by non-keratinised stratified squamous epithelium with chronic inflammation in the wall. What is the lesion?
Answer: Radicular cyst
A radicular (periapical) cyst is the commonest jaw cyst. Caries kills the pulp, inflammation spreads out of the apex, and the epithelial cell rests of Malassez proliferate to line a cyst. Its lining is non-keratinised stratified squamous epithelium, often with cholesterol clefts and inflammatory cells, and it sits on the apex of a non-vital tooth. A vital tooth with an apical lesion should raise other diagnoses.
Radicular cyst
- Commonest jaw cyst
- Apex of a non-vital tooth
- From rests of Malassez
- Non-keratinised lining with inflammation
Remember Dead tooth, apical cyst: radicular cyst.
Exam trap Always test the tooth's vitality; a vital tooth argues against a radicular cyst.
Why the other options are wrong
- Dentigerous cyst Surrounds the crown of an unerupted tooth, not the root apex.
- Odontogenic keratocyst Has a corrugated parakeratinised lining and is not caused by caries.
- Ameloblastoma A solid or multicystic odontogenic tumour, not an inflamed cyst.
- Stafne bone cavity A salivary gland indentation of the mandible, not a lined cyst.
A mother brings her 6-year-old to the dental OPD because a new tooth has appeared behind the last milk tooth in each quadrant, although no milk tooth has fallen out. Which tooth is this?
Answer: Permanent first molar
The permanent first molar is usually the first permanent tooth to erupt, at about 6 years, and it comes in behind the primary second molar without replacing any primary tooth. This begins the mixed dentition stage. Premolars replace the primary molars later (about 10 to 12 years), and the permanent second molar erupts at about 12 years. The lower central incisors are replaced at about the same age as the first molar appears.
First permanent tooth
- Permanent first molar at about 6 years
- Erupts behind the primary second molar
- Replaces no primary tooth
- Starts the mixed dentition
Remember Six years, first molar.
Exam trap Parents often think the six-year molar is a primary tooth because no tooth falls out before it.
Why the other options are wrong
- Permanent second molar Erupts at about 12 years, behind the first molar.
- Permanent first premolar Replaces the primary first molar at about 10 years.
- Permanent canine Replaces the primary canine at about 10 to 12 years.
- Permanent second premolar Replaces the primary second molar at about 11 to 12 years.
The joint between the root of a tooth and its bony socket is of which type?
Answer: Gomphosis (fibrous)
A gomphosis is a fibrous 'peg-in-socket' joint found only between the teeth and the alveolar bone. The root is held by the periodontal ligament, whose collagen fibres run from the cementum to the bone of the socket. It allows only a tiny amount of movement and is rich in pressure receptors that help control biting force. Sutures are fibrous joints between skull bones, and syndesmoses join bones by a ligament or membrane, as between the tibia and fibula.
Gomphosis
- Peg-in-socket fibrous joint
- Held by the periodontal ligament
- Cementum to alveolar bone
- Rich in pressure receptors
Remember Tooth in its socket: gomphosis.
Exam trap Gomphosis is fibrous, not cartilaginous, even though teeth are hard tissues.
Why the other options are wrong
- Synchondrosis (cartilaginous) A joint of hyaline cartilage, such as the first sternocostal joint.
- Suture (fibrous) A fibrous joint between skull bones, not a tooth.
- Symphysis (cartilaginous) A fibrocartilage joint in the midline, such as the pubic symphysis.
- Syndesmosis (fibrous) Bones joined by a ligament or membrane, such as the inferior tibiofibular joint.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Head and neck, Skull, facial bones, mandible and teeth; skull base foramina