Bones Of The Skull

Foundations and mechanisms

Structural organisation and developmental basis

The adult skull comprises 22 bones excluding the ossicles: 8 cranial bones forming the neurocranium and 14 facial bones forming the viscerocranium. Functionally, the skull is a rigid protective box for the brain, cranial nerves, cerebral vessels, special sensory organs and upper airway. For the anaesthetist, its relevance is concentrated at the cranial base, orbit, nasal cavity, temporal bone, mandible and the foramina transmitting nerves and vessels vulnerable to trauma, infection, tumour spread and regional anaesthetic techniques.

The neurocranium is divided into the calvaria and cranial base. The calvaria develops predominantly by intramembranous ossification, whereas much of the skull base develops by endochondral ossification from the chondrocranium. This distinction is clinically important: membranous bones are separated by sutures and fontanelles allowing moulding during birth, whereas skull-base synchondroses influence cranial base growth and are implicated in craniosynostosis syndromes.

Region/classification	Principal bones	Key mechanisms and exam relevance
Neurocranium: calvaria	Frontal, parietal, squamous temporal, occipital squama	Intramembranous ossification; sutures permit growth perpendicular to the suture line; premature fusion produces compensatory growth elsewhere.
Neurocranium: cranial base	Ethmoid, sphenoid, petrous temporal, basiocciput	Endochondral ossification; multiple foramina transmit cranial nerves and vessels; fractures frequently cause CSF leak, cranial neuropathy and vascular injury.
Viscerocranium	Maxillae, zygomata, nasal bones, lacrimals, palatines, inferior conchae, vomer, mandible	Determines facial skeleton, nasal airway, dental occlusion and airway management geometry; mandible is critical for mask ventilation and laryngoscopy.

Sutures, fontanelles and cranial growth

Major sutures include the coronal, sagittal, lambdoid and squamous sutures. The neonatal skull has six fontanelles. The posterior fontanelle usually closes by 2–3 months; the anterior fontanelle typically closes by 12–18 months, with normal variation up to approximately 24 months. Cranial capacity expands rapidly: brain volume is approximately 25% of adult size at birth, 50% by 6 months and 80–90% by 2 years. Craniosynostosis follows Virchow’s law: growth is restricted perpendicular to the fused suture and compensatory parallel growth occurs. Examples include sagittal synostosis causing scaphocephaly, coronal synostosis causing anterior plagiocephaly or brachycephaly, and metopic synostosis causing trigonocephaly.

Cranial fossae, foramina and transmitted structures

The internal cranial base is arranged into anterior, middle and posterior cranial fossae. The anterior fossa supports frontal lobes and contains the cribriform plate, through which olfactory fila pass; its thin structure explains anosmia and CSF rhinorrhoea after frontobasal trauma. The middle fossa accommodates temporal lobes, the pituitary fossa and cavernous sinus region. The posterior fossa contains the brainstem and cerebellum and communicates with the vertebral canal via the foramen magnum.

Foramen/canal	Bone/region	Major contents
Cribriform plate	Ethmoid	Olfactory nerve filaments; route for CSF rhinorrhoea and meningitis after fracture.
Optic canal	Sphenoid	CN II and ophthalmic artery.
Superior orbital fissure	Sphenoid	CN III, IV, V1, VI and superior ophthalmic vein.
Foramen rotundum	Sphenoid	Maxillary nerve, V2.
Foramen ovale	Sphenoid	Mandibular nerve, V3; accessory meningeal artery; lesser petrosal nerve variably.
Foramen spinosum	Sphenoid	Middle meningeal artery; clinically linked to extradural haematoma.
Internal acoustic meatus	Petrous temporal	CN VII, CN VIII and labyrinthine artery.
Jugular foramen	Temporal/occipital	CN IX, X, XI; sigmoid sinus continuation as internal jugular vein.
Hypoglossal canal	Occipital	CN XII.
Foramen magnum	Occipital	Medulla, meninges, vertebral arteries, spinal roots of CN XI, anterior and posterior spinal arteries.

Mechanical properties, intracranial implications and fracture patterns

The adult calvarium is approximately 6–7 mm thick on average, with outer table, diploë and inner table; the inner table is more brittle and may fracture despite less obvious external injury. The skull is rigid in adults, so intracranial volume is constrained by the Monro–Kellie doctrine: brain, blood and CSF occupy a near-fixed compartment of approximately 1500 mL total intracranial volume, with CSF around 150 mL and cerebral blood volume approximately 70–100 mL. Normal adult intracranial pressure is about 5–15 mmHg; sustained values >20–22 mmHg are commonly used treatment thresholds in traumatic brain injury guidelines.

Fracture mechanisms reflect applied force and skull-base anatomy. Linear fractures are most common and may cross vascular grooves; temporal fractures crossing the middle meningeal artery predispose to extradural bleeding. Depressed fractures imply focal energy transfer and dural or cortical injury. Basilar skull fractures involve anterior, middle or posterior fossae and may produce CSF rhinorrhoea or otorrhoea, haemotympanum, Battle’s sign, periorbital ecchymosis, anosmia, facial nerve palsy or lower cranial nerve deficits. The petrous temporal bone is classically divided into longitudinal and transverse fractures; longitudinal fractures are more common, whereas transverse fractures more often injure CN VII/VIII and the labyrinth.

Airway and regional anaesthetic relevance

The skull forms the fixed skeletal limits for mask fit, nasal instrumentation and laryngoscopy. The maxilla, nasal bones, vomer, perpendicular plate of ethmoid and turbinates determine nasal airway resistance; trauma or coagulopathy increases the risk of epistaxis during nasotracheal intubation. The mandible articulates with the temporal bone at the temporomandibular joint; mouth opening depends on rotation and anterior translation of the mandibular condyle, normally permitting an inter-incisor distance of approximately 35–50 mm. Sensory innervation of the face and anterior scalp is principally trigeminal, whereas posterior scalp sensation arises from cervical nerves, particularly the greater occipital nerve from C2; this anatomical division underpins scalp blocks for craniotomy and awake neurosurgical procedures.

Clinical assessment and investigations

Presentation and immediate clinical priorities

In examination terms, skull anatomy becomes clinically relevant through trauma, congenital variants, infection, malignancy and operative access to intracranial structures. In the acute setting, assessment follows ABCDE with cervical spine immobilisation, because cranial injury frequently coexists with high-energy cervical spine trauma. The anaesthetist must identify features that alter airway strategy: midface instability, mandibular disruption, active bleeding, basal skull fracture, reduced consciousness and raised intracranial pressure.

Skull fractures present with scalp laceration or haematoma, focal bony tenderness, palpable step, neurological deficit, seizures, depressed conscious level, or signs of basal skull injury. Important basal signs are periorbital ecchymosis from anterior cranial fossa injury, Battle’s sign from middle cranial fossa injury, haemotympanum, otorrhoea, rhinorrhoea, anosmia and cranial nerve palsies. These signs may be delayed by several hours and their absence does not exclude fracture.

Clinical pattern	Likely anatomical site	Key anaesthetic relevance
CSF rhinorrhoea, anosmia, periorbital bruising	Anterior cranial fossa; cribriform plate, frontal bone, ethmoid	Avoid nasal intubation, nasopharyngeal airway and blind nasogastric insertion
Haemotympanum, otorrhoea, facial nerve palsy, vertigo	Temporal bone; petrous temporal, middle ear, internal acoustic meatus	Risk of airway blood contamination, aspiration, lower cranial nerve dysfunction
Periorbital swelling, diplopia, infraorbital numbness	Orbital floor or zygomaticomaxillary complex	Occult globe injury; avoid external pressure on eye
Malocclusion, mobile maxilla, epistaxis	Midface fracture including Le Fort pattern	Difficult mask seal, difficult laryngoscopy, major haemorrhage

Differential diagnosis

The differential for skull pain, deformity or radiological abnormality is wider than trauma. In postgraduate examinations, distinguish the bone lesion from its intracranial consequences.

• Traumatic: linear vault fracture, depressed fracture, open or compound fracture, comminution, diastatic fracture across sutures, basal skull fracture, penetrating injury.
• Intracranial sequelae: extradural haematoma classically associated with temporal or parietal fracture and middle meningeal artery injury; subdural haematoma, contusion, traumatic subarachnoid haemorrhage and diffuse axonal injury may occur without visible skull fracture.
• Facial skeleton injury: nasal, orbital, zygomatic, mandibular and Le Fort I–III maxillary fractures; these primarily affect airway management and haemorrhage control.
• Non-traumatic bone pathology: osteomyelitis, mastoiditis with temporal bone erosion, metastases, myeloma, Paget disease, fibrous dysplasia, hyperostosis frontalis interna, craniosynostosis and postoperative craniectomy defects.

Investigations and imaging thresholds

Non-contrast CT head with bone windows is the first-line investigation for suspected clinically significant skull fracture or intracranial injury. Plain skull radiographs have poor sensitivity and rarely change acute management; they are largely obsolete outside specific foreign body or non-accidental injury pathways. A typical adult CT head delivers approximately 2 mSv effective dose; skull radiography is lower, approximately 0.1 mSv, but diagnostic yield is substantially inferior.

Current UK NICE head injury guidance uses time-critical CT thresholds. In adults, CT head should be performed within 1 hour for GCS ≤12 on initial assessment, GCS <15 at 2 hours, suspected open or depressed skull fracture, signs of basal skull fracture, post-traumatic seizure, focal neurological deficit, or more than one episode of vomiting. Patients with any loss of consciousness or amnesia require CT within 8 hours if age ≥65 years, bleeding or clotting disorder including anticoagulation, dangerous mechanism, or retrograde amnesia >30 minutes. In anticoagulated patients, many centres image even with apparently minor injury because delayed intracranial bleeding is uncommon but clinically important.

Investigation	Principal use	Interpretation points
CT head, bone and brain windows	Vault/base fracture; haemorrhage; pneumocephalus; mass effect	Look for fracture crossing vascular grooves, sutures, air sinuses or skull base foramina
CT facial bones/mandible	Midface, orbital and mandibular fractures	Assesses occlusion-related injury, orbital entrapment and surgical airway implications
CT angiography	Vascular injury	Indicated for fracture through carotid canal, foramen lacerum, transverse foramen, severe epistaxis, expanding haematoma or focal deficit
CT venography	Dural venous sinus injury	Consider if fracture crosses superior sagittal, transverse or sigmoid sinus, or unexplained raised ICP
MRI brain/skull base	Cranial nerves, dura, marrow, ligamentous and delayed complications	More sensitive for diffuse axonal injury and skull-base soft tissue disease; not first-line in unstable trauma
Beta-2 transferrin or beta-trace protein	Confirmation of CSF leak	Beta-2 transferrin is highly specific for CSF; reported sensitivity and specificity commonly exceed 85–95%

Interpretation and clinically important thresholds

A depressed skull fracture is clinically significant when the inner table is depressed by more than the thickness of adjacent skull, or when there is dural breach, contamination, cosmetic deformity, neurological deficit or mass effect. Compound depressed fractures generally require antibiotics, tetanus assessment and neurosurgical discussion. Fractures crossing the middle meningeal artery groove increase the risk of extradural haematoma; fractures traversing venous sinuses risk torrential bleeding or sinus thrombosis.

Basal skull fracture is a radiological and clinical diagnosis. Pneumocephalus, fluid in sphenoid or mastoid air cells, ossicular disruption and fractures of the petrous temporal bone are key CT markers. Longitudinal temporal bone fractures are more common and often produce conductive hearing loss; transverse fractures more often involve the labyrinth and facial nerve. A fracture involving the cribriform plate is a contraindication to nasal instrumentation because intracranial placement of tubes has been reported.

Raised intracranial pressure must be inferred from clinical and CT features: declining GCS, pupillary asymmetry, bradycardia with hypertension, effaced basal cisterns, midline shift, compressed ventricles or large extra-axial haematoma. In traumatic brain injury, many neurocritical care protocols treat ICP when sustained >22 mmHg and aim for cerebral perfusion pressure approximately 60–70 mmHg. These thresholds frame perioperative anaesthetic management when skull fracture coexists with intracranial pathology.

Management, pharmacology and procedures

Acute management of skull trauma relevant to anaesthesia

Management is dictated less by the bony injury itself than by associated intracranial pathology, airway risk, cervical spine injury, haemorrhage, pneumocephalus and infection risk. Initial care follows ATLS: airway with in-line cervical stabilisation, prevention of secondary brain injury, haemorrhage control and urgent CT imaging. In traumatic brain injury, secondary insults strongly worsen outcome; target SpO₂ >94%, PaO₂ >13 kPa, normocapnia PaCO₂ 4.5–5.0 kPa, normoglycaemia and temperature control. Avoid hypotension; Brain Trauma Foundation guidance recommends maintaining SBP ≥100 mmHg for age 50–69 years and ≥110 mmHg for age 15–49 or >70 years. In anaesthetic practice, cerebral perfusion pressure is typically targeted at CPP 60–70 mmHg once ICP monitoring is established.

CT head is indicated urgently in suspected skull fracture with reduced GCS, focal neurology, seizure, CSF leak, anticoagulation, penetrating injury, signs of basal skull fracture or high-energy mechanism. Classic basal skull signs include periorbital ecchymosis, mastoid bruising, haemotympanum and CSF rhinorrhoea/otorrhoea; nasotracheal intubation, nasogastric tube insertion and nasal temperature probes are contraindicated where cribriform plate or midface disruption is suspected.

Injury pattern	Key anatomical concern	Management implication
Linear vault fracture	May cross middle meningeal artery groove or venous sinus	Observe if neurologically intact; repeat imaging if deterioration
Depressed fracture	Dural breach, cortical laceration, contamination	Neurosurgical elevation if depression > skull thickness, open wound, CSF leak, contamination, neurological deficit
Basal skull fracture	Anterior/middle cranial fossa, cranial nerves, carotid canal	Avoid nasal instrumentation; assess CSF leak, vascular injury, cranial neuropathies
Temporal bone fracture	Facial nerve, ossicles, labyrinth, carotid canal	ENT/neurosurgical review; audiology follow-up; consider CT temporal bones
Penetrating/open fracture	Foreign material, dural violation, infection	Broad-spectrum antibiotics, tetanus, debridement, dural closure

Pharmacology in the injured skull and cranial vault

Drug management aims to facilitate safe airway control, reduce intracranial pressure, prevent seizures and treat contamination. Rapid sequence induction is usually required when airway reflexes or ventilation are compromised. Ketamine is no longer contraindicated in traumatic brain injury; modern evidence supports haemodynamic stability without clinically important ICP elevation when ventilation is controlled.

Indication	Drug and dose	Exam-relevant comments
RSI induction	Ketamine 1–2 mg kg−1 IV or etomidate 0.2–0.3 mg kg−1 IV; propofol 1–2 mg kg−1 only if haemodynamically tolerant	Avoid hypotension. Etomidate preserves MAP but causes adrenal suppression; propofol reduces CMRO2 and ICP but may reduce CPP.
Neuromuscular blockade	Rocuronium 1.0–1.2 mg kg−1 IV or suxamethonium 1–1.5 mg kg−1 IV	Rocuronium avoids fasciculation-related ICP concerns; suxamethonium acceptable unless contraindicated. Sugammadex reversal: 16 mg kg−1 immediate reversal.
Raised ICP temporisation	Mannitol 0.25–1 g kg−1 IV or hypertonic saline e.g. 3% NaCl 2–5 mL kg−1	Use for impending herniation or intraoperative brain swelling. Monitor osmolality, sodium and renal function; typical serum osmolality ceiling 320 mOsm kg−1.
Seizure prophylaxis	Levetiracetam 500–1000 mg IV/PO 12-hourly or phenytoin loading 15–20 mg kg−1 IV	Used for severe TBI, depressed/open fracture, cortical contusion or early seizure risk. Prophylaxis usually 7 days; no proven prevention of late epilepsy.
Antibiotics for open/compound fracture	Local policy; commonly ceftriaxone 2 g IV 12–24-hourly plus metronidazole 500 mg IV 8-hourly if contaminated	Routine prophylaxis for isolated closed basal skull fracture or sterile CSF leak is not recommended in many guidelines.
Tranexamic acid	1 g IV over 10 min, then 1 g over 8 h	CRASH-3 showed benefit when given within 3 h, particularly mild/moderate TBI; avoid delay.

Procedures involving skull anatomy

Procedural planning requires intimate knowledge of sutures, foramina, vascular grooves and cranial fossae. Burr holes and craniotomies are neurosurgical procedures but frequently demand anaesthetic anticipation of blood loss, venous air embolism, brain relaxation, positioning injury and access limitations. The pterion overlies the anterior branch of the middle meningeal artery and is clinically important in extradural haematoma. Burr holes for emergency extradural/subdural decompression are usually placed according to CT localisation; blind temporal burr holes are now exceptional and unsafe without imaging except in extreme austere circumstances.

ICP monitoring may be intraparenchymal or ventricular. External ventricular drain insertion permits measurement and CSF drainage; ventriculostomy infection, haemorrhage and malposition are major complications. Treat sustained ICP >22 mmHg according to tiered TBI algorithms: sedation/analgesia, head elevation 30°, neutral venous drainage, CSF drainage, osmotherapy, neuromuscular blockade, controlled ventilation, barbiturate coma, decompressive craniectomy in selected cases. Decompressive craniectomy reduces ICP but trial data are nuanced: DECRA showed worse functional outcomes in diffuse injury despite lower ICP; RESCUEicp reduced mortality in refractory ICP but increased survival with severe disability.

Scalp anaesthesia and skull-related blocks

The scalp is richly vascular and innervated; scalp incision, Mayfield pinning and craniotomy cause intense sympathetic responses. A scalp block reduces haemodynamic surges and opioid requirements. Local anaesthetic toxicity risk is non-trivial because of vascular uptake; always calculate cumulative dose.

Nerve	Origin	Block location
Supraorbital/supratrochlear	V1	Superior orbital rim; avoid globe injury
Zygomaticotemporal	V2	Posterior to lateral orbital rim
Auriculotemporal	V3	Anterior to tragus, avoid superficial temporal artery
Lesser occipital	C2	Posterior border of sternocleidomastoid
Greater occipital	C2 dorsal ramus	Medial to occipital artery at superior nuchal line

Typical regimen: bupivacaine 0.25–0.5% or ropivacaine 0.2–0.75%, often with adrenaline 1:200,000 unless contraindicated. Maximum doses: bupivacaine 2 mg kg⁻¹ plain, 3 mg kg⁻¹ with adrenaline; ropivacaine 3 mg kg⁻¹. Treat systemic toxicity with airway control, seizure suppression and lipid emulsion: 20% intralipid 1.5 mL kg⁻¹ bolus, then 15 mL kg⁻¹ h⁻¹, maximum approximately 12 mL kg⁻¹.

Complications and follow-up

Follow-up after skull fracture includes surveillance for delayed extradural/subdural haematoma, meningitis, post-traumatic epilepsy, cranial nerve palsy, anosmia, hearing loss, vestibular dysfunction, cosmetic deformity and persistent CSF leak. CSF leak persisting beyond 7–10 days, recurrent meningitis, pneumocephalus or large skull base defect usually requires endoscopic or neurosurgical repair. Anticoagulated patients need reversal according to agent and indication, with repeat imaging thresholds lower. Long-term review is multidisciplinary: neurosurgery, ENT/maxillofacial surgery, audiology, ophthalmology and neurorehabilitation where indicated.

Exam controversies and advanced synthesis

Why skull anatomy matters to the anaesthetist

In the Primary FRCA viva, the skull is rarely examined as an isolated osteological catalogue; it is tested through clinical synthesis: airway instrumentation in basal skull fracture, extradural haemorrhage at the pterion, venous sinus injury during neurosurgery, raised intracranial pressure, paediatric craniosynostosis, and regional anaesthetic landmarks. A high-scoring answer links bone, foramen, neurovascular content, clinical consequence and anaesthetic implication.

Landmark/foramen	Key contents	Exam-relevant implication
Pterion	Frontal, parietal, greater wing of sphenoid, squamous temporal; anterior branch of middle meningeal artery deep to it	Temporal blow may cause extradural haematoma; classically biconvex on CT and limited by sutures
Cribriform plate	Olfactory fila; thin ethmoid roof	CSF rhinorrhoea, anosmia; avoid nasal instrumentation if fractured
Foramen ovale	Mandibular nerve, accessory meningeal artery, lesser petrosal nerve	Trigeminal procedures; mandibular sensory loss after skull base pathology
Jugular foramen	IX, X, XI; sigmoid sinus to internal jugular vein	Dysphagia, dysphonia, aspiration risk; venous air embolism risk in sitting craniotomy
Hypoglossal canal	XII nerve	Tongue deviation; difficult airway and aspiration implications if bilateral lower cranial nerve dysfunction
Asterion	Junction of lambdoid, parietomastoid and occipitomastoid sutures; near transverse-sigmoid sinus junction	Burr-hole and posterior fossa surgery landmark; sinus injury may cause major haemorrhage

Guidelines and controversies: trauma, imaging and instrumentation

Current head injury practice has moved away from skull radiographs: CT head with bone windows is the diagnostic standard for clinically significant skull fracture and intracranial bleeding. NICE head injury guidance recommends CT head within 1 hour in adults with GCS ≤12 initially, GCS <15 at 2 hours, suspected open or depressed skull fracture, signs of basal skull fracture, post-traumatic seizure, focal neurological deficit, or more than one episode of vomiting. In anticoagulated patients, CT is commonly performed even with apparently minor injury, typically within 8 hours or sooner if other risk factors are present.

The classic viva controversy is nasal instrumentation in basal skull fracture. Examination doctrine remains clear: nasopharyngeal airway, nasotracheal intubation and nasogastric tube insertion are contraindicated when anterior cranial fossa fracture is suspected, particularly with CSF rhinorrhoea, periorbital ecchymosis, anosmia or radiological cribriform disruption. The evidence base consists largely of case reports of intracranial placement, so some clinicians describe this as a relative rather than absolute contraindication after CT exclusion; however, for examination purposes, choose an oral route and, where gastric decompression is required, insert an orogastric tube.

Skull compliance, decompression and raised ICP

The rigid adult skull underpins the Monro–Kellie doctrine: intracranial volume is the sum of brain, blood and CSF within a near-fixed cranial vault. Normal adult ICP is approximately 5–15 mmHg; sustained ICP >22 mmHg is the Brain Trauma Foundation treatment threshold. Recommended cerebral perfusion pressure is generally 60–70 mmHg, avoiding aggressive attempts to exceed 70 mmHg because of vasopressor-related lung injury and fluid loading.

Decompressive craniectomy illustrates how osteology becomes physiology. The operation removes calvarial constraint, increasing intracranial compliance, but outcome data are nuanced. In DECRA, early bifrontal decompressive craniectomy reduced ICP but was associated with worse functional outcome at 6 months. In RESCUEicp, decompressive craniectomy for refractory ICP reduced mortality at 6 months, approximately 26.9% versus 48.9%, but increased survival with severe disability or vegetative state. Therefore, in a viva, avoid saying “craniectomy improves outcome” unqualified; say it reduces ICP and mortality in selected refractory cases, but functional outcome trade-offs are substantial.

Paediatric and obstetric-adjacent pitfalls

Paediatric skull anatomy is frequently probed because it alters trauma, haemorrhage and surgical risk. The anterior fontanelle usually closes by 12–18 months; the posterior fontanelle by about 2–3 months. Open sutures permit some volume compensation but do not protect against brain injury. Craniosynostosis is premature suture fusion; sagittal synostosis is the commonest pattern, roughly 40–55%, producing scaphocephaly. Cranial vault remodelling may involve major blood loss, sometimes exceeding one circulating blood volume in infants. Tranexamic acid is commonly used in craniofacial surgery; regimens vary, but a typical paediatric approach is 10–15 mg kg⁻¹ loading followed by 5 mg kg⁻¹ h⁻¹, adjusted to local protocol and renal function.

Common viva traps

• Extradural versus subdural: extradural blood is limited by sutures because dura is tightly adherent there; subdural blood crosses sutures but is limited by dural reflections such as falx and tentorium.
• Middle meningeal artery: enters through foramen spinosum, not foramen ovale.
• Foramen magnum contents: medulla, meninges, vertebral arteries, anterior and posterior spinal arteries, spinal roots of accessory nerves, tectorial membrane and alar ligaments; the internal carotid artery does not pass through it.
• Scalp bleeding: profuse because vessels are tethered in dense connective tissue and cannot retract; emissary veins are valveless and may transmit infection to dural venous sinuses.
• Skull thickness: adult calvarium averages about 6–7 mm but is much thinner at the temporal squama, explaining pterional vulnerability.