L11: Osteology and radiographic appearance of the skull

Question 1

What forms the cranial floor?

Answer 1

3 bowl shaped depressions

• anterior, middle (butterfly shaped), posterior cranial fossa
• depressions in floor which seat different areas of the brain

Question 2

What bones make up the anterior cranial floor?

Answer 2

Anterior to posterior

• frontal bone
• ethmoid bone (crista galli)
• sphenoid bone (lesser wings)

Question 3

What forms the roof of the orbits?

Answer 3

Orbital plates

Question 4

What is the structure of the ethmoid bone?

Answer 4

Crista galli- pointy protrudes up into the cranial space
Cribriform plate- flat, contain many cribiform foramina to allow olfactory nerves to run upwards
(pneumatized: many air filled cavities)
Perpendicular plate: inferior extent, runs down midline of nasal cavity, forming part of the nasal septum
Concha

Question 5

What bones form the middle cranial floor?

Answer 5

• rest of the sphenoid bone (greater wings)

- temporal bone (petrous part): this is also in the posterior cranial floor

Question 6

What is found in the centre of the sphenoid bone?

Answer 6

Sella turcica- contains the pituitary fossa where the pituitary gland sits

Question 7

What bones form the posterior cranial floor?

Answer 7

• temporal bone (superior line of petrous part divides it into the middle/posterior cranial floor)
• occipital bone

Question 8

What are the different areas of the temporal bone?

Answer 8

• squamous part (lateral flat surface)
• zygomatic process
• mastoid process
• petrous bone

Question 9

Why is the petrous bone so hard?

Answer 9

Houses some of the most delicate parts of the body

• inner ear
• middle ear
• ear canal

Question 10

What are some landmarks on the occipital bone?

Answer 10

• external occipital protuberance
• superior nuchal line (trapezius attaches here)
• foramen magnum (as brain stem passes through here it continues on as the spinal cord)

Question 11

What does a skull fracture cause?

Answer 11

Risk of injury to intracranial structures
-intracranial pathology
-neurological deficits
(however significant force is required to fracture the skull, and cranial bone thickness varies)

Question 12

Can you still have intracranial injury without fracturing your skull?

Answer 12

Yes

Question 13

What are the different types of skull fractures?

Answer 13

Linear
-straight line
-no bone displacement 
Comminuted (multiple fracture lines)
-fragments may/may not be displaced inwards towards the brain (can be depressed or non-depressed)

Question 14

What part of the cranial vault is easy to fracture?

Answer 14

Pterion

• thinnest area of skull
• 4 bones meet here (parietal, frontal, greater wing of sphenoid, squamous part of temporal bone)
• beneath this area there is the middle meningeal artery: intracranial haemorrhage (extra-dural)

Question 15

What is another name for skull base fractures?

Answer 15

Basilar fractures (rarer)

Question 16

What are the signs/symptoms of a basilar fracture?

Answer 16

-periorbital ecchymosis (bruising around eye): fracture to anterior cranial fossa

-blood (external auditory meatus)/CSF (CSF otorrhoea) out of ear
-Battle’s Sign (bruising behind the ear)
-haemotympanum
These above are a middle cranial fossa fracture, involving the petrous bone

-clear fluid dripping out the nose (CSF): anterior cranial fossa fracture involving the ethmoid bone

Clinical signs that present will indicate which area of the cranial floor is involved

Question 17

What is haemotympanum?

Answer 17

Blood in the tympanic cavity of the middle ear (behind the eardrum)

Question 18

What is a meatus?

Answer 18

The passage/opening leading into the ear

Question 19

What is the most appropriate imaging for a suspected skull fracture?

Answer 19

CT Head

• allows image of soft tissue as well as the bone whereas X-Ray would just show bone and no pathology to the tissue would be detected
• MRI is too long and expensive but may be important in any underlying intracranial complications