Session 3 Flashcards
What is the neurocranium?
- The portion of the skull that completely covers the brain
- 8 bones
- Consists of calvaria, cranial floor, cranial cavity
What is the viscerocranium?
- Facial skeleton and jaw
- 14 bones
What type of ossification forms the neurocranium?
- Vault bones (skull cap) form by intramembranous ossification
- Cranial floor forms by endochondral ossification
What type of ossification forms the viscerocranium?
- Endochondral ossification
- Structures develop from the pharyngeal arches (1&2)
What are the names given to important osteological features of the bones of the skull?
- Fossae - shallow depressions or hollows
- Canals - bony tunnels
- Foramina - round(ish) holes
- Fissures - narrow slits
Why does the cranial floor contain many foramina, fissures and canals?
- To allow cranial nerves and blood vessels to enter and exit the neurocranium
What is a suture?
- Tough fibrous joint between the bones of the skull
What feature of the calvaria makes it strong?
- 2 layers of compact bone separated by a layer of spongey bone (diploe)
- Trilaminar arrangement
- Strength but lightweight
Which bones make up the calvaria?
- Frontal bone - forms part of roof of orbit
- Parietal bones - a pair
- Sphenoid bone - only greater wing can be seen from outside of skull
- Temporal bone - mastoid process
- Occipital bone - condyles articulate with C1 vertebra
What is a metopic suture?
- When fusion of the frontal bones fails
- Leaves a suture
- Don’t interpret as a fracture
What are the joints of the calvaria?
- Coronal suture between frontal bone and parietal bone
- Sagittal suture between paired parietal bones
- Lambdoid suture between parietal bones and occipital bone
What is the bregma?
- The intersection between the coronal suture and the sagittal suture
What is the lambda?
- The intersection between the sagittal suture and the lambdoid suture
What is the clinical relevance of the bregma and lambda?
- Relate to areas in foetal skull that remain membranous for up to 2 years
- Sutures aren’t fully ossified, allowing bones to grow as brain develops
What are fontanelles?
- Large unossified membranous gaps between flat bones of calvaria
- Allow for alteration of skull size and shape during childbirth
- Allows brain to grow
Why is the anterior fontanelle clinically useful when examining newborns and infants?
- Inspection and gentle palpation of anterior fontanelle
- Used to assess intracranial pressure
- Fontanelle bulges when pressure is high
- Used to assess state of hydration
- Fontanelle is sunken when dehydrated
- Must also assess how well/unwell infant appears and other sings/symptoms
What ‘shrink-wraps’ each individual skull bone?
- Periosteum
- Covers surfaces of outer and inner table of skull bones
- Strongly adhered to bone edges at suture line
- Continuous through suture and onto inner table of same bone
Why is the periosteum clinically important in haemorrhages?
- Specific shape of haemorrhage indicates blood vessel and type of haemorrhage
- Periosteum is really strongly adhered at sutures but blood can strip it away from inner table of bone at all other points
- This means that extra-dural haemorrhages have a lentiform shape
What forms the cranial floor?
- 3 bowl- shaped depressions
- Anterior, middle and posterior cranial fossae
- Seat different parts of brain and associated structures
- Made up of numerous bones
- Numerous foramina, canals and fissures
Which bones form the anterior cranial floor?
- Frontal bone
- Ethmoid bone
- Sphenoid bone
Outline the features of the ethmoid bone
- Superiorly has a sharp pointy bony prominence called crista galli
- Cribriform plate is flat and rectangular
- Contains many cribriform foramina through which olfactory nerves pass
- Perpendicular plate helps form part of nasal septum inferiorly
- Contains air filled cavities called paranasal air sinuses
Outline the features of the sphenoid bone
- Lesser wings form posterior part of anterior cranial fossa
- Greater wings form part of vault and middle cranial fossa
- Sella turcica
- Pituitary fossa, where pituitary gland sits, found in centre of sella turcica
Outline the features of the temporal bone
- Squamous part - extensive round part
- Zygomatic process helps to form shape of cheek
- Mastoid process acts as attachment for sternocleidomastoid
- Very superior edge of petrous bone forms boundary between middle cranial fossa and posterior cranial fossa
What are the holes in the sphenoid bone?
- Superior orbital fissure - transmits lots of different nerves, communicates with orbit
- Optic canal - transmits optic nerve
- Foramen rotundum - transmits maxillary division of trigeminal nerve (Vb)
- Foramen ovale - transmits mandibular division of trigeminal nerve (Vc)
- Foramen lacerum - filled with cartilage
- Foramen spinosum - transmits middle meningeal artery
- Carotid canal - transmits internal carotid artery
Why is the foramen lacerum not a true foramen?
- Arises due to a small gap between petrous part of temporal bone and sphenoid
- Filled with cartilage and nothing of significance passes through it
- Internal carotid artery passes over the top
Why is the petrous part of the temporal bone so important?
- Houses delicate middle and inner ear structures
Outline the features of the occipital bone
- External occipital protuberance can be palpated
- Superior nuchal line - attachment for trapezius
- Foramen magnum - brainstem passes through this
What is required to fracture the skull?
- Significant trauma and force
- Resistance to fracture varies depending on thickness of cranial bones
- Fractures carry risk of injury to intracranial structures
What are the different types of intracranial fracture?
- Linear - fairly straight, involves no bone displacement
- Comminuted (multiple fracture lines) - fragments may or may not displace inwards towards the brain
What is the clinical relevance of the pterion?
- Pterion is intersection between frontal, parietal, sphenoid and temporal bones
- Very thin, so very easily fractured
- Middle meningeal artery lies directly beneath
- Blows to lateral side of head can fracture bone in this area and injure artery lying immediately below
- This causes an extra-dural haemorrhage
What happens if the ethmoid bone is fractured?
- Clear fluid may drip through nose
- This is CSF
What is the specific sign of fractures of the orbital plates?
- Periorbital ecchymosis (panda/racoon eyes)
- May take hours-days to develop
What are the specific signs of a middle cranial fossa fracture of the petrous bone?
- blood and CSF coming out of ear
- Battle’s sign (mastoid ecchymosis)
- blood pooling in middle ear cavity
Which bones make up the facial skeleton?
- Zygomatic x2
- Maxilla
- Nasal
- Lacrimal
- Mandible
Which facial bones are most commonly fractured?
- Nasal bones
- Zygomatic bones and arches
- Mandible
- These are most prominent on the face
What types of X-ray might you use to look at facial fractures?
- Bucket handle view - looking at zygomatic arches
- Panoramic radiograph (OPG) to look at mandible
- Mandible normally has more than one fracture
Why are maxilla fractures concerning?
- The maxilla could fall backwards and occlude the airways
- These fractures are rare
What is the temporomandibular joint?
- Articulation between temporal bone and mandible
- Synovial hinge-type joint
- Fibrocartilaginous disc divides joint into 2 synovial cavities
- Innervated by auriculotemporal nerve - branch of mandibular division of trigeminal nerve (Vc)
What conditions can affect the TMJ?
- TMJ disorder
- Dislocation
- Arthritis
Outline the anatomy of the TMJ
- Articular surface of bones lined with fibrocartilage and do not come into direct contact
- Separated by fibrocartilaginous disc
- Divides joint into two synovial-lined cavities filled with synovial fluid
- Stabilised by joint capsule and three extracapsular ligaments
What are the movements of the TMJ?
- Hinge (rotational action) allowing opening and closing of jaw
- inferior joint capsule - Gliding forward action
- condyle slides onto articular tubercle
- superior joint capsule - Simple retraction and protraction of mandible
- gliding action at superior joint capsule
Which muscles elevate the mandible?
- Masseter
- Temporalis
- Medial pterygoid
Which muscles depress the mandible?
- Lateral pterygoid
- Gravity
- Platysma assisted by suprahyoid (against resistance)
Which muscles protract the mandible?
- Lateral pterygoid
- Assisted by medial pterygoid
Which muscles retract the mandible?
- Posterior fibres of temporalis (inserts onto coronoid process of mandible)
What can cause TMJ dislocation?
- Facial trauma (especially when jaw is open)
- Yawning
What happens as jaw dislocates?
- Jaw locks as joint fixes in open position
- This is due to anterior dislocation of condyle over articular tubercle
- Muscles of mastication contract
- Keeps jaw locked in anterior displacement
What are the meninges?
- 3 membranous layers that surround and protect the brain and the spinal cord
- Completely continuous around brain, brainstem, spinal cord
- Pia mater, arachnoid mater, dura mater
Describe the pia mater
- Microscopic
- Follows every dip and fold of the surface of the brain
- Can’t be peeled away from underlying tissue structure
Describe the arachnoid mater
- Pushed up against inner surface of dura mater
- Doesn’t extend into sulcae or lateral fissures
- Delicate + translucent + fibrous layer
What makes up the leptomeninges?
- Pia mater and arachnoid mater
- Meningitis affects this area
Describe the dura mater
- Really tough fibrous membrane
What is the subarachnoid space?
- The space between the arachnoid mater and the pia mater
- Filled with CSF - this exerts pressure to keep 2 layers separated
- Cerebral veins run through this space
- Blood vessels entering through the base of the skull set up an arterial blood supply in this space - cerebral arteries arise from here
Which part of the skull does the dura fuse to?
- Periosteum lining inner table of skull bones
Why is the dura like ‘two-ply’ tissue?
- It has 2 layers while within the skull:
1. Periosteal - part against the inner table of bone
2. Meningeal - part adjacent to arachnoid mater - these 2 layers are closely adhered but there are areas where they separate
What does the separation of the layers of dura form?
- Dural folds
- Dural venous sinuses
