SKULL BASE ANATOMY
The anatomy of the skull base is complex and its knowledge is essential for the treatment planning of the different processes that could affect this area.
The skull base is formed by two paired bones,
frontal and temporal,
and 3 unique,
ethmoid,
sphenoid and occipital.
It is divided in anterior,
middle and posterior cranial fossa and contains several canals of practical relevance (Fig. 1).
ANTERIOR CRANIAL FOSSA
Formed by:
- orbital plates of the frontal bone
- ethmoid with its components: horizontal part (cribriform plate),
vertical part (perpendicular plate,
crista galli) and lateral masses
- lesser sphenoid wings and anterior part of sphenoid body – posterior limit of anterior cranial fossa.
MIDDLE CRANIAL FOSSA
Formed by:
- sphenoid body and its lesser wings – anterior limit of the middle cranial fossa
- petrous part of temporal bone – its superior border (with the sulcus of superior petrous sinus) and the dorsum sellae constitute the posterior limit of middle cranial fossa
- body and greater wings of sphenoid.
Sphenoid
- represents the fundamental structure of the skull base,
situated in the centre,
between the frontal,
temporal and occipital bones
- contains the sella turcica and the cavernous sinuses
- it has a complex structure being a route of many neurovascular structures that connect the skull and the cervical/pharyngeal region
- it is formed by different parts: body,
two greater and two lesser wings and two pterygoid processes that descend from the union of the body with the greater wings.
Sphenoid body
It has a cuboid form and contains the sphenoid sinuses,
separated by a septum.
The antero-superior part of the body,
sphenoid planum o jugum,
joins with the cribriform plate (spheno-ethmoid suture) and in the posterior part presents relation with the chiasmatic sulcus.
Behind this groove there are situated the tuberculum sellae,
the sella turcica and the dorsum sellae.
The sphenoid body joins in its posterior aspect the basilar part of occipital,
between them being located the spheno-occipital syncondrosis,
and together forming the clivus.
Hypophyseal fossa – borders:
- anterior –chiasmatic sulcus,
tuberculum sellae
- anterolateral – medium clinoid processes
- posterior – dorsum sellae with posterior clinoid processes
- lateral – cavernous sinuses
- inferior – superior plate of sphenoid sinuses
The pituitary fossa is covered by the diaphragma sellae,
a part of duramater that inserts in the anterior and posterior clinoid processes.
Cavernous sinus – borders:
- anterior – superior orbital fissure
- posterior – petrous apex
- middle – sphenoid body with sphenoid sinus and pituitary fossa
- lateral – temporal uncus
Within the sinus,
in the inferomedial part,
it is situated the internal carotid and the sympathetic plexus,
and in the immediate vicinity,
the VI nerve.
At this level,
the III,
IV,
V1 and V2 nerves,
in cranio-caudal order are located in the external part of the sinus (Fig. 2).
Greater wings
They represent a superior and lateral continuation of the sphenoid body and are made up of two parts:
- lateral,
divided by the infratemporal crest in the temporal surface (superior) and the infratemporal one (inferior).
The first one presents insertions for the temporal muscles,
and the second,
together with the infratemporal crest,
for the lateral pterygoid muscle.
- orbital, constitutes the posterolateral part of the orbit.
Lesser wings
They originate in the anterosuperior part of the body and connect with it by two roots: one anterior,
a thin one,
and the other posterior,
thick,
between them being the optic canal.
The medial extreme of the lesser wings form the anterior clinoid process,
which sometimes can join with the middle clinoid process originating the carotico-clinoid foramen.
Pterygoid processes
They originate from the union of the greater wings with the body.
They have a lateral and a medial plate that join in the anterosuperior part.
Between them one can find the pterygoid fossa,
origin of the medial pterygoid muscle and of the tensor veli palatini; the last one has also insertions in the scaphoid fossa which is situated at the posterosuperior border of the medial plate.
The root of the pterygoid process forms the posterior limit of the pterygopalatine fossa,
where there is the anterior foramen of the pterygoid canal.
Lateral pterygoid lamina – anatomic relations:
- lateral – infratemporal fossa; lateral pterygoid muscle
- medial – pterygoid fossa; medial pterygoid muscle
- antero-superior – pterygo-maxillary fissure
- antero-inferior – palatine bone.
Medial pterygoid lamina – anatomic relations:
- lateral – pterygoid fossa
- medial – posterior nasal orifices; vomer plate
- anterior – sphenoid process of the palatine,
together forming a border of the palatovaginal canal through which pass the pharyngeal branches of the maxillary artery and the pterygo-palatine ganglia.
- posterior – palatine bone; pharyngobasilar fascia
- superior – pterygoid tubercle
- inferior – superior pharyngeal constrictor muscle; uncinate process (hamulus) providing insertions for the pterygomandibular raphe and the tensor veli palatini muscle.
POSTERIOR CRANIAL FOSSA
Limits:
- anterior – dorsum sellae,
posterior aspect of sphenoid body,
basiocciput
- antero-lateral – petrous and mastoid parts of temporal bone,
petrooccipital fissure,
inferior petrous sinus
- lateral – mastoid angles of parietal,
lateral part of occipital,
spheno-occipital synchondrosis
- posterior – squamous part of occipital
The skull base contains many foramina of high clinical relevance (Table 1).