Corpus: Petrous bone

The petrous portion of temporal bone or in Latin Pars petrosa ossis temporalis is a pyramid-shaped section of bone at the base of the temporal bone (Os temporale) that contains the inner ear.

In contrast to some animal species, the temporal bone in humans is not an independent bone, but part of the temporal bone. Therefore, the correct term is Pars petrosa ossis temporalis. The terms "Os petrosum" or "petrous temporal bone" used in clinical parlance are therefore not actually correct.

3.1. Overview[Bearbeiten]

The petrous bone has the basic shape of a three-sided pyramid and is located at the base of the skull between the sphenoid bone and the occipital bone. Its tip is directed medially and rostrally.

Macroscopically, the petrous bone can be divided into the following sections:

• apex of the petrous bone (apex partis petrosae)
• Base: is fused with the inner surface of the pars squamosa and the pars mastoidea of the temporal bone.
• Posterior surface (facies posterior)
• anterior surface (facies anterior)
• lower surface (facies inferior)
• Processus mastoideus

The osseous structures of the inner ear, the bony labyrinth, are formed inside the petrous temporal bone. The petrous temporal bone is present throughout life as plexus bone and is not remodelled into lamellar bone. The pars petrosa ossis temporalis is the hardest bone in the human skull.

3.2. Petrous bone tip[Bearbeiten]

The rough and uneven tip of the petrosal bone fits into the angle between the posterior edge of the large sphenoid wing (ala major ossis sphenoidalis) and the pars basilaris of the occipital bone. It extends medially to the sphenoid body (corpus ossis sphenoidalis). It surrounds the orificium internum of the carotid canal and forms the posterolateral edge of the foramen lacerum, which is formed together with the ala major ossis sphenoidalis. This is where the sphenopetrous ligament (Gruber ligament) attaches.

3.3. Posterior surface[Bearbeiten]

The posterior surface is part of the posterior fossa. The porus acusticus internus, which merges into the meatus acusticus internus, lies in the centre. The facial nerve and vestibulocochlear nerve enter here. The sigmoid sinus sulcus is located laterally adjacent to it. The posterior surface participates with the occipital bone in the formation of the jugular foramen.

3.4. Anterior surface[Bearbeiten]

Together with the pars squamosa of the temporal bone, the anterior surface forms part of the floor of the middle cranial fossa. The upper edge of the petrous bone (margo superior partis petrosae) forms the border to the posterior surface or posterior fossa. The roof of the tympanic cavity (tegmen tympani) separates the middle ear from the middle cranial fossa. Inflammation of the middle ear can thus spread per continuitatem to the dura and the brain parenchyma. A partial or complete, congenital or postoperative absence of the tegmen tympani favours this.

3.5. Processus mastoideus[Bearbeiten]

The mastoid process behind the auricle communicates with the tympanic cavity (cavitas tympani) via the antrum mastoideum. The mastoid cells are only separated from the medial sigmoid sinus by a thin bony lamella. Mastoiditis can thus easily lead to septic sinus thrombosis per continuitatem.

3.6. Middle ear[Bearbeiten]

The tympanic membrane (membrana tympani) separates the middle ear from the external auditory canal. It contains the tympanic cavity with its 3 sections:

• Hypotympanum: below the level of the tympanic membrane
• Mesotympanum: at the level of the eardrum
• Epitympanum: above the level of the eardrum

The three auditory ossicles are located in the tympanic cavity: Malleus (hammer), Incus (anvil) and Stapes (stirrup). It transmits the sound waves from the eardrum to the inner ear.

The tympanic cavity is connected to the epipharynx via the auditory tube. This serves to equalise pressure, but also represents a potential infection pathway.

3.7. Inner ear[Bearbeiten]

The inner ear lies medial to the middle ear and lateral to the internal auditory meatus. The cochlea and the vestibular organ with its three semicircular canals are located here. The horizontal semicircular canal runs parallel to the longitudinal axis of the petrous bone. The superior semicircular canal is perpendicular to it and runs parallel to the contralateral superior semicircular canal. The lateral semicircular canal runs perpendicular to the other two.

3.8. Bone canals[Bearbeiten]

Various bone canals run through the petrous bone. These include, among others:

• Canalis caroticus: Contains the internal carotid artery
• Meatus acusticus internus: Carries the vestibulocochlear nerve and other structures
• Canalis nervi facialis: Carries the facial nerve
• Canalis musculotubarius: Contains the tensor tympani muscle and the auditory tube
• Canaliculus vestibularis: For the endolymphatic duct
• Canaliculus tympanicus: Carries the tympanic nerve and the inferior tympanic artery
• Canaliculi caroticotympanici: Contains the arteriae and nervi caroticotympanici

Various anatomical variants occur on the petrous bone that are relevant in imaging and especially in surgical procedures. These include, among others:

• Pneumatisation variants of the apex or the mastoid cells, sometimes asymmetrical
• Vascular variants such as an aberrant course of the internal carotid artery in the middle ear, a protruding or lateralised jugular bulb or a persistent stapled artery
• Variants in the course of the facial nerve including dehiscences and positional anomalies
• Spatial configurations such as a deep tympanic sinus or a prominent subarcuate canal
• Widening of bony canals such as the aqueductus cochleae

These norm variants can lead to surgical difficulties, favour pathological processes or simulate pathological findings in imaging.

See main article: Anatomical norm variants of the petrous bone

The petrous temporal bone can be injured in skull injuries. Fractures of the petrous temporal bone are usually burst fractures in the context of polytrauma or craniocerebral trauma.