The petrous part of the temporal bone is a dense pyramidal wedge of bone that projects anteromedially toward the centre of the skull base, forming the boundary between the middle and posterior cranial fossae. It contains the cochlea, semicircular canals, internal auditory meatus (with facial and vestibulocochlear nerves), the carotid canal (internal carotid artery), jugular foramen, and the facial nerve canal from the geniculate ganglion to the stylomastoid foramen.
Petrous bone fractures are classified as longitudinal (along the long axis, 80% of cases) or transverse (perpendicular, 20%). Longitudinal fractures commonly cause conductive hearing loss from middle ear haemorrhage and disrupt the ossicular chain; transverse fractures more often injure the facial nerve at the geniculate ganglion and cause sensorineural hearing loss from cochlear concussion. CSF otorrhoea from petrous fracture indicates dural tear requiring observation for spontaneous closure. Acoustic neuromas arise from the vestibular nerve within the internal auditory meatus of the petrous bone.
Lateral impact fractures the petrous bone along its long axis, producing conductive hearing loss from haemotympanum and ossicular disruption, with facial nerve injury in approximately 20% of cases, usually at a bony spur near the geniculate ganglion.
High-energy frontal or occipital impact fractures the petrous bone transversely across the otic capsule, producing immediate complete sensorineural hearing loss, vertigo, and facial nerve paralysis from direct geniculate ganglion trauma in up to 50% of cases.
