The otic capsule or osseous (bony) labyrinth refers to the dense bone of the petrous temporal bone that surrounds the membranous labyrinth of the inner ear. It is surrounded by the less dense and variably pneumatized petrous apex and mastoid part of the temporal bone.
From anterior to posterior, the osseous labyrinth is comprised of:
- cochlea, containing the cochlear duct
- vestibule, containing the utricle and saccule
- semicircular canals, containing the semicircular ducts
It is lined by periosteum. As the membranous labyrinth is slightly smaller than the osseous labyrinth, the two are separate by perilymph, which does not communicate with the endolymph contained in the membranous labyrinth.
There are 4 openings:
- into the middle ear cavity
- into the posterior cranial fossa
- petrous temporal bone fractures: involvement of the otic capsule is important to recognize due to increased risk of complications