mastoido-, mastoid- +

(Greek: breast; used in the specialized sense as "of or pertaining to the breast-shaped mastoid process of the temporal bone)

A reference to the sternum (breastbone) and the mastoid process.
Pertaining to the temporal bone and the mastoid process.

The mastoid process is a conical protuberance of the posterior portion of the temporal bone (protrusion of bone) that is situated behind the ear in humans and many other vertebrates and serves as a site of muscle attachment.

Relating to the tympanic cavity (air chamber in the temporal bone containing the ossicles or small bones of the middle ear) and the mastoid cells (small intercommunicating cavities in the mastoid process or a nipple-like projection).
A mastoidectomy (excision of the mastoid air cells) with tympanectomy (excision of the tympanic membrane).
Inflammation of the middle ear and the mastoid cells (small intercommunicating cavities).
An operation to exteriorise (expose an organ temporarily for observation) and to join the mastoid air cells (small intercommunicating cavities), the middle ear space, and the external meatus (natural passage or canal), often for extensive cholesteatoma (benign condition involving an expanding mass of cholesterol crystals and keratinised, or horny, skin in the middle ear space; with common symptoms causing hearing loss, ear fullness, and pain).