Describe the applied anatomy of the mastoid and temporal bone.

The Temporal Bone and the Mastoid

• The temporal bone has four parts — the squamous, petrous, mastoid and tympanic parts, with the styloid process. The petrous part houses the inner ear and is the densest bone in the body
• The mastoid process is absent at birth and develops during the first two years under the pull of sternocleidomastoid. The mastoid antrum, however, is present and adult-sized at birth, lying superficially beneath the suprameatal triangle
• Pneumatisation begins in fetal life and continues through childhood, spreading from the antrum. It is arrested by childhood middle ear infection, so a sclerotic (poorly pneumatised) mastoid on a radiograph is evidence of long-standing childhood ear disease — and it is characteristically found with cholesteatoma

Applied Aspects

• Radiology — the plain schuller view is largely historical but shows pneumatisation and the sigmoid sinus position; high-resolution CT of the temporal bone is the standard, showing bony erosion, the ossicles, the tegmen, the facial nerve canal, the labyrinth and the extent of cholesteatoma
• MRI with diffusion-weighted imaging distinguishes cholesteatoma (which restricts diffusion) from granulation tissue and fluid, and is particularly valuable in detecting residual or recurrent disease after surgery without a second-look operation
• Why pneumatisation matters clinically — a well-pneumatised mastoid provides an air reservoir buffering middle ear pressure changes, and its cells are a route by which infection spreads; a sclerotic mastoid signals past disease and is associated with cholesteatoma and with poorer surgical outcomes
• The temporal bone as a route of spread — middle ear infection reaches the intracranial cavity by direct bone erosion, through preformed pathways (the oval and round windows, a labyrinthine fistula, congenital dehiscences), and by retrograde thrombophlebitis along the emissary veins
• Temporal bone fractures — longitudinal (80%, along the long axis of the petrous bone, from a lateral blow), causing conductive loss, bleeding from the ear and a delayed, incomplete facial palsy; and transverse (20%, from a frontal or occipital blow), crossing the petrous axis and the labyrinth, causing sensorineural deafness, vertigo, haemotympanum without bleeding from the canal, and an immediate, complete facial palsy.
• Battle’S sign (postauricular bruising) and CSF otorrhoea indicate a base of skull fracture
• The antrum is adult-sized at birth though the mastoid process is not, and lies superficially in the infant
• Pneumatisation is arrested by childhood infection, so a sclerotic mastoid is evidence of long-standing disease
• Macewen triangle overlies the antrum at 1.5 cm depth in the adult, and is where cortical mastoidectomy begins