Following a cortical mastoidectomy, exposure and decompression of the tympanic and mastoid segments of the facial nerve. It is also called transmastoid facial nerve decompression.

Iatrogenic facial paralysis and facial paralysis due to cholesteatoma are the main indications. It is also undertaken for facial paralysis resulting from a transverse fracture, as part of a combined procedure.

The cortical mastoidectomy including posterior tympanotomy has been completed. The facial canal and all three semicircular canals have also been skeletonized. The temporal bone is ready for the next steps of postauricular facial nerve decompression.

DR: Digastric ridge

EAC: External auditory canal

MCF: Middle cranial fossa (dural plate)

SS: Sigmoid sinus (dural plate)

Drilling with a medium-sized diamond burr, the digastric ridge is followed anterosuperiorly to expose the periosteum of the stylomastoid foramen, which is continuous with the mastoid facial canal. The critical structures are shown in the figure.

II: 3–4mm diamond burr

MIL: Periosteum of stylomastoid foramen

HSC: Horizontal semicircular canal

PSC: Posterior semicircular canal

SA: Subarcuate artery

SPI: Short process of incus

SSC: Superior semicircular canal

Drilling is continued until the mastoid segment of the facial nerve remains covered only by an eggshell-thin layer of bone. Note the relationship of the mastoid segment of the facial nerve, the periosteum of the stylomastoid foramen, and the digastric ridge.

DR: Digastric ridge

EG: External genu

FR: Facial recess

MFN: Mastoid facial nerve

PSF: Periosteum of stylomastoid foramen