The digastric notch (mastoid notch) is a groove on the medial surface of the mastoid process that provides origin for the posterior belly of the digastric muscle. It lies immediately medial to the mastoid process tip and lateral to the occipital groove (for the occipital artery). The mastoid emissary vein traverses the bone just posteriorly.
The digastric notch is the key landmark for identifying the facial nerve trunk during parotidectomy and mastoid surgery. The posterior belly of the digastric muscle, traced to its mastoid origin at the notch, defines the inferior limit of the mastoid and directs the surgeon to the stylomastoid foramen 1-1.5 cm superolateral to the notch. In modified radical and radical mastoidectomy, the digastric ridge (the bony crest marking the inner wall of the digastric notch) is the inferior drilling limit protecting the facial nerve descending in the mastoid segment above.
The digastric ridge (bony crest of the digastric notch) on the inner mastoid surface is the inferior limit of safe drilling in mastoidectomy, indicating the level of the facial nerve mastoid segment emerging from the vertical canal above; drilling inferior to this ridge risks injury to the nerve or the jugular bulb below.
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