Applied anatomy of the mastoid segment of facial nerve

Objectives To provide anatomic data for identifying the mastoid segment of facial nerve during mastoidectomy through the lateral approaches.Methods The research was applied on 8 cadaveric specimens.Electric drill was used to remove the superficial bone of the mastoid process to expose the underlying semicircular canals and the mastoid segment of the facial nerve.The length and diameter of the vertical segment of the facial nerve were measured.The distances from the vertical segment of the facial nerve to surrounding structures were measured.Results The length of the mastoid segment of the facial nerve was about(11.04±1.03)mm,and the diameter(2.29±0.39)mm.The distance from the beginning of the mastoid segment of the facial nerve to sigmoid sinus,Helen's spine,durra mater of the posterior fossa,the junction of sigmoid sinus and transverse sinus,posterior semicircular canal was(9.41±2.06)mm,(15.61±2.11)mm,(5.96±1.51)mm,(20.20±3.44)mm,(3.84±0.14)mm separately.The distance from the end of the mastoid segment of the facial nerve to the sigmoid sinus,Helen's spine,mastoid tip,durra mater of the posterior fossa,the junction of sigmoid sinus and transverse sinus was(7.05±2.01)mm,(20.32±2.69)mm,(16.48±2.48)mm,(7.97±2.61)mm,and(27.38±4.21)mm respectively.Conclusions Horizontal semicircular canal,posterior semicircular canal and the digastric ridge are important structures for identifying the mastoid segment of the facial nerve.The mastoid segment of facial nerve lies in the triangle formed by the posterior semicircular canal,the horizontal semicircular canal and the anterior edge of the digastric ridge.