Surgical Implantation

Cochlear implantation in children requires meticulous attention to the delicate tissues and small dimensions. Skin incisions are designed to provide access to the mastoid process and coverage of the external portion of the implant package while preserving the blood supply of the postauricular skin. The incision employed at the Indiana University Medical Center has eliminated the need to develop a large postauricular flap. The inferior extent of the incision is made well posterior to the mastoid tip to preserve the branches of the postauricular artery. From here the incision is directed posterosuperiorly and is then directed superiorly without an superior anterior limb. In children, the incision incorporates the temporalis muscle to give added thickness. A subperiosteal pocket is created for positioning the implant induction coil. A bone well tailored to the device being implanted is created, and the induction coil is fixed to the cortex with a fixation suture or periosteal flaps.

After the development of the skin incision, a mastoidectomy is performed. The horizontal semicircular canal is identified in the depths of the mastoid antrum, and the short process of the incus is identified in the fossa incudis. The facial recess is opened using the fossa incudis as an initial landmark. The facial recess is a triangular area bounded by (1) the fossa incudis superiorly, (2) the chorda tympani nerve laterally and anteriorly, and (3) the facial nerve medially and posteriorly. The facial nerve can usually be visualized through the bone without exposing it. The round window niche is visualized through the facial recess approximately 2 mm inferior to the stapes. Occasionally, the round window niche is posteriorly positioned and is not well visualized through the facial recess or is obscured by ossification. Particularly in these situations, it is important not to be misdirected by hypotympanic air cells. Entry into the scala tympani is best accomplished through a cochleostomy created anterior and inferior to the annulus of the round window membrane. A small fenestra slightly larger than the electrode to be implanted (usually 0.5 mm) is developed. A small diamond burr is used to "blue line" the endosteum of the scala tympani, and the endosteal membrane is removed with small picks. This approach bypasses the hook area of the scala tympani allowing direct insertion of the active electrode array. After insertion of the active electrode array, the round window area is sealed with small pieces of fascia.

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