10Securing the Receiver-Stimulator

FSeating the internal device

The receiver-stimulator is placed on the flattest available part of the skull, angled roughly 45 degrees off the canthomeatal line and far enough behind the postauricular incision that its leading edge does not sit under the ear-level processor, where rubbing can thin the skin and expose the device. A traditional bony seat (well) is drilled to the thickest part of the package, generally 2 to 3 mm deep, with a trough drilled forward into the mastoid to carry the electrode lead. The goals of placement are to minimise protrusion (reducing vulnerability to external trauma) and to restrict device movement, which can shear the connecting leads. The mastoidectomy edges are deliberately left acute rather than saucerised so they help retain the electrode leads within the mastoid cavity.[2009][2014]

TBony well versus subperiosteal tight pocket

The classic technique drills a bony well and passes non-absorbable tie-down sutures through holes in the bone superior and inferior to the package to lock it in position. Many surgeons now omit both the bony seat and the drill holes; eliminating them reduces the (already rare) risk of CSF leak and intracranial haematoma associated with deep drilling near the dura. If the well is omitted, a tight subperiosteal pocket is essential to prevent movement, and a bony ridge or an anterior tie-down suture is placed in front of the device to stop it sliding forward. Suture fixation can be achieved without bony tie-down holes by passing sutures through the periosteum above and below the device, a less invasive alternative to drilled-bone anchors.[2014][2009]

CBone depth, dura, and the lead channel

Creating a deep well and stabilising the stimulator with permanent suture to the bony cortex is strongly advised in adults, where the lateral skull is usually thick enough to accommodate the package. An operating microscope is recommended when drilling the well and the suture holes to avoid dural injury, and minimal dural displacement from a slightly shallow well is well tolerated. The lead runs in a drilled channel from the well into the mastoid cortex; a bony overhang or bridge at the trough-mastoidectomy interface protects the lead, and the lead is positioned so there is no tension at the cochleostomy. Fibrous packing should not be relied upon to retain a tenuous array; the lead can instead be tucked medial to the short process of the incus, or secured with mesh ties or a clip. The flap is closed in layers beginning with periosteum so the leading edge, the lead takeoff, and the leads are fully covered; an exposed lead in subcutaneous tissue can erode through skin.[2009][2014][2003]

CPaediatric skull, magnets, and minimal access

In a child under 1 year the skull may be only 2 to 3 mm thick, forcing a choice between a shallow well and taking the bone down to dura; a mobile island of thin bone can be left over the dura in the centre of the well for protection. From age 1 to adulthood the mastoid grows on average about 2.6 cm in length, 1.7 cm in width, and 0.9 cm in depth in males, so roughly 2.5 cm of redundant electrode lead is left coiled in the mastoid to accommodate skull growth and avoid extrusion. Studies in the young primate show cochlear implantation has no adverse effect on skull growth, supporting implantation in infancy. For stable magnetic retention of the external coil the overlying scalp is thinned to less than about 1 cm (or per the manufacturer's specification); the internal magnet also dictates MRI compatibility and may need removal or special protocols for later imaging. The internal device should be oriented to avoid a position too far posterior or too high on the lateral skull, particularly in children.[2009][2003][2014]