7Reaching the Scala Tympani: Round Window vs Cochleostomy

FWhy the scala tympani, and three ways in

The cochlea has three fluid channels (scala vestibuli, scala media and scala tympani), and the array is targeted to the scala tympani because that is where it sits closest to the spiral ganglion neurons it must drive. There are three access routes: insertion through the round-window membrane, an extended round window (removing the anteroinferior bony rim), and a separate anteroinferior cochleostomy on the promontory. Auditory performance correlates with how much of the array lies within the scala tympani, so the route is chosen to keep the electrode in that scala and out of the scala vestibuli. The classic cochleostomy is placed anterior and inferior to the round window to skirt the basal 'hook' region of the cochlea and give a straight line for the array. Full insertion within the basal turn corresponds to an insertion depth of about 25 to 30 mm depending on array length.[2009][2005]

TFinding and exposing the round window

Through the facial recess the round-window membrane lies roughly 1 to 1.5 mm inferior to the stapedius tendon; the round-window overhang should be removed to confirm the membrane positively. If the niche anatomy is obscure, the round window is never more than 2 mm from the inferior margin of the oval window and usually lies directly inferior to it. Misreading this anatomy can lead the surgeon to mistake a hypotympanic air cell for the cochlea, a reason to obtain an intra-operative skull film if insertion seems wrong. Removing the bony overhang (the subiculum / round-window 'lip') is the key exposure step shared by both the round-window and the immediately-inferior cochleostomy routes. When a true round-window insertion fails it is most often because the mastoid facial nerve runs directly lateral to the window and obscures more than half of the niche.[2009][2012]

CTrauma, the hook, and scala-vestibuli mis-insertion

In post-mortem implanted-bone studies, electrodes placed anterior to the round window more often tore the basilar membrane or crossed into the scala vestibuli, whereas more inferior insertions stayed atraumatically in the scala tympani. On that basis the recommended cochleostomy site is directly inferior to the round-window membrane, after removing the subicular overhang. A cochleostomy placed too anterosuperiorly drills toward the scala vestibuli, the wrong channel, so the access point, not just the depth, governs which scala the array enters. Drilling on the promontory generates bone dust and risks introducing blood into the cochlea, an acoustic-trauma argument in favour of the no-drill round-window route. Buckling from over-aggressive insertion can injure the spiral ligament, basilar membrane and local neurons; if resistance is met the array is withdrawn slightly and re-advanced rather than forced.[2006][2003][2009][2005]

CPerforming the opening and the modern shift

For a cochleostomy a small diamond burr makes a fenestra slightly larger than the array; reported sizes range from about 1.0 to 1.4 mm, with a 0.8-mm diamond burr used in some minimal techniques. The promontory surface is thinned to the bluish endosteum of the scala tympani, edges are smoothed and the endosteum elevated, then incised with a fine needle just before insertion to keep bone dust and blood out of perilymph. The round-window route has regained favour as the unambiguous landmark that leads straight into the scala tympani with essentially no drilling on the cochlea; one large series achieved round-window insertion in about 90% of cases across all three manufacturers. Soft-surgery and residual-hearing preservation goals reinforce the round-window preference, since avoiding promontory drilling limits acoustic and mechanical trauma. After insertion the opening is sealed around the lead with fibrous tissue (pericranium, fascia or muscle); incomplete sealing can theoretically predispose to meningitis, and fibrous packing must never be relied on to retain a tenuous insertion.[2006][2012][2009][2011]