Noise exposure of the inner ear during drilling a cochleostomy for cochlear implantation

OBJECTIVES: Inserting an electrode array into the cochlea may cause inner ear trauma, which has to be minimized, particularly in cochlear implant patients with substantial residual hearing. Another potential inner ear trauma has, to a large extent, been neglected so far: the acoustic trauma that can occur during cochleostomy using different techniques. In this study, the noise exposure of the inner ear during the drilling procedure was re-evaluated. In experiments on temporal bones, quantitative measurements of sound pressure level (SPL) were carried out while a cochleostomy for cochlear implantation was drilled. STUDY DESIGN: Experimental study. MATERIALS AND METHODS: Acoustic measurements during different drilling procedures were carried out on four human temporal bone preparations equipped with microphones attached to the round window. Special calibrations were carried out, which allowed determination of SPLs affecting the cochlea during the drilling procedure. RESULTS: The highest SPLs measured on the cochlea were recorded when a still-intact endosteal membrane was touched by the burr. The SPL exceeded 130 dB and reached a level almost comparable with the situation when the ossicular chain is touched by a running burr. CONCLUSIONS: In the drilling procedure for a cochleostomy, the inner ear may be affected by very high SPLs, particularly if the endosteal membrane is left intact and comes into contact with the running burr. Of course, the resulting SPLs depend on the drilling speed and the size and characteristics of the burr (larger burrs cause higher SPLs); however, we are of the opinion that the cochlear function is at risk, anyway, if special precaution is not exercised. Even when working with reduced drilling speed, the surgeon should be aware of the high risk in the form of an acoustic trauma, which may endanger residual hearing. Recommendations in terms of "soft surgery" are given in the paper (e.g., the use of microhooks instead of a drill to remove the very last shell of bone covering the cochlea).