Surgical pitfalls encountered with the semi-implantable middle ear transducer ossicular stimulator (MET-Otologics)

To determine whether surgical pitfalls can be anticipated through a preoperative CT-Scan in case of middle ear implantation with a semi-implantable middle ear ossicular stimulator from Otologics (SIMOS). A second outcome measure was to assess the auditory gain obtained in the series.

A retrospective analysis of 19 consecutive surgical procedure for a SIMOS implantations was achieved. Preoperative Ct-Scans were analyzed and several measurements of the mastoid were taken out from axial transverse views, ie. the distance between the incus and the cortical mastoid bone [d(i-c)], the distance between the sigmoid sinus and the incus [d(i-ss)], the distance between the sigmoid sinus and the cortical bone of the mastoid [d(c'-ss)]. On coronal images, the dura of the middle fossa was described as procident if it was lower than the head of malleus and/or body incus. Auditory gain was calculated on pure tone and speech audiometry with bisyllabic words.

All cases were successfully implanted. d(i-c) was on average at 33.1 +/- 2.6 mm in cases easy to implant. One case of contracted mastoid was difficult and led to the impaction of the sigmoid sinus. In this case the distance d(i-c) was shorter at 25 mm than the average minus 2 standard deviations of the others (27.9 mm). Also d(ss-c') was lower at 7 mm than the average minus 2 standard deviations of the others (7.6 mm). Two other cases led to difficult surgical procedures because of a low middle fossa dura. This too low middle fossa dura could be identified on preoperative coronal CT-scans in these two cases. Audiometric gains were remarkable, with an average of 39 +/- 16 dB. In case of severe sensorineural hearing loss the average gain was higher at 46 +/- 9 dB. Two revision-procedures were required, one for a device failure, the second because of skin alteration due to systemic inflammatory disease requiring corticosteroids. Mean follow-up was 50 +/- 22 months.

The SIMOS is a powerful middle ear implant that can undoubtedly give a huge audiometric gain. The implantation procedure requires a precise microsurgical operation that can be hampered by anatomical conditions of the petrous bone. A thorough examination of preoperative CT-scan is highly recommended in order to obtain measurements of the mastoid in axial views, and the analysis of the position of the middle fossa dura in coronal views.