Anatomic limitations in implantation of middle ear transducer and carina middle ear implants

The objective of this study was to examine any anatomic limitations in implantation of the semi-implantable middle ear transducer (MET) and fully implantable Carina middle ear implants (Otologics, Boulder, CO).

Retrospective case series.

This study involved high-resolution computed tomography (HRCT) of the temporal bone and surgical findings in 22 middle ear implantations (17 MET, five Carina). The distance between the dura and the superior-posterior wall of the external auditory canal (dura-meatal distance) on the incus projection level was measured in coronal high-resolution computed tomography (HRCT) sections. Extensive bone removal from the tegmen for the fitting of the implant was intraoperatively documented, using as criteria the dura exposure. The correlation between HRCT measurements and dura exposure was examined.

In 10 implantations (45.5%) the dura was exposed. In nine of 10 cases (90%) the dura-meatal distance was less than 8 mm. In 11 out of 12 implantations that were performed without exposing the dura (91.7%), the dura-meatal distance was greater than 8 mm. In two cases with dura-meatal distance less than 5 mm, extensive dura exposure and surgical time were needed. In one of these cases, opening of the dura occurred during later explantation.

When dura-meatal distance is greater than 8 mm, implantation of the MET or Carina is a safe procedure. By contrast, in cases with a dura-meatal distance of less than 8 mm, the surgery introduces a high risk of complications. When dura-meatal distance is less than 5 mm, MET or Carina implantation is not recommended.