Eur Arch Otorhinolaryngol (2017) 274:2155–2160 DOI 10.1007/s00405-017-4504-0

OTOLOGY

Mastoidectomy dimensions for direct acoustic cochlear implantation: a human cadaveric temporal bone study Francesco Fiorino1   · Maurizio Amadori2 

Received: 17 September 2016 / Accepted: 9 February 2017 / Published online: 28 February 2017 © Springer-Verlag Berlin Heidelberg 2017

Abstract  The objective of the present paper was to acquire information about the mastoidectomy size necessary to obtain an optimal placement of the direct acoustic cochlear implant actuator and fixation system. Ten human cadaveric temporal bones were dissected and implanted with direct acoustic cochlear implant. Mastoidectomy size was determined after implantation in each temporal bone. A bone bed for the receiver/stimulator, mastoidectomy and a large posterior tympanotomy were drilled out. The mastoidectomy was progressively enlarged posteriorly in small steps until the actuator template was judged adequately oriented to enable passage of the rod through the posterior tympanotomy without any contact with the bony walls. The distance between different landmarks in the mastoidectomy was measured. All measured values showed a high degree of consistency, with limited median absolute deviation values. One of the most critical measure, i.e. the distance between the posterior margin of the mastoidectomy to the superior rim of the bony external ear canal wall, ranged from 13 to 16  mm with a median value of 15  mm. Prior knowledge of the ideal size of the mastoidectomy for direct acoustic cochlear implant facilitates the positioning of the fixation system and may save time during implant surgery.

* Francesco Fiorino [email protected] 1

Unità Operativa Complessa di Otorinolaringoiatria, Department of Otolaryngology, Ospedale Mater Salutis, Azienda Unita Locale Socio Sanitaria 9 Scaligera, Via Gianella 1, 37045 Legnago, VR, Italy

2

Department of Otolaryngology, Ospedale di Mirano, Azienda Unita Locale Socio Sanitaria 3 Serenissima, 30035 Mirano, VE, Italy





Keywords  Active middle ear implant · Direct acoustic cochlear stimulation · Mastoidectomy

Introduction Severe to profound mixed hearing loss caused by otosclerosis can be treated with a direct acoustic cochlear stimulation (DACI) [1–3]. The only DACI currently on the market, consists of an external behind-the-ear unit sound processor and the implant, which includes the receiver stimulator and an electro-magnetic actuator held by a fixation component in the mastoid cavity. The actuator generates the vibration, which is transmitted to the perilymph by a piston prosthesis crimped to the artificial incus, and inserted into the inner ear through a stapedotomy [1–3]. Sound is picked up by the directional microphone of the sound processor and converted into digital signals which are sent via a radiofrequency link to the implant. The implantable part of the DACI system consists of a receiver coil, the implant electronics, and the electro-magnetic actuator [1–3]. The implant decodes the incoming radiofrequencies and sends a stimulus to the electro-magnetic actuator. The artificial incus of the actuator vibrates and with it the stapes prosthesis, thereby mechanically stimulating the perilymph in the inner ear and leading to sound perception [1–3]. The fixation system that keeps the actuator firmly in place within the mastoid cavity consists of a titanium bone plate, a ball joint, and a clamping mechanism, which holds the actuator. During implantation, the bone plate is fixed to the temporal bone with bone screws. The ball joint allows a precise positioning of the clamping mechanism determining its fine positioning and optimal orientation.

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Eur Arch Otorhinolaryngol (2017) 274:2155–2160

A mastoidectomy and a wide posterior tympanotomy must be performed to house the actuator and to enable free passage of the rod into the middle ear with precise positioning of the artificial incus so that it is close to the position of the natural incus. The actuator must be locked in the proper position, avoiding any contact with the bony walls. Therefore, the mastoidectomy size must be precisely tailored to permit free orientation of the actuator tip and to enable the artificial incus to be perfectly positioned above the stapes footplate. To the best of our knowledge, the available literature does not provide precise information about the mastoidectomy necessary to fulfill these objectives. We, therefore, performed a study on 10 human cadaveric temporal bones to measure the dimensions of the mastoidectomy prepared for DACI implantation.

Materials and methods The investigation was carried out on 10 human cadaveric temporal bones fixed in 10% formalin. Bones were taken from Caucasian humans and were dissected in the “Otology Today” temporal bone laboratory (Noventa Padovana, Italy). The Codacs™ implant and the Codacs fixation system (both Cochlear Ltd., Sydney, Australia) were utilized during the investigation (Fig.  1a, b). The Codacs implant has a receiver/ stimulator, which receives and decodes the electrical signal from the sound processor, and an actuator, which delivers the signal to the cochlea The fixation system is used to hold the actuator in place. It consists of three main parts: the bone plate, the ball joint and the clamping mechanism. A partial mastoidectomy in the shape of a kidney was performed with exposure of the incus body at the aditus ad antrum. The bone of the posterior external ear canal wall was thinned as much as possible taking care to avoid any breach. Initially, the size of the mastoidectomy was just sufficient to perform a wide posterior tympanotomy, which was realized exposing the chorda tympani down to its convergence with the facial nerve, dissecting the facial nerve canal, and maintaining the buttress. The incudo-stapedial joint and the stapes were adequately exposed. The fixation system was positioned over the cortical bone posterior to the mastoidectomy with the portion containing the ball joint pointing towards the posterior tympanotomy. The width of the fixation system was drawn on the bone at the posterior margin of the mastoidectomy. A template of the actuator was used to find a good position for the clamping mechanism of the fixation system. The mastoidectomy was progressively enlarged

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Fig. 1  Codacs correctly implanted in a mastoidectomy of adequate size (a). The rod with the artificial incus passing through the posterior tympanotomy (b). The stapes piston is crimped to the incus and perpendicular to it

posteriorly in small steps until the actuator template could be adequately oriented to enable free passage of the artificial incus through the posterior tympanotomy without any contact with bone. Only after this step was the canal for the portion of the fixation system containing the ball joint drilled out in the mastoid. The fixation system was positioned and the actuator template inserted into the clamping mechanism for the last check. At this stage, the posterior tympamotomy was minimally enlarged, if there was any suspicion of contact with the bone. Once a good position for the actuator template and clamping mechanism was found, i.e. without any contact to the surrounding structures, the fixation system was fixed with bone screws and the ball joint was locked to stabilize the clamping mechanism, after the last check with the template. A bone bed for the receiver/stimulator and the cable was drilled posterior-superiorly to the mastoidectomy, as usually performed in cochlear implant surgery, at a distance of 8–10 mm from the fixation system.

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The incus was then removed and the stapes superstructure sectioned using cutting scissors via an endocanal approach. To visualize the stapes footplate, it was necessary to remove a limited portion of the posterior–superior bony rim of the external auditory canal. The receiver-stimulator was put in place and the actuator was carefully inserted into the clamping mechanism of the fixation system with the help of the actuator forceps, so that the artificial incus reached into the middle ear cavity and in a parallel position to the stapes footplate analogous to that of the normal incus. The actuator was then locked with the dedicated screwdriver. The distance between the stapes footplate and the artificial incus was then measured. Subsequently, the stapes footplate was perforated by drill and a 4.5 mm × 0.6 mm stapedotomy piston was inserted and crimped using McGee forceps to the artificial incus. After the last check and the confirmation that no contact between the actuator and the bony walls had developed following surgical maneuvers, the Codacs system was explanted and a series of measures recorded. For this purpose, a surgical pick pointed the distal end of the investigated parameter and was marked in correspondence of the proximal end. The distance between the tip of the pick and marker was then recorded, using a double decimeter. The following measures were taken (Fig. 2a, b): 1. From the posterior margin of the mastoidectomy at the position of the fixation system to the superior rim of the bony external ear canal wall, parallel to the superior margin of the mastoidectomy (a); 2. from the posterior margin of the mastoidectomy at the position of the fixation system to the superior rim of the posterior tympanotomy (b); 3. from the posterior margin of the mastoidectomy at the position of the fixation system to the inferior rim of the posterior tympanotomy (c); 4. from the superior margin of the posterior wall of the bony ear canal to the superior margin of the posterior tympanotomy at its mid portion (d); 5. from the superior margin of the posterior wall of the bony ear canal to the inferior margin of the posterior tympanotomy at the level of the facial nerve, at the mid portion of the tympanotomy (e); 6. from the posterior border of the mastoidectomy, at the position of the fixation system to the buttress (f).

Results During the dissection of temporal bones, the most likely contact zones of the clamping mechanism, actuator and the rod with the bony walls were the posterior canal wall and the margins of the posterior tympanotomy including

Fig.  2  a, b Cadaveric temporal bone with the final measures determined after Codacs implantation. See text for description. One star bone bed for the fixation system. Two stars bone bed for the receiverstimulator cable

the buttress. Progressive posterior enlargement of the mastoidectomy and adequate posterior tympanotomy enabled optimal positioning of the fixation system and the actuator, without any contact with bone and free passage of the rod and artificial incus through the posterior tympanotomy. Table 1 shows the individual distances measured in each temporal bone and the median values. All measured values show a high degree of consistency, with median absolute deviation values (MAD), i.e. a measure of statistical dispersion, equal to 1 for all of the parameters investigated. One of the most critical parameters investigated was the distance between the posterior margin of the mastoidectomy at the position of the fixation system to the superior rim of the bony external ear canal wall. The value of this parameter ranged from 13 to 16  mm with a median value of 15 mm. Of significant importance was also the distances from the posterior margin of the mastoidectomy at the position of the fixation system to the sides of posterior tympanotomy, with median values of 24 mm for the distance to the

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2158 Table 1  Individual distances a-f measured in 10 temporal bones

Eur Arch Otorhinolaryngol (2017) 274:2155–2160

1 2 3 4 5 6 7 8 9 10 Median MAD

a

b

c

d

e

f

14 13 15 14 15 16 15 14 16 15 15 1

24 25 25 24 24 25 23 23 24 25 24 1

22 23 20 20 20 21 22 21 20 22 21 1

19 17 17 15 16 18 18 17 16 19 17 1

16 15 14 16 17 16 16 15 14 17 16 1

26 25 23 22 25 24 23 23 26 23 23.5 1

Letters a–f refer to the various differences measured (see text for more information). Values are expressed in millimeters

superior margin of the tympanotomy, 21 mm to the inferior margin, and 23.5 mm to the buttress. The median distances from the superior rim of the bony canal wall to the superior and inferior margins of the posterior tympanotomy were 17 and 16 mm, respectively.

Discussion Active hearing implants improve hearing while maintaining a high degree of sound quality compared with traditional hearing aids [3–6]. The DACI used in this investigation is based on the principle of a power-driven stapes prosthesis. In contrast to other active hearing implants, it directly stimulates the perilymph of the cochlea. Patients with severe-to-profound mixed hearing loss due to otosclerosis can benefit from the DACI [3, 6, 7]. It is crucial to place the actuator of the DACI so that the artificial incus is positioned over the stapes footplate at a distance similar to that of the natural incus, to enable placement of the stapes prosthesis perpendicular to the stapes footplate (Fig.  3a). It is also necessary to avoid any contact of the actuator tube, the rod and the artificial incus with surrounding bone to avoid damping of the signal or sound transfer through the eardrum back to the microphone of the sound processor which could cause feedback [1, 2]. To this end it is of paramount importance to thin the bone of the posterior ear canal wall as much as possible and to perform a mastoidectomy of an adequate size. The clamping mechanism is 13.3  mm long and has a diameter of 7  mm at the ring housing the actuator. The actuator with the rod and artificial incus measures 17.5  mm. The possible sites of contact are between the clamping mechanism/actuator and the posterior wall of the external auditory canal and/or the border of the posterior

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tympanotomy. This may occur if the mastoidectomy size is too small and the clamping mechanism is positioned towards the bony external ear canal wall (Fig. 3b). On the other hand, the mastoidectomy must not be too wide, as this brings the clamping mechanism with the actuator in a more horizontal position, which may cause contact with the margins of the posterior tympanotomy or inability for the artificial incus to reach its optimal position over the stapes footplate (Fig. 3c). In this case, to overcome the drawback, the portion of the fixation system containing the ball joint should be placed in an advanced position compared to the posterior border of the mastoidectomy, thus reducing the stability of the system. In the present investigation the size of the mastoidectomy was measured in 10 temporal bones with the fixation system being in an ideal position for DACI implantation. The average length of different distances gives an indication of the ideal dimension of the mastoidectomy for DACI implantation. The mastoidectomy should have an anterior–posterior distance, i.e. from the posterior ear canal wall to the rim of the mastoidectomy where the fixation system is positioned, of 15 mm, when measured in parallel with the superior border of the mastoidectomy. The tympanotomy must be accurately tailored with skeletonization of the facial nerve canal exposure of the chorda tympany. After completion of the dissection, the average distance from the superior margin of the posterior tympanotomy to the superior margin of the canal wall and posterior margin of mastoidectomy should be 17 and 21  mm, respectively. The inferior margin of the posterior tympanotomy was distant to the aforementioned landmarks 16 and 24  mm in average, respectively. Finally, the distance of the buttress form the posterior edge of the mastoidectomy was 23.5 mm. It must be stressed, however, that this last parameter may be overlooked since the buttress may be broken

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down if it hinders free positioning the caudal portion of the actuator or rod. Realization of mastoidectomy and implant seat according to precise measures, as standardized in the present paper, is able to facilitate surgery and reduce operative time. To this end, repeated intraoperative measurements up to get the ideal values of the different distances determined in the present paper may realize a mastoidectomy of adequate size to house a DACI regardless the degree of pneumatization and presence of minor anatomic variants. Nevertheless, preoperative CT scan imaging is mandatory, since it tests the applicability of these measures, predicts the feasibility of surgery and anticipates possible problems arising from major anatomic variants such us a very prominent sigmoid sinus, low middle fossa dura or high jugular bulb.

Conclusions Prior knowledge of the ideal size of the mastoidectomy before DACI implantation has the potential advantage of reducing the operating time by avoiding empirical attempts to find a good position for the fixation system, with cautious progressive enlargements of the mastoid and repeated and tedious verification of the adequate position of the actuator template. In addition, it prevents hasty preparation of the mastoidectomy and the channel for the fixation system, which may compromise precise positioning and orientation of the clamping mechanism, rod and artificial incus, with the consequence of less adequate efficiency of acoustic coupling. Acknowledgements  The author is grateful to Christiane D’hondt for her assistance in the English revision of the manuscript. Compliance with ethical standards  Conflict of interest  The author has no financial relationship for the manuscript and work. Ethical approval  This article does not contain any studies with human participants.

Fig.  3  a–c Scheme of the different orientations of the clamping mechanism in relation to the width of mastoidectomy. Correct positioning of the fixation system with an ideal anterior-posterior width (a). The clamping mechanism makes contact with the posterior canal wall in a too small mastoidectomy (b). A large mastoidectomy increases the angle between the bone plate and the clamping mechanism, resulting in an excessive distance between the artificial incus and the platina (c)

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Eur Arch Otorhinolaryngol (2017) 274:2155–2160 cochlear implant in subjects with severe to profound mixed hearing loss. Audiol Neurotol 19:164–174 7. Kludt E, Büchner A, Schwab B, Lenarz T, Maier H (2016) Indication of direct acoustical cochlea stimulation in comparison to cochlear implants. Hear Res 340:185–190