Учебники / Middle Ear Surgery

28 Cochlear Implantation 147

Fig. 28.1

Fig. 28.2

148 28 Cochlear Implantation

Preparation of the Implant Bed for the Receiver

The entrance of the outer ear canal is identified by showing the spine of Henle. Next, the site for the internal receiver inside the skull is prepared using cutting and diamond burrs. The bony excavation is drilled according to the shape of the provided template (Fig. 28.2). As shown in the drawing, burr holes are drilled at the border of the implant bed for the sutures fixing the implant. The surgical procedure for the different products is basically identical, but since the size of the implant and the size of the electrodes vary, the size of the implant bed and the handling of the electrode differ slightly (Figs. 28.2, 28.3). Drilling the posterior hole needs a larger exposure. Therefore we often use the periosteum behind the implant bed for posterior placement of the fixing suture. Just showing the implant under the periosteum does not seem sufficient for us because the implant may displace. More important is the prominence, especially in children, and the greater chance of damage due to trauma. A groove is drilled into the bone to lead the electrode from the bony seat of the receiver to the mastoid cavity. The size of the bed and the amount of bone work vary according to the different types of implants. However, we believe that an implant bed must be prepared in the bone to avoid prominence of the implant. This increases the risk of damage due to trauma especially in children.

Mastoid Work

A typical cortical mastoidectomy is done after identification of the spine of Henle and the temporal line. The landmarks are consecutively exposed and identified: the sigmoid sinus, the bone covering the middle fossa, the lateral semicircular canal and the short process of the incus.

Posterior Tympanotomy

The facial nerve must be identified under a thin layer of bone before opening the facial recess between the facial nerve and the chorda tympani, which usually can be preserved. This has become more important since bilateral implantation has proven to be advantageous, especially with background noise. The facial recess is widened inferiorly and posteriorly, respecting the previously identified facial nerve. The distal end of the long process of the incus, the incudostapedial joint, the stapedial tendon, the promontory and the round window niche are seen. Under unfavourable anatomical conditions the round window cannot be identified under the facial nerve. During the drilling procedure, continuous irrigation is of great importance in removing bone dust and avoiding heat damage to the surrounding tissues especially to the facial nerve. The revolving shaft of the burr must not touch the nerve or the bone covering it.

The bony buttress between the facial recess and fossa incudis is preserved by most surgeons and this is the standard procedure. The buttress is used to fix

28 Cochlear Implantation 149

Fig. 28.3

Fig. 28.4

150 28 Cochlear Implantation

the CI electrode. Revisions have shown that this fixation is not necessary. Consequently it may be removed if necessary. This can be helpful in unclear anatomical situations. The incus is exposed and the fossa incudis opened following the long process of the incus. This procedure protects the underlying facial nerve (Fig. 28.4). If necessary the long process of the incus can be removed, but the annulus of the tympanic membrane and the posterior meatal wall need to remain intact during this procedure.

Cochleostomy

To determine the precise location of the cochleostomy, the burr hole on the promontory is placed according to Helms at a distance twice the diameter of the head of the stapes (Fig. 28.5). The cochleostomy towards the scala tympani is drilled through the posterior tympanotomy without touching the facial nerve with the 1.2-mm diamond. During this phase of surgery the shaft of the diamond burr is close to the facial nerve and may cause heat damage while the surgeon is concentrating on the promontory. The position of the cochlea in relation to the axle of the temporal bone varies slightly. A larger diamond burr can be used on the promontory until the grey white endostium of the cochlea is seen. The cochlea is opened with a smaller diamond burr. A small hook may also be used. The edges of the opening to the basal coil of the cochlea must be smoothened. This makes the insertion easier and avoids bending of the electrode during insertion. Bone dust should not enter the cochlea to reduce the risk of ossification, which may make revisions, and reinsertion of a new electrode, impossible. The position of the electrode must be clearly seen within the cochleostoma and routinely checked before proceeding to avoid false placement of the electrode (Fig. 28.6). The insertion of the electrode into the cochlea through the round window is no longer used because the electrode has to be bent to follow the course of the cochlea. The area of the round window should remain intact.

Placement of the Receiver and Electrode Insertion

Prior to electrode insertion, the internal receiver is secured inside its bony bed by sutures and the neutral electrode is positioned under the temporal muscle and the periostium with contact to the bone for the MEDEL and Nucleus Systems (Figs. 28.7, 28.8). If the electrode is placed on the muscle, it moves while the patient chews. This will cause a break of the electrode. The cochlear electrode is then gently inserted into the scala tympani via the cochleostoma without any administration of force. We like to use a long straight forceps. After full electrode insertion, pieces of soft tissue are placed around the electrode at the cochleostomy site to create a seal and prevent labyrinthitis.

In cochlear implantation, open communication with the endocranial space is more common than in otosclerosis patients. If the liquorrhoea cannot be stopped by sealing with connective tissue (too much force can damage the

152 28 Cochlear Implantation

Fig. 28.8

Closure

The periostial flap then replaced over the receiver implant, and the wound is closed by suturing the skin muscle flap. Rubber foam under the ear bandage is useful as it applies slight pressure and prevents haematomas.

Mastoiditis

If mastoiditis develops on the implanted side, the indication for opening the mastoid is the same as in other patients. The surgeon should obtain a lateral skull X-ray and a computed tomography scan to identify the exact position of the electrodes. Normally the implant does not have to be removed unless there are signs of labyrinthitis. If labyrinthitis is suspected the diagnosis is difficult, due to the absence the usual signs such as inner ear hearing loss and vertigo. The lateral semicircular canal may be diagnostically opened.

29 Revision Surgery After Cochlear

Implantation

Joachim Müller, Holger Sudhoff, Henning Hildmann

For decades cochlear implants have been the standard treatment for deaf adults, teenagers, and children. As with all technical devices, the lifetime of cochlear implants is limited, and as with all surgical procedures, complications during or after cochlear implantation may occur. Also malfunction of the device may happen earlier than expected due to device failure or for other reasons.

In the literature cochlear implant failure rates or revision rates vary from ~2 % to 18 %. A direct comparison of revision rates or failure rates published in the literature or given by the manufacturers is practically impossible on account of the different inclusion criteria and the extreme variations in data sources (such as different age groups: adults or children, implant generations, sample size, period under review). For detailed failure analysis it is necessary to specify implant failures more precisely and to differentiate between “patient related failures” and “failures which come within the manufacturer’s field of responsibility”.

Revision Surgery Can Become Necessary for Several Reasons

Medical Reasons

Medical reasons are all those related to surgery including flap problems, infections, surgical complications, incorrect positioning of the active or reference electrode, failures following fixation of the electrode, meningitis cases or cochlear implantation due to incorrect diagnosis.

Electronic Failures

Failures of the implant system can be due to electronic faults.

Trauma

An external trauma may lead to a fracture of the cochlear implant, such as may occur with ceramic implants, or may result in a broken housing, which was previously described to result from external forces in implants with metal housings.

Electrode Failures

A trauma can also damage the electrode and lead to fractures of the electrode wires. This category also includes damage to the electrode during insertion or surgery.

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Soft Failures

With increasing numbers of implantations we may be faced with a new category of “soft failures” which lead to revision. This includes revisions where the patient complains about clinical symptoms of malfunction and postoperative device testing showed device function within the manufacturer’s specifications. In general it can be said that trauma is a common reason for revision surgery.

Cochlear implant revision seems to occur more often in children than in adults.

Lower revision rates for the latest implant generations reflect the increasing surgical expertise and the continuous modifications and improvements in cochlear implant hardware made by the manufacturers. Recent studies on the latest implant generation showed remarkably low failure rates of less than 2 % in Würzburg.

During revision surgery, the reinsertion of the new electrode remains the crucial point. For the Nucleus System it has been reported in the literature that in 7 % of reimplantations the insertion of the new electrode was limited [1], whereas other studies found that the reinsertion of the electrode carrier in 21 revisions was as deep as achieved in primary surgery. In single cases an even deeper electrode insertion was observed during upgrade surgery. In principle, the risk and the tendency of ossification of the basal turn of the cochlea remain possible. It has been discussed that the initial insertion trauma may lead to ossification and impedes reimplantation. There is a small chance of this but the option for implanting the contralateral side should be mentioned before surgery. Usually, a soft tissue tube surrounds the electrode carrier. This soft tissue envelope also protects against infection from the middle ear to the inner ear at the side of the cochleostoma.

Especially in children the mastoid plane shows a high tendency for neoosteogenesis and ossification. The new bone formation may close the cavity partially or completely and surround the electrode on its way into the mastoid. We have never, however, observed bone new formation in the cavity or in the facial recess.

New bone formation may also cover parts of the implant housing. Bony fixation of the electrode carrier inside the growing mastoid does not usually present any difficulties.

It is important to do all the preparations during revision surgery systematically and leave the electrode carrier in place. As a last step the electrode should be removed and the new electrode should be introduced immediately to avoid a collapse of the soft tissue tube around the electrode.

In general, the aim is to remove the implant completely without cutting off any of the electrodes or damaging the device to allow for postoperative evaluation. However, in selected cases it can be helpful to cut off the active electrode at the side of the facial recess to facilitate better preparations in the mastoid and the facial recess.