19 Active Middle Ear Implants: Vibrant Soundbridge The Vibrant Soundbridge (VSB): Symphonix/Med-El Vibrant Soundbridge is a middle ear auditory implant. It was originally designed for fixation to the incus with direct amplification of the natural ossicular movements, but it soon became clear that the vibratory stimulus produced by the implant could also be effectively delivered to the inner ear by direct stimulation of the round window (RW) membrane. The VSB is composed of a surgically implantable part and an external component (Fig. 19.1). The external component is the audio processor (AP), which magnetically couples with the receiver-demodulator or VORP (vibrating ossicular prosthesis) implanted in the temporal region. The AP consists of a microphone, a microprocessor by which the incoming auditory stimulus is converted into an electrical signal, and a telemetry magnetic coil. The signal is amplified and modulated in the AP and transmitted across the skin to the VORP. The receiver coil of the VORP demodulates the signal and transfers it to the floating mass transducer (FMT) through the conduction cable. The FMT is responsible for transduction of signals into vibrations and directly stimulates the inner ear fluid through the ossicles. The FMT was originally developed to be secured to the long process of the incus through an incorporated titanium clip. Alternative coupling methods were subsequently developed; the FMT can be situated and fixated directly upon the round window membrane or, less frequently, to the stapes footplate or stapes remnants. The FMT is an electromagnetic transducer containing a magnet of inertial mass within two electromagnetic coils; when the device is activated, the magnet mass vibrates, causing the entire unit to vibrate. The FMT has a cylindrical shape and measures ~2.3 mm in length and 1.6 mm in diameter, with a weight of 25 mg.1 Fig. 19.1 The Vibrant Soundbridge system is depicted with the audio processor, and the VORP composed of the receiver coil, the connection link, and the floating mass transducer (FMT). Positioning of the FMT depends mainly on the type of hearing loss and on the status of the ossicular chain. There are three main indications for implanting a VSB, depending on the type and degree of hearing loss and on the status of the middle ear of the patient (Fig. 19.2). • Moderate to severe sensorineural hearing loss especially in the high frequencies, with a speech discrimination threshold better than 50% at a comfortable listening or conversational speech level.2,3 This audiologic indication overlaps with those of conventional hearing aids; the VSB should be offered to patients with stable sensorineural hearing loss (SNHL), normal middle ear function, absence of retrocochlear compromise, chronic external auditory canal disease, and/or poor compliance with hearing aids. Professionals in some fields may also need an open ear canal (e.g., physicians using stethoscopes, musicians, singers). In this category of patients the FMT is positioned on the long process of the incus through a posterior tympanotomy approach. • Mixed or conductive hearing loss, secondary to2,4 – Radical cavities – Middle ear obliteration/subtotal petrosectomy – Chronic otitis media and cholesteatoma surgery sequelae with failure to restore a normally aerated middle ear and reduce/close the air–bone gap after properly performed tympanoplasty and ossiculoplasty. • Mixed hearing loss (MHL) or conductive hearing loss due to external auditory canal atresia and malformations of the external ear.2,5 In these patients, most of whom are in the pediatric age group, a VSB can be implanted after plastic reconstruction of the pinna, or even before it if an appropriate and careful skin and soft tissue sparing technique is adopted to allow a later-stage aesthetic reconstruction. In malformation cases the FMT is placed on the round window membrane and a subtotal petrosectomy technique is adopted. In mixed hearing loss the tonal threshold for bone conduction should not be worse than 40 to 60 dB with a 30 to 40 dB air–bone gap and speech intelligibility results of at least 50%. Patients offered or searching for a middle ear implant such as the Vibrant Soundbridge for SNHL with normal middle ear function should have undergone at least a trial period with conventional hearing aids. The role of the VSB in this category of patients is prominent in patients with chronic external ear disease and patients who do not tolerate the occlusion of the external canal. Specific contraindications include: • Fluctuating or rapidly progressive SNHL or decrease in bone conduction in MHL (i.e., autoimmune SNHL) • Benefit from conventional hearing aids with no external canal disease • Inner ear malformations • Medical conditions requiring multiple MRI evaluations • Failure of previously attempted auricle reconstruction, if a second attempt is planned As MRI is becoming a common diagnostic procedure, all surgical candidates should be made aware of its possible effects on the implant. Possible side effects include loud noise, pain, and/or pressure in the ear region. The most serious complications are represented by FMT dislocation, involving surgical revision, implant demagnetization, and cochlear hearing loss. Moreover, MRI images are disturbed by the implant itself, with artifacts and consequent diagnostic difficulties.6,7 MRI is thus not advised in patients undergoing VSB implantation. All patients should undergo clinical and radiologic assessment of their hearing loss, with special attention focused on excluding retrocochlear disease and on evaluating the status of the middle ear. In SNHL and MHL the bone conduction should be stable in the last 12 months. Complete audiologic testing should be performed, including threshold and speech discrimination audiometry, impedance measurement, and a CT scan of the temporal bones. MRI may be needed to rule out retrocochlear disease such as vestibular schwannomas.2 After surgery, standard care practices are adopted for the retroauricular wound. If the postoperative period is uneventful, the implant is activated by a manufacturer’s technician after 6 to 8 weeks. The surgery requires proper positioning for mastoidectomy, hair shaving, and facial nerve monitoring. No special instruments are required for FMT positioning. Surgery is usually carried out under general anesthesia, allowing facial nerve monitoring and accommodating the patient’s and the surgeon’s comfort. Surgical Steps • Skin incision • Creation of a musculoperiosteal layer • Canal wall up mastoidectomy • Posterior tympanotomy • Exposure of the incudostapedial joint • Drill out of the receiver well and implant fixation • Positioning of the FMT on the incus • Intraoperative testing • Closure The same principles described for skin incision in cochlear implantation are applied in VSB placement to prevent extrusion and to create adequate coverage of the receiver-demodulator. The positioning of the skin incision does not need to take into account a behind-the-ear speech processor as in cochlear implant surgery. A standard Lazy-S retroauricular incision is adopted and the lateral surface of the mastoid process is identified after incision of the musculoperiosteal layer. The receiver should be situated posterior to the skin incision to minimize the possibility of extrusion. Using the manufacturer’s template, the area of the bony cradle for the receiver is marked posteriorly and superiorly to the mastoid process at the occipitoparietal junction and the size of the subperiosteal pocket is measured with a Freer elevator. A simple cortical mastoidectomy is performed. The approach should be limited but sufficient to identify the main surgical landmarks for posterior tympanotomy. The antrum is opened widely until the lateral canal is seen. Care should be taken when exposing the incus, moving the head of the bur from medial to lateral. A small bony buttress is left inferior to the short process of the incus to protect it while drilling the facial recess approach. The utmost attention should be paid to the ossicular chain as iatrogenic disruption will hamper implant positioning and possibly cause SNHL or even anacusis. The posterior canal wall is thinned out; aggressive drilling of the canal wall should be avoided as it may cause postoperative atrophy and skin retraction. The position of the third portion of facial nerve is inferred from the incus, the lateral canal, and visualization of the second tympanic portion. Exposure of the digastric ridge can be helpful but is rarely necessary. When the posterior wall is being thinned the chorda tympani can be identified, leaving the fallopian canal from inferior to superior and from medial to lateral. These surgical landmarks delineate a triangular area with the base on the incus and the apex pointing inferiorly. Drilling in this area will open the facial recess, allowing exposure of the middle ear and incudostapedial joint. Correct placement of the FMT in the tympanic cavity may sometimes require widening of the facial recess approach, with transection of the chorda tympani nerve or skeletonization of the third portion of the facial nerve. When widening the posterior tympanotomy one should take care not to injure the facial nerve medially or the tympanic annulus laterally. After the opening of the facial recess with adequate exposure of the incus is achieved, the bony well for the receiver is drilled using a large cutting bur, taking care not to uncover the dural layer. A channel is drilled in the cortical bone toward the mastoid cavity to stabilize the implant while allowing passage for the connection wire and FMT. The subperiosteal pocket and the bony cradle should secure the implant in position tightly to avoid migration of the receiver. Fixation with nonresorbable sutures may further stabilize the implant. The floating mass transducer (FMT) is then guided into the posterior tympanotomy and carefully clipped onto the incus. The FMT should move freely with the ossicular chain and the clip is secured and closed only after correct placement is achieved. In the correct positioning the long axis of the FMT is perpendicular to the footplate. The positioning and crimping maneuvers should be performed carefully in order not to apply unnecessary force on the long process of the incus or the stapes. The crimping force should be uniform and steady to achieve the maximal coupling between the FMT and the long process of the incus. Intraoperative tests are performed after crimping of the FMT. There are two main types of tests when the ossicular chain is intact: • Device integrity test • Reversal transfer function test: this test measures the firmness/stability of the coupling between the FMT and the long process of the incus. A microphone is placed into the external auditory canal and measures the sound produced by the tympanic membrane moving in response to the stimulated FMT. Correct placement is indicated by a sound response identical to the stimulating sound in terms of frequency, intensity, and duration. After intraoperative testing, the musculoperiosteal flaps, subcutaneous tissue, and skin are closed in layers. A head bandage is maintained for 24 hours. Before discharge, a bone conduction threshold tonal audiogram is obtained. Female patient, 50 years old. Audiologic evaluation showed bilateral moderate SNHL with a mean bone conduction PTA of 45 dB and an SDS of 85 at 60 dB. During a trial period of 6 months with conventional behind-the-ear hearing aids the patient presented recurrent episodes of external auditory canal infection, especially in the right ear. MRI was negative for retrocochlear disease. Fig. 19.1.1 After incision of the skin and myoperiosteal layer, the submuscular pocket for the receiver-demodulator is prepared with a round flat instrument.
19.1 Indications
19.2 Contraindications
19.3 Preoperative Evaluation
19.4 Postoperative Management and Follow Up
19.5 Surgical Steps: Incus Vibroplasty
19.5.1 Incision of Skin and Soft Tissues
19.5.2 Mastoidectomy and Posterior Tympanotomy
19.5.3 Implant Positioning
19.5.4 Audiometric Testing
19.5.5 Closure
Case 19.1 Incus Vibroplasty (Fig. 19.1.1–Fig. 19.1.7)
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