18 Bone Conduction Implants Bone conduction implants deliver the acoustic stimulus directly through the skull bones to the inner ear, bypassing the outer and middle ear structures. They can be divided in two main groups: • Implantable devices with a transcutaneous abutment, properly called bone anchored hearing aids (BAHAs), in which sound vibrations are received and amplified by a sound processor that is connected to an osteointegrated transcutaneous fixture. The sound processor and the fixture work like an active unit transmitting the sound to the bone. • Implantable devices without transcutaneous abutment, in which the sound processor is magnetically coupled to the osteointegrated bone anchored implant and the sound is transmitted through the skin to the magnetic implant that is thus passively stimulated. Implantable bone conduction devices have been developed to restore hearing in patients with disease affecting the external or middle ear in whom conventional surgery has been performed with poor functional results or in whom surgical rehabilitation is contraindicated (e.g., otosclerosis in the only hearing ear. Due to the good results and the relative ease of the surgical procedures, the implantable bone conduction systems have rapidly gained favor since their introduction and indications have expanded to include many other conditions such as single-sided deafness. Indications for implantable bone conduction devices can be divided into audiologic and otologic subgroups. Conductive hearing loss (CHL) and mixed hearing loss (MHL): with a bone conduction pure-tone average (PTA) between 0 and 45 or 55 dB HL (depending on the speech processor) and a speech discrimination score (SDS) of more than 60%. Bone conduction implants require less signal amplification than conventional hearing aids as the conductive loss is completely bypassed; this is particularly relevant in patients with a mean conductive loss of 30 dB or more.1 Single-sided deafness (SSD): bone conduction implantable devices are also indicated in the audiologic rehabilitation of patients suffering from severe to profound hearing loss or anacusis on one side and with a contralateral bone conduction PTA not exceeding 20 dB with an SDS better than 60%. Implantation on the deaf side allows for decrease in the head shadow effect and improves sound discrimination in noise.2 Generally speaking, implantable devices with transcutaneous abutment are considered more powerful than the analog devices without transcutaneous fixtures as sound vibrations are directly transmitted into the bone. Otologic indications include deaf patients affected by congenital or acquired middle or external ear pathologies in whom surgical hearing rehabilitation is not indicated or has already been performed with poor functional results. • Chronic otitis media (COM) 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. • Radical cavities Implantable bone conduction devices avoid the need for conventional endaural or retroauricular hearing aids, reducing the risk of infection and otorrhea in chronic ear patients. Patients with radical cavities or canal wall down tympanoplasties usually have a large external auditory meatus; they often require high amplification and a custom-made mold, with the accompanying risk of acoustic feedback. Implantable bone conduction devices represent an option in patients with otosclerosis for whom surgery is not an option because of (multiple) previous unsuccessful surgeries or anatomical anomalies. The implantable systems are valid alternatives to the air conduction device if the air–bone gap exceeds 30 dB, due to their better performance and the absence of feedback. Chronic external auditory canal disease (i.e., dermatitis, recurrent infections) and/or poor compliance with conventional hearing aids (because of itching or the occlusion effect) may be considered indications for implantable bone conduction devices.2 In the pediatric age group, the most common indication for a BAHA is bilateral congenital malformations represented by congenital aural atresia, in which functional surgery is known to achieve poor hearing results and may have significant complications.2 Patients who have undergone middle ear obliteration or subtotal petrosectomy with blind-sac closure of the external auditory canal obtain significant benefit from the implantable bone conduction devices if there is a cochlear reserve according to the audiologic criteria. Implantable bone conduction devices are contraindicated if the SDS is lower than 60% and the bone PTA (in the better hearing ear) exceeds 55 dB. Implantable devices, moreover, are contraindicated in patients with previously irradiated bone and psychiatric disease. Inability to maintain adequate hygiene represents a contraindication, as poor hygiene is a common reason for adverse skin reactions. Diabetic patients do not have an increased risk of implant loss or skin problems. The BAHA and Ponto systems, developed by the Cochlear and Oticon companies, respectively, are defined as bone anchored hearing aids, being composed of an external sound processor and an implantable transcutaneous part. The implantable part is composed of a screw (the fixture) that is fastened to the bone and an abutment to which the sound processor is attached. Both the screw and the abutment length can be preoperatively decided according to each individual patient considering bone and skin thickness. In implantable devices such as the BAHA and Ponto systems, the external part (sound processor) actively transmits sound vibrations to the fixture screwed to the parietotemporal cortical bone; this allows for preoperative trials using cutaneous couplers. Specific preoperative evaluation includes complete audiologic and otologic evaluation (i.e., otomicroscopy, tonal threshold, and speech discrimination score) and may be extended to include free field tonal audiometry and discrimination with and without the use of the device, allowing professionals and patients to objectively evaluate the change in auditory performances. There are many cutaneous couplers available: • Rod-test: a small rigid rod attached to the sound processor is kept in contact with the tip of the mastoid bone, the forehead, or the incisor teeth while using the device. This test allows the patients to have a quick estimation of possible benefits. • Test band/head band: these are hairband-like steel springs attached to the sound processor. The test band is used for pre-operative evaluation, while the softer head band may allow the use of the sound processor in patients who present temporary or permanent contraindications to the surgical implantation of the fixture (i.e., bone thickness less than 3 mm). • Soft band: the sound processor may also be tested preoperatively mounted on a bandeaux-like band. The soft band may be used in children younger than 5 years of age who require auditory rehabilitation (i.e., in case of bilateral aural atresia). The use of the soft band and head band or the test rod offers an easy, fast, and objective way of assessing bone conduction capacity that allows the patient to directly experience bone-conducted sound. It should be made clear to the patients and parents that the subjective benefit obtained in these tests will be less than achieved by using the processor coupled to the osseointegrated fixture. Preoperative temporoparietal bone thickness may be evaluated in young children (>5 years of age) and in adults undergoing revision surgery or surgery in a multioperated site by the means of a standard unenhanced temporal bone CT scan. Fig. 18.1 Cochlear BAHA device: left to right, the fixture, the abutment, and the audioprocessor. Fig. 18.2 Oticon PONTO device: right to left, the fixture, the abutment, the screw connecting the abutment and fixture, the audio processor. Audiologic indications for both devices include: • Conductive hearing loss (CHL) or mixed hearing loss (MHL), with an average bone conduction threshold ≤45 dB (Ponto Pro and BAHA BP100) or 55 dB (Ponto Pro Power and BAHA BP110) and a speech discrimination score (SDS) of more than 60%. For bilateral fitting, the criterion is symmetric bone conduction thresholds (i.e., there is less than a 10 dB difference on average across 0.5, 1, 2, and 3 kHz, or less than 15 dB difference at individual frequencies). • Single-sided deafness (SSD): bone conduction implantable devices are also indicated in the audiologic rehabilitation of patients suffering from severe to profound hearing loss or anacusis on one side. The contralateral average bone conduction threshold should not exceed 20 dB with a SDS better than 60%. Implantable bone conduction devices with transcutaneous fixture are contraindicated in children younger than 5 years and in patients with a skull bone thickness of less than 3 mm. Since Tjellstrom and colleagues first introduced the BAHA system in 1977,3 the surgical procedure has been modified, with several teams reporting their adjustments. There are two major goals: osseointegration and prevention of soft tissue reactions. Currently two techniques are commonly used, one with skin (subcutaneous) thinning and the other without skin thinning. Thinning of the skin has been developed to prevent soft tissue movement around the implant, to reduce the amount of scar tissue formation, and to provide a thin hairless skin area directly in contact with the deeper bony layer. The second technique is more straightforward as it does not require skin thinning; recent findings have demonstrated that patients treated without skin thinning showed favorable results in terms of skin reaction and implant integration after 2 years of follow-up.4 After surgery, standard care practices are adopted for the wound. If the postoperative period is uneventful, a manufacturer’s technician activates the implant after 4 to 6 weeks. The implant should be located ~50 to 55 mm from the ear canal so that the sound processor will not touch the pinna in its final position. We usually mark the implant site on the temporal line. The skin incision is marked 10 mm anterior to the implant site, parallel to the hairline. An elliptical/rectangular area of 40 to 60 mm is then marked around the incision line for subsequent subcutaneous tissue reduction (see Fig. 18.1.1). When planning the implant site one should be certain to consider anatomical variations due to congenital malformations or previous surgery, as well as the location of future reconstructive surgery. Patients with congenital microsomia may have a contracted mastoid process, low-lying middle fossa dura, and a sigmoid sinus closer than usual to the mastoid surface. However, sufficient bone for an implant is usually found. In adults the procedure is generally performed under local anesthesia. In children general anesthesia is preferred. The marked area is injected with a local anesthetic of lidocaine and epinephrine; usually 10 mL is used to infiltrate the skin and periosteum at the implant site. A 3- to 4-cm skin incision is made and carried to the periosteal layer, which is left intact, severing the skin and muscular layers (see Fig. 18.1.2). Subcutaneous tissue around the skin incision is removed for an area of 40 to 60 mm marked out earlier. A large no. 22 knife blade is introduced into the dermis and moved parallel to the skin surface, removing the deep dermal and fat subcutaneous layers, and thus removing the hair follicles as well. The thickness of the skin around the incision should be less than 1 mm (see Fig. 18.1.3). The aim of the soft tissue reduction is to achieve a gradual tension-free edge sloping down toward the implant area. Keeping the periosteum intact is extremely important for the blood supply required for the healing of the skin flap. A percutaneous needle is used through the skin at the implant site to identify the periosteal area to be addressed. A self-retaining retractor is placed to hold the edges of the incision and a cruciate incision is made in the periosteum. The four triangular periosteal flaps are raised using a microraspatory (see Fig. 18.1.4). The implant site is prepared using the guide drill at high speed (2,000 rpm) under abundant irrigation to minimize bone heating. Bone drilling is initiated with the guide drill, keeping the spacer on 3 mm and the drill indicator attached (see Fig. 18.1.5). During drilling the bur is moved up and down and around to ensure that the cooling solution reaches the tip of the drill. While gradually deepening and widening the hole with the guide drill, it is important not to over-widen the section that will contain the implant as this may cause initial instability. The bottom of the hole is repeatedly checked visually and with a blunt dissector to check for bone at the base of the site. If there is adequate bone thickness, the spacer is removed, allowing drilling to continue to a depth of 4 mm (see Fig. 18.1.6). Drilling continues in the same manner, using the whole length of the drill to allow space for the countersink. In most adults there is sufficient bone thickness for a 4 mm implant. During this step it is possible to penetrate the wall of the sigmoid sinus and open the dura mater; therefore, the surgeon should proceed with care. Cooling is very important for all drilling procedures. Heat can damage the bone tissue and jeopardize osseointegration. It is important that the drilling is performed perpendicular to the skin surface, since this orientation will affect the orientation of the implant and the abutment. The drill indicator will facilitate correct drill orientation and should be used both with the guide drill and with the countersink drill. After preparation of the primary hole for the implant, the next step is to widen the hole to the exact diameter of the fixture. This procedure is carried out with either a 3-mm or a 4-mm countersink drill, depending on the depth reached with the guide drill. The countersink is used at high speed (2,000 rpm) and should be moved up and down during drilling while ensuring that the cooling solution reaches the tip of the drill. Bone chips should be removed from the drill flutes frequently. When the surface is uneven, the countersink allows the flange of the titanium screw to have maximum contact with the bone surface. The tip of the countersink is blunt to minimize the risk of damaging tissues (e.g., dura) at the bottom of the hole and a stop collar prevents excessive countersinking (see Fig. 18.1.7). Implant insertion is performed using the same handpiece at a low speed setting. The torque limit on the motor panel should be adjusted according to the quality of the bone. In compact cortical bone a setting between 30 and 40 N-cm is recommended. In soft bone a lower torque setting of 10 to 20 N-cm should be used. The self-tapping fixture with premounted abutment is delivered sterile and it is picked up with the abutment inserter (see Fig. 18.1.8). It is recommended that the drill indicator is also used during implant insertion. The implant is installed without irrigation until the small grooves in the distal end of the implant are well within the canal so as to avoid distal contamination. After this step the implant placement continues under abundant irrigation. If the implant enters the hole on a wrong axis, the drill motor is put into reverse. The correct angle is then found and the implant is re-inserted. Here the drill indicator may be useful for setting the correct angle (see Fig. 18.1.9). When the flange of the implant has reached the bone surface it stops automatically. The abutment inserter is then removed from the abutment. The periosteal flaps are then replaced near the implant base (see Fig. 18.1.10). The skin is sutured in layers over the abutment. A hole is punched exactly over the abutment using a 4-mm-wide biopsy punch. The skin is carefully eased down over the abutment (see Fig. 18.1.11). If necessary, two small incisions are made to facilitate the process. The healing cap is snapped onto the abutment and a long gauze soaked with antibiotic ointment, is loosely packed circumferentially around the implant. Alternatively, a custom-tailored foam dressing may be used (see Fig. 18.1.12). The healing cap should gently hold the dressing in place to compress the thinned skin in close contact with the bone. This prevents hematoma formation and allows direct apposition of the skin and periosteal layers. A mastoid dressing is applied. The head bandage is usually not necessary. Fig. 18.1.1 Male patient. The skin incision is marked 10 mm anterior to the implant site, parallel to the hairline. An elliptical/rectangular area of 40 to 60 mm is then marked around the incision line for subsequent subcutaneous tissue reduction.
18.1 Indications
18.1.1 Audiologic Indications
CHL and MHL
SSD
18.1.2 Otologic Indications
Chronic Otitis Media and Sequelae
Otosclerosis and Tympanosclerosis
Chronic External Auditory Canal Disease
Aural Atresia
Middle Ear Obliteration/Subtotal Petrosectomy
18.2 Contraindications
18.3 Implantable Devices with Transcutaneous Abutment
18.3.1 Preoperative Evaluation
18.3.2 Key Features of the Devices
18.3.3 Specific Indications
18.3.4 Contraindications
18.3.5 Surgical Technique
18.3.6 Postoperative Management and Follow-up
18.3.7 Placement of an Implantable Device with Transcutaneous Abutment Using the Skin Thinning Technique: Surgical Steps
Marking the Implant Site
Anesthesia
Skin Incision and Soft Tissue Reduction
Periosteal Incision
Bone Drilling
Countersink Drilling
Implant Positioning
Skin Closure and Exteriorization of the Abutment
Case 18.1 Bone Anchored Hearing Aid (Fig. 18.1.1–Fig. 18.1.12)
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
