After more than 25 years of clinical experience, the BAHA (bone-anchored hearing aid) system is a well-established treatment for hearing-impaired patients with conductive or mixed hearing loss. Owing to its success, the use of the BAHA system has spread and the indications for application have gradually become broader. New indications, as well as clinical applications, were discussed during scientific roundtable meetings in 2004 by experts in the field, and the outcomes of these discussions are presented in the form of statements. The issues that were discussed concerned BAHA surgery, the fitting range of the BAHA system, the BAHA system compared to conventional devices, bilateral application, the BAHA system in children, the BAHA system in patients with single-sided deafness, and, finally, the BAHA system in patients with unilateral conductive hearing loss.
Overall, the results with bilateral fitted BAHAs were better than with unilaterally fitted BAHA; the benefit is not only caused simply by bilateral stimulation but also, to some extent, by binaural hearing. Bilateral BAHAs should be considered for patients with bilateral hearing loss otherwise suitable for BAHAs.
One limitation with the Bone Anchored Hearing Aid (Baha) is too poor amplification for patients with moderate to severe sensorineural hearing losses. Therefore, we investigated if bone conducted (BC) sound transmission improves when the stimulation approaches the cochlea. Also the influence from the squamosal suture on BC sound transmission was investigated. Both sides of the heads on seven human cadavers were used and vibrational stimulation was applied at eight positions on each side with a frequency range of 0.1-10 kHz. A laser Doppler vibrometer was used to measure the resulting velocity of the cochlear promontory. It was found that the velocity of the promontory increases as the stimulation position approaches the cochlea; this was especially apparent at distances within 2.5 cm from the ear canal opening and when the stimulation position was in the opened mastoid. At frequencies above 500 Hz there was on average 10 to 20 dB greater vibrational response at the cochlea when the stimulation was close to the cochlea compared with the normal Baha position. Moreover, even if there were general indications of attenuation of BC sound when passing the squamosal suture, an effect from the suture could not be conclusively determined.
A dry skull, prepared with a layer of damping material in the brain capsule, was used to investigate the vibratory pattern of bone-conducted sound. Three orthogonal vibration responses of the cochleas were measured, by means of accelerometers, in the frequency range 0.1 to 10 kHz. The exciter was attached at different points in the temporal, parietal, and frontal bones. The results revealed a profound low-frequency antiresonance (attenuation) in the ipsilateral transmission path, which was psychoacoustically shown to yield a distinct lateralization effect. It was further shown that the direction of excitation coincides with that of maximum response at the ipsilateral cochlea. No dominating response direction was found for frequencies above the first skull resonance at the contralateral cochlea. An overall higher response level was achieved at the ipsilateral cochlea when the transducer was attached close to the posterior semicircular canal compared with the other excitation points of the skull, for the energy transmission in general and specifically for the direction of excitation. Transcranial attenuation between the cochleas was found to be highly frequency specific, with a maximum of 40 dB in a single direction. Transcranial energy attenuation was less than 10 dB for the investigated frequency range.
This study was undertaken on 100 children aged 16 years or under intended for installation of osseointegrated implants. Of these, 76 had implants installed for bone-anchored hearing aids or prostheses. The main indication for implant installation was a bilateral ear malformation. Surgery was generally performed as a two-stage procedure with a healing time of 3 to 4 months in between. Available bone thickness was on average 2.5 mm, and lack of bone necessitated bone augmentation in 12 patients. Thirty-nine percent of implants were installed in contact with the dura, sigmoid sinus, or an air cell. Implant failures were 5.8% of 170 inserted fixtures. Adverse skin reactions appeared in 9.1% of patients over a 21-year follow-up period. Revision surgery was undertaken in 22% of patients because of appositional growth of the temporal bone. Of the 24 children considered but not found suitable for osseointegration surgery, plastic surgery was considered a better option, or growth of the temporal bone was awaited. It is concluded that the rate of implant failure is lower in children; the frequency of skin reactions is the same as in adults, but revision surgery is more common in young patients because of new bone formation. Our clinical experience supported by the data presented in this article suggests that the concept could be used with good functional and aesthetic results in children.
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