The Mechanical Impedance of the Human Skull via Direct Bone Conduction Implants.

PURPOSE: The mechanical skull impedance is used in the design of direct bone drive hearing systems. This impedance is also important for the design of skull simulators used in manufacturing, service, and fitting procedures of such devices. PATIENTS AND METHODS: The skull impedance was measured in 45 patients (25 female and 20 male) who were using percutaneous bone conduction implants (Ponto system or Baha system). Patients were recruited as a consecutive prospective case series and having an average age of 55.4 years (range 18-80 years). Seven patients were treated in Gothenburg, Sweden, and 38 patients in Edmonton, Canada. An impedance head (B&K 8001), driven by an excitation transducer with emphasized low-frequency response, was used to measure the mechanical point impedance with a swept sine from 100 to 10k Hz. RESULTS AND DISCUSSION: The skull impedance was found to have an anti-resonance of approximately 150 Hz, with a median maximum magnitude of 4500 mechanical ohms. Below this anti-resonance, the mechanical impedance was mainly mass-controlled corresponding to an effective skull mass of 2.5 kg at 100 Hz with substantial damping from neck and shoulder. Above the anti-resonance and up to 4 kHz, the impedance was stiffness-controlled, with a total compliance of approximately 450n m/N with a small amount of damping. At frequencies above 4 kHz, the skull impedance becomes gradually mass-controlled originating from the mass of the osseointegrated implant and adjacent bone. No significant differences related to gender or skull abnormalities were seen, just a slight dependence on age and major ear surgeries. The variability of the mechanical impedance among patients was not found to have any clinical importance. CONCLUSION: The mechanical skull impedance of percutaneous implants was found to confirm previous studies and can be used for optimizing the design and test procedures of direct bone drive hearing implants.

Implant losses for the bone-anchored hearing devices are more frequent in some patients.

INTRODUCTION: Our knowledge of implant losses for bone-anchored hearing devices (BAHD) is still limited. This study examined the long-term survival rate, the reasons for implant loss, and the need for implant replacement in a large cohort with a long follow-up. METHOD: Retrospective study of patients who received a BAHD between January 1977 and December 2011 as identified from records of operations performed. All data were retrieved from patient records. RESULTS: A total of 571 patients were found in the surgical records. Seven patients were excluded because of incomplete data. The follow-up period was up to 32 years, with a median follow-up of 6.6 years. Implant failures were identified in 46 (8.2%) of the patients. A total of 763 implants were inserted. Of these, 141 (18%) implants were lost: 109 (14%) due to lost osseointegration, 21 (3%) due to trauma, while 11 were electively removed. Twenty-seven patients had 1 implant reinstallation, whereas 5 patients had 2 reinstallations. Nine patients were reinstalled three times and five patients were reinstalled 4 to 6 times. Of 141 implants lost, 78 (55%) were lost in 19 patients with 2 or more implant losses. CONCLUSION/IMPLICATIONS: Implant failures were identified in 8.2% of the patients with a median follow-up time of 6.6 years. A total of 141 (18%) of 763 implants were lost; the majority of losses took place in patients with multiple implant failures.

A vibration investigation of a flat surface contact to skull bone for direct bone conduction transmission in sheep skulls in vivo.

HYPOTHESIS: Bone conduction implant (BCI) attached with a flat surface contact will offer efficient and linear vibration transmission over time. BACKGROUND: Despite that percutaneous bone conduction devices (PBCD) are successful in treating patients with conductive hearing loss, there are some drawbacks related to the need of a permanent skin penetration. The BCI system is designed as an alternative to the PBCD because it leaves the skin intact. METHODS: BCI dummy implants were installed in 3 sheep skulls in vivo to study the vibration transmission characteristics over time. Mechanical point impedances and vibration transfer response functions of the BCI implants were measured at the time of surgery and after a healing period of 8 months. RESULTS: In 1 sheep both implants healed without complications. In the other 2 sheep, the implants were either partially loose or lost to follow up. In the sheep with stable implants, it was found by the resonance frequency shift of the mechanical point impedance that a firmer integration between the implant and bone tissue as seen in osseointegrated surfaces developed over time. It was also shown that the transcranial vibration transmission remains stable and linear. Providing bone chips in the contact between the implant and the bone did not enhance vibration transmission. The surgical procedure for installing the BCI dummy implants was uneventful. CONCLUSION: The mechanical point impedances and vibration transfer response functions indicate that the BCI implants integrate and that transmission conditions remain stable over time.

Bone-anchored hearing device surgery: a 3- to 6-year follow-up with life table and worst-case scenario calculation.

BACKGROUND: Published studies on complications for the bone-anchored hearing devices can present different outcomes. Unfortunately, data are often difficult to interpret, compare, or use in meta-analysis owing to missing data. This study presents a sensitivity analysis of presenting cohort data, which considers incomplete follow-up data. An example is given using life table calculations and worst-case scenarios for implant losses and revision surgeries after installing BAHA. METHODS: A retrospective case review of all patients consecutively operated on for BAHA between 2005 and 2007 with a 1-stage procedure. Life table calculations were used to handle incomplete follow-up data, including 50% of patients lost to follow-up as failures. In addition, a worst-case scenario was calculated, in which 100% of patients lost to follow-up were considered to be failures. RESULTS: A total of 138 patients were operated on at Sahlgrenska, with a follow-up time of 3 to 5 years. Within the study cohort, 9 patients (6.5%) lost the implant. Reasons for incomplete follow-up data were as follows: "lost to follow-up" (n = 4), "not using BAHA" (n = 3), and "patient was deceased" (n = 2). Life table calculation gave a calculated implant loss of 9.8%. The worst-case scenario gave an implant loss frequency of 13%. Revision surgery was performed in 3 (2.2%) of 138 patients in the study cohort. With life table and worst-case scenario calculations, the frequency of revision surgery was 6.2% and 10.1%, respectively. CONCLUSION: Incomplete follow-up data can theoretically affect outcome data considerably. Therefore, outcome data should also be presented with life table and worst-case scenario. This would enhance the possibility to interpret and compare data.

Sleeping implant in the temporal bone: report of a case with 20-year follow-up.

BACKGROUND: There is limited knowledge of the long-term fate of "sleeping" or nonloaded implants in the temporal bone. PURPOSE: This article describes the fate of a fixture installed in the temporal bone that remained unloaded for 20 years. PATIENT AND METHODS: A 25-year-old male with hemifacial microsomia had three osseointegrated implants installed for an auricular episthesis and bone-anchored hearing aid (BAHA) in the left temporal bone in 1988. Two of the implants for the ear episthesis were activated the same year, but the fixture for the hearing aid was not uncovered until 2008. When the patient experienced hearing problems at his office, he wanted to reactivate the sleeping implant. An audiogram showed a maximum conductive hearing loss with good preserved cochlear function. Before reactivation, an Accuitomo three-dimensional, cone beam computed tomography was performed. Resonance frequency analysis (RFA) using the Ostell technique was done when the implant was uncovered. RESULTS: Preoperative x-ray investigation showed the sleeping implant to be well integrated in the temporal bone, covered with 1 mm bone, and with no signs of resorption. Geometric measurements correlating to the two loaded implants showed the sleeping implant to be positioned too close to these to be able to anchor a BAHA without interference with the episthesis. Surgical exploration was done to analyze the implant. The clinical status correlated well to that diagnosed from the x-ray investigation. RFA revealed the implant to be well integrated. A new fixture and abutment for BAHA was installed in the temporal line and activated 2 months after surgery. The patient is today supplied with a BAHA. CONCLUSION: It seems possible to use sleeping implants in the temporal bone even 20 years after installation.

Historical background of bone conduction hearing devices and bone conduction hearing aids.

During the last 20 years, bone-anchored hearing aids (Baha(®)) became a familiar solution in the treatment of some types of hearing loss. The aim of this chapter is to present the different historical steps which have permitted the production of this new bone conduction hearing device. The recognition of bone conduction hearing is old and was known at least in Antiquity. During the Renaissance, Girolamo Cardano demonstrated a method by which sound may be transmitted to the ear by means of a rod or the shaft of a spear held between one's teeth: this was the beginning of teeth stimulators to improve hearing, firstly in connection with a musical instrument and then, in the second part of the 19th century, with the speaker. The development of the carbon microphone at the beginning of the 20th century allowed the construction of the bone conduction vibrator placed on the mastoid area, notably supported by eyeglasses since the 1950s. Confronted by various problems, and notably the loss of part of sound in the soft tissue of the external mastoid, the idea to implant the vibrator into the mastoid bone was developed in Göteborg, and the first Baha was implanted in 1977 by Anders Tjellström. From that date, various improvements allowed the development of the actual Baha. These different steps are presented in this study, supported by original documentation.

Transmission of bone-conducted sound in the human skull measured by cochlear vibrations.

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.

Skin reactions after BAHA surgery: a comparison between the U-graft technique and the BAHA dermatome.

OBJECTIVE: To compare and evaluate 2 surgical methods for handling the soft tissues in Bone-Anchored Hearing Aid (BAHA) surgery: the U-graft technique versus the dermatome designed for BAHA site preparation. BACKGROUND: Skin reactions surrounding the percutaneous titanium abutment of the BAHA are a matter of clinical concern. Excessive residual soft tissue surrounding the implant is thought to be the principal cause of this complication. To address the challenge of adequately thinning the soft tissue, a dermatome has been introduced to facilitate BAHA site preparation. METHODS: All patients fitted with a BAHA between 2001 and 2004 at our clinic were included in the study. Resection of soft tissue associated with the use of the U-graft technique or the dermatome was documented. At follow-up, skin reactions were registered according to Holgers. Here, we comparatively analyze the fate of the implant site according to the soft tissue resection technique used. RESULTS: We used a U-shaped graft in 45 patients; the dermatome was used in 25 patients. A total of 373 observations were recorded in follow-up. In the U-shaped graft group, 29 (64%) of 45 patients experienced no adverse skin reactions. In the BAHA dermatome group, 21 (84%) of the 25 patients experienced no skin reactions. The difference in adverse skin reactions between the 2 groups was 19.6% (p = 0.14; 95% confidence interval, -3.6 to 42.7%). CONCLUSION: The BAHA dermatome is a tool, which achieves BAHA skin-healing outcomes at least as good as U-graft flaps created by long-experienced BAHA surgeons. Perhaps, this tool allow other BAHA surgeons to achieve similar outcomes without having to experience as many skin reactions as occurred in the evolution of skin management around BAHA abutments.

High age at the time of implant installation is correlated with increased loss of osseointegrated implants in the temporal bone.

BACKGROUND: The implant failure rate in temporal bone has been reported to be about 5 to 10% over a 10-year period. A number of our elderly patients have shown increased failure rates over a long time period, which is the reason for the present study. PURPOSE: The aim of the present study was to find out if age is correlated with implant failure and to measure blood flow in implant sites. MATERIALS AND METHODS: The long-time survival of 131 osseointegrated implants installed in the temporal bones of 81 patients was correlated with the age of the patient at the time of installation. The blood flow in 37 fixture installation sites in 22 patients was recorded by means of laser Doppler flowmetry. RESULTS: The mean implant failure rate in the study group was 9.8% after a mean follow-up time of 7.6 years. There was a significant increase of implant failure in patients above 60 years of age. There was further a trend that implants used for the bone-anchored hearing aid were lost to a higher proportion than implants used for bone-anchored episthesis. There was also a trend that female patients lost fewer implants than males. Blood flow in the temporal bone correlated well with the age of the patient in that the highest values were recorded from the youngest patients. CONCLUSIONS: Increasing age affects failures of osseointegrated implants in the temporal bone. Blood flow is higher in the child's temporal bone, a factor that can be of importance to understand why age influences implant survival.

Osseointegration timing for Baha system loading.

OBJECTIVES: The process of osseointegration for creating a biological bond between titanium oxide and bone is time dependent. However, different surgeons have used very varied time frames before loading the implant. The waiting time in dental implant loading ranged from immediate to 6 months. The Baha system (Cochlear Limited, Englewood, CO) traditional waiting period consisted of 3 months for adults and 4 to 6 months for children. The purpose of the study was to evaluate the safety of reducing the waiting time to 6 weeks in adults. STUDY DESIGN: Retrospective study of patients who underwent Baha implantation and exteriorization between March, 2004 and July 2005. METHODS: Twenty-six adult patients underwent Baha titanium implantation and exteriorization in a single stage. They were loaded with the external processor after an average of 6.5 weeks. The etiology of their hearing loss (HL) included conductive HL, mixed HL, and unilateral sensorineural HL. Follow-up period ranged between 6 and 20 months. RESULTS: All patients were successfully implanted with the titanium implants, loaded at the 6 week interval, and have safely retained their prosthesis. The only reported complication was dermatitis, which occurred in three patients. Patients were pleased to receive the external processor earlier. CONCLUSION: The reduction of the waiting period from 3 months to 6 weeks did not result in any failure of osseointegration of the titanium implants. The earlier activation resulted in enhanced patient satisfaction. A larger series would be needed to definitively recommend shortening of the interval between Baha implantation and device loading.

Consensus statements on the BAHA system: where do we stand at present?

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.

Bilateral bone-anchored hearing aids (BAHAs): an audiometric evaluation.

OBJECTIVES: Since the technique to implant bone-anchored hearing aids (BAHAs) with the use of osseointegrated implants was developed in 1977, more than 15,000 patients have been fitted with BAHAs worldwide. Although the majority have bilateral hearing loss, they are primarily fitted unilaterally. The main objective of this study was to reveal benefits and drawbacks of bilateral fitting of BAHAs in patients with symmetric or slight asymmetric bone-conduction thresholds. The possible effects were divided into three categories: hearing thresholds, directional hearing, and binaural hearing. STUDY DESIGN: Prospective study of 12 patients with bilateral BAHAs. METHODS: Baseline audiometry, directional hearing, speech reception thresholds in quiet and in noise, and binaural masking level difference were tested when BAHAs were fitted unilaterally and bilaterally. RESULTS: Eleven of the 12 patients used bilateral BAHAs on a daily basis. Tests performed in the study show a significant improvement in sound localization with bilateral BAHAs; the results with unilateral fitting were close to the chance level. Furthermore, with bilateral application, the improvement of the speech reception threshold in quiet was 5.4 dB. An improvement with bilateral fitting was also found for speech reception in noise. CONCLUSIONS: 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.