Surgery and Healing
Surgery was performed under general anesthesia and was uneventful in all patients. Good implant stability was achieved at insertion, with a mean ISQ of 75.7 (SD, 8.8) (mean of highest value out of two perpendicular measurements in each patient). The mean soft tissue thickness was 6.0 mm (SD, 1.1 mm). Flap thinning was performed in three patients. The average surgery time was 45.0 minutes (SD, 14.6 min) from first incision to last suture. The surgical site healed satisfactorily in all patients. No implants or implant magnets were lost, replaced, or removed.
Fitting of the SP to the magnetic implant was performed at 4 ± 1 weeks after surgery on all but one patient, for whom fitting was delayed 3 weeks because of trauma to the implant site 10 days after surgery. Six and 21 patients selected the BP100 and BP110 Sound Processor, respectively. Table 2 shows the distribution of SP magnets per visit. After initial magnet selection, 14 patients changed to weaker and two patients to stronger magnets. Four patients changed magnets more than once.
Insufficient magnetic retention was reported for five patients with SPM5, who all had preoperative soft tissue thicknesses exceeding 6 mm; in three of these patients, flap thinning was performed at implant surgery. Sufficient retention force was achieved by removing the soft pad while awaiting availability of a stronger magnet. Three of the patients were able to return to using the soft pad after a period of adaptation of the skin.
Free-field Hearing Tests
Pure-tone audiometry showed a statistically significant improvement in PTA (mean of 500, 1,000, 2,000, and 4,000 Hz) of 18.4 dB HL (SD, 6.9 dB; p < 0.0001) with the test device at 9 months compared with unaided hearing. The corresponding improvement for the subgroup of patients with conductive hearing loss and SSD was 17.9 dB HL (SD, 6.6 dB; p < 0.0001) and 19.1 dB HL (SD, 7.7 dB; p = 0.0005), respectively. No statistically significant difference in PTA compared with softband tests was recorded. Table 3 shows PTA values per visit for all tested conditions.
Statistically significant improvements with the test device compared with unaided hearing were recorded at all frequencies up to and including 6,000 Hz (Fig. 3A). The mean improvement was largest in the frequency range 500 to 3,000 Hz: up to 25.2 dB improvement (SD, 8.4 dB; p < 0.0001). Overall similar hearing thresholds were obtained with the SP on a softband, with a slight advantage for the test device between 750 and 1,000 Hz and an advantage for the softband at and above 4,000 Hz.
Speech recognition tests in quiet showed statistically significant improvements at all tested intensity levels with the test device compared with unaided hearing. At 9 months, the mean improvement in percentage correctly repeated words at 50, 65, and 80 dB SPL was 50.0, 46.4, and 24.2 percentage points, respectively. Comparison with softband tests showed no significant differences (Fig. 3B). The percentage improvement for the subgroup of patients with a conductive hearing loss and SSD were similar: 55.6, 45.3, and 23.3 percentage points and 40.1, 48.3, and 25.8 percentage points, respectively, at increasing SPL.
A mean SNR of −4.9 dB (SD, 5.1 dB) was recorded for the test device in adaptive sentence in noise tests at 9 months, providing statistically significant improvements of 15.0 dB (SD, 12.8 dB; p < 0.0001) and 3.8 dB (SD, 7.0 dB; p = 0.0092) compared with unaided hearing and softband tests, respectively. A slight gradual improvement in SNR from the time of initial fitting to the 3-month follow-up visit was recorded (Fig. 3C). Although there were differences in test language and methodology, the four study sites were all consistent in terms of the improvement compared with both unaided and softband conditions. Similarly, results per type of hearing loss were in line with the global score. The SNR improvement compared with unaided hearing was 17.9 dB (SD, 15.2 dB; p < 0.0001) for patients with conductive hearing loss and 10.2 dB (SD, 4.7 dB; p = 0.002) for patients with SSD and 3.8 dB (SD, 7.6 dB; p = 0.05) and 3.7 dB (SD, 6.1 dB; p = 0.09), respectively, compared with softband.
Statistically significant improvements with the test device compared with the preoperative unaided situation were obtained for the APHAB subscales Reverberation (p = 0.016), Background noise (p = 0.035), and the Global score (p = 0.038). A nonsignificant improvement and a nonsignificant deterioration were recorded for the subscales Ease of Communication and Aversiveness, respectively (Fig. 3D).
Magnetic Force and Pressure
The mean magnetic retention force across all visits was 0.99 N, with a relatively large variation between patients (SD, 0.23 N); the mean force remained stable across time (Table 3). The mean pressure between the SP magnet and the underlying skin remained relatively constant across time with an average of 0.14 N/cm2 (SD, 0.04 N/cm2) across all visits; no single value exceeded 0.4 N/cm2, which corresponds approximately to the capillary blood pressure. The mean peak pressure across all visits was 0.44 N/cm2 (SD, 0.27 N/cm2). For the patients who used the magnet with a soft pad, as indicated, the peak pressure did not exceed the target maximum value of 0.6 N/cm2 (corresponds approximately to the diastolic blood pressure in children), except at one or two occasions in three patients (only one of the recorded values exceeded 0.8 N/cm2, which approximates to the diastolic blood pressure in adults). In patients, who used SPM5 without a soft pad, however, significantly higher values were recorded (up to 1.95 N/cm2).
Daily Use and Retention
The patient-reported average daily use was 7.0 h/d (SD, 3.8 h/d) and ranged between 3.4 and 15.4 h/d. The daily use for the subgroups of patients with conductive hearing loss and SSD was 7.6 (SD, 4.0 h/d) and 6.0 h/d (SD, 3.3 h/d), respectively. Incidences of insufficient retention were rare and reported to occur on average less than once every third day during normal daily activities.
Soft Tissue Status, Numbness, and Pain
Overall low and decreasing POSAS scores were recorded, indicating satisfactory soft tissue status. At the last visit, the mean overall opinion of the skin was rated as 1.52 (SD, 0.93) by the investigators and 1.81 (SD, 1.21) by the patients on a scale from 1 to 10, with low values indicating good outcomes. The proportion of patients experiencing numbness was highest at the time of initial fitting (62.9%) and decreased gradually thereafter (22.2% at the last visit). Overall mean pain scores were low, indicating no or limited pain in the majority of patients. See Table 3.
No cases of pressure-related skin necrosis or significant soft tissue reactions were reported. Four cases of mild erythema were reported. Three events resolved without medical treatment; in one patient, this was achieved by changing to a weaker magnet. The last case was reported as initiated at the time of the last visit and, hence, was ongoing at study end. Four cases of pain at the implant site were reported, two of which resolved within 1 week without treatment. Two patients reported mild/moderate pain after continuous use of the device. One patient reported discomfort in the magnet area, which resolved without medical treatment. No other device-related local adverse events were reported. All patients continue to use and benefit from the device.
The investigation evaluated the clinical performance of a novel magnetic bone conduction hearing implant in 27 adult patients with conductive or mild mixed hearing loss or SSD. The study showed statistically significant improvements in hearing performance compared with unaided hearing and similar or improved outcomes compared with tests performed with the SP on a softband. No major pressure-related soft tissue complications were reported and no implants were lost or removed, suggesting that the device is efficacious and safe for the tested indication.
Magnetic bone conduction hearing implants have the advantage over skin-penetrating systems of providing improved cosmetics and eliminating the daily cleaning of the site (25). With modern SP technology, it is possible to obtain good sound transmission despite the soft tissue attenuation that is inherent to magnetic bone conduction hearing implants. Although the system must provide reliable retention of the SP to ensure good clinical outcomes, it should not cause irritation of the skin or discomfort. Threshold audiometry showed that the test device provides significant functional gain at all frequencies. The improvement is largest in the important speech frequency range up to and including 3,000 Hz. Above 3,000 Hz, the performance drops gradually as expected because of the soft tissue attenuation, which is known to mainly affect the high frequencies (26,27). It is anticipated that aided high-frequency thresholds could be improved further (particularly by prescribing more amplification in the high frequencies) by less conservative SP settings than were used in the present investigation. It would be expected, however, that some attenuation of sound through soft tissue will remain. In the sentence tests in noise, which represents the most difficult listening situation, significant improvement in SNR was recorded compared with unaided hearing and compared with softband tests. Speech recognition in quiet was significantly better than for the unaided situation and similar to softband. Although not statistically verified, a gradual improvement in speech understanding was noted up to the 3-month visit, followed by relatively stable levels. A possible improvement in hearing performance may be explained by adaptation as patients get used to the sound; it may also be an effect of fine-tuning of the SP by the audiologist. The fact that overall comparable outcomes were obtained with the SP on a softband as with the test device suggests that preoperative softband tests are a good predictor of the patient’s postoperative hearing performance; the importance of preoperative testing to achieve successful clinical outcomes has been reported by several authors (5,28,29).
APHAB scores showed that the test device provides good subjective benefit in terms of the patient’s listening experience compared with the unaided situation. Improvements were obtained for the subscales related to reverberation, background noise, and ease of communication. A nonsignificant deterioration was observed for the subscale aversiveness, which quantifies negative reactions to environmental sounds; slightly worse aversiveness scores are a known effect with hearing devices (30,31) and have been attributed to unwanted sound also being amplified (30).
Soft tissue complications were minimal, as reflected by good POSAS scores and only four reports of mild skin irritation. The result suggests that the test device is associated with significantly less adverse soft tissue reactions than implants involving a skin-penetrating abutment (32). Favorable pain and numbness scores together with a high mean daily use (7 h/d) suggest good wearing comfort. Some patients reported average daily use exceeding 15 h/d; however, other patients were only part-time users while still reporting good benefit from the device. The relatively lower usage time in some patients may be reflective of the non–skin-penetrating nature and flexibility of the device, which allows patients to easily attach the SP to the invisible implant site when exposed to challenging listening situations. The ease of use of the device may provide significant advantages for patients with disabilities and/or reduced dexterity.
As with any surgical procedure involving incising soft tissue, a certain degree of transient (or in some cases permanent) numbness can be expected. In the present investigation, gradually reducing numbness was reported. Possibly the degree of paresthesia could be further reduced by placing the incision superior rather than anterior to the planned magnet position.
Assessment of the magnetic retention showed that the patients on average chose a retention force of around 1 Newton. However, the variability between patients was relatively large and most likely relates to different comfort levels and lifestyles of individual patients. For the same reasons and because of different soft tissue thicknesses, the patients chose SP magnets of varying strength. More than half of the patients required a change of SP magnet at some point during the investigation. The majority of these patients changed to a weaker magnet, which suggests that the tissue gradually compresses under the load of the magnet during the initial period after fitting. Similar observations have been reported for other implants incorporating a magnetic coupling (33,34).
The reported rate of insufficient retention was low. A few patients experienced retention difficulties with the strongest available SP magnet (SPM5); sufficient retention was obtained by removing the soft pad to increase the magnetic force. Removing the soft pad may cause areas of higher peak pressure to appear on the skin, as demonstrated by pressure measurements performed in this investigation. To maintain a healthy implant site, peak pressure areas should be avoided because the blood supply in the soft tissue may be affected. Areas of high peak pressure were not seen in the presence of the soft pad, demonstrating its ability to distribute the pressure evenly. All patients with retention difficulties had preoperative soft tissue thicknesses greater than 6 mm, highlighting the importance of flap thinning if the thickness exceeds this value. The need for extra magnet strength also in patients who had flap thinning at surgery suggests the presence of transient postoperative swelling/edema in these patients. Although the majority of patients were successfully fitted with the available range of magnets, additional strength may be required in specific situations as a temporary or permanent solution; hence, the manufacturer has developed a stronger magnet (SPM6) to meet this need.
The magnetic bone conduction hearing implant evaluated in the present investigation was shown to be safe and effective because it provides good hearing performance in patients with conductive and mild mixed hearing loss or SSD, with good wearing comfort and minimal soft tissue complications. Future investigations may be considered to address the question of maximum audiometric fitting range for these systems. Magnetic systems constitute a viable alternative for patients who cannot or will not use an implant system that involves skin penetration. Although the investigation was limited to adult patients, it is expected that the device is equally suited for pediatric patients who are candidates for bone conduction surgery.
The following coinvestigators and audiologists are acknowledged for great contributions throughout the investigation: Michael Tong, Gordon Soo, Willis Tang, Terence Wong, and Joannie Yu (Chinese University of Hong Kong, Hong Kong, China); Amit Wolfovitz, Rabia Shihada, Noam Yehudai, Riad Khnifies, and Talma Shpak (Bnai Zion Hospital, Haifa, Israel); Gloria Ribalta, Raquel Levi, and Pilar Alarcón (Clínica Las Condes, Santiago, Chile); and Kerrie Plant and Michelle Knight (HEARing Cooperative Research Centre, Melbourne). Thanks also to Johan Blechert (Cochlear Bone Anchored Solutions AB) for ensuring a high-quality study conduct in compliance with applicable guidelines and regulations.
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Keywords:Copyright © 2015 by Otology & Neurotology, Inc. Image copyright © 2010 Wolters Kluwer Health/Anatomical Chart Company
Bone conduction; Bone conduction hearing implant; Clinical outcome; Hearing performance; Magnetic system; Osseointegration; Bone-anchored hearing aid; Baha