Evaluation of a bone groove geometry for fixation of a cochlear implant electrode.

PURPOSE: Electrode migration is a rare, but relevant complication in cochlear implant (CI) surgery. An effective countermeasure is to create a bone groove in the facial recess to secure the electrode lead. We use this method routinely since 2013, but still experienced sporadic electrode migration events most likely due to an improper surgical execution. The aim of this study was to determine the optimum groove geometry. METHODS: Grooves of defined geometry were created in specimens of fresh porcine femur compacta by use of a CNC milling machine. Electrode dummies were fixed in the groove and then exposed to tensile stress. Force measurements were carried out to examine the effect of groove diameter and opening width on the holding force. The mechanical impact on the electrode cable during insertion into the groove was recorded and the electrode lead was examined under microscopic magnification to assess potential structural damage. RESULTS: Optimum groove geometry (diameter 1.10 mm, opening width 0.90 mm) ensured an average holding force of 830 mN which is equivalent to the established fixation by use of a titanium clip. None of the microscopic inspections revealed any morphological deterioration of the electrode lead. CONCLUSION: The fixation of a CI electrode in a bone groove in the facial recess appears to be effective and safe. Furthermore, this method does not require additional costs or foreign material. The optimum geometry defined in this study helped us to refine our surgical standard produce and to generate more consistent results.

The Effect of a Bone Groove Against Cochlear Implant Electrode Migration.

HYPOTHESIS: The risk of electrode migration in cochlear implant (CI) surgery can be reduced by securing the electrode lead in a bone groove in the chorda-facial angle. BACKGROUND: Electrode migration is an important complication in CI surgery, which affects hearing performance, may induce pain and facial nerve stimulation, and requires surgical revision. A potential method to secure the electrode is to use a bone groove created in the facial recess to fixate the electrode lead. This surgical measure is a standard procedure for lateral wall electrodes (LWE) in our clinic since 2013. METHODS: Retrospective analysis of consecutive CI cases of a tertiary referral center from 2006 to 2016 and comparison of incidence rates between group A (without bone groove, years 2006-2012) and group B (with bone groove fixation, years 2013-2016). Clinical cases were reviewed with respect to electrode type, migration length, time interval to migration, and findings during revision surgery. RESULTS: Seventeen cases of electrode migration were found in a total of 1,603 cochlear implantations. Only LWE designs were affected by migration. The cumulative incidence for LWE in group B (0.5%) was significantly lower than in group A (3.7%, p < 0.01). The two migration cases in group B revealed insufficient geometric design of the bone grooves. Only one patient experienced a remigration after revision surgery. CONCLUSION: The fixation in a bone groove in the chorda-facial angle is an effective measure against migration of LWE. Proper surgical execution is mandatory to ensure a tight and enduring fit.

Hearing Preservation Outcomes After Cochlear Implantation Depending on the Angle of Insertion: Indication for Electric or Electric-Acoustic Stimulation.

OBJECTIVE: This study reviewed outcomes of hearing preservation (HP) surgery depending on the angle of insertion (AOI) in a cochlear implant (CI) patient population who used electric stimulation (ES) or combined electric-acoustic stimulation (EAS). STUDY DESIGN: Retrospective case review. SETTING: Tertiary referral university hospital. PATIENTS: Ninety-one patients with different degrees of preoperative low-frequency residual hearing who underwent HP surgery with a free-fitting lateral-wall electrode array (MED-EL Flex) with lengths ranging from 20.0 to 31.5 mm. INTERVENTION: Cochlear implantation using HP surgery technique and subsequent fitting with CI speech processor for ES, or combined CI and hearing aid speech processor for EAS. MAIN OUTCOME MEASURES: Individual AOI were estimated using modified Stenvers' projection. Freiburg monosyllable test in quiet (free-field presentation at 65 dB SPL) and pure-tone averages for low frequencies (125, 250, and 500 Hz; PTAlow) were evaluated during a follow-up period of 12 months after implantation. RESULTS: Estimated AOIs showed bimodal distribution: shallow insertion (SI) with mean AOI of 377 degrees and deep insertion (DI) with mean AOI of 608 degrees. Speech test scores after 12 months were comparable between AOI groups, however, they were significantly different between stimulation types with better scores for EAS. Only ES showed a positive correlation (r = 0.293) between speech test score and AOI. When HP was possible, both SI and DI showed significant postoperative PTAlow shifts with mean of 17.8 and 21.6 dB, respectively. These were comparable between AOI groups and no significant shifts were observed in follow-up intervals. Audiometric indication for HP and subsequent EAS is proposed up to 65 dB HL at 500 Hz, and up to 87 dB HL for HP. CONCLUSIONS: CI candidates can benefit from HP surgery with deep insertion when only using ES due to insufficient residual hearing. Conversely, candidates with preoperative threshold up to 65 dB HL at 500 Hz could perform significantly better with EAS which requires shallow insertion.

Preventing Facial Nerve Stimulation by Triphasic Pulse Stimulation in Cochlear Implant Users: Intraoperative Recordings.

HYPOTHESIS: Triphasic pulse stimulation of the auditory nerve can prevent unintended facial nerve stimulation (FNS) due to a different electromyographic (EMG) input-output function compared with biphasic pulses. BACKGROUND: FNS is sometimes observed in cochlear implant (CI) users as an unpleasant side effect of electrical stimulation using biphasic pulse patterns (BPP). Clinical remedies to alleviate FNS are 1) to extend stimulus phase duration or 2) to completely deactivate the electrode. In some cases, these options do not provide sufficient FNS reduction or are detrimental to subject performance. Stimulation using triphasic pulse patterns (TPP) has been shown to prevent FNS more effectively, yet the underlying mechanism remains unclear. METHODS: EMG potentials of muscles innervated by the facial nerve (orbicularis oculi and oris muscles) were recorded to quantitatively compare the effect of BPP and TPP stimulation on FNS. Recordings were conducted in five subjects during CI surgery. In two exemplary cases, different leading phase polarities in alternating and non-alternating order were tested. RESULTS: Compared with our previous study in awake patients using surface electrodes (Bahmer and Baumann, 2016), intraoperative recordings using subdermal electrodes showed lower noise content and allowed higher sampling resolution. While inter-subject variation remained high, intra-subject results for different electrode positions were comparable: FNS was strongly reduced for cathodic-first TPP stimulation. In contrast, exemplary cases showed little reduction for anodic-first TPP as well as for alternating stimulation. CONCLUSION: FNS in CI users can be reduced using TPP stimulation, but the ameliorative effect appears to be dependent on the leading stimulus polarity.

Management of Cochlear Implant Electrode Migration.

OBJECTIVE: The present study reviewed a cochlear implant (CI) patient population after surgery, which received a free-fitting electrode carrier designed for hearing preservation. The aim was to determine the rate of electrode migration of the CI electrodes and present clinical and surgical implications. STUDY DESIGN: Retrospective patient review. SETTING: Tertiary referral university hospital. PATIENTS: Two hundred seventy-eight patients implanted uni- or bilaterally with lateral wall electrodes designed for hearing preservation (358 implants). The control group was 323 patients implanted uni- or bilaterally with preformed perimodiolar electrodes (468 implants). INTERVENTIONS: Determination of CI electrode migration was conducted according to a clinical test protocol. Revision surgery was offered in confirmed patients of electrode migration. A bone groove was considered to improve the fixation of the electrode. MAIN OUTCOME MEASURES: Audiological testing including speech audiometry, subjective sound quality rating, and bilateral pitch comparison in bilateral patients, as well as radiological examinations, were conducted. RESULTS: Electrode migration was observed solely in patients implanted with lateral wall electrodes; 10 of 358 patients with free-fitting electrodes (2.8%) had electrode migration, which was successfully confirmed by the proposed clinical test protocol. Nine of the 10 confirmed patients underwent reinsertion surgery. Mean perception score decreased from 75.0% to 62.1% after electrode migration and recovered completely after reinsertion surgery. A flowchart to detect electrode migration was designed for clinical practice. CONCLUSION: Although electrode migration is a rare complication in CI surgery, long-term follow-up diagnostics should include a test protocol to detect electrode shifts of lateral wall electrode arrays. A reinsertion surgery should be conducted in confirmed patients to recover speech perception.

Long-term Hearing Preservation Outcomes After Cochlear Implantation for Electric-Acoustic Stimulation.

OBJECTIVE: This study reviewed outcomes of hearing preservation (HP) surgery in a cochlear implant patient population, with clinical follow-up results up to 11 years after implantation. STUDY DESIGN: Retrospective case review. SETTING: Tertiary referral university hospital. PATIENTS: Ninety six patients (103 ears) with partial deafness who underwent HP surgery at the University Hospital Frankfurt since 1999 were included. Electrode carriers were Cochlear Slim Straight, MED-EL Standard, Medium, Flex, and Flex. INTERVENTION: Cochlear implantation using the HP surgery technique with either the cochleostomy or round window approach. MAIN OUTCOME MEASURES: Pure-tone averages for low frequencies (125 Hz, 250 Hz, 500 Hz, PTAlow) and speech perception scores of the Freiburg monosyllable and number tests in quiet. PTAlow shifts were used to evaluate HP as complete for ≤10 dB, partial between 10 and 30 dB, and minimal for ≥30 dB. Time intervals were: preoperative, postoperative, after 12 months, and long-term (>24 months, mean 51.4 months, range 2-11 years). Impacts of electrode design and surgical approach were analyzed. RESULTS: Postoperatively (n = 103), HP was complete in 32 (31.1%), partial in 49 (47.6%), minimal in 14 (13.6%), and loss of hearing occurred in 8 cases (7.8%). After 12 months (n = 81), HP was complete in 22 (27.2%), partial in 33 (40.7%), minimal in 11 (13.6%), and loss of hearing occurred in 7 additional cases. For long-term outcomes (n = 62) HP was complete in 7 (11.3%), partial in 24 (38.7%), minimal in 9 (14.5%), and loss of hearing occurred in 7 additional cases (total 22/103, 21.4%). Cases with residual hearing who could utilize acoustic amplification (i.e., PTAlow < 80 dB HL) were 82/95 (85.3%) postoperatively, 58/66 (87.9%) after 12 months, and 38/40 (95.0%) for long-term outcomes. CONCLUSIONS: Long-term HP is feasible in a subset of patients. Patients with sufficient long-term residual hearing had the prerequisite to benefit from additional acoustic stimulation. No correlation of total hearing loss with etiology, electrode design, or surgical approach was evident. Apart from individual effects of structural damage or inflammation, genetic factors are suggested to influence HP. Cases with total hearing loss still demonstrated successful speech perception in long-term monosyllable recognition scores.

Place dependent stimulation rates improve pitch perception in cochlear implantees with single-sided deafness.

In normal hearing, the pitch of an acoustic tone can theoretically be encoded by either the place of stimulation in the cochlea or the corresponding rate of vibration. Thus spectral attributes and temporal fine structure of an acoustic signal are naturally correlated. Cochlear implants (CIs), neural prosthetic devices that restore hearing in the profoundly hearing impaired, currently disregard this mechanism; electrical stimulation is provided at fixed electrode positions with default place independent stimulation rate assignments. This does not account for individual cochlear encoding depending on electrode array placement, variations in insertion depth, and the proximity to nerve fibers. Encoding pitch in such manner delivers limited tonal information. Consequently, music appraisal in CI users is often rated cacophonic while speech perception in quiet is close to normal in top performers. We hypothesize that this limitation in electric stimulation is at least partially due to the mismatch between frequency and place encoding in CIs. In the present study, we determined individual electrode locations by analysis of cochlear radiographic images obtained after surgery and calculated place dependent stimulation rates according to models of the normal tonotopic function. Pitch matching in CI users with single-sided deafness shows that place dependent stimulation rates allow thus far unparalleled restoration of tonotopic pitch perception. Collapsed data of matched pitch frequencies as a function of calculated electrical stimulation rate were well fitted by linear regression (R(2) = 0.878). Sound processing strategies incorporating place dependent stimulation rates are expected to improve pitch perception in CI users.