Scalp thickness in the temporal region: its relevance to the development of cochlear implants

Transmuscular pocket: a modified technique to overcome thick musculocutaneous flap in cochlear implant surgery

OBJECTIVE

To describe and evaluate a modified cochlear implant surgical procedure for patients with a thick musculocutaneous flap.

MATERIALS AND METHODS

A prospective study for cochlear implant surgery in selected patients with a musculocutaneous flap thicker than 7 mm.

RESULTS

Fourteen patients with a thick scalp flap underwent cochlear implantation between July 2019 and December 2020. The patient age ranged between 17 and 53 years. The flap thickness was between 7 mm and 14 mm. The mean follow uptime post operatively was 16.5 months. The cochlear implant receiver coil was successfully implanted using the transmuscular technique without complications and with normal audiological function.

CONCLUSION

The transmuscular pocket modified technique is a safe and effective method to overcome a thick musculocutaneous flap in cochlear implant surgery.

Blinded comparison of computed tomography, ultrasound and needle methods to measure skin flap thickness for cochlear implantation

PURPOSE

Patient suitability for cochlear implant (CI) devices compatible with magnetic resonance imaging and CI processor configuration is dependent on their retro-auricular skin flap thickness. This is typically measured intra-operatively using a needle and therefore patients are not guaranteed their implant of choice prior to surgery. We aimed to identify an accurate method to measure skin flap thickness pre-operatively to streamline CI selection and simplify the consent process.

METHODS

Blinded prospective skin flap thickness measurements for patients undergoing CI surgery were recorded using pre-operative computed tomography (CT) and ultrasound (US), and intraoperative needle measurement.

RESULTS

Fifty-six adult patients (36 females, 20 males; mean age 59 years) were included. The mean flap thickness was measured highest by CT (6.9 mm, 95% CI 6.5-7.3 mm), followed by US (6.3 mm, 95% CI 5.9-6.7 mm) and lastly needle (5.5 mm, 95% CI 5.1-5.9 mm) (p < 0.0001). A strong positive correlation (p < 0.001) was noted between all three modalities: CT vs needle (r = 0.869), US vs needle (r = 0.865), and CT vs US (r = 0.849).

CONCLUSION

Accurate, non-invasive measurement of skin flap thickness prior to CI surgery can be achieved using CT or US. We recommend the routine use of US in the outpatient clinic to minimise unnecessary radiation exposure.

Influence of surrogate scalp material and thickness on head impact responses: Toward a biofidelic head-brain physical model

Advanced physical head models capable of replicating both global kinematics and intracranial mechanics of the human head are required for head injury research and safety gear assessment. These head surrogates require a complex design to accommodate realistic anatomical details. The scalp is a crucial head component, but its influence on the biomechanical response of such head surrogates remains unclear. This study aimed to evaluate the influence of surrogate scalp material and thickness on head accelerations and intraparenchymal pressures using an advanced physical head-brain model. Scalp pads made from four materials (Vytaflex20, Vytaflex40, Vytaflex50, PMC746) and each material with four thicknesses (2, 4, 6, and 8 mm) were evaluated. The head model attached to the scalp pad was dropped onto a rigid plate from two heights (5 and 19.5 cm) and at three head locations (front, right side, and back). While the selected materials' modulus exhibited a relatively minor effect on head accelerations and coup pressures, the effect of scalp thickness was shown to be major. Moreover, by decreasing the thickness of the head's original scalp by 2 mm and changing the original scalp material from Vytaflex 20 to Vytaflex 40 or Vytaflex 50, the head acceleration biofidelity ratings could improve by 30% and approached the considered rating (0.7) of good biofidelity. This study provides a potential direction for improving the biofidelity of a novel head model that might be a useful tool in head injury research and safety gear tests. This study also has implications for selecting appropriate surrogate scalps in the future design of physical and numerical head models.

Methods for measuring pre-, intra-, and postoperative skin thickness for cochlear implants

Objective

This study was conducted to determine whether there is a reliable method for measuring the thickness of the retroauricular skin before, during, and after cochlear implantation, which allows the assessment of the optimal force of the external magnet of the cochlear implant (CI).

Methods

The retroauricular skin thickness of 83 patients who received a CI was measured using three different methods. The thickness was measured on pre- and postoperative CT images, as well as intraoperatively. The magnet category chosen by the surgeon was recorded when the implant was switched on and during the first follow-up visit. Correlation analyses were performed on the different skin thickness measurements and between the skin thickness and magnet strength categories.

Results

Only six patients required an exchange of the magnet until the follow-up. Although the median absolute thickness differed significantly between the three measures (p < 0.0001), their thickness values showed highly significant correlations (Pearson's r = 0.457–0.585; p < 0.01). In addition, magnet strength, was significantly correlated with the flap thickness determined pre-, post-, and during surgery. The lowest correlation with magnet strength was found in the intraoperative needle method.

Conclusion

All three measurements methods provided a suitable base for determining the ideal magnetic force. However, of particular interest were the pre- and postoperative CT measurements. The first enabled the early assessment of the required magnetic strength and thus a timely postoperative supply, whereas the latter helped to estimate the need for magnetic strength reduction during follow-up care and the feasibility of an early swith-on.

A comparison of imaging techniques to measure skin flap thickness in cochlear implant patients to enable pre-operative device selection

OBJECTIVES

Magnetic resonance imaging (MRI)-compatible cochlear implants have weaker internal magnets than non-MRI-compatible devices. Their suitability for individual patients is limited by skin flap thickness, traditionally measured with a needle in the operating theatre. We aimed to establish the accuracy of imaging modalities to measure skin flap thickness pre-operatively, with the goal of streamlining device selection and simplifying the consent process.

METHODS

Skin flap measurements were taken using ultrasound (US), computed tomography (CT) and MRI and compared for agreement with intra-operative needle measurement.

RESULTS

Twenty-seven skin flaps were included. Absolute agreement between needle and imaging methods was low: needle/US: 44.4% (95% confidence interval [CI]: 27.7-62.7), needle/CT: 39.1% (95% CI: 22.2-59.2), needle/MRI: 20.8% (95% CI: 9.2-40.5). However, US and CT showed 95.7% agreement (95% CI: 76.0-99.8) with intraclass correlation of 0.996 (95% CI: 0.991-0.998) and narrow Bland-Altman limits of agreement (-0.37, 0.45 mm). BMI and skin flap thickness showed a significant positive correlation (r_s= 0.664, P = 0.002) but no significant correlation was observed for age (P = 0.659).

DISCUSSION

The high level of agreement between US and CT suggests that there are more accurate measurements of skin flap thickness compared with needle or MRI. Needle measurements are consistently smaller.

CONCLUSION

The use of CT or US should be considered when making pre-operative device choices.

Changes in the Area Adjacent to the Internal Receiver-Stimulator of Cochlear Implant: A Retrospective Study

BACKGROUND

In some patients with cochlear implants, bony resorption at the internal receiver-stimulator site can be observed on postoperative computed tomography. Therefore, it is essential to elucidate the effects of the internal receiver-stimulator on the scalp and bony bed over time.

OBJECTIVE

We aimed to evaluate how the internal receiver-stimulator of a cochlear implant device changed the thickness of the surrounding scalp and skull over time using computed tomography.

METHODS

This retrospective study evaluated patients who underwent cochlear implantation and received two computed tomography scans postoperatively for different indications at a tertiary referral centre. The main outcomes were scalp thickness and bony bed depth.

RESULTS

Fourteen ears were included in this study. There was very good inter-rater reliability, with a Cronbach's alpha of 0.94. The mean scalp thickness over the internal receiver-stimulator was 6.02 (+/-2.4) mm in the first scan and decreased with no significant change to 5.62 mm (+/-1.64) in the second scan (p = 0.59, paired t-test). The mean depth of the bony bed increased significantly from 1.39 mm (+/-0.93) to 2.62 mm (+/-1.24) (p = 0.03).

CONCLUSION

There was no change in the thickness of the scalp overlying the internal receiver-stimulator indicating that the scalp was more resistant than the bony skull to the tensile pressure exerted by the internal receiver-stimulator. In contrast, the bony bed depth of the internal receiver-stimulator increased over time. This can result in decreased internal receiver-stimulator protrusion and decreased risk of device displacement and migration.

Age-dependent variations of scalp thickness in the area designated for a cochlear implant receiver stimulator

Objective

The integrity of the scalp overlying a cochlear implant receiver stimulator (RS) is critical for the long‐term survival of the implant. Exposure or extrusion of the device will likely result in the need for its removal. There is a global trend of acceleration of population aging, thus raising the prevalence of cochlear implantation (CI) in the elderly. The aim of this study was to define age‐dependent changes in scalp thickness and discuss the implication of that anatomical characteristic for CI in the geriatric population.

Methods

Scalp thickness over the location of the RS in the temporo‐parietal area was measured directly with a needle in patients of various ages.

Results

Two‐hundred thirty‐six temporo‐parietal scalps were measured in patients aged 18 to 85 years. A strong inverse correlation was found between age and scalp thickness (rs = ‐0.723, P < .001). Scalp thickness decreased with age from a mean of 8 mm in the third decade of life to 5 mm in the ninth decade of life.

Conclusion

The human scalp thins with age and most likely undergoes a reduction in its strength. As a consequence, implantable hearing devices that are shielded by the scalp can be at increased risk of exposure and extrusion in the aging recipient. This needs to be taken into account when considering an implantation procedure, the surgical approach and patient instructions on need for and venues for continuing care over time.

Level of Evidence

2B

Temporal Scalp Thickness, Body Mass Index, and Suprafascial Placement of Receiver Coil of the Cochlear Implant

OBJECTIVE

The objectives of this study are to demonstrate the relationship between the thickness of the temporal scalp and body mass index (BMI), age, and sex, and to present the surgical technique which we perform in patients with a thick scalp.

MATERIALS AND METHODS

This is a retrospective, cross-sectional study. This study was performed in a tertiary referral center. Cranial computed tomography images of 469 subjects were included in the study. These subjects were evaluated according to BMI, age, and sex. These individuals were divided into 6 groups based on the BMI levels. Differences between the groups were compared in terms of temporal scalp thickness. In 5 patients with a mean scalp thickness of 9.7 mm, the receiver coil was placed over the temporal muscle fascia through a transmuscular incision without surgical thinning of the skin flap.

RESULTS

Average scalp thickness was measured as significantly higher in males than in females, with advancing age, and increasing levels of BMI. In 5 adult patients with a scalp thickness measured as ≥7 mm who underwent cochlear implantation, suprafacial placement of the receiver coil achieved successful surgical and audiological results.

CONCLUSION

Thinning of the skin flap is recommended in patients with a scalp thickness ≥ 7 mm to provide effective transmission, minimalized power requirement, and magnet retention. Suprafascial placement of the receiver coil can be recommended in patients with a thick scalp without any excisional thinning impairing integrity and vascularity of the skin.

Effect of anatomical variability on electric field characteristics of electroconvulsive therapy and magnetic seizure therapy: a parametric modeling study

Electroconvulsive therapy (ECT) and magnetic seizure therapy (MST) are conventionally applied with a fixed stimulus current amplitude, which may result in differences in the neural stimulation strength and focality across patients due to interindividual anatomical variability. The objective of this study is to quantify the effect of head anatomical variability associated with age, sex, and individual differences on the induced electric field characteristics in ECT and MST. Six stimulation modalities were modeled including bilateral and right unilateral ECT, focal electrically administered seizure therapy (FEAST), and MST with circular, cap, and double-cone coils. The electric field was computed using the finite element method in a parameterized spherical head model representing the variability in the general population. Head tissue layer thicknesses and conductivities were varied to examine the impact of interindividual anatomical differences on the stimulation strength, depth, and focality. Skull conductivity most strongly affects the ECT electric field, whereas the MST electric field is independent of tissue conductivity variation in this model but is markedly affected by differences in head diameter. Focal ECT electrode configurations such as FEAST is more sensitive to anatomical variability than that of less focal paradigms such as BL ECT. In MST, anatomical variability has stronger influence on the electric field of the cap and circular coils compared to the double-cone coil, possibly due to the more superficial field of the former. The variability of the ECT and MST electric fields due to anatomical differences should be considered in the interpretation of existing studies and in efforts to improve dosing approaches for better control of stimulation strength and focality across patients, such as individualization of the current amplitude. The conventional approach to individualizing dosage by titrating the number of pulses cannot compensate for differences in the spatial extent of stimulation that result from anatomical variability.

Controlling stimulation strength and focality in electroconvulsive therapy via current amplitude and electrode size and spacing: comparison with magnetic seizure therapy

OBJECTIVES

Understanding the relationship between the stimulus parameters of electroconvulsive therapy (ECT) and the electric field characteristics could guide studies on improving risk/benefit ratio. We aimed to determine the effect of current amplitude and electrode size and spacing on the ECT electric field characteristics, compare ECT focality with magnetic seizure therapy (MST), and evaluate stimulus individualization by current amplitude adjustment.

METHODS

Electroconvulsive therapy and double-cone-coil MST electric field was simulated in a 5-shell spherical human head model. A range of ECT electrode diameters (2-5 cm), spacing (1-25 cm), and current amplitudes (0-900 mA) was explored. The head model parameters were varied to examine the stimulus current adjustment required to compensate for interindividual anatomical differences.

RESULTS

By reducing the electrode size, spacing, and current, the ECT electric field can be more focal and superficial without increasing scalp current density. By appropriately adjusting the electrode configuration and current, the ECT electric field characteristics can be made to approximate those of MST within 15%. Most electric field characteristics in ECT are more sensitive to head anatomy variation than in MST, especially for close electrode spacing. Nevertheless, ECT current amplitude adjustment of less than 70% can compensate for interindividual anatomical variability.

CONCLUSIONS

The strength and focality of ECT can be varied over a wide range by adjusting the electrode size, spacing, and current. If desirable, ECT can be made as focal as MST while using simpler stimulation equipment. Current amplitude individualization can compensate for interindividual anatomical variability.

Electric field strength and focality in electroconvulsive therapy and magnetic seizure therapy: a finite element simulation study

We present the first computational study comparing the electric field induced by various electroconvulsive therapy (ECT) and magnetic seizure therapy (MST) paradigms. Four ECT electrode configurations (bilateral, bifrontal, right unilateral, and focal electrically administered seizure therapy) and three MST coil configurations (circular, cap, and double cone) were modeled. The model incorporated a modality-specific neural activation threshold. ECT (0.3 ms pulse width) and MST induced the maximum electric field of 2.1-2.5 V cm⁻¹ and 1.1-2.2 V cm⁻¹ in the brain, corresponding to 6.2-7.2 times and 1.2-2.3 times the neural activation threshold, respectively. The MST electric field is more confined to the superficial cortex compared to ECT. The brain volume stimulated was much larger with ECT (up to 100%) than with MST (up to 8.2%). MST with the double-cone coil was the most focal, and bilateral ECT was the least focal. Our results suggest a possible biophysical explanation of the reduced side effects of MST compared to ECT. Our results also indicate that the conventional ECT pulse amplitude (800-900 mA) is much higher than necessary for seizure induction. Reducing the ECT pulse amplitude should be explored as a potential means of diminishing side effects.

Skin flap thickness in cochlear implant patients - a prospective study

The thickness and quality of the skin overlying a cochlear implant is important for its integrity. It should be thick enough to protect the implant and prevent flap breakdown yet should not be so thick that it impedes the electronic signal or causes difficulty wearing the coil because of loss of the magnetic coupling. The principle of this study was to devise a method to assess the thickness of skin over a cochlear implant receiver stimulator package and prospectively measure this thickness during the first year following surgery. All patients studied were implanted with MED-EL COMBI 40+ implants. The first cohort consisted of 35 adults; the second 23 children. Various methods of measurement were assessed. In this study the principle of the Hall Effect electrode was used to measure the magnetic flux density of the magnet within the receiver stimulator package. Following standardization, results showed that skin thickness significantly thinned in the adult group before stabilizing. This was less obvious in children, probably due to the effect of the skin thickening as the child grows. Knowledge of skin thickness has implications relating to the functioning of an implant and avoiding potential flap related complications.

Comparisons of peak SAR levels in concentric sphere head models of children and adults for irradiation by a dipole at 900 MHz

The aim of this study is to examine the scale and significance of differences in peak specific energy absorption rate (SAR) in the brains of children and adults exposed to radiofrequency emissions from mobile phones. Estimates were obtained by method of multipole analysis of a three layered (scalp/cranium/brain) spherical head exposed to a nearby 0.4 lambda dipole at 900 MHz. A literature review of head parameters that influence SAR induction revealed strong indirect evidence based on total body water content that there are no substantive age-related changes in tissue conductivity after the first year of life. However, it was also found that the thickness of the ear, scalp and cranium do decrease on average with decreasing age, though individual variability within any age group is very high. The model analyses revealed that compared to an average adult, the peak brain 10 g averaged SAR in mean 4, 8, 12 and 16 year olds (yo) is increased by a factor of 1.31, 1.23, 1.15 and 1.07, respectively. However, contrary to the expectations of a recent prominent expert review, the UK Stewart Report, the relatively small scale of these increases does not warrant any special precautionary measures for child mobile phone users since: (a) SAR testing protocols as contained in the CENELEC (2001) standard provide an additional safety margin which ensures that allowable localized SAR limits are not exceeded in the brain; (b) the maximum worst case brain temperature rise (approximately 0.13 to 0.14 degrees C for an average 4 yo) in child users of mobile phones is well within safe levels and normal physiological parameters; and (c) the range of age average increases in children is less than the expected range of variation seen within the adult population.