Eur Arch Otorhinolaryngol DOI 10.1007/s00405-014-3352-4
OTOLOGY
Estimation of insertion depth angle based on cochlea diameter and linear insertion depth: a prediction tool for the CI422 Annett Franke-Trieger • Dirk Mu¨rbe
Received: 26 September 2014 / Accepted: 20 October 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Beside the cochlear size, the linear insertion depth (LID) influences the insertion depth angle of cochlear implant electrode arrays. For the specific implant CI422 the recommended LID is not fixed but can vary continuously between 20 and 25 mm. In the current study, the influence of cochlea size and LID on the final insertion depth angle was investigated to develop a prediction tool for the insertion depth angle by means of cochlea diameter and LID. Preoperative estimation of insertion depth angles might help surgeons avoid exceeding an intended insertion depth, especially with respect to low-frequency residual hearing preservation. Postoperative, high-resolution 3Dradiographs provided by Flat Panel Computed Volume Tomography (FPCT) were used to investigate the insertion depth angle in 37 CI422 recipients. Furthermore, the FPCT images were used to measure linear insertion depth and diameter of the basal turn of the cochlea. A considerable variation of measured insertion depth angles ranging from 306° to 579° was identified. The measured linear insertion depth ranged from -18.6 to 26.2 mm and correlated positively with the insertion depth angle. The cochlea diameter ranged from 8.11 to 10.42 mm and correlated negatively with the insertion depth angle. The results suggest that preoperatively measured cochlea diameter combined with the option of different array positions by means of LID may act as predictors for the final insertion depth angle. Keywords Cochlea implant array Insertion depth angle Linear insertion depth Cochlear size
A. Franke-Trieger (&) D. Mu¨rbe Department of Otorhinolaryngology, Saxonian Cochlear Implant Center, Technische Universita¨t of Dresden, Fetscherstr. 74, 01307 Dresden, Germany e-mail: [email protected]
Introduction The current study is a postoperative approach to investigate the insertion depth angle and its relation to cochlear size and linear insertion depth (LID) for the Cochlear Nucleus CI422. For the CI422, the recommended intended LID is not fixed but can vary continuously within 20 and 25 mm defined by two white markers. Consequently, the LID can influence the final insertion depth angle remarkably. Furthermore, it is known that the size of human cochleae varies considerably, which has been shown in several studies [1–4]. The assumption that the size of the cochlea does influence the final insertion depth angle was confirmed by studies investigating electrode position and cochlea size for several array types [5–7]. In this context, the focus of the current study evolved to answer three specific questions: firstly, how does the LID influence the final insertion depth angle? Secondly, what is the relation between cochlea diameter and final insertion depth angle? Thirdly, is it possible to develop a prediction tool for the intended insertion depth angle by means of LID and cochlear diameter measurements? Patients might benefit from the possibility of predicting the insertion depth angle with respect to residual hearing preservation and intracochlear trauma. Several studies investigating different array types have shown that the insertion depth influences the maintenance of residual hearing after CI surgery, such as successful conservation of hearing after cochlear implantation using a limited insertion depth [8–10] and poorer preservation of residual hearing for angles greater than 400° compared to lower angles [11]. For the specific implant CI422, Skarzynski et al. [12] showed very recently that low-frequency residual hearing was well preserved. Furthermore, they found a significant correlation between threshold increase
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and preoperative hearing threshold where characteristic frequencies corresponded to an angular position occupied by the electrode array. For characteristic frequencies apical to the electrode tip, the correlation between threshold increase and residual hearing decreased in amplitude considerably. Consequently, they concluded that the selection of optimal insertion depth should be patient specific by means of corresponding threshold frequency [12]. In this context, the development of a prediction tool for the intended insertion depth angle would be valuable. It has also been shown that a limited insertion depth can help avoid intracochlear trauma or interscalar dislocations, which have been shown to occur more frequently with deep insertions. Adunka and Kiefer [13] found increased intracochlear trauma with deep insertions and Finley and Skinner [14] found that increasing the insertion depth is related to a greater number of electrodes being located in the scala vestibuli. Studies investigating the influence of the insertion depth on the quality of speech perception are inconsistent in the literature. Yukawa et al. and Skinner et al. [15, 16] found a positive correlation between insertion depth and speech perception. In contrast Finley and Skinner as well as and Holden et al. [14, 17] found that increasing the insertion depth is significantly correlated with reduced speech perception, whereas other studies did not find a significant correlation between insertion depth and speech reception [18, 19]. Earlier studies investigated the correlation between insertion depth and cochlea size of other array types. Franke-Trieger et al. [7] found a negative correlation between insertion depth angle and cochlea diameter for different MED-EL electrode arrays by means of temporal bone experiments. Escude et al. [6] investigated the insertion depth for the Nucleus 24 (CA) recipients using a LID of either 17 mm (n = 9) or 19 mm (n = 7); a statistically significant correlation between insertion depth and diameter of the basal turn (R2 = 0.51, p \ 0.05) was found for the 17-mm group. Firstly, there remained a considerable dispersion about the regression line and secondly no correlation was found for the 19-mm group, indicating that there must be other factors that influence the insertion depth angle. The specification of the LID 17 or 19 mm was based on reports stating whether the electrode array has been inserted up to the first or the third elastic rib. Escude et al. did not measure the actual final linear insertion depth, thus this specification could deviate from the actual final linear insertion depth. The current study is first comprehensive characterization of the specific implant CI422 with respect to linear and angular insertion depth as well as cochlear size.
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Materials and methods Materials Subjects in this retrospective study included 20 female and 17 male, aged 15–82 years with a mean age of 51 years. All 37 subjects suffered from profound or severe sensorineural hearing loss and were implanted with a Cochlear CI422 with Slim Half-Band Straight Electrode (Cochlear Corp., Sydney, Australia). The electrode array of the CI422 incorporates two white markers. The white marker at 20 mm, henceforth labeled as marker20, at the end of the active portion of the array indicates the minimum insertion depth. The white marker at 25 mm, henceforth labeled as marker25, indicates the maximum insertion depth. After exposing the round window membrane, the round window insertion technique was applied to all subjects. Difficulties during the insertion procedure that led to incomplete insertion were reported in two subjects. In all other subjects, neither surgical problems nor incomplete insertion were reported. Approval by the ethics committee of the university hospital was obtained for this study. Technical data of FPCT imaging FPCT examinations were carried out at the first day after implantation using a Flat Panel Computer Tomograph 3D Accuitomo 80 (J. Morita MFG. CORP., Kyoto, Japan). Imaging was performed with a tube current of 8 mA and a tube voltage of 90 kV. The raw data projection images were reconstructed using the software i-dixl (J. Morita MFG. CORP., Kyoto, Japan) resulting in a voxel size of 125 lm. Measurements Diameter of the basal turn To quantify the cochlear size, the diameter of the basal turn of the cochlear was measured; see Fig. 1 for an example of this measurement. To minimize the measuring inaccuracy two different planes of the FPCT image were used: the ‘‘cochlear view’’ [20] (Fig. 1a) and a plane perpendicular to this cochlear view (Fig. 1b). The position of the perpendicular plane within the cochlear view is indicated by the dashed line and vice versa. The measurement of the diameter of the basal turn is illustrated by the arrow, starting at the center of the round window and ending at the lateral wall on the opposite as shown in earlier studies [6, 7]. The difference between both measurements ranged from 0 to 0.38 mm. The mean difference amounted to 0.22 mm and the standard deviation to 0.11 mm, which is
Eur Arch Otorhinolaryngol Fig. 1 ‘‘Cochlear view’’ of the FPCT image (a) and a plane perpendicular to the cochlear view (b) for measuring the diameter of the basal turn of the cochlear A1 and A2. The position of the perpendicular plane within the ‘‘cochlear view’’ is indicated by the dashed line and vice versa. The diameter of the basal turn refers to the mean value of A1 and A2
Fig. 2 Example for measuring the distance d using the ‘‘Cochlear view’’ of a FPCT image to determine the linear insertion depth LID = d ? 19,55 mm
Fig. 3 Example for the angle measurement of the CI422 using the ‘‘Cochlear view’’ of the FPCT image. The reference angle 0° is defined by the round window
within the voxel resolution of the images. Henceforth, the diameter of the basal turn refers to the mean value of measurements performed in these two planes. Linear insertion depth (LID) To determine the LID, the distance d between the round window and the first basal electrode contact was measured using the ‘‘cochlear view’’ of the FPCT image as before. The final LID is then the sum of the distance d and the distance between the most basal electrode and the apical end of the array amounting to 19.55 mm according to CochlearTM, see Fig. 2 for an example of this measurement.
indicates the position of the helicotrema, defining the center of the angle measurement. The high-resolution FPCT images allowed reliable measurement of the angular position of individual electrodes. Statistics To test if population means are equal, an independent t test was performed with Microcal Origin software (Northampton, Massachusetts, USA). For correlation analysis, a linear fit was performed, also using Microcal Origin software.
Insertion angle Results The insertion depth angle was measured using the ‘‘cochlear view’’ of the image as described above, see Fig. 3 for an example of the angle measurement. The reference angle 0° was defined at the center of the round window according to the consensus panel [21]. The dot
Insertion depth angles The measured insertion depth angle ranged from 306° to 579° (mean 444°; SD 63°).
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Fig. 4 Distribution of the measured LID. The 20-mm group refers to LID = 20 ± 0.5 mm corresponding to the marker20 finally positioned at the round window (RW). The ‘‘Intermediate Position’’ refers to LID = 23 ± 0.5 mm corresponding to the statistical mode of this data set and the RW finally positioned between marker20 and marker25. The 25-mm group refers to LID = 25 ± 0.5 mm corresponding to the marker25 finally positioned at the round window
Statistically different insertion depth angles have been identified (t35 = 2.93, p = 0.006) between male and female patients. For female the measured insertion angle ranged from 333° to 579° (mean 470°; SD 59°) and for male from 306° to 540° (mean 414°; SD 56°). Linear insertion depth (LID) vs. insertion depth angle The measured LID ranged from 18.6 to 26.2 mm (mean 23.2 mm; SD 1.9 mm). See the histogram displayed in Fig. 4 graphically representing the distribution of the measured LID. For two electrode arrays, the LID was found to be less than the minimum recommended LID of 20 mm, i.e. with the first basal electrode positioned outside the cochlea (LID = 18.6 mm for both of them). For both of them, difficulties with respect to complete insertion have been reported in the surgical report. Three electrode arrays featured LID larger than the maximal recommended LID of 25 mm (LID = 25.6 mm, LID = 25.83 mm and LID = 26.2 mm). Subjects whose insertion depths were identified within ±0.5 mm at 20, 23 and 25 mm were categorized as follows. Two subjects were identified in the group of data featuring 19.5 mm B LID \ 20.5 mm labeled as 20-mm group with the marker20 finally positioned at the round window. Likewise, six subjects were included in the group of data featuring 24.5 mm B LID \ 25.5 mm labeled as 25-mm group with the marker25 at the round window.
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Fig. 5 Measured insertion depth angle as a function of LID with highlighted data (filled symbols) associated with a measured diameter of basal turn A \ 9.1 mm to visualize eliminating the influence of the cochlea diameter on the final insertion depth angle
Eleven subjects were identified in the group featuring 22.5 B LID \ 23.5 mm, named as 23-mm group or intermediate position, representing the statistical mode for the dataset. Figure 5 shows the measured insertion depth angles as a function of the LID revealing a positive correlation. The filled symbols represent data with an associated measured diameter within a certain interval to control for the influence of cochlear diameter. Consequently, the correlation between linear and angular insertion depth is stronger for this specific arbitrarily chosen interval (8.1 mm \ A \ 9.1 mm) compared to the data with unlimited diameter (8.1 mm \ A \ 10.5 mm). Cochlea diameter vs. insertion depth angle The measured diameter of the basal turn ranged from 8.11 to 10.42 mm with a mean diameter of 9.39 mm (SD = 0.54 mm). The group of female patients yielded a smaller mean diameter amounting to 9.28 mm (SD = 0.59 mm) compared to the group of male patients yielding a mean diameter of 9.52 mm (SD = 0.46 mm). Figure 6 shows the insertion depth angle as a function of the diameter of the basal turn, revealing a negative correlation between insertion depth angle and cochlear diameter. A statistically significant negative correlation could be identified for the 25-mm group (R2 = 0.97, p \ 0.0001) with marker25 finally positioned at the round window (RW) and for the 23-mm group (R2 = 0.80, p \ 0.0001) showing the RW between marker20 and marker25 named as intermediate position.
Eur Arch Otorhinolaryngol
Fig. 6 Insertion depth angle as a function of the diameter of the basal turn showing a strong negative correlation for the 25-mm group with the marker25 finally positioned at the round window (RW) and the 23-mm group with the intermediate position showing the RW between marker20 and marker25
Discussion The current study collected data from 37 patients who had been implanted with the Nucleus CI422 to investigate the insertion depth angle. The measured insertion depth angles yielded a range from 306° to 579°. This huge variation confirmed the assumption that there must be factors that considerably influence the insertion angle. In this context, the current study presents data to answer three specific questions: firstly, how does the LID influence the final insertion depth angle? Secondly, what is the relation between cochlea diameter and final insertion depth angle? Thirdly, is it possible to develop a prediction tool for the intended insertion depth angle by means of LID and cochlear diameter measurements? The measured LID revealed a larger range than expected (18.6–26.2 mm) with 14 % (5/37) falling outside the recommended depths (\20 and [25 mm). However, an expected positive correlation between the LID and the final insertion angle was found. To eliminate the influence of the cochlea size a specific group of data featuring only similar cochlea diameter was considered and revealed a much stronger correlation compared to the data with unlimited diameter. The exponential shape of this correlation is consistent with the spiral shape of the cochlea since the arc radius decreases with increasing angular position. The cochlear diameters measured in this study (8.1–10.4 mm, n = 37) are in agreement with previous
work such as Escude’s study with a similar number of cochleae (7.9–10.8 mm, n = 42) [6]. Furthermore, the current study confirmed earlier publications showing that female subjects feature a lower mean diameter than male subjects [6]. With reference to the basal region of the cochlea, a larger variation (6.8–10.3 mm) was found by Martinez-Mondero and colleagues who analyzed 124 scans than identified in the 37 subjects of this study [22]. For the relation between insertion depth angle and cochlear diameter, a negative correlation was found, which is in agreement with Escude‘s study [6]. Consistent statistically significant higher insertion depth angles were found for female patients compared to male patients in the current study as well in earlier publications [5, 6]. Escude et al. presented data from 15 patients implanted with the Nucleus24 Contour Advance electrode array and found for the group of data specified with an LID of 17 mm a negative correlation (R2 = 0.51, p \ 0.05) between insertion depth angle and cochlear diameter. They did not present a negative correlation for the group of data specified with an LID of 19 mm. It should to be mentioned that the LID described in Escude‘s study was specified only by reports of whether the electrode array had been inserted up to the first or the third elastic rib, but was not quantified in terms of measuring the distance made possible through postoperative images; thus, the actual LID might differ from the specified LID. In contrast to Escude‘s study, the LID in the current study was specified in terms of absolute measurement. Indeed, a much stronger correlation was found for two groups specified with an LID of 25 mm (R2 = 0.97, p \ 0.0001) and 23 mm (R2 = 0.80, p \ 0.0001), respectively. The 25-mm group corresponds to the marker25 finally positioned at the round window (RW) and the 23-mm group corresponds to the intermediate position showing the RW between marker20 and marker25. Consequently cochlea size as well as LID is strongly correlated with the final insertion depth angle reasoning that they might be valuable predictors for an intended insertion depth angle. Relevance for patients with residual hearing A very recent study by Skarzynski et al. [12] found a significant correlation between threshold increase and preoperative hearing threshold where characteristic frequencies corresponded to an angular position occupied by the electrode array. The correlation between threshold increase and residual hearing considerably decreased in amplitude for characteristic frequencies apical to the electrode tip. Accordingly, a patient-specific insertion depth angle corresponding to individual residual hearing threshold frequency might be desired for patients with lowfrequency residual hearing, although potential progressive of hearing loss should be considered as well.
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An individual estimation of the insertion depth angle could be achieved by measuring the diameter of the basal turn preoperatively and taking into consideration different array positions as additional parameter. Table 1 provides a tool for mapping the diameter of the basal turn to the expected insertion depth angle and corresponding frequency for six different diameters of the cochlear and two different positions of the array, respectively. Marker25 finally positioned at the round window corresponds to the maximum recommended insertion depth of 25 mm. The intermediate position refers to an LID of 23 mm corresponding to the statistical mode of this collected data set. The mapping of cochlear diameter to insertion depth angle is based on the linear function by means of the regression analysis shown in Fig. 6. The mapping of angle to frequency is based on the tonotopy of the human cochlear displayed in Fig. 7. Additionally, angles listed in Table 1 are displayed in Fig. 7. Fig. 7 Corresponding frequency as a function of the angular position along the organ of corti (line), based on smoothed data of Bredberg [23] combined with calculations provided by Greenwood [24]. The symbols indicate the calculated insertion depth/corresponding frequency for different diameter of the basal turn (8, 8.5, 9, 9.5, 10, 10.5 mm) for marker25 finally positioned at the round window (RW) and the intermediate position representing the RW between marker20 and marker25 with corresponding LID of 25 and 23 mm, respectively (see Table 1)
The relation between corresponding frequency and angular position, shown in Fig. 7, can be derived from earlier studies. Greenwood [24] provided an analytical expression for the characteristic frequency as a function of percentage length. Bredberg [23] presented numerical data of percentage length vs. angle. When combining the smoothed data of Bredberg with the expression of Greenwood results, a relationship between frequency and angle can be derived. For more detailed information refer to [7, 25].
Conclusion In the current study the influence of cochlea diameter and linear insertion depth (LID) on the final insertion depth angle achieved at surgery was investigated to develop a prediction tool for the insertion depth angle by means of cochlea diameter and LID. The identified correlation between insertion depth angle and LID and the relation between insertion depth angle and cochlear diameter were used to provide a tool for estimation of the insertion depth angles by means of cochlea size and LID. This information may be of help to patients with low-frequency residual hearing for which a reduced intended insertion depth angle may decrease the risk of hearing loss during CI surgery. For these patients, in particular, preoperatively measured cochlea diameter and the option of different array positions by means of estimated LID might act as predictor for the final insertion depth angle.
Table 1 Matrix for estimation of insertion depth angle and corresponding frequency based on cochlear size and array position Diameter of the basal turn Intermediate position (LID = 23 mm) Marker25 position (LID = 25 mm)
10.5 mm
10 mm
9.5 mm
9 mm
8.5 mm
8 mm
370°
398°
426°
453°
481°
509°
820 Hz
710 Hz
615 Hz
535 Hz
470 Hz
415 Hz
407°
449°
492°
535°
578°
621°
580 Hz
550 Hz
450 Hz
370 Hz
300 Hz
250 Hz
The intermediate position refers to a linear insertion depth (LID) of 23 mm with the round window finally positioned between marker20 and marker25. The marker25 position refers to an LID of 25 mm with the marker25 finally positioned at the round window. This is based on the linear regression in Fig. 6 (relation between diameter and insertion depth angle) and the function plotted in Fig. 7 (relation between angular position and corresponding frequency)
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Eur Arch Otorhinolaryngol Acknowledgments Thanks to Dr. H. Hessel for constructive comments on the manuscript. 13. Conflict of interest
The authors declare no conflict of interest. 14.
References 1. Erixon E, Ho¨gstorp H, Wadin K, Rask-Anderson H (2008) Variational anatomy of human cochlea: implications for cochlear implantation. Otol Neurotol 30:14–22 2. Kawano A, Seldon HL, Clark GM (1996) Computer aided threedimensional construction in human cochlear maps: measurement of the lengths of organ of Corti outer wall, inner wall and Rosenthal‘s canal. Ann Otol Rhinol Laryngol 105:701–709 3. Hardy M (1938) The length of the organ of corti in man. Am J Anat 62:291–311 4. Ketten DR, Skinner MW, Wang GE, Vannier MW, Gates GA, Neely JG (1998) In vivo measures of cochlear length and insertion depth of nucleus cochlear implant electrode arrays. Ann Otol Rhinol Laryngol 175(suppl):1–16 5. Trieger A, Schulze A, Schneider A, Zahnert T, Mu¨rbe D (2010) In vivo measurements of the insertion depth of cochlear implant arrays using flat panel volume computed tomography. Otol Neurotol 32:152–157 6. Escude´ B, James C, Deguine O, Cochard N, Eter E, Fraysse B (2006) The size of the cochlea and predictions of insertion depth angles for cochlear implant electrodes. Audiol Neurotol 11:27–33 7. Franke-Trieger A, Jolly C, Darbinjan A, Zahnert T, Mu¨rbe D (2013) Insertion depth angles of cochlear implant arrays with varying length: a temporal bone study. Otol Neurotol 35(1):58–63 8. Skarzynski H, Lorens A, Piotrowska A (2003) A new method of partial deafness treatment. Med Sci Monit 9:CS20–CS24 9. Gstoettner W, Kiefer J, Baumgartner WD, Pok S, Peters S, Adunka O (2004) Hearing preservation in cochlear implantation for electric acoustic stimulation. Acta Otolaryngol 124:348–352 10. Skarzynski H, Lorens A, Matusiak M, Porowski M, Skarzynski PH, James CJ (2012) Partial deafness treatment with the Nucleus straight research array cochlear implant. Audiol Neurotol 17:82–91 11. James C, Albegger K, Battmer R et al (2005) Preservation of residual hearing with cochlear implantation: how and why. Acta Otolaryngol 125:481–491 12. Skarzynski H, Lorens A, Matusiak M, Porowski M, Skarzynski PH, James CJ (2014) Cochlear implantation with the nucleus slim
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straight electrode in subjects with residual low-frequency hearing. Ear Hear 35(2):e33–e43 Adunka O, Kiefer J (2006) Impact of electrode insertion depth on intracochlear trauma. Otolaryngol Head Neck Surg 135:374–382 Finley CC, Skinner MW (2008) Role of electrode placement as a contributor to variability in cochlear implant outcomes. Otol Neurotol 29:920–928 Yukawa K, Cohen L, Blamey P, Pyman B, Tungvachirakul V, O‘Leary S (2004) Effects of insertion depth of cochlear implant electrode upon speech perception. Audiol Neurootol 9:163–172 Skinner MW, Ketten DR, Holden LK et al (2002) CT-derived estimation of cochlear morphology and electrode array position in relation to word recognition in Nucleus-22 recipients. J Assoc Res Otolaryngol 3:332–350 Holden LK, Finley CC, Firszt JB et al (2013) Factors affecting open-set word recognition in adults with cochlear implants. Ear and hear 34(3):342 Khan AM, Handzel O, Burgess BJ, Damian D, Eddington DK, Nadol JB (2005) Is word recognition correlated with the number of surviving spiral ganglion cells and electrode insertion depth in human subjects with cochlear implants? Laryngoscope 115:672–677 Lee J, Nadol JB Jr, Eddington DK (2010) Depth of electrode insertion and postoperative performance in humans with cochlear implants: a histopathologic study. Audiol Neurotol 15:323–331 Xu J, Xu SA, Cohen LT, Clark GM (2000) Cochlear view: postoperative radiography for cochlear implantation. AM J Otol 21:49–56 Verbist BM, Skinner MW, Cohen LT et al (2010) Consensus panel on a cochlear coordinate system applicable in histologic, physiologic, and radiologic studies of the human cochlea. Otol Neurotol 31:722–730 Martinez-Monedero R, Niparko JK, Aygun N (2011) Cochlear coiling differences in cochlear implant candidates. Otol Neurotol 32:1086–1093 Bredberg G (1968) Cellular pattern and nerve supply of the human organ of Corti. Acta Otolaryngol (Stockh) 236(suppl): 1–138 Greenwood DD (1990) A cochlear frequency-position function for several species—29 years later. J Acoust Soc Am 87:2592–2605 Cohen LT, Xu J, Xu SA, Clark GM (1996) Improved and simplified methods for specifying positions of the electrode bands of a cochlear implant array. Am J Otol 17:859–865
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OTOLOGY
Estimation of insertion depth angle based on cochlea diameter and linear insertion depth: a prediction tool for the CI422 Annett Franke-Trieger • Dirk Mu¨rbe
Received: 26 September 2014 / Accepted: 20 October 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Beside the cochlear size, the linear insertion depth (LID) influences the insertion depth angle of cochlear implant electrode arrays. For the specific implant CI422 the recommended LID is not fixed but can vary continuously between 20 and 25 mm. In the current study, the influence of cochlea size and LID on the final insertion depth angle was investigated to develop a prediction tool for the insertion depth angle by means of cochlea diameter and LID. Preoperative estimation of insertion depth angles might help surgeons avoid exceeding an intended insertion depth, especially with respect to low-frequency residual hearing preservation. Postoperative, high-resolution 3Dradiographs provided by Flat Panel Computed Volume Tomography (FPCT) were used to investigate the insertion depth angle in 37 CI422 recipients. Furthermore, the FPCT images were used to measure linear insertion depth and diameter of the basal turn of the cochlea. A considerable variation of measured insertion depth angles ranging from 306° to 579° was identified. The measured linear insertion depth ranged from -18.6 to 26.2 mm and correlated positively with the insertion depth angle. The cochlea diameter ranged from 8.11 to 10.42 mm and correlated negatively with the insertion depth angle. The results suggest that preoperatively measured cochlea diameter combined with the option of different array positions by means of LID may act as predictors for the final insertion depth angle. Keywords Cochlea implant array Insertion depth angle Linear insertion depth Cochlear size
A. Franke-Trieger (&) D. Mu¨rbe Department of Otorhinolaryngology, Saxonian Cochlear Implant Center, Technische Universita¨t of Dresden, Fetscherstr. 74, 01307 Dresden, Germany e-mail: [email protected]
Introduction The current study is a postoperative approach to investigate the insertion depth angle and its relation to cochlear size and linear insertion depth (LID) for the Cochlear Nucleus CI422. For the CI422, the recommended intended LID is not fixed but can vary continuously within 20 and 25 mm defined by two white markers. Consequently, the LID can influence the final insertion depth angle remarkably. Furthermore, it is known that the size of human cochleae varies considerably, which has been shown in several studies [1–4]. The assumption that the size of the cochlea does influence the final insertion depth angle was confirmed by studies investigating electrode position and cochlea size for several array types [5–7]. In this context, the focus of the current study evolved to answer three specific questions: firstly, how does the LID influence the final insertion depth angle? Secondly, what is the relation between cochlea diameter and final insertion depth angle? Thirdly, is it possible to develop a prediction tool for the intended insertion depth angle by means of LID and cochlear diameter measurements? Patients might benefit from the possibility of predicting the insertion depth angle with respect to residual hearing preservation and intracochlear trauma. Several studies investigating different array types have shown that the insertion depth influences the maintenance of residual hearing after CI surgery, such as successful conservation of hearing after cochlear implantation using a limited insertion depth [8–10] and poorer preservation of residual hearing for angles greater than 400° compared to lower angles [11]. For the specific implant CI422, Skarzynski et al. [12] showed very recently that low-frequency residual hearing was well preserved. Furthermore, they found a significant correlation between threshold increase
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and preoperative hearing threshold where characteristic frequencies corresponded to an angular position occupied by the electrode array. For characteristic frequencies apical to the electrode tip, the correlation between threshold increase and residual hearing decreased in amplitude considerably. Consequently, they concluded that the selection of optimal insertion depth should be patient specific by means of corresponding threshold frequency [12]. In this context, the development of a prediction tool for the intended insertion depth angle would be valuable. It has also been shown that a limited insertion depth can help avoid intracochlear trauma or interscalar dislocations, which have been shown to occur more frequently with deep insertions. Adunka and Kiefer [13] found increased intracochlear trauma with deep insertions and Finley and Skinner [14] found that increasing the insertion depth is related to a greater number of electrodes being located in the scala vestibuli. Studies investigating the influence of the insertion depth on the quality of speech perception are inconsistent in the literature. Yukawa et al. and Skinner et al. [15, 16] found a positive correlation between insertion depth and speech perception. In contrast Finley and Skinner as well as and Holden et al. [14, 17] found that increasing the insertion depth is significantly correlated with reduced speech perception, whereas other studies did not find a significant correlation between insertion depth and speech reception [18, 19]. Earlier studies investigated the correlation between insertion depth and cochlea size of other array types. Franke-Trieger et al. [7] found a negative correlation between insertion depth angle and cochlea diameter for different MED-EL electrode arrays by means of temporal bone experiments. Escude et al. [6] investigated the insertion depth for the Nucleus 24 (CA) recipients using a LID of either 17 mm (n = 9) or 19 mm (n = 7); a statistically significant correlation between insertion depth and diameter of the basal turn (R2 = 0.51, p \ 0.05) was found for the 17-mm group. Firstly, there remained a considerable dispersion about the regression line and secondly no correlation was found for the 19-mm group, indicating that there must be other factors that influence the insertion depth angle. The specification of the LID 17 or 19 mm was based on reports stating whether the electrode array has been inserted up to the first or the third elastic rib. Escude et al. did not measure the actual final linear insertion depth, thus this specification could deviate from the actual final linear insertion depth. The current study is first comprehensive characterization of the specific implant CI422 with respect to linear and angular insertion depth as well as cochlear size.
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Materials and methods Materials Subjects in this retrospective study included 20 female and 17 male, aged 15–82 years with a mean age of 51 years. All 37 subjects suffered from profound or severe sensorineural hearing loss and were implanted with a Cochlear CI422 with Slim Half-Band Straight Electrode (Cochlear Corp., Sydney, Australia). The electrode array of the CI422 incorporates two white markers. The white marker at 20 mm, henceforth labeled as marker20, at the end of the active portion of the array indicates the minimum insertion depth. The white marker at 25 mm, henceforth labeled as marker25, indicates the maximum insertion depth. After exposing the round window membrane, the round window insertion technique was applied to all subjects. Difficulties during the insertion procedure that led to incomplete insertion were reported in two subjects. In all other subjects, neither surgical problems nor incomplete insertion were reported. Approval by the ethics committee of the university hospital was obtained for this study. Technical data of FPCT imaging FPCT examinations were carried out at the first day after implantation using a Flat Panel Computer Tomograph 3D Accuitomo 80 (J. Morita MFG. CORP., Kyoto, Japan). Imaging was performed with a tube current of 8 mA and a tube voltage of 90 kV. The raw data projection images were reconstructed using the software i-dixl (J. Morita MFG. CORP., Kyoto, Japan) resulting in a voxel size of 125 lm. Measurements Diameter of the basal turn To quantify the cochlear size, the diameter of the basal turn of the cochlear was measured; see Fig. 1 for an example of this measurement. To minimize the measuring inaccuracy two different planes of the FPCT image were used: the ‘‘cochlear view’’ [20] (Fig. 1a) and a plane perpendicular to this cochlear view (Fig. 1b). The position of the perpendicular plane within the cochlear view is indicated by the dashed line and vice versa. The measurement of the diameter of the basal turn is illustrated by the arrow, starting at the center of the round window and ending at the lateral wall on the opposite as shown in earlier studies [6, 7]. The difference between both measurements ranged from 0 to 0.38 mm. The mean difference amounted to 0.22 mm and the standard deviation to 0.11 mm, which is
Eur Arch Otorhinolaryngol Fig. 1 ‘‘Cochlear view’’ of the FPCT image (a) and a plane perpendicular to the cochlear view (b) for measuring the diameter of the basal turn of the cochlear A1 and A2. The position of the perpendicular plane within the ‘‘cochlear view’’ is indicated by the dashed line and vice versa. The diameter of the basal turn refers to the mean value of A1 and A2
Fig. 2 Example for measuring the distance d using the ‘‘Cochlear view’’ of a FPCT image to determine the linear insertion depth LID = d ? 19,55 mm
Fig. 3 Example for the angle measurement of the CI422 using the ‘‘Cochlear view’’ of the FPCT image. The reference angle 0° is defined by the round window
within the voxel resolution of the images. Henceforth, the diameter of the basal turn refers to the mean value of measurements performed in these two planes. Linear insertion depth (LID) To determine the LID, the distance d between the round window and the first basal electrode contact was measured using the ‘‘cochlear view’’ of the FPCT image as before. The final LID is then the sum of the distance d and the distance between the most basal electrode and the apical end of the array amounting to 19.55 mm according to CochlearTM, see Fig. 2 for an example of this measurement.
indicates the position of the helicotrema, defining the center of the angle measurement. The high-resolution FPCT images allowed reliable measurement of the angular position of individual electrodes. Statistics To test if population means are equal, an independent t test was performed with Microcal Origin software (Northampton, Massachusetts, USA). For correlation analysis, a linear fit was performed, also using Microcal Origin software.
Insertion angle Results The insertion depth angle was measured using the ‘‘cochlear view’’ of the image as described above, see Fig. 3 for an example of the angle measurement. The reference angle 0° was defined at the center of the round window according to the consensus panel [21]. The dot
Insertion depth angles The measured insertion depth angle ranged from 306° to 579° (mean 444°; SD 63°).
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Fig. 4 Distribution of the measured LID. The 20-mm group refers to LID = 20 ± 0.5 mm corresponding to the marker20 finally positioned at the round window (RW). The ‘‘Intermediate Position’’ refers to LID = 23 ± 0.5 mm corresponding to the statistical mode of this data set and the RW finally positioned between marker20 and marker25. The 25-mm group refers to LID = 25 ± 0.5 mm corresponding to the marker25 finally positioned at the round window
Statistically different insertion depth angles have been identified (t35 = 2.93, p = 0.006) between male and female patients. For female the measured insertion angle ranged from 333° to 579° (mean 470°; SD 59°) and for male from 306° to 540° (mean 414°; SD 56°). Linear insertion depth (LID) vs. insertion depth angle The measured LID ranged from 18.6 to 26.2 mm (mean 23.2 mm; SD 1.9 mm). See the histogram displayed in Fig. 4 graphically representing the distribution of the measured LID. For two electrode arrays, the LID was found to be less than the minimum recommended LID of 20 mm, i.e. with the first basal electrode positioned outside the cochlea (LID = 18.6 mm for both of them). For both of them, difficulties with respect to complete insertion have been reported in the surgical report. Three electrode arrays featured LID larger than the maximal recommended LID of 25 mm (LID = 25.6 mm, LID = 25.83 mm and LID = 26.2 mm). Subjects whose insertion depths were identified within ±0.5 mm at 20, 23 and 25 mm were categorized as follows. Two subjects were identified in the group of data featuring 19.5 mm B LID \ 20.5 mm labeled as 20-mm group with the marker20 finally positioned at the round window. Likewise, six subjects were included in the group of data featuring 24.5 mm B LID \ 25.5 mm labeled as 25-mm group with the marker25 at the round window.
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Fig. 5 Measured insertion depth angle as a function of LID with highlighted data (filled symbols) associated with a measured diameter of basal turn A \ 9.1 mm to visualize eliminating the influence of the cochlea diameter on the final insertion depth angle
Eleven subjects were identified in the group featuring 22.5 B LID \ 23.5 mm, named as 23-mm group or intermediate position, representing the statistical mode for the dataset. Figure 5 shows the measured insertion depth angles as a function of the LID revealing a positive correlation. The filled symbols represent data with an associated measured diameter within a certain interval to control for the influence of cochlear diameter. Consequently, the correlation between linear and angular insertion depth is stronger for this specific arbitrarily chosen interval (8.1 mm \ A \ 9.1 mm) compared to the data with unlimited diameter (8.1 mm \ A \ 10.5 mm). Cochlea diameter vs. insertion depth angle The measured diameter of the basal turn ranged from 8.11 to 10.42 mm with a mean diameter of 9.39 mm (SD = 0.54 mm). The group of female patients yielded a smaller mean diameter amounting to 9.28 mm (SD = 0.59 mm) compared to the group of male patients yielding a mean diameter of 9.52 mm (SD = 0.46 mm). Figure 6 shows the insertion depth angle as a function of the diameter of the basal turn, revealing a negative correlation between insertion depth angle and cochlear diameter. A statistically significant negative correlation could be identified for the 25-mm group (R2 = 0.97, p \ 0.0001) with marker25 finally positioned at the round window (RW) and for the 23-mm group (R2 = 0.80, p \ 0.0001) showing the RW between marker20 and marker25 named as intermediate position.
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Fig. 6 Insertion depth angle as a function of the diameter of the basal turn showing a strong negative correlation for the 25-mm group with the marker25 finally positioned at the round window (RW) and the 23-mm group with the intermediate position showing the RW between marker20 and marker25
Discussion The current study collected data from 37 patients who had been implanted with the Nucleus CI422 to investigate the insertion depth angle. The measured insertion depth angles yielded a range from 306° to 579°. This huge variation confirmed the assumption that there must be factors that considerably influence the insertion angle. In this context, the current study presents data to answer three specific questions: firstly, how does the LID influence the final insertion depth angle? Secondly, what is the relation between cochlea diameter and final insertion depth angle? Thirdly, is it possible to develop a prediction tool for the intended insertion depth angle by means of LID and cochlear diameter measurements? The measured LID revealed a larger range than expected (18.6–26.2 mm) with 14 % (5/37) falling outside the recommended depths (\20 and [25 mm). However, an expected positive correlation between the LID and the final insertion angle was found. To eliminate the influence of the cochlea size a specific group of data featuring only similar cochlea diameter was considered and revealed a much stronger correlation compared to the data with unlimited diameter. The exponential shape of this correlation is consistent with the spiral shape of the cochlea since the arc radius decreases with increasing angular position. The cochlear diameters measured in this study (8.1–10.4 mm, n = 37) are in agreement with previous
work such as Escude’s study with a similar number of cochleae (7.9–10.8 mm, n = 42) [6]. Furthermore, the current study confirmed earlier publications showing that female subjects feature a lower mean diameter than male subjects [6]. With reference to the basal region of the cochlea, a larger variation (6.8–10.3 mm) was found by Martinez-Mondero and colleagues who analyzed 124 scans than identified in the 37 subjects of this study [22]. For the relation between insertion depth angle and cochlear diameter, a negative correlation was found, which is in agreement with Escude‘s study [6]. Consistent statistically significant higher insertion depth angles were found for female patients compared to male patients in the current study as well in earlier publications [5, 6]. Escude et al. presented data from 15 patients implanted with the Nucleus24 Contour Advance electrode array and found for the group of data specified with an LID of 17 mm a negative correlation (R2 = 0.51, p \ 0.05) between insertion depth angle and cochlear diameter. They did not present a negative correlation for the group of data specified with an LID of 19 mm. It should to be mentioned that the LID described in Escude‘s study was specified only by reports of whether the electrode array had been inserted up to the first or the third elastic rib, but was not quantified in terms of measuring the distance made possible through postoperative images; thus, the actual LID might differ from the specified LID. In contrast to Escude‘s study, the LID in the current study was specified in terms of absolute measurement. Indeed, a much stronger correlation was found for two groups specified with an LID of 25 mm (R2 = 0.97, p \ 0.0001) and 23 mm (R2 = 0.80, p \ 0.0001), respectively. The 25-mm group corresponds to the marker25 finally positioned at the round window (RW) and the 23-mm group corresponds to the intermediate position showing the RW between marker20 and marker25. Consequently cochlea size as well as LID is strongly correlated with the final insertion depth angle reasoning that they might be valuable predictors for an intended insertion depth angle. Relevance for patients with residual hearing A very recent study by Skarzynski et al. [12] found a significant correlation between threshold increase and preoperative hearing threshold where characteristic frequencies corresponded to an angular position occupied by the electrode array. The correlation between threshold increase and residual hearing considerably decreased in amplitude for characteristic frequencies apical to the electrode tip. Accordingly, a patient-specific insertion depth angle corresponding to individual residual hearing threshold frequency might be desired for patients with lowfrequency residual hearing, although potential progressive of hearing loss should be considered as well.
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An individual estimation of the insertion depth angle could be achieved by measuring the diameter of the basal turn preoperatively and taking into consideration different array positions as additional parameter. Table 1 provides a tool for mapping the diameter of the basal turn to the expected insertion depth angle and corresponding frequency for six different diameters of the cochlear and two different positions of the array, respectively. Marker25 finally positioned at the round window corresponds to the maximum recommended insertion depth of 25 mm. The intermediate position refers to an LID of 23 mm corresponding to the statistical mode of this collected data set. The mapping of cochlear diameter to insertion depth angle is based on the linear function by means of the regression analysis shown in Fig. 6. The mapping of angle to frequency is based on the tonotopy of the human cochlear displayed in Fig. 7. Additionally, angles listed in Table 1 are displayed in Fig. 7. Fig. 7 Corresponding frequency as a function of the angular position along the organ of corti (line), based on smoothed data of Bredberg [23] combined with calculations provided by Greenwood [24]. The symbols indicate the calculated insertion depth/corresponding frequency for different diameter of the basal turn (8, 8.5, 9, 9.5, 10, 10.5 mm) for marker25 finally positioned at the round window (RW) and the intermediate position representing the RW between marker20 and marker25 with corresponding LID of 25 and 23 mm, respectively (see Table 1)
The relation between corresponding frequency and angular position, shown in Fig. 7, can be derived from earlier studies. Greenwood [24] provided an analytical expression for the characteristic frequency as a function of percentage length. Bredberg [23] presented numerical data of percentage length vs. angle. When combining the smoothed data of Bredberg with the expression of Greenwood results, a relationship between frequency and angle can be derived. For more detailed information refer to [7, 25].
Conclusion In the current study the influence of cochlea diameter and linear insertion depth (LID) on the final insertion depth angle achieved at surgery was investigated to develop a prediction tool for the insertion depth angle by means of cochlea diameter and LID. The identified correlation between insertion depth angle and LID and the relation between insertion depth angle and cochlear diameter were used to provide a tool for estimation of the insertion depth angles by means of cochlea size and LID. This information may be of help to patients with low-frequency residual hearing for which a reduced intended insertion depth angle may decrease the risk of hearing loss during CI surgery. For these patients, in particular, preoperatively measured cochlea diameter and the option of different array positions by means of estimated LID might act as predictor for the final insertion depth angle.
Table 1 Matrix for estimation of insertion depth angle and corresponding frequency based on cochlear size and array position Diameter of the basal turn Intermediate position (LID = 23 mm) Marker25 position (LID = 25 mm)
10.5 mm
10 mm
9.5 mm
9 mm
8.5 mm
8 mm
370°
398°
426°
453°
481°
509°
820 Hz
710 Hz
615 Hz
535 Hz
470 Hz
415 Hz
407°
449°
492°
535°
578°
621°
580 Hz
550 Hz
450 Hz
370 Hz
300 Hz
250 Hz
The intermediate position refers to a linear insertion depth (LID) of 23 mm with the round window finally positioned between marker20 and marker25. The marker25 position refers to an LID of 25 mm with the marker25 finally positioned at the round window. This is based on the linear regression in Fig. 6 (relation between diameter and insertion depth angle) and the function plotted in Fig. 7 (relation between angular position and corresponding frequency)
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Eur Arch Otorhinolaryngol Acknowledgments Thanks to Dr. H. Hessel for constructive comments on the manuscript. 13. Conflict of interest
The authors declare no conflict of interest. 14.
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