Scala Tympani Cochleostomy II: Topography and Histology

Transcription

1 The Laryngoscope Lippincott Williams & Wilkins 2007 The American Laryngological, Rhinological and Otological Society, Inc. Scala Tympani Cochleostomy II: Topography and Histology Oliver F. Adunka, MD; Andreas Radeloff, MD; Wolfgang K. Gstoettner, MD; Harold C. Pillsbury, MD; Craig A. Buchman, MD Objective: To assess intracochlear trauma using two different round window-related cochleostomy techniques in human temporal bones. Methods: Twenty-eight human temporal bones were included in this study. In 21 specimens, cochleostomies were initiated inferior to the round window (RW) annulus. In seven bones, cochleostomies were drilled anterior-inferior to the RW annulus. Limited cochlear implant electrode insertions were performed in 19 bones. In each specimen, promontory anatomy and cochleostomy drilling were photographically documented. Basal cochlear damage was assessed histologically and electrode insertion properties were documented in implanted bones. Results: All implanted specimens showed clear scala tympani electrode placements regardless of cochleostomy technique. All 21 inferior cochleostomies were atraumatic. Anterior-inferior cochleostomies resulted in various degrees of intracochlear trauma in all seven bones. Conclusion: For atraumatic opening of the scala tympani using a cochleostomy approach, initiation of drilling should proceed from inferior to the round window annulus, with gradual progression toward the undersurface of the lumen. While cochleostomies initiated anterior-inferior to the round window annulus resulted in scala tympani opening, many of these bones displayed varying degrees of intracochlear trauma that may result in hearing loss. When intracochlear drilling is avoided, the anterior bony margin of the cochleostomy remains a significant intracochlear impediment to in-line electrode insertion. From the Department of Otolaryngology Head and Neck Surgery (O.F.A., H.C.P., C.A.B.), University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, U.S.A., and the Department of Otolaryngology (A.R.), Bavarian Julius Maximilians University, Wuerzburg, Germany, and the Department of Otolaryngology (A.R., W.K.G.), J.W. Goethe University Frankfurt am Main, Germany. Editor s Note: This Manuscript was accepted for publication June 19, O.F.A. and A.R. contributed equally to this manuscript. Send correspondence to Oliver F. Adunka, MD, Department of Otolaryngology Head and Neck Surgery, University of North Carolina at Chapel Hill, G0412 Neuroscience Hospital, CB # 7070, Chapel Hill, NC , U.S.A. adunka@med.unc.edu DOI: /MLG.0b013e a53 Key Words: Cochlear implant; electric-acoustic stimulation; hearing preservation; neural preservation; cochleostomy. Laryngoscope, 117: , 2007 INTRODUCTION The cochleostomy for cochlear implantation was first described in the 1980s. 1 It was initially developed to provide improved insertion trajectories. 2 By using a cochleostomy, it was feasible to access the basal cochlear turn in a straight fashion for its first 8 to 10 mm. This facilitated insertions of the relatively stiff electrode arrays of this era. 1 Scala tympani (ST) is currently considered the preferred intracochlear compartment for multi-electrode array implantations. The rationale for this is based on the fact that ST insertions provide a closer and more direct access to the excitable elements (i.e., spiral ganglion cells and dendrites) than scala vestibuli (SV). ST is also reinforced superiorly by the basilar membrane and osseous spiral lamina, thereby making the cochlear duct more resistant to electrode-related trauma in this region. 3 While a number of series have reported on the safety and efficacy of both ST and SV insertions in a variety of pathologic conditions, 4 very few systematic comparisons between ST and SV insertions in patients with otherwise normal inner ears have been made. Recently, Aschendorff et al. and Skinner et al. have both shown that patients with electrode arrays located exclusively within ST perform better on speech perception tests than those patients with electrode arrays located within both ST and SV or SV alone. 5,6 With this new information, precise electrode location within ST is now paramount. While a separate cochleostomy approach is used by the vast majority of cochlear implant surgeons for ST access, the exact topographic or surgical landmarks and trajectories for drilling this opening remain poorly defined. With the advent of combined electric acoustic stimulation (EAS) 7,8 and the need for hearing preservation, maintaining normal intracochlear anatomy will likely take on even greater meaning. The aim of this study was to systematically assess two cochleostomy approaches in a human temporal bone 2195

2 model. Given the broad range of morphologic configurations of the basal cochlear turn, 9,10 it has previously been suggested to align the cochleostomy based on the round window (RW) membrane (RWM). 10 Specifically, we hypothesized that cochleostomies arising from inferior to the RWM might provide a more favorable route for atraumatic electrode insertions, 10 thereby avoiding the spiral ligament, basilar membrane, and osseous spiral lamina. This would be in contrast to the more common teaching to place the cochleostomy directly anterior and inferior to the margin of the RWM. 10 This study evaluated both inferior and anterior-inferior cochleostomies for their suitability to provide atraumatic intracochlear implantations. Specifically, in each specimen, transfacial recess promontory anatomy, the cochleostomy drilling procedure, and intracochlear electrode properties were documented photographically and histologically assessed. MATERIALS AND METHODS This study was performed in accordance with the local institutional review board (IRB). Twenty-eight fresh-frozen defrosted human temporal bones were used for this study. Bones were either implanted with regular cochlear implant electrodes (n 19) to evaluate intracochlear trauma associated with both drilling and insertion, or not implanted (n 9) to evaluate drilling trauma only. Surgical Approach In each bone, a typical cochlear implant approach was drilled including mastoidectomy and large facial recess. When drilling the latter, special care was taken to maximize the exposure of the RW niche. The RW niche overhang was then drilled until the entire anterior and inferior margin of the RWM was visible. Drilling of the cochleostomy was carried out using either an inferior approach (21 specimens) or an anterior-inferior approach (7 specimens). During cochleostomy drilling, soft surgical principles were applied in all specimens. Thus, the scalar endosteum was identified and opened separately with a micropick rather than using the rotating drill burr. Figure 1 illustrates the sequence of drilling and the respective histology for both inferior and anterior-inferior approach cochleostomies. Inferior approach cochleostomies (n 21). Drilling was commenced directly inferior to the annulus of the RWM 10 in the direction along the presumed floor of the basal turn. The cochlear endosteum was encountered and the exposed area was enlarged to about 1 mm 2. In most cases, the final cochleostomy opening was located slightly anterior to the initial starting point (Fig. 1). Also, cochleostomy openings generally included the inferior section of the round window membrane. Electrode arrays were implanted in 15 bones of this group (15 of 21, 71.4%, 7 MED-EL C40, Innsbruck, Austria; 3 Advanced Bionics Slim Helix electrodes, Sylmar, CA, USA; 2 Advanced Bionics HiFocus Slim Lateral arrays, 2 Cochlear Corporation Contour Softip electrodes, Melbourne, Australia; and 1 Cochlear Corporation electrode used for the Hybrid clinical trial, 8 see Table I). Anterior-inferior approach cochleostomies. Drilling in this group was initiated in the anterior-inferior quadrant of the RWM and continued anterior and slightly inferior to the RW annulus (n 7). Similarly, the cochlear endosteum was exposed at an area of about 1 mm 2 and a small portion of the round window membrane was included in the opening. Four specimens in this group underwent implantation (4 of 7, 57.1%) with MED-EL C40 electrodes. With both cochleostomies, electrode insertions were immediately stopped once resistance was felt during the insertion 2196 Fig. 1. Illustration of the cochleostomy sites used for this study in a right ear. The round window overhang has not been drilled, and the edge of the overhang is illustrated as well as the inferior part of the round window annulus. Dot 2 illustrates the typical site for an inferior cochleostomy. Dot 1 outlines the typical starting site of a cochleostomy anterior-inferior to the round window annulus. The light area circumjacent to dot 2 illustrates the more broadbased bone removal observed with inferior cochleostomies in contrast to the more narrow bony drill off procedures seen with anterior-inferior approaches. procedure. This was done to avoid basal buckling of the electrode array and thus to minimize intracochlear trauma not directly related to the cochleostomy site. Photographic documentation of the promontory region was performed on completion of the cochleostomy after drilling of the round window overhang and after the cochleostomy was fully opened. Figure 1 illustrates both cochleostomy locations and Figures 2 and 3 show examples of inferior and anterior-inferior cochleostomies, respectively. Histologic Processing After implantation, each electrode was fixed onto the remaining temporal bone using nonabsorbable sutures. All 28 specimens underwent histologic evaluation employing a resin embedding technique, which allows sectioning of undecalcified bone with the implanted electrode in situ. 11 For fixation, specimens were brought into 4% buffered formalin solution. Dehydration was performed with an ascending series of ethanol (70% 100%). Specimens were then resin (poly-methyl-methacrylate) embedded and relayed to further processing. All implanted specimens (n 19) underwent fluoroscopy to evaluate the position and orientation of the implanted electrode carrier within the resin block. Also, the future sectioning plane was determined to visualize the site of the cochleostomy. The location of the cochleostomy was radiographically estimated by following the longitudinal axis of the superior semicircular canal inferiorly where it met with the electrode array. 12 Finally, all implanted bones underwent serial sectioning using a special sawing, grinding-polishing technique. A description of the histologic processing was detailed by Plenk. 11 Basal cochlear trauma was

3 TABLE I. Data on All 28 Specimens. Implantation Morphologic Data Basal Cochlear Trauma N Side Electrode Type Insertion Depth* Promontory* Cochleostomy* Spiral Ligament OSL Implanted specimens 1 Right Slim helix Full Large Inferior Intact Intact 2 Right Slim helix Full Large Inferior Intact Intact 3 Right Slim helix Full Medium Inferior Intact Intact 4 Right C40 standard 20 mm Small Anteroinferior Rupture Intact 5 Right C40 standard 14 mm Small Anteroinferior Rupture Intact 6 Right C40 standard 18 mm Medium Anteroinferior Rupture Fracture 7 Right C40 standard 20 mm Medium Anteroinferior Rupture Intact 8 Right C40 standard 20 mm Large Inferior Intact Intact 9 Right C40 standard 15 mm Small Inferior Intact Intact 10 Left C40 standard 15 mm Small Inferior Intact Intact 11 Left C40 standard 20 mm Medium Inferior Intact Intact 12 Left C40 standard 18 mm Medium Inferior Intact Intact 13 Left C40 standard 20 mm Large Inferior Intact Intact 14 Left C40 standard 23 mm Medium Inferior Intact Intact 15 Right Contour softip Full Small Inferior Intact Intact 16 Left Contour softip Full Medium Inferior Intact Intact 17 Right Nucleus hybrid electrode Full Medium Inferior Intact Intact 18 Left Hifocus Slim Lateral Full Large Inferior Intact Intact 19 Right Hifocus Slim Lateral Full Large Inferior Intact Intact Drilled, nonimplanted specimens 20 Left No electrode NA Small Anteroinferior Rupture Intact 21 Right No electrode NA Medium Anteroinferior Rupture Fracture 22 Right No electrode NA Medium Anteroinferior Rupture Intact 23 Right No electrode NA Large Inferior Intact Intact 24 Left No electrode NA Small Inferior Intact Intact 25 Right No electrode NA Medium Inferior Intact Intact 26 Left No electrode NA Large Inferior Intact Intact 27 Right No electrode NA Medium Inferior Intact Intact 28 Left No electrode NA Small Inferior Intact Intact Data on all 28 specimens: all bones were drilled using a standard cochlear implant approach, and 19 temporal bones were implanted with typical cochlear implant electrode arrays, whereas 9 specimens underwent the drilling procedures only. The round window was pushed superiorly in some bones to improve the insertion angle in inferior approaches. *Insertion depth describes the estimated length of electrode insertion during the implantation process; Promontory: relative size of each promontory as seen through the facial recess compared across all specimens; Cochleostomy: either inferior or anterior-inferior to the round window. OSL osseous spiral lamina; NA not available. evaluated by assessing electrode location and integrity of intracochlear structures. All nonimplanted specimens (n 9) were radiographically evaluated by means of high-resolution computed tomography (HRCT) to approximate the relative orientation and location of the cochlea within the block. In contrast to the implanted specimens, bones of this group were evaluated by applying a grinding and polishing technique without sectioning. This was necessary since the main landmark for sectioning, usually provided by the electrode carrier, was not available. After polishing, each plane was stained (Giemsa), electronically scanned, and digitized. Images were evaluated using Adobe PhotoShop (Adobe Systems Inc., San Jose, CA). Grinding of the block was continued until the basal cochlear turn was completely imaged. In contrast to the sectioning method applied to implanted bones, this grinding-only technique allowed for a thinner slide thickness and thus improved three-dimensional visualization of the cochleostomy site. Figure 4 illustrates an example of a series of images obtained from a single cochleostomy. RESULTS Both histologic methods allowed for adequate visualization of the cochleostomy region in all 28 bones. Topographical surgical images taken during the implantation process and the corresponding histology of the cochleostomy region provided a direct comparison of the surgical technique with histologic properties. In all cases, histologic evaluation documented a ST cochleostomy. There 2197

4 Fig. 2. Specimen 8, inferior cochleostomy. Surgical image documentation and representative histologic image. (A) view of the promontory through the facial recess; the inferior part of the round window annulus is visible. Overhang has not been removed. (B) View of the exposed round window after removal of the anterior portion of the round window overhang (higher magnification than A). The dashed circle depicts the location of the round window membrane. (C) Cochleostomy drilling inferior to the round window annulus. The red circle depicts the location of the cochleostomy inferior to the round window membrane. (D) Histologic image illustrating that the electrode enters the cochlea inferior to the spiral ligament. The round window membrane is pushed toward the osseous spiral lamina (remnant shown above the electrode in the image). No basal cochlear trauma. were no openings into scala media or scala vestibuli in this series. Inferior Cochleostomies (n 21) In all cases, a broad-based bony drill-off procedure was necessary to perform this approach since the infrascalar bone in this region was often quite thick and variable. Review of the topographical images taken during surgical implantation revealed cochleostomy sites starting inferior to the round window annulus. As mentioned previously, most cochleostomies showed a tendency to sway slightly anterior of the initial starting point, although the inferior extent clearly predominated. Corresponding histology revealed completely atraumatic cochleostomy drilling in both nonimplanted (n 6) and implanted (n 15) specimens. In the implanted specimen, histology revealed that the electrode entered the basal ST inferior to the spiral ligament. The spiral ligament was clearly avoided by the cochleostomy. Also, the osseous spiral lamina remained intact and nonfractured in all 21 inferior cochleostomy specimens. Anterior-Inferior Cochleostomies (n 7) In contrast to the inferior cochleostomy group, which required a broad-based bony drill-off procedure, anteriorinferior cochleostomies were narrower and more cylindrical in shape, likely because the bone overlying the lateral scalar lumen was much thinner and more predictable in this region. Akin to inferior approaches, review of the topographical surgical images in this group revealed a tendency for a more anterior location of the cochleostomy than initially anticipated. The final locations were more anterior than the inferior approach cochleostomies. This is illustrated in Figure 1. Histologic images of implanted as well as nonimplanted bones revealed avulsion of the inferior part of the spiral ligament from the lateral wall of the scala tympani in all seven specimens. Also, two bones (one implanted and one drilled-only specimen) revealed fractures of the osseous spiral lamina. Figure 4 shows serial sections of an anterior-inferior cochleostomy resulting in a fracture of the osseous spiral lamina. DISCUSSION In this report, we were able to compare surgical and histologic data for two cochleostomy techniques in the human temporal bone. Both techniques used herein, inferior and anterior-inferior approaches, resulted in ST implantations in all bones. Although not directly studied, it can be surmised that cochleostomies placed in locations more superior than those used in this study would be expected to enter either scala media or SV. Given the fact that recent data suggests a performance advantage for patients with electrode arrays located within ST alone, this implies that superiorly-located cochleostomies should be avoided in patients without ossification who have otherwise normal temporal bone anatomy. When comparing the two cochleostomy techniques described in this study, the inferior approach required a relatively large facial recess approach to expose the Fig. 3. Specimen 4, anterior-inferior cochleostomy. Surgical image documentation and representative histologic image. (A) view of the promontory through the facial recess; the inferior part of the round window annulus is barely visible in this specimen. The bony overhang has not yet been removed. (B) View of the exposed endosteum anterior and inferior to the round window. (C) Cochleostomy drilling anterior-inferior to the round window. The blue circle depicts the location of the cochleostomy anterior and inferior to the round window membrane. (D) Histologic image illustrating that the electrode enters the cochlea through the inferior section of the spiral ligament, thus causing a trauma to this structure. The osseous spiral lamina remained intact. 2198

5 Fig. 4. Specimen 21, anterior-inferior cochleostomy. No electrode insertion, soft tissue trauma to the lateral attachment of the spiral ligament. The images depict serial histologic images through the region of the cochleostomy from posterior (A) to anterior (F); section plane similar to coronal cuts. (A) posterior extension of the cochleostomy; visible drilling cone resulting from the round window overhang drill off. OSL: osseous spiral lamina, RWN: round window niche. (B) the drilling cone (DC) of the anterior-inferior cochleostomy is clearly visible; OW: oval window with stapes footplate; RWM: round window membrane, which is barely visible with its posterior attachment below the OSL. (C) *The lateral attachment of the spiral ligament and basilar membrane onto the lateral wall of the scala tympani (ST; this also indicates the site of soft tissue traumatization). (D) further progression towards anterior; *The location of rupture of the spiral ligament on the lateral wall of ST. The osseous spiral lamina is bent superiorly indicating traumatic fracture. (E) Further progression anterior, the size of the drilling cone is diminishing. (F) anterior to the cochleostomy. The thick promontory bone is visible. promontory broadly. Inferior approach cochleostomies also required significantly more bone removal for identification of the ST endosteum when compared to those performed anterior-inferior to the RWM. Inferior cochleostomies allowed for relatively easy electrode implantation for both short and standard periomodiolar arrays. By comparison, anterior-inferior cochleostomies resulted in partial avulsions of the spiral ligament from the bony lateral wall of the ST. Furthermore, two of the seven bones with anterior-inferior approaches revealed fractures of the osseous spiral lamina. These findings are presumably related to cochleostomy drilling since trauma was equally distributed among both implanted and nonimplanted bones. Thus, findings from this study suggest that an inferior approach cochleostomy to ST might be less traumatic than an anterior-inferior opening at least in the setting of shallow electrode insertions where no extra force is needed to obtain a full implantation. This finding seems particularly relevant for cases where hearing preservation might be desirable, such as for combined EAS surgeries. 7,13 While the causes for hearing loss in hearing preservation cochlear implantation remain speculative, the fact that spiral ligament avulsions and osseous spiral lamina fractures were identified in anterior-inferior approaches seems potentially harmful to hearing. 5 Osseous spiral lamina fractures may also have adverse consequences for spiral ganglion cell survival. 14 One shortcoming of the inferior approach cochleostomy was that the labyrinthine bone adjacent to the anterior side of the opening was not entirely removable without intrascalar drilling, thus resulting in a perimodolar insertion trajectory rather than one along the antimodiolar wall. Previous studies have demonstrated that deeper electrode insertions can increase basal cochlear trauma through a buckling mechanism where the proximal part of the electrode is deflected into the basilar membrane or osseous spiral lamina. 15 This effect might be more pronounced in the inferior cochleostomy approach since remnants of adjacent labyrinthine bone on the anterior side of the cochleostomy tend to force the electrode array toward the modiolus. 10 Future studies will address this question. Finally, this study, as well as all previous electrode insertion studies, performed in cadaveric bones lacks normal tissue perfusion and cellular responses to trauma. While the findings of this study should be viewed in this light, future studies of patients having cochlear implants should be undertaken in an attempt to corroborate these findings. CONCLUSIONS Inferior cochleostomies provide an atraumatic method of cochlear opening for limited insertion depths in the human temporal bone. While none of the anteriorinferior cochleostomies directly approach scala media, ruptures of the spiral ligament were recorded in all bones. Thus, inferior cochleostomies seem safer for the preservation of intracochlear structures that are thought to be important for hearing preservation in cochlear implantation. 2199

6 Future investigations should focus on ideal insertion trajectories for deeper implantations. BIBLIOGRAPHY 1. Webb RL, Clark GM, Shepherd RK, Franz BK, Pyman BC. The biologic safety of the Cochlear Corporation multipleelectrode intracochlear implant. Am J Otol 1988;9: Banfai P, Hortmann G, Kubik S, Wustrow F. Projection of the spiral cochlear canal on the medial wall of the tympanic cavity with regard to the cochlear implant. Scand Audiol Suppl 1979;11: Adunka O, Kiefer J, Unkelbach MH, Radeloff A, Gstoettner W. Evaluating cochlear implant trauma to the scala vestibuli. Clin Otolaryngol 2005;30: Kiefer J, Weber A, Pfennigdorff T, von Ilberg C. Scala vestibuli insertion in cochlear implantation: a valuable alternative for cases with obstructed scala tympani. ORL J Otorhinolaryngol Relat Spec 2000;62: Aschendorff A, Kromeier J, Klenzner T, Laszig R. Quality control after insertion of the nucleus contour and contour advance electrode in adults. Ear Hear 2007;28: 75S 79S. 6. Finley C, Holden L, Holden T, Whiting B, Skinner M. Peripheral physio-anatomical factors influence CI outcomes. 9th International Conference on Cochlear Implants and Related Sciences. Vienna, Austria; von Ilberg C, Kiefer J, Tillein J, et al. Electric-acoustic stimulation of the auditory system. New technology for severe hearing loss. ORL J Otorhinolaryngol Relat Spec 1999;61: Gantz BJ, Turner C. Combining acoustic and electrical speech processing: Iowa/Nucleus hybrid implant. Acta Otolaryngol 2004;124: Zrunek M, Lischka M. Dimensions of the scala vestibuli and sectional areas of both scales. Arch Otorhinolaryngol 1981; 233: Briggs RJ, Tykocinski M, Stidham K, Roberson JB. Cochleostomy site: implications for electrode placement and hearing preservation. Acta Otolaryngol 2005;125: Plenk H. The Microscopic Evaluation of Hard Tissue Implants. Boca Raton, RI: CRC Press; Xu J, Xu SA, Cohen LT, Clark GM. Cochlear view: postoperative radiography for cochlear implantation. Am J Otol 2000;21: Kiefer J, Gstoettner W, Baumgartner W, et al. Conservation of low-frequency hearing in cochlear implantation. Acta Otolaryngol 2004;124: Nadol JB Jr. Patterns of neural degeneration in the human cochlea and auditory nerve: implications for cochlear implantation. Otolaryngol Head Neck Surg 1997;117: Adunka O, Kiefer J. Impact of electrode insertion depth on intracochlear trauma. Otolaryngol Head Neck Surg 2006; 135: