Cochlear implant electrode insertion

Standard CI surgery includes a mastoidectomy and facial recess approach to the middle ear and cochlea. Knowledge of facial recess anatomy is crucial for safe and adequate cochlear visualization and ideal cochleostomy position. Anteriorly, thinning of the posterior external auditory canal is the first step in creation of an ideal facial recess. A thin posterior canal will improve identification of, and thus minimize injury to, the chorda tympani nerve. This anterior border will also maximize the amount of light entering the facial recess, ultimately improving visualization of the cochlea and middle ear structures. Following mastoidectomy, the descending or mastoid portion of the facial nerve is identified and skeletonized until it can be seen through a thin layer of bone. Although thorough review of preoperative computed tomography imaging of the temporal bone can alert and prepare the surgeon for anatomic aberrations, such as an abnormal course of the facial nerve or presence of chronic ear disease, a high level of intraoperative vigilance and precise identification of anatomic landmarks remain critical to safe CI surgery.

Superiorly, the fossa incudis is enlarged until the short process of the incus can be seen, keeping a thin bone bridge or incus bar intact. Anatomically, the short process points toward the center of the facial recess and this relationship can be especially helpful in poorly developed, minimally aerated temporal bones where identification of the facial recess may be more challenging. Although the integrity of the incus bar is typically maintained, this bone bridge (and the incus itself) can be easily removed for increased visualization of the middle ear. This technique is described in more detail in the section on the obstructed or malformed cochlea.

Next, the chorda tympani nerve is identified from its origin at the facial nerve to the chorda iter posterior, where it enters the middle ear. Bone in the facial recess, between the chorda tympani anteriorly, the facial nerve posteriorly, and the incus bar superiorly, is then carefully removed using serially smaller diamond burs. An adequate facial recess includes good visualization of the stapedial tendon, round window, and cochlear promontory (Figure 1). To achieve full access to the round window niche and inferior cochlea, bone anterior to the facial nerve and inferior to the stapedial tendon must be carefully removed. Integrity of the chorda tympani can be preserved in nearly all cases and maintenance of this landmark can protect against damage to the annular ligament of the tympanic membrane, which lies more anteriorly. Damage to the facial nerve or chorda tympani can result from the bur itself, as well as the heat or pressure from the drill shaft. Copious irrigation and vigilant monitoring of both the drill and the bur position during creation of the facial recess and cochleostomy are vital to minimize injury. Intraoperative facial nerve monitoring using continuous electromyographic recording of the ipsilateral facial muscles is generally advised during cochlear implantation.

Cochleostomy position is essential to optimal, atraumatic CI electrode placement in the scala tympani. Using a 1.5- or 1.0-mm diamond bur, removal of the round window niche may be necessary to properly visualize the membrane and thus estimate correct cochleostomy location. Electrode choice may dictate cochleostomy size; this is covered in greater detail in upcoming sections. In the normal cochlea, however, cochleostomy position is independent of electrode type. The ideal cochleostomy is inferior and slightly anterior to the round window membrane (Figure 2). A more superior position results in injury to the spiral ligament and basilar membrane and risks insertion into the scala vestibuli. Inferiorly, hypotympanic air cells can be mistaken for a round window niche, leading to gross misplacement of the electrode outside the cochlea completely and/or dangerous misjudgment of the jugular bulb. Overall, the importance of maximal visualization through an adequate, carefully created facial recess cannot be underestimated. Drilling anterior and inferior to the round window membrane, the dome of the cochlea is carefully saucerized until the endosteum of the scala tympani is seen. If present, bone dust and blood are removed with irrigation and judicious suctioning. The cochlear endosteum is then carefully opened with a small pick or rasp, revealing the lumen of the scala tympani. A small rasp is then used to remove sharp edges and, if needed, can be used to enlarge the cochleostomy until the outer wall of the basal turn is visualized. Although not required, many centers use lubricants, such as 50% glycerin or sodium hyluronate to prevent the ingress of blood and bone dust and assist in smooth electrode insertion.

A variety of scientific principles underlie the principles of electrode insertion. During insertion, there are multiple force vectors acting on the CI electrode to influence its ultimate position. Following introduction of the electrode through an optimally placed cochleostomy, there are multiple anatomic regions of potential intracochlear trauma, including the lateral wall or spiral ligament, basilar membrane, osseous spiral lamina, modiolar wall, and neuroepithelium. Direct damage to these structures, most importantly the osseous spiral lamina and modiolar wall, can disrupt spiral ganglion cells or their dendrites leading to degeneration. Additionally, it can lead to suboptimal electrode placement in the scala media or scala vestibuli of the upper cochlea. Research into CI insertion dynamics, specifically the intracochlear hydraulic and mechanical forces of insertion, has influenced both electrode design and surgical technique. Figure 3A is a force diagram illustrating the insertional forces impacting the CI electrode in the basal turn of the cochlea. The outer wall of the scala tympani is upwardly sloping and any force applied will be met with a force of equal magnitude (minus frictional forces) perpendicular to wall at the point of contact. Fluoroscopically and histologically, this location is consistently found in the lower midpars ascendens. As seen in Figure 3A, the resultant force vector is directed superiorly toward the spiral ligament and basilar membrane and can drive the electrode out of the scala tympani to a final, suboptimal position in the upper cochlea (Figure 3B). Prior research in the normal cochlea comparing the Cochlear Corporation's Contour electrode (Sydney, Australia), a straight electrode, to the Contour Advance^™ perimodiolar electrode with Advance Off Stylet^™ (AOS) technique demonstrated reduced outer wall force generation with the perimodiolar electrode. Mechanical and hydraulic force measurements, as well as histologic and fluoroscopic analysis, indicated that a more reliable and less traumatic insertion was obtained using a perimodiolar electrode and the AOS technique.

Current research in electrode design and insertional forces continues to use this model to assess structural trauma and electrode placement. Prevention of damage to intracochlear structures, such as the basilar membrane, is especially crucial for the preservation of residual hearing and the use of electric-acoustic stimulation (EAS). Although a full discussion of EAS is beyond the scope of this review, it requires preservation of native acoustic function in the apical region of the cochlea, the portion corresponding to high-frequency sounds. Using both a hearing aid and a cochlear implant in the same ear, patients utilizing EAS have low-frequency acoustic stimulation with traditional amplification and high-frequency electric stimulation using the CI electrode array. Preservation of residual hearing and successful EAS require an understanding and minimizing of the forces associated with electrode insertion. Uniquely designed electrodes, such as Cochlear Corporation's Hybrid^™ L electrode and MED-EL's^® FlexEAS electrode (Innsbruk, Austria) are described in more detail in upcoming sections and are currently undergoing human clinical trials in the USA. Insertional forces associated with the Long Hybrid Array (or Hybrid L electrode) can be seen in Figure 3C. Early insertion force results indicate minimal force generation when compared with the standard straight array. Other electrodes tested in this paradigm, specifically the Modiolar Research Array (a perimodiolar electrode) and the Straight Research Array, demonstrated little or no intracochlear trauma in cadaveric temporal bone studies. Neither of these electrodes have begun human clinical trials in the USA. In Europe, multiple studies on EAS and partial deafness CI using MED-EL's EAS electrode have demonstrated minimal intracochlear trauma and preservation of residual hearing in both adults and children.^{,  ,} 

In general, fixation of the receiver/stimulator (preferably within a bony well) is desirable before electrode insertion. The reverse may result in extrusion of a perfectly placed electrode and require reinsertion. As discussed above, optimal CI position in the scala tympani requires a thorough understanding of the cochlear orientation through the facial recess. The lumen of the basal turn is oriented in a plane virtually parallel to that of the external auditory canal wall. Therefore, the direction of electrode insertion should be down the midportion of the proximal turn, avoiding both the outer wall and the medial modiolar wall. Unlike creation of the facial recess and cochleostomy, electrode insertion is best accomplished on low power with a wide view of the mastoid cavity and cochleostomy in the distance. This view allows complete visualization of the entire electrode array and insertion instrument. A variety of tools specific to each electrode type and company are available. Regardless of electrode choice, however, gentle, smooth insertion and avoidance of excessive force are universal principles. Tactile feedback during insertion is important: the perception of resistance indicates a problem and stopping to reassess and evaluate anatomic circumstances may prevent electrode tip rollover or malposition. Complete insertion is dictated by electrode type and brand and is described in detail later. Each cochleostomy should be packed with fascia or periosteum around the electrode array. This enhances scar tissue formation around the electrode and seals the cochleostomy, thereby minimizing perilymph leakage and resultant vertigo, infectious complications, and electrode extrusion. Overall, it is a surgeon's responsibility to be familiar with all electrodes used by his/her implant center and the accompanying device-specific surgical instruments. In addition, it is highly recommended that practice in the temporal bone laboratory precede insertion of a new electrode design for the first time.

Introduced in the past 10 years, perimodiolar electrodes are designed to self-coil during or after insertion to reside close to the spiral ganglion cells in the modiolus. Impetus for this design included more selective stimulation of spiral ganglion cells subpopulations, decreased thresholds for stimulation, and overall decreased power consumption and decreased cochlear damage. Research has generally supported these benefits and demonstrated that modiolar hugging electrodes led to decreased evoked auditory brainstem response thresholds, decreased evoked auditory brainstem response thresholds latencies, and less intracochlear damage.^{,  ,  ,  ,  ,  ,  ,}  Techniques specific to both perimodiolar and outer wall electrodes are discussed.

CI insertion techniques in cases of congenital cochlear malformations, such as common cavity, Mondini deformity, or hypoplastic cochlea, require deviation from the standard technique described previously. Preoperative imaging typically includes both computed tomography (CT) and magnetic resonance imaging (MRI). These films should be thoroughly reviewed for cochlear and labyrinthine malformations, abnormalities in facial nerve position, intracavity septations, overall cochlear patency, and nerve content of the internal auditory canal. Imaging may also play a role in deciding which ear to implant or the feasibility of simultaneous bilateral implantation.

In common cavity malformations, a cortical mastoidectomy is performed and the antrum opened to reveal the otic capsule bone of the middle ear cavity. Using a 1.0- or 1.5-mm diamond bur, the dome of the cavity is flattened until the endosteum is exposed for a 1-mm diameter. The endosteum is then opened with a straight or right-angle pick exposing the cavity lumen. Commonly, egress of cerebrospinal fluid (CSF) or a CSF gusher is encountered. The head of the bed can be elevated to decrease intracranial pressure and, with time, the CSF reservoir will drain and stop, allowing electrode insertion to proceed. In cases of cochlear malformation, use of fluoroscopic guidance can be beneficial by allowing real-time visualization of electrode insertion. Bending, kinking, rollover, over insertion, and placement in the internal auditory canal can be identified and addressed immediately. As neural elements are likely located on the outer wall or septations with the cochlea, delicate electrode advancement helps reduce injury to these crucial structures. Alternate techniques include making a slot-like cochleostomy and introducing the array as a slightly bent “U” shape. Regardless of cochleostomy shape, tight packing with fascia, periosteum, or muscle is crucial to prevention of postoperative CSF leak. Some surgeons advocate packing the Eustachian tube with soft tissue before making the cochleostomy in an attempt to preempt an expected CSF leak.

The approach to the hypoplastic cochlea is performed through a facial recess. Preoperative imaging can be used to assess facial nerve location and also to predict ideal cochleostomy location. In these cases, the surgeon may encounter a narrow facial recess and limited cochlear lumen. By removing the incus and taking down the incus bar, both visualization and access to the middle ear can be maximized. As with common cavities, cochleostomy size should be kept small (approximately 1 mm in diameter) and use of intraoperative fluoroscopic guidance can be advantageous.

In general, straight electrodes are most appropriate for cochlear malformations, specifically the Cochlear Corporation K electrode and the MED-EL Standard electrode (Figures 10 and 12). These arrays have circumferential and paired contacts, respectively, and may be most likely to stimulate abnormally located neural elements. While they can be used, self-coiling electrodes or those with modiolar facing contacts are not recommended. Additionally, MED-EL manufactures a medium and compressed electrode array in which the 12 paired contacts are spaced over medium and short distances, respectively (Figures 15 and 16). These may be advantageous in the hypoplastic cochlea.

Most commonly caused by meningitis, cochlear ossification can result from otosclerosis, chronic otitis media, ototoxic agents, trauma (including iatrogenic), labyrinthine artery occlusion, ototoxic medications, leukemia, temporal bone tumors, viral infection, Wegener's granulomatosis, and autoimmune and idiopathic processes. Up to 80% of patients with ossification have radiographic evidence of partial or complete obstruction of the proximal basal turn; thus, preoperative imaging serves an important role in surgical planning. Overall, MRI is more sensitive in detection of cochlear patency, specifically using the fluid signal intensity on T2-weighted images. When combined with high-resolution CT, detection of cochlear ossification approaches 90%. Degree of ossification affects both surgical technique and electrode selection. We have adopted an algorithm for implanting the obstructed cochlea (Figure 17). To begin, a normally placed cochleostomy is created. If fibrosis, uncalcified osteoid tissue, or new bone is found in the basal turn, removal is attempted using small rasps or picks or with a limited drill-out of the basal turn. Drilling should progress anteriomedially approximately 10 mm or until the internal carotid artery is encountered. If a patent lumen is discovered at or before the ascending turn of the cochlea, a scala tympani insertion is planned. MED-EL provides a test electrode that can be used to probe the lumen before opening a device onto the sterile field. Alternatively, if available, a very small boogie catheter has been used in a similar fashion. If exploration with the test catheter or boogie is successful, scala tympani electrode insertion proceeds. If no lumen is found, the cochleostomy is extended superiorly to the scala vestibuli. This lumen is often patent in cases of meningitis and otosclerosis and full scala vestibuli insertion is desirable when the scala tympani is occluded.^,  If this scala is also obliterated, an additional cochleostomy can be created in the second turn of the cochlea for use with a double-array device. To prepare for the second cochleostomy, the incus, incus bar, and stapes superstructure are removed to maximize access anterior to the oval window. Using a 1-mm diamond bur and copious irrigation, this superior cochleostomy is created immediately anterior to the oval window, adjacent to the annular ligament of the stapes footplate. The cochleariform process serves as the superior limit of dissection and a landmark for facial nerve location. Drilling below the cochleariform process parallel to the tensor tympani muscle is critical to avoid damage to the facial nerve. If a lumen is discovered, a split or double array is used. One array is placed in the basal turn tunnel (described earlier) and the other array is inserted either retro- or anterograde through the superior cochleostomy, based on patient anatomy and manufacturer recommendations. (Cochlear Corporation prefers anterograde and MED-EL encourages retrograde.) If no lumen is found, an apical tunnel can be created through the new bone using a rasp or 0.5- to 1-mm diamond bur. This tunnel should be directed toward the tensor tympani, away from the probable location of neural elements. The second array can then be placed in this superior tunnel (Figure 18). In general, the electrodes should not overlap and forceful or overinsertion, leading to kinking or tip rollover, should be avoided. Studies suggest use of a double array yields more usable electrodes than partial insertion; however, either technique can be done safely and successfully with postoperative benefit. Each cochleostomy should be effectively packed with periosteum or fascia to firmly secure the electrode position.

All available electrodes may be used in for scala vestibuli insertion. While perimodiolar electrodes can be used, the thinner, straight arrays (specifically, the Cochlear K electrode, the MED-EL standard, medium, or compressed array, and the Advanced Bionics 1J array) may be more appropriate if a lumen is found (Figures 10, 11, 15, and 16, respectively). Two split or double-array options exist: the MED-EL Split array with 7 contacts on 1 electrode and 5 on the other and the Cochlear corporation Double Array with 11 contacts on each electrode (Figures 19 and 20).

Although electrode insertion in revision CI surgery can be challenging, careful surgical planning and methodical technique can overcome most pitfalls. In general, removal of the in situ electrode array should not commence until all preparation for reimplantation is complete. Following flap elevation, the electrode array should be carefully severed laterally within the mastoid cavity. This allows removal of the receiver/stimulator, revisions of the bony well, and mastoid cavity to proceed without disruption of the in situ electrode. Within the mastoid and facial recess, scar tissue and occasionally new bone will encase the electrode and should be delicately dissected until the cochleostomy is visualized. Progressive amputation of the array will allow safe drilling and dissection in the facial recess. With the in situ electrode in place, the receiver/stimulator of the new device should be secured. Scar tissue and/or new bone is commonly encountered at the cochleostomy site. This can be delicately incised or dissected, although drilling may occasionally be required. Although removal is commonly uncomplicated, lysis of scar tissue and adhesions at the cochleostomy are prudent. It is imperative that the in situ electrode be removed in its entirety and no component or part remain lodged within the cochlea. Intracochlear fibrosis and ossification are very common. Typically, a soft tissue sheath forms around the electrode within the cochlea. After electrode removal, this sheath can collapse, obscuring and obstructing the cochlear lumen. Therefore, the new array should be positioned near the cochleostomy so that it can be easily inserted as soon as the old electrode is removed. A pick or Rosen needle may be used to probe or identify the sheath lumen, if necessary. Use of fluoroscopy may be helpful in these circumstances to verify position and guide insertion. As the entire device is ultimately sent to the company for cause-of-failure analysis, minimizing trauma to the device during explantation is helpful.

Cochleostomy and CI electrode insertion are fundamental steps in safe, successful cochlear implantation. Optimal placement is an essential first step in maximizing postoperative outcomes. Careful evaluation of preoperative imaging, attention to device/electrode selection, and judicious use of intraoperative fluoroscopy can minimize the surgical challenges associated with electrode insertion in the malformed or obstructed cochlea and in revision CI surgery.