Factors Associated with Incomplete Insertion of Electrodes in Cochlear Implant Surgery: A Histopathologic Study

Abstract

Objectives: Atraumatic and complete insertion of the electrode array is a stated objective of cochlear implant surgery. However, it is known that obstructions within the cochlea such as new bone formation, cochlear otosclerosis, temporal bone fracture, and cochlear anomalies may limit the depth of insertion of the electrode array. In addition, even among patients without obvious clinical or radiographic indicators of obstruction, incomplete insertion may occur. The current study is a histopathologic evaluation of possible sources of resistance to insertion of the electrode array using the temporal bone collection of the Massachusetts Eye and Ear Infirmary. Methods: Forty temporal bones from patients who in life had undergone cochlear implantation were evaluated. Temporal bones were removed at autopsy and fixed and prepared for histologic study by standard techniques. Specimens were then serially sectioned and reconstructed by 2-dimensional methods. Two electrode metrics were determined for each bone: the inserted length (IL: the distance measured from the cochleostomy site to the apical tip of the electrode) and the active electrode length (AEL: the distance between the most basal and most apical electrodes on the electrode array). The ratio of these two metrics (IL/AEL) was used to split the temporal bones into two groups: those with incomplete insertion (n = 27, IL/AEL <1.0) and those with complete insertion (n = 13, IL/AEL ≧1.0). Seven possible histopathologic indicators of resistance to insertion of the electrode due to contact with the basilar membrane, osseous spiral lamina and/or spiral ligament were evaluated by analysis of serial sections from the temporal bones along the course of the electrode tracks. Results: Obvious obstruction by abnormal intracochlear bone or soft tissue accounted for only 6 (22%) of the 27 partial insertions. Of the remaining 21 bones with incomplete insertions and 13 bones with complete insertions, dissection of the spiral ligament to the lateral cochlear wall was the only histopathologic indicator of insertion resistance identified with significantly higher frequency in the partial-insertion bones than in the complete-insertion bones (p = 0.003). An observed trend for the percentage of complete insertions to decrease with the number of times the electrode penetrated the basilar membrane did not reach significance. In the bones without an obvious obstruction, the most frequently observed indicator of insertion resistance was dissection of the spiral ligament (with no contact of the lateral cochlear wall) identified in 67% (14/21) of partial-insertion bones and in 92% (12/13) of complete-insertion bones. Conclusion: These results are consistent with the view that (1) electrode contact with cochlear structures resulting in observable trauma to the basilar membrane, osseous spiral lamina and/or spiral ligament does not necessarily impact the likelihood of complete insertion of the electrode array and (2) once contact trauma to the spiral ligament reaches the point of dissection to t...

Correlation between word recognition score and intracochlear new bone and fibrous tissue after cochlear implantation in the human

“…The serial sections of the temporal bones were reconstructed, and the numbers of spiral ganglion cells in Rosenthal’s canal were calculated by conventional 2-D methods (Guild, 1921; Schuknecht, 1993). The most apical section of the cochlea containing the electrode tip in which there was histologic evidence, such as fibrous sheath around the electrode (Li et al, 2007; Lee et al, 2011), and the entry point into the cochlea were determined. The electrode length within the cochlea and the electrode length located in the scala media/vestibuli or spiral ligament were also determined using the 2-D reconstructions.…”

“…The degree of electrode insertional trauma was assessed by adapting a method previously described (Li et al, 2007; Lee et al, 2011) and presented in Table 2. Damage to the osseous spiral lamina, lateral cochlear wall and basilar membrane was used as markers for insertional trauma.…”

Potential Benefits From Deeply Inserted Cochlear Implant Electrodes

“…It is well known that full insertions of any CI electrode are not always achieved, particularly by inexperienced surgeons, and that this is often (although not always) due to obstructive pathology (Lee et al 2010). Published reports of surgical experiences with the MED-EL electrode arrays suggest that full insertion is achieved in the majority of cases of apparently patent cochleae.…”

“…Furthermore, it is possible that more shallow insertions may be associated with pre-existing obstructive pathology or insertion trauma (Neumann et al 2005;Lee et al 2010), further complicating interpretation. Skinner et al (2002), however, reported a positive correlation between word recognition and insertion depth (ranging from 11.9 to 25.9 mm) in 26 users of the Nucleus 22 device.…”

A novel perfusion-based method for cochlear implant electrode insertion

“…In contemporary surgical technique, it is often difficult to achieve electrode insertion deeper than the first basal turn even if the cochlea is anatomically normal (Lee et al, 2011). Incomplete insertion may create a mismatch between cochlear place and frequency.…”