The pattern and degree of capsular fibrous sheaths surrounding cochlear electrode arrays

Abstract

An inflammatory tissue reaction around the electrode array of a cochlear implant (CI) is common, in particular at the electrode insertion region (cochleostomy) where mechanical trauma often occurs. However, the factors determining the amount and causes of fibrous reaction surrounding the stimulating electrode, especially medially near the perimodiolar location, are unclear. Temporal bone (TB) specimens from patients who had undergone cochlear implantation during life with either Advanced Bionics (AB) Clarion TM or HiRes90KTM (Sylmar, CA, USA) or Cochlear TM Nucleus (Sydney, Australia) devices were evaluated. The thickness of the fibrous tissue surrounding the electrode array of both types of CI devices at both the lower (LB) and upper (UB) basal turns of the cochlea was quantified at three locations: the medial, inferior, and superior aspects of the sheath. Fracture of the osseous spiral lamina and/or marked displacement of the basilar membrane were interpreted as evidence of intracochlear trauma. In addition, post-operative word recognition scores, duration of implantation, and post-operative programming data were evaluated. Seven TBs from six patients implanted with AB devices and five TBs from five patients implanted with Nucleus devices were included. A fibrous capsule around the stimulating electrode array was present in all twelve specimens. TBs implanted with AB device had a significantly thicker fibrous capsule at the medial aspect than at the inferior or superior aspects at both locations (LB and UB) of the cochlea (Wilcoxon signed-ranks test, p<0.01). TBs implanted with a Nucleus device had no difference in the thickness of the fibrous capsule surrounding the track of the electrode array (Wilcoxon signed-ranks test, p>0.05). Nine of fourteen (64%) basal turns of the cochlea (LB and UB of seven TBs) implanted with AB devices demonstrated intracochlear trauma compared to two of ten (20%) basal turns of the cochlea (LB and UB of five TBs) with Nucleus devices, (Fisher exact test, p<0.05). There was no significant correlation between the thickness of the fibrous tissue and the duration of implantation or the word recognition scores (Spearman rho, p=0.06, p=0.4 respectively). Our outcomes demonstrated the development of a robust fibrous tissue sheath medially closest to the site of electric stimulation in cases implanted with the AB device electrode, but not in cases implanted with the Nucleus device. The cause of the asymmetric fibrous sheath may be multifactorial including insertional trauma, a foreign body response, and/or asymmetric current flow.

Challenging aspects of contemporary cochlear implant electrode array design

“… 29 , 30 In fact, a direct comparison of perimodiolar arrays (Clarion TM or HiRes90K™) with a lateral wall array (Full-Band Straight) confirmed thickened fibrous capsule at the medial aspect of the electrode array in individuals implanted with a perimodiolar device. 31 This finding was attributed to closer proximity to the modiolus and differences in the shape of electrical field. Increased electrode impedance, which might result from such isolating encapsulation, 32 , 33 although the association between the longitudinal impedance data and the development of fibrous tissue is difficult, 31 negates any hypothetical reduction in threshold currents linked with shorter distance to the target neural tissue at perimodiolar location.…”

“… 31 This finding was attributed to closer proximity to the modiolus and differences in the shape of electrical field. Increased electrode impedance, which might result from such isolating encapsulation, 32 , 33 although the association between the longitudinal impedance data and the development of fibrous tissue is difficult, 31 negates any hypothetical reduction in threshold currents linked with shorter distance to the target neural tissue at perimodiolar location. Insertion trauma …”

Double Ring in Cochlear Otosclerosis: A Limit to Cochlear Implantation? The Solution Is the Surgical Approach

“…4 An electrode insertion through a cochleostomy determines an important inflammation responsible of the increased ossification's risk. 5 Some authors proposed to insert the CI through the round window 6 because this procedure is less traumatic 6 than cochleostomy.…”

Correlação entre volume coclear residual pós-labirintite ossificante e medidas de impedância dos eletrodos intracocleares em pacientes submetidos à cirurgia de implante coclear

“…A impedância do eletrodo reflete toda a resistência no trajeto entre o eletrodo estimulador inserido na escala timpânica (preferencialmente pela janela redonda) e o eletrodo de retorno (que na estimulação monopolar está no próprio eletrodo e na estimulação bipolar está nos eletrodos vizinhos) (Ishai et al, 2017). Seria de se esperar que o tecido fibroso ou ósseo no interior da cóclea causasse maior resistência que a perilinfa, contribuindo para o aumento do nível de impedância.…”