*3.6.1 Types of CI electrode arrays*

According to the method of insertion of the CI electrode array, there are three main types of CI electrode arrays that vary in the design and the method of the insertion:


Each of the three types of electrode array has advantages and disadvantages. In general, the LW electrodes are usually slimmer and are assumed to have less traumatic effect on the cochlea during insertion, but they are usually rest away from the spiral ganglia which are the target of the electrodes' stimulatory impulses. The PM electrodes can hug the modiolus and became very close to the spiral ganglia; but because of the need of stylet, they are usually more stiff and thick, except the new PM electrode generations such as CI532® (Cochlear Corp), so that PM electrodes usually have more traumatic potentials on the cochlea during insertion. The MS electrodes are assumed to have the advantages of both LW and PM electrodes, but they can also have the disadvantages of both of them [44–46].

#### *3.6.2 Depth of insertion of the CI electrode array*

Proper electrode insertion is achieved by both making full insertion, which entails inserting all active electrodes into the scala tympani, and by making appropriate depth of insertion. Regarding the depth of insertion, Yukawa et al. [47] reported that the better predictor of the outcome for the depth of electrode insertion is the angular depth rather than the intracochlear length of the electrode or even the number of active electrodes that is used in speech processing. It is expected that LW electrodes, especially the long types as FlexSoft™ (MED-EL Corp), can demonstrate greater angular insertion depth, more than 360°, while the PM or MS electrodes are usually designed to encircle the first basal turn making angular insertion depth nearly equal to 360° [48]. Insertion depth at 360°, or less than one cochlear turn, is usually associated with poor speech outcome; however, above 360°, there is no association between the depth of insertion and the speech outcome [49, 50]. Deep insertion is assumed to have the advantage of extending the electrical stimulation into the apical region that is responsible for low-frequency sounds; this provides better place pitch match which may improve the outcomes of CI especially in the music perception [51]. However, deep insertion is usually associated with increasing the risk of intracochlear trauma [45].

In conclusion, the best design for "ideal" CI electrode is a matter of debate; there is no "best" CI electrode for all CI cases.
