Cochlea size variability and implications in clinical practice

The objectives of this study were: 1) study cochlea size variability among age and degree of deafness; 2) calculate the length of the cochlear implant electrode needed to obtain the optimal final insertion depth angle of 270°. A total of 241 patients (482 ears) that underwent high resolution computed tomography (HRCT) of the ear in our Institution between 2003 and 2008 were included to collect temporal bone data, and were divided in 3 groups: 97 (194 ears) patients with bilateral severe or profound sensorineural hearing loss (Group A), 70 patients (140 ears) with bilateral moderate sensorineural hearing loss (Group B), 74 patients (148 ears) without sensorineural or mixed hearing loss (Group C). In each of the 3 groups, 5 subgroups were identified with the following age criteria: 1) subgroup 1: subjects ≤5 years old; 2) subgroup 2: subjects 6-10 years old; 3) subgroup 3: patients 11-15 years old; 4) subgroup 4: patients 16-20 years old; 5) subgroup 5: subjects >; 20 years old. The length of the cochlea, height of the cochlea, basal turn lumen diameter (BTLD) and volume of the cochlea were measured. The Mann-Whitney test was used to assess the alternative hypothesis that a statistically significant difference in size exists between the different groups and subgroups. The following equation was adopted to calculate the length of a straight electrode which follows the outer wall of the scala tympani required to obtain the ideal insertion depth angle of 270°( L(IC) ): [Formula: see text] . We found that the cochlea is completely developed and has reached adult size at birth. The degree of deafness does not affect the length or volume of the cochlea, while it can affect the height and BTLD. To assist the surgeon to calculate the ideal insertion depth angle of 270° in order to preserve residual hearing, it is useful to propose a straight electrode with 3 landmarks on the array (the first at 16.635 mm from the tip, the second at 17.987 mm and the third at 19.34 mm).