Estimation of angular insertion depth of a cochlear implant electrode array: 3D reconstruction versus spiral formulae.

Objective: The primary objective was to determine which of the following angular insertion depth (AID) estimation methods had the strongest correlation with the post-operative AID (AIDpost-op) of a lateral wall (LW) electrode: 1) Escudé formula based on the largest distance from the round window (RW) to the LW (distance A), 2) the elliptic-circular approximation (ECA) method based on both distance A and the perpendicular distance (distance B), 3) a 3D reconstruction method. The secondary objective was to evaluate the impact of using the actual electrode insertion length on the evaluation of the AID estimation methods.

Methods: The study included 45 cochleae implanted with the Advanced Bionics SlimJ electrode, with all 16 active electrode contacts being intracochlear. Distance A, distance B and a 3D reconstruction along the LW using 3D Slicer were evaluated on pre-operative magnetic resonance imaging (MRI). The AIDpost-op and insertion length were assessed on post-operative cone beam computed tomography (CBCT). Two sets of estimated AID values were obtained: 1) assuming perfect 23 mm insertion length with the reference electrode at the RW (AID23mm), 2) with the actual insertion length (AIDinsertionlength). Spearman's Rank correlation coefficient assessed he association between the estimated AID values and AIDpost-op.

Results: A moderately positive correlation was found between AID23mm and AIDpost-op for all three approaches, with no statistical difference among them (rs=0.410-0.579). When the analysis was repeated with AIDinsertionlength, the strength of the correlation with AIDpost-op improved significantly for all three, with the 3D reconstruction method having a stronger correlation (rs=.952) than the Escudé formula (rs=0.734) or ECA method (rs=0.787) (p<.001).

Conclusion: The 3D reconstruction approach has the potential to improve prediction of AID of a LW electrode array compared to spiral formulae based on 2D cochlear dimensions. Accurate assessment of the actual insertion length is important for proper evaluation of AID estimation methods.