Measuring the cochlea and cochlear implant electrode depth

CT imaging-based approaches to cochlear duct length estimation-a human temporal bone study

Objectives

Knowledge about cochlear duct length (CDL) may assist electrode choice in cochlear implantation (CI). However, no gold standard for clinical applicable estimation of CDL exists. The aim of this study is (1) to determine the most reliable radiological imaging method and imaging processing software for measuring CDL from clinical routine imaging and (2) to accurately predict the insertion depth of the CI electrode.

Methods

Twenty human temporal bones were examined using different sectional imaging techniques (high-resolution computed tomography [HRCT] and cone beam computed tomography [CBCT]). CDL was measured using three methods: length estimation using (1) a dedicated preclinical 3D reconstruction software, (2) the established A-value method, and (3) a clinically approved otosurgical planning software. Temporal bones were implanted with a 31.5-mm CI electrode and measurements were compared to a reference based on the CI electrode insertion angle measured by radiographs in Stenvers projection (CDL_reference).

Results

A mean cochlear coverage of 74% (SD 7.4%) was found. The CDL_reference showed significant differences to each other method (p < 0.001). The strongest correlation to the CDL_reference was found for the otosurgical planning software-based method obtained from HRCT (CDL_SW-HRCT; r = 0.87, p < 0.001) and from CBCT (CDL_SW-CBCT; r = 0.76, p < 0.001). Overall, CDL was underestimated by each applied method. The inter-rater reliability was fair for the CDL estimation based on 3D reconstruction from CBCT (CDL_3D-CBCT; intra-class correlation coefficient [ICC] = 0.43), good for CDL estimation based on 3D reconstruction from HRCT (CDL_3D-HRCT; ICC = 0.71), poor for CDL estimation based on the A-value method from HRCT (CDL_A-HRCT; ICC = 0.29), and excellent for CDL estimation based on the A-value method from CBCT (CDL_A-CBCT; ICC = 0.87) as well as for the CDL_SW-HRCT (ICC = 0.94), CDL_SW-CBCT (ICC = 0.94) and CDL_reference (ICC = 0.87).

Conclusions

All approaches would have led to an electrode choice of rather too short electrodes. Concerning treatment decisions based on CDL measurements, the otosurgical planning software-based method has to be recommended. The best inter-rater reliability was found for CDL_A-CBCT, for CDL_SW-HRCT, for CDL_SW-CBCT, and for CDL_reference.

Key Points

• Clinically applicable calculations using high-resolution CT and cone beam CT underestimate the cochlear size.

• Ten percent of cochlear duct length need to be added to current calculations in order to predict the postoperative CI electrode position.

• The clinically approved otosurgical planning software-based method software is the most suitable to estimate the cochlear duct length and shows an excellent inter-rater reliability.