Abstract
Objective
Recent studies demonstrated the utility of high-resolution computed tomography (HRCT) scans in measuring basal cochlear length and cochlear insertion depths. These studies showed significant variations in the anatomy of the cochlea amongst humans. The aim of our study was to investigate for gender and racial variations in the basal turn length of the human cochlea in an Asian population.
Method
HRCT temporal bone data from year 1997 till 2012 of patients with normally developed cochleae who reported with otologic disease was obtained. Reconstruction of the full basal turn was performed for both ears. The largest distance from the midpoint of the round window, through the midmodiolar axis, to the lateral wall was measured (distance A ). Length of the lateral wall of the cochlea to the first turn (360°) was calculated and statistically analyzed.
Results
HRCT temporal bone data from 161 patients was initially obtained. Four patients were subsequently excluded from the study as they were of various other racial groups. Study group therefore comprised of 157 patients (314 cochleae). Mean distance A was statistically different between the two sides of the ear (right 9.09 mm; left 9.06 mm; p = 0.0069). Significant gender and racial differences were also found. Mean distance A was 9.17 mm in males and 8.97 mm in females ( p = 0.0016). The racial groups were Chinese (39%), Malay (38%) and Indian (22%). Between racial groups, mean distance A was 9.11 mm (Chinese), 9.11 mm (Malays) and 8.99 mm (Indians). The mean basal turn lengths ranged from 19.71 mm to 25.09 mm. With gender factored in, significant variation in mean basal turn lengths was found across all three racial groups ( p = 0.04).
Conclusion
The view of the basal turn of the cochlea from HRCT is simple to obtain and reproducible. This study found significant differences in basal cochlear length amongst male and female Asian patients, as well as amongst various racial groups. This has implications for cochlear electrode insertion as well as electrode array design.
1
Introduction
The human cochlea reaches maturation before birth. Until recently, there has been little literature describing the anatomical variations of the cochlea . Studies thus far have found cochleae to differ significantly in shape, coiling characteristics and dimension . This variation also suggests that place-frequency maps should vary considerably between individuals . Furthermore, even amongst cochleae with similar bony anatomy, there is variation in the Organ of Corti . Understanding these variations has great utility in the design of cochlear implants and to minimize intracochlear damage during the process of electrode insertion.
The ability to minimize intracochlear damage has become increasingly important with the recent interest in electro-acoustic stimulation (EAS). Despite some conflicting evidence , early experience with EAS has shown that insertion depth beyond 360° leads to a loss of residual hearing while shallower insertions lead to poorer auditory outcomes . Unfortunately, the variations in cochlear anatomy can result in significant variations in insertion depth for the same length of electrode inserted . Understanding cochlear length then allows us to optimize outcomes for EAS. Going ahead, it is likely that when this information is used in combination with Greenwood’s frequency-position function, the length of electrode insertion can be customized for the configuration of hearing loss as well, which may not necessitate a 360° insertion .
There are various methods to quantify cochlear length. Advances in imaging technology with high-resolution computed tomography (HRCT) and 3-dimensional reconstructions can now reliably detect and quantify variations in cochlear anatomy . Escude et al. developed a method for quantitative determination of the size of the cochlea using reconstructed images from routine HRCT of the temporal bone . The full basal turn of the cochlea is visualized and calculations can be made of the length of the outer wall of the basal turn. Measurements made on CT images have been shown to be in good agreement with measurements made on histological preparations. Other methods involve using markers along the basal turn of the cochlea or using intraoperative fluoroscopy . However, these methods are either excessively intricate or necessitate more staff and equipment.
As such, our study utilizes Escude’s method to determine the basal turn lengths in our local Asian population and investigate for gender and racial variations.
2
Method
High-resolution computed tomographic (HRCT) temporal bone data were obtained from randomly-selected patients with normally-developed cochleae who reported with otologic disease. Exclusion criteria were disease process involving the cochlea and congenital cochlear anomalies.
HRCT was performed with Aquilion ONE (Toshiba Medical Systems, Tokyo, Japan) with parameters of mA, 200; kV, 120; FOV: symphysis menti to vertex; helical pitch 41; section thickness, 0.5 mm; and reconstruction index, 0.4 mm. The 3-dimensional data were reconstructed on a Vitrea workstation (version 6.2.2096.3209; Vital, Toshiba Medical Systems). A double-oblique coronal reformatted image for each ear was obtained ( Fig. 1 ). This was rendered with a 1.0 mm thick reformat in order to visualize the basal turn from the round window to the opposite outer cochlear wall on a single image. Window settings were set at window width/center of 4000/400. The cochlear distance (distance A ) was measured from the mid-round window transecting through the mid-modiolar axis to the opposite wall of the basal turn and distances were recorded to the nearest 0.01 mm. The measurements were recorded by 2 independent radiologist observers. Each observer made 2 sets of measurements of each cochlea with an interval between measurements of more than 2 weeks.
Path length of the lateral wall or outer wall of the cochlea to the first turn (360°) was calculated using the formula as described by Escude et al. . Statistical analyses were performed using Student’s t -test and two-way ANOVA. A p value of less than 0.05 was considered statistically significant. The study was approved by our local institutional review board.
2
Method
High-resolution computed tomographic (HRCT) temporal bone data were obtained from randomly-selected patients with normally-developed cochleae who reported with otologic disease. Exclusion criteria were disease process involving the cochlea and congenital cochlear anomalies.
HRCT was performed with Aquilion ONE (Toshiba Medical Systems, Tokyo, Japan) with parameters of mA, 200; kV, 120; FOV: symphysis menti to vertex; helical pitch 41; section thickness, 0.5 mm; and reconstruction index, 0.4 mm. The 3-dimensional data were reconstructed on a Vitrea workstation (version 6.2.2096.3209; Vital, Toshiba Medical Systems). A double-oblique coronal reformatted image for each ear was obtained ( Fig. 1 ). This was rendered with a 1.0 mm thick reformat in order to visualize the basal turn from the round window to the opposite outer cochlear wall on a single image. Window settings were set at window width/center of 4000/400. The cochlear distance (distance A ) was measured from the mid-round window transecting through the mid-modiolar axis to the opposite wall of the basal turn and distances were recorded to the nearest 0.01 mm. The measurements were recorded by 2 independent radiologist observers. Each observer made 2 sets of measurements of each cochlea with an interval between measurements of more than 2 weeks.
Path length of the lateral wall or outer wall of the cochlea to the first turn (360°) was calculated using the formula as described by Escude et al. . Statistical analyses were performed using Student’s t -test and two-way ANOVA. A p value of less than 0.05 was considered statistically significant. The study was approved by our local institutional review board.
3
Results
HRCT temporal bone data was obtained for 161 patients. Four patients were of various other races and excluded from analysis. Study group therefore comprised of 157 patients (314 cochleae; 92 male, 65 female; mean age, 50 years; age range, 9–96 years). Thirty-nine percent were Chinese, 38% Malay and 22% Indian. There was excellent inter-observer reliability (intraclass correlation coefficient = 0.946) and intra-observer reliability (Pearson’s correlation coefficient, r = 0.84) for measurement of distance A . Averaged results from the 4 measurements for each cochlea were used for data analysis.
Distance A varied significantly amongst the patients and ranged from 8.10 mm to 10.31 mm. This corresponds to cochlear basal turn lengths ranging from 19.71 mm to 25.09 mm, a difference of 5.38 mm.
Mean distance A was found to be statistically different between the two sides of the ears (right 9.10 mm, left 9.07 mm, p = 0.0069).
Significant gender ( p = 0.0016) and racial differences ( p = 0.04) were found between the cochleae. Mean distance A was 9.17 mm (range 8.13 mm–10.31 mm) in males and 8.97 mm (range 8.10 mm–9.89 mm) in females. This corresponds to mean basal turn lengths of 22.32 mm and 21.83 mm respectively.
With regards to the different racial groups, mean distance A was 9.11 mm (range 7.90 mm–10.24 mm) in the Chinese, 9.11 mm (8.03 mm–10.40 mm) in Malays and 8.99 mm (range 8.00 mm–10.11 mm) in Indians. Chinese males had a mean basal turn length of 22.51 mm (range 20.63 mm–24.31 mm), Chinese females 21.81 mm (range 19.71 mm–23.67 mm), Malay males 22.39 mm (range 19.87 mm–25.09 mm), Malay females 21.88 mm (range 20.42 mm–23.23 mm), Indian males 21.93 mm (range 19.78 mm–23.92 mm) and Indian females 21.81 mm (range 20.38 mm–24.21 mm). This is summarized in Fig. 2 .
