Succinct key points:
Three-dimensional computational modelling of the cochlea, Inclusion of IAC parameters; IAC shapes; Dimensions of the IAC; Deriving landmarks from CT scanning
Keywords : Cochlea; Internal acoustic meatus; Internal auditory canal; Internal auditory meatus; Landmarks; Shape; Three-dimensional computational modelling
Introduction
The use of 3D computational modelling allows for the prediction of current spread through the cochlea, resulting in a better understanding of the functioning of the implanted cochlea (1). The anatomical description of the cochlea is constantly being developed to incorporate more detail that may improve the accuracy with which the outcomes of cochlear implantation may be predicted (2). There are few studies where the morphology of the internal acoustic canal (IAC) is included in constructing 3D computational models of the cochlea (2, 3). In present 3D computational models of the cochlea the nerve has frequently been modelled as a smooth straight volume exiting the cochlea (1).
The rounded and smooth IAC is said to be on average 8.5 mm (5.5 to 10 mm) in length and about 4 mm in diameter (3, 4). Researchers found that there are variations in the length, width and shape of the IAC.
The IAC can be classified into to one of three shapes i.e. Funnel-shaped, cylindrical and bud-shaped (4). The shape of the IAC is usually noted by direct inspection of CT images. In a recent study by Marques et al., the shapes were metrically by taking the following measurements: 1) opening diameter of the IAC, which was measured from the concave part of the posterior lip of the IAC wall to the more medial border of its anterior border, 2) IAC length, measured from the midpoint of the IAC to the extremity of the canal in its concave portion, 3) anterior-posterior (AP) diameter, measured perpendicular to the straight line drawn from the midpoint of the IAC length uniting the anterior and posterior wall of the IAC, and 4) distance from the IAC to the vestibular aqueduct, which is the distance between the most concave wall of the posterior lip of the IAC and the medial border of the vestibular aqueduct (4). The IAC is said to be funnel-shaped if the opening diameter is larger in relation to the AP diameter, cylindrical if the opening diameter and the AP diameter are relatively similar in size, and lastly, bud-shaped if the AP diameter is relatively larger than the opening diameter.
Knowledge about the relationship between the IAC and the CoN is particularly important when CI surgery is considered as the CoN plays a vital role in the success of cochlear implantation. According to the existing literature a positive correlation exists between the size of the IAC and the size of the CoN (3, 5). A hypoplastic IAC has been found to be associated with CoN aplasia while CoN hypoplasia has been described in studies of human temporal bones in association with inner ear malformation and IAC stenosis (5, 6). Congenital sensorineural hearing loss has been proved to be well correlated with IAC stenosis (3, 7, 8). The most common cause of sensorineural hearing loss is said to be a lack of development of CNVIII (7). Tahir et al ., reported that the presence of IAC stenosis on CT scans images is indicative of CoN aplasia and poor performance with CI surgeries and recommended that the presence of IAC stenosis contraindicate cochlear implantation (5). This relationship between the CoN and the IAC supports the concept of using the IAC as a determinate of the path of the CoN since the size, shape and curvature of the IAC informs the shape and path the CoN.
Analysis of the IAC to determine the shape of the CoN is of significance for 3D computational reconstruction of the auditory system. The location of the CoN fibers within the IAC may affect the way in which the current spread is predicted (9-11).
No landmark set describing the shape of the IAC has been established yet. CT images are well suited to obtain the morphology of the IAC in live CI recipients (1). Therefore, it was precedent for this study to establish a landmark set to capture anatomical knowledge of the IAC using CT images. This knowledge will be applied to refine the description of the morphology of the CoN exiting the cochlea in user-specific 3D computational models and may benefit CI researchers and clinicians in understanding and predicting the outcomes of implantation.