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.