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Furthermore, the A-value can only be applied to inner ears with a normal anatomy 7, because malformed cochlea generally do not possess two and a half turns like the normal cochlea the cochlear duct is typically shorter 8.
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While the A-value has been recognized as a reliable measurement for CDL estimation for cochlear implantation, it does not account for variability in the curvature of the second turn of the cochlea. had suggested that the cochlear size influences greatly the final insertion depth of the electrode 2. If the beginning of the second turn of the cochlea is far away from the RW, the curvature of the second turn of the cochlea becomes very tight, affecting the insertion angle considerably. In the ‘cochlear view’ we have experienced that the curvature of the cochlear turn of the cochlea is highly variable 6. The position of the facial nerve in relation to the electrode array can also be identified accurately in the ‘cochlear view’.
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The ‘cochlear view’ is the best view in which to measure the A-value. This has been termed the ‘Cochlear View’ by Xu et al. In the oblique-coronal plane, the basal turn of the cochlea can be viewed fully, along with the round window (RW) entrance and the three semi-circular canals of the vestibular organ. later described how the A-value could reliably be used to estimate the CDL along the basilar membrane, after modifying the equations derived by Escudé demonstrating results that were in agreement with the outcomes of Hardy’s histological study 3, 4. introduced the ‘A-value’ measurement of the cochlea 2 and proposed mathematical equations to estimate the CDL along the outer wall using the A-value. The measurement of the cochlear duct length (CDL) was first reported by direct measurement in 1884 and many years later, the CDL was determined by indirect measurement and graphic reconstruction 1 following which Escudé et al. The novel C-value could be used to predict malformed anatomy, although it does not distinguish all malformation types. The proximity of the facial nerve to the basal turn did not relate to the type of malformation. The C-values of the normal cochleae compared to the cochleae with IP type I and IP type III were significantly different.
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The A-value does not predict the C-value. The A-values of the normal cochleae compared to the cochleae with cochlear hypoplasia, incomplete partition (IP) type I, -type II, and -type III were significantly different. 26 publications report on the A-value but they do not distinguish normal vs. The proximity of the facial nerve to the basal turn was evaluated subjectively. The A-value and novel C-value measurement were evaluated as predictors of inner ear malformation type. High resolution Computer Tomography images in the oblique coronal plane/cochlear view of 74 human temporal bones were analyzed. The objective was to determine the A-value reported in the literature, to assess the accuracy of the A-value measurement and to evaluate a novel cochlear measurement in distinguishing malformed cochlea. The A-value used in cochlear duct length (CDL) estimation does not take malformed cochleae into consideration.