4.1 Comparison to other studies
Many studied the parameters of the IAC using different modalities from radiographs, silicone casts, dissected and dry temporal bones and histological sections (13-18) and found that the dimensions and shape of the normal adult IAC varies widely. This is seen in the mean AP diameter at the base on the basal turn, the AP diameter at the midpoint of the IAC and the anterior and posterior length of the IAC in this study.
The mean of AP diameter at the midpoint of the IAC, which is referred to as the diameter of the IAC in most literature, was found to be 3.389 mm in the present study. In an earlier study by Valvassori & Pierce it was reported that the mean diameter of the IAC obtained from polytomography was 4.00 mm (19) while Olivares & Schuknecht using histological sections reported a mean diameter of 3.72 mm (20). A study on 14 Iranian male temporal bones reported a mean IAC diameter of 3.69 mm (15). Magnetic resonance imaging was used to investigate the morphology of 7572 IAC and reported a mean diameter of 5.93 mm (21). El Sadik and Shaaban (3) reported a mean diameter of 5.27mm. The diameter in the present study is smaller than reported in the literature. Variation in results suggest that there is a difference in IAC dimensions between populations and highlight that studies that exist cannot be used to infer or extrapolate information for a population. The difference in imaging modality, technique used to measure and the places in the canal where these measurements were taken may vary from one investigator to another. In the present study, the measurements were derived from the landmark coordinates.
Regarding the left and right IAC of an individual there is accordance between previous studies (4, 13, 22) and the present study. No statistically significant difference was found between the diameters of the IAC of the right and left IAC. The difference between the right and left IAC was found to be less than 1 mm in multiple studies, a difference greater than 2 mm between the two sides is indicative of the presence of a tumour (15, 23). In the present study the average difference of the AP diameter at the base of the basal turn, AP diameter at the midpoint of the IAC, and the opening diameter between the right and left side presented with no statistically significant difference which is consistent with the existing literature (24).
A significant difference was found between the APDB and APDM of the IAC between males and females, no statistical significance exists between the opening diameter anterior and posterior length of the IAC between males and females. The trend indicated that males generally have larger IAC diameters which corresponds with the findings by previous studies (4, 19, 21), while the lengths of the IAC between males and females are relatively similar. These results suggest that the diameters of the IAC could be used to differentiate between the sexes. Benson (14) found that diameter of the IAC viewed on CBCT tends to be larger and area in males however this difference tends to decrease with increasing age.
The factors that influence the differences in dimensions is indicative of the person-specificity of the IAC. This is significant for 3D computational cochlea modelling as well as for surgery (24). Most literature agree on the following:
1) An IAC with a diameter that is less than 2 mm is small (3, 19) 2) An enlarged IAC is one with a diameter greater than 8 mm (3, 21) 3) No significant difference between the right and left IAC exist (16, 21) 4) Dimensions of the IAC differ between different population groups and the sexes.
The IAC changes throughout development as it adapts to cranial growth and the final shape of the IAC is reached in adulthood (22). Recently the terms used to describe IAC shapes are funnel-shaped, cylindrical and bud-shaped (4). The shape of the IAC is usually determined by direct inspection (4, 24), however, in the present study the shape of the IAC was quantified by numerical parameters. The measurements taken, mainly the APDB and the opening diameter of the IAC, were used to classify each IAC into one of the three shapes.
A study by Kobayashi & Zusho (1987) investigated 300 IAC and reported 4.5% funnel-shaped IAC, 72.7% cylindrical and 22.8% bud-shaped IAC. Marques, Ajzen (4) reported 58.3% of 110 IAC were funnel-shaped, 30.9% were cylindrical and only 10.8% were bud-shaped IAC. In the present study the most prevalent shape was the cylindrical shape with 57.14%, 40.48% being funnel-shaped and only 2.38% being bud-shaped. This difference of proportions can possibly be accounted for by the difference in populations studied and the age distribution of the sample.