Maturation of the internal auditory canal and posterior petrous bone with relevance to lateral and posterolateral skull base approaches

The intricate interrelationship of cochlear aperture and internal auditory canal diameter in pediatric normal petrous structures

PURPOSE

The cochlear aperture and internal auditory canal dimensions play a pivotal role in cochlear nerve health, yet their normative data exhibit significant variability. This study seeks to establish definitive normative ranges for these dimensions and investigate their interrelationship in the context of normally developed pediatric temporal bones.

METHODS

This prospective study included seventy-eight children under fifteen years, diagnosed with bilateral severe-to-profound sensorineural hearing loss and free from temporal bone deformities. Utilizing high-resolution computed tomography scans, we measured the diameters of the cochlear aperture at the mid-modiolar level and the internal auditory canal at the porus, midsection, and fundus. The associations between these dimensions were analyzed using Spearman's rank correlation and linear regression.

RESULTS

The mean cochlear aperture diameter was 2.16 mm, while the internal auditory canal diameters at the porus, midsection, and fundus were 6.60 mm, 4.27 mm, and 3.88 mm, respectively. Significant positive correlations were observed between the cochlear aperture and internal auditory canal diameters at the midsection (rs = 0.246, p = 0.029) and fundus (rs = 0.338, p = 0.002), as well as the average internal auditory canal diameter (rs = 0.233, p = 0.039). Regression analysis indicated that the internal auditory canal diameter at the fundus explained 17.5 % of the variance in cochlear aperture diameter (β = 0.179, p < 0.001).

CONCLUSION

This study established normative ranges for cochlear aperture and internal auditory canal dimensions in children, addressing past variability. These findings support cochlear implant planning, with a regression model offering a noninvasive tool to estimate cochlear aperture size. We also identified a unique, significant correlation between the two, highlighting a shared developmental pathway.

Morphometric analysis of the internal auditory canal by developmental change using three-dimensional computed tomography

PURPOSE

Our aim was to analyze the developmental changes in the morphology of the internal auditory canal (IAC) in both children and adults to gain a three-dimensional understanding that could enhance various surgical approaches.

METHODS

We retrospectively investigated 120 ears of 60 patients, ranging in age from birth to adulthood, using 3D-reconstruction data from normal temporal bone CT scans. The volume of the IAC in the temporal bone was reconstructed in 3D using Vworks version 4.0 (CyberMed, Seoul, Republic of Korea). Data were classified and evaluated by age group. IAC parameters were measured in the 3D objects using the axial, coronal, and sagittal sections of the CT scans.

RESULTS

The IAC volume significantly increased with aging from group 1 to group 2 (P = 0.01). The IAC volume also significantly correlated with the studied parameters (P = 0.001, for all). The Spearman's rho values between the volume of IAC and the six parameters were as follows: Wp (r = 0.619), Wv (r = 0.763), Wm (r = 0.697), Dp (r = 0.541), Dm (r = 0.692), and Lpf (r = 0.659).

CONCLUSIONS

IAC volume continues to develop through adolescence. Understanding the normal growth pattern and three-dimensional morphology of the IAC would be helpful for different surgical approaches.

Morphometric Analysis of the Retrolabyrinthine Approach to the Posterior Fossa

INTRODUCTION

The retrolabyrinthine approach provides shorter working distance and less cerebellar retraction compared with the retrosigmoid approach to the internal acoustic canal (IAC) and cerebellopontine angle cistern. However, exposure of the ventral surface of the brainstem and petroclival region may be restricted. Trautmann's triangle (TT), an area intimately related to this region, demonstrates significant anatomical variability, which may adversely affect the ease of the approach. The aim of this study is to evaluate anatomic parameters of the posterior fossa that may anticipate a challenging situation in approaching the IAC and the petroclival region through the retrolabyrinthine approach.

METHODS

It was performed a radioanatomic analysis of 75 cerebral angiotomography exams to identify parameters that could potentially reduce areas of surgical exposure.

RESULTS

Large variations were observed in the area of exposure of the TT (553%) and the height of the jugular bulb (234%). Shorter distances from the sigmoid sinus to the posterior semicircular canal and high-riding jugular bulb were associated with smaller areas of exposure. Dominant and laterally positioned sigmoid sinuses and less pneumatized mastoids were associated with potentially unfavorable conditions, including a narrower angle of attack to the IAC. Increased petrous slopes and petroclival angles were associated with smaller petroclival areas and shallower clival depths.

CONCLUSIONS

This study of the posterior fossa reveals remarkable anatomic variation in the region. These findings should be taken into consideration during the preoperative planning of retrolabyrinthine approaches in order to offer safer and more effective surgical procedures.

Human bony labyrinth as a sex indicator in subadults

Due to the durability and good preservation of the petrous bone in archaeological and forensic contexts, the value of the inner ear as a sex indicator has been evaluated in various studies. Previous findings suggest that the morphology of the bony labyrinth is not stable in the postnatal period. In this study, we aim to assess the sexual dimorphism of the bony labyrinth via analysis of computed tomography (CT) data obtained from 170 subadults (birth to 20 y.o.) and test whether the postnatal changes of the bony labyrinth affect the level of dimorphism in the inner ear. A set of 10 linear measurements of 3D labyrinth models as well as 10 size and shape indices were analyzed. Sexually dimorphic variables were used to produce sex estimation formulae with discriminant function analysis. The produced formulae allowed for the correct classification of up to 75.3 % of individuals aged from birth to 15 years old. Sexual dimorphism was not significant for individuals between 16 and 20 years of age. This study suggests that the morphology of the subadult bony labyrinth exhibits significant sexual dimorphism in individuals under 16 years of age, which can aid the forensic identification process. Although postnatal growth of the temporal bone seems to affect the level of sexual dimorphism present in the inner ear, the formulae created in this study could be used as an additional tool for sex estimation of subadult (<16 y.o.) remains.

Internal auditory canal volume in normal and malformed inner ears

Purpose

A narrow bony internal auditory canal (IAC) may be associated with a hypoplastic cochlear nerve and poorer hearing performances after cochlear implantation. However, definitions for a narrow IAC vary widely and commonly, qualitative grading or two-dimensional measures are used to characterize a narrow IAC. We aimed to refine the definition of a narrow IAC by determining IAC volume in both control patients and patients with inner ear malformations (IEMs).

Methods

In this multicentric study, we included high-resolution CT (HRCT) scans of 128 temporal bones (85 with IEMs: cochlear aplasia, n = 11; common cavity, n = 2; cochlear hypoplasia type, n = 19; incomplete partition type I/III, n = 8/8; Mondini malformation, n = 16; enlarged vestibular aqueduct syndrome, n = 19; 45 controls). The IAC diameter was measured in the axial plane and the IAC volume was measured by semi-automatic segmentation and three-dimensional reconstruction.

Results

In controls, the mean IAC diameter was 5.5 mm (SD 1.1 mm) and the mean IAC volume was 175.3 mm3 (SD 52.6 mm3). Statistically significant differences in IAC volumes were found in cochlear aplasia (68.3 mm3, p < 0.0001), IPI (107.4 mm3, p = 0.04), and IPIII (277.5 mm3, p = 0.0004 mm3). Inter-rater reliability was higher in IAC volume than in IAC diameter (intraclass correlation coefficient 0.92 vs. 0.77).

Conclusions

Volumetric measurement of IAC in cases of IEMs reduces measurement variability and may add to classifying IEMs. Since a hypoplastic IAC can be associated with a hypoplastic cochlear nerve and sensorineural hearing loss, radiologic assessment of the IAC is crucial in patients with severe sensorineural hearing loss undergoing cochlear implantation.