Positional relationship between the external acoustic meatus and sigmoid sinus: an MRI study

Evaluation of the Sigmoid Sinus Morphology by Cone Beam Computed Tomography; Touchstone of the Posterior Cranial Fossa

OBJECTIVE

In this study, we aimed to analyze the relationship of the sigmoid sinus (SS) with the external auditory canal, facial nerve, and mastoid cells from an anatomic point of view, to define the position of the SS during transmastoid, translabyrinthine, retrosigmoid (lateral suboccipital) approaches, in tympanomastoidectomy and posterior cranial fossa surgery.

METHODS

In this study, the morphologic structures associated with the sigmoid sinus were evaluated in cone beam computed tomography images taken between 2015 and 2022. The images of 68 men and 106 women, aged 18-65 years, obtained from the archive of Ankara University Faculty of Dentistry, Department of Oral and Maxillofacial Radiology were analyzed.

RESULTS

The most common SS pattern was type II, with a rate of 60.8% (n = 209); the second was type III, with 20.6% (n = 71); and the least common was type I, with 18.6% (n = 64). Although the distance between the horizontal line passing through the external auditory canal and facial nerve and the anterior contour of the SS was highest in type I (right, 7.26 ± 1.62; left, 7.44 ± 0.97), it was lowest in type III (right, 4.40 ± 1.50; left, 4.84 ± 1.16) (P < 0.05).

CONCLUSIONS

This study highlights the importance of the SS position in surgery, with special reference to otologic, neurotologic, and posterior cranial fossa surgery. To avoid intraoperative complications, each patient should be evaluated preoperatively by appropriate radiologic methods.

The auricula as a new surgical landmark for the transverse-sigmoid-sinus-transition

Introduction

The transverse-sigmoid-sinus-transition constitutes an important landmark during a retrosigmoid craniotomy. Due to anatomical variations, the location is highly variable. Landmarks for identification of the anterior border of the sigmoid sinus have been described extensively, such as the mastoid notch, digastric point, external auditory meatus and crux of the helix curvature. There is a paucity of landmarks for the identification of the posterior border, however.

Research question

We examined the relationship between the transverse-sigmoid-sinus-transition and the most-posterior-part-of-the-auricula.

Material and methods

We performed a retrospective analysis of one-hundred patients (38 males and 62 females) who underwent cerebral MRI examinations at Antwerp University Hospital (Belgium). Using Brainlab®, the transverse-sigmoid-sinus-transition and most-posterior-part-of-the-auricula coordinates were calculated and compared. Left and right sides were compared in both the anteroposterior and craniocaudal axis.

Results

Mean age was 56.4 ± 16.1 years. Mean MPPA-TSST-distance in the anteroposterior direction was -1.93 mm (right) and -1.96 mm (left). Mean MPPA-TSST-distance in the craniocaudal direction was -5.16 mm (right) and -5.04 mm (left).

Discussion and conclusion

The transverse-sigmoid-sinus-transition seems to be located more anterior and caudal with respect to the most-posterior-part-of-the-auricula, meaning that it can be considered a save landmark. A correction of five mm needs to be applied in order to identify the inferior border of the transverse sinus. Left/right and gender had no significant influence. The most-posterior-part-of-the-auricula can be considered a fast and practical anatomical landmark for identification of the transverse-sigmoid-sinus-transition, without affecting operative fluency, especially during an emergency craniotomy.

Phenotypical variability of the sigmoid sinus in translabyrinthine and retrosigmoid surgeries

INTRODUCTION

We hypothesized that the cranial phenotype influences the shape of the posterior cranial fossa and the relative position of the sigmoid sinus.

MATERIALS AND METHODS

The topography of the sigmoid sinus was studied on 26 magnetic resonance venograms and 35 embalmed cadavers by morphometric analysis, dissection, and photo modeling techniques.

RESULTS

The data show that the transverse diameter of the posterior cranial fossa correlates positively with the laterolateral diameter of the skull. The majority of cases with the low-anterior position of the sigmoid sinus were recorded in the brachycephalic group (82%), while the high-posterior localization of the sigmoid sinus was typical for the dolichocephalic patients (63%). The results of the ANOVA test confirm the significance of differences.

CONCLUSIONS

The shape of the skull reflects the morphology of the posterior cranial fossa and influences the topographic characteristics of the sigmoid sinus that must be considered in the selection of surgical approach to the inner ear and pontocerebellar angle.