Morphometric and Topographic Features of Stylomastoid Foramen and its Clinical Significance in Facial Nerve Block

Abstract

Objective This study aims to determine the topographic localization of the stylomastoid foramen (SF) and its morphometric relationship with the surrounding bony landmarks. Design A descriptive anatomical study. Setting Anatomy Laboratory of the Faculty of Medicine. Participants Measurements were performed on 53 dry temporal bones. Main Outcome Measures On the inferior and lateral aspects of photographic images, lines and angles were defined. The most lateral end of the SF (SF1) and the transverse medial-lateral line that passes through the upper end of the anterior border of mastoid process (line 1) were used as reference points for topographic evaluation. The upper end of the anterior border of mastoid process (A) and the tip of mastoid process (B) were considered in defining angles. The dates about SF were evaluated using the ImageJ 1.46r software and digital caliper. Results SF1 was classified into three different types based on its topographical localization, stated as Type 1, Type 2, and Type 3. In Type 1, SF1 was located anterior to line 1 (54.7%). SF1 was located posterior to line 1 in Type 2 (34.0%). SF1 was located just over line 1 in Type 3 (11.3%). We also detected angular variations between these types in the inferior and lateral aspects. Conclusion The recommended angles of application are 30 degrees on the horizontal plane and 55 degrees on the sagittal plane for Type 1 when point B is considered. A needle length below 10 mm is more suitable to minimize the potential complications of the nerve block.

Keywords: Nadbath block; morphometry; stylomastoid foramen.

Fig. 1

The defined lines, reference points, and angles on left temporal bone on inferior aspect. ( A ) Line 1, Line 2, Angle 1, and morphometric reference points on inferior aspect. ( B ) Line 3, Line 4, and Angle 2 on inferior aspect; SF1: The most lateral end of the stylomastoid foramen; SF2: The most medial end of stylomastoid foramen; SF3: The most anterior end of stylomastoid foramen; SF4: The most posterior end of stylomastoid foramen. A: The upper end of anterior border of mastoid process; B: The tip of mastoid process; C: The medial end of tympanomastoid fissure; D: The anterior end of mastoid notch. MP, mastoid process; SP, styloid process; ZP, zygomatic process.

Fig. 2

The defined lines, reference points, and angles on left temporal bone on lateral aspect. A: The upper end of anterior border of mastoid process; B: The tip of mastoid process; X: The lateral projection of the most lateral end of the stylomastoid foramen. SP, styloid process; ZP, zygomatic process.

Fig. 3

The types of stylomastoid foramen were detected by localization of SF1 to Line 1 on the left side. SF1: The most lateral end of the stylomastoid foramen; A: The upper end of anterior border of mastoid process; Line 1: The transverse medial–lateral line that passes through the upper end of anterior border of mastoid process parallelly to the ground. MP, mastoid process; SP, styloid process.

Fig. 4

The recommended angles on transverse and sagittal plane and in Type 1 when the tip of the mastoid process is considered for Nadbath block. ( A ) The needle pushed forward at 30-degree angle on transverse plane with a lateromedial direction. ( B ) The needle pushed forward at 55-degree angle on sagittal plane with a posteroanterior direction.