The Tragus-Facial Angle: Proposition of a New Method to Estimate the Trajectory of the Frontal Branch of the Facial Nerve

Context: Pterional craniotomy is considered a workhorse in skull base surgery. Inferior extension of the skin incision for expanded approaches (such as orbitozygomatic craniotomies and variations), requires wider dissections and can lead to injury to the frontal branch of the facial nerve. Properly understanding the trajectory of the facial nerve and its relationship with surface anatomical structures is essential for its intraoperative preservation. Several methods with varying levels of complexity have been proposed to estimate the position of the facial nerve, with no consensus. We aim to describe a new and simple method to estimate the position of the frontal branch of the facial nerve for frontotemporal approaches.

Methods: Formalin-fixed, silicon-injected cadaveric heads were dissected under the microscope from the stylomastoid foramen to the distal part (15 cm) of the frontal branch of the facial nerve. We measured the distances between the anterior edge of the tragus (point A) and two other points: a point on the same vertical line as point A at the intersection with the facial nerve (point B) and another point on a horizontal line from point A at the point of intersection with the frontal branch (FB) (point C). From these points, the angle between AB and BC (tragus-facial angle) was measured ([Fig. 1A]). Furthermore, we calculated the distance between a point 1 cm below point A on the vertical plane (A’) and the FB horizontally ([Fig. 1B]).

Fig. 1: Cadaveric dissections showing tragus-facial angle calculation (A) and A’FB measurement method (B).

Results: A total of 10 specimens were dissected. The right and left sides were dissected in the same proportion (50%). Data are summarized in [Table 1]. The mean tragus-facial angle was 31.91° (SD ± 3.71), ranging from 26.56 to 36.87°. The mean AB distance was 26.8 mm (SD ± 2.66). Thus, to estimate the trajectory of the frontal branch with the tragus-facial angle method, the branch would follow the BC line: approximately 2.5 cm below the tragus at an angle of 30 degrees with the vertical plane. ([Fig. 2]) The A’FB ranged from 9 to 13 mm with a mean of 10.92 mm (SD +- 1.18).

Table 1 Summarized data from 10 specimens

Measures	Mean	SD	Range
AB	26.8 mm	2.66	20–30
AC	16.3 mm	2.05	13–19
BC	30.8 mm	2.56	25–34
Tragus–facial angle	31.91°	3.71	26.56–36.87
A'FB	10.92 mm	1.18	09–13

Fig. 2 Left: Estimation of the position of the FB of the facial nerve according to the tragus–facial angle on the skin of a specimen. Right: Confirmation of the position of the facial nerve after dissection.

Conclusion: The method described has acceptable variability and represents a simple alternative for estimating the position of the facial nerve during anterolateral approaches.

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Artikel online veröffentlicht:
07. Februar 2025