Landmarks of the facial nerve: implications for parotidectomy

Abstract

Facial nerve paralysis is a daunting potential complication of parotid surgery and is widely reported. Knowledge of the key landmarks of the facial nerve trunk is essential for safe and effective surgical intervention in the region of the parotid gland. In current practice, wide ranges of landmarks are used to identify the facial nerve trunk, however, there is much debate in the literature about the safety and reliability of each of these landmarks. The aim of this study, therefore, was to evaluate the relation of the surrounding anatomical structures and surgical landmarks to the facial nerve trunk. The anatomical relationship of the facial nerve trunk to the surrounding structures was determined after micro-dissection on 40 adult cadavers. The shortest distances between the facial nerve and the "tragal pointer", attachment of the posterior belly of digastric muscle, tympanomastoid suture, external auditory canal, transverse process of the axis, angle of the mandible and the styloid process were measured. In addition, these distances were compared in the right and left sides, males and females and edentulous and non-edentulous mandibles. The distance of the facial nerve trunk from each of the surrounding landmarks ranged from (mm): tragal pointer, 24.3 to 49.2 (mean 34); posterior belly of digastric, 9.7 to 24.3 (mean 14.6); external auditory canal, 7.3 to 21.9 (mean 13.4); tympanomastoid suture, 4.9 to 18.6 (mean 10.0); styloid process, 4.3 to 18.6 (mean 9.8); transverse process of the axis, 9.7 to 36.8 (mean 16.9); angle of the mandible, 25.3 to 48.69 (mean 38.1). The length of the facial nerve trunk from its point of exit from the stylomastoid foramen to its bifurcation into upper and lower divisions ranged from (mm) 8.6 to 22.8 (mean 14.0). The results demonstrated that the posterior belly of digastric, tragal pointer and transverse process of the axis are consistent landmarks to the facial nerve trunk. However, it should be noted that the tragal pointer is cartilaginous, mobile, asymmetrical and has a blunt, irregular tip. This study advocates the use of the transverse process of the axis as it is easily palpated, does not require a complex dissection and ensures minimum risk of injury to the facial nerve trunk.