Anatomical Landmarks and Branching Patterns of the Greater Auricular Nerve

Abstract

Study Design: Descriptive cross-sectional study. Background: The greater auricular nerve (GAN) courses over the sternocleidomastoid muscle (SCM) to supply the area of skin over the parotid gland (PG), the lower auricle and over the mastoid. It is vulnerable to injury during rhytidectomies and parotidectomies, resulting in sensory losses and pain. Although previous studies have identified suitable landmarks, injury to the GAN in the Kenyan setting still occurs. This study therefore aimed at identifying specific landmarks for the GAN and describing its branching pattern. Objective: To determine the anatomical landmarks of the GAN and its branching patterns. Methods: Forty six nerves were studied. The skin and fascia of the neck was carefully dissected to reveal the platysma muscle, which was reflected to expose the GAN. The distance of the emergence of the GAN on the posterior border of the sternocleidomastoid muscle as measured from the mastoid process (MP) was measured. Its perpendicular distance from the tragus to the point of branching was also measured. Its distance to the external jugular vein (EJV) was taken using a ruler and a pair of dividers. Finally, the nerve was described according to McKinney’s point. The pattern of branching was described as either type 1 (no branching), type 2 (2 branches) or type 3 (3 branches). The position of branching was classified as either anterior, posterior or middle. Collected data was coded into SPSS software (Version 21.0, Chicago, Illinois), and means ± standard deviation were calculated. Representative photos were taken. Results: The mean distance of the point of emergence of the nerve was 9.13 cm +/− 1.66 cm from the MP, while its distance from the tragus was 6.93 cm +/− 1.55. It was also located at a distance of 1.67 cm from the EJV. It mainly bifurcated into two branches (55.6%) and trifurcated in 4.4% of the cases. It remained undivided in 40% of the cases. In our study, the nerve mainly bifurcates in the anterior third of the SCM (22.2%). Conclusions: The GAN in our population mainly bifurcates, and it is more likely to divide closer to the parotid gland. The data presented in the study may be helpful in avoiding its iatrogenic injury.

Introduction

The GAN (greater auricular nerve) is the largest superficial nerve from the cervical plexus [1]. It courses posterior to the SCM to emerge behind the posterior border of the SCM at the nerve point in the neck [2]. It then courses obliquely and superiorly over the belly of the muscle [3] before terminating into an anterior and posterior branch. The nerve carries sensory innervation to parotid fascia, lower auricle and skin over the mastoid process [4]. Variations have been noted especially in the nerve’s course and branching pattern.

Studies have tried to identify landmarks for the GAN. An example of such landmarks is McKinney’s point, which lies 6.5 cm below the external auditory meatus (EAM). At this point, the GAN is said to be most superficial [5]. McKinney’s point is however highly variable and it is documented that the most superficial part of the nerve is actually the superior third of the course of the muscle [6]. Other landmarks of the GAN that have been used to describe its location include a point, 55–105 mm on the posterior border of the sternocleidomastoid muscle as measured from the tip of the EAM [7,8,9]. In addition, Ozturk et al suggested the use of a 30° angle between a vertical limb perpendicular to the Frankfurt horizontal and a second limb drawn posteriorly from the mid lobule, as a means of locating the nerve. Despite the documentation of these landmarks, there still exists iatrogenic injury to the GAN in our setting with paucity of data on its landmarks.

Studies have also reported variance in the number of branches of the GAN, ranging from no branches at all to 4 branches [10]. In addition, the position at which the nerve branches also varies. The current identified positions of branching include: at the superior third of the SCM (type 1), branching at the mid-third of the SCM (type 2), branching at the inferior third of the SCM (type 3) and no branching (type 4) [6] .Understanding the course and branching pattern of the GAN therefore is highly important in avoiding its iatrogenic injury during parotidoiectomies [11,12]. Currently, studies have shown an increase in rhytidectomies and parotidectomies done, with the number of facelifts being as many as 116,086 in 2011 in the USA [7]. In these procedures, the most commonly injured nerve is the GAN with an incidence of up to 6–7 % [5,13]. This injury may lead to sensory deficits and pain which may hinder the quality of life of patients [14].

Methodology

A total of 46 GANs obtained from 23 formalin fixed cadavers in the routine dissection laboratory at the department of Human Anatomy, University of Nairobi were studied. Ethical approval was sought and provided by the Department of Human Anatomy, University of Nairobi as per the Kenyan constitution.

First, the skin and fascia of the neck was carefully dissected, then the platysma was reflected to expose the GAN.

The distance at its emergence from the MP as measured at the posterior border of sternocleidomastoid, the perpendicular distance from the tragus to its point of branching and the distance between the nerve and the external jugular vein were measured using a ruler and a pair of dividers. The position of the GAN was further described according to McKinney’s point (Figure 1). The pattern of branching was described as either type 1(no branching), type 2 (2 branches) or type 3 (3 branches). The position of branching was also classified as either anterior, posterior or middle.

Collected data was coded into SPSS software (Version 21.0, Chicago, Illinois), and means ± standard deviation were calculated. Photographs were taken using a using Samsung ES70 digital camera (12.2 megapixel resolution). Data was presented in photographs and tables.

Results

Anatomical Landmarks

The GAN emerged 9.13 cm +/− 1.66 on the posterior border of the sternocleidomastoid muscle as measured from the tip of the mastoid process (range of 5.4–12.0 cm). Further, it branched 6.93 cm +/− 1.55 on an imaginary line drawn perpendicularly from the tragus of the EAM (range of 3.2–10.0 cm). The GAN was also constantly superior to and parallel to the EJV and the posterior border of the platysma over the body of SCM. The mean distance of the GAN from the EJV was 1.67 cm +/− 1.11 with a range of .5–4.4 cm in 16 cases. The EJV was absent in the remaining 8 cases. Some tributaries of the EJV also crossed the GAN.

Branching Pattern of the GAN

In 55.6% of the cases, the GAN bifurcated into an anterior branch that supplies area of skin over the mandible and goes into the substance of the PG and a posterior branch that courses towards the auricle (Figure 2A and 2B). The nerve did not branch in 40% of the cases, but terminated by going into the substance of the PG at the angle of the mandible (Figure 3A and 3B). In 4.4% of the cases, the nerve had 3 branches with a branch going to the angle of the mandible, the inferior part of the ear and the skin over the MP (Figure 4A and 4B). In one case, the anterior branch of the GAN divided into two primary branches and then the posterior branch divided into 3 secondary branches for the lower auricle (Figure 5A and 5B).

The nerve divided in the anterior third of the SCM in 22.2% of the cases; this is closer to the PG and to the MP in the superficial third of the SCM (Figure 6A and 6B). The nerves divide at the posterior third of the SCM in 20% of the cases, either by emerging at the nerve point as two separate branches or emerge as a main trunk (MT) and then divide into an anterior and a posterior branch (Figure 7A and 7B and Figure 8A and 8B, respectively). It divides in the middle thirds in 20% of the cases (Figure 9A and 9B). This is closer to the lower parts of the SCM. In another 17% of the cases, the nerve did not branch over its course on the SCM but instead branched within the substance of the PG (Figure 10A and 10B).

Discussion

Anatomical Landmarks

The GAN is said to be at a distance of 5.5–10.5 cm from the EAM along the posterior border of the SCM [8] which is about a third to half of the distance from the external auditory canal to the clavicular end of the SCM [7,9]. The distance of the GAN at the posterior border of the SCM from the MP in our setting was approximately 9.13 cm, which is about two thirds of the length of the SCM, given that the average length of the SCM is 12.47 cm. Thus in our setting, the GAN seems to emerge from the posterior border of the SCM lower than previously reported. This could be due to differences in the lengths of the SCM in the different populations.

The point of branching of the GAN is at a distance of 6.93 cm from the EAM. Lefkowitz et al reported this as 4.9 ± 1.1 cm in the United States. The measurements differ possibly due to the difference in the reference points. Irrespectively, with the two points suggested, one can still trace the course of the GAN over the SCM.

The GAN was at a distance of 1.67 cm from the EJV and runs superior and parallel to it in the Kenyan population. This corresponds to reports in literature that the EJV lies at a distance of 1–2 cm from the GAN [15]. The result obtained in our study is slightly higher compared to literature, which found an average of 1.17 cm [16]. Lefkowitz et al reporteda similar value of 1.6 cm distance between EJV and GAN at the posterior border of the SCM. The lowest value reported in literature is .05 cm [5]. The difference obtained may be due to difference in the position along the course where the measurement was taken.

Our study found that the nerve consistently came to lie below McKinney’s point at the mid body of the SCM. This supports claims by some studies that have suggested that the point is highly variable and therefore, not very reliable [6,10], which is contrary to what was described by McKinney.

Branching Patterns

The GAN in our setting mostly bifurcated into two (55.6%). It did not bifurcate in 40% of the cases and trifurcated in 4.4% of the cases. In cases where the nerve bifurcated, it yielded an anterior and a posterior branch. The trifurcated nerves gave one branch to the parotid fascia, another to the lower auricle and the last to the skin over the mastoid. Sharma et al [17] reported a higher frequency of trifurcations compared to our population. Our findings are similar to data from the South African [10] and American [7] populations where, the most common number of branches were 2. Second, the nerves with 3 branches in our study included 2 posterior branches and 1 anterior, while other studies have reported higher cases of 2 anterior and 1 posterior branches being given out [10].

The branching of the GAN in our case was mainly within the anterior third of the width of the SCM closer to the PG, as opposed to the posterior and the middle third. Other studies have reported branching position in terms of the length of the SCM. Lefkowitz et al reported the most common position of branching as the superior third of the SCM close to the PG.

The slight differences noted in the GAN’s branching patterns may be due to embryological factors that regulate the development of the nerve. Our findings though do correspond in major parts to previous findings, and as such, incisions close to the angle of the mandible should be done carefully to avoid damage to the nerve.

In conclusion, the present study presents a detailed account of the anatomy of the GAN among a Kenyan population. It also describes two points that can be used to landmark the GAN as it courses over the body of the SCM. The GAN in our population remains undivided more often than it trifurcates and it is more likely to bifurcate closer to the parotid gland. The data presented in the study may be helpful in avoiding injury to the nerve during surgery.