Parotid surgery still has a high facial nerve paralysis risk. It is accepted that the risk of temporary facial paralysis is between 10–50% and 0.5% for permanent facial paralysis [2, 3]. While the injuries associated the facial nerve are still at these rates, the main purpose of parotid surgery is to remove the tumor safely without nerve injury [4]. Many landmarks have been identified for this purpose, and most of them help the surgeons in finding the FNT without nerve injury. In our study, we tried to focus on this topic. We discussed a new landmark, which we call the TMG (Akil groove).
The results showed that the distance between the TMG to the FNT changed minimally from patient to patient (Table 2). In addition to this finding, the most important points are that there is no distance between the end of the groove and the FNT, and the groove rises above the truncus in each case (Fig. 1,2). Both the ease of demonstration of the TMG intraoperatively and the high directivity of this groove to the facial nerve truncus make this groove an important landmark. Ecartation of the auricula in the posterosuperior direction and the wide opening of the surgical field make this groove more prominent. Thus, the surgeon can identify this groove more easily and quickly during parotid surgery.
When we look at other landmarks in the literature, the tragal pointer was defined by Conley [5], and it was stated that the deepest part of the tragal cartilage is helpful in finding the FNT; in other words, the relationship of the facial nerve with the anteroinferomedial tip of the tragal cartilage (tragal pointer) was demonstrated. Currently, the tragal pointer is the most commonly used landmark to demonstrate FNT, and the mean distance between the tragal pointer to the FNT is 13.6 ± 11.0 mm [6]. However, as the tragal cartilage is mobile, the position of the cartilage can be changed during retraction of the auricula, which is an important disadvantage of this landmark. In our study, we found that although the inferior border of the tragal cartilage forms the superior border of the TMG, the mobile tragal cartilage does not disturb the groove anatomy, and retraction of the auricula makes this groove more prominent. The tympanomastoid suture is a more fixed landmark to identify the facial nerve. In many studies, it is accepted as one of the best landmarks due to its fixed position and close relationship with the stylomandibular foramen [7, 8]. However, the use of this suture as a landmark has some disadvantages: it requires periosteal elevation, and the dense sternocleidomastoid muscle tendon structure adhering to the mastoid tip covers the tympanomastoid suture [9–11].
The posterior belly of the digastric muscle is again an important landmark. However, it is a more variable landmark than the tympanomastoid suture, and the distance to the FNT changes often, varying between 4–12 mm [6, 12].
The styloid process is also a parameter defined for the first time by Lapthrop and used as a landmark due to its association with the FNT [13]. Although its close anatomy to the facial nerve seems to be an important advantage, the deeper localization and its variable length are the disadvantages of this landmark [7, 14].
In addition to the classic landmarks, new landmarks have been tested to define the FNT for safer and easier surgeries. The posterior auricular artery is one of them. In a study conducted in 2018 performed on 10 cadavers, it is stated that the posterior auricular artery is crossed with the facial nerve at the inferior part of the stylomandibular foramen and that by following the posterior auricular artery, the FNT can be found [15]. However, study was performed on cadavers, and it is thought that in live surgery, the posterior auricular artery is very close to the facial nerve, and bleeding and deterioration of the surgical orientation may cause facial nerve paralysis during the identification of the posterior auricular artery. In a study published in 2019 and conducted on 8 cadavers, the relationship of the FNT and the parotid-mastoid fascia was examined. It was stated that especially as the parotid fascia travels over the FNT it can be used as a landmark [16]. Borle et al. described the relationship of a triangle and the FNT at a live surgery in 2019 [17]. At a study performed in 2015, by combining radiological findings with prospective clinical and cadaveric studies, it was stated that the facial nerve can be found easily and safely at the place between the posterior belly of the digastric muscle and the styloid process [18]. Meybodi et al., in a study in 2019 [19], noted that by finding the digastric branch of the facial nerve, the FNT can be easily found. Upile et al. defined the stylomastoid artery and its relationship with the FNT; it is located lateral to the facial nerve [20]. When we compare our study with other studies in the literature the stable anatomy of the groove despite the interventions during surgery, the clear anatomic relationship with the facial nerve and its sufficiency for being a lone landmark in finding the FNT are its advantages.