Frontotemporal Branch of the Facial Nerve and Fascial Layers in the Temporal Region: A Cadaveric Study to Define a Safe Dissection Plane
Keywords: Temporal fascia, facial nerve, dissection plane
Knowledge of the anatomical landmarks is an essential prerequisite for any surgical procedure. The relationship of the temporal fascial planes with the upper division of the facial nerve remains controversial.,,,,,,,,,,,,,,,
The frontotemporal division of the facial nerve (FTFN) supplies muscles of the forehead and face through temporal and zygomatic branches. Small branches of the frontotemporal nerve supply the orbicularis oculi, frontalis, and auricularis muscles. The anatomy of FTFN in tissue layers has been debated extensively in the literature. Many of these studies lack in exact dimension and measurements from fixed landmarks to identify the nerve intra-operatively.,,,,,,,,,,,
FTFN is prone to injury in surgical procedures like pterional, frontotemporal orbitozygomatic (FTOZ) craniotomy either directly or indirectly due to the stretching of a nerve. This cadaveric study aimed to identify the safe surgical landmarks to preserve the FTFN during surgery.
The anatomical relationships of the FTFN, superficial temporal artery (STA), fascial planes, and fat pads were determined on 10 cadaveric heads, that is (20 sides). Layer by layer dissection was done from skin to bone. Different names were described in the literature for fascia, fat pads, and nerve; therefore, preferred names are taken to avoid confusion [Table 1].
The skin incision started 3 cm below the base of the zygomatic arch, 1 cm anterior to the tragus, curving forward toward the frontal region ending in the midline at the hairline [Figure 1]. Incision involved skin, subcutaneous tissue superficial to the temporoparietal fascia. The temporoparietal fascia was dissected layer by layer inferiorly up to parotidomasseteric fascia and reflected anterolaterally with the scalp flap. The STA was identified and separated from the skin by dissection in the subcutaneous plane.
Two standard lines were drawn based on the skeletal landmark, the L1 line connecting the lateral bony canthus (LBC) to the upper level of the tragus, corresponding to the upper border of the zygomatic arch (Z) and line L2 perpendicular to L1 at lateral bony canthus [Figure 2]a. Distances were measured at points, where the branches of nerve and vessels crossed L1 and L2. The frontotemporal branch of the facial nerve usually had three rami; the point where anterior rami of a frontotemporal branch of facial nerve crossed L1 is the N1 point, middle rami N2, and posterior rami N3 point. If multiple branches were crossing then thickest was selected for an objective. If only two branches crossed, then anterior and posterior were considered. The point where the uppermost nerve crossed the L2 was designated as F, and distance from lateral bony canthus to where the frontal branch of superficial temporal artery (STA) cross the L2 was A1 [Figure 2]b.
Following fascial layers and fat pads were observed: temporoparietal fascia, superficial fat pad, the superficial layer of deep temporal fascia, intermediate fat pad, deep layer of deep temporal fascia, deep fat pad and temporal muscle [Figure 3].
The temporoparietal fascia was made up of multiple (3–4) layers above the zygomatic arch and these layers integrated with thin fibrous septa deep to the subcutaneous tissue. These layers were well defined just above the middle part of the zygomatic archand condensed with each other near the superior temporal line to become a thin single layer. This thin layer merged with deep temporal fascia and was attached to the superior temporal line. Anteriorly, this fascia inserted into the lateral orbital rim [Figure 4]. The temporoparietal fascia which descended below the zygomatic arch continued as a superficial musculoaponeurotic system (SMAS) and had no attachment to the zygomatic arch. The temporoparietal fascial layers above the zygomatic arch were easier to dissect from the superficial layer of the deep temporal fascia because of the fatty layer present between them, mainly in the middle part.
The superficial fat pad was present between the temporoparietal fascia and superficial layer of deep temporal fascia. The thickness of this fat pad was variable, depending on the habitu of the cadaver. Maximum thickness was found in the middle third. The superficial temporal vessels were found in the outer part of temporoparietal fascia whereas the frontotemporal branch of the facial nerve (FTFN) was observed in the deeper part of temporoparietal fascia and superficial fat pad [Figure 4], [Figure 5], [Figure 6] and [Figure 7].
There was tight adhesion observed over the zygomatic arch between the temporoparietal fascia and superficial layer of the deep temporal fascia, which covered the zygomatic arch. The deep temporal fascia which covers the temporal muscle is attached to the superior temporal line, and inferiorly this fascia splits into the superficial and deep layer [Figure 5]a. The superficial layers continued inferiorly through the anterior surface of the zygomatic arch to form parotidomasseteric fascia, and deep layer inferiorly attached to the posterosuperior margin of the zygomatic arch. These two facial layers were well separated in the middle third by the fat pad and fused in their anterior and posterior third. The superficial and deep layers become thicker as it descends towards the zygomatic arch from the superior temporal line [Figure 5].
In between both layers of deep temporal fascia, the fat pad present termed as an intermediate fat pad [Figure 6]a. Multiple fibrous septa extend from superficial layer of the deep temporal fascia to this fat pad and this fat layer was more densely adherent to superficial layer by multiple fibrous septa and was loosely adherent to the deep layer. This fat layer was observed near the middle third of the zygomatic arch and disappeared below the arch. Sometimes deep fat pad was present below the deep layer of deep temporal fascia [Figure 6]b.
Frontotemporal branch of facial nerve
Facial nerve gave frontotemporal branch within the parotid gland, courses within parotid, emerged from the anterosuperior surface and then crossed over zygomatic arch. Itusually consisted of more than two branches in each cadaver half [Figure 7]. The FTFN crossed the zygomatic arch as a single twig in 0.0% cases, as 2 branches in 25%, as 3 branches in 65% and as 4 branches in 10% of specimens. It crossed the zygomatic arch area and curved forward inside the deeper layers of temporoparietal fascia and in the superficial fat pad. As it coursed upward, it became superficial and near superior orbital rim, pierced temporoparietal fascia to supply frontalis and orbicularis muscles [Figure 8]a. The FTFN usually had 3 branches at the upper border of the zygomatic arch, called the anterior, middle, and posterior rami. The distance of the most anterior branch of the FTFN from lateral bony canthus (LBC) to 3.2 cm and posterior was 5.1 cm.
Superficial temporal artery
It traveled in a superficial plane of the temporoparietal fascia. The branches of the FTFN were anteroinferior to superficial temporal artery [Figure 4], [Figure 8]a and [Figure 8]b. The average distance of superficial temporal artery bifurcation from the upper border of the zygomatic arch was 24 mm. In 90% of cases, the bifurcation took place above the upper border of the zygomatic arch, and in the rest of 10% cases over the arch.
The distance from lateral bony canthus (LBC) to the points N1, N2, N3 where the FTFN branches (anterior, middle, posterior) crossed L1 measured and A1, F measured on L2.
(N1 point where anterior rami of FTFN crossed L1, middle rami N2 and posterior rami N3 point, F- distance from LBC to where the uppermost nerve crossed L2, A1 distance between lateral bony canthus to where the frontal branch of STA crosses the L2) [Table 2] and [Table 3].
The average distance between LBC and N1 was 3.2 (±0.55) cm, between LBC and N2 4.2 (±0.59) cm, between LBC and N3 5.1 (±0.49) cm, between LBC and F, 2.4 (±0.54) cm, and between LBC and A1, 3.4 (±0.42) cm The average width of FTFN across the L1 (upper border of the zygomatic arch) was about 1.9 cm and the distance between each ramus was about 1.2 cm [Table 4]. The average distance between a frontal branch of superficial temporal artery (STA) and the FTFN was 1.1 cm (range: 0.5–2.3). No significant difference was observed between the two sides in the t test.
Anatomical texts of fascial planes in the temporoparietal region are confusing and controversial because of dense adhesions and there is inconsistent use of nomenclature of fascial layers.
Fascial layers and fat pads
Mitz and Peyronie described that temporoparietal fascia presents as a fibromuscular sheet between the facial muscle and the dermis and forms part of the superficial muscular aponeurotic system (SMAS). Hing et al. observed that temporoparietal fascia was attached to the zygomatic arch. We observed that temporoparietal fascia was made up of multiples layers (3–4) above the zygomatic arch and these layers were integrated by fibrous septa [Figure 4]. Our results are similar that observed by Babakurban et al. and Tellioglu et al.
Campiglio et al. and Beheiry et al. observed that above the level of the zygomatic arch, the temporal muscle fascia separated into two sheets, the superficial sheet abutting the zygoma and continuing as parotidomasseteric fascia and the deep sheet abutting the posterior surface of the arch. The above two fascial sheets were well separated in the middle third by a fat pad and fused in the anterior and posterior third.
Yasargil et al. observed that at the orbital level the deep temporal fascia split into two layers. The superficial layer was attached to the lateral border of the zygoma and deep layer to the medial border of the zygomatic arch and both layers were separated by a fat layer.
Our observations are consistent with most of the studies,,,, inthat the temporoparietal fascia was made up of multiple (3–4) layers above the zygomatic arch which were integrated with thin fibrous septa deep to the subcutaneous tissue. These layers were well defined just above the middle part of the zygomatic arch. These multiple layers condensed with each other near the superior temporal line and became a thin single layer. This thin layer merged with deep temporal fascia and was attached to the superior temporal line. Anteriorly, this fascia inserted into the lateral orbital rim.
The deep temporal fascia was a thick, dense tough fibrous layer covering the temporal muscle; superiorly it attached to the superior temporal line, anteriorly to the orbital rim as a single layer, and split into two layers 2 cm below the superior temporal line [Figure 5] and [Figure 6]. The superficial layer of deep temporal fascia crosses the zygomatic arch and continues as the parotidomasseteric fascia and deep layer inserted to the posterosuperior edge of the zygomatic arch.
Frontotemporal branch of the facial nerve
We observed that FTFN was not a single nerve, but multiple branches crossing the zygomatic arch and anastomosing with each other [Figure 7]. The finding of the multiplicity of branches was consistent with the finding of Gosain et al., Sabini et al. but different from Pitanguy et al. who observed a single branch.
The FTFN crossed the zygomatic arch as a single branch in 0.0% cases, as two branches in 25%, as three branches in 65% and as four branches in 10% of specimens. This is at variance with the findings of some other studies, [Table 5].
In our study, we observed that the frontal branch of superficial temporal artery (STA) was located on average 3.4 cm from lateral bony canthus (LBC) on L2 and distance between artery and nerve was on an average, 1.1 cm. The mean distance between lateral bony canthus (LBC) and anterior rami of the frontotemporal branch of the facial nerve (N1) was 3.2 cm, between LBC and middle rami (N2) 4.2 cm, and between LBC and posterior rami (N3) 5.1 cm [Table 3]. Our results are consistent with that observed by Ishikawa.
Beheiry reported that frontotemporal branches of the facial nerve are found within layers of temporoparietal fascia and fat pad. Ammirati et al. 16 observed that the terminal branches of the temporal branch penetrated the temporoparietal fascia at different levels. In our study, we showed that the FTFN traversed in deeper planes of temporoparietal fascia above the zygomatic arch and then pierced the fascia near the superior orbital rim to supply frontalis and orbicularis muscle.
Dissection plane for the preservation of frontotemporal branch of the facial nerve (FTFN)
In literature, different approaches have been described for safe surgical dissection to preserve the FTFN.
Ammirati et al. observed that terminal branches of the frontotemporal branch of facial nerve penetrated the temporoparietal fascia at different levels and due to wrong identification of intermediate fat pad as the superficial fat pad, frontotemporal branches were injured during interfacial dissection, so he described submuscular dissection to preserve nerve branches. Stuzin et al.9 describes that dissection should start in the superficial fat pad and 2 cm above the zygomatic arch dissection deepen into an intermediate fat pad to preserve nerve. Coscarella et al. 13 proposed submuscular dissection (deep to the temporal muscle) or subfascial dissection (deep to the deep temporal fascia layer) based on the observation that the frontotemporal branch courses within the superficial fat pad. Beheiry and Hamid observed that the frontotemporal branch of facial nerve coursed within layers of temporoparietal fasciaand recommended interfascial dissection. Yasargil described interfascial dissection which comprises splitting of two layers of deep temporal fascia through an intermediate fat pad, thus protecting the FTFN. Splitting of two layers directly exposes the zygomatic arch and subperiosteal dissection can be done. In another recent article based on cadaver dissections, authors favored the subfascial method, below both the layers of deep fascia, for preservation of facial nerve. Facial nerve danger zones during various plastic surgery procedures have been the object of study for preservation of frontal, cervical and marginal branches. A safety zone for incisions used for supraorbital key hole surgery in relation to the frontozygomatic junction has been described.
In our study, we reviewed the results of various approaches that may be used for dissection near the zygomatic arch [Figure 9]. In addition, the advantages and disadvantages of these various approached were compared [Table 6].
In the first approach, we observed that temporoparietal fascia and the superficial layer of deep temporoparietal fascia are separated by the superficial fat pad and these two fascial layers are adherent to each other by fibrous septa over and below the zygomatic arch. The fat pad disappears below the zygomatic arch so dissection between these two layers increases the risk of injury to the frontotemporal nerve (FTFN).
In the second approach, both the layers of deep temporal fascia are separated by an intermediate fat pad and this fat pad loosely adheres to the deep layer of deep temporal fascia and contains fewer blood vessels, so dissection done in this relatively avascular plane and no risk of injury to FTFN. As the superficial layer of deep temporal fascia crosses over the zygomatic arch, so dissection can be continued in a subperiosteal plane over zygoma.
In the third approach, it is sometimes difficult to differentiate between superficial fat pads and deep fat pad, so the chance of injury to the facial nerve is more compared to the second approach.
In the fourth approach, the deep layer of deep temporal fascia is densely adherent to the temporal muscle and contains multiple small blood vessels, so dissection in this plane can lead to more blood loss, but risk of injury to facial nerve branches is minimal.
The fifth approach combines skin, fascia, and muscle flap, which minimizes the risk of injury to nerve, but due to the large bulk of temporal muscle there is decreased visualization along with the sphenoid ridge and due to excessive retraction, an increased chance of post-operative atrophy of temporal muscle.
The temporoparietal fascia was made up of multiple layers (3–4) above the zygomatic arch, the superficial temporal artery, and vein traversed in superficial layers and frontotemporal branches of the facial nerve in deeper layers of temporoparietal fascia. The frontotemporal branch of the facial nerve usually contained three rami over the upper border of the zygomatic arch and its distance from LBC was nearly constant, and the branches of FTFN were found inferior to the frontal branch of STA. Interfascial dissection between two layers of deep temporal fascia through intermediate fat pad was superior to other approaches because of the lack of facial nerve fibers in this plane, with negligible chances of injury to FTFN.
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Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]