BY GRACE YEONDU LEE, MADALINA ELENA RADU, MR JEFF DOWNIE , MR MARK DEVLIN AND PHILIP FERGUSON JONES / 04 JAN 2023
Using reliable surface landmarks for localizing important facial neurovascular structures, as well as their corresponding depth to the skin, plays a crucial role in minimizing injury risk and rate of complications. This is particularly important when injecting neuromodulating agents and soft tissue fillers. Most of these structures follow a reliable trajectory that can be identified by either a vertical or a curved line.
Surface landmarks are key in planning and completing injectable treatments effectively and safely. In this article, we describe a simple method of identifying these key anatomical points.
This vertical line, drawn through the medial canthus of the eye, crosses the location of two important landmarks of the upper and middle face.
- The supratrochlear neurovascular bundle
- The point where the facial artery gives away its terminal branch – the angular artery
Supratrochlear neurovascular bundle
The supratrochlear neurovascular bundle exits the superomedial orbit through a supratrochlear notch or foramen, at approximately 2cm (1.7-2.2cm) from the midline of the face. The skin landmarks described in the literature are the glabellar frown lines.1,2 They demarcate the corrugator and procerus muscles and have been reported to lie within 6mm of the neurovascular bundle.1
The supratrochlear artery (STrA), along with the dorsal nasal artery, are the two terminal branches of the ophthalmic artery, which in turn is the first intracranial branch of the internal carotid artery.2 At its origin in the face, STrA is relatively constant, lying at 3mm around the medial canthal line.3 It pierces the orbital septum and travels superiorly between the orbicularis oculi and corrugator muscles. At this level, the artery is found 1-2mm deep within the muscle layer and makes an ‘S’-shaped bend.4 In the lower forehead, the neurovascular structures are located within a deeper plane: either exiting a bony foramen/notch, under or within the muscle layer.1,3 At this level, the intramuscular or supra-periosteal injections should be done with caution to avoid the rich network of vessels and anastomoses between the STrA, ipsilateral angular and supraorbital arteries.1,3
The STrA pierces the frontalis muscle at 15-25mm above the supraorbital rim and gradually becomes more superficial, ascending in a subcutaneous plane.4 At the junction of the inferior and the middle third of the forehead, STrA gives away medial and lateral vertical branches.4These tend to run towards the midline from the level of the middle third of the forehead superiorly.4 Within the upper and middle forehead, dermal fillers are advised to be injected deep underneath the frontalis muscle, aiming towards a supra-periosteal placement of the product as at this level, one is likely to be underneath the vessels.5
Over its course, the STrA provides small branches to the overlying skin, muscles and periosteum. The supratrochlear nerve and veins are consistently seen to accompany the STrA in the forehead.1
The angular artery
As the line continues inferiorly it passes just lateral to the nasal base. This locates the position of the facial artery as it becomes the angular artery. This terminal branch of the facial artery lies relatively superficial at this point.
This vertical line passes through the medial limbus of the iris defining the anatomical locations of three important landmarks:
- Supraorbital neurovascular bundle (SO NVB)
- Infraorbital neurovascular bundle (IO NVB)
- Mental neurovascular bundle (M NVB)
Supraorbital neurovascular bundle
The SO NVB arises in the medial third aspect of the supraorbital rim on the vertical line crossing the medial limbus, at approximately 17-22mm lateral to the midline.4 It enters the face through either a foramen or, more frequently, a notch.
The supraorbital artery (SOA) is a terminal branch of the ophthalmic artery, which in turn is a branch of the internal carotid artery. As it exists, the orbit is usually surrounded by a ligament which is constant in cases where it passes directly over the rim, and absent when passing through a foramen.6 SOA runs from medial to lateral over the supraorbital rim and divides into a superficial and a deep branch.4 The superficial branch divides into two vessels: a vertical and a brow branch which have a similar trajectory to STrA. After a short course through the muscular layer, they enter the subcutaneous plane of the forehead.4 The deep branch gives way to three smaller vessels: medial, oblique and lateral rim, which run deep, in a sub muscular plane.4,6
In the lower forehead, most branches of the SOA and STrA are located deep to the frontalis muscle, while in the upper forehead they tend to run superficial to it.6 The artery pierces the frontalis muscle 15-40mm above the orbital rim before surfacing in the subcutaneous tissue between 40 and 60mm.7 As the vessels become more superficial, the filler should be placed at the level of the periosteum to decrease the risk of vascular occlusion.
The supraorbital nerve (SON) is a branch ophthalmic nerve (V1) which is the first and smallest division of the trigeminal nerve (CNV).8 It runs along the orbital roof and divides near the orbital rim into superficial and deep branches that closely follow the course of the arteries.6The largest branch of the SON accompanies the superficial vertical branch of the SOA, often travelling lateral to it.6 The superficial branches pass medially, entering the galea-frontalis layer and supply the skin of the forehead up to the anterior margin of the scalp, while the deep branches tend to run laterally between the galea aponeurotica and pericranium, providing sensation to the frontoparietal scalp.9,10 Some of its branches innervate part of the upper eyelid.
In practice, the naso-glabellar region is a particularly dangerous area. All autologous fat grafting and filler injections into the glabella region, inner canthus and nasal dorsum come with a risk of skin necrosis.11 Studies suggest that whilst nose injections come with a higher chance of vascular compromise, the glabellar augmentation seems to be linked with an increased risk of vision loss due to retinal artery compromise.7,11,12 This can be easily explained by the anatomy located in the upper third of the face where a rich vessel network between SOA, STrA and angular artery is located.
Moreover, the ophthalmic and facial veins drain into the orbits and can allow passage of large emboli, while the presence of choke anastomoses between the posterior and anterior ciliary vessels could also react with spasms after injection of filler product.11 Commonly reported complications are ophthalmoplegia, ptosis and temporary or permanent vision loss.11
A First Line:
B Supratrochlear neurovascular bundle
C Supraorbital neurovascular bundle
D Second Line:
E Infraorbital neurovascular bundle
F Mental neurovascular bundle
G Facial artery
H Superior and inferior labial arteries
I The facial nerve ‘tree’
J Superficial temporal artery
K Angular artery
Infraorbital neurovascular bundle
The IO NVB exits the face through an infraorbital foramen, at about 6.3-10.9mm below the orbital rim.7 Clinically, this anatomical landmark is located at one-third distance between the medial and lateral canthus in the same vertical plane, crossing the medial border of the pupil (the medial limbus) and at approximately a finger-breadth below the infraorbital rim.7,13Usually easily palpable, the foramen should be located prior to any procedure.7,13
The infraorbital artery (IOA) is a terminal branch of the maxillary artery, which in turn is a branch of the external carotid artery. IOA reaches the mid-facial region via the infraorbital foramen and supplies the skin of the malar and upper lip region, as well as the vestibular and gingival mucosa of the upper lip.14 After a short supraperiosteal course, it bifurcates into three branches: superior vestibular, nasal and zygomatic-malar branch, just under levator labii superioris.15 The nasal branch runs in an upward direction and seems to remain deep, just above the periosteum, and anastomoses with the angular, dorsal nasal or STrA. The zygomatic-malar branch tends to run horizontally, becoming more superficial as it runs towards the zygomatic arch, coursing through the malar fat pad and ending in the skin of the cheek.15 The vertical branch has the most constant path, passing through the orbicularis oculi and running downwards ending in the upper vestibular mucosa.14
The infraorbital nerve (ION) is the largest sensory branch of the maxillary nerve (V2), which is the second division of the trigeminal nerve (CNV). It supplies the skin overlying the medial cheek, lower eyelid, ala nasi, lateral part of the nose and the upper lip.8,16
In practice, the mid-cheek region can be separated into two areas by using the same vertical line that crosses through the medial limbus. Medial injections to the line in a supraperiosteal layer are to be avoided to reduce the intravascular injection risk. Retrograde propagation of product through the IO nasal branch may occlude terminal branches of the internal carotid artery, inducing ocular or cerebral compromise. It is thus advised to remove the cannula at least 2mm from the periosteum and deliver the product in small boluses, pushing the product from lateral to medial.8,15 Laterally to the medial pupillary line, injections should be performed on the periosteum to avoid the shallow vessel network of the zygomatic-malar artery which could result in skin necrosis.17
Mental neurovascular bundle
The M NVB arises from the mental foramen, which is frequently located on the same line as the supraorbital and infraorbital NVBs.18 There is a great variety described in the literature regarding the specific location of the foramen: at the level of the first or second premolar, between them, or even more medially, at the level of the canines.7,19 The mental artery, terminal branch of the inferior alveolar artery, in turn is a branch of the maxillary artery, supplying the lower lip and chin.18
Additionally, the ascending mental artery (AMA) and terminal branch of the submental artery are the main arterial supply to the top of the chin.20 After crossing the mandibular margin from the submental area, it travels upwards towards the lower lip.20 Most of the time, AMA travels superficially within the dense subcutaneous tissue and would end at the level of transverse labiomental fold.20 AMA would sometimes give away a terminal branch that travels through the mylohyoid muscle, into the floor of the mouth and anastomoses with the sublingual artery.20 This close relationship could potentially result in floor of the mouth or lingual infarction following chin augmentation.20 It has been suggested that there is frequently one dominant AMA that enters the chin at approximately 6mm from the midline, within the muscular plane and at a depth of 5mm from the skin.20
The mandibular nerve (V3), the third and largest division of the trigeminal nerve (CNV), contains both sensory and motor fibres. After leaving the skull it passes though the infratemporal fossa and bifurcates into an anterior and posterior branch. The posterior division enters the mandibular foramen as the inferior alveolar nerve, passing through the mandibular canal and exiting the face via the mandibular foramen as the mental nerve, providing innervation to the skin of the chin and lower lip. It is important to highlight that although most of the sensory innervation of the face is provided by the CNV, a small area around the mandibular angle and the auricular lobe is innervated by the auricular nerve. Moreover, both the mental nerve and the marginal mandibular branch of the facial nerve provide sensory innervation to the lower face, and the role of the direct connections between the trigeminal and facial nerves that are implicated in sensory recovery following facial trauma should be acknowledged.18
Considering the anatomical relationships described above, a deep supraperiosteal injection in the midline zone is the preferred technique as it is associated with a lower risk of vessel injury. An alternative entry point is at the level of the transverse labiomental crease, where the cannula would preferably be inserted downward, parallel to the course of the artery as it is safer to inject transversely in relation to the vasculature.7,12
First curve: the facial artery curve
We can follow the trajectory of the facial artery by drawing a medial concave line from its origin at the mandible to its termination at the medial canthus. The facial artery arises from the external carotid artery, where it passes through the submandibular gland, to cross the mandible just in front of the masseter muscle, where it can be palpated as it crosses the mandible midway between the angle and mental tubercle.6,10 Note that the artery is deep to the platysma. As the platysma lies subcutaneously, the needle should not pierce the fascia when injecting the jaw, so avoid the facial artery here.4
From the jaw, the artery then curves medially to a finger breadth’s distance lateral from the commissure of the lips, where it gives off the superior and inferior labial arteries.14 Here, the artery lies deep to, or within the muscle layer. To avoid both the facial artery and its anastomoses with the labial arteries, keeping injections superficial and subcutaneous is recommended.7
As it branches from the facial artery, the inferior labial artery briefly passes deep to the depressor anguli oris at the commissure, running submucosally along the lower lip in relation to the orbicularis oris.4 The superior labial artery similarly tends to run submucosally, giving off both superficial and deep branches along its course. To avoid intra-arterial injections and complications such as tissue necrosis, injections for the lips should not be performed outside of the vermillion border and should not exceed a depth of 2-3mm.6,11
As the facial artery ascends the face from the corner of the mouth, its course can be approximated using the nasolabial fold. Although the artery initially lies deep to or just above muscle, as it ascends to the level of the alar base and branches to give the lateral nasal artery, it quickly becomes superficial, putting it at risk of injury when injecting subcutaneously.7Injections in this area should be performed intradermally or pre-periosteally to avoid complications such as tissue necrosis or vision loss, with extra care taken as the artery ascends.7,11
After it gives off the lateral nasal artery, the facial artery continues to ascend the side of the nose as the angular artery. The angular artery is superficial to the superficial muscular aponeurotic system (SMAS) and can be palpated at the junction between the nasal bone and the maxilla.10 Injections should be performed deep in order to avoid the vessels here, and, as the angular artery forms anastomoses with the ophthalmic artery, caution is required in order to avoid vision loss which can occur as a result of retrograde embolism.7 The angular artery terminates at the medial canthus by forming an anastomosis with the dorsal nasal branches of the ophthalmic artery.7,21
Second curve: superficial temporal artery curve
The course of the superficial temporal artery (STA) can be followed by drawing a lateral convex curve from the temples to the forehead. From its origin within the parotid gland, it emerges medial to the auricle and crosses over the zygomatic process of the temporal bone, bifurcating as it enters the temporal fossa to give its frontal and parietal branches around 5cm above the process.17 The artery can be palpated at the temple as it passes in front of the ear.8
At the zygomatic process, the frontal branch of the STA travels within the superficial temporal fascia, deep to the subcutaneous level, and can be found 2cm above the arch, running closely to the temporal or frontal branch of the facial nerve.22 Injections should be performed deep in the periosteal plane or superficially in the subcutis, but not within the fascia.4
As the artery travels more medially and approaches the brow, its course also becomes more superficial, and just superior to the arch of the brow, the artery travels subcutaneously.6,7 There is a danger zone as the artery approaches the lateral border of the occipitofrontalis, which may be tentatively located as the area covered by the pad of the thumb when the tip of the thumb is placed on a vertical line through the lateral epicanthus and its radial border placed on the peak of the brow.23 The artery continues to travel across the forehead superficial to the occipitofrontalis muscle, remaining subcutaneous.23
Injections into the forehead must be placed superficially, no deeper than the mid-dermis. This is because small branches of the STA extend into the deep dermis, and occlusion can result in tissue necrosis or vision loss.5 As highlighted by the intersection between the STA curve and the second vertical line, the frontal branch of the STA forms an anastomosis with the supraorbital artery. Intra-arterial injections of the STA and its branches can lead to blindness from embolisms travelling to the ophthalmic artery, so care must be taken to avoid complications when injecting along the route of the STA.4
The facial nerve ‘tree’
The course of the facial nerve and its five branches can be mapped out by drawing the facial nerve tree. After the facial nerve emerges from the stylomastoid foramen, the facial nerve surfaces to the face through the parotid gland. From here, the nerve then splits to give five branches, which are, from superior to inferior: the temporal or frontal, zygomatic, buccal, marginal mandibular and cervical branches.24
The temporal branch innervates the muscles of the forehead, and its function can be tested by asking the patient to frown or raise an eyebrow. From the parotid gland, it travels over the middle third of the zygomatic arch, into the temporal fossa, within or just below the SMAS.7,18,25 As the nerve branch courses upwards and approaches the lateral border of the frontalis, its level becomes more superficial, and therefore when injecting near the temporal branch of the facial nerve, it is important to stay above the SMAS and not pierce fascia to avoid accidentally impinging upon the nerve. As mentioned with the STA curve, this lateral border can be tentatively located using the thumb. The temporal branch runs close to the frontal branch of the STA, with the nerve branches always lying anteriorly and inferiorly to the frontal branch of the STA.7,25 Locating the STA can be a useful method of locating the temporal branch of the facial nerve, which is further demonstrated by the meeting of the temporal branch of the nerve tree with the STA curve.23
The zygomatic branch innervates the lower muscles of the orbit and can be tested by asking the patient to tightly shut their eyes. Like the temporal branch, the zygomatic branch runs below the SMAS, and therefore injections should be superficial.25 To locate the zygomatic branch, a reference point of 2.5cm in front of the inter tragic notch on a straight line from the notch to the lateral canthus can be used.26 The zygomatic branch bifurcates to give upper and lower branches, with the upper travelling above the orbit to innervate orbicularis oculi and the lower travelling below the orbit to the nose and mouth.25
The buccal branch innervates the muscles of expression around the nose and mouth, and can be tested by asking the patient to puff out their cheeks and smile showing teeth.8,27 The buccal nerves travel in the sub-SMAS fat layer, crossing the masseter to pierce into the masseteric ligaments at its anterior border, then continuing to travel below the orbit to encircle the mouth.8 Injections in the buccal area should be superficial to avoid injuring the nerves.28 It is important to note that the buccal branch shows a great degree of variation, in terms of the number of ‘sub-branches’ it may have, and care must be taken when injecting in this area to avoid injury.29
The marginal mandibular branch innervates the muscles of the chin, and its function can be tested by asking the patient to downturn their lower lip. It courses near the inferior border of the mandible, always travelling superior to the border while it is anterior to the facial artery, and while it is posterior to the artery, travelling superior to the border in most patients.30 About a finger’s breadth below the mandible, the marginal mandibular branch curves medially at the angle.30 As the marginal mandibular branch leaves the parotid, it lies deep to the fascia, and is tightly bound as it travels superficial to the facial artery.25,30 When the nerve reaches the depressors of the lower lip, the marginal mandibular nerve courses deep to the muscle layer.25 Generally, due to the deep course of the nerve, as long as the needle does not pierce deep fascia, injury to the nerve can be avoided.
The cervical branch innervates the muscles below the chin, including the platysma. After exiting the parotid, the cervical branch travels deep to the SMAS, between the platysma (which lies within the SMAS) and deeper fascia.8,25 The cervical branch travels superficially to the marginal mandibular branch where they travel together, and because of this and its superficial course, the cervical branch is at risk of injury when injections pierce fascia.25 The branching point of the cervical nerve can be identified as being 1cm below the halfway point of a line drawn from the mentum to the mastoid process, from where the cervical branch proceeds to travel downwards to innervate the neck.31
Know your danger zones
Despite the variation in anatomy found within the head and neck region, there are consistent surface landmarks that the clinician can rely upon. The diagrammatic description we present is a simple aid to these points and how they relate to the key neurovascular structures in the face. This should be a useful, easy to remember aide for the aesthetic clinician.
Test your knowledge : Complete the multiple-choice questions
1. Which main branch of the trigeminal nerve supplies sensation to the upper lip?
2. The facial artery enters the face at which point?
a. Just in front of the masseter muscle in a superficial position
b. Just in front of the masseter muscle in a deep position on the bone
c. At the angle of the mandible
d. At the mandibular ligament
3. The facial nerve is primarily responsible for:
a. Motor supply to the muscles of facial expression
b. Sensory supply to the face
c. Motor supply to the muscles of mastication
d. Sensory supply to the muscles of mastication
4. A line dropped vertically running through the medial canthus crosses:
a. The infraorbital neurovascular bundle
b. The mental nerve
c. The supratrochlear neurovascular bundle
d. The oral commissure
5. Which statement is true about the mandibular nerve (V3)?
a. It is the third and largest division of the trigeminal nerve (CNV), contains both sensory and motor fibres
b. It is the sensory supply to the maxilla
c. It is the motor supply to mentalis
d. It supplies fibres to the sub-mandibular gland
Answers: B, B, A, C, A