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Botox injections are among the most common in cosmetic procedures, but many doctors have experienced inconsistent results despite the injection sites and dosages seeming correct. The problem often isn’t about ‘how much to inject’, but rather a lack of understanding of facial muscle structure and injection diffusion patterns. In this article, we will review some of the most common areas for facial injections, covering everything from anatomy to specific injection techniques.

01｜The Basic Logic of Facial Muscles and Injection Diffusion

Before proceeding to the specific injection area, doctors need to understand that the way muscles and skin connect in different areas of the face varies, which directly affects the appearance of wrinkles and the effectiveness of botulinum toxin.
Structurally, the muscles responsible for facial expressions can be broadly classified into three categories:
Directly attached muscles: These are primarily found in the eyebrow and perioral areas and are directly attached to the subdermis. Skin movement and muscle contraction occur almost simultaneously, resulting in the most pronounced facial expressions.
Indirectly attached muscles: A typical example is the frontalis muscle. Located between fascial structures, it transmits the force of muscle contraction to the skin surface via the fascial system.
Loosely attached muscles: These muscles are found in most areas of the face and transmit force to the skin via the SMAS system and the subcutaneous fat layer. This structure causes the force of facial expressions to disperse gradually.
In addition to structural differences, the extent of toxin diffusion is also affected by the injection depth: superficial injections result in more limited diffusion, while injections that contact the bone are more likely to act on the upper muscle belly. Prior to clinical injection, the patient’s dynamic facial expressions, eyebrow position and facial symmetry should be observed. Static and dynamic photographs should also be taken to assist in determining the injection points and dosage.

02｜Forehead Injection Techniques

The frontalis muscle contracts to produce forehead wrinkles. The shape of the wrinkles can reveal the direction of the muscle fibres.

Straight wrinkles: The frontalis muscle is more evenly distributed. Wavy wrinkles: This indicates that the frontalis muscle is more concentrated on the outer side.

If the horizontal wrinkles extend significantly outwards, the location should also be shifted outwards accordingly. If necessary, they may extend beyond the temporal crest and enter the hairline area of the forehead or temple.

①C-line: The key line that determines the injection layer. The skin on the forehead exhibits bidirectional movement: The lower forehead skin moves upwards (in the eyebrow-raising area), while the upper forehead skin moves downwards (in the hairline-lowering area). These two movements converge at approximately 60% of the total forehead length to form the C-line. This line divides the forehead into two injection areas.

②Injection strategy (level and dosage):
Above the C-line (upper frontal region) – Injection level: Bone-contact injection (superior periosteal layer). Single-point dose: 2–5 U.
Below the C-line (lower frontal region) – Injection level: Subcutaneous injection. Single point dose: 0.5–2 U
Injection points are usually arranged symmetrically in a zigzag pattern: two rows in the upper frontal region and one row in the lower frontal region. If the horizontal lines extend outwards, the injection points should also be shifted outwards accordingly.

③Risk boundaries of forehead injections
The frontalis muscle is the only muscle that lifts the brow.
Excessive injection below the C-line can cause the brow to droop, while insufficient lateral inhibition can result in ‘Mephisto brows’ (an excessive upward slant to the brow).
While injections above the C-line can reduce the risk of brow drooping, they can also make the forehead appear longer.
Figure 1 shows the spatial relationship between the frontalis muscle and the periorbital muscles.

④Common complications and issues associated with forehead correction injections include:
– Eyebrow drooping: Often associated with excessive inhibition of the lower jaw. Avoid over-injection in the area intended for eyebrow elevation.
– Mephisto brow (excessive elevation of the outer eyebrow): commonly seen in cases of insufficient inhibition of the lateral frontalis muscle.
For some patients, a pre-set salvage injection point can be established, located 1–2 cm above the highest point of the lateral eyebrow at a dose of 1–3 U.
For patients with receding hairlines, the injection point can be extended to the scalp area (1–2 U per point). If there is compensatory eyebrow elevation due to ptosis, more careful evaluation is required before treatment.

03 | Injection Techniques in the Glabella Area

Glabellar wrinkles are mainly formed by the contraction of three muscles: the corrugator supercilii, the depressor supercilii and the orbicularis oculi.
Vertical glabellar wrinkles are formed by the contraction of the corrugator supercilii, the orbicularis oculi and the lateral portion of the depressor supercilii. Horizontal glabellar wrinkles are mainly formed by the contraction of the depressor supercilii. These three muscles merge with the skin in the eyebrow hair area; therefore, the glabellar area often presents complex wrinkles. Figure 2 shows the spatial relationship of these muscle groups. As the orbicularis oculi muscle is strictly subcutaneous, the supraorbital region is more suitable for superficial injections.

①Anatomical basis of the three-point injection method

The interbrow injection primarily targets the bony origins of the depressor supercilii and corrugator supercilii muscles.
The depressor supercilii originates from the nasal bone, and the corrugator supercilii originates from the deep bony surface on the inner side of the eyebrow.
Both muscles have clear bony origins, which allows for bone-contact injection. Selecting a single muscle area as much as possible can reduce the impact on the orbicularis oculi complex. The relevant spatial relationship is shown in Figure 3.

②Injection strategy and dosage

The most common method is the three-point injection method, which targets the bony origins of the depressor supercilii and corrugator supercilii muscles.
Injection layer: Bone-contact injection
Number of injection points: Three points
Single point dosage: Approximately 10 units
If significant contraction is still visible at the upper edge of the eyebrow hair area, a supplementary injection can be performed superficially at the inner third of the eyebrow.
Injection layer: Subcutaneous
Dosage: 1–5 U
The layout of the three injection points and the supplementary injection at the inner third of the eyebrow is shown in Figure 4.

③Risks and prevention of ptosis

The most concerning complication of glabellar injection is ptosis, which is primarily caused by toxin diffusion or migration to the levator palpebrae superioris muscle. The risk can be controlled by choosing the right injection site:

Glabellar midline area: Bone-contact injection;

Supraorbital area: Avoid deep injections and use superficial subcutaneous injections instead.

If ptosis occurs, topical apraclidine eye drops can be used for treatment.

04｜Masseter Muscle Injection Techniques

①Masseter muscle structure and palpation location: When the patient clenches their teeth, the boundaries of the masseter muscle can clearly be palpated. The muscle is divided into superficial and deep bellies, which are separated by an intramuscular tendon. This can limit the diffusion of drugs and affect the arrival of toxins in the deep belly. The anterior border of the muscle may overlap with the risorius muscle. The relevant structures are shown in Figure 5.

②Mechanism of masseter muscle bulging

Masseter muscle bulging is mostly due to insufficient inhibition of the deep muscle belly. If the toxin only acts on the superficial muscle belly, bulging may occur in the superficial layer as a compensatory response. Therefore, the key to masseter muscle injections is not the number of injection points, but ensuring the toxin truly reaches the deep muscle belly.

③Injection strategy

The injection should target the deep muscle belly to inhibit the muscle. The key technical points are as follows:
Needle specification: 30G
– Needle length: ≥1 inch (approximately 25 mm)
– Injection level: Bone-contact injection
– Injection area: The lower 1/4 of the masseter muscle.
– Short needles may not penetrate the intramuscular tendon, resulting in insufficient deep drug delivery. The anterior border of the masseter muscle is adjacent to the horizontal fibres of the risorius and platysma muscles. If the injection is too superficial or diffuses to the surface, asymmetrical or limited smiling may occur. See Figure 6 for relevant proximity relationships.

④Dosage and clinical adjustment

The three-point injection method is commonly used in clinical practice. The usual dosage for Asian populations is 8–10 units per point.

Dosage must be adjusted according to muscle volume, sex and ethnicity. The horizontal molar axis should be assessed prior to treatment, as the higher side typically requires a higher dose. Following masseter muscle reduction, the temporalis muscle may enlarge as a compensatory response; this should be assessed during follow-up.

05｜Chin and mandibular border shaping

①Chin: Dynamic structure of the mentis muscle

The shape of the chin is mainly determined by the mentalis muscle. It originates from the bone surface below the mentalis-labial sulcus and its fibres attach to the dermis below. Contraction can lead to:
– Chin elevation;
– everting the lower lip;
an orange peel-like appearance.
These changes often appear in the area between the depressor labii inferioris muscles on both sides.

②Chin injection strategy:

Deep injection (superiosteal): Midline bone contact, 3–7 units (U), used to lengthen the chin and improve the mentolabial sulcus. For those with a wider chin, two injection points can be used.
Superficial injection (subcutaneous): 1–3 units per point. Used to improve the orange peel appearance during contraction. Total dose ≤6 units.
The midline should be maintained as much as possible during injection to avoid diffusion to the depressor labii inferioris muscle. For those with a longer chin, an additional injection can be given further down. If the depressor labii inferioris muscle is affected after treatment, supplementary injections can be administered to the contralateral depressor labii inferioris muscle to restore symmetry.

③Principles of Jawline Shaping

The platysma muscle exerts a strong downward force on the face. Inhibiting its downward pull can result in:
– a more defined jawline;
– improved midface lifting and volume;
– improved jawline sagging.
See Figure 7 for related contour changes.

④Mandibular Border Injection Strategy

Mandibular border shaping typically employs four equidistant injection points:
Needle size: 30G
Injection depth: Subcutaneous
Single point dose: 2–4 U
Total dose: 8–16 U
The injection points are located approximately 1 cm above the mandibular border. The innermost point is located approximately 1 cm below the corner of the mouth.
If the labiomandibular sulcus is prominent, the dose at this point can be increased accordingly, as this area also affects the depressor anguli oris muscle.
However, if this area is injected more medially and deeply, it may affect the depressor labii inferioris muscle, leading to asymmetry of the lower lip.
If platysma muscle bands are present, they should usually be treated concurrently as part of the same treatment course.

Botox treatment usually takes effect within three days, with the full effect being achieved within 10–14 days. Patients should observe the symmetry and intensity of muscle contraction. If asymmetry occurs, dosage adjustments or additional injections on the opposite side can be performed during follow-up visits.
Follow-up assessments should document static and dynamic facial expressions, paying particular attention to changes in eyebrow shape and frontalis muscle contraction. If the frontalis muscle is inhibited excessively or if upper eyelid movement becomes difficult, radiofrequency ablation can be performed in the periocular area to reduce the effects of the Botox treatment.