The importance of anatomy in optimising injection safety

Introduction

In aesthetic medicine, anatomy plays an essential role in injection safety. Precise knowledge of the anatomical structures of the face helps to minimise risks and optimise results. Facial anatomy is divided into two main approaches: Descriptive Anatomy (static structures) and Dynamic Anatomy (muscle movements and facial expressions). A successful injection must take account of these two aspects to ensure a natural result and avoid complications. How can knowledge of descriptive and functional anatomy optimise the safety and efficacy of injections in aesthetic medicine?

Descriptive anatomy: understanding the structure of injected tissues

Descriptive anatomy helps to identify the structures to be protected.

A) The skin and its particularities according to the areas of the face

The anatomy of the skin influences the choice of product, the injection technique and the final result. Consideration must be given to

• Skin thickness and composition (e.g. thin eyelids vs. thick forehead).
• Presence of sebaceous and sweat glands (oily vs. dry areas).
• Injection depth adapted to the anatomical planes.

The choice of filler depends on the injection plane:

• Epidermis: low permeability → low viscosity product.
• Dermis: high density of extracellular matrix → low viscosity and high spreading capacity
• Superficial regions: low tissue pressure → product with low elasticity to avoid over-correction.

The skin of the eyelids is particularly delicate (epidermis 0.05-0.1 mm vs. 1 mm elsewhere). It contains few enzymes and glands, requiring a specific hyaluronic acid to limit irregularities and ensure a natural result.

B) The superficial musculoaponeurotic system (SMAS), the facial skin muscles and their key role

The superficial musculoaponeurotic system (SMAS) supports tissues and influences facial expression and ageing. Its anatomy guides the depth and technique of injection

• Sub-SMAS injection: recommended for volumising fillers (nose, jawline, forehead).
• Specific areas: SOOF (Sub Orbicularis Oculi Fat) for the cheekbone, SOOF labial for the chin.
• Choice of product: hyaluronic acid resistant to muscle pressure is required.

The skin muscles (Masseter, Platysma, Depressor Anguli Oris) interact with the SMAS and increase in tone with age. Botulinum toxin relaxes them and restores the balance between lowering and elevating muscles, thus treating the cause (hypertonia) before the consequence (ptosis and tissue loss).

C) Vascular structures: high-risk areas

It is crucial to know the course and depth of the facial artery and its branches. The facial artery has major anatomical variations, both in terms of its course and its depth. These variations directly influence the vascular risk of injections, particularly in critical areas such as the nasolabial fold, the lips and the nose.

a. Classic’ course of the facial artery

The facial artery arises from the external carotid artery, opposite the mandibular angle, just above the lingual artery. It first descends under the jaw, then curves upwards to cross the lower edge of the mandible. This crossing is generally made an average of 2 cm in front of the masseter muscle (the facial vein is lateral to it, an average of 1 cm from the masseter muscle). It follows an oblique path upwards and forwards, crossing the submaxillary gland before crossing the mandibular bone margin. It is therefore dangerous to inject in a deep pre-periosteal plane, on the mandibular line, in front of the masseter muscle. The facial artery then travels to the labial commissure under the platysma muscles and then under the Depressor Anguli Oris muscle. The facial artery gives rise to the superior and inferior labial or coronal arteries. The projection of these arteries to the lips is submuscular (behind the orbicularis muscle of the lips), at the junction of the dry lip and wet lip of the red lip. Injections must therefore be strictly submucosal. The bifurcation is classically submuscular, under the modiolus, leaving a safety zone of 1 cm around the commissure. However, variations have been seen in dissection: the facial artery may give a superficial branch, above the DAO, within this so-called 'safety' perimeter. The superior labial artery gives philtral and columellar arterial branches. The facial artery then ascends along the nasolabial fold. Up to the narcular angle, the facial artery lies beneath the muscles and is therefore deep. From the nostril angle, it becomes more superficial and travels along the nasal pyramid. It gives rise to superior and inferior wing branches. These arteries are at the level of the nostril SMAS, and are therefore superficial and dangerous. The facial artery becomes the angular artery, which anastomoses with the supra-trochlear artery, a branch of the ophthalmic artery.

Although its classic path (88% of cases) is described as ascending and sinuous, several studies have shown frequent variations.

b. High-risk anatomical areas

• Glabella and forehead: The arteries responsible are the right and left supra-trochlear arteries. The supra-trochlear artery arises from the ophthalmic artery and leaves the orbit at its foramen. Its course is ascending, passing beneath the head of the corrugator muscle, and therefore deep. However, 1 cm from the upper bone margin, the supra-trochlear artery becomes superficial and travels above the muscular plane at the level of the forehead. Injections into the glabella and forehead, in the paramedian area, are therefore dangerous.
• The nasolabial fold: The facial artery divides into several superficial branches. There is a major risk of damaging the main branch of the facial artery.
• The nose: The arteries of the nose are at the level of the SMAS, and the anastomotic network is very dense. An injection above the SMAS with a dense product will create a compartment syndrome responsible for necrosis: the extravascular pressure is greater than the intravascular pressure. Injections into the nose should be made below the SMAS with a suitable product.
• The naso-jugal groove: The infra-orbital pedicle is located 11 mm below the inferior orbital bone margin. It is medial to the pupillary line. The foramen is easily palpable.
• The tear trough: The skin is thin with little subcutaneous tissue. The tyndall effect should be avoided by injecting a product dedicated to the area beforehand.
• The lips: The projection of the upper and lower coronal arteries is sub-muscular, at the junction between the dry lip and the wet lip of the red lip. The injection must therefore be submucosal with a product dedicated to the region, whatever the injection technique.major risk of damaging the main branch of the facial artery.

Functional anatomy: adapting injection techniques to facial dynamics

Functional anatomy guides injections to preserve the balance of the face. It includes the dynamic anatomy of the face, which is essential for obtaining a natural result from injections and respecting facial expressions. It takes into account the movements of the facial skin muscles and their interactions with the skin and underlying tissues. It is therefore important to know each facial skin muscle, its pathway and its function. The action of agonist and antagonist muscles (frontalis vs procerus, greater zygomaticus vs depressor anguli oris...).

Clinical application: a secure protocol to optimise results and avoid complication

A) Strategies to reduce vascular risks

Mapping of areas at risk is essential (ultrasound? MRI?...). The use of cannulas to minimise arterial penetration is essential. Obviously, a visit to the anatomy laboratory is essential before any aesthetic medicine is carried out.

B) Complication management: recognising and treating accidental injections

• Early signs of intra-arterial injection (blanching, pain).
• Emergency protocol in the event of necrosis or vascular embolisation (hyaluronidase).
• Importance of post-injection monitoring and appropriate touch-ups.

In conclusion, a good knowledge of anatomy is crucial to guaranteeing injection safety, preventing complications and ensuring safe, effective injections.