Unit 3 · Depth, direction & distribution
The muscle you're aiming for is not always the muscle the needle reaches first
Diagnosis and dose only matter if the product actually arrives where you intended. This unit works through the three physical decisions that determine that outcome: how deep the needle goes, which direction it travels, and how far the product spreads once it is there. Each has its own failure mode, and each failure mode has its own fix.
Framing
From diagnosis and dose to depth, direction and distribution
Diagnosis tells you which muscle to treat. Dose tells you how much product to use. Neither decision delivers a result on its own — the needle still has to place botulinum toxin type A (BTX-A) in the right plane, from the right angle, with a spread that matches the size of the target.
Depth, direction and distribution are three separate clinical decisions, not one continuous "technique". Each is answered differently for each muscle, and getting one wrong undermines a correct answer on the other two — a well-chosen entry point at the wrong depth still misses, and a correct depth reached from the wrong direction can still put product where you did not intend it.
Learn · Depth (D3)
Superficial planes, deep planes, and the muscle that hides beneath two of them
Accurate depth determines whether the product reaches the intended target. Too superficial risks a suboptimal muscle effect and spread to non-target structures; too deep risks reaching unintended muscles or neurovascular structures.
Superficial muscles — frontalis, orbicularis oculi at the lateral canthus, nasalis, platysma — lie immediately deep to the dermis and subcutaneous fat. A superficial to mid-depth injection reaches these effectively.
Deep muscles lie beneath more superficial muscular layers. The corrugator supercilii and depressor anguli oris are the clinically significant examples. The corrugator in particular lies beneath both the frontalis and the orbicularis oculi — a superficial injection here deposits product in the wrong plane entirely. Palpation of the muscle belly before injection is essential.
Mentalis sits centrally in the chin, embedded between skin and mandibular periosteum. Neither the superficial approach used for frontalis nor the deep approach used for corrugator applies here — a mid-depth injection into the muscle belly is the appropriate plane.
Practical depth guidance follows directly from this anatomy. For superficial muscles, a 30G 4mm or 6mm needle inserted at 45° or perpendicular reaches the appropriate depth of 2–4mm. For deep muscles such as the corrugator, a 30G 13mm needle inserted perpendicularly reaches deeper tissue, combined with active palpation of the muscle belly throughout.
| Plane | Example muscles | Needle | Insertion |
|---|---|---|---|
| Superficial | Frontalis, orbicularis oculi (lateral canthus), nasalis, platysma | 30G 4mm or 6mm | 45° or perpendicular, to a depth of 2–4mm |
| Deep | Corrugator supercilii, depressor anguli oris | 30G 13mm | Perpendicular, to deeper tissue, with active palpation of the muscle belly |
Needle length must match the muscle depth — not personal preference. A practitioner who defaults to one needle length across every muscle is treating depth as a habit rather than an anatomical fact.
Because the corrugator sits beneath two superficial layers rather than one, visual assessment of the surface alone cannot confirm you are in the right plane. Palpation of the muscle belly is the only reliable depth check available before the needle is inserted.
The corrugator supercilii lies anatomically:
Select an option to commit. The reasoning appears afterwards.
The corrugator supercilii lies deep, beneath both the frontalis and the orbicularis oculi — not in a single superficial layer, but beneath two of them. That is a materially different plane from the superficial muscles that sit immediately under the dermis.
This is why palpation before injection is non-negotiable in this muscle. A needle set to a superficial depth deposits product in frontalis or orbicularis oculi tissue rather than the corrugator, which is exactly the wrong-plane failure the depth guidance is designed to prevent.
Learn · Direction (D4)
Entry point, bevel and the vessels the trajectory is built to avoid
Direction encompasses the angle of needle entry, the orientation of the bevel, and the intended trajectory to the target muscle. It is inseparable from depth — direction and depth together define where the needle tip actually ends up.
Entry point selection should account for the target muscle's location, overlying structures to avoid — vessels, nerves, glands — and the diffusion pattern from the depot point. When treating the corrugator, entering from a lateral approach and angling medially and superiorly helps avoid the supratrochlear vessels, which run medially.
Fan techniques distribute product from a single entry point along a muscle belly, reducing the number of punctures for the patient while maintaining coverage. This is commonly applied to platysma and orbicularis oculi.
The supraorbital and supratrochlear nerves and vessels exit the supraorbital foramen approximately 2.7cm lateral to midline. Injections in the glabellar region should remain medial to a line 1cm lateral to the midpupillary line, or lateral to that safety corridor, to minimise the risk of neurovascular compromise.
You are planning corrugator injection points and want to minimise the risk of striking the supratrochlear vessels, which run medially. The recommended entry strategy is to:
Select an option to commit. The reasoning appears afterwards.
Entering from a lateral approach and angling medially and superiorly keeps the needle's path oriented away from the supratrochlear vessels, which run medially through this region. The entry point and the angle work together — one without the other does not achieve the same clearance.
This is the practical expression of the supraorbital safety corridor: the corridor tells you where the neurovascular structures exit, and the lateral entry with a medial, superior trajectory is how you route the needle around them while still reaching the corrugator's deep plane.
Learn · Distribution (D5)
How far a single injection point actually reaches
Once injected, BTX-A diffuses outward from the depot in an approximately radial pattern. The radius of clinically significant diffusion from a single injection point is approximately 1–1.5cm, influenced by volume injected, dilution used, and tissue characteristics at the site.
Multiple injection points are used when a muscle belly extends beyond the diffusion radius of a single depot — frontalis is the prime example, typically requiring 4–6 points. A small muscle such as procerus, by contrast, is adequately covered with a single central depot.
Extrusion volume directly influences diffusion spread — a larger volume in a single location increases the diffusion radius. This is relevant when broad coverage with fewer injections is the goal, but it must be balanced against the risk of unintended spread to adjacent structures, such as the levator palpebrae superioris in the upper lid from a corrugator injection.
Frontalis is typically treated with 4–6 injection points, while procerus is adequately covered with a single central depot. This difference is best explained by:
Select an option to commit. The reasoning appears afterwards.
Product diffuses radially from a depot to a clinically significant radius of approximately 1–1.5cm. The frontalis muscle belly is broader than that radius, so a single point leaves parts of the muscle untreated — hence 4–6 points. Procerus is small enough that one central depot's diffusion covers the whole belly.
Treating point count as a distribution decision, separate from depth and direction, is what stops a practitioner from either under-covering a large muscle with too few points or adding unnecessary punctures to a muscle that a single depot already reaches.
Predict · Dilution and spread
Concentrated or dilute — matching the preparation to the target
Dilution is the last variable in distribution, and it works in a direction that is easy to get backwards. Before reading on, commit to a choice.
You are preparing to treat the depressor anguli oris, where diffusion into the adjacent orbicularis oris risks an asymmetric lip droop. For the same dose, do you reach for a more concentrated preparation or a more dilute one?
Hold your answer before you open this. The value is in having committed to a position before you see the reasoning.
A more concentrated preparation — a smaller volume per unit — remains more localised and reduces the risk of spread. A more dilute preparation, with a larger volume per unit, diffuses more widely. Where precision is critical — DAO, procerus, corrugator origin — a more concentrated preparation reduces the risk of spread into adjacent structures.
Where broad coverage is the goal instead — platysma, frontalis — a more dilute preparation may reduce the number of injection points required. The choice is not about potency. It is about matching the spread of the preparation to the size and sensitivity of the target, which is why the same practitioner reaches for a different preparation for DAO than for frontalis on the same treatment day.
Unit 3 summary
Clinical takeaways
- Depth is anatomy, not preference. The corrugator lies deep, beneath both the frontalis and the orbicularis oculi. Needle length — 30G 4–6mm for superficial muscles, 30G 13mm for deep ones — must match that plane, confirmed by palpation rather than habit.
- Direction and depth are inseparable. Entry point and angle are chosen relative to overlying vessels — a lateral approach angling medially and superiorly keeps the corrugator entry clear of the supratrochlear vessels, and the supraorbital safety corridor (approximately 2.7cm lateral to midline) sets the outer boundary for the glabellar region.
- Diffusion is radial and finite. Product spreads roughly 1–1.5cm from a single depot. That radius is why frontalis needs 4–6 points to cover its belly and procerus needs only one.
- Concentration is a precision lever. A more concentrated preparation stays local and suits DAO, procerus and the corrugator origin. A more dilute preparation spreads further and can reduce the point count for broad-coverage targets such as platysma and frontalis.