Vertical Versus Horizontal Lip Injection

Understanding the Anatomy Behind the Debate

The question of whether a vertical injection crossing the vermilion border at a right angle is riskier than a horizontal injection running parallel to it has produced one of the more genuinely divided debates in aesthetic practice, with famous injectors lining up on both sides and audience surveys returning results that essentially split the room down the middle. An Instagram poll on this question returned 55% to 45% in favour of vertical injections being riskier, with a Facebook poll returning 51% to 49% in essentially the same direction, which is the statistical signature of a sector that has not collectively reached a working consensus on the underlying anatomy.

The way out of this kind of impasse is not to declare a winner based on personal preference, but to look at what each side is actually claiming about the position of the superior labial artery and the path of the needle through the tissue, and to ask which mental model fits the most reliable anatomical evidence we have. The reason this matters is that the injection that deposits the most product nearest the most likely position of the artery is the one that carries the most risk of vascular occlusion, and identifying that injection accurately requires a clearer picture of lip anatomy than most early training provides.

Why mental models are the right unit of analysis

A mental model is the three-dimensional image an injector holds in their head when making decisions in the chair, and every working clinician has one whether or not they have consciously articulated it. The honest starting position is that all mental models are low resolution, oversimplified, circumstantial, or possibly just wrong in places, including the most carefully developed ones. The point of articulating a model in detail is to make it available for testing and refinement, which is how individual practitioners and the wider sector improve over time.

This particular debate is therefore better framed as a question about whose mental model of the lip is more accurate, rather than which technique is correct. The intention is not to change anyone’s injection technique unilaterally, because every injection point varies in risk and every treatment plan balances aesthetic outcome against multiple categories of harm, with vascular occlusion being only one of those categories. The narrower question worth answering precisely is which angle of entry, with everything else held constant, is more likely to produce a vascular occlusion.

Where the superior labial artery actually runs

The superior labial artery usually runs above or within the vermilion border, with the published anatomical literature describing it routinely in the border itself or slightly above. Its depth relative to orbicularis oris breaks down to approximately 60% of the time beneath the muscle, around 35% within the muscle, and roughly 5% sitting on top of it. These proportions can vary even within the same patient because the artery does not necessarily hold a single fixed plane along its full length, and some lateral movement is to be expected.

The mental image worth carrying into the consultation is that of a probability cloud, similar to the way electrons are described in physics, where the artery does not have a precise fixed point but a region of high probability surrounded by regions of lower probability. Ultrasound and careful examination can narrow that probability cloud for any given patient, but the average injector working without imaging is making decisions based on where the artery is most likely to be in the average lip.

Anomalous anatomy does exist and matters for individual cases. A caliber persistent artery, which describes an artery running near the wet-dry border, has been recognised since the 1970s as an anomaly affecting roughly 3% of patients. Julie Horne has shared imaging of a pulsating artery at this position. The normal position of the artery is not at the wet-dry border, but it can be in a minority of cases, which is part of what every injector contends with on every treatment.

What the two techniques actually look like

The comparison only holds if the entry point, depth, volume, product, and number of injections are held constant, with the only variable being the angle of needle travel. The relevant entry point for both techniques is in the pink part of the lip rather than through the white lip, and this distinction matters because the older brutal technique of multiple passes through the white lip directly over the artery is part of why many practitioners hold an instinctive dislike of vertical injections.

A modern vertical injection enters the pink lip, skirts superficially across the anterior aspect of the lip body, and rotates the lip upward without entering the deeper part of the lip envelope. The needle starts roughly parallel to the most likely position of the artery and progressively moves further away from it as it travels deeper into the lip body. The histological cross-section of the lip shows the artery sitting just inferior to orbicularis oris in most patients, with the anterior surface of the lip carrying the muscle, a thin layer of hypodermic fat, and the dermis, while the artery lives on the posterior side of the muscle.

A horizontal injection at the same entry point runs adjacent to and parallel with the most likely position of the artery, staying in that plane for the full journey of the needle. The needle spends more total time close to where the artery is most likely to be, and in the presence of any anomalous loop or variation, the probability of clipping the vessel is higher across the length of the pass.

A thought experiment that clarifies the geometry

A useful way to test which technique is more likely to cannulate the artery is to ask how you would deliberately try to enter the vessel if that were the goal. The answer is to enter parallel with the artery, because the artery runs from lateral to medial off the facial artery across the top of the lip, and to stay deep underneath orbicularis oris while probing along its expected path until a flashback appears.

Entering vertically would actively reduce the chance of getting the needle tip into the lumen, because the trajectory crosses the artery briefly and at an unfavourable angle before moving away from it. The point of the thought experiment is to clarify the geometric logic, with the conclusion being that a needle running parallel to the artery has more opportunity to engage it than one crossing it perpendicular to its course.

Where the picture changes for involuted lips

Testing the model against different patient anatomies is where this analysis becomes more useful, because lip anatomy changes meaningfully with age. Older patients tend to have much smaller lips, with atrophy of the vermilion part of the lip being one of the common reasons people seek treatment in the first place, and in some cases the lip can almost entirely involute. The space behind orbicularis oris becomes proportionally more significant in these smaller lips, which changes the geometry of a vertical injection enough to shift the risk profile.

Picturing a vertical injection on a heavily involuted lip means the needle is naturally heading towards the retro-orbicularis oris space, which is where the artery most likely sits. The smaller and more involuted the lip, the closer a vertical trajectory takes the needle to the most likely position of the vessel. This is a genuine circumstance where the claim that vertical injections are riskier may hold true, even though it is not the patient group most injectors would intuitively choose vertical placement for in the first place.

This nuance points to a wider principle that may apply across the face. Arteries take up a relatively larger percentage of the available space as facial volume decreases, because the artery itself does not shrink at the same rate as the fat pads around it. The same injection at the same angle therefore carries a different risk profile in an older lip than in a younger one, and this changing geometry is part of what makes blanket statements about which technique is safer unreliable across the whole patient base.

What to take into the chair

The honest summary of the anatomy is that a single vertical injection through the pink lip in a normally proportioned lip is, in the average case, less likely to engage the superior labial artery than a horizontal injection at the same depth and volume, because the horizontal needle spends more time parallel to the most likely position of the vessel. The picture inverts for heavily involuted lips, where vertical entry is forced closer to the retro-orbicularis space and therefore closer to the artery. Neither finding is a recommendation to adopt a single technique for all patients, because every treatment plan balances aesthetic intent, patient psychology, comfort, and multiple harm categories beyond vascular occlusion.

What this analysis is genuinely for is sharpening the resolution of the mental model an injector carries into the room. A more accurate picture of where the artery probably sits, how its position relates to the angle of needle travel, and how that geometry changes across the age range of the patient base, supports better individual decisions without committing the practitioner to a rigid technique that may suit one anatomy and fail another.

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