Facial Asymmetry — Sorting the Difference Into Force and Volume

Three-Line Summary

• What differs between the two sides comes down to force (how hard the muscles pull, how firmly the soft tissue holds) and volume (fullness and tissue thickness). Which axis it is changes the tool completely.
• If it stands out only with expression, it is muscular, and that is where botulinum toxin belongs. If it is already visible at rest, it is soft-tissue pull or volume, and that is where energy-based treatments such as Ulthera Prime, and volume treatments belong.
• Where the skeleton itself differs, a non-surgical approach has a clear ceiling. Some degree of left-right difference is present in everyone, and aiming for perfect symmetry can end up looking less natural.

What Differs Between the Two Sides Comes Down to Two Things — Force and Volume

Let me start with one premise. No face is perfectly symmetrical to begin with. A 2018 review by Srivastava and colleagues in J Oral Biol Craniofac Res set out that subclinical asymmetry — differences at a mild, everyday level — is commonly observed across the general population. Some degree of left-right difference exists normally, and in itself it is not something to treat.

There is a reason for saying this first. If the goal set in consultation is a perfectly symmetrical face, that goal was never reachable. The longer you study yourself in the mirror, the more differences you begin to notice that never bothered you before, and from that point it is no longer a question of treatment but of the standard being used. What we actually work on is the degree of left-right difference the eye lands on at ordinary conversational distance, and easing that point is the realistic goal.

Classifying the Cause Alone Does Not Produce a Plan

The same review divides facial asymmetry by when it arises — congenital, developmental and acquired — and names habitual chewing on one side built up during growth, functional factors such as crossbite, trauma and fracture, and temporomandibular joint disorders among the representative causes. A detailed breakdown by cause is set out in the review by Thiesen and colleagues in Dental Press J Orthod (2015).

In a discussion about treatment, though, knowing the cause is not enough. Whatever the origin, we have to look at what is actually different between the two sides now before it is settled which tool reaches that difference.

Force, or Volume

What differs between the two sides falls broadly into two groups. Force, or volume.

Force splits again into two kinds. One is the active pull of muscle; the other is the structural hold of the soft tissue. The first works when you make an expression. The second is working all the time, including at rest.

Volume is fullness and tissue thickness — how much of the fat pad is still there, and how thick the skin sitting over it is.

And the two are not cleanly separable. When volume changes, the pull at that spot changes with it. Lose volume in one cheek and the weight and tension there shift, so the contour runs differently; add volume back and the pull changes again. Which is why working on volume never ends at filling a hollow.

For reference, the review by Lee and colleagues (J Clin Med, 2025) on non-surgical approaches to facial asymmetry sorts cases by clinical pattern into five groups — volumetric, soft-tissue laxity, dynamic (muscular), superficial skin texture, and combined. It is the same account we give with force and volume, organised a different way.

To put it in a table.

The first three rows are what non-surgical treatment actually reaches; the last row is a different matter. I will come back to bone later.

Take Two Photographs — One at Rest, One With Expression

The most useful column in that table is the second one, how it shows. That column alone usually settles which axis is involved.

If the asymmetry stands out when you make an expression, it is force, and specifically muscle. If the difference is already there at rest, it is the pull of the soft tissue, volume, or bone.

How to Check at Home

The method is simple. Two photographs will do.

• Resting photo — facing forward, with the face completely relaxed
• Expression photo — your usual smile

What matters is the same lighting and the same distance. And the front camera on a phone distorts because of the lens, so if you can, have someone else take them on the rear camera. A selfie can create asymmetry that was never there, and it can just as easily hide a difference that is.

Look at the Face in Three Zones

When you compare the two photographs, do not take in the whole face at once. Split it into upper, middle and lower and check each zone separately — it shows up far more clearly.

• Upper — brow height and eyelid line
• Middle — cheekbone height and cheek volume
• Lower — the run of the jawline and the height of the mouth corners

Then overlay the two and the signal separates. If the resting photo already shows the midlines of the nose and chin off-axis and the bite shifted to one side, that is a sign the skeletal share is large. If the resting photo shows little difference and the gap opens only in the expression photo, that is a sign the muscular share is large.

We do the same thing in the consulting room. Hold a neutral face, now smile, now say ah, eh, ee, oh, oo — running through expressions like this separates force from volume in most cases. What one photograph cannot tell you shows itself the moment movement is added.

When the Gap Opens With Expression: The Pull of Muscle

Imbalance in the muscles of facial expression sounds technical, but the meaning is simple: the same muscle is working harder on one side than on the other.

Around the mouth, the muscle that most often creates this difference is the depressor anguli oris (DAO). It pulls the corner of the mouth down, and when activity differs between the two sides the corner can sit to one side when smiling or at rest — an uneven smile.

The Lower Side Is Not the Weaker One

This is where people are most often misled. If one corner of the mouth sits lower, it is easy to assume the muscle on that side is weak, but in practice the opposite is more common. The muscle pulling that corner down is the one working too hard.

So the direction of treatment reverses as well. Rather than building up the weaker side, we work to take force off the side doing too much. That is exactly where botulinum toxin sits in facial asymmetry.

As for why one side ends up doing more, habitual chewing on one side and expression habits built up during growth are mentioned as factors, but pinning it on any single cause is difficult, and several usually overlap.

Botulinum Toxin for Facial Asymmetry — What Has Been Established

In one sentence, what botulinum toxin does in facial asymmetry is reduce the force on the side pulling too hard so that it balances against the other.

One study measured that change directly. A 2025 study by Pradel and colleagues in J Clin Med used 3D stereophotogrammetry in 16 adults with unilateral peripheral facial palsy to compare symmetry before injection and 2 to 3 weeks after. The whole-face symmetry index (RMS) improved from 1.51 to 1.35 (p<0.001), and although improvement appeared across the whole face it was most marked in the middle third. By expression, the largest change came with the surprised expression.

What makes this study meaningful is the method. Rather than physician or patient satisfaction ratings, coordinates were measured photogrammetrically and compared. That said, keep in mind that it was an observational study of 16 people with no control group. There is no need to read those numbers across as your own expected result.

The asymmetry this study dealt with followed facial palsy, and there the side injected is in fact the unaffected one. When the marginal mandibular nerve is damaged on one side, only the other side works, which makes the mouth corner on the unaffected side look as though it has dropped. It sounds convoluted, but the principle is the same: take force off whichever side looks stronger with expression, to bring the two into balance.

The Injection Site Here Is Trickier Than It Looks

A 2022 study by Moradi and Shirazi in Plast Reconstr Surg combined a literature review, cadaveric dissection and a retrospective chart review of 275 patients to re-establish the anatomical borders of the depressor anguli oris. The study proposed that a three-point technique along the lateral border of the upper half of the muscle is favourable for obtaining the intended effect while limiting spread into neighbouring muscles.

The same study also noted that the lower part of the depressor anguli oris lies anatomically adjacent to the depressor labii inferioris, so toxin that spreads can instead produce a new form of lip asymmetry. That would mean coming in to settle an asymmetry and leaving with one of another kind, so we approach this area carefully. Depth is adjusted along with the injection site.

For that reason we prefer to start with a low dose and adjust at two weeks after seeing the response rather than trying to correct everything in one go. The muscle here is small, and going in heavily from the start takes time to undo. Too little can be topped up next time; too much can only be waited out.

One more thing. If a whole side of the face is weak, or if there is a difference in eye closure as well, the nerve has to be checked before muscle habit is considered. Here diagnosis comes before treatment.

When the Contour Differs Even at Rest: The Hold

Now the second kind of force. Not the active pull of muscle but the structural hold of the tissue.

SMAS — Its Thickness Varies by Region

The thin membranous layer running over the muscles of facial expression is called the SMAS (superficial musculoaponeurotic system). It is the layer that comes up constantly in lifting discussions.

According to a 2010 histological study of eight cadavers by Macchi and colleagues in Cells Tissues Organs, the SMAS is a thin membranous layer sitting between two fibrofatty connective layers. Above it run vertically oriented fibrous septa linking the dermis to the superficial surface of the SMAS; below it run obliquely oriented septa linking the deep surface of the SMAS to the parotidomasseteric fascia. This three-dimensional mesh is the structure that transmits muscle contraction to the skin.

The finding confirmed as statistically significant in that study was that the SMAS thins progressively as it runs from in front of the ear toward the nasolabial fold (p<0.05). If the membrane differs in thickness from region to region, the hold differs from region to region too. When that difference opens up between the two sides, the contour at rest changes with it.

Energy-Based Treatments Are Not Only for Sagging Faces

Many people know energy-based treatments such as Ulthera Prime only as a way of lifting what has descended. But lifting is not all this treatment does.

Looking at the mechanism as Lee and colleagues set it out, high-intensity focused ultrasound concentrates energy at a depth of roughly 1.5 to 4.5 mm below the epidermis and creates zones of coagulation at 60 to 70 degrees at that point alone. Collagen denatures and contracts immediately there, and over the following months new collagen and elastin are laid down. The key point is that focusing the energy spares the epidermis while still reaching the deep layers.

So what this treatment does, in effect, is raise the hold of that area itself. The same review set out that this approach suits not only moderate to severe skin laxity but also structural asymmetry and contour imbalance.

Which is why there is a use for it not only where one side of the jawline has visibly descended, but also where there is nothing you would call sagging and yet the contours run differently on the two sides. The idea is to add tension on the side holding less, to bring the lines together.

At ABLE Dermatology the tools used here are divided by the layer they act on.

• Ulthera Prime (high-intensity focused ultrasound) — focuses on the deep layers including the SMAS, working on support itself
• Density (sequential radiofrequency) — bulk-heats the dermis as a whole to drive remodeling
• Onda (microwave) — acts selectively on the subcutaneous fat compartment, for cases where fat gathered on one side adds to the weight behind sagging
• Olewave (piezoelectric extracorporeal shockwave) — approaches through acoustic shockwaves rather than heat

There is a point to be honest about, though. There is no standard figure in the literature for how differently to distribute energy between the two sides. Ultrasound lifting itself has evidence at the level of a meta-analysis (n=475), but even there superiority over radiofrequency was not established, and there is still no quantitative protocol for differential left-right distribution aimed at asymmetry. Which makes the step of confirming beforehand what the reasoning was for treating the two sides differently all the more important.

Another Option the Literature Covers: Extracorporeal Shockwave

Alongside the approaches above, the review by Lee and colleagues also introduces extracorporeal shockwave therapy (ESWT). The principle is somewhat different. Rather than heat, acoustic shockwaves create controlled microtrauma, cytokines and growth factors are released at the site, and fibroblasts are activated. Collagen and elastin are synthesised while microvascular density and blood flow increase at the same time — these are set out as its distinguishing features.

There are two modes. Radial is described as suited to superficial muscle and soft tissue at 1 to 3 cm depth, focused to deeper structural deficits or marked asymmetry at 3 to 6 cm. The interesting part is that it also acts directly on muscle tissue. It is described as modulating muscle tone and improving muscle fibre function, which in the frame of force and volume puts it across both.

The timeline given in the literature is gradual onset from 2 to 4 weeks lasting around 6 to 12 months, with 3 to 6 sessions at 1 to 2 week intervals and maintenance every 4 to 6 months. That part, though, is better read as a summary of the literature. Extracorporeal shockwave aimed at facial asymmetry is not yet in wide use in Korea, and we too understand it as a supporting option rather than a mainstay.

Masseter Volume Also Affects the Pull

One thing to add. The volume of the masseter on each side also affects how the tissue is pulled. So treating the two masseters to exactly the same degree is not always the right answer. Reducing the masseter less on the side where the pull is weaker, or keeping volume in the lateral cheek on that side, makes the pull there comparatively stronger. This is the point where force and volume being interlocked shows up in actual planning.

When the Amount Differs: Left-Right Differences in Volume and Thickness

If you look for the cause of a hollow-looking cheek in sagging alone, you see only half of it. Sometimes the hold is entirely intact and the amount of volume itself differs between the two sides.

And a difference in volume does not end at looking asymmetric. Where volume differs, the pull differs too. So evening out volume does not end at filling a hollow — the tension at that spot changes with it, and the whole contour follows.

Filling Immediately, or Building Gradually

There are broadly two routes to restoring volume.

One is hyaluronic acid filler. It creates volume at the site immediately and draws in water. The review by Lee and colleagues set out that filler improves facial symmetry and contour by stimulating collagen production on top of immediate volume restoration. In the literature the effect is reported from immediately to within a week, lasting roughly 6 to 24 months depending on the product and the site, though this varies between individuals.

The other is collagen-stimulating injectables. These do not fill immediately; they stimulate fibroblasts so that collagen builds gradually. The same review set out that this class improves collagen synthesis, skin elasticity, volume and structural support through progressive biostimulation. The effect is generally described as appearing from 4 to 6 weeks and continuing for around 12 to 24 months.

At ABLE Dermatology the tools used here are as follows.

• Filler — when volume has to be created immediately. Its role is clearest where the left-right difference is localised and well defined.
• Juvelook Volume (PDLLA), GOURI (liquid PCL), Sculptra, Radiesse — the collagen-stimulating class, chosen when a gentle left-right difference over a wide area is to be narrowed over time.
• Re2O (hADM), CellREDM (hADM) — a class that works on the structural support of the tissue itself
• HILO WAVE (Dual-HA), BELOTERO REVIVE — for the thickness and texture of the skin itself rather than volume. These are used for left-right differences that fall under superficial skin texture in the classification above.

The choice comes down to whether the two sides have to match now, or whether there is time to fill gradually.

What Matters Is Not Where and How Much

The most important question in a volume consultation is not where to place how much, but why that difference in volume arose in the first place.

Whether the fat pads themselves developed differently on the two sides, or whether sagging has shifted their position so that it only looks that way, changes the approach completely. If it is the former, filling is right; if it is the latter, the pull covered in the previous section has to be addressed first. Fill an area that looks empty because it has descended, and that area can end up heavier for what was added.

One more point. More volume does not mean a better result. Overfilling can widen the face and make the asymmetry stand out more. We tend to set the goal at easing the one or two points the eye lands on first rather than matching the two sides exactly.

How Far Is It True That Soft Tissue Can Cover a Skeletal Difference?

Here is the bone discussion set aside earlier. People who have been told their bone is off-axis often hear the same explanation: fill the thinner side with volume and the skeletal difference is covered.

Answering how far that holds means separating two questions. How much is covered, and whether volume can stand in for bone.

How Much Is Covered

A 2024 study by Supmaneenukul and colleagues in Heliyon superimposed CBCT and 3D facial photography in 24 patients with mandibular asymmetry, measuring tissue thickness at multiple points on the deviated and contralateral sides. They compared 12 patients in a mild group with 12 in a moderate-to-severe group, split by the degree of chin deviation.

The result ran slightly against intuition. The milder the asymmetry, the more the soft tissue covers the skeletal difference. But as the asymmetry becomes more marked, that compensation decreases instead, and the skeletal difference shows through.

In short, a mild asymmetry is masked to a degree while a marked one is not. But what this study showed is how much is covered, not an answer to how much filling it would take to cover it.

Can Volume Stand In for Bone

There is a principle repeated throughout the plastic surgery field. Injected volume can make up for a minor skeletal volume deficit to a degree, but it cannot replace the supporting structure of the skeleton itself. What is missing has to be made up with something of the same order.

Lay the two accounts together and the picture completes. Where the skeletal difference is minor, soft-tissue compensation occurs naturally, and adding volume on top can be expected to soften it visually. But as the skeletal difference grows, two limits overlap. The compensation itself decreases, and it is a question of supporting structure that volume could not substitute for in the first place.

So the claim that filling the thinner side covers a skeletal difference holds only in mild skeletal asymmetry, and only at the level of visually balancing the soft tissue above the bone rather than replacing the bone.

What We Can and Cannot Do

ABLE Dermatology is a dermatology clinic, and we do not perform surgical approaches such as orthognathic surgery, facial contouring surgery or facelift surgery. So where the assessment in clinic is that the skeleton accounts for a large share, we say so first.

If the resting photograph shows the midlines of the nose and chin off-axis and the bite shifted to one side, the range within which a non-surgical treatment can work is narrow. In those cases we either set out from the start, at a lower level, how much treatment can change, or advise that being assessed first in a field that deals with the skeleton would give a more accurate picture. Recommending treatment without making that distinction would not, in our view, be an honest consultation.

Level of Evidence and Order of Treatment

Read this far and the picture is drawn. Reduce the force of the muscle, reinforce the pull of the tissue, adjust the difference in volume, and cover bone through the items above it, within limits. Logically it runs smoothly. But the level of evidence varies considerably from item to item.

There is a reason for setting the table out in this much detail. In an asymmetry consultation, knowing where the established evidence ends and clinical judgement begins before starting tends to mean less disappointment after treatment.

A Rough Timeline and the Known Adverse Effects

Figures vary by product and device, and individual variation is considerable. For botulinum toxin, bruising and swelling at the injection site, and transient changes in expression or secondary asymmetry from spread into neighbouring muscles, are reported; for micro-focused ultrasound, pain during treatment, transient redness and swelling, and rarely nerve injury and fat atrophy, are reported as adverse effects. For volume treatments, swelling, nodules and vascular complications are the known adverse effects. If symptoms persist after treatment, speak with the medical team where you were treated.

Force First, Volume Last

Running several things at the same time is not a problem. That said, we generally look at force first.

• Botulinum toxin — about two weeks for the effect to settle. Photograph again at that point and it separates far more clearly whether the asymmetry that remains came from muscle, or from pull or volume. It doubles as assessment and correction.
• Energy-based devices — a longer arc. There is some immediate tightening, but the real change starts from a month on.
• Volume — the part we tend to leave until last, because once the first two are settled the amount that actually needs filling is often smaller than first expected.

It is also worth setting up a reference before treatment. Just as the 3D quantitative study above measured coordinates, at minimum keep photographs taken under the same conditions. Without a baseline photograph you end up judging from memory whether things improved, and that memory is generally not accurate.

Maintenance and Setting the Goal

Botulinum toxin wears off gradually, so maintaining the effect means repeating it, and ultrasound treatments are commonly repeated once or twice a year. Rather than fixing an interval in advance, though, it is easier to settle on coming back for a look once the effect drops off.

A last word on goals. It is better not to set perfect symmetry as the target for a first treatment. No face is perfectly symmetrical to begin with, and forcing the two sides to match can end up looking less natural. Starting small, seeing the response and adjusting tends to arrive at the point you wanted sooner.

Asymmetry varies in degree, but some of it is present in everyone. Simply knowing which axis the point you have noticed comes from makes the range of choices — including whether to have treatment at all — far clearer.

From Consultation to Treatment

A board-certified dermatologist examines you directly to identify the layer the cause sits in, decides on the device and the parameters, and the same dermatologist carries out the treatment. There is no structure here in which a consultant recommends procedures.

The number of sessions, the intervals, the timing of maintenance and the cost are all agreed together before the first treatment.