The Hidden Art of Injection Depth: A Tale of Bulges, Kidney Beans, and Asymmetrical Smiles
Imagine a patient who smiles, but something is off. One side of the lower lip doesn't pull down. It looks like a stroke. The patient becomes self-conscious, hesitant to show happiness. This is one of the strangest side effects from botulinum toxin treatments—and it often comes down to a single, misunderstood variable: injection depth.
Depth is not a fixed number. It is a relative term, entirely dependent on the size of the structures you are treating. A deep injection in a large masseter muscle might mean the needle tip is in the lower third of that muscle. But in a small, thin masseter, the same "deep" placement might be dangerously close to the periosteum. So before you ever pick up a syringe, you must examine the patient in front of you. Have them bite down. Feel the thickness of their muscle as it contracts. This simple examination tells you exactly where to aim to avoid side effects.
Why People Inject Deep in the Masseter—and the Strange Bulge That Follows
Injecting the masseter often requires a deeper approach because the risorius muscle lies on its surface. The risorius originates from the masseter's superficial fascia and inserts into the modiolus, playing a role in your smile. If you accidentally relax the risorius, you get the oval side effect—an inability to smile properly. So we hide the botulinum toxin underneath the risorius origin.
But here is the paradox: if you go very deep in a large masseter, you leave the superficial fibers contracting. Without the balance normally created by the deeper fibers, those surface fibers contract in an odd way, creating a pinching bulge—a protrusion that looks like a muscle trying to escape.
Now, a colleague once asked me: "Does that mean if you have a thinner muscle, you actually need to be deeper?" Yes, exactly. In a thin muscle, you must be as far away from the surface as possible to avoid the risorius. That means you are closer to the bone. In a big muscle, you can afford to stay further from the bone. Depth is always relative to the muscle's thickness.
The Depressor Anguli Oris: A Dance with the Depressor Labii
One of the most popular lower face treatments is the depressor anguli oris (DAO). We treat it to lift the oral commissure, making the patient look happier. But the DAO is a risky neighbor. Directly beneath and next to it lies the depressor labii inferioris, the muscle that pulls the lower lip down and out during a genuine smile. If you inadvertently relax the depressor labii, you get that asymmetrical smile—the side effect that makes people think you've had a stroke.
This happens because of a combination of depth and injection point position. When you slide the needle in to inject the DAO, if you go deep enough, you are likely to hit the depressor labii fibers. The common injection point right on the jawline is particularly dangerous because it is too close to the overlapping fibers of the depressor labii. Diffusion can easily affect the lip.
My preferred technique is to slide up and feel the lateral border of the DAO while the patient contracts. Then I inject superior and lateral to that point, as far away from the lip depressors as possible. The only other muscle nearby is a bit of the buccinator and possibly the risorius, but those are usually more superior. You are essentially finding a safe zone where you can relax the downward pull without affecting the other vectors of the face.
The Kidney Bean Chin: A Tale of Superficial Injection
Now let's look at a strange side effect that can occur when treating the mentalis muscle. We treat the mentalis to prevent the chin from being pulled upward and to remove the little indentations caused by continuous contraction. The typical result is a smooth, relaxed chin. But sometimes, a few days after injection, you see a bunching on the skin surface that looks like a kidney bean. The top of the chin is not moving, but underneath, strands of the mentalis are still pulling upward, running into the relaxed muscle above.
This is the classic result of injecting too superficially. You only relax the upper part of the muscle, leaving deeper fibers active. However, there is another cause that often panics new injectors: if you see this on day two or three, it might resolve by day six or seven. What is happening is that some muscle fibers react faster to botulinum toxin than others. This creates a natural imbalance during the "half-baked" period. You need to hold your nerve. Wait at least a week—technically two weeks, but by day eight or nine you can usually tell the final result.
If you need to fix a kidney bean chin, look for the exact indentation where the muscle fibers are still pulling. Inject directly into those contracting fibers, parallel to the muscle, aiming for its origin near the mental crease. Two units on each side should solve it within three to five days.
The Opposite Problem: Too Deep, and the Lip Goes Flat
Consider another case: a mentalis treatment that resulted in a different side effect. The depressor labii muscle has been affected, so the patient's lower lip no longer pulls down naturally when they smile, while the chin is still contracting. This happens when the needle goes underneath the muscle, perhaps a little lateral, and hits the depressor labii instead of the mentalis.
These two cases—the kidney bean from too superficial, and the lip asymmetry from too deep—illustrate the balancing act of the lower face. Precision is everything. You need a clear understanding of where each muscle originates, where it attaches, and how to stay within that muscle without affecting its neighbors.
A Better Way to Think About the Mentalis
Most textbooks show the mentalis as a flat muscle running around the chin. But it is better to think of it as a cone-shaped muscle projecting out of the chin, with its origin near the mental crease and its insertion on the skin. If you imagine it as a piece of Blu Tack stuck to the chin and projecting outward, you can see why a straight injection from the front often goes right underneath the muscle, missing the bulk and risking the depressor labii. Similarly, a superficial injection only relaxes the top part.
My preferred technique is to enter the skin parallel to the mentalis fibers, aiming for the middle of the muscle. I inject at an intermediate depth—not superficial (which causes the kidney bean) and not down to the periosteum (which risks the depressor labii). When you are in the channel of the muscle, you can treat the entire depth without diffusing to adjacent structures. This is not about deep or superficial; it is about accurately targeting the active muscle.
A Final Word of Patience
Never blame your injections in the first two to three days. The results are half-baked. Give them time to play out. Many strange side effects resolve on their own as the toxin takes full effect. Understanding injection depth—and making it relative to the patient in front of you—is the key to mastering botulinum toxin in the lower face. And remember, the same principles apply to the upper face, where depth affects everything from forehead lines to crow's feet.