The smile arc is the curve formed by the biting edges of your upper front teeth when you smile. In a smile most people find pleasing, that curve follows the curve of the lower lip, so the edges of the two front teeth sit a little lower than the canines. When the edges run flat or curve the other way, a smile can look older or more worn, even if every tooth is healthy.
What a consonant, flat and reverse arc look like
Dentists describe three broad shapes.
A consonant smile arc runs roughly parallel to the upper border of the lower lip. The central incisors are the lowest point, the lateral incisors sit slightly higher and the canines higher again. This is the shape most often seen in younger smiles.
A flat smile arc has the edges of the front teeth in a straight line, so the centrals no longer dip below the canines. It is very common in adults and is often one of the reasons a smile looks older without anyone being able to say why.
A reverse smile arc curves upward in the middle, so the front teeth look shorter than the teeth beside them. It is less common and usually has a clear cause, such as heavy wear on the front teeth.
The arc is judged together with other features of the smile. How much tooth shows at rest is covered in our article on lip line and tooth display at rest, and the dark spaces at the corners of a smile are explained in our guide to buccal corridors.
Why the arc flattens over time
The most common reason is wear. The front teeth take the most contact when the jaw slides forward and sideways, and over decades their edges shorten. Grinding and clenching speed this up, which is why a flat or reverse arc is often seen alongside chipped or translucent edges. Our article on teeth grinding in adults covers how that load is managed.
Other causes include:
- Orthodontic treatment that levelled the teeth into a flat line rather than following the lip
- The upper teeth sitting higher than ideal, so less of the front teeth shows
- A slanted bite plane, where one side sits lower than the other
- Previous crowns, veneers or bonding made with flat edges
How the smile arc is assessed
The arc is checked from a natural, relaxed smile rather than a forced grin, because a wide smile lifts the lower lip and changes the reference curve. Photos and short videos from the front, plus a look at the bite and the wear on the edges, show whether the problem is the length of the teeth, their position, or both.
During digital smile design, the lower lip line is traced on the photos and the planned edge positions are drawn against it. The same planning looks at tooth widths and proportions, which our article on the golden proportion and smile design explains. A trial smile in temporary material can then be tried in the mouth so you can see the new curve before anything permanent is done. That preview shows the plan, not a guaranteed final result, because the finished teeth are made separately.
Ways to restore the curve
The right approach depends on why the arc has flattened.
| Cause | Common approach | What to know |
|---|---|---|
| Mild wear on the front edges | Edge bonding to add length to the centrals | Little or no drilling; composite edges can chip and need upkeep |
| Teeth in the wrong vertical position | Clear aligners or braces | Moves the teeth rather than adding material; takes months |
| Moderate to heavy wear, or shape and colour changes as well | Porcelain veneers or crowns | More durable edges; involves removing some enamel |
| Slanted bite plane or gum level differences | Orthodontics, sometimes with gum reshaping | Usually planned with a full smile design |
For small changes, adding one or two millimetres to the central incisors with composite is often enough to bring back a gentle curve. Our edge bonding page covers how that is done. Where the teeth have moved or the bite has changed, moving them with clear aligners first can mean less material is needed afterwards. Where wear is heavier, or the colour and shape of several teeth need changing too, porcelain veneers give longer-lasting edges.
Why length cannot always just be added
The edges of the upper front teeth also guide the lower jaw when you bite and speak. Making them longer without checking the bite can lead to chipping or a change in how some words sound, particularly "f" and "v". This is why changes to the arc are tested in a trial smile first, with speech and biting checked, before anything is bonded or cemented.
Questions we are often asked
Does a flat smile arc mean something is wrong with my teeth?
Not necessarily. A flat arc is common and is often purely a matter of appearance. It can, however, be a sign of wear from grinding, so it is worth having the edges and bite checked.
Do people really notice the smile arc?
Studies suggest they do, even if they cannot name it. In a study where smile photos were digitally altered, both orthodontists and members of the public rated flat smile arcs as less attractive than consonant ones (Parekh SM et al., Angle Orthodontist, 2006). Earlier work found that the arc was one of the features most strongly linked with how attractive a smile was rated (Hulsey CM, American Journal of Orthodontics, 1970).
Can braces or aligners flatten my smile arc?
They can if the front teeth are levelled into a straight line. Orthodontic planning that takes the lower lip curve into account aims to avoid this (Sarver DM, American Journal of Orthodontics and Dentofacial Orthopedics, 2001).
Can I see the new curve before committing?
Yes. A trial smile made from the plan can be placed over your own teeth for a short time. Our article on the smile mock-up appointment explains what to expect.
If your smile looks flatter or older than it used to, the team at Lumi Dental can assess the arc and talk through options from bonding to aligners. Book a free cosmetic consult or see current deals.
