In smile photo studies, the versions people tend to rate highest have the edges of the two upper front teeth sitting about 1 to 1.5 millimetres lower than the edges of the teeth beside them in studies of women's smiles, and 1 to 2 millimetres lower in a study of men's smiles. When that small step disappears and all four edges line up in a straight row, the same smile is often rated less attractive. In two of these studies a deeper step of 2.5 millimetres was also among the lowest rated, so the preferred range is fairly narrow.
Studies tend to favour a step of about 1 to 1.5 mm
The teeth either side of the two front teeth are the lateral incisors. The difference in height between the biting edge of a central incisor and the lateral incisor next to it is often called the incisal step. A series of Brazilian studies from Machado's research group has measured how people react to it by editing smile photographs so the central incisors move up or down in half-millimetre increments and asking orthodontists and people without dental training to score each version.
| Study | Smiles rated | Best-rated step | Lowest-rated |
|---|---|---|---|
| Machado and colleagues, Journal of Esthetic and Restorative Dentistry, 2013 | One adult woman, 120 judges | 1.5 mm | No step |
| Menezes and colleagues, Dental Press Journal of Orthodontics, 2017 | Two adult women, 114 judges | 1.0 to 1.5 mm | No step, or a 2.5 mm step |
| Simões and colleagues, American Journal of Orthodontics and Dentofacial Orthopedics, 2019 | Two men aged 20 to 30, 106 judges | 1.0 to 2.0 mm | No step, or a 2.5 mm step |
In all three, the central incisors that sat slightly lower were preferred to those that sat higher. Because the whole tooth was moved in these images, the gum line changed too: the best-rated versions had the gum edges of the central incisors level with, or slightly below, those of the canines. These are photo studies with one or two faces each, so they show what tends to look balanced rather than a measurement every smile should match.
The two front teeth carry the smile
Dentists sometimes describe this as central incisor dominance. In a 2014 overview of smile planning in the Dental Press Journal of Orthodontics, Machado placed the vertical position of the upper central incisors first, suggesting their edges should sit below the tips of the canines so that the two front teeth lead the smile. That paper suggested a step of 1.0 to 1.5 mm for women and 0.5 to 1.0 mm for men; the later study of men's smiles found that 1.0 to 2.0 mm was rated best.
People also seem more sensitive to changes in the central incisors than in the laterals. When Machado and colleagues (American Journal of Orthodontics and Dentofacial Orthopedics, 2013) digitally shortened single teeth to imitate wear, half a millimetre of wear on one central incisor was rated unattractive by orthodontists and by lay judges. On a lateral incisor, lay judges did not mark the smile down until the difference reached 1 mm.
The step is one part of the outline of the smile. How the edges follow the curve of the lower lip is covered in our article on the smile arc, and the width of each tooth compared with its neighbours in our article on the golden proportion.
The right step depends on the face
A single number does not suit everyone. Atik and Turkoglu (Progress in Orthodontics, 2023) showed full-face smile photos of three women, one with an average, one with a long and one with a short lower face, to orthodontists, prosthodontists and lay people. Each photo was first standardised with a 1 mm step and then edited so the central incisors sat higher or lower in half-millimetre increments. For the face of average length, the standard 1 mm step scored highest with all three groups. The authors suggested that a step of more than 1 mm may suit people with a longer lower face, and a step of less than 1 mm may look better on a shorter face, although each face type was represented by only one person and the rater groups did not all agree on the long face.
Dentists and lay people do not always agree either. In a study of lateral incisor length by Al Taki and colleagues (European Journal of Dentistry, 2017), lay people preferred laterals 0.5 mm shorter than in the unaltered photo, while orthodontists and general dentists preferred 1 mm. In a French study (Stolz and colleagues, Orthodontie Française, 2017), lay people preferred lateral incisor edges that sat on the curve of the smile, while orthodontists and dentists preferred them half a millimetre higher.
Why the step can flatten or reverse
When the step is missing, it is usually for one of a few reasons, and the cause decides the fix:
- Worn central incisors. Grinding and years of use can wear the biting edges down, which can shorten the central incisors until they no longer sit below the laterals. Our article on tooth shape and perceived age looks at how wear changes the look of a smile.
- Tooth position. The central incisors may sit too high, or the laterals too low, from the way the teeth erupted or finished after earlier orthodontic treatment. In a 2021 survey (Nascimento and colleagues, Dental Press Journal of Orthodontics), 78.3% of orthodontists chose a 0.5 mm difference in bracket height between the central and lateral incisors, a choice that may influence the step between them as the teeth line up.
- Restorations. Fillings, crowns or veneers made with a flat edge line can remove a step that was there before.
- Small lateral incisors. A narrow or peg-shaped lateral can change the outline. Our article on peg lateral incisors covers the options.
Ways a dentist may restore the step
The choice depends on what is causing the flat line and how the teeth and gums sit.
Where the lateral incisors are slightly long, a small amount of reshaping of their edges may be enough. This removes enamel, which does not grow back, so it suits only small changes.
Where the central incisors are worn or short, adding length with composite bonding or edge bonding can often rebuild the step with little or no removal of healthy tooth. When the gum line is uneven as well, moving the teeth first with clear aligners or braces may give a more natural result than building length on the edges alone. Where several teeth need a change of shape and colour, porcelain veneers can set the step as part of the design, although veneers usually involve removing some enamel.
Because the preferred step depends on the face, the lips and how much tooth shows when you smile, a trial version in tooth-coloured material can help you see a proposed change before anything is altered. Our article on the smile mock-up explains that appointment. A mock-up is a preview to discuss, not a promised result.
Common questions about the incisal step
Is a straight edge line always a problem?
Not necessarily. Machado's 2014 paper described flat smile arcs as acceptable in men, and in the Al Taki study general dentists and lay people found a lateral incisor level with the central acceptable, although it was not their top choice. A flat line is mainly worth discussing if it bothers you or comes with wear, chipping or other changes.
Can my side teeth simply be filed shorter?
Sometimes, if they are only slightly long. Enamel removed by reshaping does not grow back, so a dentist will usually check how much enamel is there and whether the central incisors are the real cause before shortening the laterals.
Will a step make teeth look uneven?
A step that is too deep can. In the Menezes and Simões studies, a 2.5 mm step was among the lowest-rated smiles, so the aim is usually a small, even step on both sides.
Planning the outline of your smile
At Lumi Dental, smile photographs taken in our clinical photography room, together with a 3D intraoral scan of the teeth, can help a dentist assess the tooth edges against the lips and face. Dr James Tran has completed Digital Smile Design training with Dr Christian Coachman. A free cosmetic consult is available for cosmetic, braces and aligner treatment. If you live interstate, you can send photos first, as explained in our guide to travelling to Sydney for dental treatment, before visiting us in Melrose Park.
Sources
- Machado AW, McComb RW, Moon W, Gandini LG Jr. Influence of the vertical position of maxillary central incisors on the perception of smile esthetics among orthodontists and laypersons. J Esthet Restor Dent. 2013;25(6):392-401. PubMed 24180675
- Menezes EBC, Bittencourt MAV, Machado AW. Do different vertical positions of maxillary central incisors influence smile esthetics perception? Dental Press J Orthod. 2017;22(2):95-105. PubMed 28658361
- Simões D, Meyge de Brito G, Teixeira Cangussu MC, Machado AW. Does the vertical position of maxillary central incisors in men influence smile esthetics perception? Am J Orthod Dentofacial Orthop. 2019;156(4):485-492. PubMed 31582120
- Machado AW. 10 commandments of smile esthetics. Dental Press J Orthod. 2014;19(4):136-57. PubMed 25279532
- Machado AW, Moon W, Gandini LG Jr. Influence of maxillary incisor edge asymmetries on the perception of smile esthetics among orthodontists and laypersons. Am J Orthod Dentofacial Orthop. 2013;143(5):658-64. PubMed 23631967
- Atik E, Turkoglu H. Does different vertical position of maxillary central incisors in women with different facial vertical height affect smile esthetics perception? Prog Orthod. 2023;24(1):28. PubMed 37544965
- Al Taki A, Hamdan AM, Mustafa Z, Hassan M, Abu-Alhuda S. Smile esthetics: impact of variations in the vertical and horizontal dimensions of the maxillary lateral incisors. Eur J Dent. 2017;11(4):514-520. PubMed 29279680
- Stolz S, Lefebvre F, Bolender Y, Kanter D. Aesthetic perception of the vertical position of the incisal edges of the maxillary lateral incisors in the smile arch. Orthod Fr. 2017;88(3):251-261. PubMed 29043973
- Nascimento MHA, Brandão KMN, Menezes CC, Machado AW, Santamaria-Jr M. Orthodontists' preferences regarding the difference of bracket bonding height between the maxillary incisors. Dental Press J Orthod. 2021;26(6):e212031. PubMed 34932712
This article is general information only and is not a substitute for advice about your own situation from a dentist.
