Smile analysis explained: smile arc, lip line, tooth display, buccal corridors and midlines

Simplified Orthodontics Academy · Published 27 October 2026

The posed smile as the usual record

A smile changes from moment to moment. The posed smile, the smile a patient makes on request, is the one usually photographed for smile analysis. Ackerman and colleagues described a method to analyse the posed smile on photographs [1].

A single photograph records one moment of one smile. Sarver and Ackerman described dynamic records of the smile, which capture more than one static photograph [2], [3]. This article covers the measurements on one frontal photograph of a posed smile, the record Simplified Face uses.

Taking the frontal smile photograph

The smile is photographed from the front, straight on, so that left and right can be compared and vertical distances read without tilt. In Simplified Face, the photo is taken with the camera at eye height, and both pupils are marked: the line through them is the horizontal reference for every reading (see The midlines and cants).

Millimetre values need a calibrated photo, as on the other views: the width of one iris, the patient’s pupil distance, or a ruler held at the face. Ratios and percentages work without calibration.

Tooth display

Tooth display describes how much of the upper incisors, and of the gum above them, shows below the upper lip during the smile. It is measured vertically at the middle of the smile, from the lower border of the upper lip to the incisal edges of the central incisors and to their gingival margin.

In Simplified Face, the readings are:

• Upper incisor display: from the lower border of the upper lip (ULs) down to the incisal edges of the central incisors (U1).
• Gingival display: from ULs to the gum tip between the central incisors (U1g). It is positive when gum shows below the lip, and negative when the lip covers the top of the crowns.
• Visible crown height: from U1g to U1.
• Interlabial gap: the opening between the lips at the midline, from ULs to LLs.
• Lower lip to the incisal edge: from U1 to LLs, positive when the lower lip lies below the incisal edges.
• Smile width, from corner to corner (Ch to Ch), and the smile index, the smile width divided by the interlabial gap.

In Simplified Face, the gum point U1g is the tip of the papilla between the central incisors, not the gingival margin over each incisor. So its gingival display and visible crown height are measured from the papilla tip, and its crown height is shorter than one measured from the gingival margin. Simplified Face shows the published reference value next to each reading.

Lip line: high, average or low

The lip line describes where the lower border of the upper lip sits on the upper front teeth during the smile. Tjan, Miller and The described smile lines as high, average and low in a sample of 454 photographs of students [4]. The three words name what the photograph shows:

• High: the lip shows the full height of the front teeth and a band of gum above them.
• Average: the lip shows most or all of the front teeth and the gum between them, the papillae, but no band of gum above.
• Low: the lip covers more of the front teeth, and no gum shows.

These categories describe the smiles in that sample of students. They are a way of naming a smile, not a standard for one.

In Simplified Face, the lip line is recorded as a number: the gingival display at the midline, from ULs to U1g, in millimetres, with a lip line label next to it. A number keeps detail that a category loses; two smiles with the same label can differ by a few millimetres.

Smile arc: consonant, flat or reverse

Sarver defined the smile arc as the relationship between the curvature of the incisal edges of the upper incisors and canines and the curvature of the lower lip in the posed smile. When the two curves are parallel, the arc is called consonant [5]. Two more words describe arcs that do not follow the lower lip:

• Flat: the incisal edges form a straight line, or nearly so.
• Reverse: the curve bows the other way, with the central incisors higher than the canines.

In Simplified Face, the smile arc is measured from three points on each curve. The incisal curve depth is how far the incisal midpoint U1 lies below the line joining the two canine tips (C3). The lower lip curve depth is how far the lower lip at the midline (LLs) lies below the lip points under the canines (Ll). The tool shows both depths, compares them, and gives one of these labels: consonant, flat, reverse, or non-consonant with the incisal curve flatter or deeper than the lower lip. In Simplified Face, “flat” means that the incisal midpoint U1 lies level, or nearly level, with the line joining the canine tips, whatever the curve of the lower lip; this is a different test from comparing the incisal curve with the lip. Three points describe a curve only roughly: an uneven edge between them, such as one short lateral incisor, does not change the label.

Buccal corridors

The buccal corridor is the dark space on each side of the smile, between the last visible upper teeth and the corners of the mouth.

Published methods differ in the points they use: the inner or the outer corners of the mouth, and the tooth taken as the end of the visible dentition. A percentage can only be compared with values measured from the same points. And because the corners of the mouth move as a smile widens, the corridor also changes with the moment the photograph catches.

In Simplified Face, the corridor is measured on each side as the horizontal distance from the inner corner of the mouth (iCh) to the distal edge of the last visible upper tooth (MV), as a percentage of the inner smile width, iCh to iCh. The tool also gives the total: the inner smile width minus the visible dentition width (MV to MV), as a percentage of the inner smile width.

The midlines and cants

The facial midline is a vertical reference through the middle of the face. Methods differ in the points used to draw it. The upper dental midline, the contact between the upper central incisors, is compared with it, and the lower dental midline with the upper one. A cant is a tilt from side to side: of the incisal edges, or of the corners of the mouth, against a horizontal reference.

In Simplified Face:

• Facial midline: the line at right angles to the pupil line through the point halfway between the pupils. If the pupils are not marked, the tool uses the line from soft tissue nasion to subnasale, and says so next to the reading.
• Upper dental midline: the contact point of the upper central incisors (U1). Its distance from the facial midline is given in millimetres, to the patient’s right or left.
• Dental midline tilt: the angle between the line from U1g to U1 and the facial midline.
• Lower dental midline: the contact point of the lower central incisors, when it is visible, measured to the right or left of the upper one.
• Cants: the incisal cant is the tilt of the line through the canine tips against the pupil line; the commissure cant is the tilt of the line through the corners of the mouth. The tool also gives the height difference of the two corners and the position of the chin from the facial midline.

Right and left are always the patient’s. In an ordinary frontal photograph, the patient’s right is on the left of the picture.

Worked example

This example is a drawing, not a patient: the schematic smile of Figure 1, drawn on a millimetre grid with the pupils level, as if calibrated. We measured it with the formulas Simplified Face uses.

To see how the smile arc label follows the two depths, we then moved only U1, up or down, and kept the lower lip curve depth at 2.5 mm:

Moving U1 also changed the upper incisor display, the visible crown height and the lower lip to the incisal edge, by the same amount. One landmark feeds several readings, so a misplaced point shows up in all of them.

Published protocols

Smile analysis is often taught as a checklist of items. Machado published one such protocol for assessing the smile, and notes that protocols should not be applied universally [6]. The readings in this article describe one photograph. They are not a verdict on the smile.

Limits

• One moment. Two photographs of the same posed smile are not identical. Dynamic records exist for the smile as it forms [2], [3].
• Camera and head position. A head turned to one side moves the corridors and the midlines; a tilted head adds to the cants. A camera above or below eye height changes how much tooth seems to show.
• Landmarks are placed by eye. The corners of the mouth and the gum tip are soft edges, and a small shift changes every reading that uses them.
• Definitions differ. The same name can mean different points: the gum tip or the gum margin, the inner or the outer corner of the mouth. Compare readings only with values measured the same way.
• Reference values come from particular samples. The values Simplified Face shows come mainly from adult samples of European descent and may not suit every patient or population.
• Description, not decision. This article describes how the smile is measured and named. It does not cover diagnosis or treatment.

Educational content for clinicians and students. Reference values vary between populations. This page does not replace clinical judgement.

References

1. Ackerman JL, Ackerman MB, Brensinger CM, Landis JR. A morphometric analysis of the posed smile. Clin Orthod Res. 1998;1(1):2-11. doi: 10.1111/ocr.1998.1.1.2. PMID: 9918640
2. Sarver DM, Ackerman MB. Dynamic smile visualization and quantification: Part 1. Evolution of the concept and dynamic records for smile capture. Am J Orthod Dentofacial Orthop. 2003;124:4-12.
3. Sarver DM, Ackerman MB. Dynamic smile visualization and quantification: Part 2. Smile analysis and treatment strategies. Am J Orthod Dentofacial Orthop. 2003;124:116-127.
4. Tjan AHL, Miller GD, The JGP. Some esthetic factors in a smile. J Prosthet Dent. 1984;51(1):24-28. doi: 10.1016/s0022-3913(84)80097-9. PMID: 6583388
5. Sarver DM. The importance of incisor positioning in the esthetic smile: the smile arc. Am J Orthod Dentofacial Orthop. 2001;120(2):98-111.
6. Machado AW. 10 commandments of smile esthetics. Dental Press J Orthod. 2014;19(4):136-157. doi: 10.1590/2176-9451.19.4.136-157.sar

More to read

• Facial analysis in Simplified FaceThe smile, frontal and profile analyses in the tool
• Soft tissue profile lines explainedE-line, S-line and Burstone’s line, with the sign convention
• تحليل الابتسامة: القياسات الأساسية في صور الوجهThe same article in Arabic
• Clinical photography in orthodonticsHow the frontal smile and the other standard views are photographed
• All articlesOrthodontic analyses, explained