Smile Symmetry & Midlines

Smile symmetry and midline analysis is one of the core components of frontal facial esthetic evaluation in orthodontics. It looks at how well the dental midline lines up with the facial midline, whether the corners of the mouth (commissures) sit level with each other when the patient smiles, and how much dark space (buccal corridor) shows between the cheek-side surfaces of the back teeth and the corners of the smile. Together these observations help clinicians decide whether a treatment plan needs to correct a midline shift, a canted smile, or an overly narrow arch — issues that are often invisible on a study model but very noticeable in a photograph or in real life.

Because these features are subtle and easy to miss on a static dental cast, they are best assessed on a natural, full, frontal-view smiling photograph, ideally with the head oriented so the interpupillary line is level and the lips are relaxed into a natural smile rather than a forced one.

What It Evaluates

Dental midline vs. facial midline: whether the line between the upper central incisors lines up with the true facial midline (commonly referenced through the glabella, nasal tip/subnasale, and philtrum/chin point), and if not, how large the discrepancy is and in which direction.
Commissure levels: whether the two corners of the mouth rise to the same height during a smile, or whether one side sits higher than the other (a commissure "cant"), which can reflect muscular asymmetry, an occlusal plane cant, or underlying skeletal asymmetry.
Buccal corridors: the dark, negative-space triangles visible between the buccal surfaces of the premolars/molars and the corners of the mouth when smiling. Corridors that are too wide can make an arch look narrow or "collapsed"; corridors that are essentially absent can make a smile look artificially wide ("denture-like").

Learn more: Understanding & Evaluation of Midline Discrepancies in Orthodontic Patients: A Comprehensive Approach Concerning Facial Harmony

Key References / Landmarks

Facial midline (glabella–subnasale–pogonion line) — a vertical reference line built from stable facial landmarks; the dental midline is compared against this line rather than against the nose or chin alone, since those can themselves be asymmetric.
Dental midline discrepancy — the horizontal distance between the contact point of the upper central incisors and the facial midline. Discrepancies of roughly 2 mm or more are commonly reported as noticeable to orthodontists and dentists, while laypeople tend to tolerate somewhat larger shifts (studies report thresholds ranging from about 2–4 mm depending on the study design and what surrounding facial context is visible).
Commissural line vs. interpupillary line — an imaginary line connecting the two corners of the mouth during a smile. In an esthetically balanced smile this line runs parallel to the interpupillary line (the line connecting the pupils); an oblique commissural line can create the illusion of a canted occlusal plane or a facial asymmetry even when none exists skeletally.
Buccal corridor ratio — the width of the visible dark space on each side of the smile, expressed as a percentage of the total intercommissural (mouth corner-to-corner) width. Research generally favors narrow-to-medium corridors, roughly in the 0–15% range, with a noticeable drop in perceived attractiveness once corridors widen substantially beyond that; however, more recent studies note the effect is smaller than classically assumed and is influenced by lighting and photographic angle.
Midline-to-contact-point angulation — even when the midline is shifted, clinicians check that the line between the two upper central incisors remains vertical (perpendicular to the incisal/occlusal plane), since a tilted contact point is often perceived as more objectionable than the shift itself.

Learn more: Buccal Corridor Guide: Proven Smile Design Tips for Dentists

Clinical Interpretation

During diagnosis, the orthodontist uses the frontal smiling photograph to decide whether a midline discrepancy is purely dental (and can be corrected with tooth movement, elastics, or interproximal reduction) or reflects an underlying skeletal or functional asymmetry that tooth movement alone cannot fully resolve. A discrepancy near or below the commonly cited ~2 mm threshold is often accepted as a treatment outcome rather than pursued aggressively, since over-correcting a midline can create other undesirable side effects (such as unleveling the occlusal plane or compromising root parallelism).

Commissure asymmetry is interpreted alongside the occlusal plane and facial midline: if the commissural line is canted while the interpupillary line and facial structures are level, the cause is more likely muscular or soft-tissue in nature; if the cant tracks with a skeletal or occlusal plane discrepancy, it points toward a true asymmetry that may need to be addressed in the treatment mechanics (e.g., with asymmetric elastics, differential intrusion/extrusion, or surgical planning in more severe cases).

Buccal corridor assessment feeds into decisions about arch expansion versus extraction-based treatment plans. Excessively wide corridors after treatment can be a sign that the arch was narrowed too much (for example, from non-expansive mechanics or extractions), while very minimal corridors are generally acceptable and, contrary to older orthodontic teaching, are not strongly linked to reduced attractiveness in most modern perception studies. As with all frontal-analysis findings, buccal corridor appearance should be judged with caution, since lighting, camera angle, and lip tension can visually widen or narrow the corridors independent of the actual dental arch width.

Learn more: Correction of Midline Deviation and Unilateral Crossbite Treated with Fixed Appliance (PMC case report)