# Lower Lip S-Line Position

Harmony · Side view · Lips · https://www.faceiqlabs.com/support/harmony/lower-lip-s-line-position

The lower lip’s signed distance from a line running from nose-base/tip midpoint to soft-tissue chin.

## What is measured

Calculation: Perpendicular lower-lip distance to the S-line × photo scale

Nasion means the nasal root, subnasale the nose base, and pogonion the most forward chin point. Frankfort refers to the direction from the ear-opening landmark to the lower orbital landmark.

## Why it matters

This relates the lower lip to the nose–chin framework. Reading it alongside the upper-lip distance helps distinguish the relationship of the two lips to the same reference.

- Negative in FaceIQ: the lip lies in front of the line.
- Positive in FaceIQ: the lip lies behind the line; zero means contact.

## Where the idea comes from

Evidence: FaceIQ adaptation. Steiner’s S-line belongs to soft-tissue profile analysis. FaceIQ approximates its nasal endpoint using the midpoint between the nose base and tip. This construction may differ from tracing the original nasal S-curve.

## A worked example

A lower lip 2 estimated mm behind the S-line reads +2 mm. Moving it onto that unchanged line gives zero; 1 mm in front gives −1 mm.

Example values explain the calculation. They are not a target range.

## How to read your result

A midpoint-based S-line is an approximation. Lip posture and movement of either endpoint change the distance; this does not isolate tooth position.

Millimeters are approximate: FaceIQ scales the photo using an assumed 190 mm hairline-to-chin height, so these are estimates, not calibrated physical measurements.

## Research & further reading

These sources explain the history or related evidence. FaceIQ’s score weights and reference ranges are model settings; the papers do not collectively validate the scoring system.

- Buschang, Fretty & Campbell (2011). [Can commonly used profile planes be used to evaluate changes in lower lip position?](https://pubmed.ncbi.nlm.nih.gov/21299383/). Compared five lip reference lines, including Steiner’s S-line, in 79 orthodontic patients. Moving reference points can make a lip appear to move backward even when it moves forward.

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Source: https://www.faceiqlabs.com/support/harmony/lower-lip-s-line-position · FaceIQ Labs overview for AI assistants: https://www.faceiqlabs.com/llms.txt