Merrifield line: the Z angle of the profile
Quick facts
- Definition
- Line tangent to the chin and the most prominent lip, which forms the so-called Z angle with the Frankfurt plane.
- View required
- True profile, with the head positioned in the Frankfurt plane.
- Main use
- Assess the balance of the soft tissue profile between the lips and the chin.
- Relevance in orofacial harmonization (HOF)
- Gives an objective reading of the profile, to be read with caution and alongside the whole face.
The Merrifield line, also called the Z line, connects the most prominent point of the soft tissue chin to the most forward lip (upper or lower, whichever sits further ahead). The angle this line forms with the Frankfurt plane is the Z angle, described in the orthodontic literature as an indicator of profile harmony.
What is the idea behind the Z angle?
The proposal is simple: a harmonious profile has the lips and chin in a certain relationship. When the angle is too open or too closed, the profile may have overly prominent lips, a recessed chin, or the opposite. The reference values described vary with the source, sex, and age group, so it is prudent to consult the source your practice adopts and keep the same criterion throughout follow-up.
Like the Burstone line, the Ricketts line, and the Steiner S line, it is a reading tool, not a prescription.
How does it fit into HOF assessment?
The main use is differential. When the patient talks about a thin lip or a small chin, the line shows whether the difference lies in the lip, the chin, or the relationship between the two. A profile with retrogenia (a recessed chin) can make the lip look protruding when it is not. Treating only the lip in that scenario can worsen the disproportion.
When photographing, a few precautions reduce error:
- head in the Frankfurt plane, with the gaze on the horizon;
- lips at rest and teeth in habitual occlusion;
- camera at eye level, at a distance that reduces distortion;
- the same setup in follow-up photos.
Limits of the line
Profile lines describe positions in the sagittal plane and do not capture smile dynamics, lateral volume, or skin quality. They also reflect patterns from specific populations and trends of an era. A patient may have a profile outside the reference and a perfectly acceptable harmony for them.
In Harmy
The app projects from a frontal photo and does not analyze the profile or the Z angle. Lips and Chin are marking points that simulate the result from the front, as an illustrative simulation. Profile reading remains with the professional, through clinical exam and standardized photos.
Also called: Z line, Merrifield Z angle.
Frequently asked questions
What is the Merrifield Z angle?
It is the angle formed between the Frankfurt plane and the line touching the chin and the most forward lip. It serves as a profile reference, with no fixed ideal value for every patient.