Guide Estimating chin implant size based on chin convexity

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Estimating implant size in chin augmentation: A simplified approach

A receding chin, colloquially known as a weak chin, is a significant aesthetic impediment to a pleasing face. Multiple techniques exist to evaluate the poorly projecting chin, but most are imprecise when it comes to choosing the proper implant size. This ...

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Conventionally, the ideal chin is thought to be one that is 1mm to 3mm posterior to a vertical plane dropped from the vermilion border of the lower lip in the profile view (Figure1A). A chin that doesn't meet this requirement is considered recessed and needs to be augmented.

In correcting the deficiency, one might assume that the size of the chin implant should simply equal the difference between the ideal projection and the present deficient one. However, in practice, this calculation is unpredictable if used alone. Merely increasing the size of the chin to a level 1 mm to 3 mm posterior to the vermilion vertical line may create an exaggerated projection if the lip-chin sulcus is deep preoperatively (Figure1B).

Figure 1) A Conventionally, the ideal chin is one that extends to few millimeters behind a vertical plane dropped from the vermilion of the lower lip; B However, a potentially exaggerated projection may result in the case of a deep lip-chin sulcus with a convex chin.

For this reason we have to take into account the degree of chin convexity (curvature of the chin pad) in the profile view when choosing the size of the implant needed.

The relation between chin convexity and implant size
The insertion of a chin implant has two main effects on the chin appearance: increasing its projection and its convexity in the profile view.
However, the main limiting factor affecting the choice of the implant size is the preoperative chin convexity, or lack of it. As Figure 1 shows, there is a limit on the number of millimetres that may be added to the different chin profiles.

Chin convexity classification
The degree of chin convexity in the profile view may be assigned to one of three categories (Figure 2):

1. Convex, with the chin profile presenting mostly as a fully convex line.
2. Vertical, with the chin profile presenting mostly as a vertical line.
3. Diagonal, with the chin profile presenting mostly as a very diagonal (posteriorly angled) line.

Figure 2) Chin classification based on the chin convexity: convex, vertical and very diagonal, with the corresponding suggested midline projection (thickness) of the chin implant.

Implant sizing based on the chin convexity classification
If the chin profile is fully convex (type 1), a relatively thin implant of about 4 mm thickness (projection or anterior to posterior dimension) at the midline should be used. The vertical category of chin (type 2), in which the chin profile is mostly a vertical line, requires a moderately thick implant measuring about 8 mm at the midline. Finally, in the diagonal category chin profile (type 3), where the chin profile is markedly angled backwards, a larger implant measuring around 12 mm in thickness at its centre is indicated.

It is important to note that it is the thickness at the centre of the implant (ie, the projection of the implant) that has the greatest impact on the final result. The length and width of the implant contribute less to the outcome.

Before and After pics

Figure 5) A and B Preoperative view of a markedly diagonal chin profile type 3; C and D Postoperative result seven months later. A silastic implant with a 12 mm midline thickness was used

Figure 3) A and B Preoperative view of an almost vertical chin (type 2); C and D Postoperative result eight months later after using an 8 mm thick implant. This size is the most commonly used one