BIOANATOMY AND BIOMECHANICS
The chin is a protrusion of soft tissue overlying the distal point of the mandible. Superiorly, it is bordered by the mental crease, and inferiorly, it merges into the neck. Laterally, the chin merges into the jawline.
The soft tissues of the skin are supplied mainly by anastomosing vessels branching from the facial artery as it deflects superiorly and lateral to the oral commissure. Sensory innervation derives from the mental nerves and motor innervation is provided by the marginal mandibular branch of the facial nerve. In most cases, a larger caliber vessel is identified running horizontally at the junction of the mentalis with the orbicularis oris. The mental nerve ascends deep to the musculature to innervate the lip.
The contour of the chin varies substantially from one individual to another. In youth, the mental crease is often minimal, and a defined, sharp arciform mental crease is more common with age and sun exposure. The soft tissue of the chin may be clefted or rounded.
The chin is a soft dome of tissue. The dermis may be either thin or elastotic and cobblestoned. The underlying adipose is more fibrous than in other areas of the face as septae connect the underlying mentalis to the overlying dermis. There is not a clear plane on the chin between the adipose and the mentalis, as the two merge together.
Mobility on the chin is found in two planes. First, even though the adipose and mentalis intermingle, a plane can be created over the surface of the mentalis. This provides substantial motion and also prevents scar inversion in suturing smaller and moderate wounds. For closure of extensive defects, the mentalis may actually be separated from its insertion onto the mandible.
Small-to-moderate wounds on the chin are amenable to vertical linear closure. Because of the prominent convexity of the chin as it passes over the mandible, linear repairs often need to be longer than would be needed to avoid standing tissue cones on a flat surface. A consequence of performing a linear repair on the chin can be an elevation of the margin of the lip as the chin bows upward at closure. Similarly, if the repair is not carried over the margin of the chin, a standing tissue cone may develop which projects from the jaw (Fig. 15.1). In patients with a prominent mental crease, a vertical repair that bridges the chin and lower lip should generally be avoided. A way to mitigate the impact of crossing the crease is to perform the repair as an S-plasty (Fig. 15.2).
Linear repair on the chin. (A) Operative wound on the lower chin. (B) Linear repair coursed over the jawline to avoid a pucker. (C) Repair ...