Transparency of Mental Anatomy: Morphology of Cobblestone Chin.

Cobblestone chin is a common aesthetic concern caused by contraction of the mentalis muscle; however, its anatomic basis remains poorly understood. This study aimed to clarify the fiber architecture of the mentalis muscle and its relationship with chin surface irregularities. Anatomic dissection was performed on 6 cadavers (4 males and 2 females). In fixed cadavers, lower facial blocks were processed using potassium hydroxide and hydrogen peroxide to create transparent specimens for analysis under polarized light. In fresh cadavers, the superficial and deep muscle layers were exposed through dissection. Sagittal sections and whole blocks were evaluated to identify fiber orientation and attachment patterns. Three types of muscle fibers were identified: horizontal fibers inserted into the chin skin and supported the lower lip; transverse fibers crossed inferomedially with point attachments to the skin, contributing to the cobblestone appearance; and vertical fibers descended to the mandibular rim and acted antagonistically. Cobblestone morphology was associated with pointwise skin insertion of transverse fibers, whereas horizontal fibers formed thick bundles that did not cause surface irregularity. The mentalis muscle comprises 3 distinct fiber groups with complex, interwoven patterns. This morphologic organization underlies the formation of the cobblestone chin. Transparent visualization offers valuable insights for aesthetic surgical procedures and anatomic education.