Dynamic muscle discord in the context of non-surgical lip enhancement

The lips are a highly complex and mobile anatomical unit. The lips' aesthetic appearance is affected not only by their inherent shape and dimensions, but also by the underlying dental/skeletal framework and a complex three-dimensional assembly of muscular slips that act on the lips. These various muscle groups work together or in opposition with each other in a highly sophisticated and coordinated pattern of movement to control lip posture, as well as the shape of the smile and other perioral expressions. These muscles also control the lips' highly intricate movements that underly speech.

The mentalis muscle is a central muscle that is situated at the tip of the chin. In addition to raising the chin, the mentalis muscle elevates the lower lip and, when more strongly contracted, can evert or protrude the lower lip, providing stability during pouting. Patients with retrognathia (recessed or ‘weak’ jaw) often present with both dental malocclusion and a positional discrepancy or gap between the upper and lower lips at rest. This discrepancy reduces the ability of the lips to come together to form an adequate seal around the mouth (termed lip incompetence). Often, patients with retrognathia will present with increased mentalis muscle activity or hypertonicity (referred to as mentalis strain). Mentalis strain may occur as a compensatory mechanism, as the muscle tries to maximally elevate the lower lip, bringing it in closer approximation with the upper lip to improve the mouth seal. The increased mentalis activity causes the lower lip to become more everted, and the chin develops an upward slant at rest due to the pull of the muscle fibres. Furthermore, the mentalis strain creates a noticeable dimpling in the chin due to contraction of the mentalis muscle fibres that attach to the overlying dermis (commonly referred to as a ‘poppy chin’). Retrognathia and associated mentalis strain can be accompanied by increased tonicity of the lower lip depressor muscles, as well as attenuation of the sphincteric action of the intrinsic lip muscle (orbicularis oris or OOM). In turn, these changes can have a wider impact on the appearance of the lips, affecting not only the lower lip, but also the upper lip indirectly. The dominance of a particular group of muscles over their counterparts results in an imbalance between the muscles, which is termed dynamic muscle discord (DMD), where there is a shift in pull and, hence, the position of the lips favouring the more dominant muscles. Consequently, DMD can affect the posture, profile and shape of the lips, both at rest and during movement.