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.
Patients with retrognathia
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.
Case study
In the case presented in this article, the patient had retrognathia, dental malocclusion, mentalis strain and lip incompetence (Figure 1). Additionally, the mentalis strain was asymmetrical with the excessive lower lip eversion and the chin dimpling was more noticeable on the right side.
At rest, the dominance of the lower lip depressor muscles also indirectly affected the lip shape through their exertion on the lower fibres of the OOM. The OOM is a complex, multi-layered muscle that attaches through a thin, superficial musculoaponeurotic system to the dermis of the upper and lower lips and serves as a point of attachment for other perioral muscles. Its deep fibres have a constrictor or sphincter action on the mouth that enables it to purse the lips. Increased exertion on the lower half of the OOM by the lower lip depressors can also indirectly affect the upper half of the muscle and reduce its overall ability to purse the lips, particularly at rest. In this case, the attenuation of the OOM resulted in a subtle inferolateral elongation of the upper lip at rest. However, during smiling, the levator labii superioris (LLS) and levator labii superioris alaeque nasi (LLSAN) dominated the upper lip, evident with an increased upper lip excursion with an excessive gingival show or ‘gummy smile’.
These types of clinical presentations represent some of the most challenging cases to treat. The aesthetic appearance of the lips in these patients is impacted by issues that are both intrinsic to the lip body (volume loss, poorly defined features, asymmetry and disproportionality), as well as extrinsic forces caused by the DMD and dental malocclusion that affect the lip posture and movements.
In lip enhancement cases where DMD is present, it is important to attenuate the impact of the DMD first to improve the posture and profile of the lips before attempting to treat the lip bodies themselves. Typically, the treatment of DMD is undertaken through volume replacement to improve the underlying structural framework in the perioral region. In complex cases, this may involve a staged approach spread over time with a series of soft tissue augmentations using higher cohesivity fillers in the pre-periosteal and deep subcutaneous layers. In this case, a chin augmentation was performed in three stages using hyaluronic acid (HA) dermal fillers, which resulted in a mechanical block or inhibitory myomodulation of the mentalis muscle, while also providing vertical support to the lower lip. Following treatment, there was an attenuation of the mentalis strain, an improvement in the lip competence and reduction of the chin puckering/dimpling (Figure 2). In addition, neuromodulation of the LLSAN and LLS muscles with botulinum toxin in combination with augmentation of the piriform fossae resulted in inhibitory myomodulation of these muscles and a reduction in the gingival show (Figure 3). The patient derived a satisfactory improvement in their lip aesthetic appearance both at rest and during expression with results that have remained stable for more than 2 years post-treatment.
Summary
Patients seeking lip rejuvenation should always be assessed for DMD. Where present, it is highly advisable that this condition is treated first before attempting to augment the lip body itself. Correction of DMD will help to optimise the resting posture of the lips and excursion patterns during facial expressions. Unfortunately, medical aesthetic practitioners often fail to diagnose DMD, leading to suboptimal aesthetic outcomes, as they attempt to address the positional imbalance between the upper and lower lips through direct augmentation of the lip body. This poorly planned approach will usually result in overfilling of the lips without successfully repositioning them (Figure 4) and may even exaggerate the deficiencies in their lip posture and shape.
Key points
- Dynamic muscle discord (DMD) describes a condition where there is an imbalance in the action between opposing groups of facial muscles, resulting in a shift of the mechanical vectors favouring the pull of the dominant muscle group
- DMD typically occurs in highly mobile areas of the face that are controlled by different groups of muscles, such as the perioral, periorbital and forehead regions
- The aetiology of DMD may differ between different regions of the face. For example, in the forehead, it can be seen as increased activation of the frontalis muscle to correct brow and lid ptosis (termed compensated blepharoptosis)
- Patients with retrognathia often present with DMD in the perioral region to compensate for potential lip incompetence through increased mentalis muscle activity, leading to excessive eversion of the lower lip and an aesthetically suboptimal lip position at rest
- Patients seeking lip enhancement must be carefully assessed for the presence of DMD, and, to avoid potentially suboptimal aesthetic outcomes, this must be addressed before the lips are treated directly.
CPD reflective questions
- In this case study, the dynamic muscle discord (DMD) was caused by an underlying skeletal structural deficiency. Can you think of other causes for DMD and other areas of the face where it may occur?
- How does your concept of DMD relate to your understanding of filler myomodulation?
- What are the challenges when consulting a patient with DMD?