Date Published: March 16, 2018
Publisher: Springer US
Author(s): Maurício de Maio.
Consideration of facial muscle dynamics is underappreciated among clinicians who provide injectable filler treatment. Injectable fillers are customarily used to fill static wrinkles, folds, and localized areas of volume loss, whereas neuromodulators are used to address excessive muscle movement. However, a more comprehensive understanding of the role of muscle function in facial appearance, taking into account biomechanical concepts such as the balance of activity among synergistic and antagonistic muscle groups, is critical to restoring facial appearance to that of a typical youthful individual with facial esthetic treatments. Failure to fully understand the effects of loss of support (due to aging or congenital structural deficiency) on muscle stability and interaction can result in inadequate or inappropriate treatment, producing an unnatural appearance. This article outlines these concepts to provide an innovative framework for an understanding of the role of muscle movement on facial appearance and presents cases that illustrate how modulation of muscle movement with injectable fillers can address structural deficiencies, rebalance abnormal muscle activity, and restore facial appearance.
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Theories of facial aging have largely focused on changes in skin, underlying fat, and bone that result in sagging and folds , while the role of muscle in aging has generally been neglected . The complementary and distinct ways in which injectable fillers and neuromodulators have generally been used for rejuvenation and improvement of facial esthetics  illustrate how skin and fat are considered separately from muscle action. Injectable fillers are customarily used to fill static wrinkles, folds, and localized areas of volume loss [4–7]. Neuromodulators (such as onabotulinumtoxinA) are used to reduce muscle movement in overacting muscles, for example, to diminish hyperdynamic lines or correct position or asymmetry by reducing muscle activity [8–13]. However, long-term observations of patients with certain structural deficiencies treated only with injectable fillers suggest that fillers can also be used to alter muscle movement (myomodulation) in facial esthetic treatments and may provide another tool, in addition to neurotoxins, in the armamentarium of facial muscle modulation.
Mimetic muscles have several characteristics that differentiate them from skeletal muscles. Generally, mimetic muscles have their origin in bone and insert on the skin and among the fibers of other muscles, with no tendons, except for the sphincteric muscles . In addition, mimetic muscles appear to lack typical muscle spindles [17, 18], which function in resetting resting tone. Together with these characteristics, three key aspects underlie the role of muscle movement in facial expression: length–tension relationship, muscle pulley and lever systems, and the action of functional muscle groups.
Each of these biomechanical concepts contributes to our understanding of changes in muscle movement during aging, as well as in cases of facial palsy. In the typical youthful face, there is a clear convexity of the upper cheek due to intact zygomatic arch, malar fat, and SOOF. Under these ideal conditions there is optimal pulling force of the zygomaticus major muscle. In aging, midface volume loss, displacement of fat pads, and loss of skin elasticity may alter zygomaticus major muscle action. The distance between the zygomaticus major origin in the zygomatic bone and insertion in the modiolus area at the corner of the mouth  increases when skin sags due to deflation of fat pads, and as a result, the corner of the mouth falls (Fig. 1). The stretching of the fibers of the zygomaticus major results in a loss of resting tension and power in contraction. At the same time, any mechanical advantage of the lever effect over the lateral SOOF is reduced as SOOF volume is depleted. Consequently, the zygomaticus major can no longer adequately counterbalance the downward gravitational pull and the contracting force of the DAO [11, 26].Fig. 1Aging and hypothetical treatment effects on muscle action. Normal youthful facial structure (underlying bone and fat) gives muscle fibers convexity, which allows powerful contraction of levator muscles. The levator and depressor muscles are balanced, and the structures are in their normal/youth position. In aging, underlying support for muscle is lost with deflation of facial structure. I believe that the mechanical advantage provided by the lever fulcrum is diminished and the levator loses lifting power to counteract gravity. Muscle fibers are stretched as skin sags. With reduced opposition, the depressor increases in tone over time and pulls facial structures downward in a domino effect. A filler bolus replaces lost structure (fulcrum), increasing mechanical advantage of the levator muscle. The levator’s movement is facilitated. This reduces sagging and balances contraction of the depressor, halting the chain of events triggered by aging
Deficits in facial structure can yield abnormal muscle action reflected at the skin and across the face. When structural support is absent or lost, muscle action is altered, affecting the balance in activity between muscles. Examining the interactions between facial structure and muscle movement—and recognizing unbalanced action in muscle synergists and antagonists—allows the clinician to understand the effects on appearance both at rest and on animation. The cases presented here provide support for the use of myomodulation, addressing muscle movement with injectable fillers in the treatment of facial structural deficiencies. Injectable filler treatment can be used to support muscle movement or block overaction regardless of whether the imbalance is due to a structural deficiency or a loss of volume in aging, and indeed, the clinician does not need to isolate the contribution of these factors to an imbalance in order to use fillers to treat it.