INTRODUCTION
The glabella is the first area to be noticed in facial expression, and its contraction is usually associated with negative feelings.1,2 It is also the most frequently area studied and the first to receive US Food and Drug Administration (FDA) approval for cosmetic botulinum toxin treatment.3,4
Contraction of the glabella changes the position and height of the eyebrows, that are fundamental in face recognition and facial expression.5,6 It involves the action of several muscles such as corrugators and orbicularis oculi (which approximate and depress the eyebrows), procerus and depressor supercili (depressors), and the frontalis muscle (the only lifter of the upper face).7
Understanding the importance and necessity of this research lies in recognizing that glabellar expression lines stem from repetitive muscle contractions, which evolve from dynamic lines into static wrinkles over time.8-10 These wrinkles manifest differently due to various factors such as gender - where men typically exhibit thicker, oilier skin, larger muscle mass, well-defined superciliary arches, a more pronounced glabella,11 wider facial movements, and more severe facial wrinkles, particularly excluding the perioral area.12-14 Additionally, factors such as aging contribute, with volume loss and changes in muscle tone or laxity influencing wrinkle formation. Ethnicity, sun exposure, and physical activity further contribute to the variations observed in the development of these wrinkles.15-17 Understanding these multifaceted influences is crucial for developing effective treatment and prevention strategies.
Although the anatomy is similar among individuals, how individuals engage their musculature varies.8 A study showed that Europeans have generally larger facial movements than Asians, especially in the eyebrow, nose, and mouth regions. An exception must be made to the eye region, where Asians have a larger excursion of the eyelids.19 It is important to
