Dynamic Evaluation of Skin Displacement by the Frontalis Muscle Contraction Using Three-Dimensional Skin Displacement Vector Analysis

ABSTRACT

Purpose@#Understanding the muscle actions and resultant skin movement can enable more safe and effective botulinum toxin injection for the treatment of forehead wrinkles. We aimed to investigate skin displacement patterns of the forehead and adjacent skin due to frontalis muscle contraction using three-dimensional skin vector displacement analysis. @*Materials and Methods@#Thirty healthy individuals were enrolled. Photographs of the face were taken at rest and during maximal contraction of the frontalis muscle. Each expression image was aligned to its respective static image to compute the differences in the skin position. @*Results@#When frontalis muscle contracts, forehead skin displacement vectors were mostly vertical (63.4%), followed by lateral oblique (33.3%) and medial oblique (3.3%). In 53.3%, only the lower part of the forehead moved upward, while 40.0% showed bidirectional skin movement with transition line at a mean distance of 59.4 mm above the pupil. Moreover, 86.7% showed asymmetric skin displacement, and 83.3% showed both glabellar and eyebrow skin displacement. Frontalis muscle contraction also induced medial 2/3 (50.0%) or entire (33.3%) skin movement of the temple. @*Conclusion@#Botulinum toxin injection into the forehead can be individualized by considering the vector and asymmetry of skin displacement. Vertical or medial vector requires more centrally located injections, while laterial vector requires more laterally located injections. The presence and location of the vertical transition line are important for preventing ptosis when treating forehead lines with botulinum toxin. Glabellar movement during frontalis contraction suggests the need for a concomitant injection into the glabella to prevent glabella wrinkle accentuation.