RESUMEN
BACKGROUND: From early dynamic lines in smiling and squinting young people, crow's feet lines in the lateral canthal region evolve through prolonged tissue remodeling into mature, stable rhytids after the mid-to-late thirties, and requests for rejuvenating treatment. Because of the heterogeneous fanning behavior of crow's feet lines, individualizing neuromodulation with botulinum toxin injections might be a sound strategy to maximize aesthetic outcomes. AIMS: To investigate the prevalence of variable crow's feet line patterns at maximum contracture and while smiling and comment on the relative distribution of such patterns and the influence of aging. These considerations will be the basis and rationale for future studies that will individualize the total 24-unit dose according to the distribution of crow's feet lines. METHODS: Identifying CFL orientation at maximum frown and while smiling; assessment tool: the four-class 2015-version of Kane's classification further modified by converting the former "central fan pattern" into a new, more precisely defined "extended full-fan pattern" class. RESULTS: The 323 cohort subjects had a mean age of 51.5 ± 9.28 years. Although with some differences, outcomes in a central-southern real-world European Caucasian population appear similar to the distribution first studied in 2003 in a highly selected population in New England involved in double-blind clinical trials. The full-fan pattern predominated both at maximum contraction and when smiling, followed in prevalence by the upper-fan pattern at maximum contraction and the extended full-fan pattern while smiling. Age significantly influenced the crow's feet line pattern-lower-fan patterns were more frequent than other patterns in the real-world younger middle-age group (40-49 years old); full-fan and extended full-fan patterns progressively more prevalent with aging. CONCLUSIONS: The study confirms but somewhat modifies the previous evidence about the non-homogeneous distribution of crow's feet line fanning patterns in different age groups and reinforces the case to individualize the topography of botulinum toxin injections.