A quantitative analysis of the brow shape after frontalis muscle contraction

To measure the changes of the eyebrow's contour after frontalis muscle (FM) contraction.

Two consecutive pictures of the eyes and frontal region of 36 volunteers (15 men and 21 women) were obtained with the forehead relaxed and upon maximal frontalis muscle contraction. Bézier lines representing the brows' contours, obtained with the ImageJ software, were graphically sampled with a resolution of 0.025 mm. The contours of the relaxed and elevated brows were compared regarding the position of the contour peak (CP), the degree of elevation of brow central point, and the medial and lateral areas outlined between the brow contours before and after frontalis contraction. The asymmetry of the brow contour after FM contraction was calculated by the Naeije formula as (Medial-Lateral)/(Medial + Lateral). A difference of 10% or less between the lateral and medial areas was taken as a cutoff value for symmetrical brow elevation.

In 62 (86.1%) of the eyes, the location of the brow's CP was lateral to the brow's midpoint. When the brows were raised, the CP of 60% of the brows was displaced medially and 40% laterally. The motion of the CP with medial displacement (mean = 3.7 mm) was significantly larger than those displaced laterally (mean = 1.9 mm) decreasing the number of brows with lateral peaks from 62 (86.1%) to 54 (75%). No difference was observed between the mean elevation of the central point of the brow for males (7.9 mm (3.2 SD) and females (7.4 mm, 3.0 SD). In 58.3% of the sample, the positional change of the medial and lateral sectors of the brows was symmetrical, and among the 41.7% asymmetrical cases, the elevation of the medial side was higher than the lateral in 66.7% of the brows.

The brow changes during FM contraction concur well with the anatomic data of the FM shape. The contour of most brows is displaced toward the medial direction when the brow is elevated. The pattern of brow shape change is an indication of the FM anatomy and must be carefully analyzed before any procedure involving the FM, such as chemodenervation injections or frontalis slings for blepharoptosis correction.