Anatomy and histology of the frontalis muscle.

ABSTRACT

PURPOSE: To determine the gross and histologic configurations of the medial and lateral frontalis muscle. METHODS: After making a midcoronal incision and bluntly dissecting to the orbital rim, the frontalis muscle was marked and measured. A protractor was used to measure the frontalis-orbicularis angle (FOA) and, when present, the angle of central bifurcation (AOB). Three strips of full-thickness forehead soft tissue measuring 0.5 cm × 8 cm were excised 3, 4.5, and 6 cm above the supraorbital notch and analyzed histologically for the presence of skeletal muscle fibers. Data were analyzed using 2-sample t tests, paired t tests, Pearson correlations, and mixed effect models. A p value of ≤ 0.05 was considered statistically significant. RESULTS: Sixty-four hemifaces of 32 cadavers (16 males) were dissected. All specimens were Caucasian. The average age was 78.2 years (range, 56-102 years). The average FOA was 88.7° (13.0°), and the average AOB was 90.0° (26.4°). A visible midline bifurcation occurred in 28 of 32 subjects (88%) at an average height of 4.7 cm (range, 2.4-7.2 cm) superior to the supraorbital notch. Continuous skeletal muscle fibers were present within the midline bifurcation histologically in 89%, 75%, and 11% of specimens 3.5, 5.0, and 6.5 cm above the supraorbital notch, respectively. In 46% of individuals, skeletal muscle fibers were continuously present microscopically within the gross bifurcation. CONCLUSION: While a medial frontalis muscle bifurcation occurs grossly in most senescent Caucasians, muscle fibers exist microscopically within this zone in nearly half of individuals.