Frontalis Muscle Asymmetry and Lateral Landmarks.

PURPOSE: To investigate frontalis muscle asymmetry and characterize its lateral interdigitation with the orbicularis oculi muscle.

METHODS

After making a mid-coronal incision and bluntly dissecting to the orbital rim, the frontalis muscle was exposed, marked, and photographed. The right and left muscle bellies were analyzed and compared in both pixels and cm ratios generated with NIH ImageJ software. A ratio of ≥1.5 was considered significantly asymmetric. The lateral interdigitation of the frontalis and orbicularis oculi muscles was measured from the supraorbital notch with a metric ruler. Data were analyzed using 2-sample t tests, paired t tests, log scales, and nonparametric tests were performed for sensitivity analyses. A p value of ≤0.05 was considered statistically significant.

RESULTS

Fifty-eight hemifaces of 29 Caucasian cadavers were studied for muscle belly asymmetry. Thirty-six hemifaces of 18 Caucasian cadavers (9 males) were dissected for lateral landmarks and average age of these specimens was 73 years (range: 35-91 years). Significant asymmetry in muscle belly area was found in 6/29 (20%) specimens, with the right muscle belly larger in all 6 specimens. On average, the right muscle belly area was 1.23 times that of the left (p = <0.001). The average frontalis-orbicularis interdigitation occurred 3.4 cm lateral to the supraorbital notch.

CONCLUSIONS

Significant frontalis muscle belly asymmetry exists in 20% of Caucasians cadavers. The right muscle belly was larger on average and in all cases of significant asymmetry. The frontalis muscle interdigitates with the orbicularis oculi on average 3.4 cm lateral to the supraorbital notch.