Anatomical consideration for botulinum toxin injection of the frontalis muscle based on analysis of intramuscular innervation

The facial nerve is the seventh cranial nerve, and its temporal branch (TBFN) innervates the frontalis muscle. Peripheral nerve disorders involving the facial nerve can lead to facial palsy, for which a common non-invasive treatment approach is to inject a chemodenervation agent such as botulinum toxin (BoNT). The purpose of this study was to provide anatomical suggestions for BoNT injection sites in the frontalis muscle based on the intramuscular innervation pattern of the TBFN as identified objectively using Sihler's staining. Nineteen hemifaces containing the TBFN and the frontalis muscle were harvested from 15 embalmed cadavers according to facial landmarks. The frontalis muscle was divided into 16 areas to identify the prevalence rates of distal nerve endings and the arborization pattern of the TBFN after applying Sihler's staining. Distal nerve endings of the TBFN were most commonly found in area B2 (17 of 19 specimens, 89.5%), followed by in area B3 (n = 15, 78.9%). No distal nerve ending was observed in area A1. Two types of the arborization pattern of the TBFN were observed. We propose four BoNT injection sites based on the intramuscular innervation pattern of the TBFN in the frontalis muscle as identified using Sihler's staining.