Anatomical proposal for the injection of botulinum neurotoxin for horizontal forehead lines
The present study (Yi et al. Plast Reconstr Surg . 2023 Mar 27. doi: 10.1097/PRS.0000000000010469) examines the anatomical factors for the injection of botulinum neurotoxin (BoNT) into the frontalis muscle for the treatment of horizontal forehead lines. Precise knowledge of the anatomy of the frontalis muscle is important in order to avoid unexpected adverse effects such as ptosis and “samurai eyebrows”. A precise injection into the frontalis muscle allows maximum effects with a minimum amount of BoNT. The study provides updated information on locating the injection point according to the anatomy of the frontalis muscle. Optimal injection sites are proposed on the basis of external anatomical landmarks of the forehead. More precise injection techniques can avoid undesired effects of botulinum neurotoxin. The results of this study contribute to a more accurate and safer procedure for BoNT injection.
Anatomically, the forehead is an exceptionally thin region, with an average thickness from the skin to the periosteum of 0.5 cm. As a result, a minimal injection of botulinum neurotoxin (BoNT) can easily be distributed to all layers. A subcutaneous injection distributes the BoNT and produces the same effect as a targeted injection into the muscle. A careful injection into the dermis, including the formation of a papule and a raised area of skin with BoNT, can reduce complications.
A further factor that should be taken into account is the position of the lateral border of the frontalis muscle. The frontalis muscle lies 1 cm lateral to the superior temporal line and connects with the superficial temporal fascia. In 2008, the recommended dosage for horizontal forehead lines in women was 6-15 U in total (Botox units). To preserve natural facial expression, the appropriate dosage for women is now 4-6 U. Because of an anatomical tendency towards more strongly developed muscles in men, a dosage of up to 6-8 U BoNT in total is recommended for men. The dilution is 100 U mixed with 4 ml of saline solution.
It is important to measure the width of the patient's forehead precisely. For a typical forehead width (less than 6 cm from the apex of the eyebrow arch to the hairline), two imaginary horizontal lines are drawn to divide the height of the forehead into three sections. Two midpupillary lines as imaginary vertical lines and two further imaginary vertical lines crossing the medial and lateral canthus are drawn. With six vertical and two horizontal imaginary lines, 12 different intersection points are created. It is recommended to inject at 6 of the 12 points in order to reduce pain and adverse effects, on the assumption that the BoNT spreads a few centimetres from each injection site. As the BoNT spreads more than 1 cm from the injection point, injection at a minimum of six points (lower point of the medial and lateral canthus and upper point of the midpupillary line) with 1 U BoNT each is recommended, 6 U in total (see Figure 1).
Fig. 1
Source: Yi et al. Plast Reconstr Surg . 2023 Mar 27. doi: 10.1097/PRS.0000000000010469