Pivotal role of injection volume on sunken cheek prevention in masseter muscle BoNT-A injection: A cadaver study

Background Botulinum neurotoxin A injection is a popular noninvasive alternative for the treatment of masseter hypertrophy. This study was conducted to identify a safe injection volume to avoid sunken lateral cheek after botulinum neurotoxin A injection in the masseter muscle. Methods One milliliter or 0.5 ml of indocyanine green was randomly injected into either side of the masseter muscles of 18 fresh cadaver heads. The bilateral spreads of indocyanine green within the masseter were observed by a fluorescence imager. The masseters were then dissected, and the spreading distance was measured. Results Dye spreading showed an oval shape parallel to the long axis of the masseter muscle. The lower edges of the masseters were all stained with indocyanine green. The upward spreading exceeded the mouth corner-tragus line in 94.44% (17/18) of the masseters on the 1-ml side, and 11.11% (2/18) of the masseters on the 0.5-ml side. The spreading area and upward spreading distance on the 1-ml side (9.95 +/- 0.48 cm(2), 3.18 +/- 0.31 cm) were significantly larger than the 0.5-ml side (7.13 +/- 0.80 cm(2), 2.08 +/- 0.32 cm). Conclusions The spread of indocyanine green within the masseter occurs in direct proportion to its volume. A bolus of 1 ml easily exceeded the mouth corner-tragus line and cause sunken cheek.