Anatomical etiology of bunny lines based on cadaveric dissection and ultrasonographic evaluation

Bunny lines are vertical and diagonal wrinkles attributed to contractions of the nasalis muscle. Common management involves botulinum toxin (BoNT) injections into wrinkles immediately below the nasal bridge. Here, we investigated the anatomical structures underneath bunny lines and their relationships with adjacent muscular structures. The area between the medial canthus and nasal dorsum was examined by ultrasonography in 13 adult volunteers (mean age, 29.4 years) and dissected in 39 cadavers. The findings consistently showed a non-muscular area (bunny triangle) between the borders of the procerus, nasalis, orbicularis oculi (OOc), and levator labii superioris alaeque nasi (LLSAN) muscles. The size and shape of the bunny triangle vary according to the width of the procerus and nasalis muscles. The muscles neighboring the bunny lines affect the thinning of the thin skin between the medial canthus and nasal dorsum. Our results indicate that conventional BoNT injection into bunny line wrinkles is likely to target connective tissue rather than a muscle, potentially producing suboptimal results. Therefore, the BoNT injection points probably need to target the borders of the procerus, nasalis, OOc, and LLSAN muscles to be most efficient.