Anatomical Relationship of the Inferior Bundle of the Incisivus Labii Inferioris With the Depressor Labii Inferioris and the Platysma.

The current study examined the inferior bundle of the incisivus labii inferioris muscle (ILI) and its morphologic and spatial relationships with regard to the depressor labii inferioris muscle (DLI) and the platysma. Incisivus labii inferioris muscles and their inferior bundles were investigated in 50 specimens from embalmed Korean adult cadavers. The inferior bundle of the ILI was divided from the originating fibers of the ILI, and it was present in all specimens (100%). The inferior bundle of the ILI could be distinguished into the transverse and inferolateral slips according to their courses and locations. The inferior bundle of the ILI was classified into 3 types according to the presence of the transverse and inferolateral slips. In Type I (44%), the inferior bundle of the ILI had both the transverse and inferolateral slips. In Type II (48%), the inferior bundle of the ILI had only the transverse slip. In Type III (8%), the inferior bundle of the ILI had only the inferolateral slip. The transverse and inferolateral slips were usually attached to the deep fibers of both the platysma lateral to the DLI and the lateral portion of the DLI via aponeurosis, just below the ILI and above the inferior margin of the mandible, respectively. The new anatomical findings of the present study might contribute to the understanding of the detailed movements required to raise the lower lip. This knowledge will be useful for botulinum toxin type A therapies, various facial surgeries, orthodontic treatment, and electromyographic analyses in this area.