The incisivii labii are small paired accessory muscles of facial expression arising from the alveolar processes adjacent to the lateral incisors and converging on the modiolus (the fibromuscular knot at the lip commissure). They are sometimes absent and are considered minor contributors to commissure mechanics. They are relevant in cleft lip repair and in the anatomical basis of the modiolus.
| Origin | Upper incisivii: anterior maxillary alveolar process adjacent to the lateral incisor; Lower incisivii: anterior mandibular alveolar process adjacent to the lower lateral incisor |
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| Insertion | Angle of the mouth (modiolus), blending with orbicularis oris and other modular muscles converging at the commissure |
| Nerve Supply | Buccal branches (upper incisivii) and marginal mandibular branches (lower incisivii) of the facial nerve (CN VII) |
| Blood Supply | Superior and inferior labial arteries |
| Actions | Draw the angle of the mouth medially and superiorly (upper) or inferiorly (lower); contribute to the complex mechanics of the lip commissure and the modiolus during speech and facial expression |
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The modiolus at the lip commissure is a three-dimensional fibromuscular complex where seven pairs of facial muscles converge, including the incisivii labii. Accurate restoration of modiolus anatomy in cleft lip repair and in commissure reconstruction after tumour resection ensures natural commissure position and movement. The incisivii labii contribute to the inward pull of the commissure during pursed-lip expressions and are disrupted in extensive lip resection requiring commissure reconstruction with Karapandzic or Estlander flaps.
Resection or reconstruction of the lip commissure for carcinoma or congenital deformity disrupts the convergence of the incisivii labii and other modular muscles at the modiolus; accurate commissuroplasty recreates the fibromuscular knot position to ensure symmetric commissure elevation during smiling and correct lip competence at rest.
