The Visualization of the Functional Role of the Depressor Septi Nasi Muscle by Ultrasound.

Supplemental Digital Content is available in the text.

The depressor septi nasi muscle (DSN), which arises from the incisive fossa of the maxilla into the medial crura, is considered to have the function of pulling the nasal tip downward[1]; however, its significance is ambiguous. The objective of this study was to visualize the contraction and to evaluate the function of the DSN using ultrasonography.

Five healthy adult volunteers were recruited. The subjects had no history of facial trauma, neuromuscular disorder, or any other congenital disorder. All examinations were performed using a diagnostic ultrasound system (Aplio500; Toshiba Medical Systems, Japan). A 12 MHz linear-array transducer was applied with a polymer gel pad (Sonagel; Takiron, Japan) as a coupling medium. The transducer was positioned perpendicular to the skin surface and parallel to the philtrum at the middle of the nostril floor. Videos of the repetitive motion of the upper lip between resting and protrusion or smiling were recorded as avi files. The ethical committees of Kansai Electric Power Hospital approved the study protocol before it was performed.

The DSN, an arcuate structure oblique to the maxilla, could be observed under the orbicularis oris muscle (OOM) at rest (Fig. 1). It was observed to continue to the columella with the transducer shifted inward. At the middle of the DSN, it merged with the cranial end of the OOM. During lip protrusion, the DSN was observed to thicken and straighten, and the junctional portion of the DSN and the OOM formed a T shape. The origin of the DSN was clearly observed on the incisive fossa. The muscle belly of the DSN and the junctional portion moved toward the caudal side as the DSN contracted (see video, Supplemental Digital Content 1, which demonstrates a lip protrusion, http://links.lww.com/PRSGO/A475). In contrast, the DSN was stretched by the rushed OOM during smiling (see video, Supplemental Digital Content 2, which demonstrates smiling, http://links.lww.com/PRSGO/A476).

Fig. 1.

A) Resting. B) Lip protrusion. Att, attachment of the OOM to the DSN; BS, buccal sulcus; Epi, epithelium; In, incisor; Max, maxilla; NS, nostril sill; Or, origin of DSN.

The DSN is well known to influence the results of rhinoplasty.[2,3] However, there is confusion about the origin and insertion of the DSN, even in cadaver studies.[2-4] Although the effects of surgical treatment on the DSN are obvious, the functional significance of the DSN remains to be clarified. The observation of the obtained images revealed that at the time of lip protrusion the DSN not only pulls down the bottom of the nostril but also pushes down and fixes the cranial end of the OOM to the maxilla. The previous study reported that the crossing fibers of the OOM increase the thickness of the philtrum during protrusion.[5] These results suggest that the synergy of the DSN and the crossing fibers of the OOM actually enables lip protrusion and simultaneously prevents nostril obstruction.

The DSN is a topical structure that has received a great deal of attention from cosmetic surgeons in relation to procedures to improve nasal or smiling deformities. The present results, which revealed new information about the function of the DSN, provide useful information not only for cosmetic surgery but also for lip surgery for conditions such as cleft lip and facial nerve paralysis.

See video, Supplemental Digital Content 1, which demonstrates a lip protrusion, http://links.lww.com/PRSGO/A475.

See video, Supplemental Digital Content 2, which demonstrates smiling, http://links.lww.com/PRSGO/A476.