An awareness of the SMAS is crucial in order to prevent injuring the facial nerve during facial surgery. The SMAS is a fascial layer that attaches the facial muscles towards the overlying dermis. In the lower face, the SMAS is continuous with the platysma. Superiorly, the SMAS reaches the level of the zygomatic arch where its fibers are anchored. Above the arch, the temporoparietal fascia may be the same as the SMAS. It joins the deep temporal fascia which extends into the scalp region.
In the lower face and neck, the facial nerve runs deep towards the platysma and also the SMAS. Toward the midline, the nerve continues deep towards the SMAS and runs superficial towards the masseter muscle. In the midface and cheek region, the nerve runs inside the SMAS, but since it passes within the zygomatic arch, it becomes more superficial. In the temporal region, the facial nerve travels inside the temporoparietal fascia. Thus, it might be apparent why facelifts are performed in the subcutaneous or sub-SMAS planes.
The facial nerve enters the face upon exiting the stylomastoid foramen. At this point it's purely a motor nerve. The nerve then travels 15-20 mm sandwiched between the digastric and stylohyoid muscles before entering the parotid gland. Just before its entrance into the parotid, it sends off a little branch to the posterior digastric and stylohyoid muscles. Additionally , it gives off the posterior auricular nerve which travels to the posterior auricular and occipitalis muscles.
During total parotidectomy, it is essential to locate the facial nerve. This is done by mobilizing the parotid superiorly and also the sternocleidomastoid laterally, that will reveal the posterior belly from the digastric. A vital landmark in identifying the main trunk is the cartilaginous tragal pointer.
It's located by following the posterior belly of the digastric towards its insertion at the mastoid, and releasing the parotid attachment to the cartilage from the external auditory canal. The primary trunk lies about 1 cm deep and slightly inferior and medial towards the tragal pointer. A branch of the occipital artery often are available in close proximity, lateral to the nerve.
Within the substance from the parotid, the facial nerve travels in a fibrous plane between your deep and superficial lobes. In the pes anserinus, it divides into two major divisions. One division travels superiorly and the other inferiorly. These major divisions become five branches that exit the parotid: temporal (frontal), zygomatic, buccal, marginal mandibular and cervical.
The buccal and zygmotic branches have many interconnections; however the temporal and marginal mandibular branches are usually terminal endings that don't arborize along with other branches. In addition, the temporal and marginal mandibular branches are at the greatest injury risk during surgery. The incidence of injury during rhytidectomy is under 1%.
The temporal branches run inside the SMAS up to the level of the zygomatic arch. Cranial to this point the temporal (frontal) branch becomes more superficial and enters the temporal region inside the temporopariatel fascia. The frontal branch runs roughly along an upward sloping line extending from 5 mm below the tragus to 15 mm above the lateral aspect of the eyebrow.
After crossing the zygomatic arch, the frontal branch travels in the superficial layer of the deep temporal fascia (temporopariatel fascia). It penetrates the undersurface of the frontalis muscle. To avoid injuring the nerve during elevation from the facial flap, one should dissect in the subcutaneous plane (superficial to the frontal branch) or in the sub-SMAS, "deep" plane (deep towards the nerve).
These branches result from the mandibular division that runs across the inferior border of the body from the mandible in 80% of cases, or within 1-2 cm below the mandible in the remaining cases. These branches run deep to the platysma and course more superficially about 2 cm laterally towards the corner of the mouth. In children the facial nerve anatomy is not as predictable as in the adult, and the described landmarks might not be accurate. In fact, the facial nerve might be located in a more superficial plane then in adults.
There are 17 main paired muscle groups in the face. The facial nerve innervates these muscles using their deep surface, with the exception of three muscles: the buccinator, levator anguli oris and mentalis.
Our website is not responsible for the information contained by this article. Articleinput.com is a free articles resource thus practically any visitor can submit an article. However if you notice any copyrighted material, please contact us and we will remove the article(s) in discussion right away.
Note: This article was sent to us by: Keith Hayes at 02142011
1. Clinical evaluation in humans
All articles are property of their respective authors. Please read our Privacy Policy!
© 2009 ArticleInput.com.
Partners: Damenmode