External Landmarks of The End Point of The Marginal Mandibular Branch of The Facial Nerve
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Date
2023
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Saudi Digital Library
Abstract
Although the general course of the marginal mandibular nerve has been established for many years, this nerve is often subjected to injury during head and neck surgeries. Several studies have described the anatomical locations with various techniques to protect the nerve; however, the incidence of nerve injury is relatively unpredictable and ranges from 15-20% [1]. The consequences of these injuries are both aesthetic and functional, may be either temporary or permanent, and the medicolegal consequences are typically quite substantial.
The classic landmarks to protect the nerve are the mandible and facial vessels. These landmarks are used routinely during neck surgeries. Notwithstanding, recent facial and neck surgeries are minimally invasive and do not typically expose theses deep and internal landmarks. Numerous studies have been devoted to the topography of the marginal mandibular branch of the facial nerve and its location in relation to the mandible, facial artery, and vein. However, the end point of the marginal mandibular nerve is not fully described, and the need to correlate it to external landmarks would definitely minimize iatrogenic injury to the nerve.
The objective of this study was to identify the location of the end point of the marginal mandibular nerve and correlate it to reliable and reproducible external landmarks. This would enable surgeons to better predict the location of the nerve and avoid inadvertent damage during minimally invasive facial and neck surgeries.
Measurements of the nerve in relation to external and internal landmarks along the nerve path were recorded from fresh cadaveric dissections. The trans-cervical approach, the most common approach employed by surgeons, was used. Nerve variations, accuracy of landmarks, prediction of the end point of the nerve, and differences in sides (right vs left) were noted and assessed for significance. Additionally, comparisons with previous studies and literature review were completed.
The data produced in this study should provide a useful tool in facial and neck surgeries to prevent iatrogenic damage to the nerve. Also, it should have a conceptual and technical impact in these surgeries.
Description
2.1 Surgical Significance
The marginal mandibular branch of the facial nerve originates from the cervicofacial branch of the facial nerve [1] (Fig 1). Injury to the nerve can result in substantial functional and cosmetic deformity [1, 2]. The marginal mandibular nerve innervates the depressor labii inferioris, depressor anguli oris, mentalis, part of the orbicularis oris muscle, and risorius [2, 3]. Several studies have reported anatomical locations of the nerve; however, the incidence of nerve damage is unpredictable and ranges from 16-20% [4, 5]. The deformity caused by nerve injury could be temporary or permanent, and the medicolegal consequences are significant [1]. The injury results in flattening and inversion of the affected lower lip with an inability to move the lower lip downward and laterally [5]. The asymmetry is apparent when the patient cries or smiles [5] (Fig 2, 3).
The mandible and the facial artery and vein are commonly used as a surgical landmarks to protect the nerve during neck surgeries, based on Dingman and Grabb’s study of 100 embalmed cadaver facial halves [6]. They identified two or more rami of the nerve at the level of the angle of the mandible. In 98% of the specimens, the nerve passed downward and forward over the posterior facial vein (Anterior division of the retromandibular vein) and, in 100% of the specimens, the nerve crossed the anterior surface of the anterior facial vein [6]. Anterior to the facial artery, all branches of the nerve were above the inferior border of the mandible. Posterior to the facial artery, the nerve ran above the inferior border of the mandible in 81% of the specimens and, in the other 19%, the nerve or one of its branches ran inferior to the inferior border of the mandible in an arc of 1cm or less [6]. Incisions placed 2 cm below the inferior border of the mandible should avoid injury to the nerve in all instances [5, 6]. They also identified the nerve as a single branch in 21% of the specimens, two branches in 67%, three branches in 9% and four branches in 3% of the specimens [6].
Thorough knowledge and understanding of the depth of the marginal mandibular nerve from the skin incision are essential to avoid nerve injury [4]. The fascial layers of the neck are divided into superficial and deep cervical fascia [7]. The superficial cervical fascia encloses the platysma muscle. The deep cervical fascia, which surrounds the whole neck, has four components: Investing layer (also known as superficial layer of deep cervical fascia), prevertebral layer, pretracheal layer, and carotid sheath layer [7]. The marginal mandibular nerve leaves the parotid runs deep to the parotid-masseteric fascia and the platysma and lies in the plane lateral to the superficial layer of the deep cervical fascia [3,7,8].
Identification of the nerve after reflection of subplatysmal flap is the most common approach during oncologic neck dissection surgeries to protect the nerve [8,9] (Fig 4). Another approach frequently used in a non-oncologic surgeries is known as the Hayes Martin maneuver [8,9]. This maneuver protects the nerve by ligating the facial vein approximately two finger-breadths below the mandible and retracting the superficial layer of the deep cervical fascia superiorly; thereby, the nerve will not be encountered or damaged [8,9] (Fig 5). These approaches protect the nerve at its mid-course over the mandible; but they don’t necessarily protect the distal portion of the nerve.
2.2 Application of Research
While not always 100% consistent, human anatomy follows the same general layout. Many essential anatomic structures, like bones and vasculature, are identified during head and neck surgeries. Notwithstanding, the incidence of marginal mandibular nerve injury varies depending on the type of surgical intervention. It is about 7.3% following submandibular gland excision, 2.5-16.6% following mandibular fracture, 5.6-16.3% post-parotidectomy, and 18-23% after neck dissection [10].
Recently developed soft tissue surgical procedures in the face and the neck, including injection of filler materials or botulinum toxin, facelift, neck lift, liposuction, and others in which the standard deep internal landmarks like the mandible or the facial vessels are neither exposed nor identified, have resulted in new interest in the study of the marginal mandibular nerve. The mimetic muscles and superficial musculoaponeurotic system (SMAS) are known to function intimately as a single unit [11]. Muscle contracture translates into movement of the facial skin via vertical fibrous septa extending from the SMAS into the dermis [11]. The mimetic muscles can be divided into superficial and deep muscle groups [12]. The mentalis, buccinator, and levator anguli oris muscles are deep muscles and innervated from their outer surface. On the other hand, all superficial mimetic facial muscles are supplied from their deep surface [12]. From a surgical perspective, an important concept to remember is that the end point of the marginal mandibular nerve is at great risk of iatrogenic injury during soft tissue procedure. Thus thorough knowledge and understanding of the basic anatomy and potential anatomical variations within this area are essential to avoid nerve injury.
Summary:
Numerous studies have described the topography of the marginal mandibular nerve and its relation to the mandible and facial vessels; however, the end point of this nerve tract still needs to be better demarcated. The end point of the marginal mandibular nerve is the most superficial point of its course and more susceptible to injury during these soft tissue surgeries. The key landmarks necessary for protecting the endpoint of this nerve during soft tissue facial and neck surgeries have not been well defined in the literature. At present, the specific location of the end point is distinctly ambiguous and that need to be elucidated. We predict that deciphering the location of the end point in relation to external landmarks will aid in improvement of the surgical outcomes and minimize the incidence of nerve injury.
Keywords
External Landmarks of The End Point of The Marginal Mandibular Branch of The Facial Nerve
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