RESEARCH ARTICLEAnastomotic patterns of the facial parotid plexus (PP): A human cadaver study☆
Introduction
The human facial nerve (FN) emerges from the base of the skull through the stylomastoid foramen. Thereafter it takes an arched course which is concave upward and median-ward entering the parotid gland high up on its postero-medial surface (Monkhouse, 1990). Within the parotid space (Proctor, 1984), FN branches into a larger superior or temporo-facial and a smaller inferior or cervico-facial trunks (Fujita, 1934, Davis et al., 1956). The temporo-facial and cervico-facial trunks branch further to form a parotid “mesh-like” plexus (Liu et al., 2010), which exhibits variations in branching pattern. Five main terminal branches arise from the plexus and diverge within the gland. They leave the parotid gland by its antero-medial surface, medial to its anterior margin and supply the muscles of facial expression. Whereas the temporo-facial trunk has a plexiform arrangement formed by dichotic and anastomotic divisions, the cervico-facial trunk resembles merely a simple large loop (Berkovitz, 2005).
Because of their small calibre and lack of time for a careful and precise dissection during anatomical teaching courses, the morphology and especially the syntopy of the facial nerve branches (parotid plexus, PP) is difficult to depict. A clear separation of the nerve into five main terminal branches (temporal, zygomatic, buccal, mandibular, and cervical) may sometimes be very difficult or even impossible. The terminal division of FN shows significant variations and the number of its main divisions or trunks (bifurcation or trifurcation) may also vary (Shoja and Tubbs, 2016). This can explain the lack of extensive information on it in basic anatomical textbooks as well as in general surgical literature.
Beginning in the 19th century, attention has been increasingly focused on descriptive anatomy of PP (Henle, 1879, Gegenbaur, 1888, Spalteholz, 1909). Later Haller von Hallerstein Graf (1934) described the phylo- and embryogenesis of the facial nerve developing from the ectoderm (external epithelial covering originating from a strip lateral to the rhombencephalic neural fold) of the second (postmandibular or hyoid) pharyngeal arch. Like all pharyngeal-arch-nerves, it consists of 3 branches: post-trematic, prae-trematic and dorsal pharyngeal. The last two branches unite to build the palatine nerve (anterior ramus) which innervates the rostral portion of the oral cavity. The post-trematic (hyo-mandibular) ramus develops into the facial nerve proper which grows together with the pharyngeal endoderm (internal covering) and mesenchyme (derived from the neural crest, angiogenic and paraxial mesodermal cell populations) into the hyoid arch.
Later, the hyo-mandibular ramus divides further into a hyoid ramus (remains close to the hyoid bone and contains most motor axons) and an internal mandibular ramus (chorda tympani) which contains the viscero-sensible axons of the post-trematic ramus.
All rami of the facial nerve which, together with branches of the auriculo-temporal nerve (from N. trigeminus) build PP in mammalia, originate from the hyoid ramus. They develop synchronously with the muscles of the second arch deriving from the 4th and 5th somitomeres (portions of the paraxial mesenchyme beneath the neural plate). Most of them migrate widely, but retain their innervation by the facial nerve. The facial nerve enters the primordial muscle mass before it begins to differentiate, migrating with the developing muscles, and dividing when they divide. In this way the facial nerve “follows” the muscles and achieves its typical fan-shaped appearance (Haller von Hallerstein Graf, 1934, Angelov, 2016).
In the course of time, classic anatomical textbooks have either only briefly mentioned the PP, or restricted it just to drawings (Von Lanz and Wachsmuth, 1979, Leonhardt et al., 1987, Schuhmacher, 1991, Schuhmacher, 1997, Tillmann, 1997, Tillmann, 2005, Tillmann and Sperlich, 1997, Sobotta et al., 2005, Bommas-Ebert et al., 2006, Tortora and Derrickson, 2006, Anderhuber et al., 2012, Kahle and Frotscher, 2013, Rohen et al., 2015).
Other sources provide descriptions, but show neither photographs, nor drawings (Rohen et al., 2010; Lippert, 2006; Schiebler and Korf, 2007; Kirsch et al., 2011; Faller and Schünke, 2008; Rohen, 2000).
Finally, in some photo-atlases and anatomical textbooks PP was not even mentioned (Tandler, 1926, Lang, 1981, Frick et al., 1992, Tillmann and Schünke, 1993, Yokochi and Rohen, 1996, Gosling et al., 2005, Valerius, 2005, Rohen and Lütjen-Drecoll, 2006, Rohen and Lütjen-Drecoll, 2011, Martini et al., 2012).
Perusing scientific publications in journals, we found only a few mainly clinically oriented reports with macroscopic photographs (Kopuz et al., 1994, Salame et al., 2002, Saylam et al., 2006, Keefe et al., 2009, Smith and Ross, 2012, Khaliq et al., 2016, Swanson et al., 2017). Hence, the aim of the current study was to provide a profound characterization of the anastomotic patterns in PP by performing a detailed analysis on its location, syntopy and morphology.
Section snippets
Human cadaveric material
This anatomical study was carried out on 95 human cadaveric heads and necks that had been dissected and prepared by students in practical dissection courses at the Anatomical Institute, University of Cologne, Germany. The additional dissectional work by the authors Dr. H. Bendella, Dr. B. Spacca and Dr. D.N. Angelov only distinguished and identified the demonstrated structures more clearly.
For the anatomical teaching courses, all heads had been fixed in 6% (winter semesters 2014/15 and 2015/16)
Results
Throughout this section, we shall start each chapter with an overview of the relevant literature data. On the one hand, the immediate reference to earlier results will emphasize our contributions. On the other hand, this will relieve the discussion of unnecessary repetitions.
A total of 158 human head-halves were studied. In all of them, a distinct structure was seen which corresponded to the description of PP.
Discussion
The present study focused on presenting detailed photo-documentation of the parotid plexus (PP) to specifically show its anastomotic patterns and its connections with some adjacent structures. We feel convinced that this description will be very helpful to students in medicine and dentistry during anatomical dissection courses as well as to head and neck surgeons working in this field.
The main result is clinically relevant and thus very important. Whereas Davis et al. (1956) reported on
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This work is dedicated to the fifth anniversary of Professor Dr. Jürgen Koebke’s death. It confirms that his endless enthusiasm for surgical research based on sound anatomical knowledge will never be forgotten.
- 1
These authors contributed equally and share first authorship.