Construction and course of the retromandibular vein 
The retromandibular vein was formed by the union of the superficial temporal and maxillary veins. The majority of the retromandibular veins divide into the anterior and posterior branch backstreaming, and the minority of the retromandibular veins have no branches or valves. The present study mainly corrected the venous blood from facial pleural and frontal regions. As for the course of the retromandibular veins, we divided them into two types based on backstreming: one type consisted of the anterior and posterior branches: the anterior branch passed through the anterior border of the sternocleidomastoid and united with the anterior facial vein, and then mostly drained into the internal jugular vein and minorly drained into the external jugular vein. The posterior branch descended backward on the surface of the sternocleidomastoid, was joined by the posterior auricular vein, and became the external jugular vein. Another type of retromandibular vein without the anterior and posterior branches directly connected with the posterior auricular vein and entered the external jugular vein. These findings are similar to previous findings[20]. In a previous study[21], the retromandibular veins were divided into two types according to their locations: superficial or deep area of the facial nerve: one type was designated as the retromandibular veins in the superior and inferior trunks of the facial nerve or in the medial superior, middle, inferior facial nerve; the other type was the retromandibular veins in the medial superior trunk of the facial nerve or in the lateral inferior trunk of the facial nerve. The retromandibular vein is also subdivided into several subtypes according to its relation to the inferior trunk branches of the facial nerve. Type A: the retromandibular vein passes through the medial region between facial nerve bifurcation and inferior trunk bifurcation. Type B: the retromandibular vein passes through the bifurcation of facial nerve inferior trunk into marginal mandibular branch and cervical branch. Type C: the retromandibular vein crosses the medial branches of facial nerve inferior trunk. Type D: the retromandibular vein is located in the medial region between facial nerve bifurcation and superior, middle, and inferior trunk bifurcation. Type E: the retromandibular vein is located between facial nerve bifurcation and inferior trunk bifurcation and corssedthe facial nerve at its medial inferior trunk. Type F: the retromandibular vein crosses the proximal lateral inferior trunk of the facial nerve. Type G: the retromandibular vein crosses the distal lateral inferior trunk of the facial nerve.

Location of the retromandibular vein and the facial nerve during mandibular angle ostectomy
A key to successful mandibular angle ostectomy is to avoid injury to the retromandibular vein and facial nerve surrounding the mandibular angle. Therefore, it is extremely important to precisely locate the marginal mandibular branch and cervical branch of facial nerve as well as the retromandibular vein surrounding the mandibular angle. Results from this study demonstrated that the position relationship between the retromandibular vein and the facial nerve was relatively constant. The majority of the retromandibular vein (88%) was located in the superior and inferior trunks of the facial nerve and in the medial branches, and the minority of the retromandibular vein (13%) passed through the medial superior trunk and the lateral inferior trunk of the facial nerve. The retromandibular vein mostly directly connected with facial nerve trunk or major branches in a decussated fashion, without separation of parotid gland tissue. In the medial segment of the parotid gland, both the marginal mandibular branch and cervical branch of the facial nerve originate from the jugular trunk of the facial nerve, and after passing through the parotid gland, the facial nerve divides into the marginal mandibular branch and cervical branch. Present results demonstrated that 88% of the marginal mandibular branches of the facial nerve ran alone and 13% of the marginal mandibular branches share the facial nerve trunk with cervical branch in the parotid gland and divide into the marginal mandibular branches and cervical branches after running out of the parotid gland. The present study divided the coursing modes of main trunk of the marginal mandibular branch into two types: isolated and sharing, to know the coursing characteristics of the marginal mandibular branch and clarify the adjacent relationship between the marginal mandiblar branch and cervical branch, so as to avoid the injury to the main trunk of marginal mandibular branch as well as cervical branch.
 
Retromandibular vein in relation to mandibular angle
The retromandibular vein is a relatively constant vessel, but controversy exists in its construction and brackstreaming. However, results from this study revealed that the middle and inferior segments of posterior mandibular ramus and the posterior region of mandibular inferior border was encased by the veins. That is to say, the posterior and inferior borders of the mandibular angle to be ressected are always encased by the veins. The retromandibular vein and facial nerve formed an arch-shaped structure which encases the mandibular angle. 75% of the arc-shaped structures coursed close to the inferior and posterior borders of the mandibular angle and 25% of the arc-shaped structures were not(averagely 0.58 m distance away from the lowest point of the mandibular angle). In the middle and inferior segments of the posterior border of the mandibular ramus, 100% of the retromandibular vein and its branches were tightly close to the posterior border of the mandibular ramus and there was only a layer of thin periosteum between the venous wall and cortical bone. The thin periosteum was just the position of mandibular angle ostectomy line, which should be paid more attention by surgeons because venous injury is easily caused no matter in vein dissection or ostectomy.

Caution taken to avoid injury to the retromandibular vein during mandibular angle ostectomy
Results from this study demonstrated that the retromandibular vein was located in the posterior border of the mandibular ramus and was easily covered by the posterior border of the mandibular ramus. For this reason, a periosteal elevator with a greatly curved blade is needed during the mandibular angle ostectomy because only such an elevator can safely strip the retromandibular vein. The retromandibular vein has thin vascular wall and large caliber, so the periosteum of posterior border of the mandibular ramus should be stripped gently and finely to avoid the injury to the retromandibular vein. During the mandibular angle ostectomy, the region 0.8 cm larger than the ostectomy line is suggested to be resected to avoid the adhesion of mandibular angle to be resected to adjacent tissues.