Copyright © 2017 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited. Preoperative Exposure of Sigmoid Sinus Trajectory in Posterolateral Cranial Base Approaches Using a New Landmark Through a Neurosurgical Perspective Ihsan Dog˘an, MD,  Onur O ¨ zgu ¨ral, MD,  U ¨ mit Erog˘lu, MD,  Eyyub S. M. Al-Beyati, MD,  Cemil M. Kilinc ¸, MD,  Ayhan Co ¨mert, MD, y and Hasan C ¸ ag˘lar Ug˘ur, MD, PhD  Abstract: The location of burr holes in posterolateral cranial base approaches should be appropriate to provide an adequate opera- tive field, and surgical freedom is crucial for bone window opening. The aim of this study was to search for more convenient and easily detectable landmarks in comparison with current landmarks in posterolateral cranial base surgery. Twenty 3- dimensional reconstructed head and neck computed tomography angiography images (group 1) and 20 cadaver heads (group 2) were evaluated. An imaginary line connecting the angle of the mandible and the mastoid tip was extended upward. A second line passing through the lateral edge of the zygomatic arch was also extended posteriorly. The authors examined if the first line met with the sigmoid sinus throughout its course and determined the location of the intersection point of these 2 lines relative to the sigmoid-transverse sinus junction. The intersection point did not correspond to the sinus region in 3 images from group 1 and 4 specimens from group 2. The matching of the mandibula- mastoid line trajectory with the sigmoid sinus course was unac- ceptable in 4 images and 5 cadavers. For venous anatomy preservation and anatomic skull base fossa orientation during posterolateral cranial base approaches, upward extension of the mandibula-mastoid line can be a proper landmark for surgical planning in this region. The authors’ proposed superficial ana- tomical line and intersection point over the skull could be used as a reliable indicator for the external projection of the sigmoid sinus and an appropriate initial burr-hole location. Key Words: Anatomy, cadaver, external projection, landmark, posterolateral cranial base approaches, retrosigmoid approach, sigmoid sinus, skull, suboccipital supratentorial approach (J Craniofac Surg 2017;00: 00–00) T he sigmoid sinus is not only a venous structure, but also a structure present throughout the posterolateral edge of the petrous temporal bone; therefore, it acts as an anatomical separator between the middle and posterior cranial fossae. 1 This anatomical location of the sigmoid sinus enhances its importance and engen- ders it to be described as a strategic surgical structure. The success of surgical approaches related to the sigmoid sinus depends on almost certainly knowing the location and course of the venous ‘‘pathway’’ preoperatively and preserving it with its surroundings intraoperatively. 2 The sigmoid sinus acts as an entry zone over the access path to a lesion in all lateral skull base approaches. Thus, knowing its course and location and being aware of its anatomical variations are crucial surgical nuances that may positively affect the surgical results. Intraoperative exposure of the sigmoid sinus and its connections while preserving its structural integrity during the craniectomy/craniotomy stage is a critical step for the continuity of surgery. 3 Excessive redundant bone window opening, sinus injury, or dural lacerations may result in poor patient outcomes and increase the potential risk of operative complications, even if the entire tumor has been removed. The primary aim of this study was to search for more conve- nient and easily detectable landmarks in comparison with current landmarks in posterolateral cranial base surgery. Here, a new projection line above the external surface of the scalp that overlaps with the sigmoid sinus course is described. Furthermore, accord- ing to the relation of this line with the modified previously defined zygomatic line, a novel point is defined. The anatomical relation- ship of this point with the sigmoid-transverse sinus junction was also examined, and it was assessed whether this point would be a safe reference point for posterior fossa craniotomies. METHODS Our study involved morphometric measurements of radiologic images and cadaveric specimens in group 1 (radiologic group) and group 2 (cadaveric group), respectively. The results in each group were initially interpreted for the left and right sides. Later, the results from the same side in both the groups were evaluated and compared between the groups. The asterion, mastoid process, angle of the mandible, inferior tip of the mastoid process (tMP), zygomatic arch, and sigmoid-transverse sinus junction region were the main elements of our research. The artificial landmarks and measurements were as follows: upward extension of a line that connects the angle of the mandible and tMP (eMML), posterior extension of a line that passes through the most lateral protruding edge of the zygomatic arch (LZL), intersection point of these 2 defined lines (IP) over the lateral cranial surface, angle between these 2 lines at IP, distance between the IP and posterior end of the zygomatic arch (z), distance between the IP and tMP (m), distance between the asterion and eMML (y), and distance from the IP to the intersection point of a line perpendicular to the From the  Department of Neurosurgery; and y Department of Anatomy, School of Medicine, Ankara University, Ankara, Turkey. Received June 15, 2017. Accepted for publication August 30, 2017. Address correspondence and reprint requests to Ayhan Co ¨mert, MD, Assoc. Professor, Department of Anatomy, School of Medicine, Ankara University, 06100 Sihhiye, Ankara, Turkey; E-mail: comertayhan@yahoo.com Preparation for publication of this article is partly supported by Turkish Neurosurgical Society. The authors report no conflicts of interest. Copyright # 2017 by Mutaz B. Habal, MD ISSN: 1049-2275 DOI: 10.1097/SCS.0000000000004182 ANATOMICAL STUDY The Journal of Craniofacial Surgery  Volume 00, Number 00, Month 2017 1