The retrosigmoid craniotomy is widely adopted to gain access to the CPA area. However, only depending on superficial anatomic landmarks usually has difficulty in determining the accurate location of venous sinuses, which leads to unsafe and/or complicated surgical access [5–7]. In such procedure, the exact localization of venous sinuses, particularly the transverse and sigmoid sinuses junction (TSSJ) is imperative to decrease the risk of the venous sinus injury, and available through osteal landmarks combined with the preoperative image-guided sinuses localization method. In our clinical practice, the method using preoperative image information with intraoperative skull anatomical landmarks had been confirmed to be accurate and practical for identifying the TSSJ and establishing the relationship between the location of the asterion and the TSSJ in retrosigmoid craniotomy.
A burr hole medial to the TSSJ can reveal the margin of the TSSJ with the least risk. However, no recognized initial burr-hole has been recommended until now [8]. How to gain the right orientation before drilling the key hole for retrosigmoid craniotomy has been widely explored by most neurosurgeons [9–11]. Classically, the neurosurgeons intend to determine the surface projection of TSSJ by the asterion to place the initial burr hole. However, more and more literatures reported the asterion has been considered as a unreliable landmark [12–14]. In order to precisely expose the inferior edge of the transverse (superiorly) and the posterior margin of the sigmoid sinuses (inferiorly), an optimal initial burr-hole is in proximity to the margin of the TSSJ, allowing their exposure without their damage.
In the present study, we observed the asterion lie directly over the inferior margin of the transverse sinus in 97.87% of cases, so the distance from the intersection of the asterion and occipitomastoid suture to the TSSJ is the shortest between the occipitomastoid suture and the sigmoid sinus. The posterior border of the mastoid process can be used as a parameter to identify the posterior margin of the sigmoid sinus [15]. The illumination of mastoid air cells correlates well with the sigmoid sinus in the retrosigmoid approach [16]. In clinical practice, we found these methods do not reflect the exact position of the sigmoid sinus, and mastoid air cells vary largely in different patients.
In retrosigmoid craniotomy, neurosurgeons commonly rely on surface landmarks and their experience to evaluate the position of venous sinuses and estimate an appropriately initial burr-hole, which is not accurate because of variability in different patients. In the previous study, many neurosurgeons have established their own methods to locate the sigmoid sinus. The author measured the x and y coordinates of the anterosuperior point of transverse-sigmoid sinus junction and the squamosal-parietomastoid suture junction to define a rectangular coordinate system [11]. However, the measurement is based on skull samples instead of patients alive, where there might be some measurement bias. The sample size was also too small to generalize this method in clinical application. The author locates the TSSJ based on 3D-CT in retrosigmoid craniotomy, but not all hospitals are equipped with 3D-CT images, and its accuracy is not high [17]. In our present study, we introduce a simplified procedure based on MRI and CT to localize the TSSJ in retrosigmoid craniotomy, and the location of the sigmoid sinus. 94 patients who underwent retrosigmoid craniotomy were analyzed. On the internal view of the skull in MRI, we measured the distance between TSSJ and the intersection of the asterion and occipitomastoid suture of every patient. Such distance is reflected as a red line labeled on the outer surface of cranium indicated in Fig. 1B. This simple method could help in localizing the sigmoid sinus and TSSJ and avoiding the risk of sinus injury and reducing the bone defect. The method does not need to establish any coordinate system, so it is convenient but sufficiently precise for practical application at surgical planning.
We have illustrated that the distance between the TSSJ and the occipitomastoid suture is not associated with age or gender, which pushes our work into a more universal level and makes it easier to apply to most patients. Due to anatomic heterogeneity in different cases, it is not reliable to traditionally depend on the asterion for the margin of TSSJ. For clinical practice, individual anatomic information from preoperative imaging data is necessary for different patients. CT scan, especially bone window, is superior to MRI to study bone features, but the T1WI MRI sequence was clearer in indicating the transverse and sigmoid sinuses, which was helpful for determining the distance from the intersection of the asterion and occipitomastoid suture to the TSSJ [11, 15]. In our series, the distance data from the asterion to the margin of TSSJ on the base of CT scan images combined with MRI images can be obtained preoperatively. According to these information and intraoperative osseous anatomical landmarks, the location of the burr hole could be determined on the cranium surface and its location corresponds to the projection of the margin of TSSJ.
Because surface landmark in the literature for identifying the transverse TSSJ junction is unreliable, we have attempted to refine this location method with the largest sample size to date. These data can assist neurosurgeons in localizing the preoperative projection and intraoperative location of the TSSJ when the surface landmark is not accurate. We studied relations of the external landmarks with the venous sinuses, of which the anatomic position was variable. Knowing the location of the venous sinuses avoids inadvertent entry into the venous sinuses and limitation of the size of the bony opening.
Interestingly, we found a significant difference between left sides and right sides for the distance from occipitomastoid suture to TSSJ (P = 0.006) in our series. The distance on the right side is longer than that on the left side at a distance of 2.76 mm on average. This might be due to the different sizes of the transverse sinuses in between the left sides or the right sides. Hwang RS et al. found that the right and left TS were constantly different in size, and the right TS was more often larger than the left TS [18]. This may guide neurosurgeons to pay attention to such a difference during craniotomy since the tumor is on different sides. Ribas et al. performed measurements on 50 sinuses from 25 dried skulls [17]. He found that TSSJ occurred approximately 1 cm in front of the asterion, which is in accordance with our result. Anatomical landmarks complemented with preoperative images offer a simple and reliable method in the identification of the TSSJ position for retrosigmoid craniotomy. This method significantly promotes speed and safety in the retrosigmoid approach and decreases venous sinus injuries, which displayed the reliability of our study in the location of TSSJ in all cases of our series.
Some authors described a high incidence of venous injury simply based on image-guided retrosigmoid craniotomy [2, 3]. This is not case in our series and the sigmoid sinus burst did not occur. In the present study, the umbriferous position of TSSJ onto the external surface of the cranium, indirectly located by the preoperative image with intraoperative landmarks, can confirm the burr hole place during surgery. Every patient has a distinctive location relationship between the asterion (occipitomastoid suture) and the TSSJ. Morphometric data from CT and MRI scans makes precise localization of venues structure possible in individual patient.
Although neuronavigation can achieve an accurate orientation of the target area, it brought on additional damage to patients because of installing Mayfield clamps and added preoperative and intraoperative procedures [16]. Briefly speaking, neuronavigation is invasive, time-consuming and produces more costs.
One of the limitations of our study is that we just measured the distance from TSSJ to the occipitomastoid suture in the asterion plane. It is reported that in some cases, the TSSJ was below the asterion plane, which is uncommon. Another limitation was that most of our patients were adults, which limits our study applied to other patients. However, the method combining preoperative image data and intraoperative anatomical landmarks discards complicated coordinate systems and provides a simple method for neurosurgeons to locate the TSSJ practically and precisely.