Asterion as a surgical landmark for lateral cranial base approaches

A novel theory for rapid localization of the transverse-sigmoid sinus junction and "keyhole" in the retrosigmoid keyhole approach: micro-anatomical study, technique nuances, and clinical application

To determine a rapid and accurate method for locating the keypoint and "keyhole" in the suboccipital retrosigmoid keyhole approach. (1) Twelve adult skull specimens were selected to locate the anatomical landmarks on the external surface of the skull.The line between the infraorbital margin and superior margin of the external acoustic meatus was named the baseline. A coordinate system was established using the baseline and its perpendicular line through the top point of diagastric groove.The perpendicular distance (x), and the horizontal distance (y) between the central point of the "keyhole" and the top point of the digastric groove in that coordinate system were measured. The method was applied to fresh cadaveric specimens and 53 clinical cases to evaluate its application value. (1) x and y were 14.20 ± 2.63 mm and 6.54 ± 1.83 mm, respectively (left) and 14.95 ± 2.53 mm and 6.65 ± 1.61 mm, respectively (right). There was no significant difference between the left and right sides of the skull (P > 0.05). (2) The operative area was satisfactorily exposed in the fresh cadaveric specimens, and no venous sinus injury was observed. (3) In clinical practice, drilling did not cause injury to venous sinuses, the mean diameter of the bone windows was 2.0-2.5 cm, the mean craniotomy time was 26.01 ± 3.46 min, and the transverse and sigmoid sinuses of 47 patients were well-exposed. We propose a "one point, two lines, and two distances" for "keyhole" localization theory, that is we use the baseline between the infraorbital margin and superior margin of the external acoustic meatus and the perpendicular line to the baseline through the top point of the digastric groove to establish a coordinate system. And the drilling point was 14.0 mm above and 6.5 mm behind the top point of the digastric groove in the coordinate system.

Anatomical Landmarks for Transverse-Sigmoid Sinus Junction: A Cadaveric Study

BACKGROUND AND OBJECTIVE

Accurately identifying and avoiding crucial anatomical structures within the posterior cranial fossa using superficial landmarks is essential for reducing surgical complications. Our study focuses on the top of the mastoid notch (TMN) as an external landmark of the cranium, aiming to assist in the strategic placement of the initial burr hole. In this study, we present a method for predicting the path of the transverse sinus (TS) and explore the relationship between the junction of the transverse-sigmoid sinus and the TMN.

METHODS

Following anatomical dissections of the brain in cadaveric specimens, we conducted intracranial drilling from the inside surface of the cranium on 10 adult skulls (20 sides). A coordinate system was established on the posterolateral surface of the skull to assist the analysis. Using a self-leveling laser level, we set up a horizontal Frankfurt line (X-axis) and identified a vertical perpendicular line passing through the TMN to serve as the Y-axis. To identify the course of the TS, we divided the segment between the two inferomedial points into six equidistant points along the Frankfurt line.

RESULTS

No significant difference was observed between the inferomedial points of the transverse-sigmoid sinus junction (TSSJ) on the left and right sides. The inferomedial point was positioned at a median of 6.6 mm (Q1: 3.7 mm, Q3: 9.4 mm) dorsally and at a median of 19.2 mm (Q1: 16.1 mm, Q3: 23.2 mm) cranially from the TMN. The upper edge of the TS was located at distances of 6.4 mm (5.7; 12.7), 10.3 mm (8.8; 12.3), and 13.8 mm (11.9; 16.3) on the right, and 4.9 mm (4.1; 7.9), 8.6 mm (7.6; 13.0), and 12.8 mm (11.7; 17.5) on the left side from the Frankfurt horizontal plane at the ¼, ½, and ¾ line points, respectively. The bottom edge was positioned at distances of 0.6 mm (-2.7; 2.0), 2.1 mm (-0.8; 3.8), and 4.8 mm (2.4; 6.7) on the right, and 1.1 mm (-3.4; 2.4), 2.0 mm (0.2; 4.8), and 3.9 mm (3.7; 5.3) on the left from these respective points. The upper edge of the right TS was found to be statistically more distant from the Frankfurt horizontal plane at the ¼ line point (p-value = 0.027) compared to that on the left side. The confluence of the sinus center was identified as having a median distance of 7.8 mm (4.5; 8.3) and an inferior point of 1.5 mm (0.1; 3.0) cranially to the inion. In all examined bodies (n = 10), the confluens sinuum was consistently 4.7 mm (3.3; 5.6) to the right in relation to the inion. Notably, the median of the right transverse sinus diameter (median = 9.3 mm) was found to be significantly larger than that of the left transverse sinus (median = 7.0), with a statistically significant p-value of 0.048.

CONCLUSIONS

The literature regarding the external identification of the TSSJ and the course of the TS varies. In our efforts to provide a description, we have utilized the TMN as a reliable landmark for locating the TSSJ. To delineate the trajectory of the TS after its exit from the confluence of sinuses, we employed a Frankfurt horizontal plane to the inion. These findings may assist surgeons by using external skull landmarks to identify intracranial structures within the posterior fossa, particularly when image guidance devices are not available or to complement a neuronavigational system.

Morphometric Evaluation of Sutural Patterns at the Pterion and Asterion in Dry Indian Skulls: Surgical Relevance

INTRODUCTION

The pterion and asterion serve as crucial landmarks on the skull, representing the antero-lateral and postero-lateral fontanelles in neonates, respectively. In clinical practice, these points play a pivotal role in guiding the understanding of deeper structures and their relationships to the head's surface. The thin calvarium at these junctures is susceptible to fractures, and the underlying vessels are prone to tear, often leading to extradural hematoma formation, necessitating burr hole surgery for evacuation.

MATERIAL AND METHODS

The study involved 40 human dry skulls in Indians (n= 27 male (10.8%), n= 13 female (5.2%)) of unknown age, evaluating morphometric characteristics of 80 pterions and asterions. Measurements were conducted using a digital caliper (SKADIOO±0.2mm/0.01") in millimeters on both sides of each skull. The investigation also included an examination of sutural patterns in Pterion and asterion.

RESULTS

Three types of sutural patterns were identified in the pterion, the most common being the sphenoparietal variety (75% on the right, 70% on the left), followed by the epipteric variety, which was the second most common (11.3% on the right, 12.5% on the left), and then the frontotemporal type (1.3% on the right, 2.5% on the left). Two sutural patterns were observed in the asterion: type 1 (presence of sutural bone) in 17.9% of skulls and type 2 (absence of sutural bone) in 82.1%.

CONCLUSION

The differences in pterion and asterion positions across various populations explored in previous studies motivated us to conduct this research in the Indian population. Our findings revealed that among Indians, the predominant pterion type is predominantly sphenoparietal, whereas type 2 is the most prevalent in asterion. Understanding the clinical significance of the pterion and asterion is crucial for healthcare professionals to ensure precise and safe surgical procedures, particularly for the effective treatment of head trauma patients.

Is Asterion a Reliable Surgical Landmark for the Transverse and Sigmoid Sinus Junction in Indian Skulls?

Background

Asterion is the junction of lambdoid, parietomastoid, and occipitomastoid sutures. In traditional anatomy teaching, it is believed that asterion sits over the transverse-sigmoid sinus junction (TSSJ). It is a significant surgical landmark for the placement of a burr hole in the retrosigmoid approach.

Objective

The purpose of this study was to evaluate the asterion's accuracy as a TSSJ-specific external surgical landmark in the Indian population.

Materials and Methods

16 human dried skulls were obtained from the Department of Anatomy, AIIMS, New Delhi. The point of TSSJ was taken between the two lips and just posterior to the entry of the superior petrosal sinus. Asterion internally was localized with the help of a geometrical divider. The x- horizontal/ anterior and y- vertically superior (+)/ inferior (-) distances were measured from asterion (internally) to the TSSJ by a digital vernier caliper.

Results

The mean horizontal distance (x) of the left side asterion & TSSJ was 10.3±1.0mm whereas the vertical distance (y) ranged between +2 to -4.3 mm. The mean horizontal distance (x) of the right side asterion & TSSJ was 13.5±1.4mm whereas the vertical distance (y) ranged between +3 to -4.2 mm.

Conclusion

Asterion is not a reliable landmark for TSSJ in Indian skulls. The TSSJ with respect to asterion was found on average 10mm and 13.5mm anterior on the left and right side respectively, and mostly inferior (average 4.2mm) in 75% of the skulls. The TSSJ was closer to the asterion on the left side in comparison with the right side. However, further studies with a larger sample size will be needed to evaluate the population-specific relation of asterion with the TSSJ.

Application of Surface Landmarks Combined with Image-Guided Sinus Location in the Retrosigmoid Approach and their Clinic-Image Relationship Analysis

Objectives: During craniotomy for cerebellopontine angle (CPA) lesions, the exact exposure of the margin of the venous sinuses complex remains an essential but risky part of the procedure. Here, we revealed the exact position of the asterion and sinus complex by combining preoperative image information and intraoperative cranial landmarks, and analyzed their clinic-image relationship. Methods: Ninety-four patients who underwent removal of vestibular schwannoma (VS) through retrosigmoid craniotomies were enrolled in the series. To determine the exact location of the sigmoid sinus and the transverse sinus and sigmoid sinus junction (TSSJ), we used preoperative images, such as computed tomography (CT) and/or magnetic resonance imaging (MRI) combined with intraoperative anatomical landmarks. The distance between the asterion and the sigmoid sinus was measured using MRI T1 sequences with gadolinium and/or the CT bone window. Results: In 94 cases of retrosigmoid craniotomies, the asterion lay an average of 12.71 mm on the posterior to the body surface projection to the TSSJ. Intraoperative cranial surface landmarks were used in combination with preoperative image information to identify the distance from the asterion to the sigmoid sinus at the transverse sinus level, allowing for an appropriate initial burr-hole (the margin of the TSSJ). Conclusions: By combining intraoperative anatomical landmarks and preoperative image information, the margin of the TSSJ, in particular, the inferior margin of the transverse sinus, can be well and thoroughly identified in the retrosigmoid approach.

Dermoid Cysts of the Asterion: An Unusual Location for Unusual Dermoids, Radiological Findings and Neurosurgical Implications

Asterion is an uncommon site for lesions, especially dermoid cysts. We report a case series of three asterional intracranial dermoid cysts, which, to the best of our knowledge, have never been described before. Patients presented with non-specific symptoms and underwent surgical excision of the lesions. It is crucial to correctly diagnose intracranial masses and to identify their relationships with surrounding anatomical structures, especially if the location is unusual as the asterion, to plan surgery. The challenge of this tumor location is to preserve the venous drainage system during surgical procedures, because of the contiguity between the asterion and the transverse–sigmoid junction. Rupturing or damaging of the venous drainage system have been proven to be catastrophic because they lengthen surgical time and present dire consequences for patients. In conclusion, it is crucial to familiarize with atypical dermoid presentation to ensure proper diagnoses and to perform adequate imaging for optimal surgical planning.

History, Variations, and Extensions of the Retrosigmoid Approach: Anatomical and Literature Review

The retrosigmoid approach is the workhorse for posterior fossa surgery. It gives a versatile corridor to tackle different types of lesions in and around the cerebellopontine angle. The term “extended” has been used interchangeably in the literature, sometimes creating confusion. Our aim was to present a thorough analysis of the approach, its history, and its potential extensions. Releasing cerebrospinal fluid from the subarachnoid spaces and meticulous microsurgical techniques allowed for the emergence of the retrosigmoid approach as a unilateral variation of the traditional suboccipital approach. Anatomical landmarks are helpful in localizing the venous sinuses and planning the craniotomy, and Rhoton's rule of three is the key to unlock difficult neurovascular relationships. Extensions of the approach include, among others, the transmastoid, supracerebellar, far-lateral, jugular foramen, and perimeatal approaches. The retrosigmoid approach applies to a broad range of pathologies and, with its extensions, can provide adequate exposure, obviating the need for extensive and complicated approaches.

The Radiological Evaluation of the Mastoid Process and Its Implications for Surgical Approaches

Background and objective In various surgical procedures, in approaching the posterior fossa and the posterolateral cranial base, surface markings are used to locate the groove for transverse and sigmoid sinuses, and their junction. Determining these surface landmarks, especially the mastoid bone and its surrounding anatomical formations, is extremely valuable. The purpose of this study was to examine the anatomical features and the relationship between the mastoid process and surrounding structures on cone-beam CT (CBCT) images. Methods Before starting this retrospective study, approval was obtained from the Ethics Committee of Gaziantep University (approval date: 04/12/2019; number: 470). Individuals who consulted the Department of Oral and Maxillofacial Radiology of Gaziantep University's Faculty of Dentistry between 2015-2018 for any reason and whose CBCT images were taken were included in this study. Frankfort horizontal plane was used for the standardization of the images. Measurements were made on three different sections: coronal, transverse, and sagittal. Results The cohort consisted of 149 females and 98 males; the mean age of the participants was 45.72 ± 17.12 years. It was determined that homogeneity was achieved in terms of data distribution between genders according to age (p=0.777). Additionally, it was determined that there was a statistically significant difference between the genders in all parameters except anterior inclination angle (AIA), and higher values were found in males. Conclusion We believe that the results obtained from this study may contribute toward decreasing the complication rate and increasing success in surgical procedures, especially with respect to approaching the posterior fossa and the posterolateral cranial base.

Morphometry of Asterion and its Proximity to Dural Venous Sinuses in Northwest Ethiopian Adult Skulls

INTRODUCTION

The asterion is a craniometric point in the norma lateralis of skull situated at the confluence of parietal, temporal, and occipital bones. The aim of this study is to determine the type and mapping of asterion and its relation with transverse-sigmoid venous sinus junction (TSSJ).

MATERIALS AND METHODS

Sixty-one dry and intact adult human skulls of unknown sex and age were used. The measurements were implemented using stainless steel sliding vernier caliper. The data were analyzed using SPSS version-20 and independent t-test analysis was implemented. P < 0.05 was considered as statistically significant.

RESULTS

In the study, type II was found to be the predominant asterion type. There was statistically significant difference in measurements of central thickness of right and left sides of asterion (P = 0.04, Eta squared = 0.025). Besides, the TSSJ was situated above the asterion and at the level of asterion in 39.34% and 60.66 of the cases, respectively.

CONCLUSIONS

The Northwest Ethiopian population has a high incidence of type II asterion. In most of the cases, the TSSJ is situated at the level of the asterion. The finding of the present study may be useful to neurosurgeons to reduce the risk during posterior cranial fossa surgeries. This work may also offer worthy information to radiologists, anthropologists, anatomists, and forensic scientists.

The Longissimus Capitis Insertion as a Superficial Landmark for the Sigmoid Sinus: An Anatomical Study

Objective Many external anatomical landmarks have been used for approximating deeper, intracranial structures. Herein, we evaluate the attachment of the longissimus capitis (LC) on the mastoid process as a landmark for the underlying sigmoid sinus.

Methods Adult cadavers underwent dissection of the posterior occiput with special attention to the attachment of the LC muscle. Once the periphery of the muscle's tendon of attachment was determined, a burr hole was made in this area and evaluated internally for its relationship to the sigmoid sinus.

Results From an intracranial view, burr holes on all sides were over the sigmoid sinus and just slightly lateral to the center of the sinus. The distance from the midline to the medial border of the insertion of the LC had a mean of 63.0 ± 7.2 mm. The width of the tendon of insertion of the LC on the mastoid process had a mean of 17.6 ± 5.7 mm. The length of the tendon insertion of the LC had a mean of 14.7 ± 4.7 mm. The distance from the inferior border of the insertion of the LC to the tip of the mastoid process had a mean of 6.2 ± 4.5 mm.

Conclusion To our knowledge, use of the attachment site of the LC on the mastoid process as an external landmark for the underlying sigmoid sinus has not previously been reported. Based on our cadaveric findings, the sigmoid sinus is centered under the attachment of the LC regardless of the width of its tendon.

Retrosigmoid approach assisted by high-resolution computed tomography: a cost-effective technique to identify the transverse and sigmoid sinus transition

Background

When utilizing the retrosigmoid approach (RA), accurately identifying the transverse and sigmoid sinus transition (TSST) is a key procedure for neurosurgeons, especially in developing countries restricted by the lack of expensive devices, such as the neural navigation system and the three-dimensional volumetric image-rendered system. Before operations, a computed tomography scan is a common and cost-effective method of checking patients who suffer lesions located at the cerebellopontine angle. Therefore, we present a technique using only high-resolution computed tomography to identify the transverse and sigmoid sinus transition.

Methods

This retrospective study included 35 patients who underwent retrosigmoid approach operations to resect an acoustic neurinoma with the assistance of our technique. In brief, our technique contains 4 steps: (1) All patients’ 1-mm, consecutive, high-resolution computed tomographic images that clearly displayed landmarks, such as the inion, lambdoid suture, occipitomastoid suture, and the mastoid emissary foramen, were investigated initially. (2) We selected two particular slices (A and B) among all of these high-resolution computed tomographic images in which scanning planes were parallel with the line drawn from the root of the zygoma to the inion (LZI). Slice A contained both the root of the zygoma and the inion simultaneously, and slice B displayed the mastoid emissary foramen. (3) Four points (α, β, γ, δ) were arranged on slices A and B, and point α was located at the inner surface of the skull, which represents the posterior part of the sulci of the sigmoid sinus. Point β was located at the outer surface of the skull, and the line connecting them was perpendicular to the bone. Similarly, on slice B, we labeled point γ as the point that represents the posterior part of the sulci of the sigmoid sinus at the inner surface and point δ as the point located at the outer surface of the skull, and the line connecting them was also perpendicular to the bone. The distances between point β and the lambdoid suture/occipitomastoid suture and between point δ and the mastoid emissary foramen were calculated for slices A and B, respectively. (4) During the operation, a line indicating the LZI was drawn on the bone with ink when the superficial soft tissue was pushed away, and this line would cross the lambdoid suture/occipitomastoid suture. With both the crosspoint and the distance obtained from the high-resolution CT images, we could locate point β. We also used the same method to locate point δ after revealing the mastoid emissary foramen. The line connecting point β and point δ indicated the posterior border of the sigmoid sinus, and the intersection between the line and LZI indicated the inferior knee of the transverse and sigmoid sinus transition (TSST).

Results

All 35 patients underwent the RA craniectomies that were safely assisted by our technique, and neither the sigmoid sinus nor the transverse sinus was lacerated during the operations.

Conclusion

Our cost-effective technique is reliable and convenient for identifying the transverse and sigmoid sinus transition (TSST) which could be widely performed to guarantee the safety of RA craniectomy.

Mastoid notch as a landmark for localization of the transverse-sigmoid sinus junction

Background

The top of the mastoid notch (TMN) is close to the transverse-sigmoid sinus junction. The spatial position relationship between the TMN and the key points (the anterosuperior and inferomedial points of the transverse-sigmoid sinus junction, ASTS and IMTS) can be used as a novel method to precisely locate the sinus junction during lateral skull base craniotomy.

Methods

Forty-three dried adult skull samples (21 from males and 22 from females) were included in the study. A rectangular coordinate system on the lateral surface of the skull was defined to assist the analysis. According to sex and skull side, the data were divided into 4 groups: male&left, male&right, female&left and female&right. The distances from the ASTS and IMTS to the TMN were evaluated on the X-axis and Y-axis, symbolized as ASTS&TMN_x, ASTS&TMN_y, IMTS&TMN_x and IMTS&TMN_y.

Results

Among the four groups, there was no significant difference in ASTS&TMN_x (p = 0.05) and ASTS&TMN_y (p = 0.3059), but there were significant differences in IMTS&TMN_x (p < 0.001) and IMTS&TMN_y (p = 0.01), and multiple comparisons indicated that there were significant differences between male&left and female&left both in IMTS&TMN_x (p = 0.0006) and in IMTS&TMN_y (p = 0.0081). In general, the ASTS was located 1.92 mm anterior to the TMN on the X-axis and 27.01 mm superior to the TMN on the Y-axis. For the male skulls, the IMTS was located 3.60 mm posterior to the TMN on the X-axis and 14.40 mm superior to the TMN on the Y-axis; for the female skulls, the IMTS was located 7.84 mm posterior to the TMN on the X-axis and 19.70 mm superior to the TMN on the Y-axis.

Conclusions

The TMN is a useful landmark for accurately locating the ASTS and IMTS.

The Surface and Intracranial Location of Asterion

BACKGROUND

Asterion is identified as the connection point of sutura parietomastoidea, sutura occipitomastoidea, and sutura lambdoidea. The location of asterion, which is primarily preferred as a landmark during posterolateral surgical approach for intracranial operations, shows many variables. The aim of this study was to identify the surface location of the asterion and determine the distances between intracranial anatomical structures and asterion.

METHODS

At this present study, 11 hemicraniums (22 asterion points), situated at the laboratory of Department of Anatomy, Faculty of Medicine, Bursa Uludag University, were used. The asterion points which the sesamoid bone located were classified as type I and the ones which sesamoid bone did not locate were classified as type II. According to the proximity of asterion with sinus transversus, 3 groups were classified. About 19 parameters were measured related to asterion. The obtained data were analyzed in SPSS 22.

RESULTS

As a result of findings, while sesamoid bone was seen at 7 asterion points (type I), at 15 points sesamoid bone was not detected (type II) (respectively, 31.81%, 68.19%). It was identified that 15 asterion points were at the surface, 5 ones were average 3.42 ± 2.52 mm over, 2 ones average 3.21 ± 2.26 mm below of the projection of sinus transversus. Statistically significance was not seen between the measurements taken from left and right sides.

DISCUSSION AND CONCLUSION

Asterion is an important landmark for the retrosigmoid approaches. The surface and intracranial location of the asterion and proximity with dural sinuses are important for surgeons not to cause fatal subdural hematomas during the approaches using "Burr Hole" technic in neurochirurgie operations.

Morphology of asterion and its proximity to deep vein sinuses in Iranian adult skull

Background: The asterion is located at the posterior lateral side of the skull at the junction of the parietal, temporal and occipital bones.Methods: We examined the morphology of the asterion, its association with deep vein elements, the mastoid apex and inion in 105 adult cadavas (210 hemicraniums) including 146 males and 64 females at the anatomy lab of the Legal Medicine Organization.Results: Two types of asterion were observed. Type I was found in 14.7%, and type II in 85.3% of cases. In 70% of cases, the asterion was at or above the venous sinus. The distance between the asterion and the mastoid appendage on the right side was 47.03 mm and on the left side was 46.5 mm. The distance between the asterion and the inion at the right side was 70.55 mm and on the left side was 70.2 mm.Conclusion: The asterion in 70% of cases was at or above the level of the transverse sinus. For this reason, in posterior fossa surgical approaches, the first burr hole is preferred to start in the lower parts of the asterion.

Baha Attract Implantation Using a Small Incision: Initial Report of Surgical Technique and Surveillance

OBJECTIVES

To determine the appropriate anatomical borders of implantation on the temporal bone in a cadaver study, and to develop a simplified surgical technique for Baha Attract implantation through a small incision along the hairline using anatomical evidence and a navigation system.

METHODS

In a cadaver study, 20 human adult dry skulls were used to find flat areas of the temporal bone for Baha Attract magnet implantation. Four borders of the "optimal surgical site" were defined: Asterion line, occipitomastoid suture line, sigmoid sinus line, and digastric groove line. In three patients, we implanted the Baha Attract according to the newly developed surgical procedure and validated the feasibility of this technique with a navigation system.

RESULTS

We identified the appropriate position of the implant on the temporal bone, suggesting a simplified surgical technique for Baha Attract with a small incision. We determined the spot of implantation, and the implants were inserted through a small surgical incision (<2.5 cm) under local anesthesia; the procedure lasted approximately 30 minutes.

CONCLUSION

The optimal surgical site of the temporal bone is a safe and easily accessible location for implantation of the Baha Attract.

Evaluation of the inion and asterion as neurosurgical landmarks for dural venous sinuses: osteological study on a sample of South African skull specimens

BACKGROUND

Sub-Saharan neurosurgeons most likely need to perform invasive procedures without the latest imaging and navigation technology in the operating room. Therefore, these surgeons need to utilize other methods such as superficial surface landmarks for neuro-navigation. Bony landmarks, including the inion and asterion, are commonly used during invasive procedures to pinpoint the location of the confluence of sinuses and transverse-sigmoid sinus junction, respectively. The purpose of this study was to investigate whether the inion and asterion can be used as superficial landmarks for the confluence of sinuses and the transverse-sigmoid sinus junction, respectively, in a South African population.

METHODS

Fifty South African human skulls were used (25 male, 25 female). The micro-focus X-ray radiography and tomography facility (MIXRAD) at Necsa scanned and created three-dimensional virtual images of the skull specimens. Reference points were then inserted on the images and the relation between bony landmarks and venous sinuses was documented.

RESULTS

The inion was directly related to the confluence of sinuses in 4% of the sample, whereas the asterion was directly related to the transverse-sigmoid sinus junction in 28% of the cases, on both the right and left sides.

CONCLUSIONS

This study confirmed that neither the inion, nor the asterion, are directly related the confluence of sinuses and transverse-sigmoid sinus junction, respectively. These bony landmarks are more likely to be located either inferior, or not related at all, to the investigated dural venous sinuses.

Preoperative identification of the initial burr hole site in retrosigmoid craniotomies: A teaching and technical note

BACKGROUND

When fashioning a retrosigmoid craniotomy, precise placement of the initial burr hole is crucial to avoid iatrogenic sinusal injury and to facilitate a corridor that allows for minimal cerebellar retraction.

METHODS

3D CT reconstructions of 16 cadaveric sides were used to identify and measure three discrete anatomical points. These three points and distances between them were plotted onto the surface of the skull using a digital caliper to identify the optimal burr hole location. This technique was subsequently applied in 20 clinical cases.

RESULTS

Optimal burr hole placement was achieved in 87.5% of specimens and, with minor refinement, 100% of clinical cases with no significant increase in operative time. Preoperative planning took an average of 10 minutes.

CONCLUSION

This technique for localizing the location of the initial retrosigmoid burr hole is a simple, safe, reliable, rapid, and inexpensive solution for surgeons who do not have regular access to neuronavigation.

Relationship Between the Horizontal Part of the Sigmoid Sinus and the Line Through the Digastric Point and Posterior Edge of the Condyle: An Anatomic and Radiologic Study

OBJECTIVE

This study aims to determine whether the line between the digastric point and posterior edge of the occipital condyle (DC line) could be a new surface landmark for the posterior margin of the horizontal part of the sigmoid sinus.

METHODS

Cadaveric specimens were used to show the relationship between the DC line and retrosigmoid craniotomy. Three-dimensional computed tomography angiography images of adult heads (56 sides) were analyzed to measure the distance between the DC line and the horizontal part of the sigmoid sinus at the digastric point, posterior edge of the condyle, and midpoint of the line.

RESULTS

The DC line was roughly parallel and posterior to the posterior margin of the sigmoid sinus. The distance between the DC line and the posterior edge of the sigmoid sinus at the digastric point, condyle, and midpoint of the line measured 4.7 ± 3.3 mm, 5.9 ± 2.6 mm, and 1.3 ± 2.2 mm, respectively. All sigmoid sinuses coursed anterior to the digastric point and condyle but in 17.9% (10/56 sides) the posterior edge of the sigmoid sinus extended a maximum of 4.1 mm posterior to the midpoint of the DC line.

CONCLUSIONS

The DC line can be used as a new surface landmark for estimating the position of the horizontal part of the sigmoid sinus. The posterior edge of the sinus may extend posterior to the line at the midpoint; thus, care should be taken to prevent sinus injury when drilling around the midpoint of the line.

Relationship of the sinus anatomy to surface landmarks is a function of the sinus size difference between the right and left side: Anatomical study based on CT angiography

BACKGROUND

Several cadaveric studies demonstrate reliable localization of the transverse sinus and the transverse sigmoid junction (TSJ). These studies use the line drawn from the inion to the posterior root of the zygoma (IZ) and the asterion, respectively. We investigated how the size difference between the right and left transverse sinuses (TS) and sigmoid sinuses (SS) affected the accuracy of their respective superficial landmarks, particularly with regards to where this relationship may result in unsafe and/or complicated surgical access.

METHODS

We utilized Vitrea software to render three-dimensional images based on computed tomographic angiography (CTA). We measured the relationship between the TS and SS to their respective superficial landmarks.

RESULTS

We analyzed 50 patients in this study. The distal TS was found caudal to the inion-to-zygoma (IZ) line on average by 5.0 ± 4.3 mm on the right and 6.4 ± 9.3 mm on the left. The mid TS was found cranial on average 3.5 ± 5.7 mm on the right and 3.2 ± 6.0 mm cranial on the left to the superior nuchal line (SNL). The distance from the asterion to the SS was 11.5 ± 2.4 mm medial on the right and 12.1 ± 4.4 mm medial on the left. The average distance was greater on the left than the right when using the IZ landmark. This was directly proportional to the size difference of the sinuses (r² = 0.15, P = 0.03).

CONCLUSIONS

Statistically significant differences between the right and left TS and SS were seen in terms of size. This appeared to correlate nicely to the differences observed between the locations of the TSs' and their respective superficial landmarks.

Morphologic Study of Positional Relationship Between Transverse-Sigmoid Sinus and Extracranial Bony Landmarks With Reconstructed Computed Tomographic Image

OBJECTIVE

By using the line between the lowest point of the mastoid process and the external occipital protuberance as landmarks, to locate the projection of the transverse-sigmoid sinus (TSS) on the skull surface using three-dimensional reconstruction technique, to provide morphological basis for avoiding TSS injuries during surgeries.

METHODS

A total of 120 volunteers underwent computed tomography scan, and computed tomography reconstruction was used to reconstruct the 3D model of the skull for structural landmark and measurement. The line between the most prominent point (A) of external occipital protuberance and the lowest point (B) of mastoid process was used as the landmark to depict distance between the TSS sulcus and the landmarks, as well as the width of the TSS sulcus.

RESULTS

The widths of the transverse sinus sulcus, denoted as d, at its central landmark J were measured to be significantly different between the right and left sides (t = 6.291, P < 0.05); no statistically significant difference was found in the measurements of indicators including h1, h2, h3, h4, h5, h6, h7, h8, d1, α, s1, s2, s3, s4, s5, s6 between the right and left sides (P > 0.05), or between the males and females (P > 0.05).

CONCLUSIONS

These above-mentioned results can help to locate the projection of the TSS sulcus on the skull surface accurately, which is simple and convenient in guiding the surgeons to protect the TSS during surgeries.

A novel reference coordinate system to locate the inferomedial point of the transverse-sigmoid sinus junction

BACKGROUND

A coordinate system was previously developed to identify landmarks on the skull surface to help locate the transverse-sigmoid sinus junction in order to reduce surgical morbidity in retrosigmoid craniotomy; however, in practice we found that this system has important flaws.

OBJECTIVE

To develop and evaluate a novel reference coordinate system to precisely locate the inferomedial point of the transverse-sigmoid sinus junction (IMTS) and evaluate the effect of gender and skull side (left or right).

METHODS

Forty-two adult skulls (84 sides) were obtained for analyses. The X-axis was defined by point A (where the upper edge of the zygomatic arch joins with the frontal process of the zygomatic bone) and point B (where the upper edge of the zygomatic arch blends posterosuperiorly into the supramastoid crest). The Y-axis was defined by the line perpendicular to the X-axis and extending across the tip of the mastoid. The x and y coordinates of IMTS (IMTS-x and IMTS-y) were measured in this coordinate system.

RESULTS

There were 20 male skulls and 22 female skulls. The mean IMTS-x measurements were significantly higher on the right side compared with the left side in both males and females. For the left skull side, the mean IMTS-y measurements were significantly lower in females compared with males.

CONCLUSION

This novel reference coordinate system may be a reliable and practical method for identifying the IMTS during retrosigmoid craniotomy. There are significant differences in location of the axes with regard to gender and skull side.

[CT-assisted navigation for retrosigmoidal implantation of the Bonebridge]

The Bonebridge is an active bone conduction implant (BCI) that is primarily indicated in patients with conductive and combined hearing loss. However, many of these patients present with a radical cavity as a result of previous surgery. In these cases, the implant should not be introduced into the mastoid region, but rather via a retrosigmoid approach to maintain separation from the pathological alteration. To ensure the best possible acoustic transduction, the Bone Conduction-Floating Mass Transducer (BC-FMT) should be positioned near to the cochlea. This requires precise identification of the sigmoid sinus, which cannot be achieved accurately enough using external anatomical landmarks. We thus report on two patients in whom the Bonebridge was implanted via a retrosigmoid approach using CT-guided navigation.

Determination of the keyhole position in a lateral suboccipital retrosigmoid approach

Appropriate placement of the keyhole at the transverse and sigmoid sinus (T/S) junction is important for performance of safe and accurate lateral suboccipital craniotomy with minimum bone loss. Here, we report a method for predicting the position of the T/S junction and investigate the relationship between the T/S junction and asterion. The subjects were 88 patients treated surgically via a lateral suboccipital approach. These cases included 78 acoustic neuromas, 4 meningiomas, 1 trigeminal schwannoma, 1 epidermoid cyst, 2 trigeminal neuralgias, and 1 hemifacial spasm. To expose the T/S junction, we usually place the keyhole lateral to asterion by a half diameter of the burr hole. The distance of the T/S junction from asterion was investigated using three-dimensional computed tomography (3DCT) images. We investigated the differences between the actual and predicted positions of the T/S junction based on skull landmarks, and we compared our method with other literature methods. The mean distances were 5.7 mm caudal and 6.6 mm lateral. The difference between the actual and predicted positions was significantly smaller in our approach compared to other methods. Placing the keyhole lateral to a provisional burr hole just caudal to asterion and lateral by half the diameter of the burr hole was useful for exposure of the T/S junction. The best approach is to use preoperative 3DCT, but this may be limited by equipment problems, emergency cases, or allergy to contrast medium. Determination of the appropriate keyhole position with reference to skull landmarks is a universally useful method.

Surface Landmarks for the Junction Between The transverse and Sigmoid Sinuses: Application of the “Strategic” Burr Hole for Suboccipital Craniotomy

Objective:

Localization of internal cranial anatomy based on superficial landmarks is paramount in identifying and avoiding various important structures and, thus, decreasing surgical morbidity. We have studied external skull bony landmarks to facilitate the placement of the initial “strategic” burr hole just inferior and medial to the junction of transverse- sigmoid venous sinuses during standard retrosigmoid craniotomy.

Methods:

One hundred adult skulls (200 sides) underwent intracranial drilling of a small hole from the inside surface of the cranium, 5 mm inferior and medial to the border of the transverse sigmoid sinus junction (defined as the ideal location for the center of the strategic burr hole). Localization of this hole from the external surface of the skull was made based on easily identifiable superficial landmarks, including the mastoid process and zygomatic arch. A horizontal line was established parallel to the superior border of the zygomatic arch (“zygomatic line”), and a vertical line was fashioned by connecting the mastoid notch superiorly to the squamosal suture (“mastoid line”).

Results:

For left sides, 81% of the strategic burr holes were inferior to the zygomatic line and 86% were medial to the mastoid line. For right sides, 91% of the strategic burr holes were inferior to the zygomatic line and 97% were medial to the mastoid line. For left and right sides, the mean distance for the center of the burr holes from the zygomatic line was 4.5 and 7.7 mm, respectively. For left and right sides, the mean distance from the mastoid line was 9.1 and 9.8 mm, respectively.

Conclusion:

Because landmark data in the literature for externally identifying the transverse sigmoid sinus junction is variable, we have attempted to refine this location with the largest sample size to date. These data can assist surgeons to localize the external cranial projection of the area just inferior and medial to the junction between the transverse and sigmoid sinuses when image guidance devices are not available. This localization is important in creation of appropriate size for craniotomy/craniectomy during the posterolateral approaches to the cranial base.

Window anatomy for neurosurgical approaches. Laboratory investigation

OBJECT

Knowledge of the cranium projections of the gyral structures is essential to reduce the surgical complications and to perform minimally invasive interventions in daily neurosurgical practice. Thus, in this study the authors aimed to provide detailed information on cranial projections of the eloquent cortical areas.

METHODS

Ten formalin-fixed adult human skulls were obtained. Using sutures and craniometrical points, the crania were divided into 8 windows: superior frontal, inferior frontal, superior parietal, inferior parietal, sphenoidal, temporal, superior occipital, and inferior occipital. The projections of the precentral gyrus, postcentral gyrus, inferior frontal gyrus, superior temporal gyrus, transverse temporal gyri, Heschl gyrus, genu and splenium of the corpus callosum, supramarginal gyrus, angular gyrus, calcarine sulcus, and sylvian fissure to cranial vault were evaluated.

RESULTS

Three-fourths of the precentral gyrus and postcentral gyrus were in the superior parietal window. The inferior frontal gyrus extended to the inferior parietal window in 80%. The 3 important parts of this gyrus were located below the superior temporal line in all hemispheres. The orbital and triangular parts were in the inferior frontal window, and the opercular part was in the inferior parietal window. The superior temporal gyrus was usually located in the inferior parietal and temporal windows, whereas the supramarginal gyrus and angular gyrus were usually located in the superior and inferior parietal windows. The farthest anterior point of the Heschl gyrus was usually located in the inferior parietal window. The mean positions of arachnoid granulations were measured as 3.9 +/- 0.39 cm anterior and 7.3 +/- 0.51 cm posterior to the bregma.

CONCLUSIONS

Given that recognition of the gyral patterns underlying the craniotomies is not always easy, awareness of the coordinates and projections of certain gyri according to the craniometric points may considerably contribute to surgical interventions.

A 3-dimensional computed tomographic procedure for planning retrosigmoid craniotomy

OBJECTIVE

The planning of retrosigmoid craniotomies often relies on anatomic landmarks on the posterolateral surface of the cranium, such as the asterion. However, the location of the asterion is not fixed with respect to the underlying transverse-sigmoid sinus complex. We introduce a simple procedure that uses 3-dimensional (3D) computed tomographic imaging to project the transverse-sigmoid sinus complex onto the external surface of the cranium.

METHODS

We enrolled 8 patients scheduled for retrosigmoid craniotomy (Group 1) and 30 patients without posterior fossa lesions (Group 2). The procedure consists of 3 steps: 1) marking the sinus on the internal surface on 3D images of the cranium, 2) transferring the marks to the external surface on axial images, and 3) checking the transferred marks on the external surface of the cranium on 3D images.

RESULTS

In Group 1, the craniotomies planned with the aid of our procedure coincided with findings made at surgery, indicating the accuracy of our procedure. When we applied it in morphometric studies in Group 2, we found that the relative location of the transverse-sigmoid sinus junction to the asterion, the superior nuchal line, and the posterior edge of the mastoid process exhibited a high degree of individual variation.

CONCLUSION

Retrosigmoid craniotomy standardized according to anatomic landmarks raises the risk for sinus injury because of variations in their location. We offer our 3D computed tomographic imaging-based procedure as a useful device for the planning of safer retrosigmoid craniotomies.