A new landmark for finding the sigmoid sinus in suboccipital craniotomies

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.

Key role of microsurgical dissections on cadaveric specimens in neurosurgical training: Setting up a new research anatomical laboratory and defining neuroanatomical milestones

Introduction

Neurosurgery is one of the most complex surgical disciplines where psychomotor skills and deep anatomical and neurological knowledge find their maximum expression. A long period of preparation is necessary to acquire a solid theoretical background and technical skills, improve manual dexterity and visuospatial ability, and try and refine surgical techniques. Moreover, both studying and surgical practice are necessary to deeply understand neuroanatomy, the relationships between structures, and the three-dimensional (3D) orientation that is the core of neurosurgeons' preparation. For all these reasons, a microsurgical neuroanatomy laboratory with human cadaveric specimens results in a unique and irreplaceable training tool that allows the reproduction of patients' positions, 3D anatomy, tissues' consistencies, and step-by-step surgical procedures almost identical to the real ones.

Methods

We describe our experience in setting up a new microsurgical neuroanatomy lab (IRCCS Neuromed, Pozzilli, Italy), focusing on the development of training activity programs and microsurgical milestones useful to train the next generation of surgeons. All the required materials and instruments were listed.

Results

Six competency levels were designed according to the year of residency, with training exercises and procedures defined for each competency level: (1) soft tissue dissections, bone drilling, and microsurgical suturing; (2) basic craniotomies and neurovascular anatomy; (3) white matter dissection; (4) skull base transcranial approaches; (5) endoscopic approaches; and (6) microanastomosis. A checklist with the milestones was provided.

Discussion

Microsurgical dissection of human cadaveric specimens is the optimal way to learn and train on neuroanatomy and neurosurgical procedures before performing them safely in the operating room. We provided a “neurosurgery booklet” with progressive milestones for neurosurgical residents. This step-by-step program may improve the quality of training and guarantee equal skill acquisition across countries. We believe that more efforts should be made to create new microsurgical laboratories, popularize the importance of body donation, and establish a network between universities and laboratories to introduce a compulsory operative training program.

Anatomical Predictors of Transcranial Surgical Access to the Suprasellar Space

Objective  The suprasellar space is a common location for intracranial lesions. The position of the optic chiasm (prefixed vs. postfixed) results in variable sizes of operative corridors and is thus important to identify when choosing a surgical approach to this region. In this study, we aim to identify relationships between suprasellar anatomy and external cranial metrics to guide in preoperative planning. Methods  T2-weighted magnetic resonance images (MRIs) from 50 patients (25 males and 25 females) were analyzed. Various intracranial and extracranial metrics were measured. Statistical analysis was performed to determine any associations between metrics. Results  Interoptic space (IOS) size correlated with interpupillary distance (IPD; a  = 7.3, 95% confidence interval [CI] = 4.5-10.0, R ²  = 0.3708, p  = 0.0009). IOS size also correlated with fixation of the optic chiasm, for prefixed chiasms ( n  = 7), the mean IOS is 205.14 mm ² , for normal chiasm position ( n  = 33) the mean IOS is 216.94 mm ² and for postfixed chiasms ( n  = 10) the mean IOS is 236.20 mm ² ( p  = 0.002). IPD correlates with optic nerve distance (OND; p  = 0.1534). Cranial index does not predict OND, IPD, or IOS. Conclusion  This study provides insight into relationships between intracranial structures and extracranial metrics. This is the first study to describe a statistically significant correlation between IPD and IOS. Surgical approach can be guided in part by the size of the IOS and its correlates. Particularly small intraoptic space may guide the surgeon away from a subfrontal approach.

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.

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.

En-bloc craniotomy for the pre-sigmoid infra- and supratentorial approach: technical note

INTRODUCTION

The combined supra-infratentorial approach as described some 30 years ago is to date considered a standard procedure for skull base procedures. Several variants have been devised, including preservation of the mastoid process. We herein present the cosmetically most sophisticated and fastest solution.

OBJECTIVES

The authors describe an en bloc supra- and infratentorial pre-sigmoid combined approach. This variant of surgical technique involves a one-piece bone flap (temporal-suboccipital-mastoideal flap). We present another variant of craniotomy for the combined supra- and infratentorial pre-sigmoid approach that preserves the mastoid process and thus appears to be cosmetically much more acceptable.

MATERIALS AND METHODS

Eight dry cadaveric skulls were used to develop an ideal one-piece excision of the cranial vault across the transverse sinus, including portions of the mastoid. Our aim was that no further drilling of the basal skull was needed. The procedure thereafter was practiced on a fresh prepared cadaveric specimen where its feasibility was again confirmed and was then applied to a patient suffering from a huge petroclival meningioma. It was very well tolerated and produced an excellent long-term cosmetic result.

DISCUSSION

The combined supra- and infratentorial pre-sigmoid approach offers the possibility of resecting complex petroclival lesions. The variant presented herein is less time consuming than previously described methods and probably offers the best possible cosmetic result.

CONCLUSION

The en-bloc cranioplastic approach with preservation of the mastoid process is a new, interesting variant of a classical technique that is easy to perform and has the intention of achieving the best possible cosmetic result.