Anatomical Step-by-Step Dissection of Complex Skull Base Approaches for Trainees: Surgical Anatomy of the Retrosigmoid Approach

Anatomical Step-by-Step Dissection of Complex Skull Base Approaches for Trainees: Surgical Anatomy of the Endoscopic Endonasal Approach to the Anterior Cranial Fossa

Introduction  The development of endoscopic techniques has made endoscopic endonasal approaches (EEAs) to the anterior cranial fossa (ACF) increasingly popular. Still, the steps and nuances involved in the approach may be difficult to understand for trainees. Thus, we aim to didactically describe the EEAs to the ACF in an anatomically based, step-by-step manner with supplementary clinical cases. Methods  Six cadaveric head specimens were dissected. Endoscopic endonasal Draf I, IIA, IIB, and III frontal sinusotomies, endoscopic endonasal superior ethmoidectomy, and endoscopic endonasal transcribriform and transplanum approaches were modularly performed. The specimens were photodocumented with endoscopic techniques. Results  Draf I frontal sinusotomy started with the complete removal of the anteromedial portion of the agger nasi cell, exposing the medial orbital wall, cranial base, and anterior cribriform plate. Draf II frontal sinusotomy proceeded with the removal of the floor of the frontal sinus between the lamina papyracea and the middle turbinate (IIa), and the nasal septum (IIb) until the first olfactory filaments were exposed. Draf III proceeded by creating a superior septal window just below the floor of the frontal sinus. The bone of the ACF bounded by the limbus sphenoidale posteriorly, frontal sinus anteriorly, and the medial orbital walls bilaterally was removed; the cribriform plate was removed; and the crista galli was dissected free from the dural leaflets of the falx cerebri and removed. Conclusion  We provide a step-by-step dissection describing basic surgical steps and anatomy of the EEAs to the ACF to facilitate the learning process for skull base surgery trainees.

Modified colored three-dimensional posterior and temporal cranial fossa model with mobility of the joint between C1 and occipital condyle

BACKGROUND

Skull base surgery requires anatomical knowledge and appropriate surgical technique in bone drilling. We developed a newly modified three-dimensional (3D) model of the posterior cranial fossa as a learning tool that improves knowledge of skull base anatomy and surgical approaches, including skull base drilling techniques.

METHODS

This bone model of the posterior cranial fossa was created based on computed tomography data using a 3D printer, and incorporates artificial cranial nerves, cerebral vessels, bony structures, dura mater, and cerebellar tentorial dura. These anatomical components are differentiated with various colors. In addition, the atlanto-occipital junction can be mobilized to fully expose the surface of the cartilage between the C1 condyle and occipital condyle to allow drilling to open the hypoglossal canal under a wide surgical field. The usefulness of the model for practicing skull base surgical approaches was evaluated.

RESULTS

Experience of bone drilling, dural dissection, and 3D positioning of important structures, including cranial nerves and blood vessels, was identical to that in actual surgery.

CONCLUSIONS

This model is designed to facilitate teaching anatomical knowledge and essential epidural procedure-related skills, and is useful for teaching the essential elements of posterior skull base surgery.

Surgical Anatomy of the Retrosigmoid Approach With Transtentorial Extension: Protecting the 4th Cranial Nerve

BACKGROUND AND OBJECTIVES

The retrosigmoid approach with transtentorial extension (RTA) allows us to address posterior cranial fossa pathologies that extend through the tentorium into the supratentorial space. Incision of the tentorium cerebelli is challenging, especially for the risk of injury of the cranial nerve (CN) IV. We describe a tentorial incision technique and relevant anatomic landmarks.

METHODS

The RTA was performed stepwise on 5 formalin-fixed (10 sides), latex-injected cadaver heads. The porus trigeminus's midpoint, the lateral border of the suprameatal tubercle (SMT)'s base, and cerebellopontine fissure were assessed as anatomic landmarks for the CN IV tentorial entry point, and relative measurements were collected. A clinical case was presented.

RESULTS

The tentorial opening was described in 4 different incisions. The first is curved and starts in the posterior aspect of the tentorium. It has 2 limbs: a medial one directed toward the tentorium's free edge and a lateral one that extends toward the superior petrosal sinus (SPS). The second incision turns inferiorly, medially, and parallel to the SPS down to the SMT. At that level, the second incision turns perpendicular toward the tentorium's free edge and ends 1 cm from it. The third incision proceeds posteriorly, parallel to the free edge. At the cerebellopontine fissure, the incision can turn toward and cut the tentorium-free edge (fourth incision). On average, the CN IV tentorial entry point was 12.7 mm anterior to the SMT base's lateral border and 20.2 mm anterior to the cerebellopontine fissure. It was located approximately in the same coronal plane as the porus trigeminus's midpoint, on average 1.9 mm anterior.

CONCLUSION

The SMT and the cerebellopontine fissure are consistently located posterior to the CN IV tentorial entry point. They can be used as surgical landmarks for RTA, reducing the risk of injury to the CN IV.

Anatomical Step-by-Step Dissection of Complex Skull Base Approaches for Trainees: Surgical Anatomy of the Frontotemporal and Orbitozygomatic Craniotomies

Introduction  Although many neuroanatomic atlases have been published, few have detailed complex cranial approaches and pertinent anatomic considerations in a stepwise fashion intended for rapid comprehension by neurosurgery students, residents, and fellows. Methods  Five sides of formalin-fixed latex-injected specimens were dissected under microscopic magnification. The frontotemporal and orbitozygomatic approaches were performed by neurosurgical residents and fellows at different training levels with limited previous experience in anatomical dissection mentored by the senior authors (M.P.C. and M.J.L.). Meticulous surgical anatomic dissections were performed until sufficient visual and technical completion was attained, with parameters of effectively demonstrating key surgical steps for educational training purposes. Following the completion of dissection and three-dimensional photography, illustrative case examples were reviewed to demonstrate the relative benefits and optimal applications of each approach. Results  The frontotemporal and orbitozygomatic approaches afford excellent access to anterior and middle skull base pathologies, as well as the exposure of the infratemporal fossa. Key considerations include head positioning, skin incision, scalp retraction, fat pad dissection and facial nerve protection, true or false MacCarty keyhole fashioning, sphenoid wing drilling and anterior clinoidectomy, completion of the craniotomy and accessory orbital osteotomy cuts, dural opening, and intradural neurovascular access. Conclusion  The frontotemporal and orbitozygomatic approaches are core craniotomies that offer distinct advantages for complex cranial operations. Learning and internalizing their key steps and nuanced applications in a clinical context is critical for trainees of many levels. The orbitozygomatic craniotomy in particular is a versatile but challenging approach; operative-style laboratory dissection is an essential component of its mastery and one that will be powerfully enhanced by the current work.

Retrosigmoid versus middle fossa approach for hearing and facial nerve preservation in vestibular schwannoma surgery: A systematic review and comparative meta-analysis

BACKGROUND

Vestibular schwannomas (VS) are benign tumors arising from vestibular nerve's Schwann cells. Surgical resection via retrosigmoid (RS) or middle fossa (MF) is standard, but the optimal approach remains debated. This meta-analysis evaluated RS and MF approaches for VS management, emphasizing hearing preservation and Cranial nerve seven (CN VII) outcomes stratified by tumor size.

METHODS

Systematic searches across PubMed, Cochrane, Web of Science, and Embase identified relevant studies. Hearing and CN VII outcomes were gauged using the American Academy of Otolaryngology-Head and Neck Surgery, Gardner Robertson, and House-Brackmann scores.

RESULTS

Among 7228 patients, 56 % underwent RS and 44 % MF. For intracanalicular tumors, MF recorded 38 % hearing loss, compared to RS's 54 %. In small tumors (<1.5 cm), MF showed 41 % hearing loss, contrasting RS's lower 15 %. Medium-sized tumors (1.5 cm-2.9 cm) revealed 68 % hearing loss in MF and 55 % in RS. Large tumors (>3cm) were only reported in RS with a hearing loss rate of 62 %.

CONCLUSION

Conclusively, while MF may be preferable for intracanalicular tumors, RS demonstrated superior hearing preservation for small to medium-sized tumors. This research underlines the significance of stratified outcomes by tumor size, guiding surgical decisions and enhancing patient outcomes.

Micro-Doppler for venous sinus localization in approaches to the cerebello-pontine angle

BACKGROUND

Main anatomical landmarks of retrosigmoid craniotomy are transverse sinus (TS), sigmoid sinus (SS), and the confluence of both. Anatomical references and guidance based on preoperative imaging studies are less reliable in the posterior fossa than in the supratentorial region. Simple intraoperative real-time guidance methods are in demand to increase safety.

METHODS

This manuscript describes the localization of TS, SS, and TS-SS junction by audio blood flow detection with a micro-Doppler system.

CONCLUSION

This is an additional technique to increase safety during craniotomy and dura opening, widening the surgical corridor to secure margins without carrying risks nor increase surgical time.

Experience and Balance: Long-Term Trends in Preferred Skull Base Approach for a Case Series of Cavernous Malformation Resections

BACKGROUND

Skull base approaches are a foundation of modern cerebrovascular surgery; however, their application over time has varied.

OBJECTIVE

To assess trends in skull base approach selection for cavernous malformation (CM) resection.

METHODS

This is a retrospective case series of all first-time CM resections by a single surgeon from 1997 to 2021. Cases were classified by craniotomy and approach. Four sets of common comparator skull base approaches were identified by coauthor consensus: pterional and orbitozygomatic; retrosigmoid, extended retrosigmoid (xRS), and far-lateral; suboccipital and torcular; and trans-cerebellar peduncle (MCP) and transcerebellopontine angle. Counts were binned by 5-year or 10-year clusters for descriptive statistical assessment of temporal trends.

RESULTS

In total, 372 primary CM resections met the study criteria and were included. Orbitozygomatic approach use increased during the second 5-year period, after which the pterional approach rapidly became and remained the preferred approach. During the first two 5-year periods, the far-lateral approach was preferred to the retrosigmoid and xRS approaches, but the xRS approach grew in popularity and accounted for >50% of operations in this comparator group. Trans-MCP use compared with the transcerebellopontine angle approach closely mirrored the change in xRS use. The midline suboccipital approach accounted for a larger proportion (range, 62%-88%) of cases than the torcular approach (range, 12%-38%) across all periods.

CONCLUSION

The xRS and trans-MCP approaches have been increasingly used over time, while the orbitozygomatic and far-lateral approaches have become less common. These trends seem to reflect versatility, efficiency, and safety of these techniques.

Anatomical Step-by-Step Dissection of Complex Skull Base Approaches for Trainees: Surgical Anatomy of the Far Lateral Approach

Introduction  Skull base neuroanatomy is classically taught using surgical atlases. Although these texts are critical and rich resources for learning three-dimensional (3D) relationships between key structures, we believe they could be optimized and complemented with step-by-step anatomical dissections to fully meet the learning needs of trainees. Methods  Six sides of three formalin-fixed latex-injected specimens were dissected under microscopic magnification. A far lateral craniotomy was performed by each of three neurosurgery resident/fellow at varying stages of training. The study objective was the completion and photodocumentation of the craniotomy to accompany a stepwise description of the exposure to provide a comprehensive, intelligible, and anatomically oriented resource for trainees at any level. Illustrative case examples were prepared to supplement approach dissections. Results  The far lateral approach provides a wide and versatile corridor for posterior fossa operation, with access spanning the entire cerebellopontine angle (CPA), foramen magnum, and upper cervical region. Key Steps Include  The study includes the following steps: positioning and skin incision, myocutaneous flap, placement of burr holes and sigmoid trough, fashioning of the craniotomy bone flap, bilateral C1 laminectomy, occipital condyle/jugular tubercle drilling, and dural opening. Conclusion  Although more cumbersome than the retrosigmoid approach, a far lateral craniotomy offers unparalleled access to lesions centered lower or more medially in the CPA, as well as those with significant extension into the clival or foramen magnum regions. Dissection-based neuroanatomic guides to operative approaches provide a unique and rich resource for trainees to comprehend, prepare for, practice, and perform complex cranial operations, such as the far lateral craniotomy.

Anatomical Step-by-Step Dissection of Complex Skull Base Approaches for Trainees: Surgical Anatomy of the Middle Fossa Approaches and Anterior Petrosectomy, Surgical Principles, and Illustrative Cases

Introduction The middle fossa (MF) approaches encompass a group of versatile surgical accesses to pathologies in the MF, internal auditory canal (IAC), and superomedial aspect of the posterior fossa. Although many descriptions of the MF approaches have been published, a practical surgical guide that allows an easy understanding for Skull Base trainees is needed.

Methods Three formalin-fixed, colored-injected specimens were dissected under microscopic magnification (six sides). A MF craniotomy followed by IAC drilling was performed on three sides, and anterior petrosectomy (AP) was performed in the remaining three sides. The anatomical dissection was documented in stepwise three-dimensional photographic images. Following dissection, representative case applications were reviewed.

Results The MF approach provides direct access to the MF structures and IAC. The AP provides excellent access to the superomedial aspect of the posterior fossa. Key common steps include: positioning and skin incision; scalp and muscle flaps; burr holes; craniotomy flap elevation; dural dissection along the petrous ridge; division of the middle meningeal artery; and exposure of the greater superficial petrosal nerve, tegmen tympani, and V3. Then, to approach the IAC: superior IAC drilling, and longitudinal dura opening. The area drilled in the AP approach forms a pentagon limited by the petrous internal carotid artery, cochlea, IAC, petrous ridge, and lateral border of V3.

Conclusion The MF approaches are challenging. Operatively oriented skull base dissections provide a crucial foundation for learning these techniques. We describe comprehensive step-by-step approaches intended to develop familiarity in the cadaver laboratory and facilitate understanding of their potential for skull base disorders. Basic surgical principles are described to help in the operating room as well as illustrative cases.

Application of Subperineural Resection Technique in Vestibular Schwannomas: Surgical Efficacy and Outcomes in 124 patients

Objective

We aimed to explore the application and prospects of the subperineural resection technique for tumor separation and removal under the perineurium during surgery for vestibular schwannomas (VSs).

Methods

This study retrospectively analyzed 124 patients with VSs who underwent surgery via a retrosigmoid approach from July 2015 to October 2020 in the Department of Neurosurgery, Tangdu Hospital, Air Force Military Medical University. The data will be discussed with regard to the following aspects: clinical features, surgical strategies, tumor resection extent, facial-acoustic function preservation, and postoperative complications.

Results

Gross total resection (GTR) of the tumor was achieved in 104 patients, with a GTR rate of 83.9%, and subtotal resection (STR) of the tumor was achieved in 20 patients. There was no significant difference in facial and acoustic nerve functional preservation between GTR and STR, as well as in tumor resection between solid and cystic tumors. The retention rate reached 97.6% in terms of complete anatomical facial nerve preservation. Facial nerve function was assessed using the House-Brackmann (HB) grading score. Consequently, HB grades of I-II, III-IV, and V-VI were determined for 96 (77.4%), 25 (20.2%), and 3 (2.4%) cases, respectively, 1 week postoperatively and accounted for 110 cases (88.7%), 13 cases (10.5%), and 1 case (0.8%), respectively, at 6 months. Fifteen of 35 (42.9%) patients with serviceable hearing before the operation still had serviceable hearing at 6 months postoperatively. There were 5 cases of cerebellar or brainstem bleeding after the operation, and one patient died. Multivariate logistic regression analysis showed that older age (≥60 years, p = 0.011), large tumor (>3 cm, p = 0.004), and cystic tumor (p = 0.046) were independent risk factors associated with the extent of adhesion between the tumor and the brainstem and facial-acoustic nerve.

Conclusion

We successfully applied the subperineural resection technique to a large series of patients with VSs and achieved satisfactory results. Accurate identification of the perineurium and subperineural resection of the tumor can effectively reduce the disturbance of the facial-acoustic nerve during the operation and provide an intuitive basis for judging the tumor boundary. The subperineural resection technique may be conducive to improving the rate of total tumor resection and facial-acoustic nerve functional preservation in the surgical treatment of VSs.