How I do it: inside-out bony window design in the retrosigmoid suboccipital approach

Abstract

Background

The conventional retrosigmoid approach uses external landmarks like the asterion to locate the transverse-sigmoid sinus junction, but anatomical variability often makes these landmarks unreliable, risking inaccurate craniotomy and sinus injury.

Method

We describe an individualized "inside-out" technique using preoperative 3D reconstruction to identify the transverse sinus, sigmoid sinus, and their junction on the inner skull table and project them onto the outer table, enabling precise burr hole placement and patient-specific bone flap design.

Conclusion

This inside-out approach offers a reliable, individualized method for safe and efficient retrosigmoid craniotomy, reducing sinus injury risk and optimizing surgical exposure.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1007/s00701-026-06958-5.

Introduction

The retrosigmoid approach is a fundamental corridor to the cerebellopontine angle and posterior fossa [7]. Accurate localization of the transverse-sigmoid sinus junction (TSSJ) is critical for safe burr hole placement [2, 5]. Traditional external landmarks such as the asterion are often unreliable due to anatomical variability, risking sinus injury or inadequate exposure [4]. We propose an individualized "inside-out" strategy using preoperative 3D reconstruction to project the TSSJ from the inner to the outer skull table, enabling a truly patient-specific craniotomy.

Relevant surgical anatomy

A thorough understanding of the layered anatomy from superficial to deep is essential for the retrosigmoid approach [1–3, 6]: occipital artery, sternocleidomastoid muscle, splenius capitis muscle, digastric muscle (posterior belly), asterion, parietomastoid, occipitomastoid, lambdoid sutures, mastoid process, digastric groove, mastoid emissary foramen, transverse sinus, sigmoid sinus, transverse-sigmoid sinus junction (TSSJ), mastoid emissary vein.

Description of the technique

Preoperative planning

High-resolution CT venography or contrast-enhanced MRI is obtained. DICOM data are imported into 3D Slicer(open-source software).

Step 1: Orientation to surgical position

The reconstructed skull is rotated to match the intraoperative head position: 30–45° contralateral rotation and 10–20° flexion. All subsequent measurements are performed in this orientation.

Step 2: Segmentation of venous sinuses

Using the segment editor, the transverse sinus, sigmoid sinus are segmented. The transverse-sigmoid sinus junction (TSSJ) is identified on the inner table where the transverse sinus curves downward to become the sigmoid sinus.

Step 3: Reconstruction of cranial tables

The inner and outer tables of the skull are segmented separately to create two distinct 3D models. The inner table model includes the impression of the venous sinuses.

Step 4: Identification of external landmarks

On the outer table model, two bony landmarks are identified:

• The asterion: junction of parietomastoid, occipitomastoid, and lambdoid sutures.

• The apex of the digastric groove: the highest point of the digastric groove on the medial aspect of the mastoid process.

Step 5: Projection of TSSJ to outer table

On the inner table model, the TSSJ point is marked at the junction where the transverse sinus curves downward to become the sigmoid sinus. Using the "markups" module, a point is placed at the TSSJ. The surface-normal vector at this point is calculated. The point is projected along this vector to the outer table surface, yielding the projected TSSJ point, accounting for skull thickness at the projection site (Fig. 1).

Fig. 1.

Illustration of the preoperative planning of inside-out bony window. A External cranial landmarks visible on the outer surface of the skull. B Internal cranial landmarks on the inner surface of the skull. C Bony window design on the inner surface of the skull. D Projection of the bony window onto the outer surface of the skull. E Transverse-sigmoid sinus junction defined based on the bony window boundaries. F Location of the transverse-sigmoid sinus junction on the inner surface of the skull

Step 6: Measurement to reference landmarks

Using the "measurement" tool, two distances are recorded:

• Distance A: From the asterion to the projected TSSJ point, including direction (e.g., 14 mm anteromedial).

• Distance B: From the apex of the digastric groove to the projected TSSJ point, including direction (e.g., 18 mm superoposterior).

Step 7: Documentation

All measurements are documented with exact distances and directional vectors. A 1:1 scale template can be printed, or the measurements are recorded in the surgical plan. The entire 3D reconstruction and inside-out projection workflow takes on average 12–15 min per patient.

Patient positioning

Patient is placed lateral or supine. Head is rotated contralaterally and flexed to replicate the preoperative orientation. Ipsilateral shoulder is taped down. Head is fixed in a head clamp.

Skin incision and soft tissue dissection

A curvilinear retroauricular incision is made. Suboccipital muscles (sternocleidomastoid, splenius capitis, digastric) are dissected posteroinferiorly. The occipital artery is identified and preserved. The squamous occipital bone, digastric groove, and mastoid process are exposed. Asterion and apex of the digastric groove are identified.

Inside-out guided craniotomy

Step 1: Mark TSSJ projection

From the asterion, measure Distance A in the predetermined direction. Mark the point. From the digastric groove apex, measure Distance B in the predetermined direction to the same point. Adjust until both distances are satisfied. This point is the external projection of the TSSJ.

Step 2: Burr hole placement

A single burr hole is placed at the marked point. A cutting burr is used initially. Near the inner table, switch to a diamond burr and advance millimeter by millimeter. When a thin eggshell layer remains, lift it with a blunt dissector to expose the dura over the TSSJ.

Step 3: Inner table beveling

Using a curette or 1–2 mm Kerrison punch, enlarge the inner table opening circumferentially, particularly posteriorly and inferiorly away from the sinus. Remove 3–5 mm of inner table beyond the original margin. Keep the instrument tip angled away from the sinus (Fig. 2).

Fig. 2.

Inner table beveling. A Step 1: Burr hole placement on the skull. B Step 2: Enlarging the inner table opening with a diamond burr. C Step 3: Bone flap elevation with a craniotome

Step 4: Sinus verification

Palpate the sinus through the opening with a blunt dissector.

Step 5: Mastoid emissary vein management

Using a diamond burr under irrigation, thin the bone posterior to the sigmoid sinus until the mastoid emissary vein is visualized. Expose the vein fully. Coagulate with bipolar forceps at a safe distance from the sigmoid sinus and divide it. Inspect the sinus-side stump for hemostasis.

Step 6: Dural dissection

From the beveled edge, separate the dura from the inner table using a dissector. Direction of dissection is posterior and away from the sinus.

Step 7: Bone flap elevation

Using a craniotome, cut a 3–4 cm bone flap. The cut is directed posterior and medial to the sinus. The footplate remains in contact with the inner table, sliding along the beveled surface. The cut is placed posterior to the ligated mastoid emissary vein stump.

If additional exposure is required—such as in an extended retrosigmoid approach—the edges of the sigmoid and transverse sinuses are further skeletonized using a diamond burr under continuous irrigation (Fig. 3).

Fig. 3.

Inside-out bony window design in a patient with left-sided vestibular schwannoma. A Surface landmarks of the asterion and cranial sutures. B Determination of the bony window and transverse-sigmoid sinus junction on the inner surface of the skull. C Projection of the bony window and transverse-sigmoid sinus junction onto the outer surface of the skull. D Projection of the transverse-sigmoid sinus junction onto the scalp. E Postoperative view of the bony window and transverse-sigmoid sinus junction on the inner surface of the skull. F Postoperative view of the bony window and transverse-sigmoid sinus junction on the outer surface of the skull

Dural opening and intradural exposure

The dura is incised in a C-shape based on the sinus. The cerebellomedullary cistern is opened to drain CSF. The surgical corridor is directed according to the target pathology.

Closure

The dura is closed watertight with 5–0 non-absorbable suture. Opened mastoid air cells are sealed with bone wax. The bone flap is reattached with titanium plates and screws. Muscles, subcutaneous tissue, and skin are closed in layers.

Indications

• Cerebellopontine angle tumors: Vestibular schwannomas, meningiomas, and epidermoid cysts requiring surgical resection

• Petroclival lesions: Meningiomas, chordomas, and chondrosarcomas involving the petroclival region

• Vascular lesions: Aneurysms of the vertebral artery, vertebrobasilar junction, and proximal posterior inferior cerebellar artery (PICA)

• Jugular foramen tumors: Schwannomas and selected meningiomas with limited extension into the jugular foramen

• Trigeminal neuralgia: Microvascular decompression for neurovascular conflict at the trigeminal root entry zone

• Hemifacial spasm: Microvascular decompression of the facial nerve

• Brainstem lesions: Cavernous malformations and selected intrinsic tumors with lateral pial presentation

• Lower cranial nerve disorders: Exploration and decompression of glossopharyngeal, vagus, or accessory nerves

Limitations

• Limited access to lesions anterior to the brainstem: Petroclival lesions with significant anterior extension beyond the midline are difficult to adequately expose through this corridor alone.

• High-riding jugular bulb: A high jugular bulb obscures the inframeatal region and limits access to the jugular foramen and lower clivus.

• Restricted access to the middle fossa and Meckel's cave: Lesions with significant extension into the middle cranial fossa cannot be adequately addressed through this approach alone.

• The technique requires basic 3D software skills; it is a complementary tool and not a replacement for intraoperative navigation in complex cases.

How to avoid complications

• Skin and soft tissue: Curvilinear incision, preserve occipital artery, layered open and closure.

• Bone and mastoid air cells:Precise TSSJ localization with inside-out method, stepwise drilling, inner table beveling, ligate mastoid emissary vein before craniotomy, cut posterior/medial to sinus, seal air cells with bone wax, rigid bone flap fixation.

• Dura mater: C-shaped incision, sharp dissection, watertight closure with 5–0 suture, Valsalva maneuver, patch if needed.

• Intracranial vessels: Ligate mastoid emissary vein pre-craniotomy, cut posterior to stump, gelatin sponge/muscle patch for sinus tears, confirm contralateral sinus patency, neurophysiological monitoring

• Brain tissue: Early CSF drainage, dynamic retraction, maintain SBP < 120 mmHg, arachnoid plane dissection, early postoperative CT.

• Cranial nerves: Continuous neurophysiological monitoring, sharp dissection, low-power bipolar with irrigation, internal debulking first, accept thin tumor remnant if plane unclear, ENT evaluation before oral intake.

• CSF circulation: Multilayered closure (watertight dura + bone wax + muscle approximation), head elevation at 30°, avoid Valsalva, lumbar drain for persistent leaks, re-explore if refractory.

Advantages over intraoperative navigation systems

• It provides direct bony-based anatomical understanding rather than relying on digital registration, which is less affected by registration error or brain shift.

• It offers a tangible, reproducible measurement scheme (Distance A and B) that can be verified intraoperatively without screen reference.

• It works without registration time or equipment dependency, and can be used in centers without navigation.

• It improves anatomical insight for the operator, especially trainees, by clarifying inner-outer table relationships.This technique is best used as a complement to navigation rather than a replacement, combining anatomical certainty with digital guidance.

Educational value

• It teaches the three-dimensional relationship between inner venous sinuses and outer skull landmarks, improving anatomical literacy.

• It provides a step-by-step, reproducible workflow for trainees to learn safe craniotomy principles.

• It allows preoperative visualization and rehearsal of the sinus location, reducing risky trial-and-error drilling.

• It builds confidence in landmark-based surgery for young surgeons, even when navigation is unavailable.

• The 3D reconstruction process itself serves as a teaching tool for understanding cranial base anatomy and individual variability.

Specific information to patient

• Preoperative evaluation is essential

• Understand the layered risks and how they are managed

• Define your goals and verify surgical feasibility

Relevant points

• Preoperative imaging with 3D reconstruction is mandatory for inside-out localization

• Orient the 3D model to the exact surgical position (30–45° rotation, 10–20° flexion)

• Two reliable external landmarks: asterion and apex of the digastric groove

• Inside-out projection principle: project TSSJ from inner to outer table along surface-normal vector

• Dual-reference verification intraoperatively using both landmarks

• Stepwise burr hole drilling: cutting burr → diamond burr → eggshell layer lifted with dissector

• Inner table beveling with curette or Kerrison punch is critical to prevent dural tears

• Always identify, coagulate, and divide the mastoid emissary vein before craniotomy

• Craniotome cut directed posterior and medial to the sinus, posterior to ligated vein stump

• Early CSF drainage from cerebellomedullary cistern minimizes cerebellar retraction

• The average additional time for 3D modeling is 12–15 min per case

• This method enhances training safety and anatomical comprehension

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Nian Jiang: Investigation, Validation, Formal analysis, Writing – review & editing.

Yuanyang Xie: Software, Resources, Data curation, Writing – review & editing.

Xuejun Li: Supervision, Project administration, Funding acquisition, Writing – review & editing, Final approval.

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 82270825) and the High talent project of Hunan Province (Grant No. 2022WZ1031) and the National Key Clinical Specialty Major Research Program(Grant No. 20230422).

Data availability

Data availability Not applicable. No datasets were generated or analyzed during the current study.

Declarations

Ethical approval

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Informed consent

Informed consent was obtained from all individual participants included in the study. Additional informed consent was obtained from all patients for whom identifying information is included in this article.

Consent for publication

The authors affirm that human research participants provided informed consent for publication of the images in all figures and videos.

Human and animal rights

This article does not contain any studies with animals performed by any of the authors.

Competing interest

The authors declare no competing interests.