Navigation-guided opening of the internal auditory canal via the retrosigmoid route for acoustic neuroma surgery: cadaveric, radiological, and preliminary clinical study

Inner Ear Breaches from Vestibular Schwannoma Surgery: Revisiting the Incidence of Otologic Injury from Retrosigmoid and Middle Cranial Fossa Approaches

OBJECTIVE

To assess the rate of iatrogenic injury to the inner ear in vestibular schwannoma resections.

STUDY DESIGN

Retrospective case review.

SETTING

Multiple academic tertiary care hospitals.

PATIENTS

Patients who underwent retrosigmoid or middle cranial fossa approaches for vestibular schwannoma resection between 1993 and 2015.

INTERVENTION

Diagnostic with therapeutic implications.

MAIN OUTCOME MEASURE

Drilling breach of the inner ear as confirmed by operative note or postoperative computed tomography (CT).

RESULTS

21.5% of patients undergoing either retrosigmoid or middle fossa approaches to the internal auditory canal were identified with a breach of the vestibulocochlear system. Because of the lack of postoperative CT imaging in this cohort, this is likely an underestimation of the true incidence of inner ear breaches. Of all postoperative CT scans reviewed, 51.8% had an inner ear breach. As there may be bias in patients undergoing postoperative CT, a middle figure based on sensitivity analyses estimates the incidence of inner ear breaches from lateral skull base surgery to be 34.7%.

CONCLUSIONS

A high percentage of vestibular schwannoma surgeries via retrosigmoid and middle cranial fossa approaches result in drilling breaches of the inner ear. This study reinforces the value of preoperative image analysis for determining risk of inner ear breaches during vestibular schwannoma surgery and the importance of acquiring CT studies postoperatively to evaluate the integrity of the inner ear.

How to Precisely Open the Internal Auditory Canal for Resection of Vestibular Schwannoma via the Retrosigmoid Approach

Objective

The aim of this study was to investigate how to precisely expose the intrameatal portion of vestibular schwannomas (VSs) without damaging the labyrinth.

Methods

This was a retrospective study of patients who had undergone retrosigmoid resection of a VS in our institution from April 2018 to December 2021. The patients were divided into microsurgery (MS) and navigation endoscopic-assisted (combined surgery, CS) groups and the effects of image guidance and endoscopy evaluated. The tumors in the CS group were then divided into medial and lateral types by fusion imaging and the differences between the two types analyzed.

Results

Data of 84 patients were analyzed. Residual tumor was detected by postoperative MRI at the fundus of the internal auditory canal in 5 of the 31 patients in the MS group and 1 of the 53 in the CS group. The labyrinth was damaged in four patients in the MS group but was not damaged in any of the CS group patients. The CS group included 29 lateral type and 24 medial type schwannomas. Endoscopic-assisted resection of residual tumor in the IAC was performed significantly more often on medial than on lateral tumors.

Conclusion

Navigation and endoscopy are useful in assisting the exposure of the intrameatal portion of VSs. Preoperative MRI/CT fusion imaging is helpful in preoperative evaluation and surgical planning in patients undergoing VS surgery. Tumors of the medial type require endoscopic assistance for resection.

Accuracy of a Modern Intraoperative Navigation System for Temporal Bone Surgery in a Cadaveric Model

Objectives

To determine the accuracy of a modern navigation system in temporal bone surgery. While routine in other specialties, navigation has had limited use in the temporal bone due to issues of accuracy, perceived impracticality, and value.

Study Design

Prospective observational study.

Setting

Temporal bone laboratory.

Subjects and Methods

Eighteen cadaveric specimens were dissected after rigid fiducials were implanted and computed tomography scans were obtained. Target registration and target localization errors were then measured at various points.

Results

The mean overall target registration error was 0.48 ± 0.29 mm. The mean target localization error was 0.54 mm at the sinodural angle, 0.48 mm at the lateral semicircular canal, 0.55 mm at the round window, 0.39 mm at the oval window, and 0.52 mm at the second genu of the facial nerve.

Conclusion

A modern navigation system demonstrated submillimeter accuracy for all points of interest. Its use in clinical as well as training settings has yet to be fully elucidated.

Microsurgical Anatomy of the Internal Acoustic Meatus as Seen Using the Retrosigmoid Approach

HYPOTHESIS

To show via a retrosigmoid approach the bony labyrinth anatomy and its relationship with the internal acoustic meatus so as to provide guidelines for a safer drilling to the fundus using this approach.

BACKGROUND

Few studies deal with the complex anatomy of petrous bone structures as observed by a retrosigmoid approach.

METHODS

Ten retrosigmoid approaches were performed bilaterally on five fresh cadaveric heads. Afterward high-resolution computed tomographic scans were obtained. Measurements of landmarks and distances between important topographic structures of the pyramid were obtained on its surface using a navigation system. Semicircular canals, vestibular aqueduct, and internal acoustic meatus were dissected to show their anatomy and relationships.

RESULTS

The anatomy of the inner ear structures was shown. Opening of the internal acoustic meatus was accomplished without injury to the labyrinth in 9 out of 10 sides. The distance between the drilled bone of the internal acoustic meatus and the vestibule was calculated on the postoperative computed tomographic scan. The mean value was 1.43 mm (SD, 0.30 mm; range, 1.0-1.8 mm).

CONCLUSION

A better knowledge of the anatomy of the semicircular canals and of the vestibular aqueduct as observed by a retrosigmoid approach, together with their relationships to the fundus and other petrous bone landmarks, can be useful to get a general orientation in acoustic neuroma surgery. Using this information together with the neuronavigation, we were able to successfully open the internal acoustic meatus without entering labyrinthine structures in 90% of the study dissections.

Exposing the Fundus of the Internal Acoustic Meatus without Entering the Labyrinth Using a Retrosigmoid Approach: Is It Possible?

OBJECTIVES

To evaluate the feasibility of performing a labyrinth-sparing neuronavigation-assisted retrosigmoid approach to the fundus of the internal acoustic meatus (IAM) and to describe the anatomy of the structures embedded in the posterior meatal wall.

METHODS

Ten surgical dissections were performed bilaterally on 5 fresh cadavers. Cadavers were subjected to preoperative computed tomography scans and spatial coordinates of inner ear structures were recorded. A retrosigmoid craniectomy was performed. The IAM was drilled towards the fundus until no more than 1 mm of bone covered the labyrinthine structures. Specimens underwent a new computed tomography scan to verify the length of opened IAM and the status of the labyrinth. We then opened the labyrinthine structures and recorded their coordinates using navigation. These were compared with the radiologic coordinates to verify the neuronavigation accuracy.

RESULTS

In 9 sides, the IAM was opened to the fundus without injuring the labyrinth; in 1 side, the vestibule was opened. The mean residual bone on the fundus was 0.97 mm. The average length of the accessible IAM was 88.95%. The best accuracy of the navigation was for the identification of the common crus, with a mean value of 0.73 mm.

CONCLUSIONS

This surgical technique could facilitate the opening of the IAM with preservation of inner ear structures. We opened a mean of 88.95% of the IAM without entering the labyrinthine structures in 90% of cases. These results confirm the feasibility of the retrosigmoid approach for the exposure of the IAM fundus with preservation of labyrinthine structures.

Real-time imaging with the o-arm for skull base applications: a cadaveric feasibility study

Introduction Although intraoperative imaging/navigation has established its critical role in neurosurgery, its role in cranial base surgery is currently limited. Due to issues such as poor bony resolution and accuracy, surgeons have to rely on anatomic landmarks that can be distorted by pathology when drilling out critical structures. Though originally developed for spinal application, we hypothesized that the O-Arm could address the above issues for use in cranial base procedures. Methods A cadaveric study was performed in which heads underwent a preprocedure scan via the O-Arm, a fluoroscopic device capable of providing three-dimensional images through the use of cone-beam technology. Preprocedure scans were taken and then registered to a Stealth S7 machine (Medtronic, Inc., Minneapolis, MN, USA). Key cranial base landmarks were identified on these scans and then subsequently identified under direct visualization after (1) endoscopic endonasal dissection and (2) a middle fossa approach. We then quantified the difference in distance between the preplanned and identified structure during surgery. This difference was considered the error. Results For anterior cranial fossa structures, the mean error was 0.25 mm (anterior septum), 0.27 mm (left septum), and 0.32 mm (right septum). For middle fossa structures, the errors were: 0.11 mm (foramen spinosum), 0.44 mm (foramen rotundum), and 0.21 mm (foramen ovale). Conclusion Based on this preliminary cadaveric study, we feel the O-Arm can provide the necessary imaging resolution at the skull base to be employed for intraoperative navigation during cranial base approaches (open and endoscopic). This study warrants further investigation into its clinical use in patients undergoing similar surgical procedures.

Dural Landmark to Locate the Internal Auditory Canal in Large and Giant Vestibular Schwannomas: The Tübingen Line

BACKGROUND:

In cases of large and giant vestibular schwannomas (VS), the visualization of the internal auditory canal (IAC) opening is difficult or impossible.

OBJECTIVE:

To describe the Tübingen line and explore its relationships with the IAC as a landmark to help locate the IAC.

METHODS:

Ten cadaveric heads were used in this study. Between 2004 and 2009, the senior author (M.T.) used the Tübingen line as a landmark to recognize the IAC in 300 consecutive patients with VS. To locate the Tübingen line, the initial step was to identify several vertical foldings of dura located around the area of the vestibular aqueduct. After this, foldings upward consistently reached a linear level where all of the foldings ended and the dura tightly adhered to the bony surface in a smooth, foldless shape.

RESULTS:

The Tübingen line was identified in all temporal bones studied and in all 300 patients operated on, with the exception of 2 cases (<1%). Removal of the bone just above the Tübingen line located the IAC in all temporal bone specimens studied. Similarly, the surgical cases showed that the Tübingen line helped locate the IAC in all patients.

CONCLUSION:

The Tübingen line is an easy, consistent, and safe method to locate the projection of the IAC along the posterior surface of the temporal bone.

Identification of venous variants in the pineal region with three-dimensional preoperative magnetic resonance imaging navigation in patients harbouring tumors in this area: significance for surgical approach to the lesion

OBJECT

The purpose of this study was to identify the anatomy of pineal region venous complex using neuronavigation software when distorted by the presence of a space-occupying lesion and to describe the anatomical relationship between lesion and veins. Moreover we discuss its influence on the choice of the surgical strategy.

METHODS

Of the 33 patients treated at our Institute for pineal region tumors between 2003 and 2008 we used the neuronavigation software to depict the venous system of the pineal region in 14 patients. We focused on depiction of the basal vein of Rosenthal (BV), the internal cerebral vein (ICV) and the vein of Galen: connection patterns between the veins and the type of anatomical distortion caused by the lesion were investigated and classified.

RESULTS

Using the neuronavigation software for three-dimensional (3D) reconstruction of MRI images the ICV was clearly depicted in all patients on both sides (100%). Last segment of the BV was identified in 25 sides on a total of 28 (89.3%) and absent in 3 of the 28 sides (10.7%). Studying the distortion effect of the tumor on the galenic venous system, three directions of displacement were observed: craniocaudal, anteroposterior and lateral. Seven patients presented a cranial dislocation, 5 patients caudal dislocation and there was no craniocaudal shift in 2 patients. Considering the anteroposterior displacement: 3 subjects showed an anterior shift of the veins, 5 subjects posterior shift and no anterioposterior shift was present in 6 patients. Only 2 of the 14 patients presented lateral displacement of the veins. The principal approaches used in this series were: supracerebellar infratentorial and interhemispheric parieto-occipital. The craniocaudal displacement of the pineal veins seems to be the most important for the choice of the approach.

CONCLUSION

The galenic venous system has a central role in the surgery pineal region tumors. Our study demonstrates that the architecture of the pineal veins and their anatomical relationship with the lesion can be depicted with great accuracy by using 3D neuronavigation software in order to facilitate surgical planning and intraoperative orientation.

Image-guided, endoscopic-assisted drilling and exposure of the whole length of the internal auditory canal and its fundus with preservation of the integrity of the labyrinth using a retrosigmoid approach: a laboratory investigation

OBJECTIVE

Hearing loss after removal of vestibular schwannomas with preservation of the cochlear nerve can result from labyrinthine injury of the posterior semicircular canal and/or common crus during drilling of the posterior wall of the internal auditory meatus. Indeed, there are no anatomic landmarks that intraoperatively identify the position of the posterior semicircular canal or of the common crus. We investigated the usefulness of image guidance and endoscopy for exposure of the internal auditory canal (IAC) and its fundus without labyrinthine injury during a retrosigmoid approach.

METHODS

A retrosigmoid approach to the IAC was performed on 10 whole fresh cadaveric heads after acquiring high-resolution computed tomographic scans (120 kV; slice thickness, 1 mm; field of vision, 40 cm; matrix, 512 x 512) with permanent bone-implanted reference markers. Drilling of the posterior wall of the IAC was executed with image guidance. Its most lateral area was visualized using endoscopy.

RESULTS

Target registration error for the procedure was 0.28 to 0.82 mm (mean, 0.46 mm; standard deviation, 0.16 mm). The measured length of the IAC along its posterior wall was 9.7 +/- 1.6 mm. The angle of drilling (angle between the direction of drill and the posterior petrous surface) was 43.3 +/- 6.0 degrees, and the length of the posterior wall of the IAC drilled without violating the integrity of the labyrinth was 7.2 +/- 0.9 mm. The surgical maneuvers in the remaining part of the IAC, including the fundus, were performed using an angled endoscope.

CONCLUSION

Frameless navigation using high-resolution computed tomographic scans and bone-implanted reference markers can provide a "roadmap" to maximize safe surgical exposure of the IAC without violating the labyrinth and leaving a small segment of the lateral IAC unexposed. Further exposure and surgical manipulation of this segment, including the fundus without additional cerebellar retraction and labyrinthine injury, can be achieved using an endoscope. Use of image guidance and an endoscope can help in exposing the entire posterior aspect of the IAC including its fundus without violating the labyrinth through a retrosigmoid approach. This technique could improve hearing preservation in vestibular schwannoma surgery.

Application accuracy of computed tomography-based, image-guided navigation of temporal bone

OBJECTIVE

Although frameless stereotactic techniques have become indispensable in neurosurgery, their technical complexity requires careful definition and evaluation. Navigation is of particular concern when it is applied to approach a complex, tight surgical area like the temporal bone, where every millimeter is important. Theoretically, the temporal bone is an ideal location in which to use image-guided navigation because its bony construct precludes pre- and intraoperative shift. In this context, the feasibility of using a navigational system is determined by the system's accuracy and by the spatial characteristics of the targets. Literature addressing the accuracy of image guidance techniques in temporal bone surgery is relatively sparse. Accuracy of these systems within the temporal bone is still under investigation. We investigated the application accuracy of computed tomography-based, frameless, image-guided navigation to identify various bony structures in the temporal bone via a retrosigmoid approach.

METHODS

In a total of 10 operations, we performed a retrosigmoid approach simulating operative conditions on either side of 5 whole, fresh cadaveric heads. Six titanium microscrews were implanted around the planned craniotomy site as permanent bone reference markers before the surgical procedure. High-resolution computed tomographic scans were obtained (slice thickness, 0.6-mm, contiguous non-overlapping slices; gantry setting, 0 degrees; scan window diameter, 225 mm; pixel size, >0.44 x 0.44). We used a Stryker navigation system (Stryker Instruments, Kalamazoo, MI) for intraoperative navigation. External and internal targets were selected for calculation of navigation accuracy.

RESULTS

The system calculated target registration error to be 0.48 +/- 0.21 mm, and the global accuracies (navigation accuracies) were calculated using external over-the-skull and internal targets within the temporal bone. Overall navigation accuracy was 0.91 +/- 0.28 mm; for reaching internal targets within temporal bone, accuracy was 0.94 +/- 0.22 mm; and for external targets, accuracy was 0.83 +/- 0.11 mm. Ninety-five percent of targets could be reached within 1.4 mm of their actual position.

CONCLUSION

Using high-resolution computed tomography and bone-implanted reference markers, frameless navigation can be as accurate as frame-based stereotaxy in providing a guide to maximize safe surgical approaches to the temporal bone. Although error-free navigation is not possible with the submillimetric accuracy required by direct anatomic contouring of tiny structures within temporal bone, it still provides a road map to maximize safe surgical exposure.

Image-Guided Lateral Suboccipital Approach: Part 2—Impact on Complication Rates and Operation Times

Objective:

Image-guidance systems are widely available for surgical planning and intraoperative navigation. Recently, three-dimensional volumetric image rendering technology that increasingly applies in navigation systems to assist neurosurgical planning, e.g., for cranial base approaches. However, there is no systematic clinical study available that focuses on the impact of this image-guidance technology on outcome parameters in suboccipital craniotomies.

Methods:

A total of 200 patients with pathologies located in the cerebellopontine angle were reviewed, 100 of whom underwent volumetric neuronavigation and 100 of whom underwent treatment without intraoperative image guidance. This retrospective study analyzed the impact of image guidance on complication rates (venous sinus injury, venous air embolism, postoperative morbidity caused by venous air embolism) and operation times for the lateral suboccipital craniotomies performed with the patient in the semi-sitting position.

Result:

This study demonstrated a 4% incidence of injury to the transverse-sigmoid sinus complex in the image-guided group compared with a 15% incidence in the non-image-guided group. Venous air embolisms were detected in 8% of the image-guided patients and in 19% of the non-image-guided patients. These differences in terms of complication rates were significant for both venous sinus injury and venous air embolism (P < 0.05). There was no difference in postoperative morbidity secondary to venous air embolism between both groups. The mean time for craniotomy was 21 minutes in the image-guided group and 39 minutes in non-image-guided group (P = 0.036).

Conclusion:

Volumetric image guidance provides fast and reliable three-dimensional visualization of sinus anatomy in the posterior fossa, thereby significantly increasing speed and safety in lateral suboccipital approaches.

Surgical planning for retrosigmoid craniotomies improved by 3D computed tomography venography

OBJECTIVE

It is impossible to precisely anticipate the crooked course of the transverse and sigmoid sinuses and their individual relationship to superficial landmarks such as the asterion during retrosigmoid approaches. This study was designed to evaluate this anatomical relationship with the help of a surgical planning system and to analyze the impact of these in vivo findings on trepanation placement in retrosigmoid craniotomies.

METHODS

In a consecutive series of 123 patients with pathologies located in the cerebellopontine angle, 72 patients underwent surgical planning for retrosigmoid craniotomies based on 3D volumetric renderings of computed tomography venography. By opacity modulation of surfaces in 3D images the position of the asterion was assessed in relationship to the transverse-sigmoid sinus transition (TST) and compared to its intraoperative localization. We evaluated the impact of this additional information on trepanation placement.

RESULTS

The spatial relationship of the asterion and the underlying TST complex could be identified and recorded in 66 out of 72 cases. In the remaining 6 cases the sutures were ossified and not visible in the 3D CT reconstructions. The asterion was located on top of the TST in 51 cases, above the TST in 4 cases, and below the TST in 11 cases. The location of the trepanation was modified in 27 cases due to the preoperative imaging findings with major and minor modifications in 10 and 17 cases, respectively.

CONCLUSION

Volume-rendered images provide reliable 3D visualization of complex and hidden anatomical structures in the posterior fossa and thereby increase the precision in retrosigmoid approaches.

Preservation of function in vestibular schwannoma surgery

The management and surgical technique for microsurgical tumor removal of vestibular schwannomas (acoustic neuroma) with the suboccipital retrosigmoid approach and semi-sitting patient positioning is described. An emphasis is placed on the preservation of auditory and facial nerve function with a stepwise description of the technical and operative nuances, including presurgical evaluation, positioning, anesthesiological and neurophysiological aspects, approach, microsurgical techniques, and postsurgical care.

Neuronavigational endoscopic endonasal sellar and parasellar surgery using a 2-mm-diameter lens rigid-rod endoscope: a cadaver study

OBJECTIVE

Most of the endoscopes used for endonasal transsphenoidal surgery use 4-mm diameter lenses. The applicability of a newly developed neuroendoscope with a lens diameter of only 2 mm was tested in endonasal transsphenoidal pituitary surgery.

METHODS

The newly developed rigid-rod neuroendoscope with a 2-mm lens and an endoscope with a 4-mm lens were coupled with a navigation system and used for this comparative study. Comparison between the views obtained with these two devices was performed in a model and in formalin-fixed cadaver heads. A pure endonasal approach was used to reach and explore the sellar and parasellar regions. The navigation system was used to locate the same position in both lenses for image comparison.

RESULTS

The sellar and parasellar regions could be reached and explored using the new endoscope with the 2-mm lens and an oval-shaped irrigation and suction channel. The visual field appeared to be reduced compared with that of the 4-mm lens. However, this reduction was compensated by greater mobility and easier introduction and maneuvering of the instruments at the sellar level. Reduced image size and brightness were also found using the 2-mm lens compared with the 4-mm lens. These differences could be overcome by increasing the amount of light and enlarging the image but with subsequent reduction in image resolution.

CONCLUSION

The small diameter of this neuroendoscope resulted in good maneuverability and maintained a fine quality of vision. Children and patients with small nostrils are good candidates for the use of such a device.

Endoscopy in neuro-otologic surgery

Endoscopy offers several distinct advantages over the operating microscope during neuro-otologic surgery that make it an excellent adjunctive tool to the microscope or independent modality during cranial base surgery. The high magnification gives excellent definition of perforating blood vessels, cranial nerves, and neural structures, which in many cases is superior to that achieved with the microscope. Furthermore, the use of angled or flexible endoscopes allows one to look around corners and behind anatomic structures blocking the view seen via a 0 degree microscope. Endoscopy also has the theoretical advantage that a less invasive operative procedure is required, which should reduce the operative morbidity. Several notable disadvantages of endoscopy include the problems associated with blood soiling the endoscope, making visualization difficult or impossible, the lack of readily available instrumentation designed specifically for endoscopic neuro-otology, and the poor overview of the operative field. This last point is an important one because the endoscope is placed adjacent to the lesion and does not allow one to look backward to prevent [figure: see text] injury to structures next to the shaft of the telescope. Furthermore, the surgeon must be cognizant of potential thermal injury to structures caused by the heat generated by the light source. The present endoscopic technology limits the image that the surgeon sees to two dimensions, which results in certain unique problems when operating in a three-dimensional milieu. Because of this, there is a steep learning curve to acquire endoscopic dexterity and three-dimensional orientation. Finally, bimanual operation requires the use of an articulated endoscope holder or the commitment of the co-surgeon to hold the endoscope. One of the limitations of the operative microscope is that the angle of view is determined by the distance of the lens to the skull, retractor, or obstructing tissue, which is a function of the lens focal length; the longer the focal length, the narrower the viewing angle. During most microsurgical procedures, the focal distance varies between 200 and 400 mm. Using a previous analogy, if one looks through a door's keyhole at close range, nearly the entire room on the opposite side of the door can be seen, although nothing can be seen when the hole is viewed from a long distance. This is similar to what happens when using the endoscope with focal lengths ranging from 5 to 20 mm: a wider angle of view can be achieved. Based on their, experience the authors believe that endoscopes can be used safely during neuro-otologic surgery. As an adjunct to or substitution for the operative microscope, this modality does improve visualization of bony, neural, and vascular structures while minimizing cerebellar retraction.