Image-guided Localization of the Internal Auditory Canal via the Middle Cranial Fossa Approach

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.

Future of Endoscopic Ear Surgery

Endoscopic ear surgery has gained popularity in recent years, becoming standard practice in otology centers around the world as an adjunct to conventional microscopic surgery and as a sole tool for limited disease. During the last years, technical improvements and growing expertise in the handling of the endoscope allowed introducing an exclusive endoscopic approach to the middle ear, lateral skull base, middle cranial fossa, and posterior fossa/cerebellopontine angle pathologies. Endoscopic instrumentation, techniques, and knowledge have improved during the last few years, and in the future, endoscopic surgical techniques will gain even more importance in otologic surgery.

Prospects and limitations of different registration modalities in electromagnetic ENT navigation

The present study examined electromagnetic tracking technology for ENT navigation. Five different registration modalities were compared and navigation accuracy was assessed. Four skull models were individually fabricated with a three-dimensional printer, based on patients' computer tomography datasets. Individual silicone masks were fitted for skin and soft tissue simulation. Five registration modalities were examined: (1) invasive marker, (2) automatic, (3) surface matching (AccuMatch), (4) anatomic landmarks, and (5) oral splint registration. Overall navigation accuracy and accuracy on selected anatomic locations were assessed by targeting 26 titanium screws previously placed over the skull. Overall navigation accuracy differed significantly between all registration modalities. The target registration error was 0.94 ± 0.06 mm (quadratic mean ± standard deviation) for the invasive marker registration, 1.41 ± 0.04 mm for the automatic registration, 1.59 ± 0.14 mm for the surface matching registration, and 5.15 ± 0.66 mm (four landmarks) and 4.37 ± 0.73 mm (five landmarks) for the anatomic landmark registration. Oral splint registration proved itself to be inapplicable to this navigation system. Invasive marker registration was superior on most selected anatomic locations. However, on the ethmoid and sphenoid sinus the automatic registration process revealed significantly lower target registration error values. Only automatic and surface registration met the accuracy requirements for noninvasive registration. Particularly, the automatic image-to-world registration reaches target registration error values on the anterior skull base which are comparable with the gold standard of invasive screw registration.

Use of bone anchoring device in electromagnetic computer-assisted navigation in lateral skull base surgery

CONCLUSION

The use of the bone anchoring device associated with a fiducial marker, both fixed close to the operating field, improves the reproducibility and effectiveness of the computer-assisted navigation in lateral skull base surgery.

OBJECTIVES

Computer-assisted navigation in lateral skull base surgery using the electromagnetic system Digipointeur(®) needs an external fiducial marker (titanium screw) close to the operating field to increase position accuracy (PA) to about 1 mm. Displacement of the emitter placed in the mouth (Buccostat(®)) induces a drift of the system, leading to at least 20% of unsuccessful procedures. The aim of this study was to evaluate the PA, stability, and reproducibility of computer-assisted navigation in lateral skull base surgery using a bone anchoring device to provide a fixed registration system near the operating field.

METHODS

Forty patients undergoing a lateral skull base procedure with the Digipointeur(®) system performed with both the titanium screw and bone anchoring device were included in this prospective study. They were divided in two groups. In the first one (n = 9), the PA was measured before and after screw registration for five intratemporal landmarks, during a translabyrinthine approach. In the second group (n = 31), all lateral skull base procedures were included and the PA was evaluated visually by the surgeon on different landmarks of the approaches as well as the stability of the system.

RESULTS

In the first group, the PA was 7.08 ± 0.59 mm and 0.77 ± 0.17 mm (mean ± SEM, p < 0.0001) before and after screw registration, respectively. In the second group, the PA was considered as accurate by the surgeon in all cases and no drift of the system was observed. Computer-assisted surgery was never abandoned due to increased stability of the bone-anchored emitter.

Validation of exposure visualization and audible distance emission for navigated temporal bone drilling in phantoms

Background

A neuronavigation interface with extended function as compared with current systems was developed to aid during temporal bone surgery. The interface, named EVADE, updates the prior anatomical image and visualizes the bone drilling process virtually in real-time without need for intra-operative imaging. Furthermore, EVADE continuously calculates the distance from the drill tip to segmented temporal bone critical structures (e.g. the sigmoid sinus and facial nerve) and produces audiovisual warnings if the surgeon drills in too close vicinity. The aim of this study was to evaluate the accuracy and surgical utility of EVADE in physical phantoms.

Methodology/Principal Findings

We performed 228 measurements assessing the position accuracy of tracking a navigated drill in the operating theatre. A mean target registration error of 1.33±0.61 mm with a maximum error of 3.04 mm was found. Five neurosurgeons each drilled two temporal bone phantoms, once using EVADE, and once using a standard neuronavigation interface. While using standard neuronavigation the surgeons damaged three modeled temporal bone critical structures. No structure was hit by surgeons utilizing EVADE. Surgeons felt better orientated and thought they had improved tumor exposure with EVADE. Furthermore, we compared the distances between surface meshes of the virtual drill cavities created by EVADE to actual drill cavities: average maximum errors of 2.54±0.49 mm and −2.70±0.48 mm were found.

Conclusions/Significance

These results demonstrate that EVADE gives accurate feedback which reduces risks of harming modeled critical structures compared to a standard neuronavigation interface during temporal bone phantom drilling.

Morphometric analysis of the internal auditory canal by computed tomography imaging

Background

Many clinical and experimental studies have been done to analyze the anatomical and functional aspects of the internal auditory canal (IAC) in human beings since there are great inter-individual variability and structural variations that may occur regarding the other adjacent structures.

Objectives

The purpose of this study was to characterize the morphology of the internal auditory canal (IAC) during development using high resolution computed tomography (CT) and to analyze its dimensions, which will be determined by measuring the nearby areas and structures using a system of digital image processing.

Patients and Methods

CT images of the IAC of 110 normal subjects aged 1 to 92 years (mean age, 46.5 years) of both genders were reviewed to determine the shape, area, opening width (OW), longitudinal length (LL), vertical diameter (VD) and distance from the vestibular aqueduct.

Results

The shapes observed in children and adults were funnel-shaped (74% and 58.3%, respectively), cylindrical (22% and 30.9%, respectively) and bud-shaped (4% and 10.8%, respectively). The measurements by CT in children were: area= 50.30 mm², OW = 7.53 mm, length = 11.17 mm, VD = 4.82 mm and the distance between the IAC and the vestibular aqueduct (VA) = 12.63 mm. In adults, the measurements were: area = 44.64 mm², OW = 7.10 mm, length = 9.84 mm, VD = 4.47 mm and the distance between IAC and VA = 11.17 mm.

Conclusions

CT images showed that the IAC has different shapes and when the measurements obtained for children were compared with those of adults, the parameters that presented statistically significant differences in either gender were length and diameter.

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.

Localization of the internal auditory canal by identifying the intersection of the posterior border of the trigeminal ganglion and the superior petrosal sinus in cadavers

The identification of the internal auditory canal (IAC) has relied on visualization of the arcuate eminence (AE). However, it is not uncommon that the topographic markers on the middle cranial base are featureless and difficult to identify, including the AE. "Point T", the intersection of the posterior border of the trigeminal ganglion (TG) and the superior petrosal sinus (SPS) has been presented as a marker to localize the IAC. Thirty-four sides from 17 dry skulls and five formalin-fixed latex-injected cadaver heads were studied. In the dry skull, the imaginary line of the IAC was defined by connecting the uppermost point of the rim of the external auditory canal and the uppermost point of the porus acousticus on the petrosal ridge. Point T was defined as the posterior margin of the trigeminal impression on the petrosal ridge. For cadaver heads, a standard middle fossa approach was performed, and the line of the IAC was defined by joining the tip of Bill's Bar and the midpoint of the dura on the porus acousticus. Point T was expressed as the intersection of the posterior border of the TG and the SPS. The distance between point T and the medial end of the IAC was termed "segment TI", and the angle spanning from segment TI to the IAC was "angle theta (theta)". In dry skulls, segment TI (mean+/-standard deviation [SD]) measured 9.74+/-0.71mm and angle theta was 135.56+/-3.21 degrees ; in cadaver heads, segment TI measured 10.25+/-0.58mm and angle theta measured 133.43+/-2.00 degrees . An alternative for localization of the IAC is proposed when the AE is difficult to identify in the middle cranial fossa. As a mnemonic, the IAC can be located by identifying point T first, and then tracing 1cm posteriorly along the SPS and turning laterally 90 degrees plus half of 90 degrees (135 degrees total).

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.

Placement of Intraventricular Catheters Using Flexible Electromagnetic Navigation and a Dynamic Reference Frame: A New Technique

BACKGROUND

Catheterization of narrow ventricles may prove difficult resulting in misplacement or inefficient trials with potential damage to brain tissue.

MATERIAL AND METHODS

The application of a new module for navigated ventricular catheterization using flexible electromagnetic navigation and a dynamic reference frame is presented.

RESULTS

Navigated catheter placement was successful and accurate in a pilot study. Electromagnetic interferences had to be taken into consideration.

CONCLUSION

Flexible electromagnetic navigation with a dynamic reference frame is a useful tool for catheter placement as it reduces the risk of misplacement or repeated catheterization trials.

Application accuracy of an electromagnetic field-based image-guided navigation system

OBJECTIVE

We tested the application accuracy of an electromagnetic field-based image guidance system to compare it to traditional optically tracked systems.

METHODS

A plastic skull phantom was fitted with fiducial markers rigidly attached via self-drilling bone screws. Volumetric CT scan was obtained to simulate the clinical condition. A metal disc marked in 1-mm increments was placed at the expected target point. Following registration and alignment of a trajectory guide, radial and depth localization errors were measured after both freehand and stabilized approaches on both the right and left sides. Statistical analyses of the localization errors were performed.

RESULTS

Total target localization error ranged from 0.71 to 3.51 mm with a mean +/- SEM of 2.13 +/- 0.11 mm. The radial error averaged 0.98 +/- 0.11 mm, depth error 1.74 +/- 0.13 mm. The freehand procedures produced a statistically greater radial, depth and total error than the fixed procedures.

CONCLUSIONS

Accuracy of image-guided localization using an electromagnetic field guidance system is similar to that reported for optically guided systems.