Navigation with the StealthStationtrade mark in Skull Base Surgery: An Otolaryngological Perspective

Augmented Reality for Retrosigmoid Craniotomy Planning

While medical imaging data have traditionally been viewed on two-dimensional (2D) displays, augmented reality (AR) allows physicians to project the medical imaging data on patient's bodies to locate important anatomy. We present a surgical AR application to plan the retrosigmoid craniotomy, a standard approach to access the posterior fossa and the internal auditory canal. As a simple and accurate alternative to surface landmarks and conventional surgical navigation systems, our AR application augments the surgeon's vision to guide the optimal location of cortical bone removal. In this work, two surgeons performed a retrosigmoid approach 14 times on eight cadaver heads. In each case, the surgeon manually aligned a computed tomography (CT)-derived virtual rendering of the sigmoid sinus on the real cadaveric heads using a see-through AR display, allowing the surgeon to plan and perform the craniotomy accordingly. Postprocedure CT scans were acquired to assess the accuracy of the retrosigmoid craniotomies with respect to their intended location relative to the dural sinuses. The two surgeons had a mean margin of davg = 0.6 ± 4.7 mm and davg = 3.7 ± 2.3 mm between the osteotomy border and the dural sinuses over all their cases, respectively, and only positive margins for 12 of the 14 cases. The intended surgical approach to the internal auditory canal was successfully achieved in all cases using the proposed method, and the relatively small and consistent margins suggest that our system has the potential to be a valuable tool to facilitate planning a variety of similar skull-base procedures.

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.

The Assessment of Distortion in Neurosurgical Image Overlay Projection

BACKGROUND

Numerous studies have demonstrated the superiority of neuronavigation during neurosurgical procedures compared to non-neuronavigation-based procedures. Limitations to neuronavigation systems include the need for the surgeons to avert their gaze from the surgical field and the cost of the systems, especially for hospitals in developing countries. Overlay projection of imaging directly onto the patient allows localization of intracranial structures. A previous study using overlay projection demonstrated the accuracy of image coregistration for a lesion in the temporal region but did not assess image distortion when projecting onto other anatomical locations. Our aim is to quantify this distortion and establish which regions of the skull would be most suitable for overlay projection.

METHODS

Using the difference in size of a square grid when projected onto an anatomically accurate model skull and a flat surface, from the same distance, we were able to calculate the degree of image distortion when projecting onto the skull from the anterior, posterior, superior, and lateral aspects. Measuring the size of a square when projected onto a flat surface from different distances allowed us to model change in lesion size when projecting a deep structure onto the skull surface.

RESULTS

Using 2 mm as the upper limit for distortion, our results show that images can be accurately projected onto the majority (81.4%) of the surface of the skull.

CONCLUSION

Our results support the use of image overlay projection in regions with ≤2 mm distortion to assist with localization of intracranial lesions at a fraction of the cost of existing methods.

Is image guidance accurate in children sinus surgery?

OBJECTIVES

To compare the precision of the calibration of a 3D navigation system for endoscopic sinus surgery in children and adults. To compare the complication and calibration failure rates of the system in both populations.

METHODS

The precision of the calibration of the Stryker navigation system (Stryker Neuronavigation ENT 2.0 Software) was found in the charts of children and adults operated on for various nasosinusal procedures between May 2008 and February 2013 in a tertiary care center. Demographic characteristics and complication rates were also noted.

RESULTS

Thirty-eight adults and 21 children were included in the study. No statistically significant difference was found between the two groups with an identical mean precision of 0.7mm (p=0.90). The rate of precision unreliability and calibration failures was not statistically different between the two groups (14% children vs 5% adults). No major complications occurred in both groups (p=1.00). No demographic characteristic predicted a failed calibration (height, weight, BMI, age).

CONCLUSION

The Stryker image-guided system can provide a precision level that is equivalent in both children and adults. This study also demonstrated an absent/low complication rate respectively for children and adults post endoscopic surgery.

Navigation-assisted surgery for bone and soft tissue tumors with bony extension

BACKGROUND

The navigation system was introduced to orthopaedic surgery in the 1990s. More recently, CT-based navigation systems have been used more commonly in spine and joint replacement surgery because of their precision.

QUESTIONS/PURPOSES

The aim of our study was to evaluate the accuracy and efficacy of navigation-assisted excision of bone and soft tissue tumors.

METHODS

From 2006 to 2009, we performed navigation-assisted surgery in 16 patients, 11 males and five females, with a mean age of 39 years (range, 13-70 years). We diagnosed nine benign bone tumors and seven malignant bone and soft tissue tumors. In two patients, the malignant soft tissue tumors infiltrated the adjacent bones. Nine excisional biopsies for benign tumors and seven en bloc excisions for malignant tumors were performed. In all cases, the point registration method was performed using 10 skin markers, which were placed around the tumor. Each excisional difference between the preoperative and postoperative plans was evaluated histologically or by postoperative CT.

RESULTS

The mean accuracy of this system, which was determined using skin markers, was 0.93 mm (range, 0.6-1.2 mm). All biopsy and excision samples were evaluated by pathologic examination and postoperative CT imaging. The mean difference between the planned margin and postoperative CT or excised histologic specimen was 0 mm to 4 mm. The mean followup was 34 months (range, 10-54 months). There were no local recurrences, except for excision of skip metastases in a patient with a chordoma.

CONCLUSION

We report our experience with navigation-assisted surgery for bone and soft tissue tumors. Navigation-assisted surgery could be indicated for sufficiently reliable, accurate, and minimally invasive resections.

CT radiation dose for computer-assisted endoscopic sinus surgery: dose survey and determination of dose-reduction limits

BACKGROUND AND PURPOSE

Computer-assisted navigation is increasingly used in functional endoscopic sinus surgery (FESS) to prevent injury to vital structures, necessitating preparative CT and, thus, radiation exposure. The purpose of our study was to investigate currently used radiation doses for CT in computer-assisted navigation in sinus surgery (CAS-CT) and to assess minimal doses required.

MATERIALS AND METHODS

A questionnaire inquiring about dose parameters used for CAS-CT was sent to 30 radiologic institutions. The feasibility of low-dose registration was tested with a phantom. The influence of CAS-CT dose on technical accuracy and on the practical performance of 5 ear, nose, and throat (ENT) surgeons was evaluated with cadaver heads.

RESULTS

The questionnaire response rate was 63%. Variation between minimal and maximal dose used for CAS-CT was 18-fold. Phantom registration was possible with doses as low as 1.1 mGy. No dose dependence on technical accuracy was found. ENT surgeons were able to identify anatomic landmarks on scans with a dose as low as 3.1 mGy.

CONCLUSIONS

The vast dose difference between institutions mirrors different attitudes toward image quality and radiation-protection issues rather than being technically founded, and many patients undergo CAS-CT at higher doses than necessary. The only limit for dose reduction in CT for computer-assisted endoscopic sinus surgery is the ENT surgeon's ability to cope with impaired image quality, whereas there is no technically justified lower dose limit. We recommend, generally, doses used for the typical diagnostic low-dose sinus CT (120 kV/20-50 mAs). When no diagnostic image quality is needed, even a reduction down to a third is possible.