The comparisons of optic and electromagnetic navigation system using

2.2.1 Patients

Thirty patients (24 men and 6 women) with chronic paranasal pansinusitis were enrolled in this study. The severity of the disease was classified according to the

Lund-Mackay CT scan classification systems of chronic rhinosinusitis. All patients were scored as Grade III-IV nasal polyposis according to the endoscopic grade system

proposed by Meltzer et al. ⁽²⁷⁾. The total score for each case was greater than 15 in the Lund-Mackay CT scan classification system. Exclusion criteria were the same as section 2.1. Preoperative sinus CT was done and the image data were then transferred to the navigation unit using a compact disc. The same medical team performed bilateral endoscopic sinus surgery using the Medtronic S7 navigation system on all patients. The S7 system has both optic and electromagnetic tracking devices in one machine to facilitate the operation. We used the optic navigation system when performing one side endoscopic sinus surgery and the electromagnetic navigation system to assist the other side endoscopic sinus surgery in a random allocation through the use of a random number table. The test statistics measured intraoperatively in this study indicate the times taken for surface registration and the surgical procedure, as well as the navigation errors (NEs) along the 3 axes.

2.2.2 Equipment set-up

After the induction of general anesthesia, a head reference frame was attached to the patient’s forehead using an elastic strap. The head frame was equipped with a headset patient tracker to provide a continuous point of reference for the navigation system. The head frame was placed at the center of the forehead and manipulated gently to ensure that it was attached securely, and that its position relative to the head would be

maintained during the registration process and subsequent procedure.

The localizer (i.e. the optic system camera or the electromagnetic field emitter) was set up before the surgery on each side. For optical tracking, the system camera

triangulates the spatial coordinates of the optical markers on the tracked devices. The position the camera head is approximately 1.75 m from the patient reference, and the camera must have a clear line of sight to the optical markers (Figure 3). For

electromagnetic tracking, a low-energy electromagnetic field emitter was fixed on a holder and attached to the operating table bed frame. The emitter holder was positioned at least 20 cm above the operating table and pointed directly at the patient’s nose from a distance of 20 cm. No part of the holder was extended into the emitter’s electromagnetic field (Figure 3). All of the instrument tracker cables for intraoperative electromagnetic navigation were connected to the navigation unit ports.

2.2.3 Surface registration

A surface registration technique was used in both the optic and electromagnetic systems. Synergy Cranial Software was employed for the registration and measurement in both systems. The points that were used for the surface matching computer algorithm were distributed along various locations at the nose, forehead and orbital rim. After collecting a sufficient number of points on the scanned area for computer calculation, the three-dimensional location information was automatically matched with the CT image. After completing the registration procedure, the correlation between the position of the instrument in the surgical field and the corresponding location on the CT images was established to allow real-time orientation during surgery. No additional registration processes were needed during the operation to adjust for anatomical drift. The time required for surface registration in both systems was recorded.

2.2.4 Navigation error

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(a) (b)

Figure 3. The set-up of (a) the optic navigation system (b) the electromagnetic navigation system in endoscopic sinus surgery.

During the operation, a best effort was made to avoid moving or tilting the

patient’s head or the operating table. The bulb press testing was sometimes performed to check the integrity of the lamina papyracea. However, the head reference frame must remain attached and stable until navigation is complete to ensure an accurate

navigational reading. We cleared the abnormal sinonasal tissue on both sides to achieve adequate exposure of the skull base, lamina papyracea and sphenoid sinus. A

navigational probe was then used to determine the NE. The probe was used to touch a predetermined anatomic landmark in the patient. The NE was measured as the vertical distance from the navigated tip of the probe to an imaginary plane passing the

predetermined CT landmark and perpendicular to the measuring axis (Figure 4). The NE in the cranial-caudal (CC) direction was measured on the sagittal plane using the central points of the anterior and posterior ethmoid roofs (AE, PE, respectively) as the

predetermined CT landmarks, whereas the NE in the medial-lateral (ML) direction was measured on the coronal plane with the most medial point of the lamina papyracea (LP) and the insertion point of the superior turbinate (ST) as the CT landmarks. The NE in the anterior-posterior (AP) direction was measured on the axial plane of the CT scan relative to the opening of the sphenoid sinus (OS) reference points and the upper border of choana (UC). For all patients, the distance measurements were performed 3 times without repeating the registration process. The mean of these values was considered the NE for specific locations. All the NE results were expressed as absolute values for the purpose of statistical analysis.

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Figure 4. The measurements of the navigation errors at the central points of the anterior and posterior ethmoid roofs (AE, PE, respectively), the most medial point of the lamina papyracea (LP), the insertion point of the superior turbinate (ST), the opening of the sphenoid sinus (OS) and the upper border of choana (UC). Dotted lines indicate imaginary lines passing the predetermined CT landmarks and perpendicular to the measuring axes.