Decompression operation of the optic canal via the nasal path under endoscope has been widely performed in the recent years (Kountakis et al., 2000; Onofrey et al., 2007; Pletcher and Metson, 2007); however, optimizing its effects remains a challenge (Chen et al., 2006; Pletcher et al., 2006). Unsatisfactory results, besides the choosing of an unsuitable indication, were largely associated with otolaryngologists' limited understanding of the real anatomical situation of the optic canal before operation. Multislice helical CT scanning and reconstructive technique has proved to be a new and noninvasive technique in the detection of diseases. By choosing the different reconstructive technique, multislice helical CT could clearly display the three-dimensional image of the normal anatomic structures, the location, size, surface morphology, and extent of the lesion. To provide otolaryngologists with the real anatomical situation and data preoperation, we carried out a comparative study of the optic canal as measured by multislice helical CT and anatomical methods.
Decompression operation of the optic canal via the nasal path under endoscope is widely used, but it is both a challenging and controversial method. Unsatisfactory results were largely associated with otolaryngologists' limited understanding of the real anatomical situations of the optic canal before operation. To provide otolaryngologists with the real situations and data preoperation, multislice helical CT was used to reconstruct the images of the optic canal. Using multislice helical CT-aided three-dimensional reconstructive methods in combination with direct anatomic measurement, we dissected and analyzed the shape of the optic canal and its anatomic relationship with the adjoining structures in 40 intact postmortem skull samples. The In-Space technique clearly showed the structure and the related region of the optic canal. The virtual endoscopy technique showed superbly the spatial appearance and topography of the inner optic canal and also gave the inner structure of the optic canal optically. There was no statistic difference in three-dimensional reconstructive data with that obtained by anatomical measurements and thus can be used to directly instruct the clinic operation. These results demonstrate that a combined In-Space technique with virtual endoscopy can accurately define the subtle structure and the related region of the optical canal. In conclusion, multislice helical CT-based three-dimensional reconstruction is of important value for clinical operations. Anat Rec, 2008. © 2008 Wiley-Liss, Inc.
MATERIALS AND METHODS
Forty adult postmortem skulls, 24 men and 16 women, were fixed in 10% formalin. All specimens were intact in structure, without pathological findings such as tumors, myxoma, or apparent inflammation.
A sliding caliper (size 0–125 mm and precision 0.02 mm) was purchased from Shanghai Measurement and Tooting Factory (Shanghai, China). Compasses were purchased from Shanghai Pufa Pantography General Factory (Shanghai, China).
The postmortem skulls were scanned with a helical CT (America General Electric Company); a 16-row helical CT with 120 kV of tube tension, 300 mA of current flow, and interval coronal scanning thickness 1.0 mm, distance 1.0 mm, window width 1,000, and window level 200. The scan line met the line linking the tuberculum sellae and the infraorbital border at a right angle. The scanning area started from the anterior nasal spine through to the posterior clinoid process. Three-dimensional reconstruction was performed using surface shaded display (SSD), In Space, and virtual endoscopy (VE), with a thickness of 0.75 mm and a distance of 0.1 mm.
After scanning, each skull specimen was first sawn along the line linking the superior border of the arcus superciliaris and the point 1 cm over the occipital tuberosity, and then sawn along the midline
Observation and Measurement
The imageological and real anatomical observations by rhinological endoscope were performed over the inner wall adjoining relationship of the optic canal and the sphenoidal sinus. The thickness and length of the bulges inside the optic canal were measured.
All data were analyzed using SPSS11.5 software. Measurement data were expressed as mean ± SD ( ± S). A t test was used for statistical analysis among group data. P < 0.05 was considered statistically significant.
Reconstruction by SSD and In Space
The optic canal was observed from different angles via SSD reconstruction; however, its inner wall and peripheral structures were unable to be visualized well (Figs. 1–6). In contrast, the general spatial resolution of In Space reconstruction was high, with clear visualization of the inner wall and peripheral structures of the optic canal as required. Thus, the data measured were accurate and reliable.
Reconstruction by VE
The reconstructed images by VE were in accordance with the real anatomic form of the optic canal, with global stereopsis, from which the inner wall structures were clearly displayed (Figs. 7–10, and the internal sclerotic succession could be dynamically monitored (Figs. 10–12). In our studied cases, a congenital defect of the inner wall of the optic canal was discovered with the image reconstruction by VE (Fig. 11)
Combination of the Optic Canal Inner Wall and the Sphenoidal Sinus
Based on the adjoining relationship between the optic canal inner wall and the sphenoidal sinus, three types were classified: (1) ethmoidal type: the whole optic canal adjoined with posterior ethmoidal sinus; (2) ethmoidal-sphenoidal type: the whole optic canal adjoined with posterior ethmoidal and sphenoidal sinuses; and (3) sphenoidal type: the whole optic canal adjoined with sphenoidal sinus. The developmental types of sphenoidal sinus were based on the criteria by Fan et al. (1997). The reconstruction results by In Space were in exact agreement with that observed by real autopsy. The combination of the optic canal inner wall and sphenoidal sinus are summarized in Table 1.
|Conchaltype||Antero-sellar type||Semisellar||Pensellar type||Sellar-occipital|
The Combination of the Bulges Forms of the Optic Canal and Their Adjoining Structures
Based on the classification criteria by Shi et al. (1997), the bulge forms were classified into four types: canal type: more than 50% of the circumference of the optic canal intrudes into the sinuses; semicanal type: less than 50% of the circumference of the optic canal intrudes into the sinuses; impression type: optic canal intrudes into the sinuses only a little; and no impression type: optic canal does not intrude into the sinuses at all and separate from the latter with a thick bone plate. Reconstruction results by In Space were in exact agreement with that observed by real autopsy. The combination of the bulges forms of the optic canal and their adjoining structures are summarized in Table 2.
|Ethmoid type||Ethmoid-sphenoid type||Sphenoid type|
|No impression type||8||3||10|
Thickness and Length of Various Bulges Forms of the Optic Canal Inner Wall
In-space technique is a reliable method for measuring the spatial distance. In this study, we used the In-Space technique to reconstruct the optic canals and measured the thickness and length of various bulge forms of the optic canal inner wall. The measured data were compared with that measured by real autopsy. There were no significant differences in these measured data between the two methods, indicating that the reconstruction by In Space was in accord with that by anatomical measurement (Tables 3 and 4).
|Anatomic measurement||CT measurement|
|Canal type||0.45 ± 0.23||0.43 ± 0.23|
|Semicanal type||0.53 ± 0.25||0.51 ± 0.27|
|Impression type||0.71 ± 0.26||0.70 ± 0.30|
|No impression type||0.91 ± 0.31||0.89 ± 0.31|
|Anatomic measurement||CT measurement|
|Ethmoid type||9.30 ± 0.30||9.25 ± 0.23|
|Ethmoid-sphenoid type||9.50 ± 0.40||9.40 ± 0.34|
|Sphenoid type||9.0 ± 0.45||8.95 ± 0.39|
Application of In-Space Three-Dimensional Reconstruction in the Study of the Optic Canal
In-space three-dimensional reconstruction is a powerful technique, which can simultaneously display on-screen images reconstructed by other three three-dimensional methods (Cheng et al., 2005). In this study, the optic canal model reconstructed by the In-Space technique accurately reflected the real measurement and also the real spatial resolution and had the capability to indicate the adjoining structures very well. In contrast, the SSD technique had poor resolution when used for image reconstruction. Thus, the In-Space technique can provide otolaryngologists with detailed measured data related to the operation of the optic canal and is of important value in the clinical setting.
Application of the VE Technique in the Study of the Optic Canal
The VE technique is different from that using the real endoscope, and the observed area can also be moved to the outside canal (Cheng et al., 2005). In this study, VE was used to observe the inner wall of the optic canal and its adjoining structures with satisfactory results. In particular, we found a congenital defect of the inner wall of the optic canal, suggesting that the VE technique could be used to judge bone fracture based on the sclerotin succession observed in the inner wall of the optic canal, thus avoiding potential misdiagnosis owing to the irregular inner wall. This would be of great practical clinical value.
Clinical Significance of the Inner Wall of the Optic Canal and Its Adjoining Structures in the Decompressive Operation
The relationship between the optic canal and its adjoining ethmoidal/sphenoidal sinus is very complex (Zhang et al., 2006). In this study, we demonstrated that in the case where the whole optic canal adjoined with the ethmoidal sinus, the bulges in the sinus become apparent. In this situation, an operation would be less problematic, as for the situation where the whole optic canal adjoined with the sphenoidal sinus, and the latter developed well. However, in the case where the whole optic canal adjoined with the ethmoidal and sphenoidal sinuses, and the former's inner wall was thick, an operation would be considerably difficult. In this case, the decompressive operation of optic canal needs to be performed safely with the aid of an endoscope and an electrodril. Prior to a decompressive operation of the optic canal, it is necessary to perform an imageological examination to clarify possible variations of the ethmoidal and sphenoidal sinuses and their association with the inner wall of optic canal. In this study, we demonstrated that the three-dimensional reconstruction with combined use of In Space and VE techniques can provide otolaryngologists with precise information preoperation.