Analysis of pneumatization and neurovascular structures of the sphenoid sinus using cone-beam tomography (CBT)

Abstract

Background

The sphenoid sinus is a frequent target of paranasal sinus surgery. Because of the high risk of injuring the surrounding structures (e.g. internal carotid artery, optical nerve) a preoperative imaging is absolutely necessary.

Purpose

To analyze the possibilities of cone-beam computed tomography (CBCT), which is especially quite a new technique in ENT, in the evaluation of the sphenoid sinus, its surrounding structures, and the corresponding anatomical variations.

Material and Methods

This was a retrospective, single-centre study of 580 patients (1160 sides = cases). The Accu-I-Tomo-F17 was used. Pneumatization of sphenoid sinus, course of internal artery, course of optical nerve, and dehiscence of the bony canals were evaluated.

Results

In the case of pneumatization a type I (completely missing or minimal sphenoid sinus) was found in two patients (0.3%), type II (posterior wall of sphenoid sinus is in front of the anterior wall of the sella) in 38 patients (6.6%), type III (posterior wall is between anterior and posterior wall of sella) in 332 patients (57,2%), type IVa (posterior wall is behind the posterior wall of sella without air dorsal the sella) in 104 patients (17.9%), and type IVb (similar to type IVa but with air dorsal the sella) in 104 patients (17.9%). In 1025 cases (89.5%) a smooth course of the internal carotid artery was found whereas a free course could be detected in 120 cases (10.5%). Defects of the bony canal of the optical nerve were found in 16.7% and of the internal carotid artery in 2.7% of the cases. The optical nerve showed a free course through the sphenoid in 151 cases (13.7%) and a smooth course in 1007 cases (87.0%).

Conclusion

CBCT could evaluate all relevant anatomic structures and answer the questions of different anatomical variants. A modified classification of the pneumatization of the sphenoid sinus could be described. Frequencies of anatomical variations are in accordance with the current literature of CT research.

Keywords

Cone-beam computed tomography (CBCT)pneumatization sphenoid sinus internal carotid artery optical nerve

Pathologies of the sphenoid sinus and surrounding structures are the cause of frequent endoscopic operations by ENT surgeons and/or skull-base surgeons. Knowledge about the individual anatomy is essential to prevent intra-operative complications. The different courses of the optical nerve and the internal carotid artery in particular have to be respected. A preoperative imaging of the paranasal sinuses is a standard procedure in ENT surgery. A navigation is used only in single specialized cases, so the diagnostic work-up proceeding paranasal sinus surgery routinely includes coronal and axial computed tomography (CT) scans to get an exact representation of the anatomy of the region of the lateral nasal wall and the anterior skull base (1). At present, CT is the gold standard for radiological examination of the paranasal sinuses (2).

The sphenoid sinus is located in the middle of the skull and is divided according to the embryologic origin in the orbito-sphenoid (deriving from cephalic mesoderm) and basis-post-sphenoid structures (deriving from neural crest) (3). The intersphenoidal septum, which separates the right from the left side, has large variation. This leads to extreme differences between the sizes of the right or left sphenoid sinus. Furthermore, an extreme variable pneumatization of the sphenoid sinus can be observed (4), which has a high relevance for surgeons operating on pathologies of the sphenoid sinus or the surrounding structures. These are important structures like the optical nerve, internal carotid artery, pituitary gland, and skull base (5).

Cone-beam computed tomography (CBCT or digital volume tomography [DVT]), is a relatively new imaging technique for the diagnosis of paranasal sinus diseases (6, 7). The disadvantage of CBCT in comparison to CT is the limited differentiation of the soft tissue (8). An advantage of CBCT is a lower purchase price for the equipment proportional to modern high-resolution CT (6, 9, 10). The geometric accuracy is sufficient for CBCT-based image-guided surgery (11–13). The use of CBCT in ENT is a useful tool, for example, in the diagnostics of trauma of the nasal skeleton (14), the detection of anatomical variants of the anterior skull base (15, 16), the detailed analysis of anatomical variations in healthy and pathological anatomy of the temporal bone (17–20), and the exact analysis of the cochlea and the electrodes after cochlea implantation (21, 22).

Although some anatomical and radiological reports deal with the sphenoid sinus and its surrounding structures based on cadaver or CT studies, these reports are not concerned with the possibilities of CBCT imaging of such structures. For this reason, the aim of the current study was to analyze the possibilities of the CBCT, which is, especially in ENT, a relatively new technique, in evaluation of the sphenoid sinus, its surrounding structures and the corresponding different anatomical possibilities.

Material and Methods

Between January 2009 and December 2010, 644 patients received a CBCT scan of the paranasal sinus in cause of preoperative diagnostics of chronic rhinosinusitis. Sixty-four patients had to be excluded because of incomplete detected sphenoid sinus CBCT scans. According to this, 580 patients (313 women, 267 men; mean age 39.6 ± 18.1 years) were completely analyzed.

During CBCT examination (Accu-I-tomo F17, Morita, Kyoto, Japan) the patient sits in an upright position with the head fixed on an adjustable chair. The field of view (FOV) is marked by target laser beams that can be positioned arbitrarily. An emitter-detector unit moves along the patient's head for about 18 s. During this time 588 single slices of a section width down to 0.08 mm are performed. The extension of the X-ray beam is conical: the top of the cone is the emitter of radiation, the base is the detector. The intersection plane of the course of the beam is rectangular. The cylindrical volume (10 cm in height and 10 cm in transverse diameter for the evaluation of the paranasal sinuses and the anterior skull base) is created by the rotation during examination. The tube voltage was set at 80 kV, the tube current at 8 mA. Calculating the FOV with special software (Idixel, Morita, Kyoto, Japan) allows reconstruction of the slices that can be visualized on a monitor in three orthogonal planes in a frontal, axial, and sagittal manner.

The different types of the pneumatization of the sphenoid sinus are evaluated in the sagittal view and differentiated by the position of the posterior wall of the sphenoid sinus in relation to the anterior and/or posterior wall of the sella turcica (Fig. 1). The actual described classification of the pneumatization is a modification based on the classification of Hardy et al. (23). Therefore, a conchal type = type I (complete missing or minimal sphenoid sinus), presellar type = type II (posterior wall of sphenoid sinus is in front of the anterior wall of the sella turcica), sellar type = type III (posterior wall of sphenoid sinus is between anterior and posterior wall of sella, turcica), and a postsellar type = type IV (posterior wall of sphenoid sinus is behind the posterior wall of sella turcica) are divided. The postsellar type is furthermore divided into type a = type IVa, where no air/sphenoid sinus is directly behind the sella turcica, and type b = type IV b, where it is (Fig. 1). This classification is a clinically important one, because different types lead to different risk profiles. Therefore, preoperative planning and case-specific information about complications is possible.

Fig. 1
Presentation of different type of the pneumatization of sphenoid sinus in relation to the sella turcica: (a) I = conchal type; (b) II = presellar; (c) III = sellar; (d) IVa = postsellar without air behind the sella; (e) IVb = postsellar with air behind the sella

The analyses of the bony canal of the optical nerve and the internal carotid artery are divided into right and left sides, resulting totally in 1160 cases. Anatomic variations and incidental findings of the bony canal of the internal carotid artery were analyzed in axial and coronal views. The course of the bony canal was divided in two possibilities. First, the smooth course in the posterior wall of sphenoid sinus (called ‘smooth course’) and second, a distinct protrusion (called ‘free course’) into the sphenoid sinus (Fig. 2), whereas the last one was defined by more than the half diameter of the carotid diameter prolonged in the pneumatized sphenoid sinus. The course of bony canal of the optical nerve was evaluated in the same way in axial and coronal views (Fig. 3). Furthermore, bony defects in the bony courses of the optical nerve and internal sphenoid artery were analyzed and defined by a bone gap of more than 0.3 mm.

Fig. 2
Two different possibilities of the course of the bony canal of internal carotid artery and one incidental finding: (a) Course complete in the dorsal and lateral wall of the sphenoid sinus (‘smooth course’); (b) Course of internal carotid artery with a protrusion in the pneumatized sphenoid sinus (‘free course’); (c) Incidental finding of a calcified internal carotid artery

Fig. 3
Possibilities of the bony canal of the optical nerve (white arrows): (a) Prolonged course of the optical nerve in relation to the lateral and superior wall of the sphenoid sinus (‘free course’); (b) Course direct located in the walls of sphenoid sinus (‘smooth course’)

Statistics were generated with SPSS (Version 15.0.1, SPSS Inc., IBM, Armonk, NY, USA). All facts were analyzed with the descriptive statistics and frequencies analysis. Differences between the sexes were analyzed with a t-test for independent pairs. Further analyses were done by t-test for matched pairs and bivariate correlations.

For comparison with the current literature, a PubMed literature search was done with the following keywords: anatomic variation, sphenoid sinus, internal carotid artery, optical nerve, pneumatization sphenoid sinus, pituitary gland.

Results

In 18% of the patients (106/580) a complete or partial pathology of the sphenoid sinus was found. Despite this, in all of these cases a complete evaluation of the mentioned anatomic structures was possible.

In the case of pneumatization, a type I was found in two patients (0.3%), type II in 38 patients (6.6%), type III in 332 patients (57.2%), type IVa in 104 patients (17.9%), and type IVb in 104 patients (17.9%). Typical images are viewed in Fig. 1 and the results are summarized in Table 1. There was no significant difference in the frequencies of the different types between female and male patients.

Table 1
Overview about the current literature and present results of the pneumatization of sphenoid sinus in relation to the sella turcica. The different types are presented in Fig. 1

n Modality Type I (conchal) (%) Type II (presellar) (%) Type III (sellar) (%) Type IVa (postsellar without air) (%) Type IVb (postsellar with air) (%)

Hardy et al. (1967) (23) –* –* 3 11 86^†

Muhr et al. (1981) (35) 205 Autopsy 18 82^†

Banna et al. (1983) (4) 70 Autopsy 2.8 11.4 85.7^†

Liu et al. (2002) (27) 45 CT/MRI 2 20 78^†

Batra et al. (2004) (36) 64 Autopsy 4.7 4.7 25 65^‡

Hamid et al. (2008) (26) 296 CT/MRI 2 21 54.7 22.3^‡

Present study (2010) 580 CBT 0.3 6.6 57.2 17.9 17.9

No data available

^†Refers to Types III, IVa, and IVb

^‡Refers to Types IVa and IVb

In 120 cases (10.5%) a free course of the internal carotid artery was found whereas a smooth course could be detected in 1025 cases (89.5%). In 15 cases imaging was incomplete in part of the posterior wall of the sphenoid sinus, therefore the course of the internal carotid artery could not be evaluated. Bony defects could be found in 31 cases (2.7%). In detailed analysis according to the different courses, 16 of the 31 cases were found in patients with a free course whereas 15 of the 31 cases were found in patients with a smooth course. No significant differences were found between left and right sides or female and male patients. Patients with a free course of the internal carotid artery showed significantly more free courses of the optical nerve than patients with a smooth course of the internal carotid artery (P < 0.01).

The optical nerve showed a free course through the sphenoid sinus in 151 cases (13%) and a smooth course in 1007 cases (87%). In two cases it was not possible to evaluate the course of the optical nerve because of incomplete imaging. In 187 cases (16.1%) a bony defect of the canal wall was found. In differentiation between the possible courses, 47 of the 187 cases with a bony defect were in patients with a free course of optical nerve and 140 of the 187 cases in patients with a smooth course. There was no significant difference between left or right sides, and no difference between women and men. According to the results of the course of the internal carotid artery, a significant positive correlation was found between the free course of the optical nerve and the free course of the internal carotid artery, highlighting the high risk of injury of the important structures.

The results of the different anatomical variants of the bony canal of the optical nerve and the internal carotid artery are summarized in Table 2.

Table 2
Overview about the different courses of internal carotid artery and optical nerve in present study and current literature. Typical images of the smooth or free course are presented in Figs. 2 and 3

n Modality Optical nerve Smooth (%) Prolonged (%) Internal carotid artery Smooth (%) Prolonged (%)

Bansberg et al.* (1987) (33) 80 CT 77 23 –* –*

Dessi et al. (1994) (34) 150 CT 92 8 –* –*

Basak et al. (1998) (29) 111 CT 87 13 88 12

Kazkayasi et al. (2005) (30) 267 CT 96 4 95 5

Davoodi et al. (2009) – men (31) 210 CT 62 38 52 48

Davoodi et al. (2009) – women (31) 189 CT 65 35 66 34

Present study 1180 CBT 87 13 89.5 10.5

No data available

A systematic analysis of the cases with a free course of the optical nerve and the internal carotid artery in dependence of the pneumatization of the sphenoid sinus is shown in Table 3. A free course of optical nerve could be evaluated in type I pneumatization of the sphenoid sinus in no cases (0%), in type II in 1 of 76 cases (1.3%), in type III in 66 of 664 cases (10.1%), in type IVa in 31 of 208 cases (15.4%), and in type IVb in 53 of 208 cases (25.5%). Analyzing the internal carotid artery, a free course could be found in types I and II of sphenoid sinus pneumatization in no cases, in type III in 22 of 662 cases (3.9%), in type IVa in 30 of 208 cases (14.4%), and in type IVb in 68 of 208 cases (32.7%).

Table 3
Overview about the different possibilities of the bony canal of the optical nerve and the internal carotid artery in dependence of the pneumatization of the sphenoid sinus. Right and left sides are taken together

Type of sphenoid sinus Optical nerve Smooth Prolonged Internal carotid artery Smooth Prolonged

I 4 (100%) 0 (0%) 4 (100%) 0 (0%)

II 75 (98.7%) 1 (1.3%) 65 (100%) 0 (0%)

III 597 (89.9%) 66 (10.1%) 638 (96.1%) 22 (3.9%)

IVa 176 (84.6%) 31 (15.4%) 178 (85.6%) 30 (14.4%)

IVb 155 (74.5%) 53 (25.5%) 140 (67.3%) 68 (32.7%)

Discussion

There is no doubt about the indication of a preoperative imaging before intervention in the sphenoid sinus or surrounding structures, but what type of imaging to do is the question. The answer depends on the disease and the planned therapeutic consequences. In case of chronic rhinosinusitis, today CT is the gold standard. A relatively new imaging technique used in ENT is CBCT. This relatively new technique first has to show that it allows analysis of the frontal and lateral skull base. The advantages of CBCT are excellent detailed resolution superior to most CT scans, low radiation exposure in most instances, and lower costs compared to flat panel CT scanners (8, 24). The disadvantages are in some modalities the necessity of a compliant patient who is able to sit in an upright position without any motion for about 18 seconds and the physical background because CBCT depends on X-rays and can differentiate between hard contrast objects like bone-air surfaces extremely well, but differentiation between different soft tissues is not possible in a precise manner. The use of CBCT in the diagnosis of ENT disease has been described for the past few years, for example for the postoperative evaluation of cochlea implants (22, 25), the diagnosis of nasal bone fractures (8), the anatomic variations of the temporal bone (18, 19), and the anterior skull base (15).

The sphenoid sinus is located in the middle of the skull and is divided according to its embryologic origin in the orbito-sphenoid (deriving from cephalic mesoderm) and basis-post-sphenoid devices (deriving from neural crest) (3). The intersphenoidal septum, which separates the right from the left side, has a large variation. This leads to extreme differences of the size of the right or left sphenoid sinus. A high variability of the degree of pneumatization of the sphenoid sinus could be shown more than 50 years ago. Hardy et al.* described three different types (conchal, presellar, and sellar) (23). Parallel to the development of the surgical techniques, the description of the pneumatization became more detailed. Four types (conchal, presellar, sellar, and postsellar) were described by Hamid et al. (26). From our point of view a discrete correction makes sense because only an exact classification of the anatomical structures can lead to an optimal preoperative evaluation of the individual risk of injuring the sphenoid sinus and/or surrounding structures. Therefore we propose a differentiation in the type I (conchal), type II (presellar), type III (sellar), type IVa (postsellar without air dorsal the sella turcica), and type IVb (postsellar with air dorsal the sella turcica). An overview about the distribution of the different types of the corresponding literature and the current results is given in Table 1. The frequency of type I (conchal) is, in all publications, about 2%. Interestingly, a type II (presellar) is reported in some publications with a frequency of about 10–20% and in our results with 6.6%. One reason for this might be the interpretation of the planes registered. Liu et al., who report a frequency of 20%, and Hamid et al., who report a frequency of 21%, analyzed their images only in the axial and coronal planes (26, 27). The missing sagittal plane, which shows exactly the position of the posterior wall of the sphenoid sinus in relation to the bony walls of the sella turcica, may lead to a ‘wrong to high’ frequency of the type II in this publication. The same explanation leads to the different frequencies in types III–IVb shown in Table 1. Additionally, a typical dependence of free courses of the optical nerve and internal carotid artery on the types of pneumatization could be described, to the best of our knowledge, for the first time. Patients with extended pneumatization show a high rate of free courses of internal carotid artery and optical nerve, which results in a higher risk of complications.

Because of the high variability in the pneumatization, as shown and discussed before, and variations in the anatomical course of the surrounding structures, it is very important for surgeons to know about the frequencies of anatomical variations. In particular, to avoid complication during sphenoid sinus surgery, the optical nerve and the internal carotid artery have to be respected. In the case of the internal carotid artery, typically two possibilities can be differentiated. First, a smooth course in the lateral and posterior wall of the sphenoid sinus and, second, a relevant bulging of the course (‘free course’) into the pneumatized sphenoid sinus (Fig. 2). Furthermore, complete free courses without any bony coverage are described (28). Free course of the internal carotid artery is in the literature described with a frequency of 5–48% (29–31) (Table 2). Our current results with a frequency of 10.5% of a free course are in good accordance with the literature. Additionally, a missing bony coverage is a high risk for strong bleeding. Thomas et al. described a case with a completely uncovered internal carotid artery (28). Unal et al. found dehiscences in 5% and suspicious thin bony walls in 7% in their CT morphologic study (32). These results are comparable to our finding of 2.7% of dehiscences of the bony canal of the internal carotid artery (definition by defect more than 1 mm).

The same aspects have to be respected in the case of the course of the optical nerve. Prolonged courses or bony dehiscences can lead to serious complications. A prolonged course is, in the literature, found in about 30% of patients, which does not differ much with the 13% presented in this study (29–31, 33, 34) (Table 2). Regarding the bony dehiscences, 16.1% in the present study are equal to the results of Unal et al. who described a dehiscence in 8% of their cases (32). The differences might result from a different patient group with regards to number and ethnicity of patients. Furthermore, different frequencies of the degree of pneumatization of the study population cannot be excluded as a reason.

As expected, there was a correlation between the incidence of free courses of the internal carotid artery or optical nerve and the degree of pneumatization. The greater the sphenoid sinus the more often a free course of the internal carotid artery and optical nerve was detected. Consequently, a strong correlation between the different courses of internal carotid artery and optical nerve was found.

In conclusion, a CBCT examination of the sphenoid sinus and its surrounding structures is very helpful in the pre-operative workup of patients with diseases in preoperative/ pretherapeutic planning. In nearly all patients it was possible to analyze the pneumatization of the sphenoid sinus and to detect the individual courses of internal carotid artery and optical nerve. This special knowledge of the individual anatomy leads to an individual classification of a ‘dangerous preoperative status’ and is essential for preventing complications like injury of optical nerve, heavy bleeding, or injury of the pituitary gland.

Conflict of interest: None.

	n	Modality	Type I (conchal) (%)	Type II (presellar) (%)	Type III (sellar) (%)	Type IVa (postsellar without air) (%)	Type IVb (postsellar with air) (%)
Hardy et al. (1967) (23)	–*	–*	3	11		86^†
Muhr et al. (1981) (35)	205	Autopsy	18			82^†
Banna et al. (1983) (4)	70	Autopsy	2.8	11.4		85.7^†
Liu et al. (2002) (27)	45	CT/MRI	2	20		78^†
Batra et al. (2004) (36)	64	Autopsy	4.7	4.7	25	65^‡
Hamid et al. (2008) (26)	296	CT/MRI	2	21	54.7	22.3^‡
Present study (2010)	580	CBT	0.3	6.6	57.2	17.9	17.9

	n	Modality	Optical nerve Smooth (%)	Prolonged (%)	Internal carotid artery Smooth (%)	Prolonged (%)
Bansberg et al. (1987) (33)	80	CT	77	23	–*	–*
Dessi et al. (1994) (34)	150	CT	92	8	–*	–*
Basak et al. (1998) (29)	111	CT	87	13	88	12
Kazkayasi et al. (2005) (30)	267	CT	96	4	95	5
Davoodi et al. (2009) – men (31)	210	CT	62	38	52	48
Davoodi et al. (2009) – women (31)	189	CT	65	35	66	34
Present study	1180	CBT	87	13	89.5	10.5

Type of sphenoid sinus	Optical nerve Smooth	Prolonged	Internal carotid artery Smooth	Prolonged
I	4 (100%)	0 (0%)	4 (100%)	0 (0%)
II	75 (98.7%)	1 (1.3%)	65 (100%)	0 (0%)
III	597 (89.9%)	66 (10.1%)	638 (96.1%)	22 (3.9%)
IVa	176 (84.6%)	31 (15.4%)	178 (85.6%)	30 (14.4%)
IVb	155 (74.5%)	53 (25.5%)	140 (67.3%)	68 (32.7%)

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