Anatomical Analysis of Sphenoid Sinus and its Surrounding Structures in Cone-beam Computed Tomography Images: A Retrospective Cross-sectional Study

Abstract

Background

Anatomical analysis of the sphenoid sinus is important because of its proximity to vital neurovascular structures. The aim of this study is investigating the anatomy of the SS and its surrounding structures in CBCT images.

Methodology:

In this retrospective cross-sectional study, 201 cone-beam computed tomography (CBCT) radiographs were analyzed. The analyzed images included the type of SS, protrusion of ICA and optic nerve and foramen rotundum, vidian canal type, the Onodi cell, pneumatization to surrounding bones, the ostium, the septum. The data were analyzed using chi-square and the t-test in SPSS 24 at a significance level of 0.05.

Results

The tilted septum had the highest frequency (46.3%). Ostium was bilateral in most images (56.2%). The protrusion of the ICA (54.7%) and optic nerve (62.2%) was smooth in most images. Foramen rotundum protrusion and Onodi cell prevalence were 25% and 22.9%, respectively. The postsellar type of sinus had the highest frequency (71.1%). Vidian canal Grade II had the highest frequency (40.3%). Greater wing pneumatization had the highest frequency (51.7%). No statistically significant relationship was found between the optic nerve protrusion and Onodi cell (P-value < 0.05). There was a significant relationship between the pneumatization of the pterygoid process and the protrusion of the vidian canal, and the foramen rotundum protrusion (p-value < 0.001).

Conclusion

CBCT imaging is very effective in the anatomical analysis of the SS and its surrounding structures. The postsellar type had the highest frequency.Vidian canal Grade II had the highest frequency; greater wing pneumatization was observed in more than half of the images; the most common type of septum was tilted; ostium was bilateral in most cases; the protrusion of the ICA and optic nerve was mostly smooth.

Background:

Due to locating deep at the apex of the nasal cavity, sphenoid sinus (SS) surgery is risky and endangers the surrounding structures. The sinuses may become highly pneumatized and cover the internal carotid artery (ICA), the optic nerve, and the vidian canal. Therefore, these structures are in danger. Safe access to sella is affected by the SS pneumatization pattern. Normal anatomical differences among patients may predispose the sinus to sinusitis (1, 2). In the skull base surgery of SS, there is a common route for access to the sellar, parasellar, suprasellar, and clival regions. In a well-pneumatized SS, only a thin layer of bone may separate the sinus from its surrounding vital structures (3). The pattern of sinus pneumatization is assessed according to the position of the posterior sinus wall relative to sella turcica. Sinus overextension increases the risk of iatrogenic lesions and damage to vital structures such as the ICA (4).

The first step in transsphenoidal surgery or microscopic surgery is to locate the SS ostium, which can be traced through the anatomical landmarks around the sinus (e.g., the superior turbinate and the posterior nasal spine). The ostium allows easy access to the sinus during surgery (5). The SS septum is an important landmark during surgery to find important structures such as the ICA, optic nerve, basal scale, and midline (6, 7). The septum can attach to the bone that protects the ICA, wherein the fracture of the septum to gain access to the ICA can damage this artery (8). The Onodi cell, or esphenoethmoidal cell, is the posteriormost ethmoidal air cell. This cell is pneumatized superolaterally into the SS and surrounds the optic canal. In the case of detection of Onodi cell that extends up to 1.5 cm of the sphenoid surface, the surgical method is changed. Onodi cell expansion can cause morphological changes in the lateral wall and floor of the SS. In addition, the Onodi cell is poorly drained, leading to stasis of secretions, infection, and optic neuropathy. During endoscopy, the Onodi cell may be mistaken for the SS. Damage to the optic nerve or carotid artery can be prevented by diagnosing Onodi cells before surgery (9). The SS and surrounding structures can be observed and analyzed more commonly in MDCT images. However, cone-beam computed tomography (CBCT images), which have a higher resolution and a much lower dose than CT, is more commonly used in dentistry. Conventional dental imaging, such as panoramic and lateral cephalometry, has some limitations. For example, they provide a two-dimensional image, and the superimposition of structures does not allow accurate identification of the location of the SS.

In comparison, CBCT or CT shows a three-dimensional view of the paranasal sinuses. CBCT is an efficient method for evaluating paranasal sinuses (10). The advantages of this method over CT include lower cost, shorter imaging time, lower patient dose, and less space (11).

Objectives: This study aimed to investigate the anatomy of the SS and its surrounding structures in CBCT images.

Materials And Methods:

This retrospective cross-sectional study was conducted by analyzing 201 CBCT images of patients who visited Hamedan School of Dental Medicine for diagnosis and treatment. The CBCT images of the patients were prepared by NewTom VG (NEWTOM, VEROMAN, ITALY) FOV 12 INCH, 110 KVP conditions, and mAs variable. The images were taken based on the size and age of the patients and imported to the NNT Viewer software (NewTom, Verona, Italy). Two observers, consisting of two maxillofacial radiologists, viewed the images. These observers re-examined35 images two weeks later to measure inter-observer agreement.The inclusion criterion was an age of over 18 years. This criterion was selected because, according to Gray, although the nasal cavity extends into the body of the sphenoid bone before birth, it is likely to reach its full extension only after adulthood (2, 8).

Exclusion criteria were trauma to the areas under study, history of SS surgery, sinonasal tumors, nasal polyps, sinusitis, and evidence of sinus pathology such as mucosal thickening (2, 10).

The examined parameters included:

Position of the vidian canal relative to the SS:

Grade I: Completely surrounded by bone.

Grade II: \(\frac{1}{3}\) around the vidian canal is surrounded by air.

Grade III: \(\frac{1}{3}\)to \(\frac{2}{3}\) around the vidian canal is surrounded by air.

Grade IV: Almost completely surrounded by air (12) (Fig. 1).

Types of sinuses based on the position of the posterior wall relative to sella turcica:

1. Conchal type: No or minimal expansion of the sinus.

2. Presellar type: The posterior wall of the SS is located in front of the anterior wall of sella turcica.

3. Cellular type: The posterior wall of the sinus is located between the posterior and anterior walls of the sella turcica.

4. Postsellar type: The posterior wall of the sinus is located behind the posterior wall of sella turcica (1, 13) (Fig. 2).

Prevalence of protrusion of the ICA and the optic nerve is divided into two categories: prevalence of foramen rotundum protrusion and into the SS:

1. Smooth type: There is no invagination of the ICA and optic nerve canals into the SS.

2. Prolonged type: There is an invagination of the ICA and optic nerve canals into the SS (Fig. 3).

Onodi cells, the posteriormost air cells of the ethmoid sinus with the upper lateral extension to the SS, were prevalent. There is a close relationship between Onodi cells and the optic nerve (13) (Fig. 4).

Types of septa: 1) C-type, 2) S-type, 3) Tilt or T, 4) No septum, and 5) No deviation (Fig. 5)

Types of ostium: 1) bilateral, 2) left, 3) right, and 4) without ostium (14) (Fig. 6).

Pneumatization to the surrounding bones: 1) Greater wing and lesser wing of the sphenoid, 2) Anterior and posterior clinoid processes, 3) clivus, 4) Pterygoid processes, and 5) Nasal septum (Fig. 7).

Statistical analyses:

The Chi-square test and Fisher’s exact test were performed to analyze the data. The interclass correlation coefficient (ICC) and kappa statistic were used to determine the inter-observer correlation. All calculations were performed in SPSS 24. The significance level of the test was considered 0.05. The data were analyzed using Chi-square and T-test.

Results:

The inter-observer agreement was measured after viewing 35 radiographic images using the kappa coefficient and was evaluated as excellent (90%). The first observer’s agreement with herself was also evaluated as excellent (93%).

This retrospective cross-sectional study was performed using 201 CBCT images taken from 2012 to 2020.

The participants’ age ranged from 18 to 65 years with a mean age of 28.8 ± 9.28 years. Among the patients, 83 (41.3%) were male and 118 (58.7%) were female. The SS and its surrounding structures were examined from three axial, coronal, and sagittal planes.

Table 1 reports the frequency of septal deviation and the type of ostium in the SS. The most common type of SS septum was tilted (46.3%). The ostium was bilateral in most images (56.2%). The protrusion of the ICA was mostly smooth (54.7%). Also, the optic nerve protrusion was mostly smooth (64.2%). Foramen rotundum was protruded in 25% of the cases.

Table 2 reports the frequency of SS types according to the position of the posterior wall relative to sella turcica and vidian canal types and the frequency of SS pneumatization to the surrounding bones, and the frequency of Onodi cells. The highest frequency of SS in the posterior position relative to sella turcica belonged to the postsellar type (1.71%). The lowest frequency was related to the conchal type (1%) in both males and females. Examining the vidian canal in the coronal view demonstrated that the highest frequency was related to Grade II (40.3%), whereas the lowest frequency was related to Grade I (14.9%).

The most pneumatization occurred in the greater wing of the sphenoid (51.7%), while the least pneumatization occurred in its lesser wing (1%).

Table 3 shows the relationship between optic nerve protrusion and Onodi cells. According to the Chi-square test results, there is no significant relationship between the two variables of optic nerve protrusion and Onodi cell (p-value = 0.161).

Table 4 shows the relationship of pneumatization of the pterygoid process with foramen rotundum protrusion and the vidian canal. According to the Chi-square test results, this relationship is statistically significant (p-value < 0.001).

*: Statistically significant

Table 5 shows the frequency of the variables based on the patients’ gender. The chi-square test was used to examine this relationship.

Table 6 shows the frequency of the variables based on the patients’ age. The chi-square test was used to examine this relationship.

Discussion:

Sinus type (based on its posterior wall relative to sella turcica) and size are important factors for evaluating the risk of damage to neurovascular structures during transsphenoidal surgery. If the sinus extends to the clivus or the roof of the SS, which occurs in the postsellar type, only a thin layer remains between the dura mater and the posterior and anterior cranial cavities (17). Performing transsphenoidal hypophysectomy in the conchal type is contraindicated due to the thick posterior bony wall (18).

In the present study, the conchal, presellar, sellar, and postsellar types were estimated to be 1, 2.5, 25.4, and 71.1%, respectively. In both males and females, the postsellar type has the highest frequency. The lowest frequency is related to the conchal type.

Martina C. Schwerzmann examined the anatomy of the SS in 50 CBCT images in 2020. The frequencies of conchal, presellar, sellar, and postsellar types were estimated to be 2, 4, 38, and 56%, respectively. As can be seen, the postellar type had the highest frequency (20).

However, Nikola Stokovi´c (2016) conducted a study on different SS types based on 51 CBCT images and reported the sellar type has the highest frequency (41%). Other sinus types were reported as follows: the conchal type 2%, the presellar type 24%, and the postsellar type 33% (17).

Access to structures during closed surgery depends on the degree of sinus pneumatization. If the sinus extends to the clivus or the roof of the SS, only a thin layer remains between the dura mater and the posterior and anterior cranial cavities. Greater wing pneumatization increases the risk of accidental penetration into the middle cranial fossa and cerebrospinal fluid (CSF) leak (17). When the sinus extends to the lesser wing, it may also extend to the anterior clinoid process, enclosing the optic nerve. Overextension of the sinus increases the risk of iatrogenic lesions and damage to vital structures such as the ICA and the optic nerve, leading to internal bleeding and impaired vision (20). Sinus extension to the pterygoid process provides a great route to access the skull base. However, it simultaneously provides a space for pus accumulation and paves the way for sinusitis (4).

In our study, 51.7% of the patients had greater wing pneumatization. Pneumatization to the anterior and posterior clinoid processes were observed in 27.4 and 2.5% of the patients, respectively. Pneumatization to the lesser wing of the sphenoid, nasal septum, clivus, and the pterygoid process occurred in 1, 26.9, 32.3, and 40% of the cases, respectively.

Singh (2019) examined 84 CT images and reported the prevalence of pneumatization to the greater wing of the sphenoid and the pterygoid process to be 22.61% and 32.14%, respectively (22).

Hewaidi et al. (2008) investigated the anatomical variation of the SS in 300 PNCT images. According to their findings, the sinus was pneumatized to the anterior clinoid process in 15.3%, to the greater wing of the sphenoid in 20%, and to the pterygoid process in 29% of the patients (23).

Wojciech Ilków (2018) examined 100 CT images to observe the posterior clinoid process and found that the SS had been pneumatized in 14% of the patients (24).

A close relationship to the optic nerve sinus is an important risk factor during surgery. This risk increases when more than 50% of the optic nerve protrudes. Damage to the optic nerve can lead to diplopia. Also, damage to the carotid artery can lead to uncontrolled bleeding, retrobulbar hematoma, or acute proptosis (26).

Foramen rotundum is an important aperture that contains important neural structures such as the maxillary nerve. In addition, they are closely related to the structures around the SS (33).

In the present study, protrusion of the ICA was smooth type in 54.7% of the patients and prolonged type in 45.3% of them. Also, protrusion of the optic nerve was smooth and prolonged in 64.2% and 35.8% of the patients, respectively. Foramen rotundum protrusion was also observed in 25% of the patients.

Jaworek Troć J. et al. (2020) examined 296 CT images. In their study, the carotid canal was protruded into the sinus in 55.74% of the patients. The protrusion prevalence was more in males than in females. In our study, the carotid artery was protruded in 45.3% of the patients; 54.2% in males and 39% in females (5).

Turkdogan et al. (2017) examined 200 CT images and found that foramen rotundum was protruded into the sinus in 17.5% of the patients (27).

The Onodi cell is poorly drained, which leads to stasis of secretions, infection, optic neuropathy, or mucositis. During endoscopy, the Onodi cell may be mistaken for the SS. Damage to the optic nerve or carotid artery during surgery can be prevented by diagnosing the Onodi cell (9).

In our study, Onodi cells were recorded in 22.9% of males and females, although it was more common among females (20.48% in males and 24.57% in females).

Adnan Ozdemir et al. (2019) investigated Onodi cells in 508 PNS CT images. According to their findings, these cells were present in 21.2% of the patients (24.5% in males and 17.6% in females) (9).

Neşe Asal (2019) conducted a study on 300 PNS CT images and reported the existence rates of the Onodi cells in males and females to be 26 and 19.1%, respectively (25).

The vidian canal contains the vidian nerves and arteries. It is divided into four types based on their degree of surrounding by bones. Type I vidian canal is the most suitable type for the extended endonasal method because no damage occurs to the structures located in the canal. In types III and IV, neurovascular structures are very likely to be damaged during surgery (3).

In the present study, grades I, II, III, and IV were observed in 14.9, 40.3, 24.4, and 20.4% of the patients. Grade II had the highest frequency in both males and females.

Kurt (2019) investigated the vidian canal and its relationship to surrounding structures. In CBCT images of 100 patients, Grades I, II, III, and IV were observed in 24, 33, 23.5, and 9.5% of the cases, respectively. Grade II had the highest frequency in both genders (12).

In another study, Yegin et al. (2017) studied the vidian canal in 594 CBCT images. They reported Grades I, II, III, and IV in 56.3, 29.7, 5.1, and 8.9% of the males, and 52.2, 20.7, 9.4, 17.7 of the females, respectively. Contrary to our study, Grade-I had the highest frequency in both genders (29).

The SS ostium is an important landmark for the surgeon to access the sinus. It is difficult to find ostiums, and their openness is very important in transsphenoidal surgery (14).

The ostium is the normal site of drainage of sinus secretions and is obstructed in many cases in inflammatory and tumoral diseases of the sinus. The sphenoethmoidal recess drains the secretions of the SS and the posterior ethmoid air cells into the superior meatus and finally into the nasopharynx. The relationship between these structures is important in sinonasal diseases and can cause disease transmission from one sinus to another. Another point is that the sphenopalatine artery passes beneath the SS ostium. Therefore, it is sometimes cauterized using a cauter during surgery to prevent severe bleeding (18).

The septum is a normal variation found in the structure of the SS that can attach to the bone that protects the ICA, where the fracture of the septum to gain access to the ICA can damage this artery (8).

In the present study, the sinus septum was of the C-shape type in 19.4%, S-shape type in 22.9%, tilted type in 46.3%, and absent in 1.5% of the patients. No deviation in septum was observed in 10.0% of the patients. Ostium was bilateral in 56.2%, only on the left side in 9%, and only on the right side in 8% of the patients. In 26.9% of them, it was not observed in any direction.

Nesibeh Yilmz (2016), analyzing 200 CBCT images, found that C-shaped and tilted types of septum had the highest and lowest frequency in females, respectively, while S-shaped and Tilted types had the highest and lowest frequency in males, respectively. Ostium was bilateral in 72.3%, on the right side in 10.7%, on the left side in 8%, and absent in 8.9% of the females. Also, it was bilateral in 70.5%, on the right side in 5.7%, on the left side in 12.5%, and absent in 11.4% of the males. In total, it was bilateral in 71.5%, on the right side in 8.5%, on the left side in 10%, and absent in 20% of all of the participants (14).

Ozturan et al. (2012) analyzed 999 CT scans of patients to find a relationship between optic nerve protrusion and the presence of Onodi cells. They found that the presence of Onodi cells was significantly related to optic nerve protrusion, and sinus pneumatization was significantly related to the anterior clinoid process. They also reported that this relationship would increase the risk of damage to the optic nerve during surgery (31).

However, Adnan Ozdemir et al. (2019) analyzed 508 PNS CT images to find a relationship between optic nerve protrusion and Onodi cells and reported no significant between them (9).

The present study also investigated the association between these two variables, although it found no statistically significant relationship between them (p-value = 0.161).

Pneumatization is considered pterygoid process type if it extends below the surface between the vidian canal and the foramen rotundum. Excessive pneumatization of the pterygoid process can change the position of the vidian canal and expose it to injury during surgery. It can also lead to foramen rotundum protrusion by changing its position in the sinus (32).

The present study indicated the relationship of pterygoid pneumatization with vidian canal protrusion and foramen rotundum (p-value < 0.001).

Rahmati et al. (2016) also analyzed 103 CBCT images and found that pneumatization of the pterygoid was significantly related to vidian canal protrusion and foramen rotundum (2).

Conclusion:

CBCT images are very useful for the anatomical analysis of the SS and its surrounding structures. The postsellar type of sinus had the highest frequency. Vidian canal Grade ll was observed the most in the mages. Greater wing pneumatization was observed in more than half of the images. The most common type of SS septum was tilted. Ostium was more bilateral than unilateral. The ICA and optic nerve protrusion were smooth in most cases but slightly different from the prolonged type. No significant relationship was observed between optic nerve protrusion and the presence of Onodi cells. However, sinus pneumatization to the pterygoid was significantly related to the vidian canal and foramen rotundum.

List Of Abbreviations:

CBCT: cone-beam computed tomography

SS: sphenoid sinus

ICA: internal carotid artery

ACP: anterior clinoid process

PCP: posterior clinoid process

Declarations:

Ethics approval and consent to participate:

The study was approved by the ethics committee of Hamedan University of Medical Sciences (IR.UMSHA.Rec.1399.824). All methods were carried out in accordance with relevant guidelines and regulations (Helsinki declaration). Written informed consent was obtained from all patients.

Consent for publication:

Not applicable.

Competing interests:

The authors declare that they have no competing interest.

Acknowledgements:

This study was approved by the ethics committee of Hamedan University of Medical Sciences (No.14000124471). The authors would like to thank the Research Deputy of the School of Dentistry for the support.

Funding:

None.

Availability of data and materials:

The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

Contributions:

MF participated in Study concept, study design, literature review, editing and review.FS participated in Study concept, study design, literature review, editing and review. LK participated in Study concept, study design, literature review, editing and review.BA participated in Statistical analysis.

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