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).