Key points

Comprehension of the complex anatomic variants comprising the frontal sinus outflow tract is essential for successful surgical intervention. Similar to consideration of the other paranasal sinuses, familiarity with the surrounding anatomy in both virgin and revision cases is critical, because deviation from sound technique has the potential to result in a variety of sequelae ranging from recurrent disease to catastrophic intracranial and orbital injury. Furthermore, the frontal sinus outflow tract is typically a tight space where even a small amount of mucosal disruption can lead to the failure of any intervention. Consequently, a detailed understanding of the surgical anatomy is paramount.

Frontal sinus surgical intervention has evolved since the era of trephination and obliteration. Nonetheless, although rarely performed, these open procedures still arguably have a role in the otolaryngologist’s surgical repertoire. Hence, a thorough understanding of both endoscopic visualization and anatomic considerations relating to external techniques may be valuable for the practicing otolaryngologist. This review aims to cover the key anatomy encountered, further illustrating these concepts through a description of several advanced dissection techniques.

Surgical planning

As in any operative procedure, appropriate preoperative assessment and exhausting all nonsurgical options as appropriate are critical. Comprehensively reviewing medical management of frontal sinus disease as well as indications and contraindications for surgical intervention is beyond the scope of this review, but its importance cannot be overemphasized. With regard to frontal sinus anatomical considerations, close examination of preoperative imaging is mandatory. Nowadays, this almost exclusively encompasses computed tomography (CT), preferably involving fine cuts with axial, coronal, and sagittal views. Key structures to examine include the location of the anterior ethmoid artery ( Fig. 1 ), the presence and amount of suprabullar cells ( Fig. 2 ), the presence of the agger nasi cell (and its degree of pneumatization) (see Fig. 2 ; Fig. 3 ), the depth and pneumatization of the frontal sinuses, the uncinate bone attachment, and middle turbinate anatomy. Each of these landmarks is discussed in further detail later. Visualization of the frontal sinus recess through a sagittal view allows for appreciation of its basic anatomic features (see Fig. 2 ). In relation to the frontal recess, the agger nasi cell is often present anteriorly, suprabullar cells posteriorly, fovea ethmoidalis posteriorly (see Fig. 2 ), the middle turbinate (vertical lamella) medially (see Fig. 1 ), lateral lamella posteromedially, and the orbit laterally.

Coronal CT scan illustrating location of the right anterior ethmoid artery, adjacent to the right superior oblique muscle.

Sagittal CT scan illustrating an agger nasi cell, suprabullar cells, fovea ethmoidalis, and frontal sinus outflow tract ( arrow ).

Coronal CT scan illustrating bilateral agger nasi cells, a right type I frontal cell, and left frontal sinus outflow tract ( arrow ).

Surgical planning

As in any operative procedure, appropriate preoperative assessment and exhausting all nonsurgical options as appropriate are critical. Comprehensively reviewing medical management of frontal sinus disease as well as indications and contraindications for surgical intervention is beyond the scope of this review, but its importance cannot be overemphasized. With regard to frontal sinus anatomical considerations, close examination of preoperative imaging is mandatory. Nowadays, this almost exclusively encompasses computed tomography (CT), preferably involving fine cuts with axial, coronal, and sagittal views. Key structures to examine include the location of the anterior ethmoid artery ( Fig. 1 ), the presence and amount of suprabullar cells ( Fig. 2 ), the presence of the agger nasi cell (and its degree of pneumatization) (see Fig. 2 ; Fig. 3 ), the depth and pneumatization of the frontal sinuses, the uncinate bone attachment, and middle turbinate anatomy. Each of these landmarks is discussed in further detail later. Visualization of the frontal sinus recess through a sagittal view allows for appreciation of its basic anatomic features (see Fig. 2 ). In relation to the frontal recess, the agger nasi cell is often present anteriorly, suprabullar cells posteriorly, fovea ethmoidalis posteriorly (see Fig. 2 ), the middle turbinate (vertical lamella) medially (see Fig. 1 ), lateral lamella posteromedially, and the orbit laterally.

Coronal CT scan illustrating location of the right anterior ethmoid artery, adjacent to the right superior oblique muscle.

Sagittal CT scan illustrating an agger nasi cell, suprabullar cells, fovea ethmoidalis, and frontal sinus outflow tract ( arrow ).

Coronal CT scan illustrating bilateral agger nasi cells, a right type I frontal cell, and left frontal sinus outflow tract ( arrow ).

The uncinate process

The attachment of the uncinate process impacts the drainage of the frontal sinus. In the most common configuration, the uncinate process attaches to the medial orbital wall, resulting in a drainage pattern medial to the uncinate and directly into the middle meatus. In the minority of cases that involve uncinate attachment to the middle turbinate or skull base, drainage occurs lateral to the uncinate. Preoperative evaluation of imaging and familiarity with the uncinate attachment are helpful knowledge for the surgeon.

Agger nasi cells

The presence and extent of agger nasi cell development are among the most important factors with which to be familiar before operating on a patient for frontal sinus disease. Being the most anterior ethmoid air cell, it represents the anterior border of the frontal recess (see Figs. 2 and 3 ), and its size and pneumatization can have a significant impact on frontal sinus outflow in combination with mucosal integrity and other immunologic factors. Preoperative imaging should be examined for these factors. Although originally thought to have a lower prevalence, improvements in CT revealed that this cell is present in nearly all patients. It can be found on a coronal scan associated with the origin of the middle turbinate. As described later, removing the posterior wall of this cell is instrumental for facilitating frontal sinus drainage in advanced frontal sinusotomy. On endoscopic view, the agger nasi cell manifests as a “bulge” or “mound” anterior to the origin of the middle turbinate on the lateral nasal wall.

In patients with larger agger nasi cells comprising a significant portion of the frontal recess, this cell can be confused with the frontal sinus recess itself. Several strategies can be used if there is intraoperative uncertainty regarding whether one is in the frontal sinus recess. The surgeon can gently palpate the posterior wall of what is thought to be the frontal sinus recess as well as the middle turbinate with a probe. If a completely different bony wall is found, the surgeon can gently remove this wall. Another method that may assist in determining whether one is in the frontal sinus recess or an agger nasi cell is switching endoscopes. Switching to a 70° camera and visualizing the entire frontal recess should be attempted. Finally, in situations where preoperative imaging indicates complicated frontal sinus recess anatomy, including significant agger nasi cells, image guidance may play a role, although the investigators stress that this tool should be used as an adjunct in surgery rather than be relied on.

The importance of understanding whether one’s probes and instruments are located in the frontal sinus recess versus an agger nasi cell cannot be emphasized enough. A recent analysis of chronic rhinosinusitis patients with endoscopic sinus surgery failures noted that retained agger nasi cells were present in the majority (73.1%), likely contributing to persistent symptoms. If one mistakenly thinks they are not in an agger nasi cell and that the frontal sinus recess is not obstructed, then frontal sinus obstruction may obviously remain postoperatively. Furthermore, leaving a portion of the agger nasi cell may facilitate further outflow tract scarring, as the remnants may adhere to the posterior wall of the frontal sinus recess. Finally, with newer technologies, such as balloon catheter dilation (BCD), situations may occur where a balloon is deployed into an agger nasi cell (rather than the frontal recess), and resultant dilation may cause frontal sinus recess obstruction worse than what was initially present, necessitating a more difficult revision procedure down the road.