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Endoscopic approaches to the frontal sinus have grown significantly in the last decades and due to its complex anatomy, including the possibility of pneumatization in different cells and anatomical variations, possess numerous challenges to the endoscopic surgeon. Moreover, the proximity to noble structures such as the cribriform plate, orbit and anterior ethmoidal artery can increase the risk of injury. Unlike the maxillary, ethmoidal and sphenoidal sinuses, the frontal sinus is not in line of visualization with a zero-degree endoscope and often requires an angled endoscope. Several anatomical classification methods have already been proposed for frontal sinus, however, these previous systems present limitations of anatomical details. In 2016, the International Frontal Sinus Anatomy Classification (IFAC) was described by Wormald et al. The authors propose improved classification of the frontoethmoidal cell in diagrammatic nomenclature to facilitate greater accessibility in surgical planning.
Department of Otorhinolaryngology, Head and Neck Surgery, Faculty of Medicine, Universiti Kebangsaan Malaysia, Kuala Lumpur, Malaysia
Allergy Unit, Hospital Canselor Tuanku Muhriz, Kuala Lumpur, Malaysia
Balwant Singh Gendeh*
Pantai Hospital Kuala Lumpur, Kuala Lumpur, Malaysia
Department of Otorhinolaryngology, Head and Neck Surgery, Universiti Kebangsaan Malaysia Medical Center, Kuala Lumpur, Malaysia
*Address all correspondence to: bsgendeh@gmail.com
1. Introduction
The frontal sinus is still a challenge for rhinologists due to its anatomy. The location of the frontal sinus which is in between the eyes and superior to the nasal cavity limits accessibility. It is believed to be an extension of the ethmoidal sinus via superior pneumatization of ethmoidal air cells. Unlike the maxillary, ethmoidal and sphenoidal sinuses, the frontal sinus is not in the line of visualization with a zero-degree endoscope and often requires an angled scope (70 or 120 degrees). The frontal sinus outflow tract or ostium may not be easily identified by an untrained eye. A surgeon with little experience with the frontal sinus may easily become disoriented and confused with its anterior and posterior boundaries, resulting in frontal sinus surgery being challenging. Besides, the anatomical variation that impedes the frontal sinus drainage is challenging and will require identification with a computed tomographic (CT) scan [1]. The space limitation of the frontal sinus ostium or anterior and posterior diameter of the frontal sinus impedes access to endoscopic sinus surgery instruments to the periphery, posing a challenge for clearance in tumor surgery. Unlike the maxillary and sphenoid sinuses that have a visible ostium during endoscopic sinus surgery, the visualization of frontal sinus ostium is often hindered by air cells, making its approach more tedious and the need for a good understanding of its three-dimensional anatomy.
It is for these reasons that frontal sinus surgery often gets its own chapter in the literature and books. A functional endoscopic sinus surgery (FESS) involves ventilation and drainage of the maxillary, anterior ethmoids, posterior ethmoidal and sphenoid sinus. A full house FESS consists of a FESS and frontal sinusotomy [2]. It is vital to understand the frontal sinus anatomy both radiological and clinical prior to a surgical approach. The objectives of this chapter are firstly to discuss the latest accepted classification of the frontal sinus based on air cell anatomy and radiology, and secondly to describe the approaches of the frontal sinus based on its classification.
The frontal sinus is not present at birth but starts to form and invade the frontal bone at approximately 4 years. It extends superiorly and laterally and completes its pneumatization by the second decade of life [3]. Essentially it is located in between the anterior and posterior tables of the frontal bone being the anterior and posterior boundaries respectively. Its inferior boundary is the frontal sinus beak.
The frontal sinus ostium is the opening of the frontal sinus superiorly to the nasal cavity. It is the narrowest area bounded by the frontal sinus superiorly and frontal recess inferiorly. It is bounded anteriorly by the frontal sinus beak; posteriorly by the skull base, laterally by the lamina papyracea and medially by the vertical segment of the middle turbinate [4].
The frontal recess is located inferior to the frontal sinus ostium and it is the space at which the frontal sinus drains. It consists of air cells and space superior to the ethmoidal bulla [4]. The presence of air cells in the frontal recess may further narrow the viable drainage pathway of the frontal sinus known as a nasofrontal duct. This is a misnomer as it is not a true duct but an anatomical description of a tubular mucosal structure forming its drainage pathway. Its boundaries are the uncinate process anteriorly and ground lamella of the ethmoidal bulla posteriorly. In some cases, the posterior superior wall of the agger nasi may form the anterior boundary of the nasolacrimal duct. The nasofrontal duct may open anterior superior to the infundibulum (59%), directly into the infundibulum (40%) or rarely drain superior to the bulla (1%) [5, 6]. Stenosis of the nasofrontal duct due to scar tissue formation or residual disease leads to failure of frontal sinus surgery with recurrent frontal sinusitis.
Stammberger et al. have described the frontal sinus and recess as an hourglass figure. Here the frontal sinus is located superiorly and the frontal recess is located inferiorly. The frontal ostium forms the narrowest bottleneck of the hourglass center (Figure 1) [7]. Occasionally, a narrowed nasofrontal duct within the frontal recess may become narrower forming a tubular stricture tapering inferiorly at the nasofrontal duct [5].
Figure 1.
Sagittal view of the frontal sinus. Authors annotation of the frontal infundibulum and nasofrontal duct. The frontal recess is the nasofrontal duct (green) and surrounding air cells (blue). Appreciate the hourglass figure formed by the frontal sinus and nasofrontal duct. FS: Frontal sinus; ANC: Agger nasi cell; SP: Sphenoid sinus; IT: Inferior turbinate.
“surgical treatment of chronic frontal sinusitis is difficult, often unsatisfactory and sometimes disastrous. The many surgical techniques available are expressions of our uncertainty and perhaps of our failure.”
The approach to the frontal sinus dates back to 1750. Hassan et al. state that the best approach is one that should fulfill five criteria namely complete relief of symptoms, disease eradication, preservation of sinus function, minimal morbidity and minimal cosmetic dysfunction [9]. Over the years approaches to the frontal sinus have progressed from external approaches to combined external and endoscopic.
Frontal sinus surgery has experienced many eras in order of timeline (Figure 2) [10].
Early: Frontal wall removal and obliteration of sinuses
Conservatism: Endonasal approaches
External: Second wave of external approach involving fronto-ethmoidectomy followed by osteoplastic flap
Endonasal: Conventional endonasal ethmoidectomy
Endoscopic: Endoscopic approach to frontal sinus disease
Figure 2.
Coronal views of the frontal sinus. A: Draf I involving a frontal sinustomy with clearance of the frontal recess and leaving the floor intact; B: Draf IIa resection of the floor to the middle turbinate; C: Draft IIb resection of the frontal sinus floor of the frontal sinus from the lamina papyracea to the nasal septum; and D: Resection of the floor of the frontal sinus from one lamina papyracea to the other.
3.1 Early
In the 1890s, Schaeffer described the intranasal approach of opening the nasofrontal duct for ventilation and drainage of the frontal sinus in chronic suppurative frontal sinusitis. Ingals, Halle, Good and Wells went on to describe techniques to enlarge the nasofrontal ducts that were associated with high morbidity and mortality. Ogston in 1884 described anterior table trephination, dilatation of nasofrontal duct, placement of drainage tube and curation of the frontal sinus mucosa to aid drainage [11]. This was described by Luc several years later, forming the Ogston Luc technique [12]. This technique failed due to restenosis of the frontal-ethmoidal drainage pathway [9]. Lyan in 1895 described the Khunts technique involving the removal of the anterior table, stripping of the superior nasofrontal duct mucosa, placement of nasofrontal duct stent and sinus obliteration via placement of skin on the remaining posterior table [13]. However, this was associated with significant deformity of frontal cosmesis. Riedel-Schenke in 1898 introduced a complete obliteration of the frontal sinus by removal of anterior table and floor, stripping of frontal sinus mucosa and sinus obliteration via placement of skin on the remaining posterior table [14]. Once again poor cosmesis resulted in its failure. Killian in 1903 tried to improve cosmesis by preserving the superior orbital rim, harvesting a nasal mucosa and rotating it into the frontonasal duct and placing a nasofrontal duct stent [15]. This was associated with necrosis of the supraorbital rim and still significant cosmesis defect.
3.2 Conservatism
Due to the cosmetic deformity caused by external approaches, many have tried intranasal approaches with little success due to the mortality associated as a result of poor visualization. Lothrop in 1915–1917 introduced a technique involving resection of bilateral anterior ethmoids, resection of the anterior floor of the frontal sinus, resection of the superior nasal septum and internisus septum to widen a narrow nasofrontal duct which is a cause of frontal sinus disease. It was technically challenging and associated with restenosis of the nasofrontal duct [10, 16, 17]. This was a combined intranasal and extranasal ethmoidectomy.
3.3 External
Both Lynch and Howarth described an external ethmoidectomy in 1921 in the United States and the United Kingdom respectively [18, 19]. It involved a medial periorbital incision, excision of ethmoidal air cell, removal of lamina papyracea, removal of the frontal process of maxilla, removal of the floor of the frontal sinus, curettage of frontal sinus mucosa, placement of drain for 5 days followed by postoperative dilatation. This technique showed initial success but it was later learned that herniation of orbital contents from the lateral boundary to the medial aspect of the nasofrontal duct led to restenosis.
Nasofrontal duct widening had failed and many then considered closure of the frontal sinus by obliterating it. This then led to the popularization of the osteoplastic flap as it provided good visualization and access to the frontonasal duct. Although the osteoplastic flap was first reported by Schonborn in 1894 and Brieger in 1895, it was only first performed in the US by Goodale and Montgomery in 1958. Complications involved, CSF leak, frontal paresthesia, headaches and lack of exposure to the ethmoids [10].
3.4 Endonasal
Endonasal ethmoidectomy gained popularity based on the understanding that marsupialization of the ethmoidal air cells restored ventilation and drainage of the frontal sinus. Mosher in 1912 introduced radical ethmoidectomy, middle turbinectomy and curettage of anterior ethmoidal air cells [20]. In 1972, Eichel described a technique of posterior to anterior ethmoidectomy, probing of the nasofrontal duct and removal of the anterior ethmoidal air cells, he believed in mucosal preservation to ensure its function in mucociliary clearance and disease eradication via ventilation and drainage [21, 22]. They indicated that intranasal techniques should be used for recurrent frontal sinusitis sinusitis and frontal sinusitis with impending complications, failure of which will require a second transnasal or external approach [10].
3.5 Endoscopic
Masserklinger popularized the use of nasal endoscopy in the 1970s in Europe. His teachings were then brought to America by Kennedy and the rest of Europe by Stammberger through the English medium. In 1990, Schaefer and Close used an endoscope for frontal sinus disease. Shortly after, Draf in 1991 combined the use of microscope and endoscopy in performing frontoethmoidectomy. The types of procedures are shown in Figure 3 and remain popular until today [22].
Figure 3.
Timeline of significant historical progress in frontal sinus surgery.
Draf I: Simple drainage via anterior ethmoidectomy and opening of the nasofrontal duct in pansinusitis without frontal sinus opacification.
Draf II: Extended drainage unilateral resection of the floor of the frontal sinus from the lamina papyracea to the nasal septum in pansinusitis with frontal sinus opacification.
Draf III: Median Drainage. Bilateral frontal sinus floor resection from the lamina papyracea of one orbit to the other and removal of the internisus septum. To be considered in revision surgeries, intracranial or orbital complications and failed open procedures.
The Draf technique saw less frontal sinus mucocele post procedure. However, during the time of publishing his work, Draf had proposed the combination of external and intranasal micro-endoscopic approaches in recurrent frontal disease, presence of ethmoidal disease in previously operated patients. Draft preferred an external approach should the frontal sinus be large, orbital complications, intracranial complications, lateral mucocele, large fractures and presence of osteomas. Draf also preferred obliterating the frontal sinus in revision cases [22]. However, re-evaluation of an obliterated frontal sinus is still challenging today despite being aided by imaging. The success of endoscopic frontal sinus surgery was possible owing to the increased availability of CT scans of the paranasal sinuses allowing surgeons to identify the frontal sinus recess and anatomical variations.
The Draf II was modified to include the following.
Draf IIa: Unilateral resection of the floor from the ipsilateral lamina papyracea to the axilla of the middle turbinate.
Draf IIb: Unilateral resection of the floor from the ipsilateral lamina papyracea to the nasal septum.
Gross in 1995 rekindled the Lothrop’s procedure in resecting the medial floor of the frontal sinus, superior nasal septum and intersinus septum via an external and intranasal technique. Gross pointed out that the Lothrop’s technique had been lost in translation over the years and could be performed in the twentieth century via an endoscope and completely intranasal. Gross had also acknowledged Draf’s median drainage technique via a microscope and endoscope with or without an external approach as similar. Essentially both of them described a similar technique at different times, one in the early twentieth century and the other later. This gave rise to the Draft III being termed as the endoscopic modified Lothrop procedure [23, 24].
Harvey in 2023 demonstrated the Crolyn’s window approach whereby an axillectomy lateral to the middle turbinate is performed with a high-speed drill leading straight into the frontal sinus. This is a simplified approach to the Draft IIa frontal sinus with a zero-degree rigid scope, particularly useful when there is a small anterior posterior diameter limiting access to instruments [25].
Many have pushed the boundaries to rely solely on endoscopic intervention. In today’s world, open procedures are only reserved for distorted intranasal landmarks, failed multiple endoscopic surgeries, frontal sinusitis with intracranial or intraorbital complications and disease extending to the lateral boundaries [9]. With the latter, a combined approach of external and endoscopic surgery may suffice.
3.6 Frontal sinus classification
“The sinus frontalis is in the vast majority of cases a derivative (a) of the recessus frontalis directly, (b) of one or more of the cellulae ethmoidales anterior which have their genesis in frontal pits, or (c) of both, when present in duplicate or triplicate.”
The frontal sinus is an extension of the anterior ethmoids into the frontal region. It consists of varying anatomy of air cells in different individuals. In the early days, frontal cells were identified during dissection intraoperatively or in cadavers. Improved imaging in the 90s and a shift from radiographs to CTs have allowed for better identification of frontal cells in large volumes of individuals.
Bent in 1994 identified the four variations or possibilities among frontal cells which are above the agger nasi; which is the anterior most ethmoidal air cell (Types I to IV). These cells arise from posterior to the agger nasi cell and may obstruct the frontal sinus drainage [27, 28]. The cells are as follows.
Type I: Single frontal cell superior to the agger nasi (Figure 4).
Figure 4.
Coronal view of the frontal sinus. Khun’s type I and II frontal air cell. ANC: Agger nasi cell.
Type II: Several layers of cells superior to the agger nasi (Figure 4).
Type III: Large single cell extending superiorly into the frontal sinus (Figure 5).
Figure 5.
Coronal view of the frontal sinus. Khun’s type III and IV frontal air cell. S: Septum; FS: Frontal sinus; IT: Inferior turbinate; ANC: Agar nasi cell.
Type IV: Isolated single cell in the frontal sinus (Figure 5).
This was known as Bent and Khun’s classification of the frontal sinus and was referred to by many books and literature for two decades. The authors learned the frontal sinus anatomy based on Bent and Khun’s classification.
Over the years, many have been able to further detail the origins of these frontal cells and the role of frontal recess obstruction. This has allowed for a better understanding of the frontal sinus drainage and the role played by the surgeon to individualize their surgery based on the anatomical variations and requirements of the disease. Wormald et al. published the International Frontal Sinus Anatomy Classification (IFAC) in 2016 that classified frontal cells into anterior, posterior and medial cells. It identifies specific cell types and explains how these cells affect the drainage of the frontal sinus [4]. Anterior cells tend to distort the frontal sinus drainage to medial, posterior or posterior medial, posterior cells distort it anteriorly while medial cells distort it laterally. The IFAC should be made by viewing a CT on its three planes of axial, coronal and sagittal simultaneously to accurately appreciate its origins and extensions. This takes practice for good coordination. Below is the IFAC [4].
3.7 Anterior cells
Agger nasi: The agger nasi cell (ANC) may sit anterior to the middle turbinate or superior to the anterior insertion of the middle turbinate to the lateral nasal wall (Figure 6).
Supra agger: The supra agger cell (SAC) sits superior to the ANC and posterior to the frontal sinus beak. It is laterally based and does not extend into the frontal sinus (Figure 6).
Supra agger frontal: The supra agger frontal cell (SAFC): An anterior-lateral ethmoidal cell extending into the frontal sinus. It may be small at the level of the frontal sinus floor (Figure 7) or large that may extend to the roof of the frontal sinus (Figure 8). A large SAFC may either require an extended endoscopic approach or a combined endoscopic and external approach.
Sagittal view of the frontal sinus. Small SAFC (blue) situated above the ANC pneumatization into the frontal sinus. FS: Frontal sinus; FB: Frontal beak; ANC: Agger nasi cell; IT: Inferior turbinate; SP: Sphenoid sinus.
Figure 8.
Sagittal view of the frontal sinus. Large SAFC (blue) pneumatization into the frontal sinus. FS: Frontal sinus; FB: Frontal beak; ANC: Agger nasi cell; IT: Inferior turbinate; SP: Sphenoid sinus.
3.8 Posterior cells
Supra bulla: Supra bulla cell (SBC) is a cell that sits just superior to the ethmoidal bulla. This cell does not extend into the frontal sinus and its anterior wall will be in continuation with that of the ethmoidal bulla (Figure 9).
Supra bulla frontal: The supra bulla frontal cell (SBFC) originates from the supra bulla region. Unlike the SBC, it extends into the posterior part of the frontal sinus. Therefore, its posterior wall is the skull base (Figures 10 and 11).
Supra orbital ethmoid: The supra orbital ethmoid cell (SOEC) anterior ethmoid cell located anterior, around or posterior to the anterior ethmoidal artery over the orbital roof. It differs from the SBFC based on its pneumatization superior and over the orbit (Figure 11).
Figure 9.
Sagittal view of the frontal sinus. SBC (blue) situated above the BE and not extending into the frontal sinus. FS: Frontal sinus; FB: Frontal beak; BE: Bulla ethmoidalis; MT: Middle turbinate; IT: Inferior turbinate; SP: Sphenoid sinus.
Figure 10.
Sagittal view of the frontal sinus. SBFC (blue) situated above the BE and pneumatization into the frontal sinus. FS: Frontal sinus; FB: Frontal beak; BE: Bulla ethmoidalis; MT: Middle turbinate; IT: Inferior turbinate; SP: Sphenoid sinus.
Figure 11.
Coronal view of the frontal sinus. SBFC (blue) in the right FS situated above the BE and pneumatized into the frontal sinus. In the left frontal sinus. The SOEC is situated above the AEA and orbital rim. Appreciate the nipple sign (arrow) FS: Frontal sinus; AEA: Anterior ethmoidal artery; BE: Bulla ethmoidalis; M: Maxillary sinus; NLD: Nasolacrimal duct; S: Septum.
3.8.1 Medial cells
Frontal septal: The frontal septal cell (FSC) is a pneumatization within or at the interfrontal sinus septum. It may be an anterior ethmoid medial cell or frontal sinus inferior cell (Figure 12).
Figure 12.
Coronal view of the frontal sinus. FSC (blue) located medially and originating from the interfrontal sinus septum. FS: Frontal sinus; S: Septum.
3.9 Updated surgical classification
The IFAC has allowed for an updated surgical approach based on the anatomy and obstruction caused by frontal air cells. Together with the IFAC, Wormald et al. also proposed a termed classification of the extend of endoscopic frontal sinus surgery (EFSS). The benefit of this improved classification is that it considers the anatomical variation resulting in frontal sinus obstruction and surgical difficulty encountered in the operative procedures. This allows for a step-wise approach to Otorhinolaryngology trainees or surgeons in tackling operative procedures to the frontal sinus and recess. For the ease of understanding, it can be divided into three groups with increasing difficulty and invasiveness (Table 1).
Grade 0
Sinus dilation with balloon without any tissue removal
No tissue removal
n/a
Grade 1
Clearance of SACs and SBC within the frontal recess without surgery to the frontal ostium
Clearance of frontal recess with tissue removal
Draf I
Grade 2
Clearance of SACs or SBCs that obstruct the frontal sinus ostium
Grade 3
Clearance of SAFCs, SBFs and FSCs extending to the frontal sinus via frontal ostium without enlarging the frontal ostium
Grade 4
Clearance of SAFCs, SBFs and FSCs extending to the frontal sinus via frontal ostium with enlargement of the frontal ostium
Bone removal from frontal beak
Draf IIa
Grade 5
Unilateral frontal sinus drill out involving enlargement of the frontal sinus from the lamina papyracea to the nasal septum with removal of the floor of the frontal sinus
Draf IIb
Grade 6
Frontal sinus drill out involves the removal of the floor of the entire frontal sinus and joining bilateral ostium to perform a common ostium with drill out of the septum and frontal beak
Draf III/Modified Lothrop
Table 1.
EFSS classification and comparisons to the Draf classification.
Comparing it to the Draf classification, The authors are of the opinion that Grades 1 to 3 may correspond to a Draf I, Grades 4 and 5 being a Draf II and Grade 6 represents a Draf III or modified Lothrop. The ambiguity in the extent of air cell removal still persists until today for a Draf I procedure. How much of or little is adequate for a Draf I? This is where the classification of EFSS has better defined the frontal recess clearance to aid classification and accurate reporting of surgical outcomes. Grade 6 surgery is preferred in revision surgery where there was previously failed endoscopic sinus surgery to the frontal sinus, disease recurrence or tumor surgery. The frontal sinus drill out simply allows for better postoperative surveillance and delivery of topical medications to the frontal sinus.
The frontal sinus is no more of a mystery. Imaging and endoscopic sinus surgery has allowed for an improved and better understanding of the origins and three-dimensional anatomies of air cells within the frontal recess and frontal sinus and its obstruction to drainage. This has led to an improved classification allowing for a more individualized surgery to be performed for the ventilation and drainage of the frontal sinus.
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Written By
Hardip Singh Gendeh and Balwant Singh Gendeh
Submitted: 08 December 2023Reviewed: 14 February 2024Published: 03 April 2024
Open access peer-reviewed chapter
