Frontal Angle: A New Predictor of Difficulty in Endoscopic Frontal Sinus Surgery

Abstract

Objectives:

The possibility of visualization of the frontal sinus during endoscopic surgery depends on 3-dimensional configuration of the frontal sinus opening (FSO). We aimed to determine the prevalence of unfavorable angulation of the lower part of the posterior wall of the frontal sinus and its relation to FSO diameter.

Methods:

One hundred and twenty-eight computed tomography (CT) scans were retrospectively reviewed to measure (1) the angle between the nasal floor and the plane tangent to the posterior table of the frontal sinus above the most posterior aspect of the anterior buttress (frontal angle, FA) and (2) dimensions of the FSO.

Results:

The FA ranged from below 30° to 90°. Nearly 13% of sinuses (16.4% of patients) showed FA about 90°, which should enable good visualization of the sinus with the 30° scope after opening and clearing the frontal recess, while 4% (6.25% of patients) showed FA ≤ 30°. The anterior–posterior diameter (A-PD) was below 5 mm in 17.6% of sinuses (26.6% of patients). There was a significant correlation between FA and A-PD. Unfavorable combination of FA and A-PD (<45°, <5 mm) was present in 5.2% of sinuses (8.6% of patients), and extremely unfavorable combination (<30°, <5 mm) in 0.8% (0.8% of patients).

Conclusions:

The FA shows great interindividual variability, which is very likely to reflect the possibility of inspection of the frontal sinus. This implies a need for further prospective clinical studies to validate FA as a predictor of difficulty in frontal sinus surgery.

Keywords

anatomy frontal sinus frontal opening endoscopic sinus surgery

Introduction

Despite improvement in imaging techniques and surgical tools, endoscopic surgery of the frontal sinus is still a challenge due to a narrow-curved drainage pathway. Recognition of the anatomical variants of the uncinate process insertion, the depth of olfactory groove, and the types of frontoethmoidal cells improved understanding of anatomy and surgical performance.^1
-3 Most of primary endoscopic sinus surgeries are performed with Draf I or Draf IIa technique using 30° or 0° endoscopes. During Draf IIa procedure, the frontal sinus drainage pathway (FSDP) limited by neighboring frontoethmoidal cells is widened using cutting forceps or a microdebrider. After the frontal recess is cleared, the frontal sinus opening (FSO) can be visualized between the anterior buttress and the anterior ethmoidal artery. The possibility of visualization of the FSO and the inside of the sinus depends on the protrusion of the anterior buttress toward the skull base, angulation of the posterior table of the frontal sinus, and dimensions of the FSO. The anterior–posterior diameter (A-PD) of the FSO below 5 mm is regarded as a factor increasing the difficulty of the surgery.⁴

The scope resting on the limen nasi introduced just below the axilla of the middle turbinate is at an angle of about 55° to the floor of the nasal cavity.⁵ Under these conditions, the 30° scope lens is positioned nearly parallel to the floor of the nasal cavity. The surgeon trying to localize the FSO is looking upward along the line perpendicular to the floor of the nasal cavity passing just behind the anterior buttress. Thus, angulation of the posterior table just above the anterior buttress seems to be decisive not only for the possibility of inspection of the frontal sinus but also for maneuverability of the tools within the sinus (Figure 1). Unfavorable angulation of the FSO can be overcome to some extent by the use of more angulated scope (45° or 70°), which decreases spatial orientation.⁶

Figure 1.

Scheme illustrating visibility of the frontal sinus with the 30° scope (90° angle of view) inserted under the axilla of the middle turbinate at the angle of 55° to the floor of the nasal cavity for different frontal angle (FA). Green vertical line indicates line 1. A, Line 1 passing anterior the posterior table of the frontal sinus—FA ≥ 90°—good visibility of the posterior wall and the anterior table and good access to the sinus. B, 90° > FA > 60°; good visibility of the entire posterior wall and good access to the sinus. C, 60° > FA > 45°; good visibility of the lower posterior wall and slightly limited access to the sinus. D, 45° > FA > 30°; limited visibility of the lower posterior wall and limited access to the sinus. E, FA < 30°; poor visibility of the frontal sinus opening and poor accessibility to the sinus.

The aim of this study was to determine the prevalence of unfavorable angulation of the lower part of the posterior wall of the frontal sinus and its relation to the FSO diameter.

Material and Methods

This study was approved by the ethics committee of the Medical University. The analysis was performed using retrospective data (computed tomography [CT] scans had to be performed for diagnostic process). No informed consent was requested from the patients.

Paranasal sinus CT examinations of patients referred to otorhinolaryngology department were reviewed. Examinations acquired with slice thickness greater than 1 mm, with a nonregular slice interval, performed using cone-beam CT were eliminated. The patients with frontal sinusitis, bilateral frontal sinus agenesis, after frontal sinus surgery, frontal bone fracture, gross hard palate asymmetry, and those with neoplastic or expansile lesions of the frontal recess and sinus were excluded. This resulted in identification of 128 CT examinations (250 sides after excluding 6 one-sided frontal sinus agenesis).

Multiplanar reconstruction (MPR) images were analyzed in the bony window (window level—300; window width—1500) using Horos (free DICOM viewer) software.

Posterior Table Angulation: The Frontal Angle

Vertical line (line 1) perpendicular to the nasal floor, passing just behind the anterior buttress was drawn up to the posterior wall of the frontal sinus, defining point A (Figure 2). The second line (line 2) was drawn tangent to the posterior wall of the frontal sinus at point A. Then the angle between the plane of the nasal floor and line 2 (frontal angle, FA) was measured. If line 1 did not reach posterior table and crossed anterior table, FA was assigned as “>90°.” The measurements were performed twice at 3 points: (1) the most lateral aspect of the anterior buttress, (2) the midpoint between the lamina papyracea and the middle turbinate, and (3) just lateral to the middle turbinate (Figure 3). The 2 measurements for each point were averaged. The maximum value of the FA from these 3 points was used for further analysis of correlation between A-PD and FA.

Figure 2.

Paranasal sinus computed tomography—sagittal view: Line 1 perpendicular to the hard palate passing just behind the anterior buttress, point A defined by crossing of line 1 and posterior table of the frontal sinus, line 2 parallel to the posterior table at point A, and frontal angle between the plane of the nasal floor and line 2.

Figure 3.

Scheme illustrating the measurements of A-PD and FA. The yellow line on the sagittal section (A) shows the anterior–posterior dimension (AP-D, axial scan (B), and coronal scan (C): 3 red lines indicate 3 points of FA measurement. A-PD indicates anterior–posterior diameter; FA, frontal angle.

Anterior–Posterior Dimension (A-PD) of the FSO

The A-PD of the FSO was measured using MPR between the most posterior part of the anterior buttress and the most proximal point of the skull base (Figure 3).

Statistical Analysis

Statistical analysis was performed with Prism version 7.0 software. A Shapiro-Wilk test was used to assess normal distribution of continuous variables. Continuous variables were presented as mean and SD or median with interquartile range. A Spearman rank correlation coefficient was used to compare relationships between first and second measurements of the FA and to evaluate the relationships between the FA and A-PD. A P < .05 was considered significant.

Results

Among 128 adult Caucasian patients, there were 77 females and 51 males, with a mean age of 42.3 years (range 17-86). The FA values were subcategorized into 5 types (Table 1).

Table 1.

Frontal Angle Distribution.

Angle range (degree)	Number of sinuses (%)	Number of patients (%) at least on 1 side	Number of patients (%) on both sides
FA ≥ 90° (Figure 1A)	33 (13.2)	21 (16.4)	12 (9.4)
60° < FA < 90°	91 (36.4)	64 (50)	27 (21.1)
45° < FA ≤ 60°	86 (34.4)	67 (52.3)	19 (14.8)
30° < FA ≤ 45°	30 (12)	26 (20.3)	4 (3.1)
FA ≤ 30° (Figure 1E)	10 (4)	8 (6.25)	2 (1.6)

Abbreviation: FA, frontal angle.

The median FA value was found to increase from lateral aspect of the anterior buttress (50.5°) to the level of the middle turbinate (60°; P < .001).

The mean A-PD of the FSO was 7.5 ± 2.7 mm (range 2-15 mm). The A-PD < 5 mm was found in 44 (17.6%) sinuses in 34 (26.6%) patients, and above 1 cm in 38 (15.2%) sinuses in 25 (19.5%) patients. A significant correlation was found between FA and A-PD of the FSO (r = 0.50, P < .0001; Figure 4).

Figure 4.

Correlation between frontal angle versus anterior–posterior diameter of the frontal sinus opening.

The most favorable and unfavorable combinations of FA and AP-D are presented in Table 2.

Table 2.

Most Favorable and Unfavorable Combinations of FA and AP-D.

Combinations of FA and AP-D	Number of sinuses (%)	Number of patients (%) at least on 1 side	Number of patients (%) on both sides
FA ≥ 90°, AP-D > 10 mm	15 (6)	10 (7.8)	5 (3.9)
FA ≤ 45°, AP-D < 5 mm	13 (5.2)	11 (8.6)	2 (1.6)
FA ≤ 30°, AP-D < 5 mm	2 (0.8)	1 (0.8)	1 (0.8)

Abbreviations: A-PD, anterior–posterior diameter; FA, frontal angle.

A significant correlation was found between the first and second measurements of the FA (r = 0.84, P < .0001).

Discussion

The complexity of the frontal sinus surgery was classified according to the presence of the frontoethmoidal cells and dimension of the FSO.⁴ According to this classification based on assessment of 10 selected cases, the most difficult frontal sinus to access is the one with the A-P dimension below 5 mm and the presence of frontoethmoidal cell extending into the frontal sinus. About 25% of our study population has “difficult frontal sinus” at least on 1 side, based only on a single criterion of AP-D.

Results of our study suggest that FA is another important factor that may influence the difficulty of the frontal sinus surgery. Moreover, low values of FA are more common in patients with narrow FSO, which makes the access even more challenging in these cases.

Our preliminary findings are based on CT measurements alone; however, there are data from the literature supporting our hypothesis that the possibility of inspection of the frontal sinus depends on the combination of FA and A-PD.

The line between the limen nasi and the anterior ethmoidal artery reflects the position of the endoscope rested upon the limen nasi passing just below the axilla of the middle turbinate. The angle between this line and the nasal floor is 54.5° ± 6.8°.⁵ If a 30° scope is used, the center of view is directed upward, about 85° (55 + 30 = 85°) to the nasal floor. The field of view for the scope is 90°. Thus, the maximum angle, at which anterior part of the frontal recess is still in the field of view, is at 130° from the nasal floor (90 / 2 + 85 = 130°, angle opened posteriorly). This angle corresponds to FA of 50° (180° − 130°).

For FA greater than 60° visualization of the inside of the frontal sinus with 30° scope should be clear. If FA is below 60°, visualization will be impaired and more dependent on the size and shape of the FSO. In case of FA below 30°, visualization of the sinus is rather impossible without removal of the anterior buttress (Figure 1).

Hilger et al described the angle between the FSO, frontal recess, and the infundibulum “frontonasal angle.” This angle, measured in sagittal plane, should indicate which endoscope provides the best view of the FSO.⁷ The frontonasal angle depends on the configuration of the frontoethmoidal cells, which can be opened with classic FESS tools. The FA depends on outer frame of the frontal recess and the FSO.

Gheriani et al⁸ introduced Frontal Ostium Grade (FOG), a method similar to ours, which is based on relation of 2 parallel lines: line R and line S. Line R is drawn in a sagittal plane through the medial aspect of the lacrimal sac along the vertical axis of the frontal process of maxilla. Line S is drawn at the deflection point of the anterior skull base in proximity to the FSO. If the S-line is anterior to the R-line, the access to the frontal sinus is difficult. The authors proved that the time needed to complete frontal sinusotomy is longer for these sinuses compared to those with the R-line anterior to the S-line. The drawback of this classification is that FOG is measured in only 1 sagittal plane, which corresponds to the most lateral measurement from our study. This can be misleading as the shape of the skull base and internal surface of the anterior buttress changes from lateral to medial. The “deflection point” described by Gheriani et al can be difficult to establish in patients with smooth oval-shaped upturn of the anterior skull base.

The median value of FA increases from lateral to medial about 10°. Thus, the visibility of the sinus should be on average slightly better just lateral to the middle turbinate. This implies that Draf I procedure provides better accessibility to the frontal sinus in patients with the most common, medial variant of the FSDP compared to those with lateral FSDP.

We chose the maximum FA value from 3 measurements (medial, midpoint, and lateral) for further assessments because this value is decisive for possibility of visualization of the posterior table in patients with lower values of FA. The disadvantage of this approach is that in a patient with 3 nearly equal values of FA, the insight to the sinus is better than in a patient with 1 high and 2 low values of FA. Thus, the overall possibility to visualize the frontal sinus may be overestimated in our study for lower values of FA.

Angulation of posterior table of the frontal sinus at the level of the anterior buttress may range between less than 30° and 90°. This may significantly influence the visualization of the frontal opening and sinus. Careful preoperative assessment of CT with the use of MPR allows the identification of the frontal sinuses, which are potentially difficult to access due to unfavorable angulation of the posterior table (<30°) and narrowed frontal opening (<5 mm) or a combination of these 2 features. In these cases, the need to use 45° and 70° scopes, image guidance, extended approaches such as Draf IIb or Draf III or even external procedures, is much more likely. In contrast, for patients with FA over 60° and large A-PD, it is highly likely to visualize the sinus with a 30-degree scope and maneuverability of the tools within the sinus should be satisfactory. The disadvantage of our study is lack of clinical validation. The exact values of “favorable” and “unfavorable” FA should be established intraoperatively or in a cadaveric study. The main advantage of FA measurement is that it reflects wide range of FSO configurations compared to 3-grade scale introduced by Gheriani et al.

Classification of frontal sinus anatomy for surgical purposes is challenging. Hourglass-shaped FSO narrowed by small bony protuberance of the anterior buttress can be widened without the use of powered tools in selected cases. In contrast, low skull base and massive anterior buttress may create the angulated bottleneck-shaped FSO, with thick bony walls making it difficult to access the frontal sinus without the use of the drill. Although clinically important, the difference between these 2 configurations eludes classifications based on the measurements of the FSO dimensions, FA, or the presence of frontoethmoidal cells pneumatizing into the frontal sinus.

Conclusions

The FA shows great interindividual variability, which is very likely to reflect the possibility of the frontal sinus inspection. This implies a need for further prospective clinical studies to validate FA as a predictor of difficulty in frontal sinus surgery.

Footnotes

Authors’ Note

This study was approved by the ethics committee of the Medical University of Warsaw, Poland.

Acknowledgments

The authors thank Aleksandra Gąsecka for the help with the statistical analyses.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Paulina Kołodziejczyk

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Frontal Angle: A New Predictor of Difficulty in Endoscopic Frontal Sinus Surgery—A Preliminary Computed Tomography Study