Analysis of Olfactory Fossa Anatomy using Cone Beam Computed Tomography

1. Postgraduate Student, Department of Oral Medicine and Radiology, Sinhgad Dental College and Hospital, Pune, Maharashtra, India. 2. Reader, Department of Oral Medicine and Radiology, Sinhgad Dental College and Hospital, Pune, Maharashtra, India. 3. Professor, Department of Periodontics, Sinhgad Dental College and Hospital, Pune, Maharashtra, India. 4. Professor and Head, Department of Oral Medicine and Radiology, Sinhgad Dental College and Hospital, Pune, Maharashtra, India.

Abstract

Introduction: Olfactory fossa is an important structure in the anterior skull base. It is made of Lateral Lamella of Cribriform Plate (LLCP) and fovea ethmoidalis which are very delicate parts that can get damaged during surgical procedures causing numerous serious complications. To avoid such complications, the knowledge of anatomical variations of these parts is mandatory. As there are very few studies which have used Cone Beam Computed Tomography (CBCT) for evaluation of olfactory fossa, the authors have assessed the olfactory fossa with the help of Kero’s and Gera’s classification for lateral lamella of cribriform plate.

Aim: To evaluate the anatomy of olfactory fossa using CBCT.

Materials and Methods: A retrospective observational study was conducted in the Department of Oral Medicine and Radiology, from 1^st January 2019 to September 2021. The CBCT scans of 107 adults were analysed to evaluate the height of LLCP according to Kero’s classification and the angle between the LLCP and the true horizontal plane according to Gera’s classification. Comparison of height of lateral lamella (mm) and Gera angle (degree) among right and left sides were done using Student’s Independent t-test. The comparison of Gera angle with Kero’s type was analysed by one way Analysis of Variance (ANOVA) test. Chi-square test was performed to assess categorical data on the right and left sides.

Results: A total of 107 CBCT scans consisting of 59 males (55.14%) and 48 females (44.86%) were included in the study. The mean age of the study population was 31.60±11.17 years. Kero’s type II (69.6%) and Gera’s class II (92.1%) LLCP were found to be most commonly seen. The Gera angle on right side (60.26±9.84) was greater than on left side (56.38±10.16) which was statistically significant (p-value=0.005). On right side, Kero’s type II was more common in males (71.2%) and on left side, Kero’s type II was more common in females (77.1%).

Conclusion: Kero’s type II Olfactory fossa (OF) was found as most common type, with no significant difference between gender and /or side. Similarly, class II Gera angle was most common and the values were higher on right side.

Ethmoid roof, Fovea ethmoidalis, Gera angle, Kero type, Lateral lamella of cribriform plate

Olfactory fossa (OF) is a depression or valley present in an anterior cranial cavity. A floor of the OF is formed by ethmoid bone and cribriform plate (1),(2). It is laterally bounded by Lateral Lamella of Cribriform Plate (LLCP) and medially by crista galli (1),(3). The lateral lamella where ethmoid artery penetrates anterior cranial fossa is known as the thinnest and delicate bone present in the anterior skull. This bone is dehiscent in about 14% of the population (4),(5),(6). The LLCP laterally articulates with fovea ethmoidalis (FE). The FE which is a part of frontal bone helped in the development of roof of the ethmoid bony labyrinth (2),(7). Thus, LLCP and FE are one of the most delicate and vulnerable parts of the skull base which can contribute to various complications while performing endoscopic surgeries or surgeries of paranasal sinuses (8),(9),(10).

Functional Endoscopic Sinus Surgery (FESS) is the frequently used technique to treat chronic or recurrent sinusitis, mucoceles, CSF leak closure, nasal polyposis, sellar tumors, optic nerve decompression, management of epistaxis and epiphora originated by lower lacrimal duct obstruction (11),(12),(13). Serious complications like herniation of orbital fat, injury to extraocular muscles, optic nerve injury and intracranial injuries like injury to major blood vessels, etc. can occur in 0-1.5% of the cases (5),(13). A thorough knowledge of different anatomical variations in patients will help to reduce the unwanted iatrogenic complications by knowing the risks while performing surgeries (14).

The term “dangerous ethmoid” was introduced to define the position of anterior ethmoid artery and the position of lateral lamella in relation to the cribriform plate (3),(11),(15). The relationship between OF and ethmoid roof was studied by Kero in 1962 and he derived a three category classification system for assessing the depth of OF in relation to ethmoid roof (16). Even if the role of OF depth has been given importance earlier, only limited data regarding the slope of the anterior skull base, particularly the angulation of the LLCP in the coronal plane are known. This angulation might have a role when approaching the paranasal sinuses during the dissection of more medial ethmoidal cells (17). A classification system given by Gera which focuses on the angle formed by LLCP and the horizontal plane passing through cribriform plate may serve this purpose. Some authors suggests that the risk of complications is directly proportional to the depth of OF, while others propose that the complications are closely associated with the angle between LLCP and horizontal plane (15),(17),(18).

Though Multislice Computed Tomography (MSCT) is the gold standard in the estimation of anatomy and pathologies of cranial structures, paranasal sinuses and nasal cavity, CBCT can definitely be used as a promising alternative by dental surgeons and otolaryngologists to assess cranial and nasal structures and paranasal air sinuses. The CBCT is preferred because of its advantages such as low radiation exposure, high quality images with lower costs (19),(20),(21). Though the role of radiographic analysis of OF was emphasised earlier, there is very limited data available till date. Thus, the aim of this study was to assess the anatomy of OF using CBCT.

Material and Methods

A retrospective observational study was conducted in the Department of Oral Medicine and Radiology using CBCT scans taken during the period of 1^st January 2019 to 31^st January 2021. The data collection including the statistical analysis and interpretation was done from February 2021 to September 2021. The study protocol was reviewed and approved by the Institutional Review Board of Ethics Committee (SDCH/IEC/2021/49) performed according to Helsinki Declaration guidelines.

The CBCT scans of 107 adults above 18 years were obtained retrospectively from the Department of Oral Medicine and Radiology. The CBCT images obtained using a Promax 3D unit (Planmeca, Helsinki, Finland), operating at 84 kVp, 9-14 mA, with a 0.16 mm voxel size, exposure time of 12 seconds and a field of view of 8×8 cm².

Inclusion and Exclusion criteria: The CBCT images showing the medium and superior regions of the face including the crista galli of the ethmoidal bone and nasal fossa were included in the study. Patients having a history of maxillofacial trauma, pathologies in the paranasal sinuses and paranasal sinus surgery were excluded. The CBCT scans with inferior quality images or images with artifacts, producing visualisation of anatomical forms difficult were excluded.

Study Procedure

Two investigators evaluated the CBCT images with inbuilt software (Planmeca, Romexis viewer 4.3.0.R) on a 24-inch Nvidia Quadro FX 380 screen with 1280×1024 resolution in a quiet room with subdued ambient lighting.

Investigators were checked for intra and inter-examiner variability to refine the intra and interpersonal reliability. Before the study analysis in between each measurement both examiner examined all measurements with an interval of one week. The average was considered, if the variability in between two examiners was set up to be upto 10%. If the variability was more than 10%, another investigator reassessed it. The coronal slices (thickness: 1 mm) were used and linear measurements were done by using the software’s ruler to evaluate the following parameters:

1. Kero’s classification: Olfactory fossa depth was decided by the Height of lateral lamella of cribriform plate , measured as the distance between fovea ethmoidalis (F) to cribriform plate (P) of ethmoid bone using the Kero’s classification (Table/Fig 1) (16).
(i) Type I-height lower than 3.0 mm
(ii) Type II-height between 4.0 and 7.0 mm
(iii) Type III-height between 8.0 and 16.0 mm

2. Gera’s classification: Angle between lateral lamella of cribriform plate and the continuation of the horizontal plane passing through cribriform plate was measured using the Gera’s classification (Table/Fig 2) (17).

• 80°, low risk)
• Class II (45 to 80°, medium risk)
• Class III (<45°, high risk)

Statistical Analysis

The data analysis was performed using Statistical Package for Social Sciences (SPSS) version 23.0. The mean and percentage were used to assess the prevalence and gender distribution. Comparison of height of lateral lamella (mm) and Gera angle (degree) among right and left sides were done using Student’s Independent t-test. The comparison of Gera angle with Kero’s type was analysed by one way ANOVA test. Chi-square test was performed to assess categorical data on right and left side. Spearman’s correlation test was applied 33to find the correlation between Kero’s and Gera’s Class. The p≤0.05 was considered as statistical significant.

Results

A total of 107 CBCT scans consisting of 59 males (55.14%) and 48 females (44.86%) were included in the study. The mean age of the study population was 31.60±11.17 years.

The mean height of LLCP was 4.63±1.68 mm. The mean Gera angle was 58.32±8.50°. The mean height of LLCP in males was 4.69±1.76 mm and females 4.55±1.59 mm, respectively. The mean Gera angle was 59.03±8.56° in males and females 57.44±8.43°, respectively. The difference was not statistically significant (Table/Fig 3).

The height of LLCP on the left side (4.73±1.91 mm) was slightly higher than the right side (4.52±1.87 mm) but the difference was not statistically significant (p-value=0.401). Similarly, the Gera angle on right side (60.26±9.84°) was greater than the left side (56.38±10.16°) which was statistically significant (p-value=0.005) (Table/Fig 4).

On comparing the sides, Gera angle and height of lateral lamella between males and females showed no statistically significant difference (Table/Fig 5).

The Kero’s Type II (69.6 %) was most commonly seen. The Gera Type II (92.2%) was more common followed by Type III (4.7%) and Type I (3.3%), respectively (Table/Fig 6).

On comparing with sides, Kero’s Type II was prevalent in the population studied followed by type I and III. The difference in Kero’s type between right and left side was statistically non significant (p-value=0.678) among the types. Similarly, Gera’s Class II angle was prevalent followed by Class III and Class I. However, there was no statistically significant difference in Gera angle between right and left side (p-value=0.085) among the all the classes (Table/Fig 7).

Kero’s type II was more prevalent in males than females on right side. The comparison of different types according to gender was statistically non significant. Similar findings were noted on left side (Table/Fig 8).

There was no significant difference in Gera angle between males and females on both sides (p-value=0.281) and (p-value=0.097). Gera Class II angle was slightly more prevalent in males than females but the difference was statistically non significant (Table/Fig 9).

There was no significant difference in mean age among Kero’s types and Gera angle between right and left side (p-value >0.05) (Table/Fig 10),(Table/Fig 11).

On left side, there was a statistically significant difference when Kero’s type was compared with Gera angle (Table/Fig 12). When the height of LLCP was compared between the Gera’s type, statistically significant (p-value=0.047) was found on the right side (Table/Fig 13).

(Table/Fig 14) shows negligible non significant correlation between Kero’s and Gera classification.

Discussion

An ethmoid roof is one of the most complex and delicate bones in the skull. The thinnest structure where lateral lamella of cribriform plate attaches to middle turbinate is considered as “locus minoris resistentiae”. Understanding the complex anatomical relationship of the ethmoid roof, the anterior skull base and olfactory zone is of paramount importance as the knowledge of these structures will avoid unnecessary complications (2),(10),(12),(17),(18),(19),(22).

The CBCT can be used as a potent alternative to CT for analysis of paranasal sinuses and adjacent structures as it gives high resolution spatial images with remarkable reduction in radiation dosage to the patient (19),(20),(21),(23). In the present study, viewing that the coronal plane was best for estimating, the ethmoid roof anatomy, CBCT images were selected that came up with detailed information about this segment that normally presents many dissimilarities (right and left side) in a same individual and as a result, risks were minimised in surgical interventions (12),(22).

The levels of the ethmoid roof and cribriform plate can vary considerably in different individuals and also in the same individual on right and left sides (3),(4),(5),(7),(10),(15). Analysing the Kero’s types, based on population, it is seen that majority of the studies have reported Kero’s type II to be more prevalent (Table/Fig 15) (1),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12),(13),(14),(15),(17),(20),(22),(24),(25),(26). Shows the list of various authors that have studied the depth of OF using the Kero’s classification.

Likewise, studies reported from India have also found Type II to be more prevalent except for a study by Deepa G and Shrikrishna BH., (28) who reported Type I to be more prevalent (1),(13),(27),(28). The Kero’s type III is considered to be the most vulnerable for iatrogenic injuries due to its long length of lateral lamella (2),(28). In the literature Kero’s type III has been reported to be ranging from 0-32.3% (14),(22),(25). In this study, Kero’s type III was reported to be 11.2% which is slightly higher than previously reported in Indian population (1),(13),(27),(28). Few authors have reported difference between Kero’s type and gender (1),(4),(5),(7),(8),(9),(11),(14),(27), while others have reported no difference which was in agreement with the present study (15),(17),(20).

The possibility of injury to the skull base increases with increasing height of LLCP. As the LLCP height increases, OF will become narrower and deeper; also the ethmoid roof will be more low lying (2),(28). The mean height of LLCP was 4.63±1.68 mm in the current study which is slightly lesser than that reported in the previous studies by Skorek A et al., (15) 5.77 mm, Erdem G et al., (25) 6.1 mm; Meloni F et al., (29) 5.9 mm and Jacob TG et al., (3) 4.9 mm, respectively (Table/Fig 16) (1),(8),(10),(13),(25),(28).

Studies reported from India by Babu AC et al., (5.26±1.69 mm) Murthy AV and Bollineni S (5.21 mm) have shown mean height to be more than that reported in the present study, except that reported by Deepa G and Shrikrishna BH (3.3±1.63 mm) (1),(13),(28).

In the present study, no difference was noted in the height of OF between the right and left sides which is in accordance to the previous studies (3),(9),(13),(15),(20),(25),(29). It is possible to speculate that the differences in the anatomic development in the ethmoidal roof might be related not only with heredity, environmental factors and previous chronic infections that could have affected the development of the sinuses, but also with ethnicity (30). Similarly, according to Jacob TG et al., (3) the degree of pneumatisation of frontal sinus and ethmoid labyrinth varies in different populations which could be a factor for different results (3). Furthermore, differences in technique may influence the apparent measurements.

Some studies have demonstrated that the LLCP is uniformed in less than 50% of people, and that this asymmetry is associated with flattening of the Fovea Ethmoidalis (FE) with angulation of the LLCP, which may result in surgical difficulties (4),(12). Whereas other authors noted a high percentage of asymmetry and describe a wide range of 9.5-93% (3),(4),(5),(6),(7),(10),(15),(31),(32). In the present study no asymmetry was seen on the right and left sides. Shorek A et al., (15) and Abdullah B et al., (14) observed that only Kero’s classification was not enough to identify the ‘dangerous ethmoids’; since the Kero’s classification does not describe the risk of intracranial entry. So, the angle between the LLCP to horizontal plane, which is called as Gera angle was proposed to analyse the theoretical risk of iatrogenic injuries (2),(12),(14),(18).

In the present study, the mean degree of Gera angle was reported as 58.32±8.50º which is lesser than reported in the previous studies (Table/Fig 17) (14),(17),(18),(26).

Class I angle denotes a higher risk of iatrogenic injury followed by Class II and Class III (17),(26). In the present study, most commonly reported Gera angle was Class II (92.1%) followed by Class III (4.7%) and Class I (3.3%) respectively which is in accordance with previous studies (14),(17),(18). The prevalence of Class II in the present study is more as compared to previous studies (Table/Fig 17) (14),(17),(18). Also, there was no significant difference in Gera angle with gender which is in accordance to previous studies (14),(17).

Thus, it can be noted that individuals classified as low risk according to Kero’s classification may show high risk according to the Gera classification. Considering the risk of iatrogenic injury, in this study authors reported 16.7 % having Gera Class III as Kero type III on left side which is not in accordance with the previous study (14).

In the present study we found no significant correlation between Kero and Gera classification which is in accordance with Abdullah B. et al., (14) while Gera R et al., (17) found a positive correlation. This difference could be due to anatomical variation in different populations.

Limitation(s)

The sample size was small with unequal distribution among gender including age wise distribution of study participants was limitation of the study.

Conclusion

Thus, in the present study, Kero’s type II OF was found as most common type, with no significant difference associated with gender and/or side. Similarly, class II Gera angle was also most common and the values were higher on right side with no difference in age and gender. The occurrence of substantial relationships between multiple ethmoidal measurements emphasises the need of examining more than simply the height of the ethmoidal skull base. Authors believe that the implementation of categorisation systems could be effective in the preoperative assessment of ethmoid sinus imaging in order to avoid serious issues.

Large scale collaborative studies including classification systems that are based on axial, sagittal and coronal planes are required to improve our knowledge of anatomical variations and their distribution. The estimate of the depth of the olfactory fossae and ethmoidal roof asymmetry presence should be included in routine description of CBCT reports because it constitutes a significant facet in endoscopic surgeries. Further prospective longitudinal studies or retrospectively analysing patients who showed iatrogenic complications during surgical procedures will help to validate the postulated results.

DOI and Others

DOI: 10.7860/JCDR/2022/53527.16494

Date of Submission: Dec 03, 2021
Date of Peer Review: Feb 18, 2022
Date of Acceptance: Apr 02, 2022
Date of Publishing: Jun 01 2022

AUTHOR DECLARATION:
• Financial or Other Competing Interests: None
• Was Ethics Committee Approval obtained for this study? Yes
• Was informed consent obtained from the subjects involved in the study? NA
• For any images presented appropriate consent has been obtained from the subjects. NA

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ETYMOLOGY: Author Origin