Introduction

Sigmoid notch (SN) is a deep concavity on the mandible's upper border encircled by the condyle and coronoid process1. The coronoid process and the condyle are divided by the SN, also known as the mandibular notch (MN). It allows blood to travel to and from the masseter muscle via the masseteric artery and vein. The coronoid process and mandibular condyle both have an impact on the shape of SN. Shakya et al. have described morphological variations of the SN (broad-wide, round, and sloping). These variations occur due to the genetic background or functional changes with the growth progression2.

The term ‘medial sigmoid depression’ (MSD) was first used in a study on dry human skulls by Langlais et al. in 19833. MSD occurs by reduced radiation absorption due to the bone's thinning on the lingual side of the sigmoid notch4. Foramen-like radiolucency can be seen in the upper ramus immediately below and somewhat anterior to the sigmoid notch, indicative of MSD. Bilateral or unilateral radiolucency is conceivable. It can be confused with various pathologies due to its location and appearance. Clark called this depression a pseudocyst in 1984 and examined it in one hundred skulls5.

Recently, Nyer Firdoose identified one such variation in the structural morphology of the mandibular coronoid process, which has been elaborated as the coronoid foramen in the oro-facial region6. Even though MSD has long been understood to be an anatomical variation, it might be mistaken for an abnormal coronoid foramen when seen on panoramic radiography6,7. Several other articles in the literature also mentioned MSD as a possible radiographic finding or a differential diagnosis for other pathologies8,9,10,11,12. A dentist may suggest that the patient undergo three-dimensional imaging since MSD can visually resemble a radiolucent lesion. In contrast to panoramic radiography, cone beam computed tomography (CBCT) imaging offers superior image quality and permits precise three-dimensional visualization of anatomic variations like mandibular accessory foramina and minute nutrient canals. This is because real anatomy exists in three dimensions6. Although salivary gland-related cortical defects (stafne bone defect) have been reported to occur in more posterior and/or inferior locations in the medial or lateral aspect of the mandibular ramus, Defects involving the ascending ramus have also been reported. Thus, while making an MSD differential diagnosis, these lesions should also be considered13,14,15.

MSD area is thin; therefore, it may increase the potential for complications in orthognathic surgery16. While performing orthognathic surgery, particularly sagittal split osteotomies and gross bone resections for mandibular corpus cancers, MSD is crucial in determining the osteotomy locations with the lowest risk of fracture17. Furthermore, it has been shown that MSD is linked to high levels of muscular activity, which raises the risk of recurrence after orthognathic surgery18. In addition, the fusion of the medial and lateral cortical plates induced by MSD may raise the risk of difficult or undesirable fractures18. It needs no additional radiological study or intervention if no surgical treatments are planned at this spot (such as sagittal split osteotomy orthognathic surgery). Surgery complications might be avoided by knowing this anatomical landmark's prevalence and shape variability19. Carvalho et al. classified the morphology of MSD into tear-drop, semilunar, circular, and triangular4. Most studies investigating MSD have focused on panoramic radiographs, which have several limitations, such as distortions, superpositions, and magnifications. The sigmoid notch region is displayed on the panoramic images along with the nasopharyngeal airway shadow, pterygoid plate, and soft palate. Therefore, the observer may overlook medial sigmoid depression19. On the other hand, cone-beam computed tomography (CBCT) devices providing three-dimensional (3D) imaging eliminated these limitations.

MSD is a relatively recently described anatomical variation. Because of this region's fragility, separating the ramus will likely be more challenging16. Despite being in a location at risk for surgical complications, there are not many studies that assess this anatomical variation4,20,21. Moreover, no studies can be found comparing the MSD on CBCT and panoramic. Whether panoramic imaging, which contains a lower radiation dose than CBCT imaging, is sufficient to detect this structure must be determined. The morphology of anatomical structures is affected by nearby structures. The MSD is just in front of the SN. We could not find a study examining the association between the SN and MSD presence and morphology. This study aimed to assess the prevalence of MSD area and whether the prevalence differs between panoramic and cone beam computed tomography (CBCT) images. Additionally, this study aims to assess different sigmoid notch types and determine whether there is a relevance between sigmoid depression and notch morphology.

Material and methods

This study was performed retrospectively. The Hacettepe University's Local Ethics Committee approved the experimental procedure (GO 21/705). For the sole purpose of this study, no individuals were exposed to radiation, the included images had been acquired for routine therapies and clinical evaluations in 2021. The CBCT images were obtained using an i-CAT Next Generation CBCT scanner (Imaging Sciences International, Hatfield, PA, USA) with the following settings: 5 mA (tube current), 120 kVp, 14.7–17.8 s (exposure time), 16 × (8–13) cm and 23 × 17 cm (fields of view), and 0.20–0.30 mm (voxel size). All CBCT scans were obtained in a standardized head posture (the Frankfort plane parallel to the floor). The CBCT scans were reconstructed 3-dimensionally so that they could be sectioned at any plane and position. The panoramic images were obtained using a Veraview IC5 HD (Morita Corporation, Osaka, Japan) with the following settings: 12 mA, 18 s, and 70 kV. CBCT and panoramic images were selected from the digital archive of the Department of Dentomaxillofacial Radiology, Faculty of Dentistry, at the University of Hacettepe. The inclusion criteria were individuals older than 18 years, panoramic and CBCT imaging availability, and clear imaging of both full sigmoid notches. The exclusion criteria were: A lesion or tumor in the SN region, or poor imaging quality (asymmetrical magnification, inappropriate patient positioning, and blurring).

Panoramic and CBCT images of 129 individuals met the inclusion criteria. The same observer (maxillo-facial radiologist with 5 years of experience) evaluated anonymous CBCT and panoramic images separately. The same computer and darkroom were used to review every image. The observer could only adjust the brightness and contrast to evaluate. The assessments of the CBCT images were conducted on the CBCT reformatted panoramic screen and axial section. Frankfort Horizontal was utilized to standardize head locations anteroposteriorly in the reconstructed images. Maximal overlap of bilateral components in the mandibular body, ramus, and maxilla was used in sagittal orientation. By choosing a unique focal trough that extended posterior to the condyles and went through the coronoid process, panoramic images were generated from CBCT volumes. Focal trough width was varied to ensure it encompassed the entire length and height of the ramus mandibula. Axial serial slices were reviewed to ensure the focal trough encompassed complete of the ramus mandibula. The right and left sides were assessed.

Firstly, the presence of MSD was confirmed in the axial section of the CBCT image then the presence or absence of MSD was recorded on the reformatted panoramic screen (Fig. 1).

Figure 1
figure 1

Confirmation of MSD presence in CBCT axial section.

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Secondly, if it exists, the shape is noted. The literature has characterized the geometric shapes of MSD as tear-drop, semilunar, circular, and triangular, which are the kinds evaluated for interpretation4 (Fig. 2).

Figure 2
figure 2

The cropped panoramic image of the MSD (a showing tear-drop shape, b showing semilunar shape, c showing circular shaped, d showing triangular shape; The cropped CBCT panoramic image of the MSD (e showing tear-drop shape, f showing semilunar shape, g showing circular shape, h showing triangular shape).

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Thirdly, the sigmoid notch morphology was analyzed by the classification given by Shakya et al.2 (Fig. 3) as sloping, wide and round and noted. The aforementioned forms were also noted using panoramic images.

Figure 3
figure 3

The cropped panoramic image of the SN (a showing sloping shape, b showing wide shape, c showing round shape); Cropped CBCT reformatted panoramic image of the SN (d showing sloping shape, e showing wide shape, f showing round shape).

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Twenty-five randomly selected CBCT and panoramic images were re-evaluated to explore intra-observer consistency three weeks after the first evaluation.

Statistical analysis

A non-parametric framework was employed for the analysis of the qualitative data in order to determine the significance of research parameters on a categorical scale between two or more groups using the Chi-Square/Fisher Exact test. The Fisher Exact test was applied to examine the normal distribution of the variables with small cell samples. The significance level for interpreting the data was set at 0.05; when p 0.05, a significant relationship between variables was found. However, when p > 0.05, there was no significant correlation with the variables on a categorical scale. Cohen's kappa coefficient was calculated for intra-observer consistency.

McNemar’s and McNemar Bowker tests compared the findings detected on panoramic and CBCT images, with a significance level of 0.05 (α = 5%). The images were analyzed using SPSS (Statistical Package for Social Studies) version 23.00. The statistical significance was set at p˂ 0.05.

Ethical approval and ınformed consent

The Local Ethics Committee of Hacettepe University, retrospectively registered, with the number of GO 21/705. All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2008 (5). Informed consent was obtained from all patients for being included in the study.

Results

Among 129 individuals consisting of 258 sides, the prevalence and morphology of MSD and the sigmoid notch (SN) morphology were evaluated using CBCT and panoramic images. The sample comprised 48 males and 81 females, aged 18 to 78 years (mean age of 46.6 ± 15.4 years). Except for gender, all data and tables are provided for 258 sides (n = 258). There is no distinction between right and left.

The Kappa test was used for intra-observer variability, and The Kappa value was found between 0.75 and 0.85, showing a near-perfect agreement for CBCT and panoramic images.

The prevalence of sigmoid notch (SN) morphology was as wide at 51.9% (n = 134), followed by a round at 29.1% (n = 75), and sloping at 19% (n = 49) in CBCT. The prevalence of sigmoid notch morphologies was detected as wide in 47.7% (n = 123), followed by a round in 31% (n = 80), and in sloping 21.3% (n = 55) in panoramic.

MSD was more prevalent in females than males in both techniques, but this difference was not statistically significant. Table 1 shows the prevalence of MSD on panoramic and CBCT images with regard to gender.

Table 1 The prevalence of MSD on panoramic and CBCT images according to gender.
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The absence and presence of MSD on both techniques were insignificant (Mc nemar p = 0.81 Table 2).

Table 2 The prevalence of MSD in panoramic and CBCT imaging.
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MSD was detected in 63.3% (n = 164) of CBCT images. Triangular was the most prevalent morphology identified in 36% (n = 59) of MSD by semilunar in 34.1% (n = 56), tear-drop in 24.3% (n = 40), and circular in 5.5% (n = 9). MSD was detected in 62.4% (n = 161) of panoramic images. Triangular was the most prevalent morphology identified in 37.2% (n = 60), followed by tear-drop 33.1% (n = 53), semilunar in 24.2% (n = 39), and circular in 5.5% (n = 9). The incidence of MSD shape was not significantly different between both imaging modalities (McNemar Bowker p = 0.07 Table 3).

Table 3 The prevalence of MSD shape on panoramic and CBCT imaging.
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There was no association between the shape of MSD and the morphology of the sigmoid notch in both techniques (Table 4 for panoramic imaging, Table 5 for CBCT imaging).

Table 4 The relationship between the shape of MSD and the morphology of the sigmoid notch on the panoramic image.
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Table 5 The relationship between the shape of MSD and the morphology of the sigmoid notch on CBCT.
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Discussion

We retrospectively assessed MSD and SN in panoramic and CBCT images in the present study. In order to evaluate the repeatability of the method in CBCT and panoramic images, the intra-observer agreement was evaluated, and the consistency was found to be relatively high in all of the evaluations. The panoramic technique is widely used as a dental imaging modality. Various studies of SN and MSD on panoramic images have evaluated the relationship between demographic and etiologic factors2,20.

Developmentally, the coronoid process, mandibular condylar process, and mandibular notch configurations are interconnected22. The mandibular notch formation is highly reliant on the coronoid and condylar processes contours23. Nevertheless, genetics, hormones, eating habits, and temporalis muscle activity may all affect how these structures shape morphologically. In the literature, there have been previous researches in which various configurations of these structures have been demonstrated23,24,25. According to the study Shakya et al., SN was classified into the following types: wide, round, and sloping2. Similar to Sahithi et al. and Manoj et al.'s studies, our study observed the most common type of wide SN on panoramic images23,25. By contrast, Shakya et al. and Dar et al., reported sloping SN as the most frequent type2,26.

In our study, the most common type of wide SN was observed on CBCT, similar to panoramic images. By contrast, Tassoker et al. reported round SN as the most frequent type27. Nevertheless, different sections were evaluated in two studies. Unlike our study, we could not find a study evaluating sigmoid notch in the reformat panoramic screen of CBCT.

The association between MSD and SN morphology was evaluated in our study. No previous research has investigated the association between SN and MSD. No statistically significant relationship was found in either panoramic images or CBCT.

The present study's 62.4% prevalence of MSD in panoramic images was higher than the associated findings from Langlais et al. (10%), Carvalho et al. (20.3%), Kumar et al. (23.2%), and Kang et al. (33%)3,4,19,28. Like us, Asdullah et al. reported a prevalence of 70% for MSD in panoramic20. Varying techniques and the subjectivity of radiographic interpretation might explain these discrepancies. MSD may not be seen due to positioning errors20. According to Langlais et al., when the MSD is absent, it may occasionally be because the location is not in the focal trough3.

According to the study Carvalho et al., MSD was classified into the following types: tear-drop, semilunar, circular, and triangular. Similar to Carvalho et al.’s study, triangular MSD was the type most frequently seen, whereas circular shape was the least frequently seen on panoramic images4. By contrast, Asdullah et al. reported semilunar, the type most often20.

Storey et al. have suggested that the size and shape of the depression may be a result of variations in muscle function29. The functional adaptation in the ramus caused by the insertion of the medial and posterior attachments of the temporal muscle into this region plays a significant role in characterizing the features of MSD. Adisen et al. found higher maximum biting force values in MSD patients, demonstrating a correlation between MSD and maximum bite force18. We could not evaluate the subjective factors affecting muscle activity since our investigation was retrospective.

MSD can be confused with various pathologies due to its radiolucent appearance. As a potential radiography finding or as a differential diagnostic for other conditions, MSD was mentioned in passing in several additional literature articles8,9,10,11,12. Although rare, studies are reporting that stafne bone cyst is detected in the ascending ramus. It has been reported that MSD may be confused with a stafne bone defect. However, these conditions can be easily distinguished on panoramic radiographs13,14,15.

Considering that MSD can mimic a nonodontogenic cystic lesion in appearance, a dentist may refer the patient to undergo three-dimensional imaging. CBCT yields three-dimensional (3D) images, eliminating limitations, and thus has attracted increasing interest. CBCT-reformatted panoramic images used in the current study were free of magnification, superimposition of surrounding structures, and other issues endemic to panoramic radiography since they were reformatted slices of the maxilla and mandible30. CBCT equipment is easily accessible in many dental clinics. For this reason, the possibilities of acquiring CBCT images from the regions that clinicians consider suspicious have become considerably easier. Nonetheless, it is essential to carefully consider if the patient's request for an advanced examination is necessary and whether it is preferable to alternative methods.

MSD can be misinterpreted as a coronoid foramen. Even after a CBCT examination MSD may still be misinterpreted as a foramen because of inappropriate thresholding settings in transparent volume-rendered images, or it may be misinterpreted as a break in the bone surface in computed tomography (CT) sections because of metal artifact from metallic objects in the oral cavity, such as dental prosthetic components or filling materials6,7. The threshold, opacity, and translucency settings in volume-rendered (VR) CT images were selected to display structures deep within the bone surface; consequently, MSD can be erroneously interpreted as a foramen since very thin bone is not visible in such VR images because of the thresholding process13,31. The detection of pseudo foramina in VR CBCT images has been classified by the European Commission Guidelines on CBCT as a post-acquisition manipulation error because of improper thresholding, and operator training in the proper application of windowing controls is advised32. Multiplanar reformatting (MPR) images provide the most accurate results when evaluating thin bone, with measurements that have the highest agreement with anatomical findings33. Volumetric reformatting of CT has been shown to be less accurate than MPR, and it should therefore be borne in mind that such reformatting of CT or CBCT examinations is for general visualization purposes only and not for diagnosis and analysis. For this reason, volumetric reformatted images were not used in our study13,34.

The present study's prevalence of MSD was 63.6% in CBCT images. We found that Hasani et al. was the only author to analyze the prevalence and morphology of depression on CBCT images. Like us, Hasani et al. reported a prevalence of 60.45% for MSD21. In our study, triangular MSD was most frequently observed, while circular was less frequently observed in CBCT. By contrast, Hasani et al. reported semilunar, the type most often. Hasani et al. assessed the reconstructed 3D images, while we assessed the reformatted panoramic images.

We evaluated whether CBCT imaging was needed to detect MSD. The prevalence of MSD in CBCT and panoramic images showed remarkably similar results in our investigation. Although we found that MSD prevalence was high and similar in panoramic and CBCT images, this finding was insignificant. The presence of MSD identified in CBCT images was sometimes not detected in panoramic images or MSD detected in panoramic imaging could not be confirmed in CBCT images, which is the gold standard for identification. The differences in the results may be related to the subjectivity of radiographic interpretation of the morphology due to the panoramic image techniques limitations and the observer's skill. It is important to evaluate intraobserver and interobserver agreement in radiographic studies. Although intraobserver agreement was found to be high in our study, the fact that interobserver agreement was not evaluated is one of the limitations of the study. MSD In our study, triangular MSD was the most frequently seen, whereas circular shape was the least frequently seen on panoramic and CBCT images. By contrast, the prevalence of tear-drop and semilunar shapes of MSD differed markedly in panoramic and CBCT images. The MSD shape was quite difficult to identify both on panoramic images and on CBCT images. These results can be impacted by the expert's subjectivity and experience in interpreting the radiograph to define the shape.

In addition, ethnic differences, variances in the device used, discrepancies in the method used to analyze MSD, and variations in sample size may all contribute to the disagreements in the results.

The similar prevalences in the two methods resulted in the conclusion that advanced imaging is not necessary to identify MSD. However, CBCT imaging may be necessary to obtain more information about the MSD morphology for surgical procedures.

Conclusion

According to our results, the most common SN shape was wide observed in panoramic and CBCT images. We found the prevalence of MSD to be high and similar in panoramic and CBCT. There was no correlation between SN morphology and MSD shape or prevalence in both techniques. In our study, triangular MSD was the most frequently observed, while circular was less frequently observed in CBCT and panoramic. We consider that the similar prevalence of the two methods reveals that the panoramic image has been sufficient to define MSD. The high prevalence of MSD showed that it is essential for clinicians to define this anatomical variation well. If surgery is needed, patients with MSD could need further investigation with CBCT.