INTRODUCTION
The sinus tympani (ST) is a depression localized in the retrotympanum (the posterior area of the tympanic cavity) and forms its medial portion. The mean depth of the ST in adults was estimated to be 2.6–2.7 mm, but there are also reports stating that it can reach even up to 10 mm [8, 16, 41]. The ST lies below and posteriorly to the promontory, posteriorly to the round window, extends medially to the pyramidal eminence, stapedial muscle, and facial canal (its mastoid segment) and laterally to the posterior semicircular canal [19]. The superior boundary of the ST forms the ponticulus, whereas the subiculum is the inferior boundary. The subiculum is the posterior extension of the promontory that separates the ST from the round window [34].
Many studies have emphasized the clinical importance of the ST because of pathological processes which may occur inside, and highlight difficulties in surgical approaches and endoscopic explorations of this region of the middle ear [5, 7]. One of such pathologies is cholesteatoma that is an abnormal accumulation of keratinized squamous epithelium inside the ear. It has a destructive effect on the auditory structures causing their inflammation that might eventually lead to erosion of the bone. Therefore, a timely treatment is required, involving surgical removal of the cholesteatoma [48].
Epidemiological data revealed that cholesteatomas occur more often in Caucasian than African or Asian populations. The cholesteatoma affects all age groups. The annual incidence of cholesteatoma is estimated to be about 3 per 100,000 in children and about 9 to 12 per 100,000 in adults, with a male predominance (1.4:1) [39]. However, its occurrence has changed over time, and varies across countries showing different annual trends [27, 47]. Moreover, this disease can manifest unilaterally or bilaterally. The bilateral occurrence of cholesteatomas has been found in 10–20% of cases, male children and adolescents are affected more often than females [47].
Although cholesteatoma has been recognised for more than three centuries [20], the nature of this disorder has yet not been fully determined, particularly the factors regulating its progression [28]. Some studies revealed that sex predilection for cholesteatoma might be dependent on the expression of female sex hormone receptors in the middle ear that act as an auditory protectant [43]. It has also been noted that various expression of genes may control pathogenesis of cholesteatomas [3, 29, 40].
Pathophysiological and epidemiological data encourage undertaking various studies aimed at identifying factors that favour implantation of cholesteatoma, and those that restrict its complete removal from the middle ear. It is particularly true for its excision from the ST which is regarded as the so-called “hidden structure in the ear,” and endoscopic and surgical access to its interior can be troublesome. Therefore, detailed three-dimensional (3D) presentation of the ST topography and anatomy may enhance perspectives in treating cholesteatoma, which often invades this part of the middle ear.
In the recent years, high-resolution computed tomography (CT) and endoscopic examinations have been used to show the internal anatomy of the ear [14, 15]. These imaging techniques allow to perceive morphological changes related to the ear pathologies or structural malformations important from the clinical point of view. Notwithstanding, some anatomical details can be omitted in visual assessment or hardly recognised due to limited spatial imaging attained in clinical tomography or due to limited field of view during an endoscopic inspection.
In pre-clinical studies such technical limitations of viewing anatomical structures in detail can hinder appropriate evaluation of the morphological variants, especially if their dimensions are in range of a millimetre or even less. Formerly, only manual sectioning of the ear could reveal the entire spectrum of its morphological details and varieties [25]. Nowadays, this tedious and time consuming procedure has been substituted by the micro-computed tomography (micro-CT) which delivers images with microscopic precision. Hence, series of the micro-CT scans allows to perform 3D reconstructions of the studied samples and to section them virtually in order to display their internal morphology [18, 44].
Therefore, in the present study the authors applied micro-CT imaging of the ST, so as to display its anatomy in the 3D mode. The said approach facilitated multidirectional viewing and comparison of its morphological features, allowing for their subsequent assessment and classification of possible variations.
MATERIALS AND METHODS
Three-dimensional imaging of the ST was performed on 44 adult temporal bones (25 male and 19 female of Caucasian origin) whose age at death ranged from 30 to 70 years. The examined bones have been dated back to the XVIIth century. In order to obtain the temporal bones, we used fragmented human skulls found during archaeological excavations conducted in the Southern Poland that were deposited in the Department of Anatomy of the Jagiellonian University Medical College. Henceforth, no skulls were neither deliberately damaged nor destroyed in order to conduct the present study. The specimens were well preserved and did not show any pathological changes, nor deformations that would exclude them from the present study.
The petrous part was cut off from each temporal bone and scanned with micro-CT using a Nanotom 180N equipped with 180 kV/15 W ultra-high performance nanofocus X-ray tube, with set parameters: I = 250 µA and V = 70 kV (GE Sensing & Inspection Technologies Phoenix X-ray Gmbh). The tomograms were registered on a Hamamatsu 2300 × 2300 pixel detector.
The reconstructions of the scanned specimens were created with the aid of the GE software datosX ver. 2.1.0 using the Feldkamp algorithm [22]. The obtained micro-CT scans had dimensions of 1796 × 2284 × 1100 pixels, whereas the image pixel size was 18.6 μm. The post-reconstruction data processing included: denoising, cropping and 16 to 8 bit conversion, performed by the means of the VGStudio Max 2.1 software (https://www.volumegraphics.com).
In order to obtain stand alone 3D models of the ST, serial micro-CT scans of the petrous bone images representing the retrotympanum were segmented with the help of the thresholding algorithm. This procedure was performed with the aid of the 3D Slicer software (https://www.slicer.org) which is a free, open source package dedicated for medical and biomedical applications [21].
In the next step, the Autodesk Meshmixer free software (https://www.meshmixer.com) was used for virtual extraction of the ST from the retrotympanum and other anatomical structures included in the initial 3D model of the petrous bone. Such pre-prepared 3D models of the retrotympanum with the ST were displayed on the computer screen and processed interchangeably by the means of Meshlab and Meshmixer software to obtain isolated 3D models of the ST. Using clipping planes and tools dedicated for virtual cutting of the 3D models, the authors removed these parts of the model which occluded the ST. The MeshLab software (https://www.meshlab.net) was indispensable for accomplishing the 3D model of the ST represented by the mesh of triangles, stored in the STL file format [17].
With the aid of the MeshLab software, the authors decimated and smoothed the original mesh of triangles representing the retrotympanum which was further used for creating the final 3D model of the ST. Mesh decimation reduces the number of vertices, edges, and triangles in a mesh, thus the file including 3D model attains smaller size and can be processed quicker by the computer software [6]. Decimation was necessary because the original meshes approximating geometrical configuration of the entire retrotympanum consisted of a huge number of triangles (in the order of 10 million) which tremendously effected the size of the STL files. In our material, the size of the original STL files ranged approximately from 300 Mb to 500 Mb, whereas decimated meshes by half yielded twice smaller files but still preserved the mesh’s topology. To simplify the initial mesh of the triangles we applied quadratic edge collapse decimation strategy implemented in the MeshLab software [24]. Thus, the target meshes representing standalone 3D models of the ST usually comprised 20,000–100,000 triangles, depending on the volume of the ST.
The mesh smoothing reduced noise in the model surface and resulted in a more user friendly visual perception. For this purpose, the authors applied the Laplacian smoothing algorithm which creates a new mesh according to local information on the position of the vertexes in the mesh [32]. However, this operation was taken with the outmost precaution, because the smoothing algorithm may inadvertently alter the surface properties of the object and shrink the mesh. To prevent the unwanted changes of the mesh geometry, the authors experimentally adjusted the smoothing force at each iteration applied for mesh processing and compared the changes with reference to the original mesh topology. This was a crucial step in the present study in order to obtain reliable results of visual observations of the ST’s interior. Overly smoothened model may erroneously reflect geometrical and morphological properties of the anatomical structures due to extreme elimination of tiny elements like spikes, folds or depressions which naturally exist on the ST’s surface.
The process of creating 3D models used for evaluation ST’s morphology has been visualized in the flowchart (Fig. 1).
By rotating the 3D models and viewing the ST at different angles on the computer screen, the authors evaluated resemblance of their shapes to geometrical figures. In truth, the shapes reflected by the 3D models should be considered to be virtual endocasts of the STs.
Surface configuration of the ST’s interior was evaluated visually with reference to the material texture (smooth — homogeneous, rough — heterogeneous). Outstanding features characteristic for the ST configuration were measured on the 3D models by means of the ruler tool available in the MeshLab software.
The occurrence of each anatomical variant of the ST was estimated as the percentage of the defined types in the whole data set, determined separately for the female and male temporal bones. Statistical analysis was completed using a Fisher’s exact test processed with the Statistica package ver. 13 (https://www.statsoft.pl). The level of statistical significance for the results was set at p < 0.05.
Ethical consideration
The study was conducted with approval (KBET/198/B/2014) of the Bioethics Committee of the Jagiellonian University.
RESULTS
For better orientation in the ST topography, the authors present a representative 3D model showing the location of the ST in relation to the surrounding osseous structures of both the middle and inner ear (Fig. 2).
Regarding the shape of the ST, the authors distinguished two basic morphological forms that were regarded as saccular or bowl. The saccular shaped ST characterizes postero-inferior elongation. This means that one of its diameters is considerably bigger than the other two. In turn, the bowl shaped ST resembles an irregular ovoid, in which three dimensions are similar (Fig. 3). The authors also found that the bowl shaped STs were usually wide and relatively shallow compared to the saccular types of the STs which were sitted deeply in the retrotympanum. The saccular STs were usually cylindrical in shape, except for a singular case in which a form, reseambling triangular pyramids (tetrahedra) of variable size, was noted (Fig. 4).
Regardless of the shapes of the STs, their interior structure revealed variable configuration which was defined as homogeneous or heterogeneous, depending on the presence or absence of tiny bony structures located in their walls (Fig. 5). The irregular bony structures found in the heterogeneous STs looked like: trabeculae, crests or laminae with diameters of around 1.0 mm. It should be noted that some of the heterogeneous STs were characterised by the presence of recesses or diverticula emerging from their walls. Such structural modifications were chiefly found in the saccular type of the STs. However, their existence was not included in the STs classification because their size and configuration did not affect the overall shape of the ST.
In turn, homogeneous configuration was attributed to these STs in which their interior was relatively smooth. Their walls were characterized by the lack of bony protrusions. Nonetheless, in some of them mild folds and depressions or pits were observed.
The present study demonstrated that both the saccular and bowl-shaped STs had a similar prevalence (47.7% and 52.3%, respectively) in the combined sex analysis. Furthermore, the authors did not find significant differences between the occurrence of these shapes in the male and female subgroups (Fisher’s exact test, p = 0.557). As for the internal characteristics of the ST, the heterogeneous surface was typical for the entire sample (70.5%), whereas homogeneous surface was less common with respect to all the samples (29.5%); however, almost half of the female subgroup (47.4%) was characterized with the latter type of the ST. Although male STs were largely heterogeneous (84.0%), their prevalence just about predominated in females (52.6%). The differences in prevalence of the homogeneous and heterogeneous surfaces of the STs in male and female subgroups were statistically significant (Fisher’s exact test, p = 0.044). The percentage of all observed morphological variants of the ST in male and female subgroups are graphed in Figure 6 and presented in Table 1.
|
Sex |
Shape |
Surface configuration |
||||||
|
Saccular |
Bowl |
Homogeneous |
Heterogeneous |
|||||
|
N |
% |
N |
% |
N |
% |
N |
% |
|
|
Male (n = 25) |
13 |
52.0 |
12 |
48.0 |
4 |
16.0 |
21 |
84.0 |
|
Female (n = 19) |
8 |
42.1 |
11 |
57.9 |
9 |
47.4 |
10 |
52.6 |
|
Male + female (n = 44) |
21 |
47.7 |
23 |
52.3 |
13 |
29.5 |
31 |
70.5 |
Finally, the authors estimated the number of cases in which the ST revealed a combination of the following morphological features: saccular shape and heterogeneous surface. Such a complex anatomy may hinder surgical operations and lead to pathological processes by facilitating the harbouring of pathogens. Among all the examined sinuses, 36.4% involved this specific anatomy (Table 2). However, the said combination occurred in 13 out of 25 male samples (52.0%), whereas only 3 out of 19 (15.8%) female samples had this composition, a statistically significant difference (Fisher’s exact test, p = 0.025).
|
Sex |
Saccular and homogeneous |
Saccular and heterogeneous |
Bowl and homogeneous |
Bowl and heterogeneous |
||||
|
N |
% |
N |
% |
N |
% |
N |
% |
|
|
Male (n = 25) |
0 |
0 |
13 |
52.0 |
4 |
16.0 |
8 |
32.0 |
|
Female (n = 19)) |
5 |
26.3 |
3 |
15.8 |
4 |
21.1 |
7 |
36.8 |
|
Male + female (n = 44) |
5 |
11.4 |
16 |
36.4 |
8 |
18.2 |
15 |
34.1 |
DISCUSSION
So far, morphological variation of the ST has been evaluated utilising either CT or endoscopic images, or dissected temporal bones. Therefore, its evaluation was actually limited to the description of only the shape of the ST’s orifice visible on the dry specimens, or the ST was assessed in the cross-sectional CT scans, or its interior was viewed by the means of endoscope [8, 10, 11, 15, 30, 35, 41].
Each of these techniques have some technical limitations which may hinder detailed evaluation of the STs’ shapes, especially in the 3D way, as neither are capable of simultaneous display of the topographical relationships. Actually, this can be attained in volume renderings obtained from CT scans. Nevertheless clinical CT data are not accurate enough to create 3D reconstructions which could reveal ST anatomy and topography in great details. Therefore, the authors applied micro-CT for exploration of the ST anatomy because this modality offers extremely high spatial resolution for imaging objects which dimensions range on a millimetre scale.
The present study revealed that both the shape of the ST and its interior surface configuration vary considerably amongst human adults. The factors affecting the shape and volume of the ST still remain unclear; however, degree of the temporal bone pneumatisation is regarded as one of the physiological reasons that can affect the retrotympanum, as well as the ST itself [10, 26].
Bekci et al. [10] noticed that the volume of the ST has been greater in patients with pneumatised petrous apex. In turn, Baklaci et al. [9] found that a well-pneumatised mastoid process is associated with a deep ST. The effect of pneumatisation on the ST’s shape and size has also been noticed by Abou-Bieh and Haberkamp [1]. The aforementioned authors recognised a few types of the ST from CT scans of adult temporal bones, classified as: shallow (15.8%), deep (33.8%), and intermediate (50.5%). Detailed descriptions of small cavities within the retrotympanum, including: the sinus subtympanicus, lateral tympanic sinus, and posterior tympanic sinus were presented by Cheiţă et al. [16] and Marchioni et al. [33]; however, their studies did not include a description of the ST interior.
Up-to-date internal morphological features of the ST have only been briefly mentioned by Nitek et al. [37]. The said authors noticed presence of bony trabeculae within 3 of 30 examined samples, but they did not analyse their size and sex in which these structures occurred. Bonali et al. [13] reported on the rare existence (6%) of bony crests as part of a description of anatomical variants of the ST.
In principle, there is scanty information in the anatomical literature describing the internal morphology of the ST; therefore, this issue was raised in the present study. Our classification of the ST differs from those presented in literature, because it combines shape of the ST with its internal configuration. It is based on the anatomical features which seem to be clinically very important, hence explored in 3D space thanks to the virtual models which can be viewed from any direction. A huge number of triangles comprised in the mesh that approximated surface geometry of the ST and other structural components of the temporal bone provided accurate exhibit of their anatomical features.
Previous reports yield information about morphological features of the ST obtained usually from CT images or direct visual observations of the ST using endoscopes or operative microscopes. Thus, Cheiţă et al. [16] performed endoscopic examinations and classified the ST as oval, rectangular, round, trapezoidal or rhomboidal. Nitek et al. [37] recognised the ST as either vertically oval, horizontally oval, round or polygonal. Wang et al. [49] used a CT analysis of the ST in patients with congenital aural atresia, and distinguished three types of the ST shape: cup, pear and boot-shaped. The boot-shaped ST was an unusual variant characterized by a large cavity, and appeared only in patients affected by congenital aural atresia. As part of an endoscopic study of the retrotympanum, Nogueira et al. [38] identified the ST to have the following shapes: classical, confluent, partitioned or restricted.
Our classification of the ST was simplified to two basic morphological types defined as: saccular or bowl. We assume that both saccular and bowl form of the ST results from individual developmental pattern of the retrotympanum, which might be slightly differentiated in females and males regarding the overall size of the temporal bone. The proposed division of the STs into two basic forms: saccular or bowl corresponds with the data presented by Niemczyk et al. [36] who recognized two main developmental forms of the ST — deep and shallow. In our classification, the saccular form of the ST has always been deep, due to its inferior elongation, whereas the bowl shaped ST were shallow and wide, in contrast to the saccular one.
Bilińska et al. [12] compared the ST shape to a trough. This seems to be a very general approximation of the variety of shapes the STs present, especially if magnified in the 3D space. Nevertheless, from the practical point of view classifications of the ST should be based on the morphological features which can impact surgical access to its interior (peculiar shape and size) or may promote progression of the pathological processes (configuration of the internal surface).
Therefore, explored here internal morphology of the ST by means of micro-CT expands knowledge about variation of bony structures that may exist inside the ST. Furthermore some of them may render clinical implications if they are extensively developed. Besides the bony trabeculae, the ST may contain spikes, crests, small depressions and recesses. The presence of the abovementioned structures was predominant in male STs (84.0%), significantly higher than their occurrence in females (52.6%), as confirmed by Fisher’s exact test (p = 0.044). Accessory bony structures within the ST, such as secondary recesses emerging from its wall, complicate the anatomy of the ST — particularly in the case of the saccular shape. The possible implications of such a configuration include increased bacterial biofilm adherence to the irregular surface which may lead to middle ear inflammation [46].
It has been estimated that over 20 million people worldwide are afflicted with middle ear infection (otitis media), whereas approximately one-fourth of them have cholesteatoma [4].
According to Saunders et al. [45], bacterial biofilms are common in suppurative otitis media and infected cholesteatomas. The biofilm is most often produced by Staphylococcus aureus and Pseudomonas aeruginosa. Such biofilms have been found in 75–85% samples taken from patients with chronic middle ear infection associated with cholesteatoma [23, 45].
The rough surface of the ST may also favour the formation of cholesteatoma (abnormal skin growth), and may prove to be an obstacle in irrigating it from the small cavities and recesses.
Effectively, complete removal may not be possible, due to the difficulties of reaching this region for endoscopic observation or surgery [7, 42, 48]. The literature suggests that the prevalence of cholesteatoma is 1.4 times greater in males than in females [31, 39]. In addition, epidemiological studies performed by Aquino et al. [4] showed that the incidence of cholesteatoma in males is predominant (64.7%) over females (35.3%). Very similar results concerning the prevalence of cholesteatoma were reported by Ajalloueyan [2] (64.0% male vs. 36.0% female).
In an attempt to find possible correlations, the present study included an investigation on the occurrence of a complex ST anatomy comprised of a saccular shape and heterogeneous surface. This combination of morphological features may constitute complications during surgical treatments, as they are the difficult to access. Furthermore, the irregularities of the ST wall favour pathological processes ongoing inside the ST. It was demonstrated that such type of the ST is more common in males (52.0%) than in females (15.8%), being statistically significant (p = 0.025). This morphological feature of the ST may explain the reason for the higher prevalence of cholesteatoma in males in juxtaposition to females. Thereby, a detailed anatomical demonstration of the ST may be beneficial for better understanding the interactions between ear structures and diseases, as well helpful for simulating the route for surgical access to the ST by presenting its potential anatomical variants and local variation of the inner surface configuration.
Limitations of the study
Nevertheless, we perceive some limitations of the performed study. Firstly, micro-CT imaging of the ST is only applicable for the specimens which have been removed from the human body; therefore, the proposed classification of the STs may be rather of cognitive meaning than serve directly in the clinical practice supported by the CT imaging of lower spatial resolution. Secondly, the assessed configuration of the ST interior reflects details of bony wall morphology without the inclusion of the mucosae, which is routinely observed during endoscopic examination. Lastly, the sample size used in the current study is too small in number in order to conclude objectively that only such types of the ST as have been recognized may exist in the general population.
Thereby, it would be valuable to perform similar studies on cadaveric temporal bones which preserve structures composed of soft tissues, and are numerous enough to be representative for each gender subset taken from the general population.
CONCLUSIONS
Micro-CT of the petrous bones yielded image data that allowed to build accurate 3D models of the ST and compare their shapes in 3D space.
Upon visual assessment, the STs typically revealed two distinct morphological forms classified as saccular and bowl, regarding their shapes. Hence, internal inspection of the ST allowed to recognise variable configuration of their walls, and define them as homogeneous or heterogeneous, depending on the presence or lack of tiny extensions or depressions localised on the walls of the ST.
A statistically significant difference was found between males and females for internal surface configuration, but not for the shape of ST. A complex combination of ST features composed of a saccular shape and heterogeneous surface seems to be clinically important. This may rise negative impact on health outcomes after surgery in the case of cholesteatoma, and may favour chronic pathological processes.
Acknowledgements
The authors would like to thank Andrzej Stanisz, M.Sc. from the Department of Bioinformatics and Telemedicine of the Jagiellonian University Medical College for his help with statistical analysis.
Funding
This study has been funded using Jagiellonian University Medical College statutory funds N41/DBS/000151.
