Abstract
The sinus tympani is a deep pocket of varying dimensions situated in the retrotympanum. It lies medial to the facial nerve thereby making surgical access difficult. It is this area which is frequently involved in chronic otitis media attico-antral type. Removal of disease from sinus tympani is cumbersome and expedites Cholesteatoma Recidivism. In India, very limited studies have been conducted describing the variations of Sinus tympani. Hence, this study is dedicated to estimating it’s variations in volume. This is a one-year Observational Cross-sectional study conducted between January 2020 to December 2020 in Temporal Bone Dissection Lab of Department of Otorhinolaryngology and Head and Neck Surgery, J. N. Medical College, KAHER, Belgaum. 40 Human temporal bone (both left and right) have been dissected. The middle ear cavity was filled carefully with a moulding material i.e. Otoform. Once set, a 3-dimensional model of the middle ear cavity was made, which was removed carefully without causing any damage. The volume of the middle ear cavity was calculated. The other contents and parameters were measured using a measuring probe. Forty bones were dissected. The volume of the sinus tympani was measured to be 8.77 cubic mm with a range of 6.5 cubic mm to. 10.9 cubic mm hence showing that sinus tympani show a larger variability in terms of size, shape and volume. The ponticulus was Complete in 65% of the cases (26 bones) and incomplete in 35% of the cases (14 bones). The ponticulus and subiculum were found to be complete in a well pneumatized mastoid bone. The volume of the middle ear cavity was measured to be 1.17 cubic cm. The sinus tympani volume was calculated in this study employing a novel technique called the Otoform mould in cadaveric temporal bones, enabling for accurate measurement. The study concluded that sinus tympani showed a wide range in volume thus owing that it is the structure which has highest variability in terms of size and shape in the middle ear.
Keywords: Volume, Sinus tympani, Moulding, Otoform
Introduction
The posterior wall of the middle ear cavity contains a space between the subiculum inferiorly and the ponticulus superiorly known as the sinus tympani. With the advancement made in Ear microsurgery, an elaborate and accurate study of the cavity is imperative owing to the difficult access and anatomical variations of sinus tympani. It lies medial to the vertical segment of facial nerve. These variations can escalate chances of Recidivism [1]. The normal anatomical knowledge is essential to study the middle ear disparities which thereby aid in management and treatment of middle ear infections [2].
Retrotympanum is divided in to the lateral space and medial space. The medial space is further divided by the ponticulus into the sinus tympani and posterior tympanic sulcus. The sinus tympani is bounded laterally by the vestibule, medially by the facial nerve, stapedius, and the pyramidal eminence and posteriorly by the semicircular canals. The posterior tympanic sinus is bounded by the pyramidal eminence and the facial recess medially and located below the ponticulus. When the ponticulus is incomplete, it is a single sinus with the sinus tympani [3, 4].
One of the challenges in middle ear surgery is clearing the sinus tympani, which is one of the body’s most concealed locations and has distinct anatomical variations in terms of size, shape, and depth. The sinus is classified into 3 types based on its depth as: Type A, Type B and Type C. Type A is the shallow sinus tympani, Type B is the intermediate depth sinus tympani and Type C goes deep and posteriorly to the vertical section of the facial nerve which is seen mostly in only well—pneumatized mastoids [2, 5].
Sinus tympani is of enormous clinical interest to otologists due to its susceptibility to be invaded by cholesteatoma, the difficulties in visualizing and due to the absence of direct operative access channel. The structural variabilities demonstrate how challenging it is to perform surgery and to address the lesion from the region of the sinus tympani [6].
In India, very limited studies have been conducted describing the variations of Sinus tympani despite the high incidence of residual disease of the middle ear pathology owing to these anatomical variations. Since the previous studies were done using radiological investigations primarily, more anatomical studies are a necessity to give an insight into the variations of the middle ear cavity. Hence, this study was done to determine the variations of the sinus tympani and its dimensions by a novel technique using the Otoform mould to reduce the prevalence of lingering middle ear disease and extrapolating the knowledge in live surgery.
Materials and Methods
Our study is a cross sectional study carried out over a period of 1 year in the department of Otorhinolaryngology and Head & Neck surgery, J. N. Medical college, KAHER, Belagavi from 1st January 2021 to 31st December 2021.Institutional ethical committee approval was obtained for the study. This study was done on 40 wet temporal bones. Damaged and diseased temporal bones, temporal bones removed properly, and temporal bone of pediatric age group were excluded from the study. The temporal bones were fixed on the temporal bone holder. The soft tissues and external ear were removed after recording the presence of pinna, pre auricular and post auricular region, external auditory canal and the status of the tympanic membrane. The spine of Henle was identified. MacEwen’s triangle was exposed and drilled using handheld motor and drill under the operating microscope. Mastoid antrum was identified and extended cortical mastoidectomy done (Fig. 1). Canaloplasty was done to view the incudostapedial joint and the eustachian tube in one view and measurements were taken (Fig. 2). The incus and malleus were removed, and stapes left behind for identification of sinus tympani. The Otoform, a moulding agent was prepared by mixing with the hardening agent and was injected into the middle ear cavity through the external auditory canal (Fig. 3). After 5–7. Minutes the Otoform was carefully removed using circular knife and a ball probe. The sinus tympani were identified and marked using a permanent marker in the mould removed (Fig. 4). The volume of sinus tympani was calculated by the measuring the length, breadth and depth of the sinus tympani in the mould using a vernier caliper. Along with this the volume of the middle ear cavity was also determined from the mould taken. These recordings were recorded in a proforma sheet.
Fig. 1.
Specimen showing cortical mastoidectomy
Fig. 2.
Specimen showing incudo-stapedial joint after canaloplasty
Fig. 3.
Middle ear cavity filled with otoform mould
Fig. 4.
Mould showing sinus tympani marked using permanent marker
Results
In this study, a total of 40 wet cadaveric temporal bones were dissected. Out of the 40 bones dissected 21 was left sided and 19 were right sided. The mastoid cavity in 85% of the samples was found to be diploic, whereas 7.5% were well pneumatized and 7.5% were sclerotic.
The Mean Volume of the cavity of the ST (Table 1) was 8.77 ± 1.25 Cu millimetres and ranged from 6.5 ± 1.25 Cu millimetres to 10.91 ± 1.25 Cu millimetres.
Table 1.
Summary or descriptive statistics of parameters related to
| Parameters | Min | Max | Average | SD |
|---|---|---|---|---|
| Volume of ST(cu. millimetres) | 6.5 | 10.91 | 8.77 | 1.25 |
The ponticulus (Table 2 and Fig. 5) was Complete in 65% of the cases (26 bones) and incomplete in 35% of the cases (14 bones).
Table 2.
Ponticulus—complete/incomplete
| Ponticulus | Number of patients | Percentage |
|---|---|---|
| Complete | 26 | 65.0 |
| Incomplete | 14 | 35.0 |
| Total | 40 | 100 |
Fig. 5.
Ponticulus—complete/incomplete
Disscusion
This study aimed at finding out the variations of the sinus tympani. It is a known fact that ST is a very frequent location for remanent cholesteatoma wherein it’s volume and variations are of highest importance [7, 8].
As the middle ear is an irregular structure in this study, it was estimated using a mould of the MEC. The similar moulding technique was used by Shakeel et al. in 2015 in Dundee, United Kingdom to measure the RW niche wherein he used the Aquasil putty which is used very commonly by the dentists. However, in this study the Otoform has been utilised which is a silicone mould generally used by the Otorhinolaryngologists and audiologists for estimations of the external ear cavity which aids in the preparation of moulds for hearing aid measurements [9].
Mastoid Cavity
The mastoid tip was prominent in 90% of the samples and not prominent in 10% of the samples. Deniz Baklaci et al. in 2019 concluded that a mastoid which is well pneumatized was related to deep and posteriorly placed ST. In this study this comparison was not possible as the computerised tomography of the cadaveric bones was not performed however this study elaborates that well pneumatized mastoid bones had complete ponticulus and subiculum [10].
Ponticulus
The ponticulus which is the bone extending from the promontory to the PE was complete in 65% of the bones. This study was consistent with the study in 2004 by Holt et al. which states that ponticulus was present in 66% of their specimens [11].
Subiculum
The subiculum creates the lower boundary of the ST and is a smooth projection that extends from the lower lip of the recess of the RW up to the promontory. It is complete in 60% of the samples and incomplete or absent on 20% respectively. This study found that subiculum and ponticulus are interrelated and are complete in 60% of the samples. After the above observations, the silicone mould was injected and removed carefully and the following parameters were measured.
Volume of the ST
The ST which is the largest sinus of retrotympanum, is separated by the posterior sinus tympanicus by the ponticulus [12, 13]. The average volume of the ST was found to be 8.77 ± 1.25 cubic millimetres which was concurrent with the study by Hizli et al. in 2019 [14].
It was a found in this study that higher range of the volume of the ST was seen in 40% of the samples which helps in concluding that the incomplete ponticulus leads to ST to be confluent with the posterior tympanicus sinus giving rise to large volume sinus tympani.
The moulding process was useful for determining the ST's volume, but the middle ear volumes varied [15].
The limitations of this study include the arduous learning curve for injecting and removing the mould without damaging or altering the structures as the measurements were made on the mould. Further studies with the moulding technique with a larger sample size will be favourable in learning process of resident doctors and ENT surgeons.
The Advantages of this study include the analysis of variation in ST and volume compared to variations to other middle ear structures can help in complete removal of cholesteatoma and reduce the chances of recurrence. Also, the temporal done dissection is pragmatic and is of immense value for the post graduates/residents. The study of the volume of MEC with accurate measurements along with the Otoform mould can also be utilised on a larger scale to get more accurate models of the middle ear implants.
Conclusion
The otologist's primary issue is getting rid of middle ear illness without it recurring. Considering the high incidence of residual disease of middle ear pathology that is triggered by irregularities in the structure of the sinus tympani, very few anatomical investigations, especially in India, have been done to study this variation. The sinus tympani volume was calculated in this study employing a novel technique called the Otoform mould in cadaveric temporal bones, enabling for accurate measurement. The study concluded that sinus tympani showed a wide range in volume thus owing that it is the structure which has highest variability in terms of size and shape in the middle ear.
Funding
This study was not funded by any institution or organisation.
Declarations
Conflict of interest
The author declares that they have no conflict of interest.
Ethical Approval
Institutional ethical committee approval was obtained for the study.
Footnotes
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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