Abstract
Objective: To compare the computed tomography findings with intraoperative findings of the level of tegmen plate with respect to the superior most part of the lateral semicircular canal in patients with Chronic Otitis Media. This study was attemptted to provide an objective assessment of the level of tegmen mastoideum. Materials and Methods: The level of tegmen plate with respect to the superior most part of the lateral semi-circular canal was measured preoperatively using HRCT scan of slice thickness of 0.6 mm with a software - RadiAnt DICOM Viewer 64-bit version in sagittal plane. The distance between these two planes was measured intraoperatively using micro ear straight pick and Vernier Calipers. Results: 75 participants (thirty-three male & forty-two female) with chronic otitis media underwent computed tomography preoperatively and surgery. No significant difference was found in the height of tegmen measured preoperatively using HRCT temporal bone and intraoperatively (p value – 0.16). The tegmen plates were classified as low lying (2.0 -2.49 mm), intermediate lying (2.49–2.99 mm) and high lying (3.0 -3.49 mm). Conclusion: Computed tomography findings of tegmen height correlates well with the intraoperative findings. An objective assessment of the level of tegmen mastoideum can provide the surgeon an idea of the severity of low lying level of tegmen to be expected and hence likely surgical problems. Based on this study, a classification system of level of tegmen plate has also been proposed.
Keywords: Tegmen, High Resolution Computed Tomography, Lateral Semicircular Canal, Chronic Otitis Media
Introduction
The tegmen is a thin bony plate that separates the mastoid and middle ear space from the middle cranial fossa. It is formed by the petrous and squamous portions of the temporal bone. The tegmen has a variable shape with no reliable external landmarks. It can be divided anatomically into the following three parts:
Tegmen tympani forming the roof of epitympanum,
Tegmen antri forming the roof of the mastoid antrum, and.
Tegmen tubari forming the roof of the eustachian tube.
Mastoidectomy is a procedure which involves drilling out the mastoid air-cell system, connected to the middle ear through the aditus ad antrum. Common indications include chronic otitis media with and without cholesteatoma. It can also be a part of the approach for procedures like translabyrinthine resection of the acoustic neuroma, cochlear implantation, or endolymphatic sac decompression [1].
The tegmen, facial nerve, and sigmoid sinus are the key anatomic structures which should be identified for performing a safe mastoidectomy. The temporal bone development decides the position of these structures. The mastoid antrum is about 1-1.5 cm2 in size and well developed at birth. Rapid pneumatization in the first-year results in the mastoid cells increasing to about 3.5-5cm2 in size followed by linear growth until the age of six (1-1.2cm2/ year), and then a slower growth to reach the final adult size at puberty (~ 12cm2) [2]. Insufficient mastoid development may lead to low-lying tegmen. The underlying etiology of insufficient mastoid development is debated in the literature. Several authors suggest a hereditary theory which was first proposed by Diamant in 1940. This theory states that the mastoid aeration is genetically determined and small cellular systems may be predisposed to chronic and acute otitis media [3–6] while other authors favour environmental theory developed by Wittmaack in 1918. This theory suggests that chronic inflammation and Eustachian tube dysfunction during the early years of development predispose to insufficient mastoid development [7, 8].
An injury to the tegmen can cause a cerebrospinal fluid leak [9–13], pneumocephalus, brain herniation, and cerebral abscess [14–17]. Hence, tegmen serves as a basis for surgical discourse or preoperative planning and acquaint novice surgeons with the complexity of its anatomy and variations.
Even though computed tomographic (CT) scans can preoperatively determine the level of the tegmen for any individual patient, an accepted classification system is currently not available in the literature for discussing this with other surgeons or trainees. The present study is intended to serve as a guide as to what to expect when drilling on the tegmen during mastoidectomy.
Materials and Methods
This was an observational study conducted in the Department of Otorhinolaryngology & Head Neck Surgery in a tertiary care centre from April 2021 to October 2022. The clearance from the Institutional Ethical Committee was obtained.
The inclusion criteria entailed both male and female patients aged 18 years and above with Chronic Otitis Media, planned for surgery, who have undergone computed tomography as routine preoperative evaluation. The patients of chronic otitis media with Tegmen or Lateral semicircular canal erosion or any contraindication for surgery were excluded from this study.
The subjects were counseled about their participation in the study and informed consent was taken after explaining the study and procedure for evaluation and management.
A total of 75 cases (thirty-three male & forty-two female) diagnosed as Chronic Otitis Media as per inclusion and exclusion criteria were the study cohort. The sample size was calculated as 75 in consultation with an epidemiologist, based on the hospital records. It was expected to get 75 patients with chronic otitis media during the study period, who required both computed tomography and surgery. The HRCT scan available with the patient was studied using a software - RadiAnt DICOM Viewer 64-bit version which provides basic tools for the manipulation of images and measurement. A slice thickness of 0.6 mm was taken in all the scans. Each axial CT scan was reconstructed in the sagittal plane. After reconstruction, the distance between the plane passing through the lowest point on the tegmen and the superior point on the lateral semicircular canal was measured (Fig. 1).
Fig. 1.
Measurement of tegmen height preoperatively using computed tomography and intraoperatively using Vernier Calipers
All the patients enrolled in the study underwent mastoidectomy (intact canal wall or canal wall down). After delineating the dural plate (tegmen), posterior canal wall and the lateral semicircular canal, the plane of the lowest level of the tegmen plate and the superior point of the lateral semicircular canal was identified. Micro ear straight pick was placed at the superior-most part of the lateral semicircular canal and a second straight pick was placed at a point corresponding to the lowest level of tegmen. The distance between the two picks was measured using Vernier Callipers. The slope of tegmen plate was taken into consideration while measuring its distance from the superior most point of lateral semicircular canal, both radiologically and intraoperatively. All the data was recorded and tabulated. The tegmen height obtained preoperatively and intraoperatively was subjected to statistical analysis. Statistical Package for the Social Sciences version (SPSS) 25 was used to analyze results. We regarded p value less than 0.05 as statistically significant.
Results
A total of 75 patients, 33 males and 42 females were enrolled in the study. The mean age of the study participants was 32.84 years. Most of the participants were diagnosed to have Chronic Otitis Media Squamous disease corresponding to 64%. The participants with Chronic Otitis Media Mucosal disease accounted for 36% of the cases. 28 patients underwent Canal wall down mastoidectomy corresponding to 37%, 20 patients underwent Intact canal wall mastoidectomy corresponding to 27% and 27 patients underwent Cortical mastoidectomy corresponding to 36% (Fig. 2). No significant difference was found in the height of tegmen measured preoperatively using HRCT temporal bone and intraoperatively (Fig. 3). The P value was not significant [Table 1].
Fig. 2.
Surgical approach in our cohort of patients
Fig. 3.
Box and Whisker plot showing Tegmen height on HRCT and intraoperatively
Table 1.
Comparison of the height of tegmen preoperatively and intraoperatively
| Tegmen height | Pre-operative | Intraoperative |
|---|---|---|
| Mean | 2.51 | 2.42 |
| Std dev | 0.3442669 | 0.49347088 |
| P value | 0.169421 | |
| Inference | No significant difference in height measured preoperatively and intraoperatively | |
The height of tegmen measured in HRCT scan and intraoperatively was divided into two groups < 2.5 mm; ≥2.5 mm based on the study done by Elkhalidy et al. 36 out of 75 participants had tegmen height < 2.5 mm preoperatively corresponding to 48% and 43 participants had tegmen height < 2.5 mm preoperatively corresponding to 57%.
Comparison of the height of tegmen measured radiologically and intraoperatively based on the type of surgery has been depicted in Tables 2 and 3. On comparing the findings obtained, the radiologically measured height in patients who underwent cortical mastoidectomy were not comparable with the intraoperative height. A larger proportion of patients were found to have low lying tegmen intraoperatively than radiologically. This could be attributed to the thinning of tegmen plate intraoperatively which varied from surgeon to surgeon.
Table 2.
Comparison of the height of tegmen radiologically and the type of surgery
| Tegmen height (Radiological) | < 2.5 mm | ≥ 2.5 mm |
|---|---|---|
| Cortical mastoidectomy | 06 | 21 |
| Intact canal wall mastoidectomy | 13 | 7 |
| Canal wall down mastoidectomy | 12 | 16 |
Table 3.
Comparison of the height of tegmen intraoperatively and the type of surgery
| Tegmen height (Intraoperative) | < 2.5 mm | ≥ 2.5 mm |
|---|---|---|
| Cortical mastoidectomy | 18 | 9 |
| Intact canal wall mastoidectomy | 11 | 9 |
| Canal wall down mastoidectomy | 13 | 15 |
Discussion
During the transmastoid approach, the first step is to locate the mastoid antrum. Apart from tegmen height, the other variations that can add to the complexity of the case include small, sclerotic mastoid cavity or an anterior sigmoid sinus. A low-lying tegmen can mislead the surgeon while attempting to locate the antrum by obscuring the antrum.
Comprehensive removal of pathology is key to preventing recurrence from the residual pathologic tissue. With disease close to the dura, removal of all tegmental cells is a much more difficult task in a patient with a deep overhang dura than in a patient with a flat tegmental plate.
Makki et al. categorized groups as low, intermediate, or high for potential risk of injury to tegmen based on the distance between the tegmen and the superior semicircular canal (SSC) and the tegmen and the external auditory canal (EAC) in the coronal and sagittal planes respectively. Their radiological classification (Amoodi-Bance classification) of the tegmen mastoideum is based on the Lateral-Medial slope posteriorly and the Posterior-Anterior slope laterally, as visualized in the coronal and sagittal CT scans, respectively [14]. The use of the external ear canal may not be a reliable radiologic landmark, due to its variability in size and shape and the superior semicircular canal is not often sought out in mastoid surgery.
Karatag et al. showed in their series of 50 patients who underwent chronic otitis media surgery, that the tegmen height was lower in patients where the dura was exposed as a complication of surgery (n = 12) than in patients without dural exposure (n = 38). The tegmen height was measured from the spine of Henle to the lowest point on the tegmen in the coronal plane of HRCT scans. The spine of Henle is not an optimal radiological landmark as is difficult to identify on HRCT images and is often absent in patients who have undergone mastoidectomy [18].
A study by Anup Singh et al., proposed a classification system of the anatomic variations of the tegmen plate by microdissection of the human cadaveric temporal bones (n = 93). The Tegmen Mastoid (TM) was classified into Grade-A and B as per the position of the tegmen plate and the visibility of the superior semicircular canal (SSCC) and the aditus. The latter two structures were well visualized in Grade A and poorly visualized in Grade B [19].
Elkhalidy et al. measured the distance between the LSCC and the lowest point of the tegmen tympani. Their results indicated that there is one average height in the coronal (4.4 mm ± 1.2 mm) and sagittal (2.66 mm ± 1.38 mm) planes. They proposed a classification system where tegmen tympani heights below 4.5 mm on coronal measurements and 2.5 mm on sagittal measurements be considered “low” (type A) whereas tegmen tympani heights above these parameters are considered “high” (type B) [20].
Comparison of Radiological Tegmen Height with Intraoperative Values
The main purpose of this study was to calculate expected tegmen height in sagittal plane of HRCT scan and compare them with the intraoperative height.
The LSCC was chosen as the landmark for measurement because it is a stable radiological as well as an important surgical landmark easily identifiable when performing any mastoid surgery. It is not destroyed by surgery or by the diseases of the temporal bone.
Comparison of Radiological and Intraoperative Tegmen Heights with the type of Surgery
The height of tegmen measured radiologically and intraoperatively were also divided into low lying and high lying based on the type of surgery performed – Canal Wall Down mastoidectomy, Intact Canal Wall mastoidectomy and Cortical mastoidectomy (Tables 2 and 3). The patients who underwent cortical mastoidectomy were found to have low lying tegmen intraoperatively than radiologically. This could be due to the variable thickness of the tegmen plate left behind while drilling which varies from surgeon to surgeon.
New Classification System
Based on the above-mentioned results and findings, we propose a classification system for the tegmen height at the level where the tegmen mastoideum continues as tegmen tympani [Table 4].
Table 4.
Classification system for the level of tegmen plate
| Tegmen height | Interpretation |
|---|---|
| 2.0–2.49 mm | Low lying tegmen |
| 2.5–2.99 mm | Intermediate lying tegmen |
| 3.0–3.49 mm | High lying tegmen |
It is based on measurement in the sagittal plane as it is appropriate while drilling from the antrum to the attic region anteriorly. This region is important while drilling in Intact canal wall mastoidectomy as there is increased risk of injury to the dura in cases with low lying tegmen. This classification is more elaborate compared to the classification proposed by Elkhalidy et al. It helps in predicting the accurate level of tegmen and anticipating the level of difficulty during drilling. No such classification has been described in the literature.
Tegmen Height classification (From lower-most point on tegmen to superior-most point on the lateral semicircular canal).
A low lying tegmen pre-operatively should alert the surgeon as it poses a high risk of injury to the dura while drilling and increased risk of recurrence due to incomplete clearance of disease.
An intermediate lying tegmen poses an intermediate risk of injury to the dura and recurrence of disease.
A high lying tegmen comparatively ensures a safe mastoidectomy with lower risk of injury to the dura and lower chances of recurrence.
Conclusion
The computed tomography findings of tegmen height correlates well with the intraoperative findings. The tegmen height from the superior-most point of lateral semicircular canal to the lower-most point of tegmen has a bearing on surgery as it corresponds to the level of entering from antrum to attic. A classification system between these two points is proposed which can accurately assess the tegmen height. We recommend this classification system to be included in the imaging parameters which can guide a surgeon effectively prior to the surgery.
Declarations
Conflict of Interest
The authors declare that they have no conflict of interest.
Ethical Approval
Institutional ethics committee clearance was obtained before the start of the study.
Informed Consent
The subjects/ patients were counselled about their participation in the study and informed consent was taken.
Footnotes
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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