|Year : 2021 | Volume
| Issue : 3 | Page : 131-134
Nonmetrical variants of the sigmoid sulcus: A study on dry human skulls
Rakesh Kumar Diwan, Rakesh Kumar Verma, Arvind Kumar Pankaj, Anita Rani, Navneet Kumar
Department of Anatomy, King George's Medical University, Lucknow, Uttar Pradesh, India
|Date of Submission||19-Sep-2020|
|Date of Decision||29-Dec-2020|
|Date of Acceptance||21-Mar-2021|
|Date of Web Publication||30-Jul-2021|
Rakesh Kumar Diwan
Department of Anatomy, King George's Medical University, Shah Mina Road, Chowk, Lucknow - 226 003, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
Background: Sigmoid sinus is an area of interest for anatomists, neurosurgeons, and radiologists. Different procedures such as mastoidectomy, thrombectomy in the sigmoid, and transverse sinus thrombosis have realized the need of proper morphological knowledge of the sigmoid sinus and its variations and their important relations. Therefore, the present study was conducted to report the prevalence of various nonmetrical attributes of the sigmoid sulcus in dry human skulls in India. Methodology: This study was conducted on 224 human skulls in which sex and origin were not known and these were taken from the departmental osteology laboratory. Interior of the skull base at the area of the sigmoid sulcus was carefully observed for the presence of any osseous projection on its anterior or posterior lips such as crest, plate, or bars. Shape, position, and side of projection were noted and frequency was calculated in percentage. Results: Out of 224 skulls, 80% of the sigmoid sulci showed one or other form of bony features. A total prevalence of 43.08% was observed for crests, whereas plates showed lesser frequency (28.57%) on the anterior lip of the sigmoid sulcus. Bony bars were the least common (9.14%) features, but were characteristically located at the angle region of the sigmoid sulcus. In 16.74% of skulls, sigmoid sulcus was shallow. Conclusion: Occurrence of bony attributes in the form of plates, crests, and bridges along the course of the sigmoid sulcus is quite high and shows frequent variability in location and laterality, which may adversely affect the morphology of the sigmoid sinus.
Keywords: Cerebellopontine angle, sigmoid sinus, sigmoid sulcus, translabyrinthine, transpetrosal
|How to cite this article:|
Diwan RK, Verma RK, Pankaj AK, Rani A, Kumar N. Nonmetrical variants of the sigmoid sulcus: A study on dry human skulls. Natl J Clin Anat 2021;10:131-4
|How to cite this URL:|
Diwan RK, Verma RK, Pankaj AK, Rani A, Kumar N. Nonmetrical variants of the sigmoid sulcus: A study on dry human skulls. Natl J Clin Anat [serial online] 2021 [cited 2021 Sep 21];10:131-4. Available from: http://www.njca.info/text.asp?2021/10/3/131/322804
| Introduction|| |
The sigmoid sulcus is located in the posterior cranial fossa between petrous temporal and occipital bone. Direction of the sigmoid sulcus is forward and downward initially, then downward and medially, and in the last, it is directed forward toward the jugular foramen. It contains the sigmoid sinus, a continuation of transverse sinus which itself drains as the internal jugular vein. These channels present within the dura mater drain the venous blood from the brain.,
Sigmoid sinus is an area of interest for anatomists, neurosurgeons, and radiologists. Many procedures such as mastoidectomy and thrombectomy require complete and proper reevaluation of morphological knowledge, anatomical variations and their important relations to adjacent structures. Transpetrosal approach with partial labyrinthectomy has shown variation in the course of the sigmoid sinus. Researchers emphasize the role of dry bone observations for documenting morphological variants as these are not easy to observe during dissections by surgeons and anatomists. In addition, radiological studies also need evidence for confirming the diagnosis. Therefore, this study was conducted to find out the prevalence of various nonmetrical attributes of the sigmoid sulcus in dry human skulls in the Uttar Pradesh region of India.
| Materials and Methods|| |
This study was conducted in the osteology laboratory of anatomy department on 224 dry adult human skulls. Skulls were collected from the region in and around Lucknow district of Uttar Pradesh over the period of 80 years. The study was conducted in the period from January 2019 to March 2019. The origin, age, and sex of these skulls were not known. Interior of the skull base at the area of the sigmoid sulcus was carefully observed for the presence of any osseous projection on its anterior or posterior lips such as crest, plate, or bars. Bars were subtyped into complete and incomplete variety. Shape, position, and side of projection were noted and frequency was calculated in percentage. For describing location, sinus was divided into lateral, angle, and medial zones. Depth of the sulcus was observed and classified as deep or shallow and association of depth was made with laterality. Depth of the sulcus was subjectively perceived as shallow or deep on comparing various skulls.
| Results|| |
Out of 224 skulls, 80% of the sigmoid sulci showed one or other form of bony features. Left-sided sulcus was exhibiting more features as compared to the right. A total prevalence of 43.08% was observed for crests, whereas plates showed lesser frequency (28.57%). Crests and plates were distributed randomly anywhere along the anterior lip [Figure 1] and [Figure 2]. Bony bars were the least common (9.14%) features, but were characteristically located at the angle region of the sigmoid sulcus. These osseous bars were spanning between the posterior margin of petrous temporal and posterior lip of the sigmoid sulcus [Figure 3] and [Figure 4]. None of the skulls exhibited any additional osseous feature on the posterior lip of the sigmoid sulcus. Various features exhibited different combinations in each case.
|Figure 1: Basis cranii interna showing crest at the tip of the sigmoid sulcus on the left side (arrow)|
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|Figure 2: Basis cranii interna showing plate on the both sides of the sigmoid sulcus (arrow)|
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|Figure 3: Basis cranii interna showing unilateral bridge on the right side (arrow)|
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|Figure 4: Basis cranii interna showing bilateral bridge at the tip of the sigmoid sulcus (arrow)|
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The crests were located on the left side in 46.42% of skulls in comparison to 39.73% on the right side. Plates were present in 33.92% of cases on the left side and in 23.21% of cases on the right side of the skull. Bilateral complete bridges over the sigmoid sulcus were present in 0.89% of skulls. These bridges were ring-like structures [Figure 4]; the unilateral bridges were observed in 2.24% of skulls on the left side and 1.33% on the right side. The incomplete bridges were present in 7.58% of skulls on the left side and 5.35% on the right side [Table 1]. In 16.74% of skulls, sigmoid sulcus was shallow [Figure 5]. Out of total 75 shallow sulci, 27 (36%) were observed on the right side and 48 (64%) on the left side.
|Table 1: Prevalence of various types of bony attributes on the sigmoid sulcus|
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|Figure 5: Basis cranii interna showing shallow groove on the right side of the sigmoid sulcus (arrow)|
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| Discussion|| |
The abnormal bony connection between occipital and temporal bone over the sigmoid sulcus has been reported by many researchers. Singh et al. conducted a study in 318 adult Indian skulls and reported different morphological bony projections on the anterior lip of the sigmoid sulcus. They found crests, plates, and bridges in 42.9%, 22.3%, and 13.5%, respectively. These values are almost in the similar range as we have observed in the present study for crests (43.08%) and plates (28.57%) except for frequency of bridges which are in the lower range in comparison to the above study, i.e., 9.04%. Singh et al. reported 11.8% prevalence of incomplete bridges and 1.7% of complete type, whereas we observed 6.47% of cases of incomplete bridging and 2.67% of cases with complete bridges. It is interesting to note that they observed higher frequency of various bony traits on the right side n contrast to our findings, where all features were more frequent on the left side. Singh et al. did not find complete bilateral bridging, whereas we noted it in six sides. Kumar et al. reported identical bilateral bony bar at the sigmoid sulcus in a case report.
In our study, the depth of the sigmoid sulcus was shallow in only 75 sides (mostly on the left). The depth of the sigmoid sulcus is primarily contributed by the occipital bone. In a study conducted using high-resolution computer tomography, it was observed that the area of the sigmoid sinus was greater on the right side. Our observation strengthens once again the fact that as in the majority of subjects, the superior sagittal sinus continues into the right transverse sinus which in turn drains into the right sigmoid sinus. Therefore right sigmoid sinus is larger and hence justified to have a corresponding deeper sulcus.
Altered morphology of the sigmoid sulcus may serve as proxy to the variant anatomy of the sigmoid sinus. The sigmoid sulcus exhibited differences in its length and width in the same skull base on the right and left sides, but studies on nonmetrical variations are very few. These variants are thought to be genetically determined and often used as population markers too.
Two types of bridging (ponticuli foraminis jugularis) are observed in relation with jugular foramen. This bony bridge is contributed by the intrajugular process of the temporal bone situated posterior to the triangular depression and the bony process of the occipital bone. Now, on the basis of location of bony process of occipital bone in relation to hypoglossal canal, bridging is classified as Type I (anterior type) and Type II (posterior type)., Researchers proclaim that formation of osseous bar at the sigmoid sulcus may be due to a venous sinus anomaly, dural ossification, or intracranial calcifications. Dura mater develops from the mesenchyme around the perineural tube which is invaded by neural crest cells. These neural crest cells are also the source of neurocranial bones. Therefore, few cells of the dura mater have the potency to behave like osteoblast cells and may calcify. Calcification may occur secondary to vascular malformations and aneurysm. Calcification is often observed in cases of cavernous angiomas vessel wall and adjacent parenchyma. In cases of basal cell nevus syndrome early calcification of various regions of dura mater is quite common and are also common sites of physiologic calcifications.
These bony projections have a great clinical relevance for neurosurgeons. The role of various osseous attributes at the sigmoid sulcus becomes important as the sigmoid sinus and the area associated with it are used for several surgical approaches. While doing retrosigmoid approach for acoustic neuroma if any osseous bar is present, it can lead to dilatation of the sigmoid sinus proximally and may cause difficulty in dissection during various surgical approaches. Variations in shape and position of the sigmoid sinus and its distance from the external acoustic meatus are important during the surgical approach to the tympanic cavity, mastoid antrum, membranous labyrinth, internal acoustic meatus and pontocerebellar trigone surgery, and transmastoid cisternoscopy.
During translabyrinthine approach, exposure of desired area depends on high placement of jugular bulb, large sigmoid sinus, and inferiorly placed middle fossa. The morphological variations of the sigmoid sinus may affect adversely the success of surgical approaches for cochlear implantations, the cerebellopontine angle, and the excision of glomus jugulotympanicum tumors. The significance of these bony features can be understood by one of the recent studies carried out by Singh et al. (2019) who performed microdissection on 94 formalin-preserved cadavers and classified sigmoid sinus using presigmoid bony plates into favorable, intermediate, and unfavorable. As these features are not constant or uniformly distributed across all skulls, could mislead surgeons intraoperatively and necessitates thorough preoperative assessment. The knowledge of variations in the anatomical location of the sigmoid sinus in the temporal bone is very important for performing mastoid surgeries, especially transmastoid approaches to the cerebellopontine angle.
The data represented in the present study generate need for study on larger sample size and necessitate evaluation of gender differences of such attributes with known ethnicity due to their association with genetic factors.
| Conclusion|| |
Occurrence of bony attributes in the form of plates, crests, and bridges along the course of the sigmoid sulcus is quite high and shows frequent variability in location and laterality, which may adversely affect the morphology of the sigmoid sinus, thus adversely affecting the procedures carried out through this sinus.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]