POST-GLENOID FORAMEN: A RARE EMISSARY FORAMEN IN HUMAN SKULL

Objective: The aim of the present study was to determine the presence of post-glenoid foramen in human skulls. Methods: The study was carried out in 100 adult dry human skulls with 200 temporal bones on each side (right and left) which were collected from the Department of Anatomy at RVS Dental College and Hospital, Coimbatore. The selected skulls were placed on a flat table and the considerable foramen was visually identified by two investigators. The foramen was examined by a probing method to find out its patency. Results: Of 100 skulls (200 temporal bones), only one skull showed a unilateral post-glenoid foramen on the mandibular fossa of the left side temporal bone (2% of 200). Conclusion: The emissary veins are considered as an important encephalic venous drainage that connects the intracranial and extracranial venous pathway through emissary foramen. If there is little variation, and rare occurrence of the emissary foramen may cause serious pathological condition. Thereby, the present study highlights one of the rare emissary foramen, the post-glenoid foramen. This anatomical study of post-glenoid foramen may help the surgeons while operating temporomandibular joint and inner ear malformation to avoid the risk of accidental bleeding during surgery.


INTRODUCTION
The post-glenoid foramen is an emissary foramen which is rarely found in humans. It lies anterior to the external acoustic meatus in the line of fusion of the squamous and tympanic parts of the temporal bone [1]. It is also called as foramen jugulare spurium or foramen retroarticulare. If present, it transmits the petrosquamous sinus. The petrosquamous sinus is an emissary vein connecting the intra and extracranial venous networks. It arises from the dorsolateral portion of the transverse sinus, before the confluence of the transverse sinus with the superior petrosal sinus. Then, courses over the superolateral surface of the petrous bone, either within an osseous groove or a complete canal called a temporal canal of Vergi [2].
It has two drainage pathways: One anteroinferiorly into the retromandibular vein through the foramen retroarticulare and another one anteromedially into the pterygoid venous plexus through the foramen ovale. The petrosquamous sinus collects the middle and superficial temporal veins, the masseterine veins, and the internal maxillary veins and drains into the external jugular vein. It usually disappears during the development of adult venous patterns in the past 3 months of prenatal life in humans [3].
Although this drainage pathway is prominent in most mammals where it constitutes the major route of cerebral venous drainage, it is rarely of functional significance in humans, in which the internal jugular vein and vertebral venous system represent the major outflow pathways. With the evolution of primates, the foramen retroarticulare regressed as the pterygoid plexus increased its functional value [4]. In humans, the petrosquamous sinus, found during embryonic development, usually has disappeared by the time of birth. If this sinus persists, it passes through post-glenoid foramen. In the present study, a skull with post-glenoid foramen was found and discussed.

METHODS
A total of 100 human adult dried skulls with 200 temporal bones on each side (right and left) which were collected from the Department of Anatomy at RVS Dental College and Hospital, Coimbatore. The skulls were randomly selected of unknown age and sex. The broken or partly damaged skulls were excluded from the study, only clearly demarcated temporal bones of the skull were included in the study. The selected skulls were placed on a flat table in the dissection hall and the considerable foramen was visually identified by two investigators. The foramen was examined by inserting a probe. The probe is a steel wire measuring about 0.5 mm in diameter and is inserted into the foramen to find out the patency of the foramen. The wire is passed on to the foramen from exterior of the skull and removed it through the interior of the skull cavity to examine the communication between the foramen and the cranial cavity.

RESULTS
Of 100 skulls (200 temporal bones), only one skull showed a unilateral post-glenoid foramen on the mandibular fossa of the left side temporal bone (2% of 200). The foramen was found backwards from the glenoid fossa of the temporal bone (Fig. 1). It lies anterior to the external acoustic meatus in the line of fusion of the squamous and tympanic part of the temporal bone. A needle was inserted to see the patency of the foramen. It was a patent opening. Since it was found in dry human skull, the structure passes through it was deemed to be persistent petrosquamous sinus.

DISCUSSION
The petrosquamous sinus was first described by Knott in 1881 and was named a Knott sinus. It was found in fetal life and developed from a pro-otic group of veins, connecting deep temporal vein to transverse sinus. This sinus runs in the angle between petrous and squamous part of temporal bone and emerges out through post-glenoid foramen [5]. The post-glenoid foramen is an emissary foramen which involves in extracranial venous drainage of the dural venous sinus. Res, Vol 13, Issue 4, 2020, 42-43 Priya and Yasodai of the transverse sinus with the superior petrosal sinus. It runs over the lateral superior surface of the petrous bone. It drains either anteroinferiorly into the retromandibular vein through the foramen retroarticulare or anteromedially into the pterygoid venous plexus through the foramen ovale. Until now, its pathway has been described in animals, predominantly in quadrupedal gait. It was rare in humans and is reported only by anthropologists and anatomists. In the present study, a unilateral post-glenoid foramen was observed. Since it was found in adult dry human skull, the structure passes through it may be persistent of petrosquamous sinus.

Asian J Pharm Clin
Conroy observed post-glenoid foramen in about 10% of human cadaver dissections, most often unilaterally. Through this foramen, the petrosquamous sinus is directed into the retromandibular vein and then into the external jugular vein [6]. Boyd observed a post-glenoid foramen in 0.6% of 1400 human skulls examined most of them <1 mm in size, with only one measuring 1.5 mm [7]. Wysocki detected this structure in 3.5% of the human skull with 0.97 mm 2 as mean area [8]. These findings corroborate with the findings of the present study.
Giesemann et al. reported post-glenoid foramen measuring more than 5 mm in two cases [10]. Ruiz et al. demonstrated a CT angiogram with a post-glenoid foramen in a 7-year-old patient with severe headaches but no inner ear malformation [11]. These studies correlate with the present study that of 100 skulls (200 temporal bones), only one skull showed a unilateral post-glenoid foramen on the mandibular fossa of the left side temporal bone (2% of 200) which may transmit petrosquamous sinus.

CONCLUSION
Post-glenoid foramen is relatively a rare anatomical variation in human skulls. Knowledge of such variation will be useful for the surgeons while operating middle ear anomalies and while operating posterior cranial fossa.

AUTHORS' CONTRIBUTIONS
Dr. G. Priya conceptualized, designed, and drafted the manuscript. Mrs. R. Yesodai gathered the data with relevant photographs.