The foramen may be absent (in approx. 2% of the cases), in which case the middle meningeal artery enters the cranial cavity through the foramen ovale.
The foramen spinosum and ovale may be continuous, and the foramen spinosum may be duplicated. Wood-Jones (1931) found the foramen spinosum to be more or less incomplete in approx. 44% of the cases. The foramen spinosum was small or altogether absent in 0.4% of Lindblom's (1936) cases. This is especially true when the middle meningeal artery arises from the ophthalmic artery (the foramen would be near to empty in that case). In rare cases, early division of the middle meningeal artery into a posterior and anterior division may result in a duplication of the foramen spinosum.
In the newborn, the foramen spinosum is about 2.25 mm and in the adults about 2.56 mm in length. The width of the foramen extends from 1.05 mm to about 2.1 mm in the adults. The average diameter of the foramen spinosum is 2.63 mm in the adult.
The earliest perfect ring-shaped formation of the foramen spinosum was observed in the 8th month after birth and the latest in 7 years after birth in a developmental study on the foramen rotundum, foramen ovale and foramen spinosum. The majority of the foramen in the skulls studies was round in shape. Ginsberg et al. (see reference below) observed asymmetry of size in 16% of their patients.
In a study under 123 CT studies, Ginsberg, Pruett, Chen and Elster did not find an inverse relationship between the size of the foramen spinosum and that of the foramen ovale (for instance, a smaller foramen spinosum did not correlate with the size of the foramen ovale).
^ ab Illustrated Encyclopedia of Human Anatomic Variation: Opus V: Skeletal Systems: Cranium - Sphenoid Bone. Illustrated Encyclopedia of Human Anatomic Variation. Retrieved on 2006-04-10.
^ Lang J, Maier R, Schafhauser O (1984). "Postnatal enlargement of the foramina rotundum, ovale et spinosum and their topographical changes". Anatomischer Anzeiger156 (5): 351-87. PMID 6486466.
^ ab Yanagi S (1987). "Developmental studies on the foramen rotundum, foramen ovale and foramen spinosum of the human sphenoid bone". The Hokkaido Journal of Medical Science62 (3): 485-96. PMID 3610040.
^ Ginsberg LE, Pruett SW, Chen MY, Elster AD (February 1994). "Skull-base foramina of the middle cranial fossa: reassessment of normal variation with high-resolution CT". Americal Journal of Neuroradiology15 (2): 283-91. PMID 8192074.
This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant.