Introduction: The foramen magnum is a transitional zone between cranial cavity and spinal canal, and it is related with the very important neuro-vascular structures like vertebral arteries, spinal accessory nerves and spinal arteries and terminal part of medulla oblongata. Thus, cranio-cervical junction surgeries require a thorough knowledge about the variations in the morphometry and morphology of the foramen magnum.

Materials and Methods: This study analyzed 70 dried human skulls from the Departments of Anatomy at TS Misra Medical College & Hospital, Era’s Lucknow Medical College & Hospital, and KGMU, Lucknow. The shape of the foramen magnum was classified, and its transverse and anteroposterior (AP) diameters were measured using a vernier caliper. The foramen magnum index (FMI) and the foramen magnum area (FMA) were also calculated.

Results: The foramen magnum showed six distinct shapes: oval (47%), round (30%), tetragonal (10%), triangular and irregular (10% each), pentagonal (2%), and hexagonal (1%). The average AP diameter was 34.3±2.90 mm, and the average transverse diameter was 28.9±2.80 mm. The average FMI was 77.80±27.80, while the average FMA was 845.90±87.50 mm².

Conclusions: The data from this study are valuable for neurosurgeons performing surgeries at the cranio-cervical junction and posterior cranial fossa. Understanding the morphometric and morphological characteristics of the foramen magnum is crucial for the prognosis and treatment of neurological conditions like Arnold Chiari syndrome, achondroplasia, and posterior cranial fossa lesions.

Keywords: Foramen magnum, Transcondylar approach, Foramen magnum index, Foramen magnum area, Transverse diameter (TD), Anteroposterior diameter, Trigonal