Morphometric Analysis of the Foramen Magnum in South Indian Dry Adult Skulls: A Cross-Sectional Study with Clinical Implications

Background: The foramen magnum (FM) is an important hole in the base of the skull through which important neurovascular structures pass, such as the medulla oblongata, vertebral arteries, and the spinal accessory nerves. Variations in the size and form of FM are of great importance in neurosurgery, forensic medicine, especially for sex determination and identification of anomalies affecting the CVJ (e.g., Chiari malformation), and in radiology, for planning transcondylar surgical approaches. Very little information is available regarding morphometry of the FM in manual vernier calipers in the south Indian population. This study was conducted with the aim of assessing the morphometric features and shapes of the FM in dry adult skulls of people from South India and the clinical significance of the same. Material and Methods: A total of 100 dry adult human skull specimens (of unknown sex) in the South Indian osteological specimens were used in this cross sectional observational study. Injured, abnormal or underdeveloped skulls were not included. To minimise inter-observer variability, a single observer was used and measurements were made with a standard manual vernier caliper with a reading of 0.01 mm. Basion to opisthion (anteroposterior diameter; APD); maximum width between lateral margins (transverse diameter; TD) were recorded. The FM index (FMI = TD × 100 / APD) and approximate surface area (APD × ¼ × π × TD - approximate formula due to Radinsky) were calculated. Oval, round, tetragonal, pentagonal, hexagonal and irregular were the visual shape classifications of the shapes. Data were analysed by descriptive statistics, comparisons being made when appropriate. Results: The mean APD was 34.2 ± 2.6 mm (range: 29.5–39.8 mm), and the mean TD was 28.8 ± 2.3 mm (range: 24.0–34.5 mm). The mean FMI was 84.3 ± 6.5, and the mean surface area was approximately 775 ± 105 mm². Most of the samples were oval (42%), followed by round (30%), and also tetragonal, pentagonal and irregular shapes (less frequently). In most of the sex known subsets, males had larger dimensions than females, indicating sexual dimorphism. All measurements were repeated twice and averages calculated. Conclusion: This is a cost-effective and simple manual Vernier caliper based study, which gives some baseline morphometric reference values for the foramen magnum area in the dry skulls of South Indian origin. These predominant shapes of oval and round are supported by studies conducted in the region in India and the observed sexual dimorphism have significant clinical implications for neurosurgical planning, forensic identification and radiological interpretation. These data highlight the importance of population specific data which can enhance the accuracy of diagnosis and surgical results in the craniovertebral area. Further studies with more larger sample size and known population are recommended.

Keywords: South Indian skulls, Chiari malformation, Transcondylar surgical approaches and Foramen magnum are the keywords.