Prevalence and Morphological Characteristics of Occipito-Atlanto-Axial Fusion among South Indian Population: A Cross-sectional Study

Introduction: The anatomy of the cervical vertebrae, along with the atlanto-occipital joint, allows for free flexion, extension and rotation, which occurs almost entirely in the first two cervical vertebrae. Assimilation of the atlas and axis is a rare or uncommon abnormality recorded in anatomical, morphological and radiological studies. Aim: To determine the prevalence and morphological characteristics of occipito-atlanto-axial fusion in the South Indian population. Materials and Methods: A cross-sectional study was conducted in the Department of Anatomy at Adichunchanagiri Institute of Medical Sciences, Mandya, Karnataka, India from May 2024 to November 2024. A total of 250 dry skulls were examined for fusion between the occiput and the cervical vertebrae and the findings were recorded. In the skulls that showed fusion, the Anteroposterior (AP) diameter and Transverse Diameter (TD) of the foramen magnum were measured using a Tesa Swiss-made digital vernier caliper accurate to 0.01 mm. Measurements of the diameters of the foramen magnum were recorded in centimeters and the prevalence of occipito-atlanto-axial fusion was expressed in terms of frequency and percentage. Results: Out of the 250 skulls, only one exhibited occipito- atlanto-axial fusion (0.4%). The fusion points were at the superior and inferior articular surfaces of the atlas with the occipital condyle and the superior facet of the axis, respectively. The anterior arch of the atlas was not fused in the middle; however, it was fused laterally on the borders of the Foramen Magnum (FM). The AP measurement of the foramen magnum was 3.3 cm and the TD measurement was 3.5 cm. Conclusion: Understanding the bone fusion between the cranial base and the first and second cervical vertebrae is essential. Such skeletal aberrations may result in sudden unexpected death due to compression of critical structures, such as the brainstem and vertebral arteries. Hence, a detailed understanding of occipitocervical synostosis is important for neurosurgeons and radiologists to prevent and avoid possible complications and achieve optimal operative results.