OAA

OAA – Occipital-Atlas-Axis

Between C1 and C2 there are two synovial joints, known as the atlantoaxial joint. These joints facilitate rotation in this plane. The axis has an upper extension (pointing upwards), which is a conical bone called the dens.

Disorders of the head and upper neck can be described as abnormalities of the craniovertebral (craniocervical) junction (CCJ). The CVC is one of the most unique and complex areas of the body, as this is where the brain transitions into the spine. The CVC consists of the occipital bone, the atlas (C1) and the axis (C2), as well as a network of complex nerve and vascular structures. The occipital bone, atlas and axis are responsible for the majority of the spine’s rotation, extension and flexion – in short, no other part of the spine moves more than the CVC.

The craniocervical junction is arguably the most critical part of the body, with various interfaces. The junction between the base of the skull and the upper part of the spine is teeming with important neurological and vascular activity.

The occipital bone rests on the atlas, the first bone in the neck. The atlas is named after the Greek god Atlas, who carried the ‘world on his shoulders’. The atlas and the skull are connected by synovial joints, known as the atlas-occipital joint. The atlas is also referred to as C1. Whilst other bones of the spine are numbered in this way (for example, the first bone of the lumbar spine is designated TH1), this vertebra differs significantly from the other vertebrae in both form and function. The atlas is ring-shaped and has the important task of supporting the head. It also serves to facilitate movement of the head and neck.

The occipital bone is a bone that covers the back of the head, an area known as the occiput. The occipital bone is the only bone in the head that is connected to the cervical spine. The occipital bone surrounds a large opening known as the foramen magnum. The foramen magnum allows important nerves and blood vessels to pass between the brain and the spine. The spinal cord passes through this foramen magnum to enter the skull. The brainstem also passes through this opening. Through the foramen magnum, two important blood vessels that run through the cervical spine – known as the vertebral arteries – can enter the cranial cavity and supply the brain with blood.