The first two vertebrae of the spinal column, known as C1 and C2, are uniquely positioned at the top of the spine, at the base of the skull. These specialized bones differ from other vertebrae, forming a complex that supports the skull and enables extensive head and neck movement.
Anatomy of the Atlas and Axis
The first cervical vertebra, C1, is commonly called the atlas. This vertebra is ring-shaped, lacking a typical vertebral body or a spinous process, which distinguishes it from most other vertebrae. The atlas consists of an anterior arch and a posterior arch, connected by two lateral masses. These lateral masses contain concave superior articular facets that articulate with the skull, allowing for nodding movements. The inferior articular facets on the atlas are flatter, designed for articulation with the axis below.
The second cervical vertebra, C2, is known as the axis. It is recognized by a prominent, tooth-like projection called the dens, or odontoid process, which extends upward from its body. The dens acts as a central pivot point around which the atlas and the head rotate. The axis also features superior articular facets that articulate with the atlas.
The Atlantoaxial Joint Complex
The connection between C1 and C2 is not a single joint but a complex of three distinct synovial joints. This intricate arrangement includes one median atlantoaxial joint and two lateral atlantoaxial joints. The combined structure of these joints allows for a broad range of head motion, particularly rotation.
The median atlantoaxial joint is classified as a pivot joint. It is formed by the dens of C2 articulating with the inner surface of the anterior arch of C1 and the transverse ligament of the atlas. This enables the atlas and head to rotate around the dens. The joint is enclosed by a loose capsule, permitting substantial movement.
The lateral atlantoaxial joints are classified as plane, or gliding, joints. These two joints are formed by the inferior articular facets of the atlas (C1) and the superior articular facets of the axis (C2). While primarily allowing gliding movements, their orientation also contributes to the complex’s rotational capacity.
Movement and Stability
The atlantoaxial joint complex is designed to facilitate extensive head movements, with rotation being its primary function. Approximately 50% to 60% of the total rotation of the head and neck occurs at this joint, allowing for movements like shaking the head “no.” During rotation, the atlas and the head move as a single unit around the dens of the axis. Limited flexion, extension, and lateral flexion are also possible at this joint.
Despite its high mobility, the atlantoaxial joint complex maintains stability through a network of strong ligaments. The transverse ligament of the atlas is especially significant, running between the lateral masses of C1 to hold the dens of C2 securely against the anterior arch of C1. This ligament prevents excessive forward displacement of the atlas relative to the axis. Other ligaments, such as the alar ligaments and the tectorial membrane, also limit extreme movements. The alar ligaments limit excessive rotation, while the tectorial membrane covers the posterior surface of the dens.
Clinical Significance
The C1 and C2 joints are notable for their location and function at the top of the spinal column. These joints are directly beneath the skull and surround the brainstem and upper spinal cord. Their unique structure and extensive range of motion mean they rely heavily on ligamentous support for stability.
This reliance makes them susceptible to injury, particularly from trauma like whiplash or falls involving axial loading to the head. Conditions like instability, fractures, or degenerative changes, such as arthritis, can arise in these joints. Damage to these joints can affect the C2 spinal nerve, the vertebral artery, or the spinal cord itself, potentially leading to pain, limited mobility, or neurological symptoms.