Spinal and Thoracic Joint Anatomy: Ligaments & Movement
Spinal Column Joints and Ligaments
The vertebrae are linked together through their vertebral bodies by interposing between them a fibrocartilage disc (fibrous synchondrosis). This synchondrosis occurs in the center, a gelatinous core known as the nucleus pulposus. Around the nucleus pulposus, which is a remnant of the embryonic notochord, is the cartilaginous ring (annulus fibrosus). The fibers of the annulus fibrosus are oriented in layers, similar to an onion, with their direction changing: vertically oriented on the outside and horizontally oriented on the inside.
The vertebrae are also joined through their articular processes. These are typically arthrodial (flat facet) joints. For example, the atlanto-axial joint allows for rotational movement and is classified as a trochoid (pivot) joint.
Both the articulations of the vertebral bodies and the apophyseal (facet) joints are reinforced by ligaments that extend continuously from the occiput to the coccyx.
Continuous Spinal Ligaments
There are three major continuous ligaments extending from the occiput to the sacrum/coccyx:
Anterior Longitudinal Ligament
The Anterior Longitudinal Ligament extends like a broad ribbon from the occiput along the anterior aspect of the vertebral bodies and intervertebral discs.
Posterior Longitudinal Ligament
The Posterior Longitudinal Ligament extends along the posterior aspect of the vertebral bodies, within the vertebral canal. It is shaped like a tape but expands slightly over the intervertebral discs, giving it a scalloped appearance.
Supraspinous Ligament
The Supraspinous Ligament extends from top to bottom, covering the apex of the spinous processes.
Discontinuous Spinal Ligaments
Besides these continuous ligaments, there are also ligaments that link individual vertebrae, often described as ‘discontinuous’ because they span between adjacent vertebrae:
Ligamentum Flavum (Yellow Ligament)
The Ligamentum Flavum (Yellow Ligament) connects the laminae of adjacent vertebrae. It is very rich in elastic fibers, giving it a yellowish color.
Intertransverse Ligaments
The Intertransverse Ligaments are located between the transverse processes, connecting each pair of adjacent transverse processes.
Interspinous Ligaments
The Interspinous Ligaments, composed of collagen fibers, are found between each pair of adjacent spinous processes.
Suboccipital Joints
Among the joints of the spine are the suboccipital joints, which include the articulation between the occiput and the atlas (C1), and the articulation between the atlas (C1) and the axis (C2). The atlas (C1) has no vertebral body; its upper surface features concave facets that articulate with the convex occipital condyles of the occipital bone. The joints between the atlas and axis are articulated through the articular processes of both vertebrae. Additionally, the atlas articulates with the axis through the odontoid process (dens) of the axis.
Atlanto-Axial Facet Joints
Atlanto-Odontoid Joint
This joint allows for significant rotational movement of the head.
Spinal Movements
The global movements of the spine include flexion-extension, lateral bending, and rotation. In the thoracic region, the presence of the ribs significantly limits the range of motion of the vertebral column.
Thoracic Cage Joints
Rib Articulations with Vertebrae
Costovertebral Joints
The costovertebral joint is formed by the head of the rib articulating with the vertebral bodies and the intervertebral disc. The head of a typical rib has two facets: one articulates with the vertebral body of the same number (n), and the other with the vertebral body superior to it (n-1). These are classified as arthrodial (gliding) joints. All ribs articulate in this manner, except for the first rib (which articulates only with T1) and the last two floating ribs (which articulate only with their corresponding vertebrae).
Costotransverse Joints
The rib also articulates with the transverse process of the thoracic vertebrae. Behind the neck of the rib, there is a tuberosity with a flat facet that forms the costotransverse joint. This is also an arthrodial joint.
Both the costovertebral and costotransverse articulations are reinforced by a series of ligaments that strengthen the joint anteriorly, posteriorly, and somewhat remotely. These include the radiate ligament of the head of the rib and the costotransverse ligaments.
Sternocostal Joints
Within the chest, the costal cartilages form the sternocostal joints with the sternum. The first seven ribs have their own articular cartilage notches on the lateral border of the sternum. The eighth, ninth, and tenth ribs join with the cartilage immediately superior to them, forming the costal margin (composed of the costal cartilages of ribs 8, 9, and 10). The last two ribs (floating ribs) do not join the sternum; they articulate only with their corresponding vertebrae.
Thoracic Cage Movements
The movements permitted at the sternocostal and costovertebral joints are often compared to the movement of a bucket handle. During inspiration, these movements increase both the sagittal (anteroposterior) diameter and the transverse diameter of the thoracic cage. Conversely, during expiration, the opposite occurs, and these diameters decrease.
Muscles of the Trunk
Latissimus Dorsi
Muscles of the trunk are often categorized by their position from superficial to deep. The Latissimus Dorsi is a large, superficial muscle of the back.