Thoracic Spine Anatomy
The thoracic region consists of 12 vertebrae, and is the largest segment of the spine. Due to its articulations with rib cage, the thoracic spine is more rigid than the cervical and lumbar regions. It protects thoracic viscera such as heart, lungs, and great vessels. Starting from early fetal period, the sagittal alignment of the thoracic spine is kyphotic, ranging between 20 to 40 degrees in adults.
The thoracic vertebrae are intermediate in size between the cervical and lumbar vertebrae. There are regional variations from Tl to T12. Tl shows some similarities to the cervical vertebrae with uncinate processes protruding from its superolateral edges. T2 to T8 are quite uniform, and are considered as typical thoracic vertebrae. T9 to T12 are the transition vertebrae, and shows some similarities with lumbar vertebrae. Thoracic region can also be subdivided as upper (T1-T4), middle (T5-T8), and lower (T9-T12) thoracic areas.
Vertebral bodies: Thoracic vertebral bodies are heart-shaped, with a deeper anterior posterior dimension than medio-lateral width. The left side of the vertebral body may be slightly flattened or depressed due to pulsations of the aorta. The dimensions of the bodies increase as one moves down the spinal column (see picture).
Pedicles: The pedicles connect the vertebral bodies to the posterior elements. They are cylinder-like structures and their medial cortex is thicker than the lateral cortex. They are oval in cross section, having larger heights and smaller widths. However, their elliptical shapes are highly variable. The medial wall of the pedicle is bounded by the exiting nerve roots and the thecal sac. Laterally the pedicle is bounded by the costovertebral ligaments, joints, and ribs. Superiorly and inferiorly, the pedicles are bounded by the adjacent neural foramen. Pedicle sizes are around 6 to 8 mm from T1 to T12. The smallest sizes of pedicles are found between T3 and T6 and may only be 10 mm in height and 4 mm in width. The largest pedicle diameter is usually in the low thoracic region, being 14 mm in height and 8 mm in height at T12 level. In the proximal thoracic spine, pedicle sizes are slightly larger than those of middle thoracic levels. Pedicle axes show convergence at the axial (transverse) plane. These transverse angles are also show regional differences. The largest transverse angulations are observed at T1 (30 degrees). A steady reduction in transverse angulations is observed as one moves caudally, with the transverse inclination of the T12 pedicles being approximately 0 degrees. The pedicles are also show downward inclination at the sagittal plane. This inclination is at the posterior superior to anterior inferior direction, and is around 20 degrees.Determining the posterior projection of the pedicle on the posterior elements is of crucial importance during pedicle screw placement. The center of the pedicle projection lies at the intersection of a line drawn parallel to the edge of the transverse process in its superior one-third, and a line drawn 1 to 2 mm medial to the lateral edge of the lamina.
Facet Joints: The thoracic facets oriented in the coronal plane. Thus, thoracic facets allow primarily lateral bending and axial rotation. The superior articular facet of the caudal vertebrae forms the roof of the neural foramen.
Transverse Processes: The transverse processes join the pedicles and the laminae at their bases. They extend lateral and posterior, to make room for the ribs to pass anterior to them. They articulate with the ribs on their anterolateral surface. T1 transverse processes is the largest and then gradually decrease in size toward T12. The angulation in the transverse plane changes from fairly flat at T1 to more posteriorly protruding at T12.
Lamina and Spinous Processes: The thoracic vertebrae typically have long, slender spinous processes that point downward and overlap vertebral arches of the vertebra below. Thus, thoracic laminectomy requires removal of the inferior portion of the rostral spinous process. Both the laminae and the spinous processes slope in a superior anterior to posterior inferior direction. They serve as attachment points for the segmental spinal muscles and the thoracolumbar fascia.
Each thoracic segment is accompanied by a pair of ribs. The ribs articulate with the spinal column posteriorly, and the sternum anteriorly. Ribs 1 to 7 are termed as true ribs. They articulate with the sternum directly. Ribs 8-10 are termed as false ribs, and they articulate with the costocartilage of the rib above. Ribs 11 and 12 are termed as floating ribs, because they do not articulate to either the sternum or the costal cartilage at their distal ends (see picture).
Costovertebral Joints (Costal facets): The first, eleventh, and twelfth pair of ribs articulate with their named vertebra only. The second through tenth ribs articulate both with their named vertebral body and with the intervertebral disk and the vertebra above. Also, each pair of ribs articulates with the anterior surface of the transverse process of its named vertebrae. The ribs are connected to the vertebral column by the costovertebral ligaments. The space between the transverse process, the lateral edge of the pedicle, and the medial edge of the rib constitutes a triangular osteoligamentous zone. This zone can be used for extrapedicular screw placement (see picture).