Thoracic back pain
Spinal pain is a well recognised condition associated with significant personal and community burdens. The most common spinal regions studied are the lumbar and cervical spine, probably because of their strong and well-established associations with pain conditions, work-related injuries, intervertebral disc degenerative conditions, headaches and psychosocial disturbances. Compared to the lumbar and cervical spine, the thoracic spine has received less attention in terms of clinical, genetic and epidemiologic research, yet pain experienced in the thoracic spine can be equally disabling, imposing similar burdens on the individual, community and workforce.
Thoracic back pain is a term commonly used to describe mid or upper-back pain. Thoracic back pain is much less common than neck or low back pain; therefore, the diagnosis of pain originating from thoracic region has been challenging. Frequently, thoracic back pain has a benign musculoskeletal origin, but it may indicate a more serious underlying problem. The neural complexity of the thoracic spine, along with referred visceral pain, leads to poor pain source localization.
Although the incidence of spinal pain has been reported as 54–80% of the general population, the number of patients suffering from chronic upper or mid-back pain secondary to thoracic disorders is rather small, ranging from 3% to 22% of outpatient pain population.
The thoracic area of the spine consists of 12 vertebrae (10 of which connect to ribs with costovertebral joints), intervertebral discs, supporting soft tissue, and 12 thoracic nerves. Thoracic vertebrae are considerably larger and stronger than cervical vertebrae. The spinous processes of T1 and T2 are long, laterally flattened, and directed inferiorly. On the other hand, the spinous processes of T11 and T12 are shorter, wider, and directed more posteriorly. In general, thoracic vertebrae have longer and larger transverse processes when compared to cervical vertebrae. Movements of the thoracic region are limited by the attachment of the ribs to the sternum. From a structural viewpoint, the 12 thoracic vertebrae have three separate shapes, with the smaller upper four thoracic vertebrae sharing some characteristics with the cervical vertebra and the larger lower four thoracic vertebrae sharing similar features with the lumbar vertebrae. The middle four thoracic vertebrae share characteristics with both the cervical and lumbar regions. The spinal canal has a similar diameter in the thoracic and lumbar area, but the space around the thoracic spine is narrower than lumbar spine. Thus, spinal cord compression from a herniated thoracic disc is more likely at the thoracic level. The upper thoracic vertebral interspaces from T1 to T2 and the lower thoracic vertebral interspaces from T10 to T12 are functionally equivalent to the cervical and lumbar interspaces, respectively, which facilitates placement of the epidural block in a similar fashion to the cervical and lumbar approaches. The thoracic vertebral interspaces between T3 and T9 are functionally unique because of the acute downward angle of the spinous processes. Epidural blockade of these middle thoracic interspaces requires use of the paramedian approach to the thoracic epidural space.
The thoracic facet joints are paired synovial joints formed by the articulation of the superior and inferior articular facets of adjacent vertebrae. They are true joints in that they are lined with synovium and possess a true joint capsule that is richly innervated and so can become a pain generator. The poor localization of facet joint pain is explained in part by the pattern of overlapping of the sensory innervation of these joints and their close proximity to one another. The posterior rami of a nerve root diverge from the spine at the intervertebral foramen and pass dorsally and caudally through the intertransverse ligament where they divide into medial, lateral, and intermediate branches. The medial branch crosses over the top of the transverse process at a variable point lateral to the point at which the transverse process meets the vertebra. The nerve then travels medially and inferiorly across the surface of the transverse process to innervate the facet joint. The medial branch supplies the lower pole of the facet joint at its own level and the upper pole of the facet joint below. Therefore each facet joint receives its innervation from a medial branch nerve of two posterior primary rami. One branch arises from the nerve at the same level as the joint, and the other from the segmental level above. This explains why the dorsal nerve from the vertebra above the affected level must often also be blocked to provide complete pain relief.