BACKGROUND AND ANATOMY

CASE REPORT

BACKGROUND Painful musculoskeletal disorders in the region of the thoracic spine can be difficult to diagnose and treat. Imaging examinations are often negative or difficult to visualize, especially in instances of internal disc disruption, costovertebral arthropathies, and zygapophyseal problems.16,23,48,53,71 Knowledge of some of the distinctive anatomical features of this region, combined with information gathered in biomechanical studies2,6,11,15,35,43,44,55,61,62 can assist the clinician in isolating a painful structure (or structures) based on clinical examination. Because spinal metastases are most often seen in the thoracic spine,73,37 and because organic problems can cause musculoskeletal-like pain, it is important that the patient first undergo a thorough medical work up prior to visiting the physical therapist.

When dealing with painful musculoskeletal conditions of the thoracic spine, there are a number of unique features to consider about this region. Anatomical construction of the thoracic spine makes it a rigid structure and, enhanced by the cage, it functions not only to protect the life-sustaining organs in the , but it also serves as the foundation for movements of the head and extremities. The presence of the alone adds a number of possible sources of pain, such as pathology at the costovertebral (dorsal) and even cost-chondro-sternal (ventral) connections.

The thoracic spine has the largest number of vertebrae of the three spinal regions. Morphological variation between the cranial and the caudal vertebrae is great, and this difference in morphology illustrates one of the primary functions of the thoracic spine, which is that of a transition “zone.” The cranial segments, (T1 to T4) and the caudal segments, (T8 to T12) have the appearance and the kinematic features similar to that of the cervical and lumbar spines, respectively. In regard to mobility, the transition between axial rotation at the cervical level and flexion-extension at the lumbar level occurs through the thoracic spine.

The intrinsic rigidity of the thoracic spine and , and the relatively long lever system of the arms, allows for the absorption and transfer of tremendous forces47 through vigorous and/or repeated movements of the upper extremities. Numerous published case studies documenting stress fractures in the ribs, and spinous - as well as transverse - processes of the thoracic spine in golfers, elite rowers, water skiers, jockeys, and power lifters exemplify this phenomenon.18,26,27,32,36,40,41

This paper reviews the clinically significant pathoanatomy and kinematics of the thoracic spine and ribs, and then further discusses it in a case study. Given the interplay of the vertical and horizontal kinematic chains in this region, several afflictions that perpetuate an individual’s painful condition can be seen concurrently. This frequently encountered clinical picture is exemplified in the patient case report.

CLASSIFICATION OF THE THORACIC SPINE The thoracic spine can be seen simply as that part of the vertebral column where the ribs are attached, however, on closer inspection three distinct parts can be observed: 1) the T1 to T6 vertebrae with ribs that have a direct attachment to the sternum, 2) the T7 to T10 vertebrae with ribs that have an indirect attachment to the sternum, and 3) the T11 and T12 vertebrae with ribs that have no sternal connection. In the segments where ribs connect directly to the sternum, there is less flexion-extension range of motion. With regard to sidebending mobility, the segments without the direct sternal connection have more sidebending. Rib connections have little influence on the axial rotation motion of the thoracic spine, and act mainly to limit sidebending. Thus, when testing trunk motions, rib pathology is most often provoked with thoracic sidebending.

Morphologically seen, T1 to T4 are similar to cervical vertebrae except that in the thoracic region the endplate becomes thinner and the spinal canal narrows. At the caudal end, T11 and T12 begin to show

features of increased spinal canal size and thicker endplates, similar to lumbar vertebrae. From T5 to T10, the vertebrae most closely resemble the “true” thoracic region, with thin endplates, narrowed spinal canal, and a critical zone of least vascularity in the spinal cord.21

Careful history taking can give the examining clinician an idea as to the localization of the patient’s problems: pain provoked by movement of the head or arms could indicate upper or mid thoracic pathology, respectively. Whereas, thoracic spine pain provoked by leg motions could be an indication of lower thoracic spine pathology.

Kinematics During uniplanar movements, the greatest degree of axial rotation occurs at the T1 to T8 segments, with significantly decreased amounts at the last three segments. In contrast, the T9 to T12 segments have the most flexion-extension range of motion (similar to the lumbar spine). Sidebending is about equal throughout the thoracic spine, with a slight increase at the last two segments.59

Head movements create spinal motion that extends caudally to T4, while arm movements create spinal motion that occurs down to T6. Leg movement, such as during walking, creates movement of the spine cranially to about T7.28

When an individual performs a motion out of the sagittal plane, in other words performs a movement into sidebending or rotation, the spinal motions are paired with other, synkinetic, motions.66 For example, during sidebending at the T1 to T4 segments, an accompanying ipsilateral rotation occurs. The main motion out of the sagittal plane and its automatic accompanying synkinetic motion are termed “coupled” motion patterns. In the same way, when a rotation is performed, an automatic ipsilateral sidebending occurs as well. In the lower thoracic spine, T8 to T12 has an ipsilateral coupling during rotation or sidebend when the spine is flexed. When it is extended, however, the coupling tends to be a contralateral movement pattern. For instance, in an extended spine, when T8 to T12 rotates, the segments automatically sidebend in the opposite direction.60 The coupling pattern in the T4 to T8 segments of the thoracic spine is not as predictable, however, partly due to different methods of measurement by investigators, differing use of terminology, and partly due to the degree to which an individual can move out of their normal kyphosis. If the kyphosis is rather mobile, and there is movement toward extension, coupling during sidebending and during rotation in an “extended thoracic spine” is likely to be a contralateral movement pattern. In instances of a more rigid kyphosis, however, with minimal to no movement toward extension, as well as in positions of flexion, the coupling at T4 to T8 during sidebending and during rotation is ipsilateral.62

In general, when comparing with other levels of the spine, coupling at the T4 to T8 level, although present, is quite minimal. Synkinetic coupled motions of the spine are guided by spinal curve, joint orientation and soft tissue connective structures including the disc. These three-dimensional coupled movement patterns allow for maximal stretch on the zygapophyseal joint capsules.

Of course, it is possible to introduce a motion into the spine that is opposite to the coupling, such as actively left rotating while in a position of flexion and right sidebend. This movement pattern is termed “combined” and is restricted by a “jamming” of the zygapophyseal joint surfaces.

The principles of coupled and combined motions can be used not only in establishing diagnoses during interpretation of the physical examination, but also during the execution of manual therapy techniques in the treatment of joint or segmental dysfunction. Since the three-dimensional coupled movement patterns maximally stretch the zygapophyseal joint capsule, it is these that will be most painful in the incidence of capsule lesions or synovitis. On the other hand, combined motions, which jam the facet surfaces, will provoke the most pain in instances of articular chondropathy. During treatment, the clinician can ensure maximal stretch to the structures of that segment when mobilizing in a 3-dimensional coupled motion. On

the other hand, when mobilizing the rib , the clinician can effectively “lock out” the thoracic spine by ensuring that it is in a combined position.

PATHOANATOMY Bony structures Bony anatomy of the differs from that of the cervical and lumbar regions in several ways. Vertebral body diameter is equal or larger in an anterior-posterior direction than in a transverse direction, functioning to carry the loads imposed on it more ventrally. Of clinical importance is the slightly wedge-shaped form of the vertebral body in the sagittal plane, approximately 3.8deg, that creates a 45deg angle of kyphosis when looking at the entire thoracic region.59 In the lumbar region, the vertebral bodies are more rectangular, whereas the discs are wedge-shaped. Thus, with disc aging and dehydration, the thoracic spine increases in kyphosis, whereas the lumbar spine becomes flatter. Endplates are thinnest in the thoracic spine, which explains why Schmorl’s nodes are seen most often here in spite of the fact that endplate disruptions are present throughout the spine. Frequently seen in the central or dorsal part of the endplate/vertebral body in lumbar spine, they often occur ventrally in the thoracic spine (again demonstrating the location of load through this area). Previously dismissed as insignificant, Schmorl’s nodes should be considered clinically important because the process of accelerated disc degeneration has been associated with endplate fractures.4,72

Partly due to the short, thick pedicles in this region, the spinal canal is small and round, particularly from the T4 to T9 segments. Furthermore, the pedicle is found at the upper half of the vertebral body, and so the intervertebral foramen lies quite high in relation to the disc. As a result, disc bulges will rarely lead to root or spinal nerve compression, thus radicular symptoms of pain radiating along the course of an intercostal nerve, and radicular signs of sensory changes and motor deficit are uncommon in the thoracic spine. Because of the small canal, however, moderate to severe posterior protrusions or prolapses would have a great chance of resulting in spinal cord compression. Fortunately, thanks to relatively thin and highly annular discs in the thoracic spine, instances of frank protrusion are an exception rather than the rule.54

The overlapping of the large flat laminae forms an entirely bony posterior wall to the thoracic vertebral column. Dural irritation is a frequently observed phenomenon even with the smallest of disc bulges because there is no room for the sac and its contents to move out of the way due to the small canal and bony back wall. Furthermore, meningovertebral attach the dural sac to the vertebral bodies as well as to the posterior longitudinal , and thus even a minor bulge can tug on the sac, creating irritation.9

Thoracic spine transverse processes, found at the junction of the superior and inferior articular processes, lie dorsal to the articular facets, which allows for increased axial rotation at the thoracic segments. The T1 transverse process is the longest, approximately 8cm, and length decreases caudally from there. Rib movement is influenced by the orientation of the transverse processes. With a relatively frontal location of the T1 to T7 transverse processes, ribs 1 through 7 elevate more in an anterior direction during inspiration, thus increasing the anterior-posterior diameter of the thorax at that level. From T8 to T12, transverse processes are oriented dorsally at an oblique angle. Ribs 9 through 10 attaching to these transverse processes move upward more in a lateral direction during inhalation, thus increasing the side- to-side diameter of the thorax. Under normal conditions, ribs 1 and 2 only move during deep inspiration, as well as with arm elevation.

Relatively flat in shape, orientation of the zygapophyseal articular facets in the transverse plane is 20deg ventrolaterally inclined in relation to the frontal plane. In the parasagittal plane, these facets are inclined 60deg in relation to the vertebral endplate. Because of the ventrolateral orientation of the facets, and the fact that they lie posterior to the axis of rotation for rotation, little resistance is offered to axial rotation. Since the ribs also provide little restriction to axial rotation in the thoracic spine, the

provides the primary constraint to this motion.55 Clinically presented, internal disc disruptions will often be provoked with axial rotation motions. Partly due to their parasagittal orientation, the articular facets are effective in controlling ventral translation during flexion.45,58

Thoracic spinous processes are long and oblique in orientation, covering each other closely. Contact between the bases of the spinous processes plays a large role in the restriction of extension.39 In addition, numerous strong muscles attach to these bony processes, leading to asymmetry during development. Because a normal thoracic spinous process can be up to 1 cm out of the midline, even a skilled practitioner cannot reliably use palpation of spinous process orientation to determine positional changes or dysfunction of individual thoracic segments. Facets can be found both at the sides of the vertebral body, and also at the transverse processes for articulations with the ribs. Ribs 2 through 9 attach to the both the corresponding and the cranial vertebrae, with the intervertebral disc in-between. Ribs 1, 10, and 12 attach to their matching vertebrae alone. Location of articular facets at the transverse process is ventral for ribs 1 through 6, and superior for ribs 7 through 10. Ribs 11 and 12 are free of transverse process attachments. Being short, strong, and rigid structures, and the site of numerous muscle attachments, ribs 1 through 4 endure large transfers of forces from the head and upper extremities through to the thoracic spine. Examples of these forces can be seen clinically, in patients who have been involved in motor vehicle accidents where they braced themselves with their arms during impact: upper rib and spine pathology can often be found. In repetitive overuse syndromes of the upper extremity, upper rib dysfunction is frequently detected.46

Intervertebral Disc Intervertebral discs of the thoracic spine are nearly uniform in shape, and thus it is the wedge-shape of the vertebral bodies that gives the thoracic spine its kyphosis, unlike the lumbar spine where the wedge- shape of the discs form the lordosis. Thus, with general disc dehydration during aging, the clinician observes increasing kyphosis in the thoracic spine, but decreasing lordosis or a flattening of the lumbar spine.

In comparison to other spinal regions, there is less nuclear material in the disc, which relates to the general decreased mobility of this section of the spine. Overall, the disc is thinner in this region of the spine, with a 1 to 5 disc to height ratio in comparison to 1:2.5 for the cervical spine and 1:3 for the lumbar spine.30 Discs in the lower thoracic spine thicken again, and this finding, along with the loss of support from ribs that are no longer attached to the sternum and the increased load of body weight may explain the increased incidence of severe disc lesions in the lower thoracic region.24,10

Unique to thoracic spine discs is their direct attachment, via intra-articular ligaments to the abutting rib heads at their posterolateral aspects. Protrusion of disc material in a posterolateral direction is rare at these thoracic levels, and can explain why this phenomenon is again seen at the lowest level of T10-11 and T11-12 where there is no longer rib attachment to the disc. Other problems can occur at the levels where there is rib attachment, however. If there is disc space narrowing, such as in instances of degeneration, pressure on the head of the rib progressively increases, in effect “pinching the rib out” and leading initially to a possible hypermobility at this joint followed by eventual osteoarthrosis. For years, disc pathology in the thoracic spine was thought to be extremely rare, given the thin and mostly annular structure of the disc, the abutment by the ribs, and the uncommonness of radicular (or root) signs. MRI studies and the advent of provocative thoracic discography have since shown the thoracic disc indeed to be a source of pain, and to a significantly greater degree of up to 11% of all disc lesions.16,23,48,71 Internal disc disruptions can occur without ever deforming the outer walls of the disc. In provocative discography followed by gadolinium enhanced CT scan, Schellhas et al48,49 and Wood et al71 found painful annular fissures in patients with previously normal MRI’s. Location of the internal disc disruption or annular fissure could be correlated with the location of pain. Anterior annular tears cause visceral-type pain in the lungs, heart, and stomach. Lateral fissures refer diffuse pain to the lateral trunk. Posterior and posterolateral tears caused local paraspinal pain.

Radicular involvement in terms of loss of muscle strength, pain that radiates along the intercostal nerve, or numbness is rare in instances of disc bulge because the thoracic roots exit the dural sac much higher in the segment and there is little chance of exposure to disc material. Dural irritation, however, is a common occurrence in the instance of even minor disc bulges because the narrowness of the spinal canal does not allow for the cord, covered by the dura, to move out of the way. Dural irritation can also be chemically mediated, coming from inflammatory substances weeping from the disc as a result of the internal disc disruption. Because of the ligamentous attachments between the posterior disc/posterior longitudinal ligament and the dural sac, it may be possible to provoke pain from dural irritation using the Neri test,65 which would, in turn, confirm the disc as the primary pain generator. Furthermore, because the test imposes a tension load on the dural sac and can thus pull on the outer wall of the disc, a painful internal disc disruption in the outer third of the annulus can also give a positive dural test. Consequently, inflamed dura as the result of a minor disc bulge or chemically mediated irritation, as well as a disc disruption in the outer third of the posterior disc can produce positive dural signs: increased intensity of the patient’s symptoms when cervical flexion is added during either trunk flexion or end-range rotation (positive Neri test). When provoked, pain from dural irritation is diffuse because of its multisegmental innervation by the sinuvertebral nerve.65

Ligamentous Structures The posterior longitudinal ligament of the thoracic spine is more developed than in the lumbar spine. In comparison to the cervical and lumbar regions, the flaval ligament has less elastin fibers and more collagen fibers, which correlates with a function that serves more to restrict than to allow mobility. Ossification of either of these two ligaments has been described in recent literature, again due to the very small spinal canal, leading to severe pain with cord compression, manifesting in progressive gait disturbances, spasticity, abnormal reflexes, and bladder dysfunction.25,51,56

The anterior longitudinal ligament provides innervation for the anterior aspect of the intervertebral disc via branches from the sympathetic communicating rami. Pain coming from a ventral disc lesion has the characteristics of visceral-type pain.50 Because the anterior and posterior longitudinal ligaments are attached to both the disc and the vertebral bodies, they deform with bulging of the disc and also when the disc translates. Traction at the attachment points, as a result of prolonged excessive forces from either the bulging or the translation can often be seen to produce an anterior lipping of the vertebra.5

Zygapophyseal joint capsules in the thoracic spine are thin, do not have menisci, and are voluminous. The number of encapsulated nerve endings is less consistent here than in the cervical spine, suggesting that the zygapophyseal joints do not play a large role in proprioception here.42 Dreyfuss et al22 demonstrated pain patterns of the zygapophyseal joints by distending the joint capsules. Symptoms provoked as a result remained fairly local at the paraspinal region, extending approximately two spinal levels above and below the injected segment.

At the costovertebral, or costocentral joint, where the rib meets the articular surface of the vertebral body to which it corresponds, along with the intervertebral disc, and the smaller facet of the cranial vertebra (for ribs 2 to 10), a fibrous capsule with synovial membrane can be found. Strong radiate ligaments reinforce costovertebral joint capsules, and during inhalation, the ligaments twist around the ribs as the ribs make a backward spin motion at that joint. Synovial joints are again found at the laterally located costotransverse connections.

Numerous other ligaments are present in the thoracic vertebral column and rib regions but appear to be of less clinical significance in terms of musculoskeletal pathology. Thus, they will not be discussed in the realm of this paper.

Clinically significant anatomical description is incomplete, however, without a brief discussion of the rib connections either directly or indirectly to the sternum. Costal cartilages connect ribs 1 through 7 to the sternum, and ribs 8 through 10 to the adjacent cranial rib. The first rib has a firm non-synovial

fibrocartilaginous connection to the sternum. Ribs 2 through 7 have synovial connections to the sternum, via thin fibrous capsules. The 7th through 10th ribs form the ventral costal arch and have numerous synovial connections between them at the ventral thorax.63

It is important to recognize that in chronic afflictions of the thoracic spine, adjacent regions should be assessed as well, particularly when addressing causal factors. Directly or indirectly, each typical thoracic vertebral body has twenty connections.

Neural Structures At the thoracic spine, discrepancy is seen between motion segments and neurological levels of the spinal cord. T1 to T6 neurological segments are approximately two levels cranial to the vertebral column motion- segments. At T7 to T10 there is up to a three-level difference. The upper thoracic roots course 3 cm caudally before entering the intervertebral foramen, whereas thoracolumbar roots course about 7 cm to reach the foramen.29

Individually, the 12 pair of thoracic ventral rami each becomes an intercostal nerve (the 12 termed the subcostal nerve), and innervates the pleural, costotransverse and , intercostal muscles and the skin at the ventral thorax. Ventral rami of T1 and often T2 also contribute to the brachial plexus, in addition, T2 to T4 ventral rami have branches, called the intercostobrachial nerves, which contribute to the cutaneous nerves of the arm and forearm. Because of these interconnections, pathology in the upper thoracic spine can sometimes lead to vague upper extremity symptoms of pain and/ or pins and needles sensations. The abdominal muscles are innervated by ventral rami from T6 to T12 and in the very rare instance of severe disc prolapse, weakness might be detected in these muscles.

Dorsal rami innervate the costovertebral joints, zygapophyseal joints, the sacrospinal muscles, and the dorsal skin of the thorax.

The sinuvertebral nerve, consisting of somatic and sympathetic fibers, re-enters the spinal canal, and not only innervates structures at that level, but also ascends or descend up to 5 segmental levels. Along its way, branches from the nerve innervate the dura mater, posterior longitudinal ligament, the posterior aspect of the disc, and the zygapophyseal joint. This explains why pain coming from a lesion to one of these structures tends to be diffuse and difficult to isolate.13 The individual often “paints” that area of the spine that is affected, rather than pointing with one finger precisely where the pain is felt.

Ganglia from the sympathetic nervous system run along either side of the vertebral column along its ventral aspect, lying very close to the rib heads. And extensive network of branching is found in the region of the somatic rami and the sympathetic ganglia; connections can be found between the dorsal rami, gray communicating rami, and the sympathetic ganglia. Branches from this paravertebral plexus also innervate the costovertebral joints.57 Costovertebral joint capsules contain mechanosensors and nociceptors; triggering these receptors can provoke responses from muscle spasm to changes in breathing.38

The following case history is a clinical picture of an individual with trunk pain as a result of a musculoskeletal lesion or lesions. Because of the functional interplay of the vertical and horizontal kinematic chains in the thoracic region, the clinician has to be prepared to tease out more than one possible pain generating structure and be able to treat each one without doing harm to another.

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