In the thoracic region the intervertebral discs are relatively thin with respect to the vertebral bodies, and together with the presence of the ribs and sternum, their movement is limited. The orientation of the articular processes of the thoracic vertebrae, which lie on the arc of a circle with its centre close to the anterior part of the vertebral body, permits flexion, extension, lateral flexion and rotation. However, the inferior processes of the 12th thoracic vertebra resemble those of the lumbar region and therefore movements at the thoracolumbar junction are similar to those between lumbar vertebrae.
Flexion and extension The combined range of flexion and extension in the thoracic part of the vertebral column is between 50° and 70°, with extension being much more limited than flexion. Flexion is much freer in the lower half of the region as the lower ribs tend to be longer and more flexible because of their longer costal cartilages.
In flexion the inferior articular processes of the vertebra above slide upwards over the superior processes of the lower vertebra. The interspace between the two vertebrae opens out posteriorly, compressing the anterior part of the intervertebral disc. Flexion is limited by the presence of the thoracic cage, and by the tension developed in the supra- and interspinous ligaments, the ligamenta flava, and posterior longitudinal ligament. From the upright position, flexion is controlled by the postvertebral muscles of both sides, while flexion from the supine position is brought about by the anterior abdominal muscles of both sides, particularly rectus abdominis. During flexion all the angles between the various segments of the thorax and between the thorax and vertebral column increase.
Figure. (A) Range of flexion and extension of the thoracic region, (B) movements between adjacent vertebrae, (C) effects on the thoracic cage
Extension of the thoracic region approximates the vertebrae posteriorly. It is limited by impact of the articular and spinous processes between adjacent vertebrae, as well as by tension in the anterior longitudinal ligament. The effect of extension on the thoracic cage is to flatten it by decreasing all angles between the various segments of the thoracic cage and the vertebral column. Extension from the upright position is controlled by the anterior abdominal wall muscles and from the prone position by the postvertebral muscles.
Lateral flexion In the thoracic region this has a range of 20-25° to each side, being freer in the lower half of the region. During lateral flexion the articular processes of the two adjacent vertebrae slide relative to each other, so that those on the contralateral side move as during flexion, while those on the ipsilateral side move as during extension. Lateral flexion is limited by the impact of the articular processes on the side of the movement, and by tension developed in the ligamenta flava and intertransverse ligaments of the opposite side. In the thoracic region lateral flexion is associated with rotation of the vertebrae. In full lateral flexion the degree of rotation is about 20°. In other words, for each degree of lateral flexion there is almost 1° of accompanying rotation; the rotation being such that the spinous processes of the thoracic vertebrae point towards the concavity of the curve, i.e. they rotate contralaterally.
Figure. (A) Range of lateral flexion of the thoracic region, (B) movements of adjacent vertebrae, (C) effects on the thoracic cage.
As with flexion and extension, lateral flexion modifies the shape of the thoracic cage. On the contralateral side the thorax is elevated, the intercostal spaces widen, the thoracic cage enlarges and the costochondral angle of the 10th rib tends to open out. On the ipsilateral side the reverse occurs: the thoracic cage is lowered and shrinks, the intercostal spaces narrow, and the costochondral angle decreases.
As in the lumbar region, lateral flexion is brought about by the contraction of muscles on the same side.
Rotation The orientation of the articular processes in the thoracic region is such as to promote rotation. This may be a necessary adaptation because of the presence of the ribs. Were it not for the presence of the thoracic cage the range of rotation of the thoracic column would be greater. Even so, it is some 35° in each direction. During rotation, the inferior processes of the upper vertebrae slide sideways outside the superior processes of the lower vertebrae leading to rotation of one vertebral body with respect to the other about a common axis. This movement of the vertebrae is accompanied by rotation and twisting of the intervertebral disc, which has a tendency to pull the adjacent vertebrae together. A small degree of lateral flexion accompanies this rotation. When the thoracic vertebral column is fully extended both rotation and lateral flexion are greatly reduced, and may be lost completely.
Rotation is limited by tension in the supra- and interspinous ligaments and the ligamenta flava and is brought about largely by the abdominal oblique muscles. For example, rotation of the trunk to the left is achieved by contraction of the right external oblique and left internal oblique.
Figure. (A) Range of axial rotation of the thoracic region, (B) movement between adjacent vertebrae, (C) effects on thoracic cage, (D) rotation in the thoracic region during walking.
Because of tlie articulation of the ribs with the vertebrae, any rotatory movement of tlie vertebrae will induce a similar movement in the corresponding ribs. However, such movement is limited due to the articulation of tlie ribs anteriorly with tlie sternum. Instead there is a distortion of tlie ribs as follows: an accentuation of the concavity of the rib on tlie side of rotation, with a flattening of tlie concavity on the opposite side. These changes are possible because of tlie elasticity of tlie rib and its costal cartilage. These movements of tlie ribs subject the sternum to shearing forces.
Rotation of the thoracolumbar part of the vertebral column is extremely important during walking. However, the rotation is not simple but rather complex, occurring in opposite directions in the upper and lower parts of the region. As one leg swings through ready for the next heel-strike the pelvis rotates about the hip joint of the supporting leg, carrying with it the trunk. In an attempt to keep the head facing forwards the pectoral girdle rotates in the opposite direction. Studies have shown that the intervertebral disc between the seventh and eighth thoracic vertebrae is not subjected to any rotation, while maximum rotation occurs (in opposite directions) in the discs immediately above and below it. The degree of rotation decreases towards the pelvic and pectoral girdles.
Accessory movements Accessory movements of the thoracic part of the vertebral column are difficult to demonstrate and usually only used therapeutically. Nevertheless, pressure on a spinous process will, because of its length and angulation, produce a slight rocking of the vertebral body and apposition of the articular surfaces of the zygapophyseal joints associated with its articular processes. A localized pressure applied to the posterior aspect of the base of the transverse process just below the zygapophyseal joint will slightly separate the joint by pushing the upper articular process forwards.