Structure of the Thoracic Region• The majority of the thoracic vertebrae adhere to the basicstructural design of all vertebrae except for some minorvariations.• The 1st and 12th thoracic vertebrae are transitional vertebraeand therefore possess characteristics of the cervical andlumbar vertebrae, respectively.• The first thoracic vertebra has a typical cervical shaped bodywith a transverse diameter practically twice theanteroposterior diameter.• The spinous process of T1 is particularly long and prominent.
• The 12th thoracic vertebra has thoracic-like superiorzygapophyseal articular facets that face posterolaterally.• The inferior zygapophyseal facets, however, are morelumbar-like and have convex surfaces that faceanterolaterally to articulate with thevertical, concave, posteromedially facing superiorzygapophyseal facets of the first lumbar vertebra.• Additional differences in T1, T11, and T12 include thepresence of full costal facets rather thandemifacets, inasmuch as ribs 1, 11, and 12 articulate onlywith their corresponding vertebral bodies.
• The pedicles in the thoracic region are generally directedmore posteriorly and less laterally than any otherregion, which creates a smaller vertebral canal in thethoracic region than in the cervical or lumbar regions.• The laminae are short, thick, and broad.• The end plates show a gradual increase in transverse andanteroposterior diameters from T1 to T12. The inferior endplate width increases by 55%, and the superior end plateanteroposterior diameter increases by 75%.• Increase in width for both superior and inferior end platesis greatest at T11/T12.
Typical Thoracic Vertebrae• Body• The body of a typical thoracic vertebra hasequal transverse and anteroposteriordiameters (Fig. 4-35), which lends to greaterstability.• The vertebral bodies are wedge shaped withposterior height greater than anteriorheight, which produces the normal kyphoticposture of the thoracic spine.
• In a study of 144 vertebrae, Panjabi andcoworkers found that the posterior height of eachvertebra increased from approximately 14.3 mmat T1 to 22.7 mm at T12, representing an increaseof 60%, or a 0.8-mm increase per vertebral level.• Demifacets(or half facets) for articulation withthe heads of the ribs are located on theposterolateral corners of the vertebral plateaus.
• Arches– Pedicles. These generally face posteriorly withlittle to no lateral projection, creating a smallvertebral canal.– Laminae. The laminae are short, thick, and broad
– Zygapophyseal Articular Processes.– The superior zygapophyseal facets are thin and almostflat and face posteriorly and slightly superolaterally.– The inferior zygapophyseal facets face anteriorly andslightly superomedially.– The facets lie nearly in the frontal plane.– The orientation of the facets changes at either T10 orT11 so that the superior facets face posterolaterallyand the inferior facets face anterolaterally, and theylie closer to the sagittal plane.
– Transverse Processes.– The transverse processes have thickened endsthat support paired large oval facets(costotubercular facets) for articulation with thetubercles of the ribs.
– Spinous Processes.– The spinous processes slope inferiorly and, fromT5 to T8, overlap the spinous process of theadjacent inferior vertebra.– The spinous processes of T11 and T12 aretriangular and project horizontally.– For most of the thoracic spine, the tip of thespinous process lies at the level of the caudalvertebral body.– Vertebral Foramen.– The vertebral foramen is small and circular.
• Intervertebral Disks• There has been little study of the structure of thethoracic intervertebral disks; however, the structure isgenerally held to be similar to disks in the lumbarregion, with differences only in size and shape.• Thoracic intervertebral disks are thinner than those ofother regions, especially in the upper thoracicsegments.• Also, the ratio of disk size to vertebral body size issmallest in the thoracic region, which results in greaterstability and less mobility for this region.
• The intervertebral disks are also somewhatwedge shaped, with the posterior heightgreater than the anterior height, whichcontributes to the thoracic kyphosis.• The thoracic intervertebral disks are primaryrestraints to movement and are consideredthe primary stabilizer of the mobile segment.
Articulations• Interbody Joints• The interbody joints of the thoracic spineinvolve flat vertebral surfaces that allow for alltranslations to occur.• The intervertebral disk allows for tipping ofthe vertebral bodies; however, the relativelysmall size limits the available motion.
• Zygapophyseal Joints• The zygapophyseal joints are plane synovial joints withfibroadipose meniscoids present.• These joints lay approximately 20° off the frontalplane, which allows greater ROM into lateral flexion androtation and less ROM into flexion and extension (see Fig.4-18B).• The joint capsules are more taut than those of the cervicaland lumbar regions, which also contributes to less availableROM.• Ligaments• The ligaments associated with the thoracic region are thesame as ligaments described at the beginning of thechapter except that the ligamentum flavum and anteriorlongitudinal ligaments are thicker in the thoracic regionthan in the cervical region.
Function of the Thoracic Region• The thoracic region is less flexible and more stable than thecervical region because of the limitations imposed bystructural elements such as the rib cage, spinousprocesses, taut zygapophyseal joint capsules, theligamentum flavum, and the dimensions of the disks andthe vertebral bodies.• Each thoracic vertebra articulates with a set of paired ribsby way of two joints: the costovertebraland thecostotransversejoints.• The vertebral components of the costovertebral joints arethe demifacets located on the vertebral bodies.• The vertebral components of the costotransverse joints arethe oval facets on the transverse processes.
• Kinematics• All motions are possible in the thoracic region, but therange of flexion and extension is extremely limited in theupper thoracic region (T1 to T6), because of the rigidity ofthe rib cage and because of the zygapophyseal facetorientation in the frontal plane.• In the lower part of the thoracic region (T9 to T12), thezygapophyseal facets lie more in the sagittal plane, allowingan increased amount of flexion and extension.• Lateral flexion and rotation are free in the upper thoracicregion.
• The ROM in lateral flexion is always coupled with someaxial rotation.• The amount of accompanying axial rotation decreases inthe lower part of the region because of the change inorientation of the zygapophyseal facets at T10 or T11.• In the upper part of the thoracic region, lateral flexion androtation are coupled in the same direction, whereasrotation in the lower region may be accompanied by lateralflexion in the opposite direction.• In this region, however, the direction of coupled rotationmay vary widely among individuals.
• Flexion in the thoracic region is limited by tension in thePLL, the ligamentum flavum, the interspinousligaments, and the capsules of the zygapophyseal joints.• Extension of the thoracic region is limited by contact of thespinous processes, laminae, and zygapophyseal facets andby tension in the anterior longitudinalligament, zygapophyseal joint capsules, and abdominalmuscles.• Lateral flexion is restricted by impact of the zygapophysealfacets on the concavity of the lateral flexion curve and bylimitations imposed by the rib cage.• Rotation in the thoracic region also is limited by the ribcage.
• When a thoracic vertebra rotates, the motion isaccompanied by distortion of the associated rib pair(Fig. 4-36).• The posterior portion of the rib on the side to whichthe vertebral body rotates becomes more convex asthe anterior portion of the rib becomes flattened.• The amount of rotation that is possible depends on theability of the ribs to undergo distortion and the amountof motion available in the costovertebral andcostotransverse joints.
• As a person ages, the costal cartilages ossifyand allow less distortion.• This results in a reduction in the amount ofrotation available with aging.
• Kinetics• The thoracic region is subjected to increased compressionforces in comparison with the cervical region, because ofthe greater amount of body weight that needs to besupported and the region’s kyphotic shape.• The line of gravity falls anterior to the thoracic spine.• This produces a flexion moment on the thoracic spine thatis counteracted by the posterior ligaments and the spinalextensors.• The greatest flexion moment is at the peak of the kyphosisas a result of the increased moment arm of the line ofgravity.