Clinical Anatomy of the Trunk and Central Neuraxis

Fig. 13.1

Cross section of a thoracic vertebral segment with a simplified spinal nerve and paravertebral space shown (right side only). DRG dorsal root ganglion

It is important to realize that the first cervical (C1) nerve leaves the spinal cord and courses above the atlas (C1 vertebra); hence, the cervical nerves are numbered corresponding to the vertebrae inferior to them (e.g., the C6 nerve exits below C5 and above C6 vertebra; C8 nerve exits below C7 and above T1). From this point on, all the spinal nerves are named corresponding to the vertebral level above. For example, T3 and L4 spinal nerves exit below the T3 and L4 vertebrae, respectively.

13.1.2 Vertebral Column

This section reviews the developmental anatomy and growth of the vertebral column (spinal column or spine) and provides a basis for appreciating the improved visibility rendered when imaging the spine using ultrasound on the pediatric patient. It begins with the development of the vertebrae and vertebral column as a whole, followed by the growth and curves of the vertebral column and finally a brief description of the developmental anatomy of the thoracic and lumbar spine and sacrum.

13.2 Development of the Vertebral Column

As with most of the other bones of the human body, the development of the human vertebral column goes through three stages: mesenchymal (precartilaginous), chondrification (cartilaginous), and ossification (bony).

Mesenchymal (precartilaginous) stage

During the fourth week of intrauterine life (IUL), sclerotomic mesenchymal cells of the somites (derived from the paraxial mesoderm) start to migrate toward the notochord which represents the primitive axial supporting structure. At this stage, the developing neural tube, notochord, and endoderm of the yolk sac are in close contact and are flanked by the paired dorsal aortae from which the intersegmental arteries branch off and course between the somites. In the thorax, these intersegmental arteries become the intercostal arteries; in the abdomen, they become the lumbar arteries. The mesenchymal tissue surrounding the notochord is subdivided by the intersegmental vessels into sclerotomic segments. The mesenchymal cells within the sclerotome become characterized by alternating regions of densely packed and loosely arranged cells between which an intervertebral fissure appears. This new segment forms the primitive vertebra consisting of a dense cranial zone separated by a loose caudal zone of cells by the intersegmental vessels.

From the dense zone, three processes arise and extend dorsally, ventrally, and laterally. The dorsal extension is called the neural process and will form the vertebral neural arch. The ventral process will form the centrum (vertebral body), while the lateral process is related to the development of the vertebral transverse process and the attachment of the ribs. Therefore, the primordia of the definitive vertebrae are not formed from the mesenchymal cells of one somite but the recombination of the lesser and more condensed zones of mesenchyme derived from two adjacent somites. The intervertebral fissures fill with mesenchymal cells that migrate from the dense zone to form the annulus fibrosus of the intervertebral (IV) disk, whereas the notochord and inner cells of the annulus fibrosus degenerate to form the nucleus pulposus.

Chondrification (cartilaginous) stage

During the sixth to seventh weeks of IUL, a pair of chondrification centers appears in each vertebral body followed by separate centers in each neural and transverse processes. The two centers in each vertebral body fuse at the end of the embryonic period (~9 weeks of IUL) to form a cartilaginous centrum or body. At the same time, the chondrification centers in the neural and transverse processes fuse with those in the centrum to form a cartilaginous model of the vertebra. The spinous and transverse processes arise from proliferation of chondrification centers in the vertebral neural arch and lateral process, respectively.

Ossification (bony) stage

Ossification of the developing vertebrae commences during the embryonic period around the eighth to ninth weeks of IUL and is usually complete by the 25th year. Three primary ossification centers appear, one for the centrum and one for each half of the vertebral neural arch, followed by five secondary (epiphyseal) ossification centers. Centers for the vertebral arches appear classically first in the upper cervical vertebrae during the ninth to tenth weeks of IUL and then in successively lower vertebrae, reaching the lumbar vertebrae at around 12 weeks. Thus, at birth, each vertebra consists of three bony parts connected by cartilage. The vertebral neural arches usually fuse during the first 3–5 years of life, commencing in the lumbar region and progressing cranially. The vertebral arches articulate with the centrum at the cartilaginous neurocentral junctions (joints) or synchondroses, which permit the vertebral arches to grow and the vertebral canal to expand as the spinal cord enlarges. These neurocentral junctions disappear when the vertebral arches fuse with the centrum beginning in the cervical region during the third to sixth years of life. Until puberty, the superior and inferior surfaces of the bodies and tips of transverse and spinous processes are cartilaginous. Five secondary centers of ossification appear at the epiphyses of the vertebra around puberty: one for the tip of the spinous process, one each for the tips of the transverse processes and two annular ring-like epiphyses, one on the superior and one on the inferior rim of the vertebral body. The vertebral body is thus made up of two annular epiphyses with the mass of bone in between them which is derived from the centrum. It is important to point out that the adult vertebral body is not coextensive with the developmental centrum. Although the centrum will form the majority of the vertebral body, in the adult, the body includes parts of the neural arches posterolaterally. These secondary centers fuse with the rest of the vertebra around 25 years of age to form the definitive vertebra and the vertebral column. The atlas (C1), axis (C2), C7, sacrum, and coccyx are exceptions to this typical ossification pattern.

The majority of people have 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused), and usually 4 (3 to 5) coccygeal (fused) vertebrae; however, about 2–3 % have one fewer or one or two additional vertebrae. When this occurs, there may be regional compensation (e.g., 11 thoracic and 6 lumbar vertebrae), so when determining the number of vertebrae, it is important to examine the entire vertebral column.

Growth and Curves of the Vertebral Column

The increase in length of the vertebral column is determined by the growth of its vertebral components. The various regions of the column and different parts of the vertebrae have differential rates of growth. The growth of the vertebral bodies begins in the thoracic and lumbar regions and extends craniocaudally. The lower part of the column grows faster than the upper as a functional prerequisite for providing better support. There are two periods of accelerated growth: the first between 2 and 7 years of age and the second between 9 and 15 years of age. The curves of the vertebral column become evident during the third month of IUL life. At first, there is only a slight curve, concave anteriorly, followed in the fifth month by the appearance of the sacrovertebral angle. Radiographic studies have shown, however, that 83 % of fetuses aged between 8 and 23 weeks already possess a cervical curve.

At birth, this embryonic anterior curvature is preserved in the thoracic and sacral regions. These areas of the column, concave anteriorly, are therefore referred to as the primary curves. The cervical curve is also present at birth although it becomes more accentuated when the baby starts supporting its head (around 3–4 months of life) and sitting upright (around 9 months of life). The lumbar curves appear later in response to the baby adopting the upright walking posture and walking unassisted (around 12 months of life). These are termed secondary or compensatory curves since they develop in response to biomechanical demands placed on the column and are convex anteriorly. Thus, the adult has four anteroposterior curves: cervical (secondary), thoracic (primary), lumbar (secondary), and sacrococcygeal or pelvic (primary), while the newborn has only three, cervical, thoracic, and sacral, the latter demarcated by the sacrovertebral angle. Between 3 and 9 months of life, the cervical curve becomes more marked in response to the biomechanical demands placed on it from the baby supporting its head and sitting upright, while after the first year, one sees the appearance of the lumbar curve in response to the need for supporting the baby’s weight.

The cervical and lumbar curves in the adult are due mainly to the shapes of the intervertebral disks, while the thoracic curve is related more to the shapes of the thoracic vertebrae, which have a greater depth posteriorly. During intrauterine life, the vertebral column represents about three quarters of total body length, whereas at birth, it is reduced to two fifths due to the relatively rapid development of the lumbosacral region and lower extremities. This variation in the proportions of the vertebral column, head, trunk, and extremities causes the center of gravity of the body to shift caudally as the infant grows. In the newborn held in the upright erect posture, the center of gravity is at about the level of the xiphoid process and remains above the umbilicus throughout early childhood. At about 5–6 years, it is just below the umbilicus, and at around 13 years, it shifts to the level of the iliac crest. In the adult, the center of gravity lies at the level of the sacral promontory (the most forward projecting aspect of the upper edge of the first sacral vertebra).

13.2.1 Developmental Anatomy of the Thoracic and Lumbar Vertebral Column (Spine)

At birth, the vertebrae of the thoracic and lumbar spine consist of three bony masses: a centrum anteriorly and two vertebral neural arches posteriorly, united by cartilage.
Development of the thoracic spine:
- The laminae typically unite in a caudo-cranial (upwards, T12 through T1) fashion, usually by the end of the first or beginning of the second year of life.
- The centrum (body) fuses with the neural arches (neurocentral fusion), also in a caudal to cranial manner, generally by the end of the fifth year of life.
- The transverse processes are present and prominent at birth; however, their tips, like the tips of the transverse processes, remain cartilaginous until puberty.
- The three facets for articulation with the ribs at the costovertebral and costotransverse joints are present at birth.
- The neurocentral and posterior synchondroses are not fused at birth; the posterior ones fuse within 2–3 months of postnatal life, and the neurocentral synchondroses close after 5–6 years of life.
Development of the lumbar spine:
- Fusion of the laminae of L1 through L4 occurs during the first year of life, with those of L5 fusing by 5 years of age.
- Neurocentral fusion is generally complete by age 4.
- Transverse processes begin to develop after the first year of life; however, their tips, like the tips of the spinous processes, are cartilaginous until puberty.
- The lumbar secondary compensatory curve (through intervertebral disk modification) does not begin to develop until 6 or 8 months of age when the infant begins sitting upright and supporting its weight and becomes more apparent after the first year of life when the infant is able to adopt the upright posture and walk unassisted.
The vertebral canal in young infants is quite small, with the thickness of the epidural space being as little as 1–2 mm.
Secondary ossification centers in vertebral bodies and arches do not fuse with the rest of the vertebra until the early twenties, allowing continuing growth and development of the vertebral structures.
In children, most vertebrae contain five secondary ossification centers (in the transverse and spinous processes as well as on the superior and inferior surfaces or rims of the vertebral bodies), which allow continual growth and remodeling of the vertebral column.
In adults (early 20s), the vertebral column is essentially complete with the exception of fusion between the bodies of S1 and S2.
In adults:
- The thoracic vertebral bodies are of medium size and heart shaped (Fig. 13.1); vertebral (spinal) canals are small and nearly circular in shape; laminae are relatively small; spinous processes are long and oriented obliquely inferiorly (Figs. 13.2 and 13.3). Spinous processes overlap from T5–T8, the latter being the longest and most oblique, and their obliquity increases from T1–T9 then decreases in T10–T12. The transverse processes are relatively broad and robust for articulation with the ribs and generally face posterolaterally. The thoracic vertebrae are adapted mostly for articulation with the ribs and their role in movements of the chest wall during respiration; however, they also play a secondary role in supporting and transmitting the weight of the head and neck and the thorax to the lumbar segment of the column.
  
  Fig. 13.2
  
  Vertebrae of the thoracic spine, illustrating inferiorly oriented spinous processes
  
  Fig. 13.3
  
  Spinal column showing the thoracic and lumbar regions
- The lumbar vertebral bodies are large and kidney shaped; vertebral (spinal) canals are triangular in shape; laminae are thick; spinous processes are short and stout with a posterior (horizontal) orientation (Fig. 13.3); intervertebral disks are thickest with respect to the rest of the spine; within the lumbar spine itself, the disks are thicker anteriorly, contributing to the anterior lumbar curvature (lumbar lordosis). The transverse processes are long, slender, and directed laterally and are mostly for muscular attachments. The lumbar vertebrae are designed primarily for weight bearing and transmission, as reflected in the large size and robustness of their bodies, laminae, and IV disks.