The Normal Spinal Column: Overview and Cervical Spine

The Osseous Vertebrae

Each osseous vertebra has a standard overall structure, composed of an anterior vertebral body and a posterior neural arch ( Figs. 3-3 and 3-4 ). This general shape is modified secondarily at each level. Each vertebra has an outer shell of dense cortical bone and an inner core of less dense, cancellous bone. The cortical bone is thick over the neural arch, less thick along the sides of the vertebral body, and thinnest where the upper and lower surfaces of the vertebral body abut the disc. The cancellous bone contains the vertebral marrow.

The vertebral body forms the front of each vertebra, anterior to the spinal canal. The body is shaped like an asymmetric cylinder with concave side walls. The top and bottom of each body resembles a drum head, with a smooth, raised periphery and a roughened flat-to-scaphoid center.

The neural arch forms the lateral and posterior portions of each vertebra, lateral to and behind the spinal canal. Each neural arch is composed of paired pedicles, paired laminae, paired articular processes, and a single, posteriorly directed spinous process. The pedicles form the two sides of the spinal canal. These narrow struts arise from the upper posterolateral surface of each vertebral body and pass posteriorly to merge into the laminae. The pedicles are roughly cylindrical in coronal section. In lateral view, they show shallow curves on their upper and lower borders (the gentler superior vertebral notch and the deeper inferior vertebral notch). The laminae are flattened, obliquely oriented plates of bone that form the posterior/posterolateral walls of the spinal canal. They merge with the pedicles anteriorly and angle toward each other posteriorly to fuse with the base of the spinous process in the midline. The spinous process arises from the medial edges of the paired laminae and juts posteriorly and inferiorly in the midline behind the spinal canal.

The superior and inferior articular processes lie posterolateral to the spinal canal. They arise where the pedicles merge into the laminae. From there, the paired articular processes project superiorly and inferiorly to form synovial articulations with the articular processes of the next-higher and next-lower vertebrae ( Fig. 3-5 ). At each articulation, the superior articular processes of the lower vertebra lie anterior to the inferior articular processes of the upper vertebra. The facets of the superior articular processes face posteriorly to fit with the anteriorly facing facets of the inferior articular processes.

The transverse processes of the vertebrae also arise where the pedicles merge into the laminae but project laterally. In the thoracic region, these processes are true transverse processes (designated diapophyses ) that articulate with the ribs (designated pleurapophyses, costal elements ). At other spinal levels the similar, laterally projecting bony elements are composed of both true transverse processes (diapophyses) and incorporated pleurapophyses (costal elements).

The neural foramina are the soft tissue and osseous channels between vertebrae that convey the spinal nerves and blood vessels into and out of the spinal canal ( Figs. 3-5 and 3-6 ). Superiorly and inferiorly the neural foramina are bordered by the periosteum of the concave margins of the pedicles. The superior border of the neural foramen is the undersurface of the upper pedicle. Therefore, the deeply curved inferior vertebral notch of the upper pedicle constitutes the roof of the neural foramen. The inferior border of the neural foramen is the upper surface of the lower pedicle. Therefore, the shallower superior vertebral notch of the lower pedicle constitutes the floor of the neural foramen. On their anterior wall, from above downward, the neural foramina are bordered by the periosteum of the posterior surfaces of the upper vertebra, the syndesmosis formed by the hyaline cartilage end plates and disc, and the periosteum of the posterior surface of the lower vertebra. Posteriorly, the neural foramen is formed by the periosteum over the pars interarticularis, the ligamentum flavum, and the synovial capsule of the facet joint. At T1 to T10 only, the anterior border of the foramen also includes the synovial capsules for the articulations of the ribs.

The marrow within the vertebra consists of both red hematopoietic marrow and yellow fatty marrow ( Figs. 3-7 and 3-8 ). Additional small (2-4 mm) intraosseous nodules of notochordal remnants are found in 14% of spinal columns. From birth to approximately age 2 years, the vertebral marrow is predominantly red hematopoietic marrow. It converts progressively, diffusely and homogeneously to yellow fatty marrow with growth and development. From early middle age onward, the conversion of red to yellow marrow progresses more slowly and inhomogeneously. The fat fraction of the lumbar vertebral marrow is usually higher in men than women and may increase from 27% to 70%, at a rate of approximately 7.5% per decade in adulthood. In the adult, the fat content of the lumbar marrow usually increases from L1 to L5. However, the distribution of the fat is variable and heterogeneous. The red marrow may cluster near the anterior margin of the vertebral body and at the end plates. Because fat and hematopoietic cells both grow by expansion of cell clusters to form nodules, fatty conversion tends to create islands of juxtaposed red and yellow marrow. The increase in the fat fraction with age correlates with the atherosclerotic decline in overall vascular perfusion and with reduced bone mineral density.

Craniocaudal Patterns of Osseous Anatomy ( Figs. 3-1 and 3-2 )