Clinical Biomechanics: General Spinal Biomechanics
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This is Chapter 6 from RC’s best-selling book:
Second Edition ~ Wiliams & Wilkins
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Chapter 6: General Spinal Biomechanics
This chapter discusses the vertebral column as a whole and serves as a foundation for the following three chapters that consider the regional aspects of the spine and pelvis. Emphasis here is on gross structure, function, spinal kinematics, and other general biomechanical implications.
The vertebral column is a mechanical marvel in that it must afford both rigidity and flexibility.
The Spine as a Whole
The segmental design of the vetebral column allows adequate motion among the head, trunk, and pelvis; affords protection of the spinal cord; transfers weight forces and bending moments of the upper body to the pelvis; offers a shockabsorbing apparatus; and serves as a pivot for the head. Without stabilization from the spine, the head and upper limbs could not move evenly, smoothly, or support the loads imposed upon them (Fig. 6.1).
Essentially because of its various adult curvatures, the bony spine is anatomically divided into the seven cervical vertebrae, the twelve thoracic vertebrae, the five lumbar vertebrae, and the ossified five sacral and four coccygeal segments. From C1 to S1, the articulating parts of these vertebrae are the vertebral bodies, which are separated by intervertebral discs (IVD’s), and the posterior facet joints. The IVD’s tend to be static weight-bearing joints, while the facets function as dynamic sliding and gliding joints.
The flexible vertebral column is balanced upon its base, the sacrum. In the erect position, weight is transferred across the sacroiliac joints to the ilia, then to the hips, and then to the lower extremities. In the sitting position, weight is transferred from the sacroiliac joints to the ilia, and then to the ischial tuberosities.
About 75% of spinal length is contributed by the vertebral bodies, while 25% of its length is composed of disc material. The contribution by the discs, however, is not spread evenly throughout the spine. About 20% of cervical and thoracic length is from disc height, while approximately 30% of lumbar length is from disc height. In all regions, the contribution by the discs diminishes with age.
Development of the Spine
In brief, development occurs in three stages: mesenchymal, chondrification, and ossification.
MESENCHYMAL AND CHONDRIFICATION ORIGINS
Just prior to the 4th week of embryonic development, a vertebral segment begins to develop as paired condensations of mesenchyme (somites) around the longitudinal notochord and dorsal neural tube. One or usually two chondrification centers appear (6 weeks) in the centrum and begin to form a cartilaginous model surrounded by anterior and posterior longitudinal ligaments which are complete by 7-8 weeks. Chondrification centers also form in the neural arches and costal processes. A thick ring of nonchrondrous cells establishes the model IVD around the longitudinal string of beaded notochordal segments (Fig. 6.2).
The approximate topographical landmarks in relation to the anatomy of the vertebral column and pelvis are as follows:
Inion: Prominence, midline of occipital base. C1 About 1/2 inch inferior and slightly anterior of the mastoid process. C2 First prominent spinous process below the inion. C4 Hyoid bone. C6 Cricoid cartilage. C7 Second prominent spinous process below the inion. T1 Most prominent spinous process in the region. T2 Jugular notch. T5 Angle of Louis. T7 Inferior angle of the scapula, third prominent spinous process below inion. T10 Xiphoid process. L1 Transpyloric plane. L3 Umbilicus. L4 Iliac crests. L5 Transtubular plane. S2 Level with the posterior-superior iliac spines (PSIS's). Sciatic notch: about 2 inches inferior and 1 inch lateral to the PSIS. Ischial tuberosity: about 2 inches inferior to apex of coccyx, on a vertical line through the PSIS.
A vertebra at term is in three primary ossifying centers: the oval centrum and the two aspects of the neural arch. However, the ossification process is well under way by the 16th week of gestation. Each of the three parts are united by hyaline cartilage. A cartilaginous ring develops around the anterior and lateral periphery of the centrum-disc interface, which firmly anchors the anulus to the centrum (Fig. 6.3). The two halves of the arch ossify posteriorly by appositional growth during the first year of age in the cervical region and complete ossification in the lumbar region by age 8. The progression is from above downward. The centrum, which ossifies endochondrally, joins the arch during the 3rd to the 6th year in the lumbar area and firmly fuses between the 5th and 8th year in the cervical region. In contrast, the progression is from below upward. During development, the superior and inferior cortex of the centrum thickens in the middle, and, with a cartilaginous plate that is thicker at the periphery, forms the vertebral plateau.
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