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Daily Archives: January 3, 2013

Clinical Biomechanics: General Spinal Biomechanics

By |January 3, 2013|Chiropractic Care, Clinical Decision-making, Education|

Clinical Biomechanics: General Spinal Biomechanics

The Chiro.Org Blog


We would all like to thank Dr. Richard C. Schafer, DC, PhD, FICC for his lifetime commitment to the profession. In the future we will continue to add materials from RC’s copyrighted books for your use.

This is Chapter 6 from RC’s best-selling book:

“Clinical Biomechanics:
Musculoskeletal Actions and Reactions”


Second Edition ~ Wiliams & Wilkins

These materials are provided as a service to our profession. There is no charge for individuals to copy and file these materials. However, they cannot be sold or used in any group or commercial venture without written permission from ACAPress.


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.


     Background


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.

      WEIGHT DISTRIBUTION

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.

      SPINAL LENGTH

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). (more…)