Sagittal curvature of the lumbar spine is discussed in terms of how it relates or responds to the thoracic spine. Unless otherwise specified, geometric and mechanical analyses refer to upright stance posture in the sagittal plane. To analyze the abnormal, we must first be familiar with what is normal. In the normal spine, there exists a long-term equilibrium between the bending moments imposed by body weight and the spine's ability (together with its attached musculature) to resist those loads. The spine maintains its configuration within the limits of normal sagittal curvatures. Gravitation forces acting upon the head, arms, shoulders, and thorax result in a flexion moment on the thoracic spinal column for virtually all common standing and sitting postures. The spine extensors act in tension, pulling the posterior elements of the neighboring vertebrae toward one another, generating a thoracic extension moment to maintain equilibrium (Figure 1). The intervertebral disks act as compression elements, maintaining space between neighboring endplates. The disks and endplates are loaded quite evenly under normal conditions of adequate bone and muscle. That is because of the partially hydraulic nature of the nucleus pulposus component of the disk.
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