Massage & Bodywork

JANUARY | FEBRUARY 2020

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100 m a s s a g e & b o d y w o r k j a n u a r y / f e b r u a r y 2 0 2 0 technique CLINICAL EXPLORATIONS The Motion Segment of the Lumbar Spine By Whitney Lowe Joint motion is usually described as a movement between two articulating bones. That is an accurate description of joint mechanics at most skeletal joints in the body. Spinal motion, however, is more complicated, as it involves multiple joints. It also includes many soft tissues spanning those joints. In this column, we look at the complexities of the lumbar spinal structure and how it influences movement. We'll also look at how dysfunctional mechanics may bring people into our treatment rooms. A key concept to understand with spinal motion is the motion segment, also sometimes called the functional unit of the spine. The motion segment includes two adjacent vertebrae and the intervertebral disk between them. The many soft tissues that span between adjacent vertebrae may also be considered part of the motion segment. We'll explore all these tissues and their influence on spinal mechanics, focusing on the lumbar region. STRUCTURE OF THE MOTION SEGMENT We can divide the motion segment into anterior and posterior components. Imagine a vertical line posterior to the vertebral bodies as the dividing line between the anterior and posterior elements (Image 1). The anterior components of the motion segment include the vertebral body, intervertebral disk, and the anterior longitudinal ligament. The intervertebral disk and vertebral bodies manage the large compressive loads of the spine. The majority of upper body weight is transmitted to the skeleton of the lower body through the lumbar spine. That is why the intervertebral disks and vertebral bodies are largest in the lumbar region. The lumbar motion segments must carry that compressive load while still being mobile. The third component, the anterior longitudinal ligament, limits spinal extension and helps maintain the structural integrity of the intervertebral disk. The intervertebral disk's primary role is as a shock-absorbing cushion between adjacent vertebrae. It is composed of an inner gel-like substance called the nucleus pulposus, and is surrounded by a firm fibrocartilage container of concentric rings called the annulus fibrosus. The concentric rings of the annulus are oriented in alternating directions, which helps the annulus resist the high bending and torsion loads on the spine. Where the vertebral body articulates with the disk, there is a strong hyaline cartilage border called the vertebral end plate. There is no direct blood supply to the intervertebral disk. The disk relies on diffusion of its nutritional supply from the vertebral end plate. High compressive loads impair diffusion of nutrients to the disk and may be one of the factors leading to chronic disk degeneration from long-term loading.

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